Why we know average speed of single photon equal speed of EM wave?

In summary: Photons are described by quantum field theory and have properties such as energy, momentum, and spin, but not mass. The speed of a photon is always equal to the speed of an electromagnetic wave, which is the speed of light, according to experimental evidence. If the average speed of a single photon were smaller than the speed of light, it would imply the existence of massive photons, which is not supported by any evidence. The concept of massive photons would also violate the U(1) gauge symmetry in quantum electrodynamics, which states that the photon must have zero mass. In some conditions, such as in a medium, the speed of light may appear to be slower, but this is due to interactions with the medium and does not change the
  • #1
fxdung
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Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
 
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  • #2
Do you mean photons have variance in their speed ? I have not heard about it.
 
  • #3
fxdung said:
Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
What we say we know is determined by experimental evidence and the success of various theories we construct to predict these results. All measurements known support the notion that the photon is massless. One cannot really say more.
 
  • #4
It is U(1) gauge symmetry in QED(that say m of photon equal 0)(I only know this at just this moment with QFT book) .But I do not clear why the term contains m must be square of gauge field the Lagrangian?Does motion equation for gauge field to be able reduce to Klein-Gordon equation?
I hear in some condition photons are massive(?). Is in this case gauge symmetry violated by enviroment?
 
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  • #5
fxdung said:
average speed of a single photon(in point particle view)
There is no such thing; there is no valid "point particle view" of a photon.
 
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1. What is the average speed of a single photon?

The average speed of a single photon is equal to the speed of an electromagnetic (EM) wave, which is approximately 299,792,458 meters per second in a vacuum.

2. How do we know that the average speed of a single photon is equal to the speed of an EM wave?

This has been confirmed through numerous experiments and observations, including the famous Michelson-Morley experiment and the more recent experiments involving quantum entanglement. Additionally, the speed of light is a fundamental constant in physics and is a key component of the theory of relativity.

3. Why is it important to know the average speed of a single photon?

Knowing the average speed of a single photon is crucial in understanding the behavior of light and how it interacts with matter. It also has practical applications in fields such as telecommunications, where the speed of light is used to transmit information.

4. Is the average speed of a single photon always equal to the speed of an EM wave?

Yes, the average speed of a single photon is always equal to the speed of an EM wave in a vacuum. However, in certain mediums such as water or glass, the speed of light may be slower due to interactions with the particles in the medium.

5. Can the average speed of a single photon be faster than the speed of light?

No, according to the theory of relativity, the speed of light is the maximum speed at which any object or information can travel. This applies to single photons as well, so the average speed of a single photon cannot be faster than the speed of light.

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