Quote:
Originally Posted by DonRotolo
OK, does a photon have mass? If yes, then it cannot travel at the speed of light, but since it does, at rest it must be massless. (Yes, it has momentum, but that's a special case. In theory you can approach the speed of light using a flashlight as your propulsion unit. Might take a while though...)
Are there any nonzero-mass particles/objects that travel at c? Can there be?
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The fact that photons travel at the speed of light is exactly what originally motivated the fact that they are massless.
Using a flashlight for propulsion would definitely work (albeit very slowly) because of the momentum of the outgoing photons, but it is no different than any other Newton III interaction; you will still accelerate only asymptotically to the speed of light.