# Speed of a photon in a vacuum

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?

• Any link to paper or article where the above claims are made? – exp ikx May 27 at 16:18
• structure of a photon: What should that be? Elementary particles have no internal structure. – Thomas Fritsch May 27 at 16:20
• 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. – StephenG May 27 at 16:23
• See arxiv.org/abs/1411.3987 dating from 2014 – John Duffield May 27 at 16:24
• @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. – G. Smith May 27 at 16:47

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.