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

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.

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...