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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 ;)
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.
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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.
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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!
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."
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Score 3? For what? Being wrong, at length? - smirkleton
The informative bit is the reference 7 where stochastic resonance is actually explained.
stochastic resonance
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).
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.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
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
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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...
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.
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...