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Faster than light? Nothing!

An article and a commentary in the journal Nature describes an experiment by Wang, Kuzmich and Dogariu. The experiment consists in passing a pulse of light through a certain material – cesium vapor – specially prepared so that it has appropriate properties of emission and absorption of photons.

These articles received an unusual attention in the media, both in our country and in others, even saying that the experiment, which (supposedly) produced speeds higher than the light, “was a challenge to ideas Of Einstein.” Poor Einstein. He despised sensationalism, he must be turning in his grave.

And the comments in the media about the challenge on Einstein were exagerated, including saying the speeds could be 300 times the speed of light, when in fact it is a change of a few percent.

The commentary by Marangos, and even the article by Wang et al., Are written, it has to be said, in a somewhat obscure way and in a more sensationalist style than should be the norm for scientific work. It should be read with some care to make sure that what moves faster than light is called average group velocity, something quite different from the speed with which the photons of the light pulse. Careful analysis reveals that if this average speed is higher than the speed of light in vacuum,  c , this is due to a wrong definition because the average group velocity which is not the same as the average velocity of the photons.

This is evident if one thinks that all photons always travel at the speed c , which is known since 1905, and thanks precisely to Einstein that, in two fundamental articles established, first, that the speed of light is a universal constant, and second, that the light pulses consist of photons that travel at the speed precisely c. Proven results in literally millions of experiments.

The call rate of propagation of light in a medium, or the average group velocity, which can be denoted by v, is a useful measurement in certain approximations. Perhaps the best way to understand this for non-professionals is with a couple of examples.

e.g 1. If someone measures the distance in a straight line between Madrid (Spain) and Geneva (Switzerland), it is 1,000 kilometers. I have traveled this route many times by car, at an average speed of 100 kilometers per hour, and it has taken me 14 hours, not the 10 that would be expected.

The reason for this, of course, is that the road does not follow a straight line, and the actual route by car, is 1,400 km. The same thing happens to a photon when passing through a material medium. It collides with other atoms and its trajectory is no longer a straight line. So ts effective rate is lower: like my effective rate on the Madrid-Geneva is about 70 kilometers per hour.

However, it seems that the example of my trip to Geneva is not valid, because in them I always came out with an actual speed which is lower. How is it possible to understand that Wang and colleagues found a higher speed than c ? I’ve explained this in another example below.

Consider a set of 10 runners running an athletics track, all at 30 kilometers per hour. Suppose that the runners of the group begin to run separated from each other by a meter. Let us now call the speed of the group to the one with which the center of the group moves, center that is five meters behind the one that goes on head at the beginning of the race.

Now comes the trick. Each time the riders pass the finish line, a new rider is placed at the height of the head and start to run at 30 kilometers per hour.

After 10 laps, all the runners that have been added go together in the head of the group, the stragglers have disappeared and the runner in the middle of the group goes five meters before what would have been half of the group if We would not have touched it; Therefore, the average speed of the group is greater than that of any of the runners.

Of course, the trick is that the runners who start the test are not the same as those who finished it. The average group velocity concept we have entered is correct if the group contains the same people at the beginning and the end; But ceases to be if we add and delete corridors.

The fact that such group speed exceeds 30 kilometers per hour does not imply that the riders could transmit the information faster than each one ran: if we give a witness to one of the runners, the witness will go at the speed of the runner . And that if we were lucky and chose one of the runners who survive to the end.

This is, in essence, also the mechanism of the photon experiment performed by Wang et al., And, as they themselves acknowledge in the references cited in their article, by quite a few other researchers before them. In the experiments performed by the predecessors, the leading photons excite atoms, which in turn produce photons, which are added to the head group; The photos that go in the queue are absorbed.

Although each photon, always moving at the speed  c ¸ the group velocity is greater than c . The merit of Wang, Kuzmich and Dogariu is to use a more subtle method to enhance / suppress photons, but neither here nor any one of them goes more speed than light in the vacuum. And if an experimenter were to say otherwise, it would be necessary to think that he had measured wrongly and asked for an independent repetition of his experiment: as Belmonte said, what can not be can not be, and besides is impossible. The photons that pass through a medium traveling, all the while, at the speed c , as, on the other hand, it is clear in the articles themselves of Nature if you read them carefully.

The definition of group speed that authors use is inappropriate. Wang, Kuzmich, and Dogariu point him out at the end of his article, though, astutely, they maintain their definition because in that way an experiment that is simply curious seems to have fundamental consequences.

I have no doubt that it is highly unlikely that the work will succeed in “leading to profound implications for the issue of signal propagation,” as Wang and Adláters suggest, or in putting Einstein’s ideas at bay, as the Media, but there is no doubt that it has tended to get publicity for the authors. This does not mean that it has no value; Above all, technical. We must recognize the remarkable ability of Wang and colleagues: it is not easy to construct an experiment that carries out the multiplication and annihilation of photons.

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