The Speed of Light: Comparing Photon and EM Wave Velocities

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Discussion Overview

The discussion revolves around the velocities of photons and electromagnetic (EM) waves, particularly focusing on the differences in their speeds in various contexts, such as in a vacuum versus a medium. Participants explore the relationship between the quantum description of light (photons) and the classical wave description (EM waves).

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the velocity of a photon is always c, as it is a massless particle, while the velocity of an EM wave in a medium is less than c.
  • Others argue that a photon is essentially an EM wave, suggesting that they are two representations of the same phenomenon, differing only in terminology (classical vs. quantum).
  • A participant questions whether the velocity of a photon differs from that of a classical EM wave, which is described as a superposition of many photons.
  • Some participants maintain that photons and EM waves move at the same speed, emphasizing that they are the same entity modeled differently.
  • There is a claim that while the speed of a photon is c in a vacuum, the speed of light in a medium is less than c, leading to confusion about their equivalence.
  • One participant suggests that mixing classical and quantum models of light leads to confusion and that this topic has been previously addressed in another thread.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are competing views regarding the relationship between the speed of photons and EM waves, and whether they can be considered the same or different in various contexts.

Contextual Notes

Participants express uncertainty regarding the compatibility of classical and quantum models of light, and there are references to previous discussions that may provide additional context.

fxdung
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Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?
 
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A photon is an EM wave. Two names for the same thing, just different languages (classical vs. quantum).
 
So velocity of photon is difference with classical EM wave(superposition of many photons)'s velocity?
 
fxdung said:
So velocity of photon is difference with classical EM wave(superposition of many photons)'s velocity?
No. Photons and EM waves move at the same speed. They are the same thing.
 
But in medium speed of photon is c but speed of light is v<c?
 
fxdung said:
But in medium speed of photon is c but speed of light is v<c?
No. THEY ARE THE SAME. It's just two different ways of modeling reality. The quantum approach (photons) is more accurate, but the classical approach (waves) is amazingly good (for something that's wrong) at solving large scale scale EM problems with easier math.

Check out this video. They'll explain it better than I can.
 
fxdung said:
Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?

You are trying to mix together a classical model of light and a quantum model of light. That doesn't work.

Furthermore, you already have an open thread where this same confusion has been addressed:

https://www.physicsforums.com/threa...t-wavetrain-corresponding-with-photon.999282/

So there's no point in having this separate thread open to rehash the same confusion.

Thread closed.
 
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