Can we compute the relativistic velocities of 2 photons or 2 neutrinos ?
Can you give a little context? What do you mean by "relativistic velocity" and which two photons or which two neutrinos do you have in mind?
Any two that originate from the same source.
The speed of photons is c. Neutrinos may have any speed < c.
Does this imply that neutrinos have rest mass ?
The implication goes in the other direction. Neutrinos have mass, therefore they can have any speed < c.
Yes, neutrinos have rest mass.
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