# Propagation of light

1. Apr 5, 2004

### hellfire

In special relativity it is often claimed that light does not need a medium to propagate.

In quantum field theory, particles (and thus also photons) are excitations of fields. The the position is not a dinamical variable as such anymore, but just a label for each of the field excitations in space.

Does then QTF tell us that light, as quanta of EM waves, propagates indeed over a medium (the electromagnetic field permeating the whole space)?

Thanks.

2. Apr 5, 2004

### chroot

Staff Emeritus
Photons are indeed the quanta of the electromagnetic field, and, in that sense, the electromagnetic field is the "medium" through which electromagnetic radiation travels. It is not a medium composed of matter or particles, however, which is the usual meaning of the term "medium."

- Warren

3. Apr 5, 2004

### hellfire

I see, so we can say that this is a kind of medium. Then, in analogy to waves propagating on a medium composed of matter, does this mean that one could determine or derive c depending on the properties of this medium, i.e. the ground state of the EM field? For example, how does light propagate in a Casimir vacuum? If different as in a usual vacuum, how does this fit with special relativity?

Regards.

4. Apr 5, 2004

### chroot

Staff Emeritus
You can determine c from two properties of the vacuum: the permeability and permittivity. I'm not sure what you mean by "Casimir vacuum."

- Warren

5. Apr 5, 2004

### hellfire

May be this is just a trivial point: if one considers that light is propagating through a medium whose properties are determined by the ground state of the EM field, one should be able to derive a relation between this energetic state and c (or may be not?). Thus, if one modifies the ground state (as done in the Casimir experiment, where some excitation modes are suppressed due to geometric conditions), then the propagation speed should be different (higher, I suppose). I’ve never seen such a relation or derivation, and, if what I am writing is correct, I am not sure how it fits in SR.

6. Apr 5, 2004

### chroot

Staff Emeritus
I don't see how creating a resonance condition (a cavity in which some modes of the fields fit, and some do not) will result in a change in the progation of disturbances in the field.

- Warren

7. Apr 6, 2004

### hellfire

After some search I have found this reference:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

Take a look to point 12, where this effect is mentioned. This is just a short remark, but a couple of questions arise for me if this is correct. For example, if photons travel faster in a Casimir vacuum, does this mean that the Minkowski metric is not longer valid inside the plates?

Regards.

8. Apr 6, 2004

### DrChinese

On thing to keep in mind: Special Relativity does not make a specific statement about an ether. Ditto for Quantum Mechanics. In other words, there is really nothing to reconcile as to the ether being a medium between these theories.

Is the ether simply another word for space? Perhaps, but the semantics of that do not affect the validity of SR or QM. Every theory has a range of applicability. A good theory need not explain everything, as long as it describes something usefully.