Stack Exchange Network
Stack Exchange network consists of 181 Q&A communities including
Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Visit Stack Exchange
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up.
Sign up to join this community
Anybody can ask a question
The best answers are voted up and rise to the top
9 years, 9 months ago
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?
188k 40 40 gold badges 491 491 silver badges 2141 2141 bronze badges
Sep 8, 2013 at 13:55
1,128 1 1 gold badge 14 14 silver badges 31 31 bronze badges
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.
Sep 8, 2013 at 15:57
4,208 2 2 gold badges 19 19 silver badges 42 42 bronze badges
Highly active question
. Earn 10 reputation (not counting the
) in order to answer this question. The reputation requirement helps protect this question from spam and non-answer activity.
By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our
Accept all cookies
Necessary cookies only