# Faster than light information

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.

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.

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.

• I feel this answer is itself rather misleading, because the minus sign in the value $-0.34c$ is all about a faster-than-light phenomenon. (The phenomenon remains one which does not transmit information). So while the words of the abstract do not say "faster than light", nevertheless the scientific information conveyed in this abstract does say "faster than light". Sep 6 at 15:30
• The abstract uses the term ‘fast light’ very carefully to avoid using ‘faster than light’ the Abstract and work is good ‘Fast light’ in the literature is pretty precisely defined. The article about the work is misleading. Given the magnitudes they list for group velocities they are not outside light cone or conveying information faster than the speed of light. Since the question was about if information traveling faster than the speed of light I thought that answered the question without trying to explain precursors or negative dispersion etc. in detail. Sep 6 at 17:39
• @AndrewSteane But fair point that more explanation is needed to properly explain. This seems to be a pretty good explanation, especially Figure 5 why the negative group velocity is important. photonics.com/Articles/… Sep 6 at 17:49

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."

• Yeah, that's the bit (well, not the only one) that I don't understand.... If they can detect the wavefront, that in itself is information? I'm guessing that the actual wavefronts are propagating at the speed of light in the medium, but the frequency of the wavefronts is modified, and propagates through the waves faster than the speed of light. Isn't that just modulation, which can be used to transmit information. Ok, I know, or rather trust that it isn't, but I don't know why. I'm also thinking that I probably lack the maths to understand it! Sep 2 at 20:02
• @Peter Page: Imagine a water wave hitting the side of a pier. Suppose it hits it at an angle. Then the splash travels along the pier faster than the speed of the water wave. But nothing is really traveling that fast, just the splash zone. This is just an analogy, so don't take it too seriously, but it might help you imagine the sort of thing that's going on here. Sep 2 at 22:55
• @PeterPage we still have to wait for each individual wave to reach the observer. Only after that, the pulse is able to propagate, which can already be deduced from the waves we waited to receive. Sep 7 at 1:30
• @Daniel this is the key. The band of frequencies comprising the FTL pulse must preexist the pulse wherever it is measured. So a bit of information carried by that pulse must have already propagated to the detector. Oct 4 at 15:59

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.

• Ahh, of course, thank you. I completely missed the fact that the article didn't claim it was faster than light in a vacuum. .... Oh, they do. anywhere from around one-tenth of light's usual vacuum speed to more than 30 percent faster. ps. that was a just noticed they did, and amended my comment, not a sarcastic comment) Sep 2 at 11:36
• To be fair, the article OP links does make the absurd claim that they reached speeds "from around one-tenth of light's usual vacuum speed to more than 30 percent faster". The writer explicitely claims that the vacuum speed has been surpassed by 30%. But he appears to have simply made that up. Nothing in the original paper about that. Sep 2 at 12:25
• The group velocity is below $c$ in ordinary media such as glass, water, ordinary gases etc. However you can get group velocities above $c$ (for some range of wavelengths) in specially prepared media. This is connected to light amplification in gain media, and electromagnetically-induced transparency, and other such 'active' effects. Sep 6 at 16:06

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.