Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Some plasmas have a refraction index of less than 1. In these plasmas the phase velocity of light can be faster than light-speed. But the phase itself won't transfer information, so no paradox occurs here. But what if I constructed a tube filled with plasma with a torch on one end and a photon sensor on the other. When I see a beam of red light travelling past me, I lit up the torch, and some guy at the other end can know there's red light coming at him before the red light arrives. Isn't that an FTL occasion?

share|cite|improve this question
up vote 4 down vote accepted

One way to look at that:

  • The envelope of the red light pulse travels at the group velocity, less than $c$.

  • The fast oscillations within the main pulse travel faster than $c$.

In the end, the torch "lighting up" is given by the group velocity, because the envelope of the pulse is what you receive.

This animation showing a train of pulses is pretty clear. You can see that each pulse goes slowly, but the fast oscillations go fast.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.