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Hi, About three or four years ago I read from a website that a researcher in Denmark was able to slow down the speed of light to about 30 miles an hour. I don't remember the details, but the way she did it was by shooting the beam of light through a highly condensed matter (I don't know what kind), thereby blocking its passage to simulate a slow-down in speed. I'm curious if anybody has any idea about that experiment and can comment on that. Also, what may be the possible application of it (if indeed the experiment was carried out correctly)?

jpetruccelli, If I understand your explanation and the Duke website material correctly, the speed-of-light, c, is really the group speed of the multitude of single-frequency waves, right? Since I don't have time right now to read the other reference material on that website, I'm going to just ask the question here: At present, at which frequency have researchers measured the fastest light wave speed? Presumably it is faster than c. And if so, what's its value? Thanks.

Jpetrucelli You have got the group and phase velocities mixed up. The phase velocity (basically the speed that the peaks or zero crossings in the electromagnetic waveform) is the one that can be faster than the velocity of light. The group velocity is the one that carries the information and cannot be faster than the speed of light.

To see why the group velocity need not correspond to the speed of information in a wave, notice that in general, by superimposing simple waves with different frequencies and wavelengths, we can easily produce a waveform with a group velocity that is arbitrarily great, even though the propagation speeds of the constituent waves are all low. A snapshot of such a case is shown below. In this figure the sinusoidal wave denoted as "A" has a wave number of kA = 2 rad/meter and an angular frequency of wA = 2 rad/sec, so it's individual phase velocity is vA = 1 meter/sec. The sinusoidal wave denoted as "B" has a wave number of kB = 2.2 rad/meter and an angular frequency of wB = 8 rad/sec, so it's individual phase velocity is vB = 3.63 meters/sec.The sum of these two signals is denoted as "A+B" and, according to the formulas given above, it follows that this sum can be expressed in the form 2cos(kx-wt)cos(Dkx-Dwt) where k = 5, w = 2.1, Dk = 0.1, and Dw = 3. Consequently, the "envelope wave" represented by the second factor has a phase velocity of 30 meters/sec.