# Question regarding photons and mass

1. Sep 8, 2011

### Molecule

Sorry if this is a noob question or if it has been answered before or if I'm just misunderstanding the physics involved but this has been keeping me up at night and it'd be nice to have it resolved.

If E=MCsquared implies that greater energy equates to greater mass in a particle, how is it that a photon, or any other particle for that matter, has no mass?

2. Sep 8, 2011

### WannabeNewton

The equation you listed is only valid for particles in their rest frames. A massless particle has no rest frame. If you were talking about a massive particle, which does have a rest frame, then that equation would be fine to use. The full equation is $E = \sqrt{(mc^{2}) + (pc)^{2}}$ where m is the rest mass. As you can see, for particles with no rest mass (or, in related terms, no rest frame), E = pc.

3. Sep 8, 2011

### agmateo

How does tachyon move faster than light

4. Sep 9, 2011

### BruceW

Its a good question. The actual energy of a moving particle is $\gamma mc^2$ and for a massless particle like a photon, $\gamma$ is infinite and m is zero. So we can't use this equation to evaluate the energy of a photon.

The equation for energy can be re-written as: $E=\sqrt{p^2c^2+m^2c^4}$ (WannabeNewton made a slight mistake). So From this equation, the energy of a massless particle can be seen to be: $E=pc$ (as WannabeNewton said).