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The Speed Of Gravity Revealed
redwolfoz writes "New Scientist is reporting that the speed of gravity has been measured for the first time. 'The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.' Researchers made the measurement of the fundamental physical constant with the help of the planet Jupiter. One important consequence of the result is that it will help constrain the number of possible dimensions in the Universe."
Wow, that's pretty cool. Now if we could only figure out why and how gravity works, we'd be in business.
Every year during my review, I just pray the words "slashdot.org" aren't mentioned.
But of course, travelling at the speed of light, all the flying colours just appeared red due to the red-shift.
Does this make my brain look big?
Sure, this experiment will "help constrain the number of possible dimensions in the Universe" ... but will it lead to new weapons?
-kgj
I'm sorry, I don't mean to ask the stupidest question ever, but how does gravity have speed? The last I was taught on the subject (and believe me, it was a while ago) was that gravity was a force, but didn't have mass. Doesn't something need to have mass in order to have speed?
Well, if gravity travels at the speed of light, wouldn't the gravitational pull of black holes be confined by the event horizon as is the case with light?
...a topic like this to be a bit more precise in the summary. There's a signifigant difference between .95 times the speed of light, and the speed of light. Not to mention the large .25 margin of error. Which theoretically shouldn't be able to get to +.25 anyhow.
You're confusion arises because you were taught elementary Newtonian physics. In general relativity, one learns that any "information" cannot travel faster than light. Gravity is considered information because if you feel a gravitational force on you, you know that there is a body out there acting on you. That is, you have information about it (you could even estimate its mass by measuring the tug it exerts on you).
In Newtonian physics, lots of things are assumed to happen instantaneously (like gravity) so they don't have a speed per se. But in general relativity, everything has a speed -- and that speed is no greater than the speed of light.
GMD
watch this
"Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity."
The theory of relativity was appearantly used to detect the speed of gravity. This would be fine if the theory of relativity didn't assume a speed of gravity. Basically, all he did was prove his given. So, if eggs are green, then eggs are green!
Karma Clown
Photons are not particles in the sense of neutrons, electrons et. al which are massy particles.
Photons are better described as 'packets of energy'. Gravity doesn't just affect mass - it affects energy as well. Light doesnt get 'pulled into' a black hole, it just gets redshifted so much (by the gravity sucking the energy out of it), that its wavelength becomes infinite, and thus immeasureable.
Photons can exert a pressure though because they have MOMENTUM. Thus they have a 'mass equivalent', but they do not have mass, and that is not why they cannot escape black holes.
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Well, now we have the speed of light AND the speed of gravity! If we can find out the Speed of Darkness, we'll be all set!
The sun couldn't suddenly disappear, although that scenario works for the purpose of explaining the speed of gravity. Consider this alternative.
Take the sun and instantly accellerate it to almost the speed of light, toward a collision course with Earth. For most of the 8 minutes between acceleration and collision, nobody would notice anything, as light, all other energy, and gravity would all present the sun as occupying its original location.
However, brief moments before the collision, the sun's change of accelleration toward earth will be noticed. Of course, you're noticing the change that happened 93 million miles away, even though the sun is about to impact. However, one second later, the sun will appear to be almost 186000 miles closer, and it will FEEL like it's 186000 miles closer. Suddenly the gravitational accelleration has increased to reflect the new position of the sun. But within that second, you get all the accumulated influences of gravity over a much larger stretch of space than just the 186000 miles it travelled in that time. Since the sun is moving at almost the speed of light, let's say 99% of it, after 99 seconds, the influence of the sun's gravity will only be 1 second ahead of the sun. However, within that one second between the position of the sun and the gravitational influence of the sun is contained the gravitational influence of the sun over the last 99 seconds. You get the combined force in 1 second that you normally would have gotten in 99. So when the Sun's influence is finally felt by Earth, you will not get a force that implies a steady rise in gravitational force of a sun massed object until impact, you'll get a very quick rise in force of an object that is, generally, about 99 times as large as the sun.
And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.
Ok, this had no purpose at all, but it was interesting to think about. Go on with your business... nothing to see here. Rant over.
-Restil
Play with my webcams and lights here
While this is very interesting, is the speed of the propogation of gravity constant or can it be affected by certain conditions? This brings to mind the experiments at slowing down light in a special supercooled gel (is this an Einstein-Bose condensate?).
I don't think I like the idea of light being the fastest anything can travel, though. Perhaps it is for many things, but what happens if some forces travel at speeds faster (or multiples), or perhaps simple fractions, and we discount those readings instead of seeing if the old model can be adapted or remade? Well, many questions, few answers from me.
Does anyone remember the 'gravity shielding' story a while back, where a spinning superconductor was supposedly responsible for changes in weight? Podkletnov comes up in a google search for 'superconductor gravity shield' but I haven't heard anything further about it.
Also, what about magnetic forces? How do those work, and at what speed do they 'travel' ?
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I prefer to think of it as having no loose ends...
___
If you think big enough, you'll never have to do it.
From that they worked out that gravity does move at the same speed as light. Their actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25.
So, really, they're triumphantly announcing that the speed of the light is somewhere between 0.7 c and 1.2 c, and just supposing it has to be c for everything to make sense.
Physicists have been accused of being loose with rigour, but this is really stretching it.
What next? The speed of magnetism?
Yes, the speed of magnetism. The particle which mediates electromagnetic interactions is the photon which propagates at the speed of light. So if a magnet is suddenly given a push in one direction then there is a delay before distant particles notice a change in the field of that magnet.
This is an analogous result for gravity and the postulated graviton particles.
It's one thing to not understand something, we all have our fields of expertise. But assuming you know everything based on some limited high schooling makes you the saddest kind of idiot.
:wq
"In general relativity, one learns that any "information" cannot travel faster than light"
What about quantum pairs? Move them apart, and a change in one is reflected intantly in the other.
That's why I specifically said "In general relativity...". Quantum pairs are from the theory of quantum mechanics, not general relativity. Physicists have been working hard to try to combine relativity and quantum into a single unified theory. However, problems arise when the two theories predict different things -- such as the quantum pairs example you listed. According to relativity, there would be a finite time lag for the change to be reflected in the second entity of the pair whereas quantum would say that the change is instantaneous.
Incidently, I heard that a few years ago an experiment was performed on quantum pairs and, sure enough, the change was indeed instantaneous. Can anyone else corroborate this?
GMD
watch this
YESSS! In your FACE, Infinite-dimension Universe theorists! You SUCK!
Yeaaaaah! Gimme one up top, bro!
pr0n - keeping monitor glass spotless since 1981.
Gravity waves have been used in many stories as a FTL communication system, now that's all out of date.
Venus is a big swampy planet, eh guys ?
I think we should assume more of the author. Think of it this way: Using General Relativity, you can predict what the gravitational field will be. His experiment measured what the field actually was. If the predictions match the measurement, the theory is confirmed (or at least not disproven).
- Strong
- Electromagnetic
- Weak
- Gravitational
I even fetched a URL on a whim, just in case you disagree for some reason....you see, it's all component forces. If you look at the free body diagram of my car travelling on the road, you'll see the normal force, force of gravity, and my velocity in the x direction. As I mentioned, one of these component forces is gravity, labelled FsubG. It was recently discovered, and posted on slashdot, that the speed of said force is 3x10^7m/s.
And THAT'S why, officer, your radar reported that I was going 60 in a 40 zone!
I asked my physics teacher the very same exact question- "if the sun disappeared, would the earth fly off from its orbit instantly, or would it take about 8 minutes?" He goes "it would be like snipping a cord- instantaneous". Discouraged, I went into the slashdot-posting, linux compiling netadmin that I am today, never knowing the true path of lab coats, leather gauntlets, and welding glasses that is physics- How dare you Stockwell! You stole my life with an assumption and I want my five years back!!!
slashdot: where everyone yells sarcastic metaphors to themselves to understand the issue
While we think of mass and momentum being related by speed, energy and momentum are related by FREQUENCY.
Here's some equations if you want to wrap your head around them: equations
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the slightly longer answer is "because in the sun's inertial reference frame (i am going to leave gen.rel out of this) the sun still has the same mass."
if you don't understand what I just said, read more about special relativity, kay?
My life in the land of the rising sun.
http://www.what-is-the-speed-of-light.com/roemer-s peed-of-light.html
burris
To perform the experiment, numerous (probably several thousand) measurements are taken, but due to imprecision in the process of taking the measurements (imperfect measuring equipment, human error, etc) you get a variety of results. These answers could vary from well below c to well above it. If Einstein was right and nothing propogates faster than c, the higher results could only be attributed to imprecise measuremements, but you can't throw those measurements out if you are trying to be objective.
At the end of the process, you have something vaguely resembling a normal bell curve, where the height of the curve at a point along the x axis (velocity) is a measurement of the relative frequency with which that speed of gravity was obtained as a measurement. The total area under the curve will be exactly 1. In many cases, the curve may not be symmetric, but for an experiment such as this, you are unlikely to obtain an assymetric curve (Central limit theorem of statistics, or some such thing). A line right down the middle of the curve shows the measured average result (.95c).
A confidence interval is then picked (it is a shame that this interval is not mentioned in the article, but it is almost assuredly at least 95%, probably even 99%, or 99.9%). This percentage is converted to decimal (95%=.95, 99%=.99, etc), and a symmetric region around the average score with that area is blocked off. This blocked off area has a minimum X component of .7125c, and a maximum X component of 1.1875c, the difference between each of these and the average measured velocity being .2375c, which is 25% of .95c.
And that's where the 25% margin of error comes from -- for their desired level of confidence, the variance in measured results was off by no more than 25% of the value that was actually obtained as the mean.
Since the value of 'c' lies WELL within the bounds of the margin of error of the experiment, and pre-existing theories support the speed of gravity being c, this experiment supports those theories. It is important to note that this experiment did not prove anything, it only failed to disprove that the speed of gravity is anything other than something very close to c.
File under 'M' for 'Manic ranting'
Well, except that in the initial design, at least, they used an intermediate stage to transfer the momentum from the radiation pressure (generated by a conventional fission bomb)...they use the radiation to ablate the outer surface of a cylinder of U-238 (natural uranium) surrounding the deuturium/tritium to use the uranium to compress it, which also trips an initiator placed with some U-235 centered in the center of the deuturium, causing it to fission, which creates two massive pressure waves, an incoming and outgoing, that compresses the deuturium mightily. This ignites fusion in it, which, in turn, releases enough fast neutrons to ignite fission in the normally unfissionable U-238 that surrounds it. The fissioning of U-238 actually produces most of the yield of this device, "Mike", which was about a megaton. Quite an intricate piece of work, really.
Now, what did this have to do with this discussion again? Oh, yeah, the momentum of photons. I guess it's marginally ontopic. (Please forgive me, I just finished reading about the development of the H-bomb and couldn't keep from showing off the neato stuff I just learned.)
Anywways, that's more or less how my high school science teacher explained it.
File under 'M' for 'Manic ranting'
Gravitomagnetic effects make everything work out consistently. Read this FAQ. No conservation laws or symmetries are violated. (By the way, some mass-energy is radiated as gravitational waves, and the objects do spin faster and spiral into each other, but this is an extremely weak process, visible only in closely orbiting neutron stars.)
Strange, 20 years ago I was taught other people had experimental evidence agreeing with a prediction that the effects of gravity move at light speed:
In 1882 Simon Newcomb observed an excessive perturbation in precession of the orbit of mercury, to the tune of 43 seconds of arc per century. In 1915, Albert Einstein showed this could be explained by the propogation of gravitic wave effects at the speed of light...
But thanks for playing.....
"Remember the Unified Field Theory? Well, forget it. Physicists have pretty much thrown in the towel on unifying gravity with the other elemental forces, so now we have the Standard Model, which says that everything works together in intricate harmony except gravity, which is on holiday in Tasmania and need not concern us further."
- Jon Carroll on the Higgs Boson
Nerd: Derogatory term typically directed at anybody with a lower Slashdot ID than you.
"You can't go faster than the speed of light."
"Of course not. That's why scientists changed the speed of light in 2208."
If Gravity's speed is equal to that of the speed of Light, then how do you explain the pull descrepencies between blackholes and low graivty environments? Go to the moon, you'll notice that the gravitational pull there is much lower than that of Earth's. And Earth's is far far less then a blackhole's gravitational pull.
:)
So how can one say that Gravity's pull is as fast as the speed of Light when Gravity itself doesn't stay constant in different environments? I never heard light not traveling the "speed of light" so it's a bit confusing.
Ao, from what I gather, blackholes have so much gravitational pull that even light can't escape. Which suggests to me that Gravtiy is stronger than light. It would also suggest to me that gravity is is faster than light because of this. I don't have any sources to back this up, all of this is just my train of thought in words here.
I'd appreciate a simple-as-possible answer as to why my train of thinking is wrong, as i said, i'm no scientist, but this topic is interesting none the less
A Penny for my thoughts? Here's my two cents. I got ripped off!
Actually, you got the order wrong.
:).
Strong, Electromagnetic, Weak, Gravitational
This depends strongly on the distance you choose to measure the force at. At a distance of 1m, as opposed to 1e-15m, the original ordering may be correct.
And as long as we're being nit-picky, I'll point out that human-observable phenomena tend to be larger than 1e-15m
The mass of photons is very real. Try this experiment, which a professor did at one of my Engineering Physics classes:
Take a relatively large gong. Make sure it is reasonably well polished.
Next, take a professional-class camera flash and set the intensity to "fry".
Third, fire the flash at the gong. As the photons bounce off the (polished) gong, it will resound as if having been struck with a solid object.
This was a very awakening demonstration to me...
Flintstones (Hanna Barbera): Sep 30 1960 (ABC)
Road-runner (Warner Bros.): 1949
Unless you have a plausible alternative hypothesis, experiments that agree with your hypothesis tell you essentially nothing.
This observation is meaningful only in hindsight. An experiment like this one has the potential to disconfirm the hypothesis as well. The fact that it did not do so is significant, albeit not so significant as the alternative.
The slur against physicists is unjustified, particularly the "elevate it to dogma" line. If you want to test a hypothesis, you first assume that it is correct, then try to prove the assumption wrong. If you have a better method, please share it.
Anyway, it's true that just igniting some nuclear fuel into fusion isn't that hugely hard, assuming that you have some tritium, not just deuturium, around. But you don't get that much from it compared to the fission bomb you've exploded to burn that small amount of fuel. In regards to power plants, of course using the heat of the core of a fission explosion is not an option for initiating fusion. And all our current technologies currently use about as much energy to initiate and contain fusion in a fuel than they are to usefully extract from it. The vast gulf seperating fission from fusion power is that once you understand the neutron-capturing cross-sections of various isotopes, cobble together a sufficient mass of an approriate fuel, and find a moderator (and moderator arrangement) to go with it, the actual physical, engineering complexity of the reactor is minimal. You could build one by hand, which is essentially what Fermi did. You can control one by winching a control rod into and out of a pile. In contrast, the fusion reaction is very different in this context and an implementation and control mechaninism is fiendishly complex. I suppose that in a way your teacher was right, in the sense that a fission reactor is very, very different from a bomb; while a fusion reactor must by necessity in some qualitative sense be pretty similar to a fusion bomb.
I wouldn't take their word for it.
This paper gives a good case for gravity traveling faster than light and I'm pretty sure all the working Newtonian gravity calculations assume instantaneous gravity:
"Standard experimental techniques exist to determine the propagation speed of forces. When we apply these techniques to gravity, they all yield propagation speeds too great to measure, substantially faster than lightspeed. This is because gravity, in contrast to light, has no detectable aberration or propagation delay for its action, even for cases (such as binary pulsars) where sources of gravity accelerate significantly during the light time from source to target"
OddManIn: A Game of guns and game theory.
It's much more likely the ringing comes from the air right next to the polished gong surface suddenly heating up.
There's a similar confusion about what drives those "solar radiometer" things - you know, a little black-and-white paddlewheel inside an evacuated glass ball that spins when you shine a light on it? People often say the reason they run is photon momentum, when the actual explanation is that the black sides of the paddles are hotter than the white sides, so when the few gas molecules left inside the ball hit the paddles, they leave the black sides going faster than the white sides.
The proof of this is the direction the paddlewheel turns - it turns white-side-first, and a photon-mass explanation would have the paddle turning black-side-first. If you put a paddlewheel inside a REAL hard vacuum, with a REAL low friction bearing, and REALLY isplate it from outside vibration, it turns the right way. See here for a more coherent and complete explanation.
To a Lisp hacker, XML is S-expressions in drag.
There was another poster that also claimed this would have been due to the air heating up near the gong.
However, this professor continued his demonstration with sooting the gong heavily (taking it from polished to near-black), and then firing the flash again. The sound was significantly softer, noticable by all attendees (around 120), and he explained this by the photon package having been absorbed instead of bouncing (the gong only got half the impulse from before).
In a scenario where heat was the cause of the sound, sooting the gong would have caused a significantly louder sound as the light was absorbed, instead of (as happened) as softer one.
What if gravity has different properties from a long way away, such as intergalactic distances?
I've often wondered lately if perhaps gravity is both a repulsive and an attractive force. For local (i.e. interstellar) distances, the attractive force prevails. But for really vast (intergalactic) distances, it might act as a repulsive force. This could partly explain why the galaxies are accelerating away from each other.
Physicists don't have much of an idea what dark energy is... maybe it's just gravity, and Newton's law needs an amendment.
I've never heard this idea proposed, but it would make a certain kind of sense to me if it turned out to be the case.
WWJD? JWRTFA!
If the gong is reflective, the air near it gets heated both by the incoming light and by the reflected light. If the gong is sooted, only the incoming light heats the air.
At least, this seems logical to me. A way to test it would be to put a vibration sensor on the gong, and try it both in air, and in a vacuum. If you're right, the sensor should read the same, if I'm right the impact in vacuum should be much less.
To a Lisp hacker, XML is S-expressions in drag.
There is no strong force. It's a myth. Just like Neutrons are a myth. No, I'm not joking. Anytime you extract a neutron from an atom, it breaks into a proton and an electron (hydrogen). A Neutron is not a true particle, it's simply a compressed proton and electron.
First of all, you're still going to have an interesting time explaining how all of those nice, positively-charged protons are bound into an incredibly tiny space without the strong force holding them together.
Secondly, the production and decay of neutrons is mediated by the weak force, not the strong force.
Thirdly, your model fails to explain mesons and the zoo of other particles that can be produced even in relatively low-energy accelerators, while the quark model explains it nicely.
Protons become neutrons when an electron and an "up" quark interact to produce a "down" quark and an electron neutrino. The inverse process - decay of neutrons into protons and electrons - happens when a "down" quark decays into an "up" quark, emitting an electron antineutrino and an electron.
The neutrino emitted during the decay has significant momentum. Its existence can be shown - and was originally inferred - by tracking the charged particles emitted when a neutron decays into a proton and an electron. In many cases, both of the charged particles are going in the same direction. To conserve momentum, something else had to be fired off in the opposite direction during the decay. That "something" is the neutrino. If a neutron was a bound proton/electron pair, there would be no third particle to explain the momentum discrepancy.
You're also overlooking the fact that a bound system has less energy than an unbound one. Which would mean that in your proposed scenario, _neutrons_ should be the stable nucleon, which is at odds with observations.
Or, you may have written that post as sarcasm. Either way, moderators have been falling for it.
The rest of your post is even sillier, so I'm not going to bother with it.
In summary, your proposed model is demonstrably incorrect.