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?

[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]

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: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?

[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.

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: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.