Speed of a photon in a vacuum

Question

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?

Answer 1

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.

Answer 2

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.