Does a photon in vacuum have a rest frame?

Bernard Boyer 2022-07-23 Answered
Does a photon in vacuum have a rest frame?
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Answers (1)

akademiks1989rz
Answered 2022-07-24 Author has 16 answers
Short answer: no.
Explanation:
Many introductory text books talk about "rest mass" and "relativistic mass" and say that the "rest mass" is the mass measured in the particles rest frame.
That's not wrong, you can do physics in that point of view, but that is not how people talk about and define mass anymore.
In the modern view each particle has one and only one mass defined by the square of it's energy--momentum four vector (which being a Lorentz invariant you can calculate in any inertial frame):
m 2 p 2 = ( E , p ) 2 = E 2 p 2
For a photon this value is zero. In any frame, and that allows people to reasonably say that the photon has zero mass without needing to define a rest frame for it.
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