https://en.wikipedia.org/wiki/Anomalous_dispersion claims:

Recently, it has become possible to create gases in which the group velocity is not only larger than the speed of light, but even negative. In these cases, a pulse can appear to exit a medium before it enters. Even in these cases, however, a signal travels at, or less than, the speed of light, as demonstrated by Stenner, et al.

Is this (''a pulse can appear to exit a medium before it enters'') a real effect, or is it due to sloppy experiments or their sloppy interpretation?


Front velocity and Superluminal group velocity

In superluminal phase velocities, what is it that is traveling faster than light?


1 Answer 1


The usual experimental situation where a group velocity greater than the speed of light occurs requires (a) an optical transmission near an absorption peak, which distorts the pulses, or (b) the presence of the Hartman Effect, which is a quantum tunneling effect seen when the barriers are thicker.

A colleague of mine, Herbert Winful, has carefully analyzed these situations in a series of papers. Nature of “Superluminal" Barrier Tunneling; also see the referenced papers.

The conclusion: faulty analysis of the situation results in unfounded claims of information transmission that is faster than the speed of light.

Some counter-analysis has been published by Günter Nimtz et al in The Superluminal Tunneling Story.

  • $\begingroup$ my question was more specifically about the anticausal claims that effects precede the cause. $\endgroup$ Mar 27, 2016 at 20:27
  • $\begingroup$ That's the same thing as FTL communication, hence ruled out. $\endgroup$ Mar 27, 2016 at 20:32
  • $\begingroup$ Well, the paper I quoted says this itself (see the last sentence of my quote). My question is about the group velocity, which is well-known to be superluminal without violating principles of relativity. $\endgroup$ Mar 28, 2016 at 16:21

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