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There have been reports of physicist Miles Padgett at the university of Glasgow that the speed of photons in a vacuum can be made to vary slightly by changing the structure of a photon. Is this true? What do you need to do to the structure of a photon to change its velocity? Won't this have implications for the Standard Model of particle physics?

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    $\begingroup$ Any link to paper or article where the above claims are made? $\endgroup$ – exp ikx May 27 at 16:18
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    $\begingroup$ structure of a photon: What should that be? Elementary particles have no internal structure. $\endgroup$ – Thomas Fritsch May 27 at 16:20
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    $\begingroup$ I believe you are referring to this and it's not something I've looked into myself, but this comment on Arxiv may be of relevance. $\endgroup$ – StephenG May 27 at 16:23
  • $\begingroup$ See arxiv.org/abs/1411.3987 dating from 2014 $\endgroup$ – John Duffield May 27 at 16:24
  • $\begingroup$ @ThomasFritsch See en.wikipedia.org/wiki/Photon_structure_function Although the photon is a point particle, interactions with virtual particles in the quantum vacuum give it a kind of structure. But this is not the “structure” this question is talking about. $\endgroup$ – G. Smith May 27 at 16:47
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The answer to your question is that in your case it is group velocity and phase velocity.

The group velocity and phase velocity can be slower then c (or faster then c), when measured locally in vacuum.

From Wiki:

Noting that c/n = vp, indicates that the group speed is equal to the phase speed only when the refractive index is a constant dn/dk = 0, and in this case the phase speed and group speed are independent of frequency, ω/k=dω/dk=c/n.[2] Otherwise, both the phase velocity and the group velocity vary with frequency, and the medium is called dispersive; the relation ω=ω(k) is known as the dispersion relation of the medium. The phase velocity of electromagnetic radiation may – under certain circumstances (for example anomalous dispersion) – exceed the speed of light in a vacuum, but this does not indicate any superluminal information or energy transfer. It was theoretically described by physicists such as Arnold Sommerfeld and Léon Brillouin. See dispersion for a full discussion of wave velocities.

Now if you are talking about this:

Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.

Then this is the answer to your question.

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  • $\begingroup$ Hi. Thanks for this. I know about group and phase velocities. But I did not realize a single photon could have a group and phase velocity. Is that the case? How can is be? Is it the wave function of the photon that has group, phase velocities? $\endgroup$ – Rory Cornish May 27 at 19:23
  • $\begingroup$ @RoryCornish Yes, a single photon, described as an EM wave, does have a phase and group velocity. Phase velocity is the peaks' and troughs' speed. Group velocity is the envelope's speed of the wave. $\endgroup$ – Árpád Szendrei May 27 at 21:04
  • $\begingroup$ Hi Arpad! Wow. I did not realize you could write down the equations for an EM wave of a single photon. Can you point me to these equations, I would love to see and understand them. I was good at EM and optics at Uni. I did know that group and phase velocities of light in a vacuum could vary from c, but I believe the speed of actual information transfer is still c. In the case of a single photon, does the actual speed of information transfer slow down? $\endgroup$ – Rory Cornish May 28 at 9:02
  • $\begingroup$ @RoryCornish In the case of single photon, the speed of information cannot exceed c. Yes the photon can be interpreted as a wave. Like the double slit experiment. Please see here: physics.stackexchange.com/a/174498/132371 $\endgroup$ – Árpád Szendrei May 28 at 15:51
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Try to not imagine a photon as a little ball of light traveling through space like a bullet. A photon is a minimal excitation of the EM field. A photon has no velocity and no structure.

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