# A photon's speed

1. Feb 26, 2009

### captain

I have seen in a description of QED that it predicts that light can move faster or slower than the speed of light, but on average it moves at the speed of light. I wanted to know if htis was true and if it is, is the reason behind it due to the fact that the phase velocity (since we are dealing with only a photon) of light can be slower or faster than c, but the group velocity (that of a wave packet, or a group of photons) has to be c. If my description is correct then it seems that classical electrodynamics is explaining how a photon can more faster or slower than the speed of light. Is there any connection between how this classical description fits a quantum description. (This is however assuming that all I have said is correct so far. There could be errors.) Thanks in advance to whoever answers any of these questions.

2. Feb 26, 2009

### map19

Well you can't argue with Feynman, he's been elevated to Godlike status.

he invented a method of integrating all possible paths called "sum over histories"
http://en.wikipedia.org/wiki/Path_integral_formulation

he says that the light travels all possible paths at all possible speeds (and impossible speeds) so that light not only travels from your light bulb to your eye in a straight line, but also goes via the walls and ceiling at -C and +C.
All of which cancel each other out so that you are only left with the direct route at C.

That's very convenient. But not very convincing.

3. Mar 1, 2009

### Piscuit

So there is argument that the speed of light is an average? I know that light has different wavelengths and thus appears to us as color. Please give a summary of aforementioned article, it looks like it would take 3 years to read.

4. Mar 1, 2009

### lightarrow

I'm wondering if in the Feynmann's path integral formulation, all paths have to start simultaneously; in case it's not, then there is no need to talk of faster than light propagations.

Last edited: Mar 1, 2009
5. Mar 1, 2009

### confinement

Feynman's path integral method is equivalent to ordinary canonical quantum physics, so there is zero concern that the theory is outlandish or speculative.

Calling it an average is overly simplistic, in fact it is always wrong to ascribe trajectories to photons: they are quantum mechanical particles, and so their position and momentum are not simultaneously observable. If you want to think of light as traveling along some path, then you are thinking of a classical ray of light.

6. Mar 1, 2009

### Piscuit

I read it, (it is now 3 years later) and I understand basically what you mean. Light particles can travel all possible paths from point A, to point B. So trying to measure the distance that photon actually travels is impossible to measure. Because that light particle is bouncing EVERYWHERE.