Can Single Photons Ever Exceed the Speed of Light?

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Discussion Overview

The discussion centers around the question of whether single photons can exceed the speed of light, exploring concepts related to the speed of light in a vacuum, the nature of photons, and the implications of quantum mechanics on their behavior.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the speed of a single photon is always less than or equal to the speed of light in a vacuum (c), regardless of the number of photons.
  • Others discuss the distinction between phase velocity and group velocity, suggesting that the group velocity of electromagnetic waves is defined as c, but the behavior of a single photon may imply a different understanding.
  • A participant notes that while the position of a photon is uncertain in quantum mechanics, the speed measured from emission to reception in a vacuum is always c.
  • There is a query regarding the definition of "vacuum" in the context of the speed of light, highlighting a lack of rigorous definitions in the literature.
  • Another participant introduces the idea that classical and modern physics may describe photons differently, suggesting that measurements in classical terms may not fully capture the quantum nature of photons.

Areas of Agreement / Disagreement

Participants do not reach a consensus; multiple competing views remain regarding the nature of photons and their speed, as well as the definitions and implications of "vacuum" in this context.

Contextual Notes

The discussion reveals limitations in the definitions used, particularly concerning the term "vacuum" and the interpretations of classical versus quantum descriptions of photons. There are also unresolved questions about the implications of measurement on the properties of photons.

toe21k
The Speed of Single Photon is less than c [?]
 
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C is the speed of light in a vacuum. This does not depend on the number of photons.
 
phase velocity and group velocity

c is defined as group velocity of EM wave:
[tex]\frac{\omega}{k}[/tex]
I think c of one photon is less meaning.
From the point fo quantum,position of one particle is uncertain.
 


Originally posted by nocturn
From the point fo quantum,position of one particle is uncertain.
That is true, but only while in transit. The speed measured between when photon is emitted and received (in a vacuum) is always exactly C.
 
russ_watters,

That is true, but only while in transit. The speed measured between when photon is emitted and received (in a vacuum) is always exactly C.

What do you mean by "vacuum", as you used it in that paragraph? I mean, the term is always used when talking about the SOL, but I don't think I have ever read a rigorus definition of "vacuum" when it is used in conjunction with the SOL.
 
It is sure media will effect on particles.I think there are two different phrases in physics, classical and modern.Here,measurement the former,photon the later.QM says photon is spreading in space. One can get its state function also its probability of some classical quantities,energy,momentum,etc. and other,spin,degenerate... which have not corresponding concepts in class.But when we measure,an action of classical,photon will collapse and has a certain E,M,Position,etc. In this meaning, photon is not a photon of "quantum",only classical.
 

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