Quote:
Originally Posted by tusenfem
You forget that change of momentum, dp/dt, be it classical or relativistic is always caused by a force and always leads to acceleration.
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If I told a physicist that a vertically moving photon was accelerated by gravity they would tell me to go back to school.
Its momentum is affected as is its frequency but the term accelerated is not used. Even in the case of a horizontal photons, I had a world expert in GR object to my use of accelerated for the bending of light. I said that the vector of its velocity was altered and that is acceleration. He didn't like it being called acceleration. So I said to him to tell me what to call that, but it was what happened whatever you called it.
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Whether or not the acceleration changes the magnitude of the momentum depends on the direction of the force, example:
I have an electron in a magnetic and electric field at an angle φ.
If φ = 0 degrees, then the electron will be accelerated along the magnetic field line from 0 to whatever value, which corresponds to the potential drop along the field line.
If φ = 90 degrees, then the electron will be accelerated perpendicular to the magnetic field, and the Lorentz force starts to work, bending the electron around the magnetic field lines. The gyrating electron feels a constant acceleration towards the field line, centripital force, which constantly changes the direction of the momentum, but not the magnitude, as the force cannot do any work on the electron.
Apart from this gyration, there is also the ExB drift, at constant velocity perpendicular to the electric and the magnetic field, which I will just let be for the moment.
If φ is neither of these values, then a combination of the two will happen, acceleration along the field (changing momentum magnitude) and rotation around the field (changing momentum direction).
Consider your horizontal photons like the gyrating electrons
Consider your vertical photons like the field aligned electrons
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Well I just look at this as the components in the three spatial directions. We have to keep track of what happens in 3D. But what happens is slightly different at different points in the Sun. In addition to the normal tidal forces there is this GR 2x (or 5/3x) effect on the relativistic content. Because of very frequent interaction with matter, the photon momentum and matter momentum are thoroughly mixed. So we can treat them as a single fluid as far as momentum per unit mass goes. Yes, that is acceleration for sure, but during the workings we have to use acceptable GR language. The fact that this case has never actually been described before means that we have to look for a way to make it very clear. But don't forget the vertical photons which cannot be referred to as accelerated.