Light Propagation in Quantum Field Theory

In summary, the theory of special relativity says that light does not need a medium to propagate. Quantum field theory says that particles (and thus also photons) are excitations of fields. It is not clear how this relates to the theory of special relativity, as the two theories are not compatible. However, it is interesting to note that the theory of special relativity does not make a statement about an ether, which may be another word for space.
  • #1
hellfire
Science Advisor
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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.
 
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  • #2
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
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
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
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
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
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
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.
 

1. What is light propagation in quantum field theory?

Light propagation in quantum field theory refers to the behavior of light at the quantum level, where it is described as a particle called a photon. In this theory, light is seen as an excitation or disturbance of the underlying quantum field, and its propagation is described mathematically using equations such as the Maxwell equations.

2. How does light propagate in quantum field theory?

In quantum field theory, light propagates as a wave-like disturbance in the underlying quantum field. This disturbance can be described by a mathematical entity called a field operator, which acts on the quantum state of the system to create or annihilate photons. The behavior of this field operator is governed by the laws of quantum mechanics and the principles of special relativity.

3. What is the role of virtual particles in light propagation in quantum field theory?

In quantum field theory, virtual particles are seen as the mediators of interactions between particles. When light propagates, it can interact with other particles, such as electrons, by exchanging virtual particles. These virtual particles do not have the same properties as real particles and only exist for a very short time, but they play a crucial role in understanding the behavior of light in quantum field theory.

4. What are some applications of light propagation in quantum field theory?

Light propagation in quantum field theory has many practical applications in fields such as quantum optics, quantum computing, and particle physics. It is used to understand and predict the behavior of light in various materials and environments, and has also played a key role in the development of technologies such as lasers and transistors.

5. Are there any current challenges or controversies surrounding light propagation in quantum field theory?

One current challenge in light propagation in quantum field theory is the development of a consistent theory that can reconcile the principles of quantum mechanics and general relativity. This is an ongoing area of research and has led to various proposed theories, such as string theory and loop quantum gravity. Additionally, there is still much debate and experimentation surrounding the nature of virtual particles and their role in light propagation.

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