# Photon mass

1. Oct 18, 2012

### umair20

why rest mass of photon is zero? how to prove it by m=m'/√(1-v^2/c^2)?

2. Oct 18, 2012

### ZapperZ

Staff Emeritus
You cannot prove something like this via analytical methods. Please read the FAQ subforum in the Relativity forum.

Zz.

3. Oct 18, 2012

### moatasim23

4. Oct 18, 2012

### Matterwave

How would you prove the mass of the electron with that formula? You can't. Elementary particle masses are experimentally verified, not proved.

5. Oct 18, 2012

### ZapperZ

Staff Emeritus
Which part of the photon did you want me to read?

Zz.

6. Oct 18, 2012

### lightarrow

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.

7. Oct 18, 2012

### bcrowell

Staff Emeritus
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.

8. Oct 19, 2012

### lightarrow

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.

9. Oct 19, 2012