Faster than light information

Question

I've seen there are a few answers on here previously about faster than light information travel, and my understanding is that it is impossible.

However, I've just read an article https://www.sciencealert.com/physicists-broke-the-speed-of-light-with-pulses-inside-hot-plasma

While the light waves are travelling at their usual speed, they indicate that ...

But impressively, last year, researchers from Lawrence Livermore National Laboratory in California and the University of Rochester in New York managed it inside hot swarms of charged particles, fine-tuning the speed of light waves within plasma to anywhere from around one-tenth of light's usual vacuum speed to more than 30 percent faster.

Other than the problems of practicality of having a conduit of hot plasma long enough to be of any use, I'm guessing that this can't be used to transfer information faster than light, especially since the article doesn't make mention of this; but why couldn't it? Especially if that information is just a binary state.

Answer 1

Just a bad sentence or poor science writing. The actual abstract of the paper reads:

Slow and fast light, or large changes in the group velocity of light, have been observed in a range of optical media, but the fine optical control necessary to induce an observable effect has not been achieved in a plasma. Here, we describe how the ion-acoustic response in a fully ionized plasma can produce large and measurable changes in the group velocity of light. We show the first experimental demonstration of slow and fast light in a plasma, measuring group velocities between 0.12c and −0.34c.

So no information going faster than the speed of light.

It is usually good to find the link for the actual publication when you see an interesting story. Good Science communication is hard and there is the perverse incentive of click bait type titles.

Answer 2

Quotes from the original paper: "The negative group velocity indicates that the peak of the input pulse appears to travel through the plasma faster than the speed of light and is sometimes referred to as backward propagation. However, the information contained in the pulse still propagates slower than c."

Answer 3

You can't travel faster than light in a vacuum. Light in a medium, however (like light passing through a glass of water), travels at a much slower speed. It is possible to travel faster than light in a medium, but this is slower than the speed of light in a vacuum and can't be used to transfer information at FTL speeds.

Answer 4

A faster-than-$c$ group velocity is a case where a medium (such as a solid, gas or plasma) has been set up in a special state.

Consider the following comparison.

Suppose I arrange a set of dominoes to fall over, one after the other. The usual procedure is to line them up and then push one over, and it falls against the next, which topples onto the next, and so on.

Let's say the dominoes are separated by 2 cm and each takes $0.2$ s to fall over and push the next. Then the 'wave' of toppling dominoes travels at the speed $2/0.2 = 10 $ cm/s.

But now suppose I set them up a different way. I place them on a set of little platforms, like the keys on a piano, separated by 2 cm as before, and arrange that each platform can tip. Then I attach a mechanism to all these platforms. Now I can have some fun. I could, for example, make all the platforms tip at the same time. Or I could make them tip over in sequence, with 10 millisecond between each one and its neighbour. So now the dominoes will fall over one after the other, and the wave of falling dominoes travels at the speed $2/0.01 = 200$ cm/s. So now I have a wave travelling at twenty times the "speed of domino"!

With light in a specially prepared plasma you can get similar effects. The leading edge of a 'priming' pulse runs through a plasma at some speed less than $c$, and sets up the plasma so that it is ready to react in such a way that each part of the plasma does something (makes an electric field or a concentration of charge), but the first part to be stimulated does it more slowly than the last part. (This could happen, for example, when the priming pulse gets amplified as it goes). For example, suppose the two ends are separated by 3 metres, and let the priming pulse travel from left to right. Then the left end gets stimulated at time $t=0$ and the right end gets stimulated at time $t = 3/c = 10$ ns. Suppose also that the left end takes about 11 ns to build up a substantial reaction, and the right end takes 2 ns to build up a substantial reaction. Suppose similarly that places in between take times in between. Then some larger disturbance will appear at the left at $t=11$ ns, and some such disturbance will appear at the right at $t=12$ ns, and overall you will see a disturbance running through the plasma such that it travels 3 metres in 1 nanosecond. Hence its speed is $(3 {\rm metres})/(1 {\rm ns}) = 3 \times 10^9$ m/s which is $10c$.

This is perfectly possible.

The information travelled at the speed $c$ (or less) when the plasma was set up. The reaction then happened as described.

One way to see that this fast-moving disturbance does not constitute a message is to ask what would happen if someone interfered with the build-up of the reaction at the left hand end at the time, say, $t = 10.1$ ns. Would this prevent the fast-travelling disturbance? The answer is no. The effect at the right (charge movement and electric field) would still happen just the same.