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On "why massless particles move at the speed of light"
It has come up a few times whether you can derive that massless particles must go the speed of light, strictly using SR. Bcrowell proposed a way that some argued against. I have a different tack for consideration.
I recently derived that, for any particle with E0 (rest energy) > 0, the following is true:
(1 - v^2/c^2) = 1 - KE / (KE + E0)
From this you can say:
1) limit as E0 -> 0 leads to v=c
2) If there is to be such a thing as a particle with E0=0 and nonzero KE, and if it should be consistent with the SR, then this relation directly requires that v=c.
It has come up a few times whether you can derive that massless particles must go the speed of light, strictly using SR. Bcrowell proposed a way that some argued against. I have a different tack for consideration.
I recently derived that, for any particle with E0 (rest energy) > 0, the following is true:
(1 - v^2/c^2) = 1 - KE / (KE + E0)
From this you can say:
1) limit as E0 -> 0 leads to v=c
2) If there is to be such a thing as a particle with E0=0 and nonzero KE, and if it should be consistent with the SR, then this relation directly requires that v=c.