So, I stumbled onto this article which really blew my mind (http://www.bbc.com/news/uk-scotland-glasgow-west-30944584). To sum it up, these researchers set up some kind of material that slowed a photon down while it traveled through the material. Once it passes through the material, it should return $c$; however, when they race it against another photon, it comes up short. To me, this sounds like it should be bigger news. The speed of light should ALWAYS equal exactly $c$ when in a vacuum. So, if this was not the case, did they just cause some issue with Einstein? Or, is there something to which I am not privy?
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$\begingroup$ Related: physics.stackexchange.com/q/153904/2451 $\endgroup$– Qmechanic ♦Commented Mar 17, 2016 at 14:53
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$\begingroup$ Photons don't move at all, only the electromagnetic waves move. $\endgroup$– CuriousOneCommented Mar 17, 2016 at 15:24
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This is a clever experiment, but nature already does something similar for all transparent materials.
Transparent materials (glass, air) transmit images; if the image is distorted or indistinct, we know that the material is altering the coherence of the optical information. That is, what started out at the beginning has not arrived all at the same time. With enough distortion the image is completely lost.
So what is required for a transparent medium to successfully transmit an image? Since light is a physical wave, the transparent medium must preserve the coherence of the phase information of the light. In a typical glass the phase front is slightly slowed while traveling through the glass; this slowing is encoded in the index of refraction, $n = c/v$.
If the material absorbs some frequencies, the material will appear to be colored; a photon that is absorbed (depending on the energy level structure) can be re-emitted, but this will be at (a) a random time later, and (b) in a random direction. No image for this color! There is an exception: stimulated emission, which is the key to building a laser. But this is not how images are transmitted in a passive material.
The process that transmits images can be summed up as Coherent Forward Scattering: Coherent, because otherwise the image integrity is reduced; Forward, because the image is transmitted in this direction, through the material; and Scattering, the remaining available generalized mechanism at the quantum level.
The result is quite like the Huyghen's wavelet model for light transmission: the photons are the waves that are scattered coherently, and because it is coherent, they are able to interfere both constructively and destructively to maintain the coherence of the overall phase front.
It is the interference that slows the phase velocity through through the material; the individual photons continue to "move" at the speed of light, $c$, but the effective motion of the phase front is slowed.
Richard Feynman devotes some time to this in his lectures on QED: The Strange Theory of Light and Matter
In the experiment referenced in the OP they have altered the phase front of the individual photons, and then tracked them.
answered Mar 17, 2016 at 13:52
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2$\begingroup$ This is a good explanation of why light/photons slow down in a material, but it's not the issue raised by the OP. The ref. quoted refers to this paper, science.sciencemag.org/content/347/6224/857, where the authors studied "a reduction in the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam" propagating in free space (air, to be precise, but would be the same in vacuum). In other words, light kept traveling at lower then c after exiting the material, and that was due to the particular transverse shaping of the beam. $\endgroup$– udrvCommented Mar 17, 2016 at 15:26
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$\begingroup$ Phase shaping is responsible for the result; I will study the paper and add some additional details. This makes the experiment more impressive in scope. $\endgroup$ Commented Mar 17, 2016 at 17:15