# Speed of light in and out of vacuum

1. Apr 9, 2010

### Jobrag

I've read ZapperZ's thread on the speed of photons through a solid and am still confused.
If you imagine an experiment where two emitters A and B are set up to emit one photon each simultaneously towards two receptors (also A and B)in a vacuum 558,000 miles away. The receptors will sense the arrival of the photons approximately 3 seconds after they leave the emitters and should “see” them simultaneously.
Are the photons that arrive at the receptors the same photons that left the emitters?
If you now put a piece of glass 186,000 miles thick between the B emitter and B receptor and repeat the experiment the B receptor will “see” the photon slightly later then the A receptor.
If the answer to my first question was yes, is the photon that hits the B receptor in the second test the same photon that left the B emitter?

2. Apr 9, 2010

### ZapperZ

Staff Emeritus
Does it matter? Is there something in the original photon that you can encode to identify it from other photons?

There is a section on quantum statistics called "indistinguishibility" which is applicable to bosons and fermions. In this case, you can no longer track particle by particle the way one can do with classical statistics, even if they all look the same (4 red balls). When you get into such a quantum statistics, it no longer makes sense to describe the system in such a way that you can track each individual particle.

So asking if it is the "same" photon, after it passes through a medium, can't be answered, because there is no way for me to check if it is, or if it isn't. Note that this is applicable also to reflection. Is that the same photon that bounced off the surface of a mirror? Or what about the supercurrent in a superconductor. Are those the same electrons that originally formed the bound pairs on one side of the superconductor?

When it makes absolutely no difference if it is the same ones or not, we tend to try not to worry too much about such things. If you can find some new physics that does depend on whether it does, then that's a different story.

Zz.

3. Apr 9, 2010

### guerom00

In a medium, the photon's « identity as a particle »* is lost. It behaves as a quasiparticle like a phonon. I like this analogy with the phonon. When you have a whole crystal vibrating, the question “where is the phonon?” doesn't make much sense. That's the same for a photon in a medium
Now, think of the vacuum as any other medium (after all, vacuum has a refraction index, a permittivity, a permeability, etc…)

* I don't like those words… Take this analogy with caution