why rest mass of photon is zero? how to prove it by m=m'/√(1-v^2/c^2)?
You cannot prove something like this via analytical methods. Please read the FAQ subforum in the Relativity forum.
REad about Photon!
How would you prove the mass of the electron with that formula? You can't. Elementary particle masses are experimentally verified, not proved.
Which part of the photon did you want me to read?
In addition to the other answers, if photon's mass were different than zero, it should be infinite (according to the formula you have written) because v = c for a photon; its energy would be infinite too. Clearly that is a nonsense.
If the photon's mass is nonzero, then it doesn't have v=c.
The mass of the photon can only be established experimentally.
R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters , 1998, 80, 1826-1829 http://silver.neep.wisc.edu/~lakes/mu.html
Luo et al., “New Experimental Limit on the Photon Rest Mass with a Rotating Torsion Balance”, Phys. Rev. Lett, 90, no. 8, 081801 (2003)
Zz, I'd suggest adding these two references to the FAQ entry.
Of course. But the answer one can give, can also depend on the level of answer the OP was looking for: starting from the fact that a photon's speed is c, then a photon's mass different than zero generate an impossible situation.
Just because you cannot divide by zero does not prove that photons must possess a nonzero mass. Einstein's ideas concerning mass and speed are not just idle speculations, they have been verified through experiments. Keeping this in mind:
Since photons, by definition, travel at the speed of light, if they did possess ANY mass their mass would have to be infinite. A particle the size of a photon with INFINITE mass would create a singularity that would consume everything it touched. Since this is clearly not the case, photons cannot have any mass.
The point is that we don't know that a photon's speed is c. We can only put a lower limit on its speed for a given energy.
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