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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 ;)
And according to Unreal Tournament, the speed of light is about 50 miles per hour.
"It is a greater offense to steal men's labor, than their clothes"
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?
--Roy
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.
That you'd only be measuring the amount of pr0n being downloaded by physics students... unless you had your own clean segment.
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.
Video for Online Dating Profiles
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.
"It take 9 months to bear a child, no matter how many women you assign to the job."
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.
Check out the first review of this story on Ping.
Last night I shot an elephant in my pajamas. How he got in my pajamas I'll never know.
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.)
Scientists restrict study to entire physical universe; creationist
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
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?
"One man's meat is another man's poison."
--Bugs Bunny
"It take 9 months to bear a child, no matter how many women you assign to the job."
Make that sqrt(1/). In my hurry to post I made an error (and a bunch of people got their two cents in first).
Scientists restrict study to entire physical universe; creationist
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.
-- Never hit a man with glasses. Hit him with a baseball bat.
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.
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!
...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!!!"
:)
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
--
The Internet is generally stupid
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!
python -c "x='python -c %sx=%s; print x%%(chr(34),repr(x),chr(34))%s'; print x%(chr(34),repr(x),chr(34))"
Over 15000 samples it's going to be pretty constant.
Perhaps you should read the paper?
Request timed out.
:-)
IMO this would mean that slashdot is in a black hole
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").
Do the same thing with pong, then I'll be impressed.
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...
General Relativity: Space-time tells matter where to go; Matter tells space-time what shape to be.
why go through all that trouble when all you need is a flashlight and a stopwatch?
Mess Stuff Up
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.
"It take 9 months to bear a child, no matter how many women you assign to the job."
Yeah, nobody can spare the 10 minutes linux --> windows conversion. At least nobody with the proper attention span to be teaching children.
"It take 9 months to bear a child, no matter how many women you assign to the job."
We Slashdotted Los Alamos!
xxx.lanl.gov.is down.
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... ;-)
"It take 9 months to bear a child, no matter how many women you assign to the job."
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.
-----
Score 3? For what? Being wrong, at length? - smirkleton
"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.
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.
-... ---
Why is that awsome? The problem is the exact same only reversed.
Rod Taylor
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.
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.
Comparing it to Windows will be a moot point, since El Dorado is going to have a 40% larger code base than XP.
[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.
"ping -i .01 > tempfile1.txt" where ">" (the so-called 'pipe' symbol)
Then what's this thing: | ?
:)
Why do I keep typing pythong?
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"
microsoftword.mp3 - it doesn't care that they're not words...
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.)
--JoeProgram Intellivision!
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
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-
Pings are used to measure things in real life.
:-) Of course, the time taken to process the ping by the target etc. must be taken into account.
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
Oolite: Elite-like game. For Mac, Linux and Windows
-Legion
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
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.
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.
The truth shall set you free!
It doesn't have to be a constant. See below.
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...
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...
Posts like these are why I still read slashdot.
Sig goes here
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
Fuck the system? Nah, you might catch something.
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.
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 does not "bounce" through single-mode fibers, and that category covers most long-distance transmission fiber.
Scientists restrict study to entire physical universe; creationist
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.
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).
Scientists restrict study to entire physical universe; creationist
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
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.
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
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
The biggest trick the devil pulled was letting lawyers become politicians so they can write the laws.
Class dismissed.
-Legion
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
-Legion