Speed of Light Measurement Using Ping

Thomas Colthurst writes "You've no doubt already read the story of ping, but have you ever used it to measure the speed of light?" Here's a case where all that cat5 on college campuses can actually be used for education ;)

Comments

Delays due to molecular friction?

[JTinMSP]

Wouldn't you get delays due to the friction of the copper? You could use fibre, but a switch or patch in the fibre would add latency/friction as well.

Re:Delays due to molecular friction?

[8bit]

Friction?! As I remember the electrions themselves move slow as molasses, but the information (current move, current not move,) travels at the speed of light. So how would any kind of friction change this?

Re:Delays due to molecular friction?

[HMC CS Major]

They took this into account.

If you look at the actual paper (pdf version here), the 9th page shows the formulas they used to calculate the result.

Re:Delays due to molecular friction?

[Hal-9001]

Where does friction enter the picture? If you have the impression that electrons are flowing through CAT-5 to carry bits of information, you're horribly mistaken. You even more horribly mistaken if you think friction has anything to do with fiber-optic communication.

In general, the speed of light pulses sent through a fiber will be approximately 2/3's the speed of light in vacuum, since the refractive index (ratio speed of light in vacuum to speed of light in that medium) of glass is approximately 1.5. You get the latency by dividing the transmission distance by this speed. I haven't had a chance to read the paper yet, but I imagine that CAT-5 latency is probably similar.

Re:Delays due to molecular friction?

[Anonymous Coward]

You're right, it wouldn't, but there's a couple of shit-for-brains moderators, so the stupid comment gets called "Insightful" and another round of circle-jerking gets propagated.

That's why people who read Slashdot comments seriously are a bunch of cum-guzzling Taco-snotters, and I hope they all die, like in that hilarious story about the Americans who got toasted in Apollo 1 by the general incompetence of American engineers.

Re:Delays due to molecular friction?

[Hal-9001]

I just read the paper, and I was right about the latency of CAT-5:

The least squares fit to these data yields a slope of 2.04 +/- .14 x 10^8 m/s , indicating that the speed of propagation in a cat-5 cable is some 2/3 the speed of light in the vacuum.(bottom of page 8 of the PDF)

Damn, I'm good... ;-)

Re:Delays due to molecular friction?

[MindStalker]

Its like a hose full of water, you turn on the water on one end, and water almost instantly starts shooting out the other end, but the internal friction and compression of the materials in the hose will effect the speed of that processes. Think of a non compressable fluid, vesus a gas. electrons arn't very compressable, so its fairly instantanious but it does happen, but we know the percent change based on how conductive the material is.

Re:Delays due to molecular friction?

[Anonymous Coward]

Hmmm college. Is that where i stole that wire?

Re:Delays due to molecular friction?

[cyberformer]

Electrons themselves can't travel at the speed of light. (They have a rest mass, and so according to relativity theory, would require infinite energy to accelerate them to this speed). The information is carried by electromagnetic waves (or photons), which pass through the electrons and do travel at the speed of light (because they have zero mass).

Of course, the speed of light (or photons, or EM waves) in a copper wire is somewhat less than that of light in free space (but, interestingly, somewhat more that that in glass fiber, despite claims that fiber optics is "networking at the speed of light").

Re:Delays due to molecular friction?

[Anonymous Coward]

Only God may judge Osama Bin Laden, It's our job to arrange the meeting.

Yes, let's kill and justify it with our religion! Sure "God" might be all powerful, but *we* are the ones who were chosen to kill Bin Laden, despite what it says in that Bible book about 'thou shall not kill.' Kill the heathens!

-bugg (posting AC for obvious reasons)

Re:Delays due to molecular friction?

[Hal-9001]

The parent post is right: electrons themselves move too slowly for them to carry information all the way from one end of a cable field. Information is carried through cable via an electromagnetic wave, which can propagate much faster. In fact, the parent post is right again: the information propagates at the speed of light (in that medium). In fact, any given electron in the cable probably doesn't go anywhere. A simple example demonstrating this to be true of wave phenomena is that if you send a wave through a string, the end of the string you're holding onto doesn't magically find it's way to the other end of the string--you were holding onto it the whole time. The fact that the wave propagates and the medium doesn't is also why a beach ball out in the ocean beyond the breakers doesn't spontaneously return to shore.

Re:Delays due to molecular friction?

[Anonymous Coward]

Shut the hell up you foreign buttpipe. If it wasn't for American engineers, your shitty country (whatever it is) wouldn't have any technology today. Zippo, nada, zilch.

So you can gladly kiss my American Engineer ass.

One more thing... shouldn't the speed of light be measured in a vaccuum?

That's all for today.

Re:Delays due to molecular friction?

[Hal-9001]

Of course, the speed of light (or photons, or EM waves) in a copper wire is somewhat less than that of light in free space (but, interestingly, somewhat more that that in glass fiber, despite claims that fiber optics is "networking at the speed of light").

Two responses:

1. Fiber optics is networking at the speed of light--it's just the speed of light in glass, which is ~2/3 the speed of light in vacuum.
2. Photonic bandgap fiber uses a hole in the center of a glass fiber to confine the light. Since this hole is presumably filled with air, which has a refractive index very close to 1, such fiber would be networking very close to the speed of light in vacuum.

Re:Delays due to molecular friction?

[Anonymous Coward]

Hahahahahahaha, another shit-for-brains American "engineer" chips in. Why not go kill some more people with your shoddy work, dickweed?

"shouldn't the speed of light be measured in a vaccuum?"

Hahahahahaha, the funniest bit is your ignorance, and where you misspelt "vacuum", you fuckwit.

Re:Delays due to molecular friction?

[Anonymous Coward]

Mark Bluemel
Software engineer, IBM Hursley Lab
January 2002

Heap exhaustion and excessive garbage collection are often the result of poor object management. Here are a few tips to ensure you're not falling into this trap:

Remember that objects are held in the heap as long as they are "reachable" (loosely speaking) -- if an in-use object has a reference to an out-of-use object, the out-of-use object cannot be garbage collected and will waste space. Vectors, HashTables, linked lists, and similar data structures can often be misused such that they hold references beyond an appropriate time for their release. Profiling tools can be helpful in identifying this sort of problem.

Garbage collection is not free; other processing will be paused. Try to reuse objects in preference to discarding and re-creating them when possible. Object pool mechanisms can be helpful in this respect.

Mark Bluemel
Software engineer, IBM Hursley Lab
January 2002

Heap exhaustion and excessive garbage collection are often the result of poor object management. Here are a few tips to ensure you're not falling into this trap:

Remember that objects are held in the heap as long as they are "reachable" (loosely speaking) -- if an in-use object has a reference to an out-of-use object, the out-of-use object cannot be garbage collected and will waste space. Vectors, HashTables, linked lists, and similar data structures can often be misused such that they hold references beyond an appropriate time for their release. Profiling tools can be helpful in identifying this sort of problem.

Garbage collection is not free; other processing will be paused. Try to reuse objects in preference to discarding and re-creating them when possible. Object pool mechanisms can be helpful in this respect.

Mark Bluemel
Software engineer, IBM Hursley Lab
January 2002

Heap exhaustion and excessive garbage collection are often the result of poor object management. Here are a few tips to ensure you're not falling into this trap:

Remember that objects are held in the heap as long as they are "reachable" (loosely speaking) -- if an in-use object has a reference to an out-of-use object, the out-of-use object cannot be garbage collected and will waste space. Vectors, HashTables, linked lists, and similar data structures can often be misused such that they hold references beyond an appropriate time for their release. Profiling tools can be helpful in identifying this sort of problem.

Garbage collection is not free; other processing will be paused. Try to reuse objects in preference to discarding and re-creating them when possible. Object pool mechanisms can be helpful in this respect.

Re:Delays due to molecular friction?

[Anonymous Coward]

I could waste time to reply to this, but I have engineering homework to finish... so when I perfect the long range missile guidance systems we can show your P.O.S. country who's boss.

Re:Delays due to molecular friction?

[anthony_dipierro]

You get the latency by dividing the transmission distance by this speed.

You're presuming that the distance travelled by light is roughly equivalent to the distance of the cable. This is grossly inaccurate.

Re:Delays due to molecular friction?

[Anonymous Coward]

If I recall correctly, the speed of light in coax is slightly faster than the speed of light in fibre... cat5 is probably faster too. Of course, the term "light" is misleading, since in normal usage that applies to visible light, and we're really talking about radio waves here...

Re:Delays due to molecular friction?

[Mr Z]

Actually, the speed of an electromagnetic wave is somewhat slower in a cable than in a vacuum. How much slower is determined by the dielectric constant for the cabling's material.

This page on transmission line theory explains things pretty well. It actually covers the concepts that students performing the described experiment need in order to actually get their results. It also describes some other neat things (such as the theoretical reasons why you need a "balun" converter to connect 75ohm coax to 300ohm twinlead. It even explains why the wire types are called 75ohm and 300ohm, if indirectly.)

--Joe

Re:Delays due to molecular friction?

[Anonymous Coward]

friction of the copper. right-o. go read a physics book fucker

Re:Delays due to molecular friction?

[Technician]

Friction equates to loss or attenuation. The signal gets weaker with distance. Attenuation does not affect speed. Propagation factor is another mater. It does affect the velocity of the signal, but does not change the amplitude. In cable both items are present. Attenuation is due to both resitance of the copper conductor and dielectric losses in the insulation. Delay is caused by the dialectric, not the conductor. The delay in cables is well known. Anyone who uses a TDR regularly is aware of the velocity factor of the type of cable they are measuring. It must be known to get an accurate length measurement. Air dielectric has a constant of 1. Coax with solid dielectric is near 2/3rd that speed. I wonder if the university study took into account the velocity factor for the cable. If not, I expect they will find light is about 20% slow.

The best experiment I saw for measuring the speed of light was done using the mirror (8 sided) out of a laser printer. At rest a laser was reflected off a face of the mirror and went to a target reflector. Oposite the laser, a detector was used to see the same target off another face of the mirror. When the mirror was spun, the laser scanned the reflector. The reflected light pulse would not reach the detector because the travel delay kept the return pulse from hitting the mirror at the right angle to reach the detector. At a certan speed the pulse reached the mirror in the right postion (1/8th rotation) to send the reflected pulse on to the detector. Light only reached the detector with the mirror at rest and at a speed where the mirror turned 1/8th of a revolution in the time the light took to travel from the mirror to the reflector and back to the mirror. It was a good class. We started with a known distance to measure the speed of light, then used an unknown reflector (stop sign down the block) much further away and used our speed results to measure the distance.

Re:Delays due to molecular friction?

[Anonymous Coward]

You're presuming that the distance travelled by light is roughly equivalent to the distance of the cable. This is grossly inaccurate.

It's quite accurate, if the cable is long. (Assuming by "light" you mean "the signal".)

Re:Delays due to molecular friction?

[JCMay]

The electrons move (as a result of an applied voltage) at what is known as the drift velocity. A example in copper is also available.

Current doesn't stop (your "current move, current not move" parenthetical). Current is not a thing, but is a description of a situation: moving charge is a current. An Ampere is defined as one Coulomb of charge passing a reference plane in one second.

How fast a signal propagates down a wire is its group velocity.

The "friction" mentioned by the original poster I interpret to be a flawed understanding of how resisivity works. Electrical signals travelling through resistive materials are attenuated, not slowed down, due to the resistance. Changes in velocity are due to changes in the dielectic constant.

Re:Delays due to molecular friction?

[anthony_dipierro]

It's quite accurate, if the cable is long. (Assuming by "light" you mean "the signal".)

I was referring to fiber optics, in which light bounces from side to side. Because light bounces from side to side, the distance travelled by light is proportional to the length of the cable, but not equal to it.

first karma whore

[Anonymous Coward]

We report on a very simple and inexpensive method for determining the speed of an electrical wave in a transmission line. The method consists of analyzing the roundtrip time for ethernet packets between two computers. It involves minimal construction, straightforward mathematics and displays the usefulness of stochastic resonance in signal recovery. Using basic electrical properties of category-five cable students may use their measurements to determine the speed of light in the vacuum to within a few percent.

i see

[TeknoTurd]

hrm makes sense to me :)

I just measured it

[Guitarzan]

And according to Unreal Tournament, the speed of light is about 50 miles per hour.

Re:I just measured it

[FallLine]

And apparently it gets SLOWER on shorter and smaller networks! Mine slowed down to 5 miles per hour on my crossover!@@#!

Re:I just measured it

[Anonymous Coward]

According to Star Trek, the speed of light coming from a phaser is about the speed of a flying rubber band.

Re:I just measured it

[Cruciform]

It's a little higher than that. Don't forget you're actually running at 55 :P

Dunno about light..

[ackthpt]

But I've certainly used it to measure the speed of lag. I wish I had some of my old traceroute logs from when sprintnet went out in Chicago. Ping times went down to 1545ms on average, going from Bay City, MI to Saginaw, to Michigan Tech, to Rochester, to Willow Springs, to Atlanta, to St. Louis, to Kansas City to Ft. Worth, to Austin. Those were some well travelled packets.

Re:Dunno about light..

[Anonymous Coward]

Go away.

Re:Dunno about light..

[GigsVT]

That's no big deal man, this message is coming to you from my house, up 60,000 miles into space, then 60,000 miles back down, and 1000 odd land miles.

My pings are never less than 650ms.

But I sure can download things real fast once they get rolling. :)

The joys of satellite internet. Ping already let me calculate the speed of light, easily, without stociastimachwastic anything.

Re:Dunno about light..

[Anonymous Coward]

Man , at 60000 miles, you must have to move your satellite constantly. 22,241 miles, maybe?

Re:Dunno about light..

[GigsVT]

Yeah, you are probably right, mine was just a rough estimate, only off by a single constant multiplier. :)

How they do it

[Alien54]

As seen in the paper:

Here's the problem; the cable range of ethernet without a repeater is about 250 feet (that is, at most a few microseconds roundtrip in cat-5 cable) and actually all the tests described here are done in much shorter cat-5 cable (more practical for typical reuse) and coaxial cable lengths so that it can be done cheaply. A typical classroom can hold several experiments of this type, the cables being shared between pairs of computers (and thus lab groups). Since ping only returns roundtrip times as measured in microseconds the actual signal (which is the additional delay in a cable path of longer length) is below the (reported) resolution of ping.

The solution is to use noise. Although noise usually hampers one's ability to measure a signal, in this experiment, noise in the form of randomly distributed small delays (microseconds) associated with machine response actually makes the measurement of the signal (nanosecond-long cable transit delays) possible. Without the noise, the experiment we describe here would be impossible! This concept of noise-assisted sub-threshold signal detection (hereafter; stochastic resonance) is of great value because it plays a role in a great variety of systems. For a readable introduction and overview of stochastic resonance see Ref. [7] and Ref. [10] for a bibliography. For example, stochastic resonance has been used to analyze climate patterns [8] and plays a role in fundamental neuro-physiology [9] . Part of the hidden pedagogic agenda of this laboratory is to introduce the concept of stochastic resonance in a hands-on way. How well this laboratory can actually get students to ponder that depends on the approach of the instructor. Our experience with this laboratory indicates that time differences on the order of 50 nanoseconds (or about 5 % of the threshold) are reliably resolvable.

Which is damn clever of them indeed.

Re:How they do it

[Anonymous Coward]

damn karma whore!!

Re:How they do it

[Score0, Overrated]

The informative bit is the reference 7 where stochastic resonance is actually explained.

stochastic resonance

Re:How they do it

[afidel]

I don't know where they got their information but the 100-BT standard calls for a 100m cable length limitation, which is considerably more than 250 feet. In addition most networking gear providers of any decent caliper will go much farther in testing. For an example when I helped our test group set up a test for our then next generation product we made some custom 150m cables and tried our product with various manufacturers nic's. With the exception of one low cost (d-link, smc or netgear can't remember) nic all were able to go 150m with no errors. Note this was over cat-5e not cheap cat-5 or cat3.

Re:How they do it

[marklark]

One laptop had 100-BT. The other had 10-BT. The lowest common denominator is 10-BT, which would be the one that applies.

It, being the slower protocol/technology, may have a the longer, 250 foot, limit.

Re:How they do it

[eples]

Which is damn clever of them indeed.

Yes - their use of the word "pedagogic" WAS quite clever...

Seriously, though it is pretty neat.

How can this be accurate?

[TommyBear]

Their recording equipment consists of laptops that are networked. Wouldn't the packet first need to hit the network iterface, be decoded. Hit the pci bus hit the CPU, hit the software, run through the os, to the bash process to be displayed on the bash console?

Doesn't sound very accurate to me.

Re:How can this be accurate?

[Score0, Overrated]

All those things are mentioned are constant - so you can do it with a long wire, and with a short wire.

Then you use the difference - and you've eliminated your constants.

Re:How can this be accurate?

[Anonymous Coward]

I was thinking the exact same thing. Unless they knew the exact route and therefore distance the signal took through all those components (which would require knowing way too much about the architecture of the systems) before reaching the network interface, or used miles of cable, there seem to be a lot of places for error.

Maybe that's why they're sending "noise," but still...sending any signal on a computer goes through a whole lot more distance than the linear length of connections between devices.

Re:How can this be accurate?

[Anonymous Coward]

Why are they necessarily constant? Even with nothing running in the backround, the cpu could be doing random memory cleanup that could inflate the times.

Re:How can this be accurate?

[Anonymous Coward]

It would be interesting to see how the Speed of Light varied Operating System to Operating System.

Re:How can this be accurate?

[Score0, Overrated]

Over 15000 samples it's going to be pretty constant.

Perhaps you should read the paper?

Re:How can this be accurate?

[TommyBear]

While were splitting hairs....

That's by no means a constant, rather a mean or average of a group of values.

This is by no means accurate, anaything can throw the values off (OS, System, Hardware, or disks). This is really a wastes of time, in it's current form, needs more thought.

Re:How can this be accurate?

[Anonymous Coward]

While we're splitting hairs :-)

Re:How can this be accurate?

[marklark]

They aren't sending noise.

The "noise" in the data is _caused_ by the components. It is what makes the whole idea work.

And, you don't have to know everything about the hardware.

Re:How can this be accurate?

[Anonymous Coward]

[The error bias is] by no means a constant, rather a mean or average of a group of values.

It doesn't have to be a constant. See below.

This is by no means accurate, anaything can throw the values off (OS, System, Hardware, or disks). This is really a wastes of time, in it's current form, needs more thought.

Except for the fact that it actually gives the right answer for the speed of light -- reliably and reproducibly, to within a few percent. I wonder how that happened. Accident? Coincidence? Fudging the data? Incompetent error analysis? Wishful thinking? No, none of the above.

You really need to learn about statistical error analysis. This happens in every scientific experiment: there are always uncontrollable, unknown sources of error "that can throw the values off" -- be they fluctuations in OS response time, or in the temperature of a material, or air currents, or whatever is relevant to your experiment. (This case is just more extreme, where the errors are larger than the signal.)

However, that doesn't prevent you from analyzing the magnitudes of the errors and getting an accurate result bounded by error bars. In this case, if you take enough measurements, it's possible to extract a signal from the noise -- you just need to make sure that the signal-to-noise ratio is good enough.

I'm reminded of a trick for improving GPS accuracy: it's only accurate to some certain number of meters. But if you leave the receiver at the same location and carefully integrate the signal for a sufficiently long period of time (hours or days), you can actually get down to centimeter accuracy -- far beyond the theoretical "accuracy" of the equipment, even though random errors throw each individual measurement off by metters.

The reason is because the error goes like 1/sqrt(N) where N is the number of measurements. Take a lot of measurements, and you can reduce the error. (Up to a point, until the noise swamps the signal beyond any statistical chance of recovery. It isn't a magic trick for providing infinite accuracy.) I remember Jerry Pournelle, in his Chaos Manor column, talking about using a GPS unit this way to locate the exact best location for a solar eclipse (just for the heck of it, not that you really need to know it down to the last centimeter).

For that matter, this is the same reason why the LIGO instrument can use laser rangefinding to measure distances on the order of 1/1000 of the diameter of a proton. No, I'm not joking. 10^-18 meters. How can it do that, if that's far smaller than the size of an atom, if the mirror the beam is bouncing off of isn't even flat to that accuracy?

It can do that because it's measuring the average distance of lots of atoms (all the atoms in the mirror), so the same kind of 1/sqrt(N) argument applies. It's another counterexample to your first remark: the measured values don't have to be constant (due to a constant systematic error bias); they can fluctuate, as long as you've got a very accurate measurement of their average. Thus, the instrument will be able to detect the minute changes in distance that occur when a gravitational wave passes by and curves space along the beam line.

(Side note: LIGO II will be sensitive that it will actually be making macroscopic quantum measurements, running up against the Heisenburg uncertainty bound on position accuracy -- as applied to a 30-40 kg object, the mirror. It's a textbook problem to verify that the HUP bound on position for a macroscopic object is utterly tiny, but for the first time, we will be able to demonstrate its applicability on the macroscale directly.)

In all of the above cases, including the case under discussion here, this trick is only possible because the SNR was low enough to permit signal extraction from the noise. If the OS/system/hardware threw off the values by too wide a spread every time, then you wouldn't be able to do this -- but they don't. (In the LIGO case, the signal is so small that they have to do amazing noise reduction in order to pull out any signal at all. The observatory is so sensitive that it can track passing aircraft from the noise they make, since it vibrates the mirrors that the lasers are bouncing off of. Fortunately, they have all kinds of ways of subtracting out noise like that, so that the remaining unavoidable noise is absolutely tiny.)

In fact, in the case under discussion, the very errors you're claiming make the experiment "a waste of time", are what make the experiment work! (As was pointed out in the paper, and by other posters here.) If you always got a consistent "ping 1 ms" or whatever, that wouldn't tell you much, since the actual transit time is much less than 1 ms. But if there are some fluctuations due to random errors, then changing the physical round trip time will have an influence on the statistical distribution of those fluctuations. (i.e., the shape of the error bars -- or, more accurately, of the statistical distribution of error -- bounding a data point depends on where the data point is. Thus, the noise tells you about the signal!)

Incidentally, I'm reminded of some amateur radio astronomers being able to measure pulsar emission rates using homebuilt experiments. There's no way you can actually see the period signal directly, but with long integration times, some Fourier transforms, and a little signal processing... It's really amazing what you can do with a little signal processing! I'm pretty sure they weren't using anything as fancy as stochastic resonance, but imagine what they could do if they could apply this technique...

Re:How can this be accurate?

[Anonymous Coward]

This is by no means accurate, anaything can throw the values off (OS, System, Hardware, or disks). This is really a wastes of time, in it's current form, needs more thought.

Anyway, we already know the speed of the light with infinite precision.

Re:How can this be accurate?

[Anonymous Coward]

You are right in the sense that today the speed of light in vacuum is a defined constant (299 792 458 m/s). The same holds true for the second, which makes the meter a derived unit. There was an interesting article on length measurements in the March 2000 issue of Physics Today.

Good as experiment.

[Bender Unit 22]

It sounds good as a experiment. You have to figure out the time the computer in the other end takes to reply and then return an answer. But don't you really need another clock than the one that comes in a standard pc. Some PC's seems to loose up to 30 secs every day. And then there is the limit to how long your cable can be. since you can't have any switches in between, can the cable be long enough so you can measure a delay with the poor accuracy of a pc? Hmm maybe counting clock cycles would be better for timetaking. oh well.

Re:Good as experiment.

[xercist]

I don't think losing a few seconds each day will make the difference needed to mess up the results of such a test, but in either case, NTPd measures the inaccuracy of your system clock by comparing it to reliable sources (stratum 1 time servers) and compensates for it automatically. If you run ntpd, you should be able to see what your offset is by looking at the file /var/lib/ntp/ntp.drift.

Re:Good as experiment.

[LoseNotLooseGuy]

Some PC's seems to loose up to 30 secs every day.

I doubt that the PCs "let loose or release" 30 seconds every day; that would imply some sort of conscious effort on the part of the PC, not to mention control over the flow of time itself. I believe the word you were looking for was lose.

Congratulations! You have been participant #9 in my campaign to rid Slashdot of this error.

Re:Good as experiment.

[Anonymous Coward]

I'm going to loose my foot up your ass!

Re:Good as experiment.

[Bender Unit 22]

Well, I have only had one year of English education in school and since it's not a language I use everyday, I think that it was close enough. heh

Re:Good as experiment.

[invenustus]

In my experiences, computers don't have a steady drift. They tend to lose time more under high use. So when Windows is f***ing up and everything is slow, I lose all kinds of time.

I hate to bury a question like this so low in a thread, but here goes: why is it that for $5 I can buy a Backstreet Boys (or whatever) wristwatch at K-Mart that will lose less than a minute each month, yet I pay thousands of dollars for computers and the clocks are useless if you don't run a program to update it CONSTANTLY? I'd love to learn how to wire a wristwatch into my CPU to be the clock....

xxx.lang.gov?

[Anonymous Coward]

xxx.lang.gov? Oh great, a government sponsored porn site.

Seems to me

[javaaddikt]

That you'd only be measuring the amount of pr0n being downloaded by physics students... unless you had your own clean segment.

Re:Seems to me

[stretch_jc]

I'd be one to agree with you given't the domain name: xxx.lanl.gov

Re:Seems to me

[mindriot]

Well, considering the host of the website is called xxx.lanl.gov, you might not be so wrong there...

I guess it would work

[Nf1nk]

While it seems like it should work I have a hard time believing that the distances are known constants (wiring can take some very odd routes) and that there aren't other bits of wierdness that could cause problems.

But I guess its no weirder than useing beer cans and watch to determine your location

Re:I guess it would work

[HMC CS Major]

They're using their own wire, not that of a building or telco (ie: running it across a field and back, or around in circles, direct from one laptop to another ... point to point, no switches/routers/hubs in the way).

TIMOTHY, no fuckin shit; you can use ping for that

[Anonymous Coward]

And all you trolls currently trolling...EAT MY SHORTS

What About This Story of Ping?

[Cheshire Cat]

Check out the first review of this story on Ping.

Re:What About This Story of Ping?

[Anonymous Coward]

hahahahhaahha you're a double-fuck dick-fart cunt-snot of an exhumed idiot's shriveled undescended testicle for not getting that. God some people are useless, you should be the next meal of a man eating lion.
People like you make me want to tear up my membership card to the human race and join the chimpanzees.

Re:What About This Story of Ping?

[Da Schmiz]

LOL! That is absolutely, unequivocally, the funniest thing I've read all day! (Okay, except for the quote at the back of the latest PC Magazine about a digital camera featuring "a high-quality 3X optical zoom lens designed for digital pornography".)

But I digress.

The text of the review in question, for you AC's who only read the part of the website above the fold:

Ping! I love that duck!, January 25, 2000
Reviewer: A reader from El Segundo
PING! The magic duck!

Using deft allegory, the authors have provided an insightful and intuitive explanation of one of Unix's most venerable networking utilities. Even more stunning is that they were clearly working with a very early beta of the program, as their book first appeared in 1933, years (decades!) before the operating system and network infrastructure were finalized.

The book describes networking in terms even a child could understand, choosing to anthropomorphize the underlying packet structure. The ping packet is described as a duck, who, with other packets (more ducks), spends a certain period of time on the host machine (the wise-eyed boat). At the same time each day (I suspect this is scheduled under cron), the little packets (ducks) exit the host (boat) by way of a bridge (a bridge). From the bridge, the packets travel onto the internet (here embodied by the Yangtze River).

The title character -- er, packet, is called Ping. Ping meanders around the river before being received by another host (another boat). He spends a brief time on the other boat, but eventually returns to his original host machine (the wise-eyed boat) somewhat the worse for wear.

If you need a good, high-level overview of the ping utility, this is the book. I can't recommend it for most managers, as the technical aspects may be too overwhelming and the basic concepts too daunting.

Problems With This Book

As good as it is, The Story About Ping is not without its faults. There is no index, and though the ping(8) man pages cover the command line options well enough, some review of them seems to be in order. Likewise, in a book solely about Ping, I would have expected a more detailed overview of the ICMP packet structure.

But even with these problems, The Story About Ping has earned a place on my bookshelf, right between Stevens' Advanced Programming in the Unix Environment, and my dog-eared copy of Dante's seminal work on MS Windows, Inferno. Who can read that passage on the Windows API ("Obscure, profound it was, and nebulous, So that by fixing on its depths my sight -- Nothing whatever I discerned therein."), without shaking their head with deep understanding. But I digress. --This text refers to the School & Library Binding edition.

Speed of light through copper...

[Omnibus]

Isn't the speed of light through the copper in cabling a fair bit below c?

Re:Speed of light through copper...

[Anonymous Coward]

Isn't the speed of light through the copper in cabling a fair bit below c?

About zero I'd say. Unless you've got some really thin copper and some really bright light.

Re:Speed of light through copper...

[Anonymous Coward]

Yes, that's why the paper measured the tranmission speed in the cable to be about 2/3 c, and not c.

Re:Speed of light through copper...

[Anonymous Coward]

Yes, but for anything sufficiently fast you don't think of say RG58 Coax as a wire... it's a wave-guide. RG58 transmission is really about (2/3 * 1.01)c -- background in particle physics where this is used ad nauseum. For example: given a detector element you split the signal -- pipe one leg down say RG59 (which transmits about 0.9c), jam it through a discriminator (costs about 10-15ns) which in turn opens a gate on a charge integrating analog to digital converter. The other analog leg from the initial split is sent down a hunk of RG58 that delays the arrival time of the pulse until the gate on the ADC is open.
As you might guess, most particle physicists think of cable length in terms of nanosec, converting to feet/meters only when necessary.

Another tidbit: to good approixmation c=1ft/ns

Re:Speed of light through copper...

[Anonymous Coward]

Well .... AFAIK, light doesn't actually travel through copper wire !!. Electrons travelling along the copper wire actually travel very slowly indeeed (from a few mm/s to a few m/s), but the electromagnetic energy flows through a copper conductor at about 0.8 times the speed of light.

Re:Speed of light through copper...

[Anonymous Coward]

Pretty sure it's called the velocity of propagation; a TDR needs that info to be useful.
Any responsible/competent cable manufacturer should have the info. to two sig. figs.

Molecules got nuttin ta do widdit

[Spamalamadingdong]

You get delays due to the velocity factor of the cable; this is mostly due to the dielectric behavior of the insulation (which is unlrelated to friction).

You'd have the same delays in fiber; light travels more slowly though glass than through vacuum, in no small part because of the dieletric properties of glass. In case you're wondering, the speed of light in a medium is equal to 1/; when and are the values for vacuum, v = c.

(Yes, I'm a physics nut and I studied this crap for my degree. About the only thing I use it for is to set people straight about physics.)

Re:Molecules got nuttin ta do widdit

[Hal-9001]

Why do I get "Insightful" mods on explanations of basic thermodynamics, or even physics? Read a science book!

I hear ya... ;-)

Re:Molecules got nuttin ta do widdit

[Anonymous Coward]

Well, the

Why not use Jupiter's moons?

[Caractacus Potts]

If you want a real experiment, measure the speed of light using Jupiter's moons. This was how the first accurate measurement was made. At least they'll be playing outside.

click me

Re:Why not use Jupiter's moons?

[Score0, Overrated]

Why not use Jupiter's moons?

1) They don't fit in a school's classroom.
2) You can only see them at night.
3) You can only see them on a clear night.
4) You can only see them on a clear night when Jupiter is in your bit of the sky.

Re:Why not use Jupiter's moons?

[toast0]

1) build a bigger classroom
2) blow up the sun
3) freeze and shatter the clouds
4) use a big magnet to get jupiter in your bit of sky (i'm sure jupiter has magnetic materials in it)

Re:Why not use Jupiter's moons?

[Anonymous Coward]

5) You have to take measurements over a long period of time to determine their orbits.
6) The calculation requires knowledge of Newtonian orbital theory to interpret the data.

Still, using Jupiter's moons was a clever idea, as is the rotating mirror idea mentioned elsewhere in the comments.

Impressive

[Neoplane Overlord]

I never thought such a seemingly simple thing as a ping command could be used in a way related to physics/the universe. At this rate, we may be able to explain the space-time continuum by using a simple chat relay message sometime within the next couple of years. Hmm... AOL and the Universe... mind boggling isn't it?

In other news...

[Blue Lozenge]

Speed of Carrier Pigeon Measurement Using Ping

Insightful????

[Anonymous Coward]

In the future, try to read the article before posting your insightful comment.

Re:The Slashdot Drinking Game

[Anonymous Coward]

j00 r0x0r my b0x0rs. j00=l33t.

Correction in the above.

[Spamalamadingdong]

Make that sqrt(1/). In my hurry to post I made an error (and a bunch of people got their two cents in first).

I thought this comment was promising

[JustAnotherReader]

From the PDF file explaining their experiment:

Software: We took data while running Linux on both computers. Although it should be possible to do this experiment with the new release of ping for Windows, because the authors were unfamiliar with Windows, Linux was chosen.

Unfamilier with Windows? Where's my checkbook? I want to send my kids to this school! That's not sarcasm, I mean it. I think the fact that the teachers and students were more familar with Linux than Windows is awesome!

Re:I thought this comment was promising

[x1l]

Good luck getting a job if you are unfimiliar with windows. That crap only flies in colleges.

Re:I thought this comment was promising

[number one duck]

Yeah, nobody can spare the 10 minutes linux --> windows conversion. At least nobody with the proper attention span to be teaching children.

Re:I thought this comment was promising

[Shade, The]

Yep, 10 minutes to work out how all that closed source MS ping software works, and then to run it in DOS, instead of the infinitely more flexible Bash (or tcsh, or whatever) shell.

Re:I thought this comment was promising

[rtaylor]

Why is that awsome? The problem is the exact same only reversed.

Re:I thought this comment was promising

[kasek]

well i gotta say, after being a student there (YSU) for two years, i never saw Linux installed on the computers in any labs in the CIS department. i didnt run into many people in the lower level programming classes that were intimately familiar with linux either. you could telnet into their unix server to get email, compile code and whatnot, but that was the extent of my *nix dealings while there.

Re:I thought this comment was promising

[kasek]

and before you flame me, yes i did read the article, and yes i see that it was in their physics department this was done, not CSIS. so maybe just send yer kid there if you want him/her to study physics in a linux environment maybe.

Re:I thought this comment was promising

[WNight]

Really? Oh damn. I must have missed the news for the past few years...

Has Linus declared Microsoft to be enemy #1?

Does Linus control a huge sales force dedicated to wiping out Windows?

Has Linux bribed politicians to get his own laws (UCITA) in order to enforce his empire and exclude smaller companies?

Has Linus illegally used his monopoly to leverage Linux into OEM hardware to the exclusion of other OSes?

Has Linus lied (and directed others to lie) to a federal judge and faked evidence to support these lies?

Probably not... Until Linus becomes a billionare and sets out to control the world's computers, destroying all competition in his wake this won't be "the exact same only reversed."

I don't care that many people never use Linux. Many people don't use Asus motherboards or drive Toyotas either. What I care about is the freedom for people people to choose. Microsoft is trying to take that freedom away, Linux is offering more choices.

Re:I thought this comment was promising

[jsse]

He might be more familiar with Mac, or even worse, OS/2. (j/k, I'm pro-Mac and former testing team for OS/2). :)

Re:I thought this comment was promising

[Anonymous Coward]

Good luck getting a job if you can't figure out how to use 'ping' under windows :)

More likely the choice was made by a professor who hasn't got his head around these 'new fangled graphicky things' yet. Or, it was from a couple of techie students who ARE familiar with windows, but would rather use Linux given the opportunity thankyou very much. Or, they realised that it makes little difference and picked whatever was handiest on their machines.

To be 'unfamiliar' with windows makes me think back to programming classes with folk 'unfamiliar' with saving files to disk or 'unfamiliar' with doing any work not copied from someone else... let alone using pico. It seems to suggest that they're non-computer people who are using Linux because it's either what the physics department teaches, or because it's what was on the instruction sheet.

Re:I thought this comment was promising

[Anonymous Coward]

Adult Meta-Gallery seeks Webmasters. Built using open source!

This link is broken.. append "index.php". And I don't think it's quite ready to go live yet, too many broken links and scripts that don't quite work.

Re:I thought this comment was promising

[Anonymous Coward]

to work out how all that closed source MS ping software works

You need the source to fucking ping to figure out how it works?

Did you eat a lot of paint chips as a kid?

Re:I thought this comment was promising

[Anonymous Coward]

shut the fuck up..... this article is not about MS. it's about doing something cool with off the self computer stuff. Take your fucking retard MS bashing to a microsoft article.

Moderators, mod me down and take this fuck stick with me.

Re:I thought this comment was promising

[Anonymous Coward]

They didn't say that the teachers and students were more familiar with Linux; they said that the authors of the paper (the teachers) were. They're physicists, and it's not at all rare for a physics professor to know his way around a Unix box better than a Windows machine. Unix is the traditional mainstay of scientific/technical academia, after all.

Re:I thought this comment was promising

[Anonymous Coward]

Well, the sad thing is I had to move it from mySQL --> access/ODBC, so the open source part is also now partially a lie. The index.php was a server misconfiguration, since fixed.

Well then

[Anonymous Coward]

I'm glad they used ping instead of ping2death.

Memories of high school

[dohcvtec]

In my high school physics II class I experimentally measured c. There was a long service hallway that ran the length of the building, about 150 feet. We had a laser at one end and a mirror at the other end. The signal output of the laser went directly into an oscilloscope, while the beam went down the hall and back to a detector at our end. We simply measured the phase shift between the two, and voila! We came within about 6% of the accepted value of c. Not bad for a high school project, but this method sounds interesting, and there may be peripheral conclusions to be drawn, due to the electrical aspects of CAT5.

Re:Memories of high school

[Anonymous Coward]

In high school, I measured geeks' resistance to pain and hunger by beating you up and taking your lunch money.

Re:Memories of high school

[Graymalkin]

You made me spit Coke all over my monitor. Thanks.

Re:Memories of high school

[Inferno]

I just 'bout spit my toothpaste all over my monitor. Thank you to the person that warned me about this thread and prevented me from doing so.

Re:Memories of high school

[Hydrogenoid]

You didn't... c is the speed of light in vacuum...
You mesured the speed of light in air, wich happen to be close to c, but isn't the same...

Re:Memories of high school

[Anonymous Coward]

Yeah! How do you like how that shit works! Take that, geek!

Slashdot Dating Game

[Anonymous Coward]

Are you just sitting around with nothign to do?
Do you want to talk with an interesting intelligent
woman from nevada, call 800-618-8255 from 8:00 p.m.
to 12:00 p.m. and ask for Arte Belle!

Re:Slashdot Dating Game

[Anonymous Coward]

being bored, and avoiding homework, i called the 800 number, and was informed that it was not available from my area (wi) :(

The ultimate Read The Article First

[dmd]

As I write this, there are 20 comments posted already. Nearly all of them are from people who quite clearly haven't read the actual article, or even just its abstract.

Please, read the article first!

Re:The ultimate Read The Article First

[Anonymous Coward]

Why should we? We don't care about the article - we're the original bunch of fuckwits - we're the Slashdot crowd!

Re:The ultimate Read The Article First

[Cato the Elder]

It's worse than that. I expect a lot of people, especially early posters, to not read the article.
But

A) A lot of them didn't even seem to read the little dept. blurb, which actually has more good info than most of the posts.

B) A lot of these assinine responses have been moded up as "informative"

Something is broken. Badly broken. Read it. It's interesting.

Re:The ultimate Read The Article First

[prockcore]

The day people read the article, is the day the slashdot effect ceases to exist.

Don't tell me to read the article while the server is currently dying a miserable death.. unless you feel like mirroring it yourself.

It looks like...

[Nathdot]

...someone is very close to getting some sweet government funding to play quake all day!!!

M: "Joel, did you get those speed of light measurements this time?"

Joel: "No, It looks like we'll have to fire up another game. You wanna play one-on-one or co-op M?"

M: "Sweeeeet!!!"

:)

Not only is the speed of light slower...

[interstellar_donkey]

then I thought...It also seems to change. Thanks Ping.

22 5:02pm ~ >ping localhost
PING localhost (127.0.0.1): 56 data bytes
--- localhost ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.033/0.046/0.054/0.008 ms

--

Re:*****IMPORTANT*****

[Anonymous Coward]

PS: anyone know good sites to go, I'm fed up of slashdot keep banning me and determine to be gone from slashdot

I've heard that this site is open for entry to people such as yourself.

In other words, fuck off and die, shitfucker. If you can't be bothered to troll properly, we don't want you. Hope you meet CmdrTaco; I can't think of a worse fate to wish on you.

Grace Hopper measured nanoseconds...

[whitefox]

... using lengths of wire

Re:Grace Hopper measured nanoseconds...

[Anonymous Coward]

A piece of wire is not a good way of specifing a delay. Propagation time is related to the dielectric constant which isn't anywhere constant when you have a simple piece of wire. Try measuring the s parameter and you would see what I mean.

Re:TIMOTHY, no fuckin shit; you can use ping for t

[Anonymous Coward]

Slashdot requires you to wait 20 seconds between hitting 'reply' and submitting a comment.

It's been 18 seconds since you hit 'reply'!

If you this error seems to be incorrect, please provide the following in your report to
SourceForge.net:

Browser type
User ID/Nickname or AC
What steps caused this error
Whether or not you know your ISP to be using a proxy or some sort of service that
gives you an IP that others are using simultaneously.
How many posts to this form you successfully submitted during the day

* Please choose 'formkeys' for the category!
Thank you.

And the Lord said:

[Anonymous Coward]

"Thou shall not kill". How do you propose to arrange the meeting, asskisser? Spite your God and win praise from your jarhead buddies?

Nationalism: contra naturam est

Re:And the Lord said:

[Anonymous Coward]

Assumming a Xtian God (it is possable that JTinMSP ment the Muslem god, I don't know if he/she likes to capertilized) then, as the Xtain God is everywhere I'm sure he and OBL have already meet.

I tried this on slashdot!

[YouAreFatMan]

D:\WINNT\system32>ping slashdot.org

Pinging slashdot.org [64.28.67.150] with 32 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 64.28.67.150:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

I guess that means that slashdot is infinitely far away...I always suspected it

Re:I tried this on slashdot!

[anpe]

Request timed out.

IMO this would mean that slashdot is in a black hole :-)

Re:I tried this on slashdot!

[Hal-9001]

It follows that slashdot is more efficient at generating black holes than nature, since a server somewhere in the world goes down every time a link is posted... ;-)

Re:I tried this on slashdot!

[GigsVT]

Everytime a bell rings, an angel gets it's wings?

I wonder if there is a connection?

IT'S A CONSPIRACY!

when using Comcast with NAT...

[Anonymous Coward]

the speed of light is 0. Yes it's true, just look at the speed of light from my house to slashdot...

$ ping -c4 slashdot.org
PING slashdot.org (64.28.67.150) from 192.168.205.3 : 56(84) bytes of data.

--- slashdot.org ping statistics ---
4 packets transmitted, 0 packets received, 100% packet loss

Maybe this is Comcast's way of "Gunning NAT Users". They try to trick us into thinking that the speed of light is 0.

Ahh, memories.

[dimator]

Ping is a little thousand-line hack that I wrote in an evening which practically everyone seems to know about.

It was a great night, after all!

i thought we already knew the speed of light

[Anonymous Coward]

so yeah, don't we already know what the speed of light is? and aren't there more important things college students could be doing with that cat5 wire, like download porn?

Yawn.

[Multiple Sanchez]

Do the same thing with pong, then I'll be impressed.

Cuckoo's Egg

[sconeu]

Cliff Stoll mentions using Kermit ack latency to measure distance in "The Cuckoo's Egg". Of course, he wasn't trying to measure c, but to figure out where his hacker was. Turns out he was pretty accurate, even though the data was ignored because it didn't fit the currently known theories...

Re:Cuckoo's Egg

[Anonymous Coward]

BTW, in "Cuckoo's Egg" Stoll also writes about how he visited an Air Force network lab or something and met Mike Muuss, who was the author of the original ping(8).

This makes no sense to me....

[antistuff]

why go through all that trouble when all you need is a flashlight and a stopwatch?

Re:This makes no sense to me....

[Anonymous Coward]

retard. go read a physics book. but, as someone noted previously, with a laser and a photodetector hooked up to an oscilliscope and a mirror at the end of a long hall, you could measure the speed of light by checking out the groovy phase shift of the two signals

Re:This makes no sense to me....

[antistuff]

Your just jelous because I thought of saying it first. No +2 Funny for you! NEXT! (Sienfeld Reference for the tv impaired)

Oh No!

[sailracer6]

We Slashdotted Los Alamos!

xxx.lanl.gov.is down.

Re:Oh No!

[Anonymous Coward]

We Slashdotted Los Alamos!

Expect one tactical nuke to the Slashdot server bunker, pronto.

now i want that book. dope review!

[Anonymous Coward]

now i want that book. dope review!

.77 C

[ruvreve]

I can't reach the article (being /.ed?) to read it but I always thought that data transfered over wire at about .77 the speed of light. I guess if you knew the exact figure you could use ping but then again if you knew the exact figure you would already know the speed of light. Maybe the article will give some insight into this...when I can actually read it.

Speed of light got /.ed

[ehiris]

Speed of light + /. = 0

Ouch

Speed of light?

[sitturat]

What they are measuring here is not the speed of light (c), but the speed of em waves in the cat 5 medium.

Most of the replies so far make the incorrect assumption that these 2 speeds are the same.

Re:Speed of light?

[quinto2000]

electro magnetic waves == light. You're right that light has a different transmission speed in different media, though.

Re:Speed of light?

[sitturat]

I didn't say light wasn't EM.

I just mean the em doesn't travel at c through all media.

Re:Speed of light?

[Anonymous Coward]

I think you mean C. And I don't believe that you really know what you're talking about, either.

Re:Speed of light?

[Chirs]

Yes, they are measuring the speed of em waves in cat5. However, by then mathematically modelling the em characteristics of the cat5 cabling, they can then work from their experimental data to determine c.

Using electrical propreties of cat 5 and ping?

[ehiris]

Using basic electrical properties of category-five cable students may use their measurements to determine the speed of light in the vacuum to within a few percent

How does his happen? Don't you have to take in consideration processing of the response?

And why aren't they using fiber optics for this?

Re:Using electrical propreties of cat 5 and ping?

[Anonymous Coward]

Wow, you know, in a different Universe I might almost say that they knew more about it than you. Of course, here, you're some sort of God and you know everything, so the explanation simply can't be that you're an ignorant pile of shit who should fuck off and die.

Re:Using electrical propreties of cat 5 and ping?

[ehiris]

I didn't try to offend anybody but im my small little brain containg one brain cell I figured that I'd measure the speed of light with light.

c the speed of light, sort of

[ryouki]

c is the speed of light traveling thru vacum, but light propagates at different speeds depending on the medium. Changeing the medium changes the speed of propagation and makes the light bend. The propagation of an electrical signal thru copper is indeed quite fast, a good percentage of c, but it is NOT c. This experement is a cool thing to do, but it isn't mesureing the speed of light.

Re:c the speed of light, sort of

[CounterZer0]

Wrong. It's measuring the speed of 'light' in the electromagnetic waveform sense. Not all measurements of the speed of light must be done in vacuo.

Re:c the speed of light, sort of

[Legion303]

Wrong. It's measuring the speed of 'light' in the electromagnetic waveform sense. Not all measurements of the speed of light must be done in vacuo.

It's measuring the speed of EM radiation through copper. The paper clearly states that the end value will give the speed of light in vacuum, which is incorrect.

-Legion

Re:c the speed of light, sort of

[CmdrSam]

Did you even read the paper? They use the measured value of the speed of light in the wire to find a value of the speed of light in vacuum.

--Sam L-L

Re:c the speed of light, sort of

[Legion303]

Of course I read the paper. Please point out where they mentioned finding c from the pings. The only place they even mention c is at the very beginning.

-Legion

Re:c the speed of light, sort of

[Anonymous Coward]

Please point out where they mentioned finding c from the pings.

Bottom of page 8.

The only place they even mention c is at the very beginning.

They mention it at the beginning, then they give lots of details about the experimental setup, then they talk about the speed of light that they measured. (They actually measured the speed of light in the cable, not c itself.)

Re:c the speed of light, sort of

[Legion303]

They mention it at the beginning, then they give lots of details about the experimental setup, then they talk about the speed of light that they measured. (They actually measured the speed of light in the cable, not c itself.)

That's what I said. And then they went on to *estimate* (not calculate, there's a difference) c through the average properties of cat5.

-Legion

Re:c the speed of light, sort of

[Anonymous Coward]

What's the difference between "estimate" and "calculate", in your mind? Is it only a "calculation" if it's done from first principles?

Re:c the speed of light, sort of

[Legion303]

What's the difference between "estimate" and "calculate", in your mind? Is it only a "calculation" if it's done from first principles?

A calculation generally gives you exact values (minus rounding approximations), Anonymous Coward. What these guys did, while still a neat party trick, was to estimate c based on estimates of wave propagation in copper.

-Legion

Re:c the speed of light, sort of

[Anonymous Coward]

A calculation is what you do in theory. What they did was perform an experimental measurement, which is not an "estimate". If anything, the theoretical calculation is the estimate to the real thing, since theoretical calculations always make idealizations.

Re:c the speed of light, sort of

[Legion303]

No, a calculation is something you do on paper. If you'll read the article, you'll see that they use the propagation of a ping through copper to (indirectly) estimate c. See, it's right there on the page you (or another AC) directed me to: "[...] indicating that the speed of propagation in a cat-5 cable is some 2/3 the speed of light in the vacuum." They arrived at this figure through speed averages. Dividing your average speed through copper by 2/3 will give you a reasonable approximation of c, but it is not the same as doing direct experimentation to calculate c.

Class dismissed.

-Legion

Re:c the speed of light, sort of

[Anonymous Coward]

They neither measured nor estimated nor calculated c. They measured the speed of propagation, and did not "calculate" or "estimate" it. They compared it to the accepted value of c. There is no way of using it to estimate the value of c, unless you already know the relationship between the speed of propagation and c.

Re:c the speed of light, sort of

[Legion303]

They neither measured nor estimated nor calculated c. They measured the speed of propagation, and did not "calculate" or "estimate" it. They compared it to the accepted value of c. There is no way of using it to estimate the value of c, unless you already know the relationship between the speed of propagation and c.

Which is why I said "indirectly." My point, which you're arguing for, is that the authors of the paper did not derive the speed of light in a vacuum from this experiment, even though they claim to in the abstract.

-Legion

Re:c the speed of light, sort of

[Anonymous Coward]

I agree with that point, but they didn't derive c in vacuo even indirectly.

Re:c the speed of light, sort of

[Legion303]

If you insist.

-Legion

Machinist's ruler?

[zer0vector]

One of the coolest low tech experiments I've done is measure the speed of light using a machinist's ruler. That's one of those metal rulers with the marks etched on the surface. By hitting the etched marks with a laser at a very low angle you can measure the speed of light using the diffraction pattern formed.

Re:Machinist's ruler?

[r6144]

How to measure the frequency of laser when c is unknown? I think using an 1Ghz oscillator will be better --- although that surely need to be done outside.

Re:omg first post

[Anonymous Coward]

liar

VA Software is filing for bankruptcy!

[Anonymous Coward]

I just heard it announced on TechTV, VA Software (Slashdot's parent company) has filed for bankruptcy! I'm sure we'll be hearing more about it later today, and it will be hard to notice when the site shuts down. It's really a shame to see such a great resourse as this go out of business. Hopefully another company will soon step forward and offer to host Slashdot.

Ping

[Banjonardo]

The creator of ping, a seemingly cool guy, didn't die too long ago

Re:Ping

[subsimian]

I hate when I find out someone died on my birthday ...

Re:Ping

[Anonymous Coward]

I hated it when I found out someone was born on your birthday ...

"educational" network

[GePS]

"Here's a case where all that cat5 on college campuses can actually be used for education ;)"


Did I just hear education implied when talking about a college campus network? All these marvelous filesharing programs do little but propogate porn.

Hell, perhaps you could somehow measure the speed of light by observing how fast the search "teen sex" on Kazaa fills up.

A few recommendations:

[swagr]

1. Ping a machine farther away for more accurate results.
2. Have the entire lab flood-ping it to collect statistics at a faster rate.
3. Get some other shools doing this at the same time so you can compare results.

I recommend slashdot.org.

Re:A few recommendations:

[danonb]

Like ping www.microsoft.com with a few hundred universities?

Ohno second - The minuscule fraction of time in which you just realised you made a BIG mistake.

Norway?

[The Ape With No Name]

My original impetus for writing PING for 4.2a BSD UNIX came from an offhand remark in July 1983 by Dr. Dave Mills while we were attending a DARPA meeting in Norway...."

Why on earth was a US Defense department group having a meeting in Norway? I need to get my boss to start having meetings in Maui. Sheesh.

Re:Norway?

[ruvreve]

You should have been working for some of the dot-bombs. They had numerous lunch meetings at condos in Cancun and look where most of them are now. So you may want to rethink your comment about your company having meetings in far-off places.

Re:Norway?

[fferreres]

Rusia was near and US Defense beign there makes sense, for whatever excuse they could think of.

Re:Norway?

[tom's a-cold]

They were a member of NATO. Norway and Denmark are the only two Scandinavian members. There was a lot of early-warning radar up there.

Of course, Maui's part of a Nato country too. But it's so much harder to get lutefisk there.

Re:Norway?

[The Ape With No Name]

I work for a University. We will never die as long as we can squeeze....

SI length of the meter?

[j-beda]

The first footnote of the paper seems to be incorrect, it reads:

[1] Since the mid eighties the meter has actually been defined in terms of a fixed, integral number of wavelengths of light from a particular optical transition. Since the frequency of that optical transition is tied up in (what are believed to be fundamental) constants of nature, the speed of light is defined through this definition of the meter.

I had thought that the meter was defined as the distance light travels in 1/299792458 of a second, with the second being so many vibrations of a particular atom (cesium?).

Yep, according to NIST the length has been defined this way for quite some time:

The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Re:SI length of the meter?

[addaon]

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Which is exactly the same as 'a fixed, integral number [namely 299_792_458] of wavelengths of light from a particular optical transition [a specific vibration of the cesium atom].' Basically, you missed a step of logic, and you're violently agreeing with the source.

Re:SI length of the meter?

[p3d0]

Nope. It was 1,650,763.73 wavelengths, not 299,792,458. The latter would have been an astounding coincidence if it were true. Imagine: what are the odds that the meter happens to be the exact geometric mean between one light-second and one wavelength of this krypton radiation.

Re:SI length of the meter?

[j-beda]

No, the older definition was a certain number of wavelengths of a particular optical transition and the current (for quite some time now) definition is based on the distance travelled by light in a well defined amount of time. They are quite different.

Re:SI length of the meter?

[Anonymous Coward]

I knew I was taller last year.

Re:SI length of the meter?

[mikeee]

Er. Doesn't the uncertainty principle mean that there is no such thing as a perfect vacuum, reducing somewhat the value of that definition?

Re:SI length of the meter?

[gilroy]

Blockquoth the poster:

Er. Doesn't the uncertainty principle mean that there is no such thing as a perfect vacuum, reducing somewhat the value of that definition?

No. Under modern theories, there is indeed a well-defined vacuum state... it's just not empty. :)

Re:SI length of the meter?

[nadaou]

Now, due to the friction of the tides on the sea floor, the Earth's revolution about its axis is slowly slowing down. Thus the days are getting longer. (Less days/year) Thus seconds (1/86400th day) are getting longer. Thus the distance light travels in that second is getter further, and the 'meter' is growing.

(yea, yea, relax. I know about leap seconds.. )

Re:SI length of the meter?

[Anonymous Coward]

No, the older definition was a certain number of wavelengths of a particular optical transition and the current (for quite some time now) definition is based on the distance travelled by light in a well defined amount of time. They are quite different.

Well... the paper's reference to an "optical transition" was, as the other poster said, a reference to the hyperfine transition in an atom. So you and the paper are agreeing on that point. The "well defined amount of time" you refer to is preciselly the period of the optical transition.

However, the paper was incorrect in saying that the speed of light was defined directly from the wavelength of that transition; as you say, it was defined using the frequency of the transition -- that is what is directly measured. (The wavelength can be inferred from that, if you define the speed of light.)

Re:SI length of the meter?

[Anonymous Coward]

To clarify: the vacuum state is the ground state, of lowest possible energy.

Re:SI length of the meter?

[j-beda]

And the second is not defined in terms of the rotation of the earth anyway.

Again from NIST:

The unit of time, the second, was defined originally as the fraction 1/86 400 of the mean solar day. The exact definition of "mean solar day" was left to astronomical theories. However, measurement showed that irregularities in the rotation of the Earth could not be taken into account by the theory and have the effect that this definition does not allow the required accuracy to be achieved. In order to define the unit of time more precisely, the 11th CGPM (1960) adopted a definition given by the International Astronomical Union which was based on the tropical year. Experimental work had, however, already shown that an atomic standard of time-interval, based on a transition between two energy levels of an atom or a molecule, could be realized and reproduced much more precisely. Considering that a very precise definition of the unit of time is indispensable for the International System, the 13th CGPM (1967) decided to replace the definition of the second by the following (affirmed by the CIPM in 1997 that this definition refers to a cesium atom in its ground state at a temperature of 0 K):

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

They need to read up on their Linux...

[Slartibartfast]

For one, their "private IP" addresses are 168.192.x.x -- sounds a little backward to me. For another, they refer to "" as a "pipe" -- last time I checked "|" did that job.

Nevertheless, I have to say that it really is a damn cool idea; sad I never thought of it myself.

Here's to hoping that VA can keep going...

First typo post!

[Karn]

"ping -i .01 > tempfile1.txt" where ">" (the so-called 'pipe' symbol)

Then what's this thing: | ?

:)

Re:First typo post!

[Hydrogenoid]

pipes can break, too...

Re:First typo post!

[WereTiger]

The > is the DOS 'redirect' syntax, but a lot of us called it 'pipe' in the old days.

serves practically the same function.

physics class

[abe ferlman]

This is a little off topic, but not much so bear with me.

A friend of mine found physics easy in high school, but found his teacher unbearable. So he would always convert his (generally correct) answers into inconvenient units, you know, pico-thises, nano-thats.

One time the question was "what is the speed of light?"

His answer? "1 lightyear/year"

Re:physics class

[Yarn]

I could never remember the average distance from the sun to the earth, so I always gave it as either 1AU or eight light minutes..

I was never much good at astrophysics

Re:physics class

[NewsWatcher]

During physics class we were studying an introduction to logic. I walked into the class five minutes late and my teacher demanded to know why I was tardy.
"Because I wasn't early, or on time," was my response.

Re:physics class

[Anonymous Coward]

Side note: gravitational physicists often do exactly that -- measure c in lightyears/year, so that c=1. Then all those pesky c's drop out of the equations. If you want to get a physical answer out, you can just stick the c's back into the final result via dimensional analysis, but there's no need for them in the intermediate stages. (They also usually set Newton's gravitational constant G=1, too.)

Re:physics class

[Hal-9001]

The average distance from the sun to the earth is 1.5 x 10^23 angstroms, plus or minus a whole lot of angstroms.

Re:physics class

[KjetilK]

Hehe, most astronomers give c in units of cm/s. Yeah. I think that is just to be annoying... :-) (OTOH, it also says a couple of things about traditionalism in astronomy)

Re:physics class

[j1mmy]

Or the easy answer: 1 Astral Unit.

Re:physics class

[Hal-9001]

Well technically an AU is an Astronomical Unit, which is 149598550000 meters or 1.495 x 10^21 angstroms. I made my parent calculation multiplying 93 million miles x 1600 meters/mile x 10^10 angstroms/meter in my head and still managed to be within 2 orders of magnitude. As an engineer, I consider that to be pretty good, especially since once in introductory physics I got an answer off by 68 orders of magnitude. A note to aspiring scientists and engineers: don't leave out negative signs, expecially ones like in h = 6.626 x 10^-34 J-s... ;-)

wireless

[coreyb]

I'd be interested to see what kind of results this technique would produce using a wireless network.

Stupid fucking yanks...

[Anonymous Coward]

Hahahahahaha...you stupid fucking yanks couldn't even find one lousy tin-horn, rag-head terrorist leader! Osama owns your asses, dickheads!

Yeah, heaven forbid . . .

[Anonymous Coward]

Heaven forbid that the college teaching your kids how to make it in the real world actually has professors qualified to teach them how to use the software they are most likely to use once they graduate.

Ethernet is not ether

[Anonymous Coward]

I don't buy the contention that c in a vacuum can be measured within a few percent by a ping, the differences in c in a vacuum and in a wire is greater than that, not to mention the additional computational delays. However, within a wire the speed of transmission clearly can be measured. This is the basis of many network TDR devices that can not only tell you that a cable is open but where it is open (or simply how long the wire is). If you know how long the pulse took to return you can compute how long the wire is, or conversely if you know the length of the wire you can determine the speed of transmission of the pulse. This can even be seen on an oscilloscope with no more extra complex equipment than a one shot to generate a pulse.

Equally interesting is that the speed of light in a wire is also the determining factor that limits the length of an ethernet trunk if collision detection is to work, the maximum length of the cable run is the distance the packet can travel and still have a legal collision in the first part of the packet (The test is only done for so many bits, 64 if I remember right for 10mb ethernet, because after this you could not have a collision on a legally sized trunk if each staion listened before transmitting).

Re:Ethernet is not ether

[Anonymous Coward]

I don't buy the contention that c in a vacuum can be measured within a few percent by a ping,

And if you read the paper, you'll find that they didn't measure c (speed of light in vacuum). They measured the speed of transimssion within the wire, as you say. They got a value of about 2/3 c.

OT, but related.

[Anonymous Coward]

A number of companies are pitching products that measure the characteristics of IP traffic over a network link using a variety of different metrics and solutions. CQOS, Brix, and Cisco, all have solutiuons that do this their own way. Disclaimer, I work for the first company on that list, but there is some interesting information regarding IP measurement at all those sites. I'm not sure about Brix and Cisco's products, but I know CQOS's measures down to microsecond accuracy.

2+2=5!!!

[Anonymous Coward]

I love the icon for education! 2+2=5!
haven't you learn that 1+1 != 2, therefore, 2+2!=5, therefore, I am!
You should reconsider your icons.

Napster is "educational" too :-)

[Anonymous Coward]

especially good for training law students in practical applications of intellectual property protection

Neat...

[Omicron]

I haven't read the story of ping yet, and that was just a cool little read. You know, weeks will go by where I don't find anything all that interesting to me on Slashdot, and then all of a sudden a little nugget like that pops up and just makes me think "cool".

xxx?

[Snover]

It sounds like someone has an alterior motive for the measurement of speed of light with PING ;)

More than just the speed of light

[HardCase]

Even though there are a few minor errors in the paper, the authors deserve a mighty pat on the back for this. Not only did they devise a rather interesting method of estimating the speed of light, but they also managed to throw in a little basic electronic theory as well.

I'm less than a semester away from graduation as an electrical engineer and I've taken more than my fair share of physics classes, in fact, more than the curriculum required. I think that an experiment like this one has a solid place in a second semester physics class, particularly one that is taken by engineers. In the second semester, the students have (hopefully) mastered classical concepts of mechanics and are moving into waves and fields. What a perfect time for a project like this.

Suffice to say that my physics experience was not nearly so fun. Oh, and eventually we did measure the speed of light, but not until I took quantum mechanics. And then we measured it directly by modulating a laser with an extremely high frequency function generator and measuring the phase shift with an equally high sampling oscilloscope. It didn't require any particular expertise in overcoming the limitations of the hardware or really any problem solving at all, other than a little bit of math to convert feet per microsecond to meters per second.

All in all, a very good job.

-h-

Re:More than just the speed of light

[Anonymous Coward]

In one of my physics labs, we measured the speed of light by performing measurements on electrical circuits to determine the permittivity and permeability of free space (epsilon0 and mu0), since c = 1/sqrt(epsilon0*mu0) as a consequence of Maxwell's equations.

TRY READING THE ARTICLE! (n/t)

[Chirs]

n/t

Practical Pinging

[Alioth]

Pings are used to measure things in real life.

For example, DME (distance measuring equipment) in aviation. This works by equipment on the aircraft sending a signal to the ground-based DME station, which replies. The round-trip is measured, giving the distance from the station.

Maybe ICMP pings can be used to find out how much Cat 5 there is between you and the target machine :-) Of course, the time taken to process the ping by the target etc. must be taken into account.

Re:Practical Pinging

[Alien Being]

Sonar pinging like you hear on submarine movies is what i always assumed the name ping came from.

Re:Practical Pinging

[Anonymous Coward]

Pings are used to measure things in real life.

For example, DME (distance measuring equipment) in aviation. This works by equipment on the aircraft
sending a signal to the ground-based DME station, which replies. The round-trip is measured, giving the
distance from the station.

Pings have been used in aviation much earlier than that: see "radar".

My favorite part of the ping story

[Horizon_99]

Dunno how many of you read it but this is hilarious:

"The best ping story I've ever heard was told to me at a USENIX conference, where a network administrator with an intermittent Ethernet had linked the ping program to his vocoder program, in essence writing:

ping goodhost | sed -e 's/.*/ping/' | vocoder

He wired the vocoder's output into his office stereo and turned up the volume as loud as he could stand. The computer sat there shouting "Ping, ping, ping..." once a second, and he wandered through the building wiggling Ethernet connectors until the sound stopped. And that's how he found the intermittent failure."

-Horizon
"The universe is not required to be in perfect harmony with human ambition." - Carl Sagan

Technical theory behind this subject.

[rice_burners_suck]

Let me tell you something. You cannot measure the speed of light using Ping. What do you think Ping is, quantum physics hard at work? That's the most idiotic thing I've ever heard in my whole entire life. Because you know what? If you want to know the speed of light, just open a damn book. It's all there, black and white, clear as crystal. You stole phizzy lifting drink and floated up to the top of the tower which must now be washed and sterilized. And you need to remember the Boromir Principle, or whatever it's called, which says that the closer you try to measure some quantum physics stuff, the more messed up your measurements will be... or wait a minute... that's not what it says. It says that you can measure X or Y closely, but not both at the same time, and the accuracy of your measurement of X is inversely proportional to the accuracy of your measurement of Y. I just don't remember what the X and the Y were... What's that called, the Heisenberg thing? I don't know... it's been FOREVER since I've put a few good hours into reading all about physics, quantum mechanics, chemestry, superstring theory (or whatever they call it today), Calabi Chow spaces or whatever they're called, and who knows what else. :-) ...

Of course, if you can get the network to work exactly at the theoretical rate, you may actually be able to extrapolate the speed of light.However, that requires that I stop being an idiot and start writing some meaningful stuff in here. You see, what I've been doing in this stupid long and boring comment is just writing a bunch of crap to make it look at first glance as if I wrote a bunch of meaningful stuff, but really, it's just what I said it was a moment or two ago, no I think it was three moments ago, or was it four? You know what? I cannot tell because by the time I write a bunch more stuff, however many moments ago it was increased by a moment or two. Because you cannot stop the time, and that's really my point.

You see, time and the speed of light are really very closely intertwined! It works like this. Suppose that time is a dimension, kind of like our three dimensions of east/west, north/south, and up/down. So there's another dimension and it's past/future, and the present is almost nonexistant. The present is like the size of a tiny piece of an atom, if you could even measure it at all. What happens is this. Why is it that if you move an object at the speed of light, it like travels into the future or some bullshit like that? Actually, if you think about it for a while, speed is a measure of distance over time, or some garbage like that. What that really means is that, and by the way, the speed of light is the so-called alleged "cosmic speed limit" because THE SPEED OF LIGHT IS ACTUALLY THE SPEED OF TIME. Now if you go and read some books on the subject, there are really good explanations that I cannot reproduce here, for various reasons, and I will, for your convenient convenience, enumerate those items herein:

• Copyright violations. If I would include, say, a whole "commercial" book on physics and time and shit, that would probably be considered a violation of copyright law, and next thing you know, the copyright POLICE would show up at my door, and drag me kicking and screaming to the electric chair, where they would fry my happy ass.
• Because it would take too long for me to sit here like an idiot and transcribe (what a fancy word for "punch in the shit that I'm reading", eh?) the entire flipping book into this stupid freaking window, eh?
• Because Guiness sucks. That's right. That stuff tastes totally wrong. It's really weird shit. I prefer Negra Modelo. It's a Mexican beer, which means it's a correct beer. Actually, Negra Modelo is an ale. That's kind of like the difference between Madeira and Port, if you know what I mean. Guiness isn't a beer, and Guiness isn't an ale. It just isn't. It's incorrect. But Negra Modelo is correct. Some other Mexican beers aren't so good. Some are much better. But I like Negra Modelo because it is really the most awesome beer/ale/whatever the hell you want to call it in existance. Good with lime and salt, or without. Do it whatever way you want. By the way, I'm not into everything Mexican... For exampple, I hate tequila. It's gross, just like Guiness. Which brings me back to what I started saying a moment or two or three or... well, you get the point because I think I went over this whole moment and time and light thing in the previous paragraph, which is what brought me here, and if I start that whole damn thing again, this will become a recursive endless forever loop like for(;;) or some garbage like that. Actually, I like to write while (1) but many compilers are really stupid and they don't optimize out the "if" that goes in there somewhere, and they check against a gosh fucking constant, for crying out loud, and you know what? I think that sucks. But what the hell was I talking about? Oh yeah, the superb taste of Negra Modelo, and the deficiencies of Guiness, which sucks. (And I'm sure this pisses off a lot of people, like Linus, who probably wants to put a contract out on my happy ass, and RMS, who is probably committing suicide right now because if Guiness ever gets outlawed, that would probably mean the end of the GPL, or on the other hand, maybe RMS is quite happy right now, because he probably figures that beer should be open source, in other words, the brewery should release their recipes and all their trade secrets under the GPL, so that anybody could piss in the beer, or some garbage like that. Oh well.
• And the third reason... Or is it the fourth? I don't know, I've lost count. A couple of Negra Modelo's (because Guiness sucks) will do that to you. Oh well. I could write my own thing, and not plagiarize or whatever that stupid word is (and I can't even remember how to spell the damn thing) but that would take thought, time, and shit. And I don't have the patience for that kind of thing. So oh well.

So I will conclude that my conclusion is that I have discovered that there is no way in the entire universe that it would somehow be possible to use PING to measure the speed of LIGHT.

Einstein didn't use ping.

NEGRA MODELO. BECAUSE GUINESS SUCKS.

And then I had a revelation. A picture in my head. A picture of this! This is what makes time travel possible. The fluxcapacitor. As you can see, we've had our eye on you for some time now Mr. Anderson. It seems that you've been living two lives. In one, you're Thomas A. Anderson, program writer for a respectable software company. You have a social security number, you pay your taxes, and you help your landlady carry out her garbage. The other life is lived in computers, where you go by the hacker alias Neo and are guilty of virtually every computer crime we have a law for. One of these lives has a future, and one of them does not. Now I'm going to be as forthcoming with you as I can, Mr. Anderson. You're here because we need your help. We know that you've been contacted by a certain individual, a man who calls himself Morpheus. Whatever you think you know about this gentleman is irrelevant, he is considered by some authorities to be the most dangerous man alive. My partners think I'm wasting my time with you, but I believe you wish to do the right thing. We're willing to wipre the slate clean, give you a fresh start, and all we're asking is your cooperation in bringing a known terrorist to justice. That sounds like a really good deal, but I think I got a better one. How 'bout I give you the finger, and you give me my phone call. Hmmm... Mr. Anderson. You disappoint me. You can't scare me with this gestapo shit. I know my rights. I want my phone call. Tell me, Mr. Anderson. What good is a phone call, if you're unable to speak? The Matrix is a system Neo. That system is our enemy. Or some bullshit like that. Oh well.

Re:Technical theory behind this subject.

[vawlk]

Either:

1: Writer's block
2: Very bored

I vote for number 2 since I am too, and I actually read the whole thing laughing my ass off.

Now pardon me while I go find my ass.

Impedance of free space

[DotComVictim]

Hmm. Isn't this constant derived from the speed of light? Very convenient, yes?

Measuring the speed of light with a ruler

[harlows_monkeys]

One day in APh 23 (the introductory optics class) at Caltech, the professor announced it was time to measure the speed of light, and pulled a ruler out of his pocket. The class laughed.

He then turned on a laser of known wavelength, and reflected the beam off the ruler onto the chalkboard. The ruler had raised lines every 1/16th of an inch, and this made it basically act as a diffraction grating, and there was a clear diffraction pattern on the chalkboard. He marked off the pattern on the chalkboard with chalk, then took the ruler and measured the distance between the lines on the diffraction pattern. Then, still using the ruler, he measured the distance to where he had held the ruler.

A quick calculation later, and he had the speed of light.

I'm not sure that this was fully legitimate, because I can't think of a way to know the wavelength of the laser that doesn't involve already knowing the speed of light, but it was interesting nonetheless.

Speaking of interesting things to do with interference patterns, that professor did some work at Hughes on an optical weapon system. It had an array of radiators. Turn them all on, and you get a classic interference pattern, so you get a strong lobe in one direction, and not enough radiation in other directions to harm anything. The cool part was how it was aimed.

You aimed the main lobe by playing with the phase of the various radiators, so you didn't have to move things around to do fine aiming.

Here's the cool part. They used a feedback system. The modulated the phase of each radiator with a sine wave, using a different frequency for each radiator. They'd point a sensor at the target, and look for variations in the intensity of the reflection. If a particular radiator was at a phase that was contributing toward putting the max lobe on the target, there would be a weak variation in the reflection at the frequency of the sine wave they were modulating that radiator with (if the radiator is at the right phase, you are near a peak, and small variations from the modulation don't lose much). If a particular radiator's phase was way off, you'd get a strong single at the frequency of the modulation.

So, they could simply do a fourier analysis of the reflection, and see what radiators needed their phase adjusted to hit the target.

The professor had a film of a test, with a small number of radiators. They were all pointing at a black background, and you saw a kind of vague shifting light pattern. Then someone tossed a small metal model of the starship Enterprise in, and blam!, the phases were adjusted in a millisecond or so, and that thing lit up. It was very cool.

Re:Oprah Caught in Sexual Tryst with Dr. Phil

[Anonymous Coward]

You are my Hero

Ping Version?

[JerriMan]

My ping (installed from netkit-base-17) just gives a precision of tens of milliseconds. Which version gives such a precision as reported in the paper? Is there a patch out there? (I installed Linux from Scratch!)

Well,

[Eigenray]

I still like Finding the Speed of Light with Marshmallows

Note to moderators

[Anonymous Coward]

I just posted some extensive comments in the "How can this be accurate?" thread, regarding error analysis. Please take a look; I'm afraid it will get buried in the thread as an AC post.

GPS and Radio Time Signals

[cobyrne]

I once inadvertantly found myself measuring the speed of light using GPS and broadcast radio time signals

My project was to use a GPS system to generate a precise time signal for an experiment. (As part of the method they use for determining position, GPS systems have to determine the time to within a few nanoseconds or so, and some OEM GPS boards - like the one I was using - provide an accurate one pulse-per-second time signal for use). Anyway, I was having trouble understanding the signal, so I wired the signal, and a broadcast time signal from Moscow, into an oscilloscope.

There was a clear 11ms delay between when the GPS produced it's time signal and when I saw the signal from Moscow. I did the experiment in the west of Ireland, approximately 3,300km from Moscow...

Amazing post. Read and MOD Parent Up

[ecampbel]

Posts like these are why I still read slashdot.

Re:xxx.lanl.gov?

[Anonymous Coward]

'xxx' was pun on 'www' (increment one letter) -- back when the Web was new enough that puns like that were novel. As you may have guessed, the LANL arXiv actually has been blocked by porn filters before, which is perhaps one reason why 'arXiv.org' is used as an alternative to 'xxx.lanl.gov'.

Vacuum

[Anonymous Coward]

What vacuum? The one between the ears of these 'researchers'? Useless comparison. They measured the delay of a signal supposedly traveling at the 'speed of light'. Science is most excellent, NOT.

Re:Vacuum

[Anonymous Coward]

They measured the delay of a signal supposedly traveling at the 'speed of light'

If you read the paper, you will see that they measured the propagation speed of a signal in a cable, and got a value equal to about 2/3 that of the speed of light in vacuum.

If you think that's hilarious...

[Mawbid]

I have a wireless internet connection at home. A guy came and installed a directional antenna on the roof. He had me ping their gateway and he oriented the antenna while I read ping times to him over a two-way radio.

Well, I wasn't happy with the latency, so later I adjusted the antenna myself. But I didn't have anyone to read ping times to me and I wasn't too thrilled about this method anyway, so I came up with something better.

I wrote a perl script that would ping a host, wait for a reply (or a one second timeout), play a tick sound, and repeat the process. It sounds like a Geiger counter. The more frequent and steady the ticks, the better the connection. Also, every five seconds the script calls Festival to speak the average ping time. So, I get a nice intuitive feel for the connection through the stream of ticks, and a concrete measurement too.

Speakers out the window, full blast. Me on the roof. Neighbours' quizzical faces in the windows :-)

A fine story with a little irony?

[ajcpi]

With a great little tidbit:

• Software: We took data while running Linux on both computers. Although it should be possible to do this experiment with the new release of ping for Windows, because the authors were unfamiliar with Windows, Linux was chosen.
  

OT: physics units

[DJK]

My favorite velocity unit in HS physics was
angstroms per fortnight.

Re:OT: physics units

[Anonymous Coward]

You must have had one of those black and white marble covered composition notebooks. They had every weird old-fashioned unit in the conversion tables in the back.

In so you could convert fluid flow to:
hogshead per fortnight

gills per hogshead was a favorite useless conversion.

takes six months

[peter303]

You look for timing discrepencies when Jupiter is closest to the earth and furthest from the earth (about six months apart). The moons will appear to be slow or fast about fifteen minutes, or the light time to cross earth's orbital diameter.

Light path through fiber

[Spamalamadingdong]

Light does not "bounce" through single-mode fibers, and that category covers most long-distance transmission fiber.

Re:Light path through fiber

[anthony_dipierro]

Interesting. If the light travels perfectly straight without bouncing off the sides, why do we bother having the sides in the first place? Just send the laser beam straight through the air. Certainly the light must bounce. Otherwise explain how it gets around curves in the fiber.

Not bad

[epepke]

Physicists working with General Relativity frequently use units where c=1. This makes a lot more sense, as in GR c is more the aspect ratio of spacetime than it is a speed. Richard Feynman pointed out that in E=mc^2, c is just there to make the units work out. The problem is that we went on for hundreds of years thinking that energy and matter were different things, but it turns out they are related in a somewhat similar way that space is related to time. It's much prettier when you look at momentum (a 3-vector) and energy (a scalar). If you put these together, they make something that isn't really a 4-vector (but physicists don't use quaternions for this) but sort of works like that, if you imagine that the scalar is imaginary. The neat thing is that this 4-whatever transforms exactly the 4-whatever for spacetime.

Anyway, 1 lightyear/year is a fine, pure unit that is quite appropriate for working at galactic scale, at least.

The other nice coincidence is that the amount light travels in a naosecond is a little bit less than a foot, so about the length of a shoe.

Re:Not bad

[Verne]

The other nice coincidence is that the amount light travels in a naosecond is a little bit less than a foot, so about the length of a shoe

the speed of light is 3*10^8m/s
divide this by 10^9 for nanoseconds and it is 30cm/ns. I don't see what the big coincidence is here?

electron drift

[smartfart]

In fact, any given electron in the cable probably doesn't go anywhere.

When I took Circuits I and II at LSU, they taught us about this. A current travelling down a wire is taken to be the movement of positive charges. This is known as "hole flow", since a place where an electron isn't is a hole, and (IIRC) is positively-charged with repect to the departed electron.

Electrons are negative, and actually move in the direction opposite from the current. It's called electron drift, and the electrons move at only a fraction of the speed that the current flows. This speed can be calculated (if I could locate my old textbook, I'd post the formula).

Current (or hole) flow is much faster than the corresponfing electron drift. A very bad analogy would be sound travelling through a medium. The molecules don't have to travel very far at all to transmit the sound waves. Current flows faster than the actual electrons because when we are discussing current, we are talking about the movement of charges, not the movement of electrons. Current is the change in charge with respect to time.

Ping Review on Amazon.com

[scubacuda]

Here's the REAL story about ping.

(Check out the first Spotlight Review by El Segundo.)

The real results

[Anonymous Coward]

$ ping wolfstar

70ms

I conclude the speed of light is 70ms. Please give me a research grant.

Think QM, not classical physics

[Spamalamadingdong]

The fiber has to have "sides" so that there is only one mode - one solution to the wave equation - that the light can take through the fiber. It's like radio travelling over a coaxial cable; the energy isn't bouncing between the inner and outer conductors, or at least it can't until the circumference of the cable approaches a wavelength (which it can with big cables and really high microwave frequencies); then you get dispersion and other strange effects. Bending the fiber doesn't make anything "bounce", it just changes the boundary conditions and forces the wave to curve with the glass (and allows some probability of photons leaking out of the core if the curvature is tight enough, which is how some fiber-tapping techniques work).

Re:Think QM, not classical physics

[anthony_dipierro]

OK. I'll buy it. :) Thanks for the info.

Mirror of "the story of ping"

[jfengel]

"The Story of the Ping" site is slashdotted, but it is available from Google's cache

Google, I suspect, is harder to slashdot.

c = 1

[Fred Ferrigno]

I thought Planck units would be a neat way to measure things, being based off of measurable constants. C would always be 1, or at least a power of 10, because Planck time is defined as the time it takes a photon to travel one Planck length, which is in turn defined by other constants and physical rules. I don't know much more about the system, but it would seem to simplify a number of calculations a great deal.

Check out planck.com for more info. I would too if my network were not eating packets right now.

Re:c = 1

[Anonymous Coward]

Planck units are used almost exclusively in quantum gravity, where the Planck scale is relevant. In other areas, the exponents are just too big and unwieldy.

Ping around the world

[SkewlD00d]

That means that to ping the other side of the world, through wire, takes a minimum of 318 +/- 22 ms (round-trip) best case.

round trip time=pi*diameter earth/propagation speed

diameter: 12,756.3 km = 12756300 m
pi: 3.141593
prop speed: 118000000 +/- 9000000 m/s

Re:Ping around the world

[SkewlD00d]

The absolute minimum delay would be through the earth and back, in a vacuum:

2 * diameter of earth * speed of light ~= 85 ms

What the heck is wrong with using an oscilloscope?

[Anonymous Coward]

Lack of knowledge? Lack of faith in its time-base calibration? Afaik, some real beauties of Tek. 450 and 460-series portables are going for heartbreakingly-low prices.

Nick Bodley, Waltham

Re:John Walker Lindh is a fucking maggot...

[Anonymous Coward]

Oh geez. How much more evidence would be enough for you, marblehead? If one of the two of you is going to be labeled a "tool", it probably oughta be you, Forrest.