Are photon energies constant for any observers?

In summary: However, the relative motion between the emitter and the absorber will cause a shift in the frequency of the photons, which is detected as a shift in their energy.
  • #1
jcap
170
12
Title should be: Are photon energies constant along null geodesics? [Mentor's note: Title corrected]

As I understand it in general relativity the paths of light rays, given by null geodesics, are scale invariant.

Is that correct?

Now Maxwell's equations in flat Minkowski spacetime are also scale invariant.

General curved spacetime is locally flat in the vicinity of any point along a trajectory in spacetime.

Do these facts imply that the energy/momentum of photons is actually constant along the trajectory of the light beam?

Observers themselves and their measuring equipment do have fixed length scales so that their energies are not scale invariant. The energies of the emitting and absorbing atoms change but the energy of the photon itself is constant. This would be an explanation of the gravitational redshift effect without the assumption of changing photon energy.

To summarise: the scale of a photon is set purely by the emitting atom - after emission neither Maxwell's laws nor general relativity change the photon's scale.
 
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  • #2
However observers modiffract different speeds relative to one another will measure different energies for the photon as they measure different frequencies. This is just the Doppler effect at work.
 
  • #3
Nugatory said:
However observers modiffract different speeds relative to one another will measure different energies for the photon as they measure different frequencies. This is just the Doppler effect at work.

What happens if the emitter and the absorber are both locally in free-fall frames?

Would there be no Doppler effect in this case - just a difference in energy due to the different clock rates at emitter and absorber?
 
  • #4
jcap said:
What happens if the emitter and the absorber are both locally in free-fall frames?

Would there be no Doppler effect in this case - just a difference in energy due to the different clock rates at emitter and absorber?

There is still Doppler if there is relative motion.
 

FAQ: Are photon energies constant for any observers?

1. What is a photon?

A photon is a fundamental particle of light that carries energy and has no mass. It is the basic unit of all electromagnetic radiation, including visible light, radio waves, and X-rays.

2. Are photon energies constant for any observers?

According to the theory of relativity, the energy of a photon is constant for any observer, regardless of their relative motion. This is known as the principle of energy conservation.

3. How is the energy of a photon calculated?

The energy of a photon is calculated using the formula E = hf, where E is the energy, h is Planck's constant, and f is the frequency of the electromagnetic wave. This means that the energy of a photon is directly proportional to its frequency.

4. Do all photons have the same energy?

No, the energy of a photon can vary depending on its frequency. Higher frequency photons have more energy than lower frequency ones. This is why different types of electromagnetic radiation, such as visible light and X-rays, have different energies.

5. Can the energy of a photon change?

Yes, the energy of a photon can change through interactions with matter. For example, when a photon is absorbed by an atom, it can transfer its energy to an electron, causing it to jump to a higher energy level. This can also happen in reverse, where an electron releases a photon when it drops to a lower energy level.

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