About news about the "slowing down" of the photon
A couple of days ago, popular-science media spread around the news, as if scientists had discovered that photons in a vacuum can propagate at a speed lower than the speed of light . To do this, just skip them through a special mask. This message caused me some skepticism, which I reflected in quotes in the title of this post, and a desire to understand what really happened there.
Picture from Glasgow University Press Release
Naturally, the first thing I decided to check if the error had crept into the translation. This happens from time to time in runet. The fact is that only a few of us work with English-speaking sources, the rest just rewrite from already translated news. There were cases when, for example, Lenta.ru made a mistake when translating, and it was replicated in a dozen other media.
Therefore, I sought out the initial press release , and after reading it, I realized that there was no error in the translation. Indeed, it is claimed that photons were obtained, whose speed in vacuum is lower than the speed of light:
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The next step was to check if there was an error in the press release. Journalists, even in very serious publications, do not disdain to embellish the results. Even the scientists themselves in their comments for the press release sometimes use a not very good analogy or hyperbole, not noticing that it distorts the essence of their work.
In general, I looked into the scientific article itself , published in the more than authoritative journal Science . To my surprise, there I saw the same statement, moreover, made directly in the title:
This meant that we need to understand the work in essence. Fortunately, my qualification allows it.
What is the essence of the work? The authors take a light source, divide its radiation into two beams, one of which is allowed along the delay line without distorting its profile, and the second is passed through a special mask - a lens of a certain shape. The length of the delay line is selected in such a way that the photons both there and there reach the output at the same time instant. After that, the recorder is moved away for some distance and they look at the time after which two photons arrive. If their speeds are the same, they must come at the same moment in time, if not - then with some difference.
It turned out that the photon that passed along the first path arrives much earlier, while its measured speed turned out to be equal to the speed of light, which is expected. But this means that the photon, which passed the second way, has a speed less than the speed of light. It seems everything converges, and the news quite correctly reflects the result of the experiment.
But the devil, as you know, is in the details.
Let's see what a used mask is. Researchers studied two types of masks. The first gave the beam an envelope in the form of a Gaussian function, and the second - in the form of a Bessel function. Further reasoning is more convenient to use the example of a Bessel beam , because, in fact, this is a case of conical focusing, for which the lens looks something like this:
Posted by: Egmason . Source: Wikimedia Commons
If you look at this picture better, you can understand that the lens, in fact, breaks the incident wave into two waves of equal amplitude, running at an angle to the original direction of propagation. If we recall that a photon is not just a particle, but also a wave object (wave-particle duality, however), then it is clear that in such a lens it also breaks into two “sub-photons” running at an angle to the original direction of propagation. If we talk scientifically, then the photon is placed in a superposition of two states with different pulses.
At the same time, on average, the photon continues to fly, where it flew, and in the experiment the speed of this averaged motion is measured. Each of the “sub-photons”, however, flies at the same speed of light, and the measured average speed is just a projection of their speed to the original direction of propagation.
For a Gaussian beam, the reasoning is similar, with the exception that the photon in it is placed in a superposition not of two, but of an infinite number of states with different momenta.
Of course, these “sub-photons” are nothing more than imaginary objects, which I introduced for clarity. Only a photon in the superposition of two states is real. Therefore, the interpretation of the result given in the work has the right to exist. But if the photographic recorder was taken away, then after a large number of photons were flown in, the experimenters would see two separated spots, and then the ambiguity of their interpretation would become more obvious.
To emphasize what has just been said, I will give one more example. If we put a photon on a translucent plate with a transmittance of 50%, then we will register it on one side of the plate, then on the other. Its average speed will thus be zero. But this is so obvious, there is no overclocking.
Summarizing. In the experiment, indeed, the speed of the photon was measured below the speed of light. But it should be borne in mind that this is a rather specific speed. The result, however, is important for some applications, and therefore had the right to publish.
PS In a sense, the opposite effect, by the way, was used in a recent article about “self-accelerating” electrons . Since electrons also exhibit wave properties, they can be placed in such a superposition of states that for most electrons the time of their arrival at the detector will be relatively long, but a small fraction of particles will arrive at the detector very quickly, thus demonstrating an increase in speed.
Picture from Glasgow University Press Release
Naturally, the first thing I decided to check if the error had crept into the translation. This happens from time to time in runet. The fact is that only a few of us work with English-speaking sources, the rest just rewrite from already translated news. There were cases when, for example, Lenta.ru made a mistake when translating, and it was replicated in a dozen other media.
Therefore, I sought out the initial press release , and after reading it, I realized that there was no error in the translation. Indeed, it is claimed that photons were obtained, whose speed in vacuum is lower than the speed of light:
')
Co-lead author Jacquiline Romero said: "... this can be slowed down below, according to meters, even when traveling in ... vacuum.
The next step was to check if there was an error in the press release. Journalists, even in very serious publications, do not disdain to embellish the results. Even the scientists themselves in their comments for the press release sometimes use a not very good analogy or hyperbole, not noticing that it distorts the essence of their work.
In general, I looked into the scientific article itself , published in the more than authoritative journal Science . To my surprise, there I saw the same statement, moreover, made directly in the title:
Spatially structured photons that travel in free space slower than the speed of light
This meant that we need to understand the work in essence. Fortunately, my qualification allows it.
What is the essence of the work? The authors take a light source, divide its radiation into two beams, one of which is allowed along the delay line without distorting its profile, and the second is passed through a special mask - a lens of a certain shape. The length of the delay line is selected in such a way that the photons both there and there reach the output at the same time instant. After that, the recorder is moved away for some distance and they look at the time after which two photons arrive. If their speeds are the same, they must come at the same moment in time, if not - then with some difference.
It turned out that the photon that passed along the first path arrives much earlier, while its measured speed turned out to be equal to the speed of light, which is expected. But this means that the photon, which passed the second way, has a speed less than the speed of light. It seems everything converges, and the news quite correctly reflects the result of the experiment.
But the devil, as you know, is in the details.
Let's see what a used mask is. Researchers studied two types of masks. The first gave the beam an envelope in the form of a Gaussian function, and the second - in the form of a Bessel function. Further reasoning is more convenient to use the example of a Bessel beam , because, in fact, this is a case of conical focusing, for which the lens looks something like this:
Posted by: Egmason . Source: Wikimedia Commons
If you look at this picture better, you can understand that the lens, in fact, breaks the incident wave into two waves of equal amplitude, running at an angle to the original direction of propagation. If we recall that a photon is not just a particle, but also a wave object (wave-particle duality, however), then it is clear that in such a lens it also breaks into two “sub-photons” running at an angle to the original direction of propagation. If we talk scientifically, then the photon is placed in a superposition of two states with different pulses.
At the same time, on average, the photon continues to fly, where it flew, and in the experiment the speed of this averaged motion is measured. Each of the “sub-photons”, however, flies at the same speed of light, and the measured average speed is just a projection of their speed to the original direction of propagation.
For a Gaussian beam, the reasoning is similar, with the exception that the photon in it is placed in a superposition not of two, but of an infinite number of states with different momenta.
Of course, these “sub-photons” are nothing more than imaginary objects, which I introduced for clarity. Only a photon in the superposition of two states is real. Therefore, the interpretation of the result given in the work has the right to exist. But if the photographic recorder was taken away, then after a large number of photons were flown in, the experimenters would see two separated spots, and then the ambiguity of their interpretation would become more obvious.
To emphasize what has just been said, I will give one more example. If we put a photon on a translucent plate with a transmittance of 50%, then we will register it on one side of the plate, then on the other. Its average speed will thus be zero. But this is so obvious, there is no overclocking.
Summarizing. In the experiment, indeed, the speed of the photon was measured below the speed of light. But it should be borne in mind that this is a rather specific speed. The result, however, is important for some applications, and therefore had the right to publish.
PS In a sense, the opposite effect, by the way, was used in a recent article about “self-accelerating” electrons . Since electrons also exhibit wave properties, they can be placed in such a superposition of states that for most electrons the time of their arrival at the detector will be relatively long, but a small fraction of particles will arrive at the detector very quickly, thus demonstrating an increase in speed.
Source: https://habr.com/ru/post/375801/
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