Slashback: Iridium, Synthesis, Drives

Slashback tonight with word on the (groan) fate of Iridium, more Speak n' Spell modding, examples of Serial ATA oozing to market, the RIAA versus mandatory DRM, and more. Read on for the updates.

In this household, we obey the laws of physics! Tuesday before last, we mentioned that two scientists had announced what they claim is the first accurate measure of the speed of gravity.

Now, Emperor_Alikar writes "In an article on Space.com, many physicists have criticized the current work on the speed of gravity, calling it 'nonsense' and 'simply incorrect.' Many of them still doubt the claims made by Fomalont and Kopeikin even before the results were even announced. Many of the physicists still hold on to the idea that gravity works instantaneously no matter what the distance, an idea that originated by Newton, but that was argued against by Einstein."

Back from the back from the back from the dead. Checkers writes "Spacedaily.com posted the following two stories about Iridium today. The first story is about the DoD committing the first of three renewal options that will use Iridium through 2005. The second story related story is about an agreement inked between Iridium and Harris Corp. that allows Iridium the right to use Harris' OS/COMET satellite command and control system for the life of the Iridium satellite network."

E.T. was also into this scene. In re: matt simpson writes "Another fantastic Speak & Spell modder is Dave Wright of the band "not breathing". You can check his work out, among other modifications to toys, at www.carrionsound.com Dave has made speak & spell/math/read for Nine Inch Nails, Meat Beat Manifesto, and many other bands. Figured you might be interested in other neat synth hackers :)"

Further evidence, never a good time to buy. SpinnerBait writes "It's seems like Serial ATA Controllers have been on the market forever but where have all the Serial ATA Hard Drives been? The wait seems to finally be over, as HotHardware shows with this review and showcase on a pair of new Seagate Barracuda V Serial ATA drives. This article covers benchmarks with the product in single drive configurations, as well as RAID 0. In addition, they show performance on two different SATA controllers, from Promise and Silicon Image. And oh, those nice thin neat little SATA cables! Gotta love 'em."

We've had a few articles about Serial ATA; I hope it lives up to its reputation.

Just to add to the confusion ... probejockey writes "A current article in the Globe and Mail claims SCO will start collecting licensing fees from some Linux users, not all Linux vendors as previously reported here."

Birds of a feather, separate rooms. Finally, Declan McCullagh sent in a few interesting links yesterday regarding the RIAA and its announced opposition to mandated DRM technologies:

"First, here are the photos from today's press conference.

Second, the supposed news of today's announcement was that the RIAA would no longer pursue mandatory-DRM technologies like the Hollings bill. But it was the MPAA that was behind Hollings from the beginning (September 2001). And when Hollings finally introduced his bill in March 2002, it was the MPAA that endorsed it, while the RIAA pointedly did not."

Thanks to Declan for the links.

Wasn't smart enough to get in, either ... Finally, thanks to the several readers who alerted me by email and in comments that the school variously rendered Cal Tech, CalTech and other things even worse is in fact properly spelled "Caltech."

Comments

For you proper Simpson's nuts -

[spaten-optimator]

"In this house, we obey the laws of thermodynamics!" Homer mutters this when Lisa, bored at being out of school, creates a perpetual motion machine.

Re:For you proper Simpson's nuts -

[dirvish]

...Monty Python's Flying Circus!

Re: For you proper Simpson's nuts -

[Roblimo]

You are getting the Simpsons' house confused with Timothy Lord's family's house.

I have been to Timothy's house and I will tell you, they obey the laws of *physics* there, and he and his family are not cartoons. (Except maybe his brother.)

- Robin

Re:For you proper Simpson's nuts -

[Anonymous Coward]

Actually.. it's not a perpetual motion machine.. it keeps getting faster and faster !

Iridium and GPS

[The Bungi]

1. Put satellites in orbit
2. ???
3. Profit!!

It seems they've filled in the '???' part - sell 'em to the DoD.

I couldn't help but think about the GPS system though. As the military shifts from laser guidance systems for bombs and cruise missiles to GPS-based ones, the GPS network becomes more and more critical and overloaded. Is the Iridium network being used only for simple voice/data communications or is there a dual-use capability (targeting, whatever) in the network as well?

Re:Iridium and GPS

[axjms]

I am not sure if you completely understand how the GPS system works. I am not sure if I do either for that matter but the GPS system is in very little danger of being "overloaded". It is the actual GPS device that does the triangulation calculations. All the sats do is keep track of their relative position to their peers and broadcast a unique signal. Doesn't matter if 1 device or 1 million devices are earthside it is just a broadcast.

Re:Iridium and GPS

[Russ Steffen]

GPS can never be overloaded like that. The SVs are broadcast only, they don't give a rat's arse how many things are using them.

And, GPS would never be the sole means of guidance for all weapons, by virtue of the fact that it only really works well against targets at known positions. Only laser and TV guidance work well against moving targets.

Re:Iridium and GPS

[The Bungi]

"Overloaded" - bad wording. I read an article somewhere about Iraq planning to jam GPS signals, especially around cities in the event of war. Supposedly it was cheap and simple to do that (as in, just a few hundred bucks per jamming device). The article went on to say that the AF might resort to laser-guided weapons if that was the case, which of course is problematic if there's smoke or clouds and so on.

Not that they would use the Iridium sats for GPS, but perhaps some other targeting system.

Re:Iridium and GPS

[angst_ridden_hipster]

The problem with jamming GPS is that, to do that, you need to transmit a signal.

When you transmit a signal, you make yourself vulverable to things that can sense that signal; e.g., missiles that home in on radio transmissions.

So yes, you could jam a wave of GPS-guided weapons. But if the wave of attacks includes a handful of gravity bombs or other weapons that seek those frequencies, you couldn't do it twice...

Still, a smart jamming strategy might help protect a hardened target.

.

Re:Iridium and GPS

[cristofer8]

About the GPS missiles, I recently took a tour of the Trident Missile facility in California (Moffet Field) with my school. The trident missiles, which are basically our doomsday machine, not only don't rely on GPS, but don't even really have any electronics. The gov is so worried about radiation and interference and such, that they use pressurized tubes to send signals, and orient themselves by actually looking for certain stars through a little hole in the side. Of course, they refused to explain how the "magic inertia" device worked.

Re:Iridium and GPS

[phriedom]

If you can really make a GPS jammer for a few hundred bucks, and the components are easily accquired, then the jammers are disposable. You don't place them just at potential targets. You just blanket the city with them in a 1 mile grid, favoring vacant lots and open fields. These targets are not valuable enough to use a radio-seeking missle on, nor would you want to expose your aircraft to danger to destroy stupid little GPS jammers. It is a pretty good idea, since it is a low cost system that frustrates a high-cost system.

The the previous post that suggested that the US military might want to have a system that doesn't rely on GPS seems pretty sharp to me.

Re:Iridium and GPS

[Otter]

Well, that's part of it. But the big problem with trying to jam JDAMs and other guided weapons is that (I forget the details of this but have it approximately right, at least) they have location information cranked into then upon loading, and during in-flight updates. So jamming might stop them from receiving fresh GPS data while they're falling toward the target, but that still would only decrease their accuracy from a meter to several meters.

Ahhh, here's a link.

Re:Iridium and GPS

[topham]

Except the only thing that interferes with is your ground troops. A GPS guided bomb can easily have it's antena situated to block ground signals. Even at the height they are dropped this would not significantly reduce the GPS satelite recept (assuming 90 degrees.

A quick redesign should minimize that risk.

Jammers also have to jam 2 frequencies, both civilian and military. If either of them work the results can be accurate enough for military operations.

(There is an advantage to turning off civilian signals in the area, but in the past during conflicts the signals were upgraded, not degraded because the U.S. military used civilian receivers in many instances.)

Re:Iridium and GPS

[briancnorton]

as has been pointed out, GPS is a passive system. Your little Garmin e-trex is doing a lot of math based on 4+ broadcast Pseudo Random Numbers (PRN) from the system. It's like a radio station. It's not like they can have too many listeners. The only thing that communicates with the satellites is the base station that tells them not to crash to earth.

Re:Iridium and GPS

[The Bungi]

They're probably worried about EMP more than anything else. Those things are expected to work even after the shit hits the fan.

Re:Iridium and GPS

[Wyatt Earp]

The problem with jamming GPS munitions is the fact that the aircraft dropping a GPS bomb has GPS up above the jamming.

The aircraft is connected to the bomb in flight with a databus. The plane gives the weapon updated location via the bus up to the point of release.

The bomb knows where it was, the bomb also has a ballistics computer updating the rate of decent and distance to target.

So when the weapon gets into the jamming region, it still knows where it was when it was dropped, and knows how fast/far is dropped and where the target was. The weapon still falls relativly close to the target, the CEP just increases.

I've read the CEP doubles when GPS is off/jammed. Which is still much better than a dumb iron bomb.

http://www.google.com/search?q=jdam+gps+jamming& ie =UTF-8&oe=UTF-8

http://www.darpa.mil/spo/programs/gpsguidancepac ka ge.htm

http://www.globalsecurity.org/military/systems/m un itions/jdam.htm

"Once released, the bomb's INS/GPS will take over and guide the bomb to its target regardless of weather. Guidance is accomplished via the tight coupling of an accurate Global Positioning System (GPS) with a 3-axis Inertial Navigation System (INS). The Guidance Control Unit (GCU) provides accurate guidance in both GPS-aided INS modes of operation (13 meter (m) Circular Error Probable (CEP)) and INS-only modes of operation (30 m CEP). INS only is defined as GPS quality hand-off from the aircraft with GPS unavailable to the weapon (e.g. GPS jammed). In the event JDAM is unable to receive GPS signals after launch for any reason, jamming or otherwise, the INS will provide rate and acceleration measurements which the weapon software will develop into a navigation solution."

http://www.boeing.com/news/releases/1998/news_re le ase_980423n.htm

"A new anti-jam Global Positioning System (GPS) developed by Boeing has successfully defeated jammed environments in two successive drop tests, allowing the test vehicles to strike well within their designated target areas."

"In the most recent test, the AGTFT test vehicle was dropped into a high-power GPS-jammer environment from 44,000 feet and achieved direct military code GPS acquisition within 8 seconds. While descending through wind shears of up to 110 mph, the test vehicle continued to track GPS satellites in the jammed environment and ultimately struck within 6 meters of the target.

In an earlier test, the AGTFT test vehicle was dropped from 44,000 feet into a low-power GPS-jammer environment and achieved direct military code GPS acquisition within 12 seconds. The test vehicle descended in the jammed environment through wind shears of up to 105 mph, continuously tracking GPS satellites and striking within 3 meters of the target."

Those tests were conducted in 1998.

Re:Iridium and GPS

[Lord Ender]

So, you could just put the jammers in baloons and float them 100's of feat above the area to be defended. Doesn't add much cost to it. What is an interesting question though, is can the US military change the signal GPS is broadcast on in case something like this happens? That would be pretty neat.

Re:Iridium and GPS

[The Bungi]

I've read the CEP doubles when GPS is off/jammed. Which is still much better than a dumb iron bomb.

True, but still a problem. In Afghanistan if you miss you hit another mountain or a patch of desert. In Baghdad you miss and you hit a hospital, or a mosque or a "baby formula factory". Not that I'm particularly concerned about that, but the rest of the world probably is. And I'm sure civilians are as well.

BTW, thanks for those links. Interesting reading.

Re:Iridium and GPS

[marcsiry]

Many GPS guided weapons have inertial guidance as a backup. If they lose guidance from the satellites, they remember where they where, and roughly what direction and how far they had to go to hit their target.

This article claims that GPS jamming reduces the accuracy of a 200 lb JDAM to +/- 100 feet; considering the destructive power of those weapons, the difference is academic against all but hardened targets.

The frequencies are fixed; they'll only change when the next generation of GPS satellites are launched, a prospect that hasn't even been planned yet. Anyhow, any sort of technological countermeasures deployed by Iraq against its much more powerful enemies are going to be a speed bump at best- they're hopelessly overmatched. Their best bet will be evasion, deceit, and propoganda- the only things that (barely) worked for them in Gulf War I.

Re:Iridium and GPS

[CharlieG]

It's a little harder than you might think - remember that Military GPS uses a second encrypted signal, so you can tell when your being spoofed

Now, what is the RANGE of your $100 jammer. Let's say 1/2 mile - 2600 ft. Your JADM up at, oh, 35,000 feet isn't jammed, and knows RIGHT where it is - all you have to know where the target is. You drop your rock, which is falling at, oh, lets call it, 240MPH (for easy math) - one mile every 15 seconds - it's in range of your jammer for only the last 7.5 seconds, and remember, the guidance package guides the bomb in like a funnel - by the time your 1/2 mile up, your already fairly well centered on the target. The system sees the spoof, and stops acquiring data (aka, you can't feed BAD data, you can just stop the good) - now how far off is that bomb going to end up? Probably worst case will be in the low order of 10s of feet. Most JADMs are mounted on 2k lb bombs - doesn't make much of a difference

You don't thing that the military hasn't thought about jammers, right? ECM, and ECCM go back into the 40s, and we're about the best in the world at it. Thank Hughes for that

Re:Iridium and GPS

[CharlieG]

I just did a little web search, the terminal velocity of a MK82 is around 1000fps - so now your jammer has only 2.8 seconds to confuse the bomb

Good luck

Re:Iridium and GPS

[Wyatt Earp]

For ops like that, there will be TV Mavericks or SLAM-ERs as an option. If it's a radar/AAA/SAM, use HARM.

Or AGM-142 Popeyes, or something else from the bullpen.

http://www.globalsecurity.org/military/systems/m un itions/agm-84.htm

http://www.globalsecurity.org/military/systems/m un itions/agm-88.htm

http://www.globalsecurity.org/military/systems/m un itions/agm-142.htm

US/UK in South Watch have taken to dropping bombs filled with concreate mix on urban AAA/SAM/Radar targets to avoid civilian casualties since 1999.

JDAM are nice and cheap, but they aren't the only thing that can be used in a jamming rich environment.

Re:Iridium and GPS

[Noren]

The other important function of the satellites and feature of the signals is that each satellite has an atomic clock and the signals sent include an accurate (at least on the order of nanoseconds) time the signal was sent. The difference in sending times of simultaneously recieved signals is then used to determine relative distances of each satellite to the reciever, which then is used to calculate the position of the reciever.

One other minor bit, more relevant to the military discussion- each satellite transmits on two different frequencies, but the other frequency is encoded and intended for use solely by the US military (and allies?). Using two different frequencies presumably allows for significantly increased accuracy, as diffraction based errors can be compensated for. In addition, this extra band makes it easy for the US military to turn off or to send intentionally deceptive signals on the public band in the event of armed conflict with an enemy who uses GPS...

Re:Iridium and GPS

[pcardoso]

perhaps you should be concerned too.

collateral damages? f*ck that, it's some iraqi that's going to die, so it's probably good, right?

"one death is a tragedy, one million deaths are a statistic".

and this saddens me....

Re:Iridium and GPS

[MythosTraecer]

It was my understanding that weapons using GPS always have backup guidance systems in case GPS fails (either through enemy jamming or some failure of the onboard GPS hardware).

Okay, answer me this:

[TClevenger]

If the Sun suddenly disappears (hypothetically), would the Earth continue to hold its orbit for 8 or so minutes, or would it go whizzing off into space instantly? Does this new "Speed of Gravity" research change that answer from what it was, say, a year ago?

Re:Okay, answer me this:

[Anonymous Coward]

Does this new "Speed of Gravity" research change that answer from what it was, say, a year ago?

No, it just changes what the physicists tell you.

Re:Okay, answer me this:

[DAldredge]

That would be a BAD THING. I line with current US economic thinking BAD THINGS do not exist. So your question is not a valid question. You must be a terrorist supporter...

Re:Okay, answer me this:

[benjcorey]

TClevenger wrote:
If the Sun suddenly disappears (hypothetically), would the Earth continue to hold its orbit for 8 or so minutes, or would it go whizzing off into space instantly? Does this new "Speed of Gravity" research change that answer from what it was, say, a year ago?

If the sun disappeared now the Earth would instantaneously...how you say, 'whizz off' into space instantly. But if the sun would have disappeared, say, last year, then the sun would have maintained orbit for 8 minutes THEN it would 'whizz off'

Re:Okay, answer me this:

[Mac Degger]

An interesting question like that deserves an interesting answer. Too bad it's "we don't know".

Welcome to the wonderfull world of science, where a smart person can ask more than a more informed person can answer :)

Re:Okay, answer me this:

[dougmc]

If the Sun suddenly disappears (hypothetically), would the Earth continue to hold its orbit for 8 or so minutes

If you believe that gravity obeys the speed of light (I do), yes.

would it go whizzing off into space instantly

If gravity is instantaneous, yes.

Of course, it's not that simple. The sun can't suddenly disappear.

It could explode. Assuming that it forms a sphere with all the mass at the outer edge, the gravity that the Earth feels wouldn't change at all until the mass reached the Earth's orbit, and then it would immediately drop to zero (I forget the law that tells us that the gravity of a spherical body is the same as if all the mass was contained at one point in the center.)

Unfortunately, the mass would come at the Earth at less than the speed of light, so this wouldn't be a good test. That, and it would kill us all, so if you do find a device to make the Sun blow up, I suggest not using it for this.

The Sun could be grabbed away by some massive force -- but the source of this `massive force' (super massive spaceship? God? Galacticus?) would have gravity too, and that would affect us. That, and the Sun couldn't leave at more than the speed of light, so even that's not a good test.

It's not easy to measure this :)

Re:Okay, answer me this:

[aafiske]

Well, let's experiment. You get a doomsday device, annihilate the sun, instantly destroying all trace. If we were still affected by gravity, the time it takes it find out about the sun disappearing should be about 8 minutes, right? But! If we instantly started whizzing off into space, it should take longer than 8 minutes for the lack of light to reach us, because we'd be further from the sun.

There we go. So why can't this actually be done? (not destroying the sun, but the same basic experiment.) Maybe we can't get the distances and precision that we need to be able to tell if the lack of the sun-type-object made any difference.

Mad scientists, where are you?

(Is there something I'm missing, btw? Wouldn't this work? In theory.)

Re:Okay, answer me this:

[Brown]

As I understand it (not particularly well :-) ), it would indeed - but the Earth wouldn't know the sun'd disappeared, as light, and any other information, is limited to, well, the speed of light..

Thus from the earth's point of view, you could say it goes 'whizzing off' (a slightly dodgey concept in itself...) the moment it knows the sun's disappeared, and instantly from our perspective.

- Chris

P.S. I'm an engineer, not a physicist...

Re:Okay, answer me this:

[Guppy06]

"would the Earth continue to hold its orbit for 8 or so minutes, or would it go whizzing off into space instantly?"

Yes and yes.

For an observer on Earth, the planet will start leaving the star system as soon as the sun vanishes.

For an observer equidistant from both the sun and the earth, the earth will start flying out of the star system ~8.5 minutes after the sun vanishes.

For an observer on/near the sun, the earth will start to leave its path ~17 minutes after the sun vanishes.

And they're all right. Ain't relativity grand?

Re:Okay, answer me this:

[CokeBear]

If the sun suddenly disappeard, the earth would not go whizzing off anywhere, since there is no opposing force pulling it out into space. An object in motion tends to stay in motion... yadda yadda yadda... (hereafter known as the yadda theory of motion) so the earth would continue along its present path through space, although it wouldn't be a spherical path anymore, it would be more like a straight line... so I guess the answer is... yeah, the earth would go whizzing off into space.

Re:Okay, answer me this:

[vadim_t]

I always had a problem with that law.

In physics we were taught to assume the earth is an infinitely small point with the mass of the earth, and that to calculate how strongly it attacts something you just calculate radius_of_earth + objects_distance_from_surface.

However, this must be an approximation because it'd mean the center of the earth is a black hole.

Re:Okay, answer me this:

[hklingon]

... What if the sun were converted to energy instead? .. Since we can create anti-protons and anti-electrons now... Would it be possible to create some relatively equal small pockets of matter and anti-matter and have them combine to test this? A third party could observe the gravitational effects.. Can this not be used to measure gravity?? The matter would become energy .. and energy doesn't create gravity .. so .....

Re:Okay, answer me this:

[Guppy06]

"The sun can't suddenly disappear."

It can if you hook it up to an infinite improbability drive.

(Douglas Adams books and Road Runner cartoons make more sense the more quantum you learn)

Re:Okay, answer me this:

[afidel]

It's worse than an estimate because it is based off a false assumtion, that the earth is a perfect sphere. In reality the earth bulges near the equator.

Re:Okay, answer me this:

[Grog6]

How about doing the experiment outside the space station, with two smaller masses orbiting each other.
Have to get far enough away that the gravity generated by the space station wouldn't affect your setup, and other details, But; Somewhere, in orbit, we should be able to do this experiment.

Two masses, one lead and dense, one made of a less dense material easy to disperse rapidly.

After the two masses are orbiting each other, we detonate one, (make one out of TNT) , while recording positions of the other, (in 3d) and see what happens.

the amount of debris hitting the orbiting body can be determined and taken into account;

The resultant path should show whether it's instantaneous or not. (does it's path change before the debris reaches it?)

What else am I missing?

Re:Okay, answer me this:

[norweigiantroll]

I still don't understand -- I thought the speed of gravity was 9.8 meters per second?

Thanks for clearing this up.
-- An Anonymous FrontPage 2000 User

Re:Okay, answer me this:

[fredrik70]

well energy is just mass in another form. dunno how the gravity would be affected though...
anyone knows?

Re:Okay, answer me this:

[Planesdragon]

And they're all right. Ain't relativity grand?

Just because no force can travel faster than c doesn't mean that time is all wacky.

If you take two syncronized clocks, travel for one light-minute in opposite diretions at equal speeds, and then smash one of the clocks when it reads 9:00 a.m., the other clock can know that it lots its twin when IT says "9:00 a.m.", not when it says 9:02 a.m.

*sigh* Of course, if you really understood it, you'd be correcting stuck up physicists, not harassing /. (Oh, wait, they understand it to--they just use an altered thinking state because it makes it easier to focus.)

Re:Okay, answer me this:

[austus]

Christian and RPG player seems a natural combination.

Re:Okay, answer me this:

[t]

I've always thought that if you could get to the center of the earth, you would feel weightless (aside from the crushing pressure!). This would be because all of the mass of the earth would cancel out.

Re:Okay, answer me this:

[Guppy06]

"Just because no force can travel faster than c doesn't mean that time is all wacky."

Yes, it does. Special relativity says that no frame of reference is any more "correct" than any other when it comes to measuring time and space. Space and time "rearrange themselves" (or, more honestly, you perceive them to be rearranged) so that c relative to you is ~3E8 m/s no matter what you do.

"If you take two syncronized clocks, travel for one light-minute in opposite diretions at equal speeds, and then smash one of the clocks when it reads 9:00 a.m., the other clock can know that it lots its twin when IT says "9:00 a.m.", not when it says 9:02 a.m."

When clock A is smashed, it will look back across the two light-minute gap and see that clock B reads 8:58. Clock A will know that clock B is two minutes slow and it will be correct.

When clock B sees clock A is smashed, it will see clock A read 9:00 while clock B itself read 9:02. Clock B will know that clock A was two minutes slow when it was smashed and it will be correct.

Both are correct. Neither one is more correct than the other because neither frame of reference is any better or more provable than the other. That's special relativity.

"Of course, if you really understood it"

If you really understood it, you wouldn't have stuck your foot in your mouth like you did with your very first sentence. Unless you have some astounding new evidence of a flaw in the theory of special relativity (something nobody has done in about a century or so), this is the way the universe works.

Re:Okay, answer me this:

[starsong]

Just because no force can travel faster than c doesn't mean that time is all wacky.

YES it DOES. Time dilation is a well-established and experimentally measured effect of relativity. Hell, we even have to correct for it to properly calibrate aircraft navigational systems!

Interestingly, it's not strictly forces that can't travel faster than c, it's energy. Which means mass, and possibly information, depending on who you talk to. Since we have good reason to believe that all forces are mediated by particles, that would indeed seem to limit them to c.

It's been a while since I studied special relativity, but there are certain things that stuck with me. One of the most amazing things Einstein said is that there is NO SUCH THING as "simultaneous" events. Such a concept would require that there be an absolute reference time frame a la Newton. If you have a couple of observers watching events happen, be they signal flares, radio waves or even detonating alarm clocks, they will in general NOT agree on WHEN the events occur. Even more spooky, even if you correct for the speed-of-light delay between the observers and the events, their observed time intevals will still not match up.

The "altered mental state" you refer to among physicists is not a convenient falsehood or an approximation to make the calculations easier to grasp. It's what the universe is really like... more amazing and harder to understand than anything else in our lives. And that includes RPGs. :)

Re:Okay, answer me this:

[ShavenYak]

That really has nothing to do with relativity. Let's say there are two huge clocks, synchronized perfectly, on on the Sun and one on the Earth. I'll also pretend they're exactly eight light-minutes apart. At midnight on both clocks, the Sun disappears (but its clock stays around). Both clocks will show 12:08 when Earth first realizes the Sun is gone and moves into space. Interestingly, an observer on Earth will see the Sun's clock saying 12:00. They will realize that the Sun has really been gone for eight minutes. An observer at the Sun sees his clock reading 12:00 when the Sun disappears, and Earth's clock reads 11:52. When the Sun's clock reads 12:08, this observer knows that Earth is moving out of orbit - but he won't see it happen until the light from Earth comes back. Sun's clock will be 12:16, and he'll see Earth's clock reading 12:08. How this all looks to the stationary observer in the middle is left as an exercise for the reader.

Where relativity comes into play is if we have a moving observer going at, let's say 0.5c. When this observer looks at the clocks on Earth and Sun, they will not be synchronized EVEN AFTER he corrects for the travel time of the light. For example, at the moment of the Sun's destruction, the Sun's clock will read 12:00 in his frame of reference. At this exact time, as measured by the traveller, Earth's clock will already be past 12:00. The traveller will actually see a time later than the 11:52 you would expect him to see, and from that he can calculate what time it "is" on Earth.

Eight minutes later by Earth/Sun time, the traveller is halfway between. The Sun's clock will read somewhat less than 12:04 and Earth's clock will read somewhat more than 12:04. Knowing that he is four light-minutes from each, he can figure out that the Sun's clock actually reads a bit earlier than 12:08, and earth's clock a bit past 12:08. The difference between the two clocks, from his perspective, will be the same. Both clocks appear to him to be running at the same speed, slightly slower than his own. Knowing the speed of light, he will figure out that slightly less than eight minutes elapsed on our clocks between the disappearance of the Sun and the Earth leaving orbit. If he stops at Earth, though, he will disagree with us about the time our clock showed when the Sun vanished. Well, actually if he can travel half the speed of light, he should know relativity and be able to figure out how it looked from our frame of reference as well.

If the traveller had a clock, as soon as the Sun winked out we'd see his and the Sun's clocks at 12:00. Four minutes later at 12:12 our time, we'd see him halfway between us and Sun, the Sun clock would read 12:04 and his clock would read LESS THAN 12:08. We'd realize that his clock was running slower than ours and the Sun's.

The thing is, relativistic time dilation is NOT just an illusion caused by waiting for light to come from somewhere. It is absolutely real. Two things that seem simultaneous to me will not be simultaneous to you if you're moving quickly relative to me. It's really hard to wrap your head around what it all means. In fact, I may very well have screwed something up in the explanations - I just hope the concept came across.

Re:Okay, answer me this:

[zod1025]

Even more spooky, even if you correct for the speed-of-light delay between the observers and the events, their observed time intevals will still not match up. What the HELL are you talking about? Thanks...

Re:Okay, answer me this:

[Guppy06]

"Interestingly, an observer on Earth will see the Sun's clock saying 12:00. They will realize that the Sun has really been gone for eight minutes."

Can he prove it? No. Until the "sun's gone" signal reaches Earth, for all intents and purposes it is still there. There is still light and heat coming from it, and there is still its gravitational influence on everything in the star system. Special relativity says that you can't prove that isn't the case.

All he knows is that, according to an observer equidistant from both the sun and the earth, the sun will disappear eight minutes before the earth reacts accordingly. But that doesn't mean a damn thing to the people on the earth, and it can't ever mean anything because that observer's communication of "Hey, the sun is gone" is also limited by relativity.

"When the Sun's clock reads 12:08, this observer knows that Earth is moving out of orbit - but he won't see it happen until the light from Earth comes back."

"Knowing" has nothing to do with it. His knowledge of special relativity allows him to predict what will eventually happen (which is the point of studying physics), but as far as the sun-based observer is concerned, it hasn't happened yet. He can still see the reflected light off the earth and he can still feel its gravitational influence.

I know the sun will rise in my time zone in about six hours. Does that mean it's already happened?

"Where relativity comes into play is if we have a moving observer going at, let's say 0.5c. When this observer looks at the clocks on Earth and Sun, they will not be synchronized EVEN AFTER he corrects for the travel time of the light."

His clock is fine. It's the rest of us that are moving at 0.5 c, and ours are the clocks that are moving slow. And there's no way you can proove that one observation is "more correct" than the other, because Einstein tells us that both observations are equally valid.

Re:Okay, answer me this:

[otmar]

Assuming that it forms a sphere with all the mass at the outer edge, the gravity that the Earth feels wouldn't change at all until the mass reached the Earth's orbit, and then it would immediately drop to zero (I forget the law that tells us that the gravity of a spherical body is the same as if all the mass was contained at one point in the center.)

If I remember correctly from school physics (and that's 15 years past, and I'm too lazy to do the integrals now.), the gravity pull of e.g. the earth (with its mass evenly distributed) has the following properties:

a) at the center of the earth, there is no gravity pull in any direction.
b) moving from the center to the surface, the gravity increases in a linear fashion and reaches 1g. (9.81 m/s^2)
c) if you move away from the earth, the force diminishes by 1/r^2, where r is the distance to the center of the earth.

(assuming of course a perfect sphere for the earth and a homgeneous mass density.)

/ol

Re:Okay, answer me this:

[myom]

The gravitational force (g) is in Norway around 9.82m/s^2. Note the ^2. An object accelerates downward with 9.82m/s per second. (in vacuum, and not exactly "down").

This discussion is whether this gravitational force is instant, or if it moves with the speed of light.

Myom

Re:Okay, answer me this:

[stud9920]

(I forget the law that tells us that the gravity of a spherical body is the same as if all the mass was contained at one point in the center.)

IAAEETA (I Am An Electrical Engineering Teaching Assistant). As Newton's fourth law has just the same form as Coulomb's law, we can safely assume that a gravitational field can be calculated just the same way as an electrical field.

The law I will invoke here is one of the four Maxwell Laws : div D = rho with rho the electric density and D the electric field.

If you integrate it you get \int divD.dS=Q_inside.

As long as you are outside the sun's radius, Q_inside is a constant (in our case something like the solar mass, or 4/3 pi r^3 rho - volume*density). the lhs on the other hand will be taken as a sphere (the sun is spherical so it is safe to assume that its field is radial, for isotropy reasons. Integrating over a sphere is a no brainer). The lhs will become 4pi r^2 * D (4pi r^2 is the surface of a sphere with radius r).

Eliminating the superfluous constants and bringing every known one to the rhs we get D=(R^3*rho)/(3r^2)

So indeed you are right in saying that. Replace D with whatever letter suist you, IANAPTA (I am not a physics teaching assistant)

Re:Okay, answer me this:

[OneEyedApe]

F=ma.

9.82m/s^2 is gravitational acceleration.

If a 100kg falls, it will be subject to a force of about 982 Newtons (Newtons being the SI unit of force). The 982 Newtons would be a gravitational force.

Additionally, when they talk about g-force in relation to rollercoasters or aircraft, what they really should say is that the person inside is moving faster than 9.82m/s^2. The actual force exerted on their body is proportional to their mass.

Re:Okay, answer me this:

[ShavenYak]

"Interestingly, an observer on Earth will see the Sun's clock saying 12:00. They will realize that the Sun has really been gone for eight minutes."

Can he prove it? No. Until the "sun's gone" signal reaches Earth, for all intents and purposes it is still there.

At 12:08 on Earth, the "sun's gone" signal has reached Earth, along with the "it's 12:00" signal from the Sun. The Earthbound observer now knows the Sun is gone, and knowing the speed of light he can figure out that the Sun disappeared eight minutes ago. This doesn't require any knowledge of relativity - even in Newtonian mechanics this would happen.

"When the Sun's clock reads 12:08, this observer knows that Earth is moving out of orbit - but he won't see it happen until the light from Earth comes back."

"Knowing" has nothing to do with it. His knowledge of special relativity allows him to predict what will eventually happen (which is the point of studying physics), but as far as the sun-based observer is concerned, it hasn't happened yet.

No, it has ACTUALLY happened in his frame of reference, he just hasn't SEEN it yet. Look at it this way, he just watched the Sun wink out eight minutes ago. Knowing the travel time of light, he can deduce that Earthbound observers are just now seeing the Sun disappear. Again, he doesn't need to know anything about relativity to determine this, it would work the same way in Newtonian mechanics.

Eight minutes from now, he will begin to see the effect of the Sun's disappearance on Earth. That does not mean that it took sixteen minutes for anything to happen - it took eight minutes to happen, and eight more minutes for him to see it happen. Again, this is the simple result of light having a finite speed; it isn't a relativistic effect. The Sun and Earth based observers have the same frame of reference and can agree on when and where events occurred.

I know the sun will rise in my time zone in about six hours. Does that mean it's already happened?

No. But, when you first see the Sun peek over the horizon, the light you're seeing left the Sun eight minutes before. The fact that you aren't seeing it until now doesn't mean it didn't happen until now.

"Where relativity comes into play is if we have a moving observer going at, let's say 0.5c. When this observer looks at the clocks on Earth and Sun, they will not be synchronized EVEN AFTER he corrects for the travel time of the light."

His clock is fine. It's the rest of us that are moving at 0.5 c, and ours are the clocks that are moving slow. And there's no way you can proove that one observation is "more correct" than the other, because Einstein tells us that both observations are equally valid.

Right. The point is, clocks which are synchronized in the Sun/Earth frame of reference are NOT synchronized in his frame of reference. When he watches the Sun wink out, in his frame of reference the Sun clock really reads 12:00 and the Earth clock really reads a later time - 12:00 on Earth is truly in the Past for the moving observer even though 12:00 on the Sun is Now.

Another point to make is that if both Earth and Sun ceased to exist at 12:00, the moving observer would see their disappearance at different times, compute the light travel time for the distance to both objects, and determine that they really did vanish at different times - the Earth slighty before the Sun. The observer on Earth would see Earth vanish, see the Sun vanish eight minutes later, and calculate that they both vanished precisely at 12:00. They would both be correct, for their own frame of reference.

Re:Okay, answer me this:

[Guppy06]

"This doesn't require any knowledge of relativity - even in Newtonian mechanics this would happen."

Newtonian mechanics says the clock disappeared 8 minutes ago. Special relativity says the clock was 8 minutes slow in the earth frame of reference.

There is no frame of reference where you can say "it actually..." because there is no frame of reference that is more correct than any other. Measurement of time and space is relative to the observer.

That's why all observers measure c relative to them as the same value in all conditions. If there were a preferred frame of reference, then c would be different for different observers. But a century of experimental evidence shows the case to be otherwise.

"No, it has ACTUALLY happened in his frame of reference, he just hasn't SEEN it yet."

If you can't see it, hear it, or otherwise proove its existence (or lack thereof), the event hasn't happened yet. The only way the observer will know that it has (will) happened is if he has some sort of prior knowledge of a pre-arranged schedule, But that foresight doesn't mean the event happens in his frame of reference any sooner. And his frame of reference is just as valid as anybody else's frame of reference.

"Again, he doesn't need to know anything about relativity to determine this, it would work the same way in Newtonian mechanics."

Newtonian mechanics say that the delay between the sun disappearing and the earth noticing will be measured as the same for all observers. Special relativity says that there is no absolute frame of reference for the measurement of time. It will be different for different people based on location and velocity relative to the event. I can give you observers that will say that that delay is anywhere between "instantaneous" and "forever," and all of these measurements will be 100% valid.

In fact, special relativity states that if an observer is able to move faster than light, they will see the earth leave its orbital path before the sun disappeared. And that observation as well would be 100% correct.

"12:00 on Earth is truly in the Past for the moving observer even though 12:00 on the Sun is Now."

You assume that one clock is "more correct" than the other two. Special relativity says that all the clocks are correct for all observers.

An earth-bound observer will see the spacecraft's clock as two minutes slow, and he will be correct. The astronaut will see the earth-bound clock as two minutes slow, and he too will be correct. They are both correct because there is no absolute frame of reference.

"No. But, when you first see the Sun peek over the horizon, the light you're seeing left the Sun eight minutes before. The fact that you aren't seeing it until now doesn't mean it didn't happen until now."

Proove it with experimental evidence. Give me hard evidence that the light I'm seeing from the sun left the sun eight minutes ago.

For every observer that you give me that says "eight minutes," I can give you an infinite number of observers that will give you an infinite number of measurements, none of which are eight minutes. And they will all have physical evidence prooving their observations.

"and determine that they really did vanish at different times"

He can compute whatever he wants. In his frame of reference, they really did disappear at the same time. His computations have no effect on the reality of the situation in his frame of reference.

"the Earth slighty before the Sun. The observer on Earth would see Earth vanish, see the Sun vanish eight minutes later, and calculate that they both vanished precisely at 12:00. They would both be correct, for their own frame of reference."

No, they're only correct for the frame of reference of the half-way point, which isn't any better than earth-based or sun-based observations. The observers can calculate whatever they want, but (barring quantum mechanics) as far as reality is concerned at their individual frames of references, there was an eight minute gap between the two occurrences. All physical evidence that can possibly be gathered by those two observers will demonstrate that eight minute gap.

It's obvious from your posts that you know little about what special relativity says. Why don't you try reading a little about the classic barn and pole scenario before you try responding again.

Re:Okay, answer me this:

[Planesdragon]

no frame of reference is any more "correct" than any other when it comes to measuring time and space.

That's because we're always in motion, and don't have the equipment to measure an absolute (i.e., relative to the universe as a whole) reference. And even if we did, we'd stick to local references for the same reason we have heliocentric models of the solar system--it's just simpler this way, and lets the Smart Folk focus on things other than unnecessary abstraction/correction.

Both are correct. Neither one is more correct than the other because neither frame of reference is any better or more provable than the other. That's special relativity

That's a fancy way of saying "no information can travel faster than light."

When clock A looks at Clock B and sees 8:58, and it knows that clock B is two light-minutes away, clock A knows that clock B _should_ say 9:00 and if we were to instantly go from B to A that we would see 9:00.

The speed of time is flexible, but that's not "wacky"--that is, one moment always follows the next. If the sun vanishes now, it doesn't vanish eight minutes later "for us," we just only notice it eight minutes later.

There really really is an objective reality and an absolute time frame & spaital system--but it's not labled, and we have no way to see it, so those that "get" astrology tend to ignore the fact that we're just dealing with what we can percieve, and that causes a lot of wannabes and confused students to harp to a nonsense idea that blab all kinds of nonsensical statements when it comes to the topic.

If we have a colony in the outer reaches of the solar system that explodes, and it takes two days for the "we're exploding" message to get to Earth, we would properly mark the anniversary on the date of the explosion, not the date that we recieved the transmission.

Re:Okay, answer me this:

[dougmc]

As Newton's fourth law has just the same form as Coulomb's law, we can safely assume that a gravitational field can be calculated just the same way as an electrical field.

You are correct. Knowing that, and the formula involved, it's just simple caclculus. But there is a name for it, I just can't remember it or find it anywhere. Actually, it wasn't a law but a theorem.

(I got my physics degree 10 years ago and have a mind like a sieve ...)

Re:Okay, answer me this:

[Planesdragon]

One of the most amazing things Einstein said is that there is NO SUCH THING as "simultaneous" events. Such a concept would require that there be an absolute reference time frame a la Newton.

Picking an absolute reference is easy, but it'd be complex, as we'd be forever locally correcting for relative speed, distance, and time dilation.

Of COURSE there are simultaneous events--we just have neither the math nor the instruments to tell which events are simultaneous.

f you have a couple of observers watching events happen, be they signal flares, radio waves or even detonating alarm clocks, they will in general NOT agree on WHEN the events occur.

Only if they're physically far apart for the speed-of-light delay to outrace their local reaction time.

Even more spooky, even if you correct for the speed-of-light delay between the observers and the events, their observed time intevals will still not match up.

Well, yeah. That's because all three points are in motion, and you need to correct for red-shift / blue-shift as well.

Also, when have we have the combination of accuracy / distance to test this, anyway? Even opposite sides of the planet can't be more than a light-second away, which would be well within the margin of human error for timing an event--and the moon can't be much more that that, but I don't know of any astrological observations from the moon...

The "altered mental state" you refer to among physicists is not a convenient falsehood or an approximation to make the calculations easier to grasp. It's what the universe is really like... more amazing and harder to understand than anything else in our lives.

That's religion talking, not science. The difference between an absolute universe and a subjective universe is one that we, as of yet, cannot test or verify.

Science limiting itself to local subjective reference isn't any more "correct" than absolute measurements, just as picking the sun as the center of the local universe is any more correct than picking the center of the Earth or the Moon or the city of Rome. It's just simpler, and so that's what parsimony tells Science to use.

Unfortunately, a lot of scientists become or start out as atheists, and have a hard time seperating their religious views from their scientific observations. (An atheist scientist saying "there is no God" is making a religious statement; if he said "we have no evidence for God" or "God is unproven" or didn't menton the great fuzzy one at all, _then_ he's scientific.)

And that includes RPGs. :)

Hey, you're a GURPS player, aren't you! That or rolemaster... ;)

Re:Okay, answer me this:

[dougmc]

If I remember correctly from school physics

You do remember correctly.

However, I simplified matters a little bit. I said that the explosion compressed the entire mass of the sun into a `shell' that came flying at us. Before the shell reaches us, we see full gravity. After the shell passes us, we'll see *no* gravity (and we'll be dead.) (You can do the calculus on this one if you want.)

If instead of making a shell, it expanded into a homogenuous sphere that got larger and larger, what you described is exactly what would happen.

(I just chose the `shell' because it would make a more `sudden' change.)

But even if the sun could disappear immediately, even so it wouldn't tell us if gravity obeyed the speed of light.

There's two possibilities :

1) suddenly the sun disappears, and we go flying off into space at the same instant.

2) we go flying off into space for no apparant reason, and then the sun disappears.

It *seems* like we'd then know if gravity obeyed the speed of light, yes? No, actually.

The problem is that we'd not know when we started speeding off into space. There would be no acceleration detected on the Earth, no jolt -- we'd not feel the loss of the Sun's gravity. And the only way we could determine our velocity would be to pick something remote (like a star, preferably many stars) and watch our relative veleocity.

We might be able to detect the change if we were already watching the relative velocity of a star (via the red/blue shift of the spectral lines.) But I don't think this is accurate enough for us to detect the orbital motion of the Earth.

In any event, in the real world, measuring the speed of gravity is tricky :)

Re:Okay, answer me this:

[dougmc]

In physics we were taught to assume the earth is an infinitely small point with the mass of the earth, and that to calculate how strongly it attacts something you just calculate radius_of_earth + objects_distance_from_surface.

That's not quite what you were taught. Close, but there's a subtle difference.

Nobody says that the mass of the Earth is all at the middle. What they say is -

1) to `correctly' calculate the gravity would require a computer billions of years, calculating the gravity for each subatomic particle. We need to simplify the matter somehow or we'd never get anywhere.

2) the `mass is all concentrated at the center' simplification is 100% accurate (and can be mathematically proven) if the mass in question is sherical and homogenuous (actually, it doesn't have to be quite homogenuous -- as long as it's 1) spherical and 2) the density at a given radius is a constant, it'll work) so it's a much simplier way of calculating that.

3) the Earth is not a perfect sphere, of course, and it's not homogenuous. But it's relatively close. If we need more accuracy, we can apply some corrections to the simple F=Gm1m2/r^2 formula -- but for most astronomical calculations, this isn't needed.

Physicists are always looking for ways to simplify problems into simpler ones. Calculus is a powerful tool for taking simplified rules and applying them to real world problems.

In the same vein, we could use Relativisitic Quantum Mechanics to calculate the trajectory of a baseball -- but for the vast majority of things, the Newtonian laws work just fine, and are zillions of times simpler, so we use those.

Re:Okay, answer me this:

[Guppy06]

"That's because we're always in motion,"

Or always at rest. Or anything in between. You can proove that we're always in motion. I can proove we're always at rest. We'd both be right.

"and don't have the equipment to measure an absolute (i.e., relative to the universe as a whole) reference."

It's not that "we don't have the technology," it's that special relativity says there is no absolute. You can't use the fabric of space-time as an absolute because measurement of space and time is relative to the observer.

If there were an absolute frame of reference, then not all observers would measure c to be ~3E8 m/s relative to themselves, and they could use that difference to extrapolate an absolute frame of reference. It doesn't matter if you're standing on the earth's surface or flying away from it at .99c, you always measure the speed of a beam of light coming from the earth's surface as ~3E8 m/s relative to you.

In terms of Newtonian mechanics, c + .99 c = c

"When clock A looks at Clock B and sees 8:58, and it knows that clock B is two light-minutes away, clock A knows that clock B _should_ say 9:00"

"Should" has nothing to do with it. All physical evidence demonstrates that the clock says 8:58.

"and if we were to instantly go from B to A that we would see 9:00."

First off, there is no such thing as "instantly." Two events occur simultaneously only to a particular frame of reference.

Secondly, according to special relativity, if you were able to travel faster than light, when you got to clock B it would still say 8:58. As far as you would be concerned, you just travelled backwards in time.

More information on this here.

"that is, one moment always follows the next"

That's not relativity, that's the concept of causality. Relativity states that the time-order of two events can be whatever you want them to be so long as you can achieve the proper velocity. For an observer moving faster than light, the two events will be reversed ("effect" precedes "cause"). Causality is just a hunch that has survived to this day because of our inability to find any tachyons.

Causality, relativity, FTL. Pick any two.

"If the sun vanishes now, it doesn't vanish eight minutes later "for us," we just only notice it eight minutes later."

Physics is all about our ability to measure and interact with the rest of the universe. Special relativity assures us that any and all scientific, physical evidence for the sun's existence is still there on the earth. Therefore, for literally all intents and purposes, the sun is still there.

Saying that the sun "really" disappeared eight minutes ago presupposes a preferred frame of reference, an absolute frame that is somehow "more right" than another. Special relativity states that there is no such frame.

(Oh, and for an observer travelling sufficiently faster than c, the earth will leave its orbit eight minutes before the sun vanishes.)

"There really really is an objective reality and an absolute time frame & spaital system"

Proove it. All physical evidence collected in the past century or so in the study of special relativity says that you're wrong, but I will welcome hard evidence to the contrary if you can produce it.

"but it's not labled, and we have no way to see it,"

If it can't be detected/measured/etc., then as far as physics is concerned it doesn't exist.

"so those that "get" astrology tend to ignore the fact that we're just dealing with what we can percieve,"

I won't harp on your use of the word "astrology." However, you're getting hung up on the term "perception." As far as physics and pretty much all science is concerned, perception is reality. Science is the study of the measurable, observable universe. If it cannot be touched, heard, seen, measured, clocked, or otherwise detected, if it doesn't interact with the rest of the universe in any conceivable way, shape or form, then it is outside the realm of science.

You can say "It's there but we just can't detect it" all you want. Until you are able to detect it, it is purely conjecture. And, in this case, it happens to be conjecture that flies in the face of a century of physical evidence pointing to the contrary.

(I'm not saying it's impossible, I'm saying I'll believe it when I see it.)

Re:Okay, answer me this:

[Scott Carnahan]

The matter would become energy .. and energy doesn't create gravity .. so .....

According to general relativity, energy does make a contribution to gravitational effects. Einstein's field equations include the stress-energy tensor, which for each point in spacetime gives information about the energy (including mass-energy) density, momentum density, and stress (e.g. pressure) associated to all forms of matter and all non-gravitational fields [MTW].

The problem is that if you assume Einstein's field equations, you automatically get the assertion that gravity "travels" at c, the speed of light in a vacuum. Any alternative theory regarding speed would have to include some change in the field equations, which have made some very strongly verified predictions in the last 85 years. On the other hand, if you had some alternative theory that did not have the same dependence on the stress-energy tensor, and if it predicted a gravitational change from an annihilation event, then you might be able to test its validity using such an experiment.

Re:Okay, answer me this:

[ShavenYak]

Newtonian mechanics says the clock disappeared 8 minutes ago. Special relativity says the clock was 8 minutes slow in the earth frame of reference.

You're missing the simple fact that the Earth and Sun observers are in the same frame of reference, because they are not moving with respect to one another. Well, Earth is orbiting Sun, but two observers who are motionless with respect to one another but are eight light minutes apart would see the same effect.

There is no frame of reference where you can say "it actually..." because there is no frame of reference that is more correct than any other. Measurement of time and space is relative to the observer.

Yes, I can say something "actually" happened at a specific time and place in a particular frame of reference. In another frame of reference, the time and place as well as the distance and elapsed time between two events will be different. You're still missing the point that unless Earth is moving toward or away from the Sun at a considerable velocity, the observers in each place are in the same frame of reference and in that frame of reference the Sun disappears eight minutes before the observer at Earth sees it.

Special relativity says that there is no absolute frame of reference for the measurement of time. It will be different for different people based on location and velocity relative to the event.

No, only relative velocity will affect the measurement of time. Distance only affects how long after an event happens that you actually see it.

In fact, special relativity states that if an observer is able to move faster than light, they will see the earth leave its orbital path before the sun disappeared. And that observation as well would be 100% correct.

Well, actually the equations work out to give elapsed time values that are negative imaginary numbers, which is one of the reasons we assume an object with real rest mass cannot exceed the speed of light. But that's neither here nor there.

Proove it with experimental evidence. Give me hard evidence that the light I'm seeing from the sun left the sun eight minutes ago.

Simple. The sun is roughly 150 million km away from Earth. Light travels at 300,000 km/s. Thus, light you see now left the Sun 500 seconds ago, or 8 minutes 20 seconds. In your frame of reference, of course. And also from the Sun's frame of reference, since the relative motion of the two bodies is negligible compared to c. Why exactly is this a poit of contention?

It's obvious from your posts that you know little about what special relativity says. Why don't you try reading a little about the classic barn and pole scenario before you try responding again.

I've already read it many times. It has no bearing on the simple scenario of two observers at rest relative to one another turning a light off. The two observers share the same inertial frame of reference, and the proper time at which the light is turned off is the same for both of them. One just doesn't see it until a bit later due to light traveling at finite speed. But both of them will agree on the time of the event. Only an observer in motion relative to these two will witness any relativistic effects, and will disagree with them on whether various events are simultaneous.

Re:Okay, answer me this:

[hklingon]

ahh.. I see. I think, after doing a bit of research, I get all the energy but I'm left with two photons from my reaction, anyway. But if gravity were not instant, how could things orbit one another? in my minds eye, I see the earth pulling on the moon.. and the moon pulling on earth, but the moon is falling past the earth. If the speed of gravity weren't instant, it is hard to imagine circular or elliptical orbits..

Re:Okay, answer me this:

[Planesdragon]

First off, there is no such thing as "instantly." Two events occur simultaneously only to a particular frame of reference.

Secondly, according to special relativity, if you were able to travel faster than light, when you got to clock B it would still say 8:58. As far as you would be concerned, you just travelled backwards in time.

Bullocks. If you were to travel "instantly" (that is, in reality with an infinite ammount of speed), you'd get there when the clock said 9:02 and not an instant earlier. You'd "pop" foward four minutes, not back two.

But if you were to drop out of reality and appear at B, you'd see it saying 9:00.

Special and General relativity deal with what is possible and observed. Time dilates, movement is relative, and the only constant is c. But were you able to do the impossible and move faster than c, you would not travel backwards in time. (You'd probably be tossed outside of the universe and die, but we're talking drunken-physicist hypothetical here, anyway.)

Physics is all about our ability to measure and interact with the rest of the universe. Special relativity assures us that any and all scientific, physical evidence for the sun's existence is still there on the earth. Therefore, for literally all intents and purposes, the sun is still there.

Yes. And up and until you tell a widow that her husband is dead, for all intents and purposes he is still alive for her--but he still really died.

you're getting hung up on the term "perception." As far as physics and pretty much all science is concerned, perception is reality.

Evidence is scientific reality. Perception is a fallible human trait.

No information can travel faster than c. However, it's foolish to conjecutre that just because we don't know about it yet, it _hasn't happened_.

Time travels in one direction; special and general realativity can made time vary its speed, but it's rather quacky science to assume that they make reverse time-travel possible.

Re:Okay, answer me this:

[Scott Carnahan]

But if gravity were not instant, how could things orbit one another? in my minds eye, I see the earth pulling on the moon.. and the moon pulling on earth, but the moon is falling past the earth. If the speed of gravity weren't instant, it is hard to imagine circular or elliptical orbits..

Try not to think of gravity as a force, but as a geometric phenomenon. The earth and moon are not really "pulling" on each other so much as the presence of their mass, momentum, energy, etc... are shaping spacetime, and the curvature that results makes their paths appear to orbit each other. For objects which don't move too much relative to each other, this looks a lot like an instantaneous force, and that's why Newton's theory was proposed in the first place.

However, for elliptical orbits such as Mercury's, Einstein's theory predicts a precession of the axes, and it deviates enough from Newton's instantaneous force theory that the differences are testable from Earth. The observations in question agreed with Einstein's prediction much better than they did with Newton's. This is one of several pieces of experimental evidence favoring Einstein's theory.

Einstein's principle of equivalence says (among other things) that uncharged objects in space will follow what is basically a straight line path (strictly speaking, a geodesic). However, when spacetime is curved (usually due to stuff being nearby), that path can appear bent to a distant observer. Hence, planets, satellites, and thrown objects feel no external gravity, and are actually travelling in "straight" paths, while we are being accelerated upward from our natural trajectory by our contact with the ground.

Re:Okay, answer me this:

[norweigiantroll]

I was kinda trolling. It was a joke. Hence the "frontpage 2000 user." If you've ever dealt with FP users, you know what I mean.

Re:Hey Y'all

[pummer]

true. what is with it??? read my journal, in which i rant about the system.

Infinite speed gravity?

[Doctor Fishboy]

All general relativists (and for that matter, all physicists) I know think that gravity propagates at the speed of light. In the linked articles, the criticism is that they've measured the speed of light by virtue of the radio photons, not the speed of gravity, which they're claiming.

There's nothing about 'infinitely' fast gravity in the article that I can see, and of the two physicists claiming to disagree with the results, the one who says it is 'nonsense' then refuses to comment any further.

Dr Fish

Re:Infinite speed gravity?

[Henry V .009]

That's the point, Doc Fishboy. Is general relativity correct about the propagation of gravity? After all, GR certainly doesn't agree with quantum mechanics--meaning that one of the theories will have to be revised. And does this experiment prove GR to be correct, or were they measuring the speed of light, as that Japanese dude, Hideki Asada, suggested in his paper last year?

Re:Infinite speed gravity?

[Doctor Fishboy]

What I was saying in my first comment was that the majority of physicists believe gravity has a finite propagation speed, whilst the /. tagline suggested that few physicists do. The experiment itself is being questioned by physicists thinking they're measuring the speed of light, not gravity. Apart from the Newton quote, none of the modern physicists thought that gravity propagated 'infinitely' fast - it's another usual /. not-quite-right headline.

> After all, GR certainly doesn't agree with quantum mechanics

Not sure if I agree with you there - I don't recall that QT is inconsistent with GR. The problem is that it is very, very hard to test a QT of gravity because the hypothesized quanta of gravity are 10^41 times smaller than those of electromagnetism and nuclear forces.

Anyway, they came up with a speed of gravity (if that is what they measured) of plus or minus 20 percent of the speed of light, so if they were measuring gravity, it is consistent with GR.

Dr Fish

Re:Infinite speed gravity?

[Henry V .009]

True--I think that they are making more controversy over this than there probably is. On the other hand, Physicists' gut feeling doesn't stack up to observation, and everyone would like this to be settled by experiment.

As for your other point, quantum mechanics isn't relativistically invarient. Quantum field theory is, but that's not finshed--though there are experimentally confirmed results. And quantum gravity remains a pipe dream

Re:Infinite speed gravity?

[Doctor Fishboy]

> quantum mechanics isn't relativistically invarient.

Ah, my bad. It's been too long since my QM undergrad course. Thanks for the correction!

Dr Fish

Re:Infinite speed gravity?

[u19925]

They have actually measured the speed of gravity. however, the experiment assumes the speed of light and then deduces that the speed of gravity is same.

Re:Infinite speed gravity?

[rsidd]

quantum mechanics isn't relativistically invarient.

So it's quantum mechanics which needs to be changed, not relativity -- as physicists realized in the 1920s.

Quantum field theory is, but that's not finshed

True enough, but for all practical purposes (ie, to any level of experimental confirmation today or in the foreseeable future) it is finished. The standard model explains pretty much everything we know. The problem is some mathematical uglinesses and arbitrary parameters.

Re:Infinite speed gravity?

[Henry V .009]

It was only last year that solar neutrino output from the sun was finally reconciled with theory--suggesting that neutrinos must have mass, even! Quantum field theory has no place for gravity. And quantum gravity has few experimental consequences that we can test, but it is fundamentally necessary to explain the initial stages of the big bang. Nobody seems to know whats up with supernova brightness results that seem to suggest that the universe is expanding faster and faster. Theory is not is such a splendid state at the moment.

Now I'm not saying that our current theories will be overthrown--but they will certainly be revised.

Re:Infinite speed gravity?

[SiliconEntity]

Right, the problem is that the "speed of gravity" really means the speed of changes to gravity. But when you have something like Jupiter, it moves along at a very steady pace, so there are very few changes.

Even if gravity moved very slowly, like at a walking pace, the gravitational field around planets would not be much different than it is today. For complicated reasons which my margin is too damn small to hold (luckily), the gravitational field doesn't "lag behind" the planet. It stays centered on the planet even if gravity were to move slowly.

When Jupiter moves in front of the distant galaxy, its gravitational field extends spherically around the planet, moving along with the planet. That's true regardless of the speed of gravity. It would only be slight irregularities in Jupiter's motion, perhaps due to tugs from other planets or its moons (but they are tiny tiny tiny next to Jupiter!) that could produce the irreglarities that would allow you to measure the speed of gravity. Maybe that's what was done here, but I doubt it.

Re:Hey Y'all

[Sloppy]

Check your user preferences, messages. Readers decide how much the bonus is worth. Set it to 1 instead of the default, 0, and you'll see the old behavior.

Me fail GPS ? That's unpossible!

[OldMiner]

GPS is a passive system. It can't be overloaded.

Re:Hey Y'all

[Stanley Feinbaum]

agreed... for some reason I no longer can post with +2 bonus even though my karma is excellent... Did this happen to any other users? Or am I being personally censored?

Can we turn gravity off?

[bleckywelcky]

I think the real problem with determining the speed of gravity if indeed it does have a speed is the fact that we can not turn gravity on and off. Some of the first very very very rough measurements of the speed of light were made by a light source standing away from an oberserver and being turned off and on in a way that an algorithm they designed would use the information to tell them the approximate time it took for the light to get to the observer from the source. The problem with gravity is that we can not turn it off and on. Perhaps even like we can with a magnetic field. Just get a wire, run some current through it and use a switch to open/close the circuit. We could then measure the speed of a magnetic field (if it has one). The inability to turn gravity off and on is the key inhibitor to any substantial calculations on its part. And, I'm sure that when we can turn gravity off and on we really won't care that much anymore about trying to determine how fast it travels :) (although we probably will have already).

Re:Can we turn gravity off?

[Absurd Being]

How's this matter? We found the speed of light, even though we can't turn off charge.

Re:Can we turn gravity off?

[Reckless Visionary]

But you can eliminate the source of light. Like the off switch on the flashlight. You can't eliminate the source of gravity. The fastest you could do so, theoretically, would be to move the massive object causing the gravity away from the measuring instrument at the speed of light, which would screw up the measurement. Up to the smarter ones here to figure out how the measurement would be affected, but it certainly wouldn't be the same as turning off the light. And, need I mention, how do you remove the massive object at anywhere close to the speed of light?

Re:Can we turn gravity off?

[cosyne]

Couldn't you collide some matter and antimatter? If you had some mass and then it co-annihilates, it should be like turning gravity off. I'm sure the resulting gravity change is small, but there's a possibility that it could be measureable.

Re:Can we turn gravity off?

[XO]

Hmm. Now, I'm no physicist, and for that matter, I hated science...

but..

Doesn't a centrifuge, or something like that, that spins at a really high rate of speed, generate it's own gravitational force?

Wouldn't there be some way that this could be measured from that point?

Re:Can we turn gravity off?

[John Harrison]

And, I'm sure that when we can turn gravity off and on we really won't care that much anymore about trying to determine how fast it travels :) (although we probably will have already).

Actually if we could do that then knowing the speed at which it travels would be important if that speed were greater than the speed of light. That would mean that you could transmit information faster than the speed of light. I think that the ability to send and receive such transmissions has such obvious benefits that we would be very interested in the speed of gravity. What do you think?

Re:Can we turn gravity off?

[Guy Harris]

Couldn't you collide some matter and antimatter? If you had some mass and then it co-annihilates, it should be like turning gravity off.

No, it shouldn't. A matter-antimatter annihilation isn't really an "annihilation" in the sense that the "nihil" in "annihilation" might suggest; instead, if, for example, an electron and positron mutually annihilate, you get a pair of photons, and the total energy of the photons is equal to the total energy (rest energy, from rest mass, plus kinetic energy) of the incoming electron and positron.

The photons have a gravitational field just as the electron and positron did. (Mass isn't the source of gravity - energy and momentum, and the flow thereof, are.)

Re:Can we turn gravity off?

[Anonymous Coward]

Doesn't a centrifuge, or something like that, that spins at a really high rate of speed, generate it's own gravitational force?

Nope. From the point of view of someone in the centrifuge it *seems* like there is a force pulling them outwards, away from the axis of rotation. It's important to emphasize that it only seems this way. There isn't any force pulling you outward at all.

Recall that things like to go (that is must go) in straight lines until they are messed with by something else. A centrifuge spins in a circle very quickly. You, standing in the centrifuge, are trying to move in a straight line (tangent to the circle) but the floor is moving in a circle, so it keeps getting in the way: thus pushing you back on the circular track. In fact, everything in the centrifuge is trying to move in a straight line, but the floor keeps getting in the way. So when you throw something, it goes in a perfectly straight line, but to you inside it seems like the object traced out a parabola and was pulled back to the floor. When in fact, it only looked that way because *you* were wizzing around this circle.

To someone outside the centrifuge, they don't feel anything at all. It's just a big spinning thing.

This is the same feeling you get when you turn a corner in your car. Your body wants to keep moving in the same direction it was (straight) but now the car is going off on the other street, in a completely different direction! So your body runs into the door which pushes you back into the right direction. That's why it seems like you are "pulled" to the left when you turn right. In fact you aren't pulled at all: you were traveling along just fine but the car runs into you from the left because it has changed direction.

Re:Can we turn gravity off?

[glenebob]

Turning off gravity = Free Energy. Unless turning off gravity consumes at least the amount of energy potentially generated by turning it back on. It's the law.

Re:Can we turn gravity off?

[ShavenYak]

This is the same feeling you get when you turn a corner in your car. Your body wants to keep moving in the same direction it was (straight) but now the car is going off on the other street, in a completely different direction!

Have you ever watched a helium balloon in a car (I knew there was a reason we had a kid)? It drifts to the INSIDE of the turn, because the air is heavier and "wants" to go to the outside of the turn more.

Same thing should happen in a centrifuge - the balloon would go towards the axis, thus completing the illusion of gravity for the occupants of the centrifuge. Assuming, of course, that the centrifuge is an enclosed cabin on a stick like NASA uses to train astronauts. It wouldn't work on the centrifuge ride at the fair because the air isn't spinning with you.

Re:Can we turn gravity off?

[XO]

Right, I thought the comment after this one was more correct - though like i said, i'm a physics idiot - a spinning cylinder with a rod in the center - 'generates' or 'appears' to generate gravity?

Maybe it's a simplistic view, but I thought that the gravitational effect was created by the fact that the Earth spins, the Sun spins, etc.

Re:Can we turn gravity off?

[Alsee]

a spinning cylinder with a rod in the center - 'generates' or 'appears' to generate gravity?

From a certain point of view that force can appear similar to gravity, but it is very different.

I thought that the gravitational effect was created by the fact that the Earth spins, the Sun spins

No, it has nothing to do with spinning. Gravity would still be there if the earth didn't spin and if the sun didn't spin. The only thing that "spining" has to do with gravity is that if the earth didn't go around the sun then gravity would cause the earth and sun to fall towards each other and collide.

-

Re:Can we turn gravity off?

[Alsee]

Turning off gravity = Free Energy. Unless turning off gravity consumes at least the amount of energy potentially generated by turning it back on. It's the law.

The funny thing is that quantum mechanics specificly says that you CAN break the law and get free energy - so long as you never get caught doing it LOL. Particles can break the law and "get" an impossible amount of energy, but only when no one is looking.

-

Re:Can we turn gravity off?

[OneEyedApe]

From what I have read, it is not so critical that they are not observed, as it is more important that the net change in energy is 0 when all is said and done.

Re:Can we turn gravity off?

[Rich0]

Now, your homework is to investigate why the Earth doesn't fall into the Sun and kill us all. This involves "orbiting" but still has nothing much to do with "spinning".

For those not wanting to do a whole lot of homewokr the answer is pretty simple. The Earth is travelling tangentally to its orbit at some speed at any given instant. It is accellerating towards the sun as well. The net result is that the accelleration of gravity bends the velocity vector towards the sun, at an angle, which is where the Earth ends up in the next moment of time. If you repeat this ad infinitum you end up with an elipse.

Here is an illustration:

1. Drop a ball - it falls straight down.

2. Throw a ball straight ahead - it falls in a parabola.

3. Throw a ball sraight ahead faster - it falls in a more shallow parabola.

4. Throw the ball at a high enough speed. It ends up a dozen miles away, the path is a parabola, but it almost hugs the curvature of the earth.

5. Throw the ball at a VERY HIGH SPEED - it does follow a parabola, but it is going so fast it flys out into space, and so the parabola doesn't hit the earth - it arcs towards it slightly, but the ball is out of sight long before it falls very far.

6. Throw the ball at just the right speed. Its path is bent towards the surface of the earth, but the curvature is the same as the surface of the earth, so it never gets closer to the ground. If there were no air resistance, it would orbit the earth at the height it were thrown from.

Re:Can we turn gravity off?

[Guy Harris]

And that makes me think that photons can self-gravitate:
They are affected by gravity
They "bend space" like mass, so produce a gravitational field.

You are entirely correct. A photon has energy and momentum, thus it generates a gravitational field, even though it has no rest mass. It is also subject to a gravitational field, as was found to be the case in 1919.

Hell, gravity should self-gravitate, and there was an experiment to try to determine whether it does, and they appear to have found that it does....

Re:Hey Y'all

[Henry V .009]

Thanks--it's user preferences, comments, just in case anyone else can't find it.

Apparently everyone gets to independantly set the karma bonus that they see on posts.

I like this change to the system. But I do think it will make it harder for new users since the default is no change. If slashdot really wanted to fix everything they'd change moderation.

Welp...

[Anonymous Coward]

It looks like the Carrion Sound is now officially dead. Gotta love a site whose title describes its hosting status.

Re:Hey Y'all

[silentbozo]

It looks like anyone who had a +2 has been nuked back to a +1. A bug, or an attempt to mess with the status quo?

Instant gravity!

[GMontag]

Many of the physicists still hold on to the idea that gravity works instantaneously no matter what the distance, an idea that originated by
Newton, but that was argued against by Einstein.

So, if I put gravitons in a microwave will they go back in time?

Man, this topic is so heavy...

Re:Instant gravity!

[GMontag]

Ah, thank you for the safety tip!

Re:Instant gravity!

[Professor Farnsworth]

No, you moron! The microwave radiation, combined with the gravitons and graviolies from the supernova blasted us through time itself!

SATA will be a while

[xyote]

according to this Inquirer article.

Add me to the list.

[nlinecomputers]

My Karma is fsck'd up as well. I not posting at the +2 level either. Now I had a few post that got modded flamebait(hey It was, I was in a bad mood) but the karma still shows excellent yet I not getting the bonus.

Any clue what is up?

Re:Add me to the list.

[anotherone]

users can decide what the bonus is worth for themselves, so if you've got the bonus set to 0 it won't show up.

Re:Add me to the list.

[CableModemSniper]

I think the good karma bonus is user-definable now in your preferences, and i think the default is zero. You notice how the check box has changed to No Karma Bonus as opposed to no +1 bonus.

Re:Add me to the list.

[djupedal]

I'm back to down 1, from 2, but I don't see any setting to bump it back up in my prefs/messages....hummm

Iridium Flares

[FrostedWheat]

If you haven't already seen one, Iridium Flares are really quite impressive.

SCO is toast

[legLess]

My brother used to run a motorcycle courier service in LA, and the only bill higher than his liability insurance (think about it - what type of guy wants to be a motorcycle courier? how safe is he going to be?) was his SCO license. These folks have been squeezing blood out of the turnip for years, and now that people have abadonded their turnip (to further torture the analogy) SCO is looking for other vegetables.

They're toast, though, no matter what half-assed "intellectual property" scheme they come up with. I mean, really - who're you going to stay friends with? A girlfriend who gave you your toothbrush back and said, "Bye, and thanks for all the fish," or one who boiled your fucking cat alive? SCO is kicking its customers in the nuts while they walk out the door; they might squeeze a little cash out of them on the way, but they're only hastening the exit.

Chris Sontag, hired in October as senior vice-president of SCO's Operating Systems division, leads the intellectual property organization, sources said. Earlier in his career, Mr. Sontag led marketing and product development for Novell...

Did I mention that SCO is toast? That quote alone should get them on FC

Our Unix IP is a significant asset. And for several months, we have been holding internal discussions, exploring a wide range of possible strategies concerning this asset," the company said in a statement Monday. SCO hasn't decided how exactly to collect more Unix revenue, the company added.

Translation: "We're desperate and rudderless, checking under sofa cushions for spare change. Got any?"

Re:SCO is toast

[IntlHarvester]

Flame on, but read the article.

Sources said SCO plans to charge for use of two software "libraries," ... A source said SCO libraries that accompany the SVR4 and OSR5 versions of Unix may be used with UnixWare and OpenServer, respectively, but using them in conjunction with Linux is prohibited by the software's licence.

"There's a little bit of ignorance on the part of some customers," a source familiar with the plan said. But at the same time, the source added, "there are customers using the libraries that know they're not supposed to be using them."

Using the libraries allows programs designed for SCO Unix to be run, unmodified, on Linux machines in conjunction with a package called Linux-ABI. That's a key step for companies moving servers from SCO Unix to Linux with minimum disruption.

For those who don't know, "Linux-ABI" used to be called IBCS -- "Intel Binary Compatibility Standard" -- and you can guess from the name that it was an (old) attempt to standardize the ABI between different x86 Unixes. A long time ago, Linux users needed this to run commercial software like Oracle or WordPerfect.

It sounds like either Linux-ABI steps on SCO patents, or certain customers are shipping SCO libraries to run on top of Linux-ABI (which is outright copyright violation). In either case, this only affects about 0.001% of Linux users.

In short, all 2000 posts eariler were probably a massive over reaction.

Re:SCO is toast

[js7a]

all 2000 posts eariler were probably a massive over reaction

Maybe those 2000 posters are the ones with software using Linux-ABI.

CALTECH

[Anonymous Coward]

Finally, thanks to the several readers who alerted me by email and in comments that the school variously rendered Cal Tech, CalTech and other things even worse is in fact properly spelled "Caltech."

And of course, "Caltech" is pronounced: sall-TEESH

Glad I could clear that up!

We wouldn't know it.

[Timothy Chu]

The thing is, you wouldn't know the Sun disappeared till 8 minutes after the fact anyways. Wouldn't it be so cool that here we are whizzing off into space for 8 minutes while the sun is still shining brightly.

"What's this strange force pulling us off into space?"
"It's actually the lack of the sun's gravity. The sun must've disappeared!"
"Let's enjoy the last 8 minutes of sunlight while we can! Woohooo!"

<tim><

Re:We wouldn't know it.

[Ramze]

Actually, if gravity is instantaneous, then you would percieve the sun as moving away from the earth instantaneously after it vanished due to the earth moving quickly out of orbit. The earth would fling off in a straight line from the sun as soon as it vanished at incredible speed, and the light from where the sun WAS would take longer to reach the earth as the earth moves away. From earth's perspective, it would be as if the sun were moving away from the earth at incredible speed instead (relativity). However, since gravity has supposedly been measured as being affected at the speed of light, then you would notice no change at all until 8 minutes after the sun was gone.

The argument itself is moot b/c it's impossible for a mass such as the sun to simply vanish & since gravity is influenced by mass and mass can only move at sub-light speeds, it would be impossible for any change in the sun to be noticed gravitationally by any object at any speed faster than light. :-)

Ahhh... Physics rocks... lol. No wait, that's geology. ;-)

Re:We wouldn't know it.

[fredrik70]

we wouldn't really speed up or anything. We would just stop our 'fall' around the sun and go in a straight line instead. in 8 min we wouldn't come very much further away from the sun as we currently are - a bit though so yeah, we might get a *tiny, tiny* bit of extra sunlight. Mind you, bettar take all we can get if that should happen! ;-)

How to Tell the speed of Gravity:

[xmlmaster]

This experiment is not too hard, and establishes whether speed(Gravity)==speed(light):

1. Using visible light, measure the position of the sun.
   
2. Using some kind of instruments, measure the gravitational pull of the sun.
   
3. Compare the arcs received from steps 1 and 2
   

My understanding is that this has been done, and continues to be done all the time. The Gravitational pull of the Sun preceeds the source of light by 8 minutes.

I would be interested in hearing from someone who has done this experiment for themselves.

Re:How to Tell the speed of Gravity:

[Dyolf Knip]

That would work if you could tell the difference between the gravitional pull of the sun now versus eight minutes ago. The elliptical nature of our orbit could do it (we'd be closer or further away gravitationally than we were visually), but the change over 8 minutes is so ridiculously small. And as the sun is a moving and changing target itself, can that really be done today?

But you're right, this really is an easy experiment. It just requires either the ability to move very large masses very quickly or detect exquisitely small changes in gravitational attraction. Or some lesser combination of both.

How about this. You put synchronized clocks at some great distance from each other. One goes with your big mass, the other with the detector. At some predetermined time, after establishing the baseline gravity well, you move the big rock and see how quickly the change shows up. It is very important that your clocks take into account both special relativity (when moving them apart) and general relativity (one is closer to a big mass). Any changes would be utterly miniscule, but then so is the effect being measured.

Re:How to Tell the speed of Gravity:

[JetJaguar]

No, that's not what your experiment does. All it does is confirm that the sun has a gravitational pull and that it is constant. It doesn't give you any information about the propogation of gravity.

Now if the sun were undergoing large pulsations, large enough to effect the sun's gravitational field, then you could start looking at the speed of propogation that way, but once you are already immersed in a constant field, the speed of propogation is pretty much impossible measure.

Measuring the speed of gravity

[Anonymous Coward]

Okay, we can't make the sun disappear and watch what happens to the earth's orbit (and really, why would you want to?)

BUT - what if scientists create an amount of matter and anti-matter, and using very sensitive equipment measure the gravity effect it has on say a hanging weight (ala the two really heavy bags suspended next to each other). Then, all of a sudden combine the matter and anti-matter and measure how quickly the gravity "disappears". (BTW, the antimatter can be "held" in a magnetic field, as opposed to wearing really thick gloves)

Okay, there's a lot of work to be done before this could even be considered, let alone be done in the garage of /.'ers, but are there any arguments against this being possible?

I'm not sure if the energy released by the matter/anti-matter combination would interfere in any way (it probably would, for all I know). Also, it's probably not practical to generate a significant amount of anti-matter, and I believe it may have a very short half-life. Also, do we have equipment sensitive enough?

There we go - I've suggested a better experiment, shot it to bits, admitted my ignorance and also taken the piss out of my suggestion a bit! This is the ultimate /. post - no replies necessary!!! (But the replies will still come. So sayeth the spider)

Re:Measuring the speed of gravity

[g4dget]

what if scientists create an amount of matter and anti-matter, and using very sensitive equipment measure the gravity effect it has on say a hanging weight

The mass doesn't disappear when you bring matter and antimatter together. All you are doing is taking two chunks of mass an make them accelerate in opposite directions at light speed. There are cheaper and easier ways of doing that, like a battery-operated lightbulb.

Thanks!

[nlinecomputers]

Ah,

Now I understand. Ok I can now set it to +1 and get the system working the way it was before.

Thanks.

The speed of gravity, a consequence

[Traa]

Hypothesis: The speed of gravity == infinite

If the above hypothesis where true then one could (theoretically) build the following device: At place A we have a measurement tool that measures the gravitational pull of an object at place B. At place B we move the object back and forth based on a coded pattern (sending information). At spot A the difference in gravitational pull allows us to decode the pattern (reading information). The time it takes to send this information is based on the time it takes for the gravity 'waves' to reach from point B to point A. Our hypothesis says that this time is 0 so it means that we can now build a device that can send information FASTER then the speed of light. Einstein allready proofed that there is nothing faster then the speed of light.

Conclusion: The hypothesis is FALSE.

(disclaimer: bah, I'm no physicist, so don't flame me for not writing the above proof in a perfect physicist lingo...I tried :-)

Re:The speed of gravity, a consequence

[SamBeckett]

Einstein proofed it?? Har har har. It's only a theory, he didn't "proofed" anything duuuude

Re:The speed of gravity, a consequence

[Traa]

actually, you CAN get a really long string from here to china. If you pull it however then each atom in the string will atract a nearby atom to 'stay close' and this information moves at best at the speed of light. So yes, your 'almost instantaneous' will turn out to be the speed of light.

Re:The speed of gravity, a consequence

[Reckless Visionary]

Well, while I do like the idea, it's a novel one, if Einstein had truly proven that "nothing" is faster then the speed of light then "The speed of gravity == infinite" was disproven years ago. Unfortunately, I don't think that's exactly true. While I'm as certain as most that gravity is not instantaneous, we wouldn't really be having this discussion if it had been proven to such a degree that physicists no longer argued over it.

Re:The speed of gravity, a consequence

[kcbrown]

Well, you're thinking along the right lines...

Here's the experiment you really want to perform:

At place A, you place a massive object in front of a light, and move the object back and forth.

At place B, you set up a light detector and a gravity detector (the means by which one builds the gravity detector is left as an exercise to the reader).

You start off with points A and B really close to each other so you can calibrate your equipment (you need to be able to account for the difference between reading the gravity detector and the light detector). Once you do that, you move the points further apart.

Now, if the speed of gravity is instantaneous, then the phase difference between the signal received by the gravity detector and the signal received by the light detector should change as you move the points away from each other.

If the speed of gravity is the same as the speed of light, then the phase difference between the two signals should always be zero (after accounting for the equipment), no matter what the distance between points A and B.

The relationship between the phase difference and the distance between points A and B will give you a clue as to the speed of gravity versus the speed of light, if that of gravity is finite but different from that of light.

Re:The speed of gravity, a consequence

[Phanatic1a]

Actually, this is also inaccurate. The speed at which the pull propagates through the string is limited by the speed of sound in the material.

Therefore, relativity also tells us that there no are infinitely rigid materials.

Re:The speed of gravity, a consequence

[ShavenYak]

Things with MASS cannot move faster than light, per Einstein. Information has no mass, therefore your logic is incorrect.

To be precise, things with either mass or energy cannot move faster than light. Things with mass (rest mass, to be more precise) must move slower than light; things with zero rest mass must move exactly the speed of light.

Actually, according to relativity information can't move faster than light either. There are some situations (admittedly a bit contrived) where FTL information transfer can violate causality. Now, that doesn't prove anything - relativity could be just a bit wrong to where those situations really don't create a causality violation. Or it may be possible that causality is not inviolable. Neither of those options appears to be true at this juncture, so it's probably safe to say that our best information currently indicates information cannot travel faster than light - and if this is the case, gravity cannot travel faster than light either.

Like I said, that isn't proof, it's just a statement that if gravity does travel faster than light, something else we think is true, ien't. Then again, we knew that much before the experiment was done.

Re:The speed of gravity, a consequence

[ShavenYak]

Note that neither of those two examples can be used to transmit information faster than light. In the scissors example, information must start at the handles of the scissors where the force is being applied. The force closing the scissors propagates no faster than the speed of sound in the material, however at the very end of the closing process the point at which the blades meet can indeed exceed c. The distance over which this happens is only a fraction of the total length of the scissors, and no information can be sent with the point from where it begins moving faster than c to the tips of the scissors.

The same is true in the spotlight example. With a powerful enough laser pointer, you could wiggle a spot across the moon that would move faster than light. There's no way two people on the moon could use this "spot" to communicate FTL.

A similar example would be marquee lights at a theater. With precise enough timing of the bulbs going on/off, you could make the chasing effect go FTL. The marquee would have to be hella big for anyone to see it work though.

Another fun example: Neptune is approximately 4.5 billion kilometers away. Thus, in a 24-hour day it appears to traverse a circular path about 28 billion km in circumference. Light can only travel about 26 billion km in 24 hours. Needless to say, other stars are going far, far faster - more than 25 light-years / day for the nearest stars.

Of course, a rotating frame of reference is not covered under Special Relativity, so this isn't a violation. And obviously there's no FTL information transfer possible.

Re:The speed of gravity, a consequence

[Anonymous Coward]

"Einstein allready proofed that there is nothing faster then the speed of light."

In the paper "On the Electrodynamics of Moving Bodies" (1905), Einstein states that "We will raise this conjecture (the purport of which will hereafter be call the "Principle of Relativity") to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body." [The first postulate is that "the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good."]

From this statement it is evident that Einstein did not prove that there is nothing faster than the speed of light, but rather he postulated that in the formulation of his theory. As a result, it is not surprising that any theory that allows for instantaneous information transfer is inconsistant with the Principle of Relativity, as it violates one of the postulates of Relativity theory.

Re:The speed of gravity, a consequence

[ShavenYak]

I meant there's obviously no FTL information transfer made possible by the apparent FTL motion of Neptune, not that FTL information transfer is obviously impossible in general. Sorry if that was unclear.

However, one thing is known: if two events happen which are separated further in space than the light-travel time between them, observers in different inertial frames of reference will disagree on which event happened first. If FTL information transfer is possible, then one of the events can affect the other, so from certain frames of reference the effect will precede the cause. This makes physicists uncomfortable; of couse, that doesn't mean it can't happen.

Re:The speed of gravity, a consequence

[jdavidb]

I hear references to this paradox, but haven't yet heard it explained. I don't think you got it quite right. You are begging the question, that is, proving your result by assuming it. "Einstein already proved there is nothing faster than the speed of light." We are trying to construct a proof, here; you can't just go assuming it. What you have proved is that if information cannot travel faster than light, then gravity cannot. But you have assumed that Einstein's theory (which I thought applied to mass and/or energy ... does anyone know if it applies to information?) However, if, in fact gravity can propagate faster than c (I doubt it, but if it could), then information could, in the way you demonstrated, travel faster than light.

newton v. einstein

[Capt Dan]

Many of the physicists still hold on to the idea that gravity works instantaneously no matter what the distance, an idea that originated by Newton, but that was argued against by Einstein."

I ain't no physicist, but I think i'm gonna go with Einstein on this one. It's like trying to block Jordan when he was in his prime.

IRIDIUM IS EVIL! BOYCOTT!!

[sulli]

I can't believe there's another positive Iridium story on slashdot. Is this due to the visual studio .NET ads?

People, we need to boycott this insidious attempt by Micro$haft and the evil **AA along with Senator Disney and the BSA to control our PCs! Join the EFF, delete your Windows partition, and FIGHT BACK against this menace! Power to the people!

oh, Iridium? oops, never mind.

Re:Hey Y'all

[GMontag]

Well, I just tried it and it did not do a damn thing for me.

Unless there is a delay, then this snotty comment will popp to the top at a +2 and I will get mod-clobbered, LOL!

big-swinging-karma

[Cadre]

hehe, you can assign negative values to people using the karma bonus. That's pretty nice, you can now mod down the big-swinging-cock-because-i've-got-karma people. :-) yaaa

Re:big-swinging-karma

[fermion]

I suppose this is an irony. A poster with lots of top rated posts using the karma bonus to post some dumb ass shit complaining about the moderation and bonus system. I understand. We that have karma to burn just occasionally feel like doing a bit of mischief. I guess having big swinging genitalia does that to a person.

In any case, lest we forget, I quoth the FAQ:
Karma is used to remove risky users from the moderator pool, and to assign a bonus point to users who have contributed positively to Slashdot in the past. It is not your IQ, dick length/cup size, value as a human being, or a score in a video game. It does not determine your worth as a Slashdot reader. It does not cure cancer or grant you a seat on the secret spaceship that will be traveling to Mars when the Krulls return to destroy the planet in 2012. Karma fluctuates dramatically as users post, moderate, and meta-moderate. Don't let it bother you. It's just a number in the database.

And I just always wanted to note that the focus on dick length (not girth?) and cup size (not shape?) certainly lends credence to the concept of computer geeks being male dominated and female unfriendly.

Re:big-swinging-karma

[Forgotten]

It is not your IQ, dick length/cup size, value as a human being, or a score in a video game.

Claiming it's not a score in a video game is just plain denial. Try and formulate an argument that truly distinguishes /. from any other MUD. Everyone knows when you get to level 50 and become a "demigod", all the fun goes out of it. All this talk about +2 and +3 bonuses (not to mention "swords" of various length - and girth) only confirms where we are.

Time to create yet another character...

Re:USA CENTRIC SHIT!!!

[CableModemSniper]

It's a site hosted in the US, run by people who live in the uS. You can always find another blog, or turn off the YRO section or ignore it, or just not read slashdot. All there is to it.

Re:Hey Y'all

[writertype]

I'm guessing someone's a bit peeved at the arrogance of your sig... Exactly who do you write for as a "professional journalist"? Google searches only turn up your Slashdot comments, and I think you need a wider audience than that ;)

Try this...

[djupedal]

Found it...try looking in 'Prefs/Comments'....

"Karma Bonus (modifier assigned to posts where the user has good karma"

Re:Try this...

[silentbozo]

It's weird. My list of latest comments shows my posts at a +1, but now my comments in context show a +2. And great, now that I've changed the Karma bonus to +2, my posts are showing as a +3. What's with all this relativism? I'm confused!!!

So if you don't have good karma, you start out at a 1 or a zero?!? Is there a difference between good karma and excellent karma, or are they the same now? Are new users starting out with a 0 score?

Speed of gravity could well be instantaneous

[Traa]

Here is a case for why the speed of gravity could (and possibly is) instantaneous:

To measure the speed of gravity you have
1) point in space WHERE you measure
2) an object in space THAT you measure

the fastest that the object could ever travel is the speed of light, so the fastest change you could ever measure is the speed of the object, thus if we measure that the gravity changes with the speed of light, it might well be that the object changes position with the speed of light while gravity changes instantaneous.

I know that the way the two physisist measured the speed of gravity was indirect, yet that still means that the fastest any object was moving in their experiment was with the speed of light.

Does this make any sense? (just thought of it...not a lot of physics to back me up yet :-)

Um, why is this worthwhile?

[SuperDuG]

"In an article on Space.com, many physicists have criticized the current work on the speed of gravity, calling it 'nonsense' and 'simply incorrect.' Many of them still doubt the claims made by Fomalont and Kopeikin even before the results were even announced. Many of the physicists still hold on to the idea that gravity works instantaneously no matter what the distance, an idea that originated by Newton, but that was argued against by Einstein."

There are still scientists that argue EVOLUTION. This is nothing new, scientists looking to ride the coat-tails of rising stars in the field by doubting them. Obviously with the results not out before the entire idea was refuted, but this doesn't surprise me. PhD's who are "experts" in their field tend to be arrogant asses when it comes to something they didn't "discover".

Don't believe me, walk on to your local university and sit in on a graduate level class. Some people love to get paid to hear themselves speak.

Argh!!!!!

[volsung]

No, no, no! The contraversy over the results of the gravity measurement surrounds the MEASUREMENT TECHNIQUE, not the conclusion. You would have to look *really*, *really* hard to find a working physicist who thought that the influence of gravity was instantaneous. You'd have an easier time finding a "Pacifists for Bombing Iraq" organization to join.

Re:Hey Y'all

[Otter]

Why Linux Isn't Ready For The Desktop: Chapter LXXIV

Check your user preferences, messages. Readers decide how much the bonus is worth. Set it to 1 instead of the default, 0, and you'll see the old behavior.

Today's exercise in open source user-friendliness is making a major change to score display with no notice or explanation. After all, users can simply download the current Slash code from SourceForge CVS, grep for "No Karma Bonus", see what variable the checkbox sets and work back through the source to figure out why display seems to be broken. Oh! It's just a new preference that needs to be set!

Re:Hey Y'all

[iggymanz]

My goodness, people worried about their slashdot Karma..... excellent slashdot Karma and $2.50 will get you a cup of coffee at Starbucks.

Re:Hey Y'all

[Henry V .009]

That's not how it works--you are setting the level at which YOU view all 'karma bonus' comments.

Re:Hey Y'all

[Angry Toad]

I'm not even clear on what the point of Karma is anymore.

Isn't this pretty much the death knell of any meaningful value being derived from a good Karma score?

First you can't watch your Karma Value climb to insanely high levels, and now you can't even let other people Check Out The Big Karma Value On Yourself with your bonus. Other people can even mod you down (in their own views) for having a decent karma score.

So what's the point? Why not get rid of it altogether?

Go ahead, mod me down as a troll or whatever, I don't see how it matters.

Re:Hey Y'all

[Kaz Riprock]

Don't forget to click the "save" button at the bottom of the page once you've decided what bonus you want to give.

In the meantime, I was wondering what this will do to moderation. If I'm surfing with a karma bonus of 3 and there's a post (+1) from an excellent karma (+3) that's been modded insightful twice (+2)...will the post show up with a score of 6? If it still stops at 5 like it used to, then if I surf at bonus of 4, who would I mod up? They're all already at five, or do I mod up the good posts (as a good mod should) but then, that's a wasted mod if everyone mods the first few good posts and we end up with posts getting 16 mods, since you have to mod it up because its good and you don't know what threshold someone is surfing at to let everyone know it's good.

In the other meantime, I think the mod system needs a ton of work. The biggest problem is lazy mods that don't skim through at a lower browsing level. If you want to get modded up to the point that your posts hold more weight (more than 1 or 2 points above a SOVIET UNION or Stephen King is dead post) then you have to get to the discussion early and catch a modder's eye, instead of waiting until there are already 300 posts and you fall into that oblivion of post titles at the bottom of the discussion that nobody responds to or reads.

'kay. That is all.

You'd have a hard time seeing movement at all.

[Christopher Thomas]

Actually, if gravity is instantaneous, then you would percieve the sun as moving away from the earth instantaneously after it vanished due to the earth moving quickly out of orbit. The earth would fling off in a straight line from the sun as soon as it vanished at incredible speed, and the light from where the sun WAS would take longer to reach the earth as the earth moves away.

While you are correct in pointing out that we'd see the light for slightly longer than 8 minutes (with a slight accompanying redshift), the time (and distance) difference is very small.

The time between gravity shutoff and light shutoff is 8 minutes. The Earth's orbital period is about 526,000 minutes. That gives an angle of about 9.6e-5 radians. Over that small an angle, the Earth's orbit is close enough to being straight already that divergence from the path would be negligeable.

Re:You'd have a hard time seeing movement at all.

[plastik55]

Keep going with that math. 8 light-minutes times (1-cos(9.6e-5)) is about 660 meters of deviation, which is not really "negligible." I think that there would be quite a few ways of detecting a movement like that.

Re:You'd have a hard time seeing movement at all.

[Christopher Thomas]

Keep going with that math. 8 light-minutes times (1-cos(9.6e-5)) is about 660 meters of deviation, which is not really "negligible."

Compared to the distance from your terminal to the break room, no.

Compared to the distance between the Earth and the Sun (about 1.5e+11 m), yes, it's most definitely insignificant.

Read the original poster's description of visual effects to see what "significant" in this context would do.

Re:Okay, I'll try:

[glenebob]

"...would the Earth continue to hold its orbit for 8 or so minutes..."

There would be no clue that anything happened to the sun until 8 minutes after it happened. Or so the scientests are telling us. And if you think about it, it seems to make some sense when compared against other relativistic theories.

For one thing, if gravity was instananeous it could conceivably be used to send information anywhere in the universe with zero ping time. Imagine a gravity-wave wireless link that would enable us to communicate with civilizations in other galaxies. Imagine playing Q3 with an alien on a planet in M3 and still having a 20ms ping.

Now imagine sending energy via gravity waves. With the right technology you (in energy form) could be beamed, Star Trek style, to another galaxy. You could go visit your alien buddy for a lan party and be back in time for dinner.

Unfortunately, the notion of energy (and indirectly, matter) moving at infinite velocity seems to violate the entire theory of relativity. Moving you from here to another galaxy instantly certainly seems to violate the theory of relativity.

SATA benchmarks pretty useless thus far

[Tumbleweed]

Out of every benchmark I've seen of the new Seagate Barracuda V S-ATA drives, _none_ of them benchmark against it's parallel ATA brother, but instead benchmark it against either an older generation drive, or a drive of another manufacturer completely.

Look, if you want to know how SATA performs, benching one of these 'cuda V drives against a western digital p-ata drive isn't going to tell you anything. Those drives from Seagate aren't all that fast compared to drives from Maxtor or WD (or IBM/Fujitsu).

Expecting SATA to speed anything up is pretty ridiculous - the drive mechanism is what determines performance in current hard drives - we're nowhere near ATA drives that can match even ATA100 speeds (even burst rates are only reaching ATA66 speeds, if that!).

SATA won't increase your speed, PERIOD. New generation drives with higher data density, etc., are what speed up drives. The interface doesn't matter in speed.

FYI.

Re:SATA benchmarks pretty useless thus far

[NickSD]

StorageReview has a good review that compares it to it's parallel ATA brother.

Speed of gravity paradox

[nebbian]

If the speed of gravity isn't infinite, then I think you get a paradox when two bodies are orbiting each other.

Let's say we have two bodies, body 1 and body 2, both orbiting a central point.

Both of them are getting pulled in towards the central point by the other one. Right?

But if the speed of gravity isn't infinite, each body will be pulled not exactly towards the center, but towards the point at which the other body used to be, a certain time ago.

Try this experiment: You will need:
1 friend
2 tennis balls
1 roundabout (the circular playground variety)

Stand on a point on the circumference of the roundabout, and get your friend to stand opposite you. Spin the roundabout so you are both orbiting the central point.

Now throw your tennis ball at your friend. Chances are you will miss, because your friend will have moved by the time the ball gets there. So now change your aim so that the ball actually hits your friend. Get your friend to do the same.

When you've got things sorted, you should get the tennis balls hitting you from slightly 'front-on' compared to the center of the roundabout.

So what this means is that if gravity has a speed, then each orbiting body will be pulled by the phantom ghost of the other one, which will appear to be slightly behind the center of rotation. Therefore, the two bodies will keep on accelerating, pulling themselves up by their shoelaces, until the orbits around the central point become so huge that the effect isn't very big at all.

In other words, orbits won't be stable if gravity has a speed.
If we assume that 2-body orbits are stable, then gravity must be instantaneous, but this introduces a communication paradox (as pointed out by many other posters).

So we have a paradox! If you were God, would you make gravity have a speed, or not? Or do you make it so friggin' hard to measure that people give up and argue over which physicist has the bigger reputation? :-)

Re:Speed of gravity paradox

[Forgotten]

Cute thought experiment, but I don't think it matters. The end result of the vector math is still a point in between the bodies (the centre if they're of identical mass). The orbits will appear the same at all times regardless of whether gravity is constrained within light cones or not, and this is why it's such a pickle of a problem. In fact I suspect it's the sort of problem that will lead to other unexpected understanding simply because one has to be so devious to try and measure it.

If you don't believe in the vector math method (that the bodies orbit a central gravitic point, just as, say, a dust ring or ringworld would) try thinking of it this way: each body is orbiting the [displaced phantom of] the other, but because their orbits are complimentary it still doesn't matter. That is, if only one body was affected then the binary system would go spinning crazily away, but because their respective motions necessarily complement one another, it again doesn't matter - with either method, the phantomicitys you're concerned about will exactly cancel each other out.

Same applies if the bodies differ in mass, of course, though the math is a bit harder. ;)

Re:Speed of gravity paradox

[Labtek]

I don't have any sources available right now but this is what I remember from my various class on relativity (I have a B.Sc. in Physics):

Some "things" can travel faster then the speed of light. These "things" do not have mass (and we probably shouldn't call them things at all). One example is the point where the blades of scissors cross. This point can move faster then the speed of light while none of the scissors mass is move faster then the speed of light. Another example has to do with the speed of light in a medium where one "portion" of the light travels faster then c while the other is slower... but that is much too complicated to explain here.

But even if some "things" can move faster then the speed of light no information can ever be transmitted by such "things". In the case of light in a medium you need both "portions" in order to transmit information. The assumption that no information can travel faster then the speed of light is fundamental to Einstein's theories. If it was found that information can travel faster then the speed of light then the theories of relativity (both special and general) would have to be re-worked.

Anyway... that is my 2cents. I wish I had a source for you but I don't.

Re:Speed of gravity paradox

[zilly]

OK... your wish is my command. See this article, written by a University of Toronto physicist, that explains in simplespeak the concept of relativistic simultaneity. To wit:

More importantly, the relativistic notion of simultaneity makes it clear that no information can travel faster than light without throwing all our concepts of cause and effect into disarray. Relativity teaches us that if two space-time events are separated so that they cannot be connected by any signal travelling at c or less, then different observers will disagree as to which of the two events came first. Since most physicists still believe that cause needs to precede effect, we conclude that no information can be transmitted faster than the speed of light.

The article continues...

Nevertheless, velocities greater than c can be observed. Suppose a lighthouse illuminates a distant shore. The rotating lamp moves quite slowly, but the spot on the opposite shore travels at a far greater velocity. If the shore were far enough away, the spot could even move faster than light. However, this moving spot is not a single "thing". Each point along the coastline receives its own spot of light from the lighthouse, and any information travels from the lighthouse at c, rather than along the path of the moving spot. Such phenomena are described as the "motion of effects", and are not forbidden by relativity.

Pretty interesting, no?

Re:Speed of gravity paradox

[NanoProf]

I'm impressed. Very clever, but you've forgotten about the coordinate transformations of relativity. Assuming I remember my grad school E&M correctly, if one does a full calculation relativistically, the force arising from a body moving in a straight line at uniform speed does in fact appear to come from where the body would be predicted to be at the time that the signal is received, not the time that it is sent. Of course, if the body curves suddenly, this simple result breaks down (since own can't anticipate how it would curve). The situation with co-orbitting bodies is more complex, but the basic idea is the same: the full relativistic calculation with retardation effects (i.e. finite signal propagation) eliminates the naive nonphysical effects. One does, however, see things like precession of the perihelion from GR, which is absent in the Newtonian approximation.

Re:Speed of gravity paradox

[Naikrovek]

When you throw tennis balls, once the ball leaves your hand, there is no longer any force acting on the ball, and it will travel in a stright line, unless acted on by another force (air resistance). this is why you miss your friend unless you aim ahead.

gravity does not aim ahead. i'd love to see some sort of gravity shield be developed (i read something about this in Wired a few years ago) so this could be tested.

Einstein said that no *information* can travel faster than light, not that *nothing* can travel faster than light. I promise you, things do move faster than light. take your laser pointer and shine it from one side of the sky to the other. that dot moved faster than light when it travelled across the sky, but you could never get information from point a to point b faster than light that way.

who knows if it moves faster than light or not. if it does, that will certainly throw general relativity for a spin in a real hurry.

Re:Speed of gravity paradox

[Mathness]

So what this means is that if gravity has a speed, then each orbiting body will be pulled by the phantom ghost of the other one, which will appear to be slightly behind the center of rotation. Therefore, the two bodies will keep on accelerating

Which suggest that gravity have a final speed. Remember that a change in speed in a direction, is a acceleration. Therefore it follows that a object can be in constant acceleration, without the objects vector speed increases. A constant speed and constant acceleration is a circle. And orbits are circular/elliptic, then it must follow that gravity is in all likelyhood a finite speed.

Re:Speed of gravity paradox

[Kaki Nix Sain]

I promise you, things do move faster than light. take your laser pointer and shine it from one side of the sky to the other. that dot moved faster than light when it travelled across the sky,

"That dot" is not a physical thing. You are making a category error. "That dot" is an abstract entity from a category different than physics.

Re:Speed of gravity paradox

[sirsex]

But gravity does not travel only from mass_A to mass_B. Gravity radiates outward in all directions from mass_A. Assuming a circular oribt, then gravity at mass_B(time=0) and mass_b(time=t) is equivalent, and mass_b cannot tell the difference.

Re:Speed of gravity paradox

[Bruce Losis]

I remember reading somewhere that while in Newtonian physics the three body problem was insoluble, under general relativity the two body problem was insoluble - so the parent may be right. The last past of the passage was that under quantmn mechanics, the one body problem is insoluble. Which is rather nice.

Re:Speed of gravity paradox

[I am Jack's username]

the full relativistic calculation with retardation effects (i.e. finite signal propagation) eliminates the naive nonphysical effects. One does, however, see things like precession of the perihelion from GR, which is absent in the Newtonian approximation.

Can someone please translate that into tennisballese?

Re:Speed of gravity paradox

[g4dget]

In other words, orbits won't be stable if gravity has a speed.

No; it just means that they won't be stable in the way you describe. What actually happens is that in the steady state, there are dynamic forces that cause the orbiting body to see a central force corresonding to the unretarded position of the central body, but there is no need for gravity to propagate faster than the speed of light.

Re:Okay, I'll try:

[Guppy06]

"For one thing, if gravity was instananeous it could conceivably be used to send information anywhere in the universe with zero ping time."

According to special relativity, if gravity (or any other form of information transmittal) were instantaneous, you could talk to yourself in the past.

If FTL anything is possible and special relativity still holds true, FTL = time travel. It also throws the concept of causality out the window (and the concepts of "consciousness" and "free will" along with it).

Re:Hey Y'all

[John Hasler]

I could care less about my karma, but it is confusing and irritating to Slashdot's behavior change like this without warning.

Re:Hey Y'all

[GMontag]

Okay, so how do I start posting at 2 again?

Yes, I have "Excellent" karma and I leave the "No Karma Bonus" box unchecked.

Re:Hey Y'all

[Anonymous Coward]

Don't moderate at level 4, duh. Turn off the bonus while you're at it. Browse at -1. That's how it's supposed to work. Don't pay any attention to what other people set their post bonuses to. That's their problem. A worthy score 5 post needs to be a natural 5.

If you mod at 4, then you are the lazy mod you speak of. It's your duty to moderate correctly if you volunteered by checking the box.

Can't turn it off, but can move it around.

[zipwow]

Why couldn't you get a mass large enough that we can measure its gravity, and move it, in a prescribed pattern back and forth in front of the measuring device?

I'll try to ASCII-diagram, where [X] is the thing with mass and D is the measuring Device. The numbers at the side are the time. The dashes are because /. doesn't seem to care about the pre tag.

t0: D ---- [X]
t1: D ---------- [X]
t2: D --------------------- [X]
t3: D ---------- [X]
t4: D ---- [X]

So, if you begin at rest, you should be able to measure the time between the moving of the mass, and the recording of the change in gravity, right?

The only problem with this approach that I can see is a lack of ability to measure time this closely. Are there others?

-Zipwow

New Karma bonus default is busted....

[rufusdufus]

The new defaults effectively disable karma; since the bonus is nothing, there is no benefit to having good karma anymore. Basically, its now opt-in and how many people are going to bother to opt-in?

Worse, there is now an incentive to always uncheck the karma bonus, so your good comments more likely to get voted up by moderators who have the karma value set to no-zero. After all, who mods up a comment that is already at the max?

Which brings us to something that must be a bug: the point cap on comments is still 5, though the bonus can go to 6. Thus, I can set the karma bonus to +6, but comments never have values above 5. I don't see how this would ever be the desired behavior.

Re:New Karma bonus default is busted....

[Henry V .009]

I agree, except that there is no incentive to uncheck the karma bonus. In fact, there is no incentive to ever think about karma again--except for those with negative karma.

Now I understand why they're getting rid of it all. With the one to five system, +2 was just a bit too loud. I wish they had tried to find a better system though, instead of giving up.

Maybe they could have let comments range from 1 to 10, with it taking progressively more mod points to raise comment scores.

Re:New Karma bonus default is busted....

[kubrick]

Which brings us to something that must be a bug: the point cap on comments is still 5, though the bonus can go to 6. Thus, I can set the karma bonus to +6, but comments never have values above 5. I don't see how this would ever be the desired behavior.

Comments can be moderated down to have a score of -1.

Global -1 + your bonus to the comment of +6 = 5

Is that clear? I feel I didn't explain it very well... :)

Wrong and Wrong again

[Kommet]

I've seen a bunch of people point out that GPS is a transmit-only system (think of a foghorn blowing in a pattern of blasts) and doesn't take in any communications itself. I haven't seen anyone smack the original poster around for the "Profit" part.

The DoD never bought GPS, and GPS was NOT created to make a profit. Its signal was even originally encoded in such a way that non-US military receivers could not use it in an effective manner to maintain its original purpose: giving the US military a vast edge in determining exactly where its units were in 3D space. Land and sea units suddenly knew exactly where they were on the map at any time, speeding up navigation or even making effective navigation possible in places like the Iraqi deserts.

It wasn't until civilian GPS receivers had been using a neat hack-ish bit of logic (have a fixed receiver in a known place take in the encoded signals from GPS to figure out how to correct for their signal skewing, then transmit this) for a while that the DoD opened the GPS system to official civilian use and stopped screwing with the signal timings on the satelites.

For GPS I have updated the list:

1. Put GPS satellites in orbit.
2. Use superior navigation abilities to drive at maximum speed through a featureless desert and flank the Iraqis who had been told you could never drive through this desert because you would lose track of your units.
3. Kick ass.

By the way, how the fuck would someone make a profit from the GPS sats? The only thing you can make money on is a one-time sale price of a receiver unit. You certainly can't charge per-use fees...

Re:Wrong and Wrong again

[The Bungi]

I've seen a bunch of people point out that GPS is a transmit-only system (think of a foghorn blowing in a pattern of blasts) and doesn't take in any communications itself. I haven't seen anyone smack the original poster around for the "Profit" part.

I'd like you to point out where I said that the government was "buying" GPS or making a profit from GPS or otherwise whatever.

Go ahead and "smack" me. And then rent a-fuckin' sense of humor.

No, I don't wonder why...

[Grog6]

I wonder what he thought it would get us...

Great sig!

Re:No, I don't wonder why...

[legLess]

Yeah, Blonde was one of the great bands of the last 15 years, IMHO. I remember being blown away by "God is a Bullet." I still listen in slack-jawed amazement to "Mexican Moon." Johnette's album ("Pretty & Twisted") was a pale shadow; "Still in Hollywood" was interesting from an archaelogical perspective. I bought their reunion album, whatever it was called, and listened to it once *gag*. It was proof positive that they broke up at the right time. *Sigh*

Re:Hey Y'all

[glenebob]

Grr, nice of em to tell us. I've been trying to figure this one out all day. Also, nice of em to set the default to match the old static behavior.

Rule number 1: WHENEVER POSSIBLE, SET THE DEFAULT OF A NEWLY CONFIGURABLE VARIABLE TO MATCH THE OLD BEHAVIOR. This way you create the least amount of chaos in the user base.

But then, who could expect a Perl programmer to follow common rules of good software design?

Re:god i wish you could punch people on the intern

[Hilleh]

That was just...beautiful. You're my new hero. Cunt-shunner...I don't even know what it means, but I'm in love with the phrase. I shall use it mercilessly on my enemies from now on. If I didn't just use my last mod-point modding some half-assed Iradium or whatnot humor up, it would have gone to you. Consider yourself granted an honorary +1, Funny.

Re:huh?

[Anonymous Coward]

First you say no one is testing PATA vs SATA,
then you say it won't make a difference anyway.

What exactly was your point? Everyone is ignoring the interface standard, and just testing like any other HDD, but with a nice cable.

BTW the reason no-one is testing the PATA vs the SATA versions of the same drives, is that the PATA version has a 2Mb buffer and the SATA has a 8Mb buffer (with a slightly lower access time). The drives are not comparable.

Re:Hey Y'all

[charon_on_acheron]

Wait a second. Slashdot Karma is worth 50 cents at Starbucks? Thanks for letting us know. :^)

More Speak and Spell Mods

[mattrowe]

In the Januaray edition of Electronic Musician:

Re:More Speak and Spell Mods

[M-G]

An article about circuit bending in St. Louis, featuring some things that Christian Oncken (aka XJN) has done. If you're into that sort of music, his band's web site is here: Urban Jazz Naturals

Re:Hey Y'all

[glenebob]

"So what's the point? Why not get rid of it altogether?"

It matters because most people will value the opinion of someone with high karma.

Just because some people choose to see karma differently than you and I do, doesn't mean it's pointless. It still shows that moderators in general like what you say more often than dislike (that's all it ever said).

If you disagree with the average moderator, then by all means, disagree. I rather like the fact that it's now confurable. Now when you disagree with the moderators you aren't forced to read what CmdTaco or whoever thinks the average moderator likes, you read what you like.

I just think it was a mistake to make the default different than the old static setting.

"Go ahead, mod me down as a troll or whatever, I don't see how it matters."

I bet you'd take a +1 without complaining tho :-)

Re:Hey Y'all

[Siriaan]

Because not everyone has the time or inclination to sit on this site sifting through the hundreds to thousands of comments that get posted everyday. The karma system and moderation system, while perhaps flawed, allow for interesting posts to get expanded to full view of everyone reading the comments page, for those who just like to skim through after reading an article to see what people think or for some light entertainment.

SCO's licensing doesn't sound THAT bad

[BlueGecko]

If you read the article, they imply--numerous times--that the issue with SCO is solely if you are using their libc on Linux in order to run legacy SCO applications. That should not be able to impact Linux in any real way, unless I am grossly misunderstanding something.

Re:Okay, I'll try:

[Planesdragon]

There would be no clue that anything happened to the sun until 8 minutes after it happened.

Unless we suddenly shift orbits 8 minutes before we see the sun go "blink."

For one thing, if gravity was instananeous it could conceivably be used to send information anywhere in the universe with zero ping time.

No. If Gravity is instantaenous, it's not a force, it's a property. (besides which, if we _could_ alter gravity, it'd be a bitch to find a sensor to pick up the minute graivty vibrations.)

Unfortunately, the notion of energy (and indirectly, matter) moving at infinite velocity seems to violate the entire theory of relativity. Moving you from here to another galaxy instantly certainly seems to violate the theory of relativity.

No, it'd just be in a differenet paradigm. If you achieve instant transmission you're not moving at instant velocity--you're taking a shortcut.

Iridium story a non-story.

[blair1q]

The military loves Iridium and will not let it die until we start hearing about the system they are producing to exceed its capabilities.

The blurb about OS/Comet doesn't really say anything, because Iridium doesn't have the capital to replace something that is a huge part of their infrastructural investment (it'd be like replacing the linux kernel and tcp/ip on your computer without changing any other files and doing it while the machine is running and, oh yeah, you have to write it yourself and nobody else has ever done anything remotely resembling it, including all the ancient, legacied bugs).

The big story would have been if Iridium had told Harris to take a hike, because then we'd get to wonder where Iridium got the "fuck you" money.

Ok, this is what i think

[FS1]

I have seen alot of people here who say that gravity can't travel faster than the speed of light and usually back their response up with relativity. But mind you, relativity is still theory not law. Say for example gravity could travel faster than light, we will say that gravity waves travel at 2x the speed of light. Now say we have a way to measure the effect of gravity waves and not of gravity itself, just going to Heisenberg's uncertainty principle to say we can't measure the intensity of gravity waves unless we change their position. Because we would have to use light or energy or something that goes slower than light to measure the speed of the effects, that renders the results of any such attempt to send information or energy this way useless.

Now getting back to what someone asked earlier, what would happen if the sun were to be removed, would the earth spin off or stay in place for 8 minutes. To answer you question i use will use einstien's theory of gravity. He equated space to a 2-d surface, like a trampoline, gravity would warp that surface and create indentions. Ok say i put a bowling ball (sun) on the trampoline, and put a baseball (earth) in orbit of it. Now lets say i pick up the bowling ball quickly (almost instaneously), the baseball does just go off in a straight line, right?. What if i did it slowly?

Now saying that, you all know that you just simply can't move a mass such as the sun faster than the speed of light, heck you can't even make an bowling ball go faster than the speed of light, but the problem is relativity doesn't quite work for very large or very small objects. My theory is that gravity can move faster than the speed of light but the mass that generates it can't so you could never use it to create any paradox that was suggested.

Re:Ok, this is what i think

[djmurdoch]

Ok say i put a bowling ball (sun) on the trampoline, and put a baseball (earth) in orbit of it. Now lets say i pick up the bowling ball quickly (almost instaneously), the baseball does just go off in a straight line, right?.

No way. It would go shooting up in the air, then come down and bounce a few times.

Re:Ok, this is what i think

[egomaniac]

You made a typo in your post.

I believe what you meant to say was "My entire understanding of physics comes from half-remembered articles in Scientific American that I didn't really understand."

Re:Ok, this is what i think

[Hepkat]

This is a very valid point and really addresses what the controversy is. Which is the measurement of "the speed of gravity". I guess maybe what we should discuss is the real definition of gravity. Is gravity the effect on space? or is gravity the effect on other objects in the universe? Or is there even truely a difference? Given that they are distinct as in the example above, I don't know that we can ever measure the speed of gravity, only it's effects, which would be the speed of light... What's beyond that... uhm... anyone good at math?

Re:Ok, this is what i think

[PhxBlue]

For three reasons (at least), this "theory" of yours is complete bunk.

First, and most seriously, you say, "Relativity is theory, not law." This indicates a fundamental misunderstanding on your part of the Scientific Method. The fundamental difference between a theory and a law is that a law reflects and may predict observations, while a theory explains observations. This link at the Mad Sci network will give you a bit more background on the distinction.

Second, Heisenberg's Uncertainty Principle deals not with intensity and position of waves, but with the velocity vs. position of particle-waves. More specifically: "The more precisely the position is determined, the less precisely the momentum is known" (American Institute of Physics). Gravity waves are theorized to exist, but to date we have yet to detect one.

Third, you call your hare-brained idea a "theory," which it is not. At the very best, it is a hypothesis; but since you present no observations to support your idea, I hesitate to give it even that distinction. In short, what you have is an opinion--and an unfounded one at that.

A good thought experiment to carry out, which may reflect what the astronomers who performed the experiment attempted to measure, is this: watch a star's position as it nears Jupiter (and watch closely, because when it moves, it won't move much). Jupiter's mass is known; and from that we can predict how far away Jupiter will be when its gravity bends the star's light. If the star's light bends before Jupiter reaches this distance, we can conclude that the effects of Jupiter's gravity reached us before the light from Jupiter reached us, and therefore that gravity is faster than light.

Re:huh?

[Tumbleweed]

There are two different versions of each 'cuda V drive - 2meg buffer versions (model numbers end in 23A and 23S) and an 8meg versions (model #'s end in 24A and 24S).

The point in saying that it won't make any difference anyway was just to clarify that for people who wouldn't know that, in general, the interface doesn't matter, assuming the interface is implemented correctly, which we won't know until a p-ata version is tested against an s-ata version.

Re:For you proper Monty Python nuts -

[Stonent1]

It's... In my sig! 20th Century Vole

You're right, but...

[Lendrick]

What if, on some wild chance, another star of similar size came whizzing in from above the solar system at near light speed and collided with the sun, splattering it downward and away from the solar system. In that case, the sun would be moving at some speed that is close enough to the speed of light for this to be noticeable.

You're missing the domain restrictions

[starsong]

Actually, that's only the approximation for points *outside* the earth's surface. Once you cross the boundary that approximation is meaningless.

What's REALLY interesting is that for a point *inside* the earth, the gravitational effect can again be represented as that from a point mass. The difference this time is that you don't give it the mass of the whole Earth, just the bit between the center and the point you're measuring. This is because the net gravitational pull of the other bit (the shell between your position and the Earth's surface) works out to ZERO. So as you approach the center, your "mini-Earth" gets smaller and smaller, and the gravitational effect drops off until it's zero at the exact center.

Re:Okay, I'll try:

[isorox]

FTL = time travel

Why? Not being a troll or flaimbait, I'm interested. If I'm on earth at 04:00UTC, Wednesday, then step through a portal to a ship 1 light day away from earth, taking 1 minute for my matter to get to the ship (while still being perfectly conscious), I emerge, according to my wristwatch, at 04:01UTC, Wednesday. I look out the window, with my powerful telescope and see yesterdays newspaper back on earth. I sync a clock on the ship to my wristwatch and step back through the portal to earth, at 04:02UTC, My wrist watch. I emerge at 04:03UTC, My wrist watch, and compare to a control watch on earth, they are the same.

I then wait for 23 hours 55 minutes, and look at the ship with my telescope. At 04:01UTC, Thursday, I see myself emerge on the ship, look through a telescope, then go back through the portal. The clock I see says 04:01UTC, Wednesday.

If the ship then travels to earth at a slow speed (say 0.001c - about 1,000,000km/h), 1000 days later, the ship enters earth orbit, I fly up on the shuttle, and look at the clock. Relativistic forces havent changed the time of the clock by more then a few seconds, if that.

Wheres the time travel? I look back in time, but astronauts on the moon looked back in time to earth when they were 1.5 light seconds away.

Not a fair critisism

[spitzak]

Closed source and commercial software changes the defaults a lot more, and usually without telling anybody.

In fact one of the big problems with Linux is the extreme fear of changing the defaults on the chance that it will be incompatable. This is why tcsh still does not do the full editing and tab completion by default, why "find" is a pain to use for simple searches, and why programs like make are such horrors to program.

Probably the criticism you wanted was that Open Source often puts *no* checkmark in the control panel. You are supposed to figure out that you can change things by setting an environment variable and you can only figure that out by reading the source.

Re:IRIDIUM IS EVIL! BOYCOTT!!

[bfree]

F*** you you d*** s*** do you f***ing not know what a f***ing * does to a f***ing word thats sprinkled with the little s***s? Or just because some f***ing piece of s*** digital systems use a c***ish * to represent multiple unknown little b****y letters that means that we all have to f***ing start to write the way some c*** wants us all to? Sorry I forgot, you just a typical f***ing /. c*** who likes to f*** with peoples heads by t****ing!

Re:Okay, I'll try:

[leonardluen]

but we can alter gravity! well sort of...

if i remember my physics correctly, the closer you are to an object the stronger the gravity.

now, all you have to do to make "gravitational vibrations" is to move the object towards and away from the object you want to send information to. and also it isn't really a "bitch to find a sensor" we do have instruments designed to measure gravity. maybe they aren't fast enough to measure the gravity waves in this example, but the point is that it exists. and just because you don't know how to do it doesn't mean it is impossible...remember how slow computers were 20 years ago?

so given the assumption that Gravity is instananeous then faster than light communication is possible!

Re:huh?

[Tumbleweed]

Maybe so, but benchmarks aren't testing assembly time. :)

Re:Okay, I'll try:

[Guppy06]

"Why?"

Because c isn't just the speed of light, it's the speed of change through the universe. Every change in the universe, every interaction between two objects can only move at the speed of light. And if that change in the universe "hasn't reached you" yet, then for every conceivable purpose it simply hasn't happened yet. Period.

"I look out the window, with my powerful telescope and see yesterdays newspaper back on earth."

Here's the tricky part: It's not just that you're seeing an image that left your home yesterday, it's that you are actually looking at yesterday. Yesterday's reality is washing over you like a wave at the beach.

If you see yourself in that telescope, that's not just an image of you, that is you. Changes you made in the universe, everything you did "yesterday" is happening now, as you watch it. As far as space, time, and the universe (from where you sit) are concerned, you are now in two places at the same time. If you were able to step back into your teleportation device, you would end up back on the earth "yesterday."

There's a more drawn-out explaination of the circumstances and the consequences using the classic example of a duel with tachyon pistols here

Speed of Gravity: Metaresearch.org

[JB318]

I just recently stumbled across Metaresearch.org's speed of gravity page.

The article "The Speed of Gravity - Repeal of the Speed Limit" says "[a binary pulsar experiment] places the strongest lower limit to the speed of gravity: 2 x 10^10 x c." It tackles the Special Relativity objection mentioned in another reply.

The article "Possible New Properties of Gravity" goes even further with it. It talks about the orbital effect you mentioned as a specific example right away. It might be a little bit easier to understand than the article above. It dives head-first into what some of the observable consequences are--and that discussion is what makes it much more believable to me. It's what moves him from the "just another crackpot" bucket to the "if he's a crackpot, he's one with a convincing case" bucket.

Re:Okay, I'll try:

[Alsee]

FTL = time travel

Why? Not being a troll or flaimbait, I'm interested.

It's not easy to explain, but I'll take a shot at it.

If gravity (or anything) is "instantaneous" (or merely FTL) that means it affects a distant point at the same instant. That is a crucial point - it implies that "now" here is the same as "now" at some distant point.

According to relativity there is no independant universal "clock". All time is relative. Different observers see time differently. Without some independant clock it is impossible to say "now" here is the same as "now" over there. Different observers will dissagree on the timing of events. The key point here is that two different observers can dissagree on the order of two events. If you have two supposedly "simultaneous" events then an observer moving to the left at 0.9C would see event B happen BEFORE event A, and an observer moving to the right at 0.9C would see event B happen AFTER event A.

What this means is that if you can send a signal faster than the speed of light then someone moving fast enough in the right direction sees the signal recieved before he sees it sent and he can carry that information back to the place the signal came from before the signal is sent. He can tell you exactly what message you are going to send which means you can use that fast moving observer to carry messages into your past for you. And if the message is the equivalent of a "startrek transporter matter stream" then you could "beam" your self into the past.

-

library licences are all you need

[boots@work]

Well, you're closer, but you're still not on the money.

It sounds like either Linux-ABI steps on SCO patents, or certain customers are shipping SCO libraries to run on top of Linux-ABI (which is outright copyright violation).

They don't *need* patents. They just need for the SCO C library licence to say that programs developed using it may only be run on genuine SCO Unix. In that case, anyone running such a program would be in trouble -- they wouldn't have to redistribute it.

I suspect the background for this story is that a few long-standing SCO customers with an eye to the future have had a bright young nerd look at how hard it would be to get their vertical application to run on Linux instead of SCO. (Perhaps it's a dental surgery management suite running on Ingres or something similar.) Probably in many cases the customer has a binary app without source access, but that can be fixed with Linux-ABI. It's probably not so hard in most cases.

It's a good deal for the customer: they cut out their SCO licence costs, they get a platform with a bright future, and they have much less trouble finding people who can support and enhance it.

This is a bit bad for SCO, though. Once word gets back to HQ that this is happening, they start to think about methods that can be used to keep their customers locked in. One technique is to exploit the licence that the customer's application vendor originally signed to get the SCO libraries. If SCO were smart enough to put in a "this can only be used on SCO" clause, then they're set!

Anybody who has the source for their applications should be easily able to move to Linux, and probably most of the commercial applications like Oracle already have native ports. Linux-ABI and this licensing strategy really just apply to people with legacy SCO apps who can't, or don't want, to port to Linux.

Microsoft could use such a clause in the Office (or DirectX or MSVC Runtime) licences to put an end to all this Wine, Crossover and Transgaming nonsense, if they wanted to. I think there are enough precedents for that kind of restriction in software licences that it would be possible. For example, lots of driver software comes with a licence saying it may only be used with the vendor's original software. I think this technique is a terrible abuse of customers, like most proprietary software licences. But it would probably work to shake down some more money.

Re:library licences are all you need

[IntlHarvester]

Microsoft already does this, I believe -- the MSVC redistributable libs are licenced for Windows only. Apple takes it a step further and only licences their software on their hardware. At least MS is only limiting distribution rights, not trying to usurp copyright by removing runtime rights.

Note that SCO is doing the decent thing and offering an shrinkwrap licence for this stuff, and will no doubt also include it in Caldera UnitedLinux at no additional cost.

Re:Okay, I'll try:

[Alsee]

No. If Gravity is instantaenous, it's not a force, it's a property.

Acording to relativity it doesn't matter if you call it a force or a property. If it has an effect that is faster than C then it allows you to send a message into the past.

it'd be a bitch to find a sensor to pick up the minute graivty vibrations

True, detecting it is really really hard, but not impossible. We believe we are on the verge of detecting a pair of black holes and or neutron stars spiraling in on each other. Now a thought experiment - imagine we send a space ship out to a neutron pair and find an asteroid in the system. We could use normal rocket engines to nudge that asteroid between the neutron stars. The asteroid can cause a detectable change in the detectable gravity signal. By carefully choosing the path of the asteroid we can embed a short message into the collapse signal.

And as I said, according to relativity any FTL signal means you are sending information into the past. At this point you might be tempted to simply say relativity is wrong on that point. While physisists admit that relativity is incomplete, they have really good reason to believe you can't throw out that part.

-

Seeing is believing

[grokster]

If the sun disappears (hypothetically) we would only see it disappear 8 minutes later. Think we would go flying off while we still see the sun in place? (Hey, like, what's happening dude, how can we be flying away from the sun?)

Typo, i don't think so

[FS1]

Well at least i forgot more about physics, than 99% of americans will ever even think about in an entire lifetime.

Re:Hey Y'all

[sirsnork]

Or, if moderating you could view at 0 or -1 as the mod guide indicates!

It can't be faster than light

[jetmarc]

If gravity "waves" would propagate faster than light (or instantly), it would be possible to communicate faster than light (or instantly). Sure, earth is small compared to the speed of light, but even here on earth you notice half-a-second of delay for example in an overseas telefone call due to the speed of light (and 36kkm geostationary orbit of satelites). It would certainly be a win to overcome the speed of light in communication systems, for example when remote-controlling a mars explorer robot (where interactivity is completely absent). Not even to mention realtime communication with alien races.. So unfortunately, gravity is not faster than light. Sorry.

alternative theories

[g4dget]

In fact, even pretty much every remotely plausible "alternative" theory of gravity assumes propagation at the speed of light.

What that really means is that this experiment doesn't tell you much of anything--the outcome is as expected, and it doesn't discriminate among plausible alternative theories.

just read the paper

[g4dget]

Search for "kopeikin" on arxiv.org; he has several papers there explaining what the idea is behind the experiment.

doesn't work

[g4dget]

The Gravitational pull of the Sun preceeds the source of light by 8 minutes.

It does, but that doesn't mean that gravity propagates instantaneously. Many theories of gravity get that effect with finite propagation velocities. There are analogous effects in electrodynamics.

but there is nothing wrong with that

[g4dget]

Unfortunately, the notion of energy (and indirectly, matter) moving at infinite velocity seems to violate the entire theory of relativity.

Yes, it does. But the justification for the theory of relativity is based on not having observed anything that violates it. If we see superluminal communications, then we have. Logically, there is no intrinsic problem with sending information faster than light.

Give them the benefit of doubt II...

[C0vardeAn0nim0]

read my previuos comment here.

is what i said. if you use only gnu/linux apps running in plain gnu/linux you don't have to pay.

now, if you use some sort of compatibility layer to run UNIX binaries on gnu/linux they'll charge you a fee.

it's a sane model. after all, it IS their code.

Re:Okay, I'll try:

[devnull17]

I always thought it wasn't. Black holes, for instance, slow the speed of light to below zero. Or the velocity of it, anyway. My head hurts.

Speed of Gravity

[shadow_slicer]

If the speed of gravity is equal to the speed of light, and all forms of energy exert a gravitational force on eachother (including light), then it should be possible to test this:
Assume an intense coherent pulse of light of known intensity is traveling through space towards an object. Since the beam is traveling at the speed of light it is not currently exerting a force on the object. The instant the light hits the object, the total gravitational force between the light and the object would occur instantaneously. This gravitational force will oppose the pressure exerted on the object by the light. If the speed of gravity is the same as the speed of light, the gravitational force will be the integral of the gravitational force between the light and the object over the entire length of time the light was traveling. If the measured pressure of the light pulse differs from the theoretical pressure of the light pulse, then gravity must have played some role. If the difference between the theoretical pressure of light and the measured pressure of light is the same as the integral gravitational force, then gravity must move at the speed of light.

Re:Okay, I'll try:

[BlueMonk]

For one thing, if gravity was instananeous it could conceivably be used to send information anywhere in the universe with zero ping time. Imagine a gravity-wave wireless link that would enable us to communicate with civilizations in other galaxies. Imagine playing Q3 with an alien on a planet in M3 and still having a 20ms ping.

... unless through some application of uncertainly-like principles it turns out that it takes longer to create a detectable gravity signal to a farther distance. For instance, if you're 1 light year away from me, maybe it takes 1 year for me to move the moon far enough out of position that you possibly could sense any information from that motion at that distance. After all, I don't know of any method to make informational "gravity waves" other than moving a lot of matter (or, as recently discovered from another post, a *lot* of photons, which can in turn only travel at the speed of light). There must be some minimum displacement that would be detectable at a certain distance. And I suspect that it could relate to the speed of light. I'm not saying I believe that gravity travels faster than light, just suggesting that this argument might not be useful in determining how fast gravity travels.

Re:Okay, I'll try:

[Rich0]

Black holes, for instance, slow the speed of light to below zero. Or the velocity of it, anyway.

I won't claim to fully understand Lorentzian transformations, but I'm guessing that this is incorrect. I imagine that an observer standing next to a photon of light would observe it flying past him at the speed of light. An observer outside of the black hole wouldn't see it at all (since you can't see anything until the photon actually reaches you - which it won't). If it arrives it will arrive travelling at the speed of light, but it might take a really really long time (relative to the observer a distance from the black hole) to get there, as time itself passes very slowly at the surface of a black hole.

A real physicist might be of more assitance here, but light going away from the singularity in the black hole does travel at the speed of light away from it - it is just that space is curved such that the singularity is in front of it as much as it is behind it. I guess you could look at it like this:

1. I leave New York, travelling as fast as I can in any direction along the surface of the Earth (which we will assume is a perfect sphere). No matter how fast I run away from New York, I eventually end up there anyway. Why? Because if you travel in a straight line (a great circle) you always end up where you started. Your velocity is not zero at any time - it could even be at the speed of light, but the curvature of the space you are travelling on takes you back to where you started.

Re:Hey Y'all

[Kaz Riprock]

The mod guide indicates that a moderator should surf with posts of 0 or -1 being displayed. This does not have anything to do with the karma bonus level which has now been instituted and does not have a bearing on which post subjects are shown in the comment list. It only changes the score of those posts written by people with excellent karma. This is a complication to the modding system which was not previously present and hasn't fully been thought through (would be my guess).

Re:Okay, I'll try:

[devnull17]

But the speed of light is relative, right? Since relativity centers around time not being absolute, and speed = time/distance...

I'll admit that i'm in way over my head here, but I'd still like to know.

Is there really a Past?

[TheLink]

Just curious, I don't get it. Is there really a past to send messages to in the first place?

Does current modern physics say there is a past?

So far the FAQs etc don't seem to touch much on this.

Re:Is there really a Past?

[Alsee]

Does current modern physics say there is a past?

The nature of time is one of the biggest puzzles of physics today. We have various equations that tell us about time, but there are different ways to interpret them and we don't know exactly what they mean. We also know there are more rules, but we don't know what they are.

It's almost like a bad scifi cliche, but our deepest understanding of time is the Causality principle. Causality says that the universe won't let you make a time machine. We have absolutely no explanation of causality, and absolty no reason to believe it other than the fact that it has never been wrong and that the universe wouldn't make sense if it was wrong.

Relativity says that if you can send an FTL signal then you can make a time machine. Scientists believe that is correct. That also believe that the universe doesn not allow you to send an FTL signal therefore you can't make a time machine.

Quantum mechanics says that wormholes exist, and that if you can enlarge a a wormhole and move it around and send anything through it then you can make a time machine. Scientists believe that is correct. They have also fond that all sorts of insurmountable problems crop up if you try to do it.

If cosmic strings exist and a pair of them fly past eachother at close to the speed of light you can use it as a time machine. Later calculations showed that a pair of strings at that velocity would require more energy than exists in the entire universe.

Physics predicts numerous ways to make a time machine. We have discovered reasons some are impossible. Causality predicts that we will eventially discover reasons for them to all be impossible. It is actually extremely usefull for making new discoveries. Scientists look at potential "time machines" and seach for an unknown law of physics that would make that time machine impossible. Either you succed and discover new physics or you "fail" and actually build a time machine (chuckle). It's a win-win.

Is there a past? What is time? What is the nature of reality? Why does the universe exist? These questions are closely intertwined and we just don't know.

-

Re:Hey Y'all

[writertype]

Yes, I'm sure Google excludes the hundreds of articles a "professional journalist" would publish over the course of a career. In case you haven't noticed, most major media organizations have web sites these days.

Unless Google is persecuting him, too, just like Slashdot.

OMG. "CONSPIRACY THEORY II"!!!!

Patent for gravity transmitter

[Dr. Cody]

Fig 1
Dude

Fig 2
Ball

Fig 3
Dude

Fig 1 tosses Fig 2 to Fig 3, and vice versa, on cue, producing a binary transmitter of gravity waves.

Re:Okay, I'll try:

[isorox]

If you see yourself in that telescope, that's not just an image of you, that is you. Changes you made in the universe, everything you did "yesterday" is happening now, as you watch it. As far as space, time, and the universe (from where you sit) are concerned, you are now in two places at the same time. If you were able to step back into your teleportation device, you would end up back on the earth "yesterday."

So what you are saying is that because you cant perceive what is going on "now", it's not going on? Isn't that more of a philosophical question (tree falls in forrest does it make a noise)? Because you havent heard an explosion 4 miles away when it goes off, it doesnt mean its gone off.

If its something to do with schrodingders cat and uncertaintly, FTL communications would break the uncertaintly about it, as sure as a CCTV or XRay will break the view of cat in the box.

As for the tacyon pistols, thats a very self centered view of the universe. Because I fire at someone doesnt mean it's there. If I track a plane via sound waves, then fire my laser, I'll miss. It wouldnt look like I had hit the person, as the tachyon "bullet" would travel away from me, and hit the person instantly. However as light is slow, the light from the bullet would take 16 seconds to reach me from where the person was when I fired. It would "look" like the bullet travelled at the speed of light (if it did travel at the speed of light it would look like it travelled at half the speed of light).

To the bloke being shot, He would fire his gun, then get hit at the same time. He'd then (if he didnt die) see the bullet he was hit by move backwards out of him at 1.0c An optical illusion, nothing more.

Re:Okay, I'll try:

[isorox]

If you have two supposedly "simultaneous" events then an observer moving to the left at 0.9C would see event B happen BEFORE event A, and an observer moving to the right at 0.9C would see event B happen AFTER event A.

If theres two simultaneous explosions, one (A) at 3 miles north of X, one (B) at X, an observer at 3 miles south of X would hear Explosion B before A, and could then radio in to the observer to the north that explosion B has occured. B doesnt know the explosion has occured because he hasnt heard it yet. He ehars A, then B, and Believes A is nearer then B, which the Southern observer denies.

If I sit here at my computer, I dont know if you are typing a reply until I load up the page. Does this mean I can stop you typing it if I havent read it?

I think this boils down to the "universal clock". Just pick a clock that is the same distance from both people at any given point.
For more then 2 people, pick a clock (Z) that is equidistant between person A and B, and a clock (Y) between C and D, and X between E and F, and W between G and H.
A will see the same time as B (the time clock Z appears to be).
C will see the same time as D (the time clock Y appears to be).
E will see the same time as F (the time clock X appears to be).
G will see the same time as H (the time clock W appears to be).
Then set the clock Z and clock Y to the time that it perceives clock V, equidistant from Z and Y. then Z will see the same time as Y.
Therefore A will see the same time as B, C and D.
Then set the clock X and clock W to the time that it perceives clock U, equidistant from X and W. then X will see the same time as W.
Therefore E will see the same time as F, G and H.
Then set the clock V and clock U to the time that it perceives clock T, equidistant from V and U. then V will have the same time as U.
Therefore A will see the same time as B, C and D, E, F, G and H.

That syncronises watches for people A throguh H.

Re:Okay, I'll try:

[Alsee]

You are missing the relativity aspect. I'll do my best to explain it, but relativistic effects are bizzare from our point of view.

I bet you've never traveled more than about 400MPH (on an airplane perhaps). 400MPH is less than 0.0000006 of the speed of light. The size of the relaivity effect at 400MPH is 0.0000000000002. You have been essentially motionless your entire life therefore you've never seen what happens when you move. That effect is real and it messes up your example.

It is not merely a matter of a delay in hearing the explosion. For a moving observer one explosion REALLY DOES happen before the other. You want to pick one clock in the middle to look at, but there is nothing special about that clock. All points of view are equally valid. Someone else's clock is just as valid as the one you picked.

That syncronises watches for people A throguh H.

Realativity says time is not fixed. For a moving observer the seperated clocks really truely are not sychronized. I assume you've heard of the twin paradox? That's the one where you age more slowly if you take a trip near the speed of light. It's the same thing, time itself gets distorted and changes. If the two explosions are seperated by a distance of one light-hour then for some observers explosion A really does happens up to one hour ahead of B, and for other observers explosion A really does happen up to one hour after B. And they both are equally correct. The only thing everyone can agree on is that your clock sitting halfway between them has a reading half way between the explosions.

The reason this matters is that we can create messengers traveling close to the speed of light. We can use subatomic particles to carry messages. And for those particles the order of events really is different. If the order of events gets too far out-of-whack then someone will see an effect happen before a cause. That is sending a signal back in time.

If two points have a distance of one light hour between them then the timing of events has a "fudge factor" of up to +/- one hour depending on who you ask. But that one hour fudge factor doesn't cause a problem because it takes at least an hour to get a signal from one to the other. One hour +/- a fudge factor of up to one hour is always greater than zero. Different observers will dissagree how long it too the signal to arrive, but everyone gets an answer greater than zero. If you could get a signal from one point to the other in less than an hour then for some observers the fudge factor of minus one hour means the message took a NEGATIVE amount of time to get from one point to the other - it arrived before it left - the signal went backwards in time.

The point is that if we arrange it right then WE can be the ones to see the signal go back in time. The observer can be a stationary human and points A and B can be two subatomic particles moving left or right at close to the speed of light. If an effect or signal can get from A to B at faster than the speed of light then WE will see that signal go backwards in time.

-

Thanks.

[TheLink]

Funny that it's not in some Physics FAQ somewhere. I've been looking for quite a while (avoided bad pun ;) ).

I'll try to understand the FTL + Relativity = time machine thing.

The FTL effects of two separated entangled particles thing is funny tho. Coz I'm thinking: Isn't the Universe already entangled too?

There should be a "So..." somewhere, I just haven't figured it out yet ;).

Link.

Re:Thanks.

[Alsee]

The FTL effects of two separated entangled particles thing is funny tho.

Yeah, I'm still trying to understand that better too. Scientists are still debating what "it" is, but "it" probably isn't what we normally think of as something causing an effect on something else. There probably isn't anything actually FTL about it.

My best stab at explaining the entangled particle effect is that it isn't an something that "happens", it is just something that "is". It is already true, it is already there.

The "FTL effect" about entangled particles isn't really about the particles themselves, it is what we KNOW about the particles. Quantum mechanics gives you different results when you know something about a particle than when you don't know something about it. Since the two particles are "the same" then when you know something about one then you know something about the other. And that changes what you can predict about the other. But did that knowledge actually affect the other particle in some way? I don't think so, although sometimes it sorta seems like it does.

One way to make sense of it is to say that one particle "reaches out" and has an FTL effect on the other one. It is by far the "simplest" explanation, but it just doesn't fit in with the rest of physics. I don't think many physists believe that's the real answer.

Another way to make sense of it is the multiverse interpretation. That says that everyting that can happen does happen. In one "universe" you see X happen and in another "universe" you see Y happen, and that in some sense universe X and universe Y overlap or interact. When you know "X or Y" is true the result is the combination of the overlap of X and Y, and that result is very different than knowing X is true and different than knowing Y is true. When you know "X or Y" we see the overlap, when we discover which it is it splits into two realities and we only see one.

Some people criticise that theory for "inventing" parrallel universes out of thin air. Instead of "parallel universes" think of the universe as a higher dimensional object - maybe a sphere - and our 3 dimentional universe is merely a single point on the surface of that sphere. All of the points on the surface are smoothly connected. Every point is an entire 3d "universe". Adjacent points are almost identical and they affect each other. This interpretation is very appealing because the math for it works out really well. The universe functions according to math, and when you come up with math that works it's usually right.

Another way to make sense of it is to say that the particles have some deeper nature that we don't understand. If this interpretation right it must have some weird properties because they have proven the "simple logical interpretation" wrong. The "simple logical interpretation" would be that the particles actually have a simple hidden values before we look at them. If the particles have some hidden aspect then there's something really peculiar about it.

This interpretation is still kinda up in the air because no one has been able to guess what that hidden nature might be.

-

Thing is

[TheLink]

My point/hypothesis was if you take the universe as a whole (and entangled), the "is"/"already" effect could become more understandable.

When you do something to a particle, you and the particle (and friends) could be like coherent light from the same source, just split a long while ago. Sure that's a bit recursive, but I doubt one should assume the observer/experimenter/experiment as being in a fully independent universe from the tested particles.

It may turn out that we have independent sources but can we assume that?

There may not be multiverses (but due to limits we may still have to resort to calculations using that concept).

Re:Okay, I'll try:

[Rich0]

The speed of light in a vacuum is constant in all frames of reference. Any observer will always measure the speed of light to be c. That means that if you turn on a flashlight just as I'm wizzing by at a speed you measure as being 99% of c, we both see the same photons moving at c relative to ourselves.

In relativity neither time nor distance is absolute in all frames of reference. A fast moving observer would measure a given distance as being different as an observer at rest (of course, even the term "at rest" is silly since to the "fast-moving" observer the "at-rest" observer is just moving fast in the opposite direction).

Right now there is no way to assign an absolute speed to an object. You could argue that the Earth is stationary in the universe, or you could argue that everything in the universe is moving in a particular direction at any arbitrary speed whatsoever. If the speed of light were relative this would not be the case. You could set up a speed-of-light measuring device and move it in various speeds and directions until you found a configuration that gave you a maximum figure. Then you would know that this frame of reference is truly at rest, as it is moving the slowest relative to light. In reality, this is not possible, as the experiment would come up with the same answer in every configuration. This is similar to the experiment that demonstrated that light does not propagate in the "Ether" - as the speed of light did not change with respect to the motion of the earth.