# Waves having energy with no mass

1. Aug 7, 2014

### Scimaster

Waves have energy without mass which can't be true in Einstein's equation
E=mc2
If you work out that E=0 (mass of waves) times 300,000 c2
then its zero.

Please can someone tell me why waves can have energy but without mass which in einstein's equation cannot be right.

Thank you

Scimaster

2. Aug 7, 2014

3. Aug 7, 2014

### Staff: Mentor

The proper equation is
$$E^2 = (p c)^2 + (m c^2)^2$$

I guess that when you talk about "waves without mass," you mean light (photons). Photons have momentum, $p^2 >0$, so $E^2 > 0$.

4. Aug 7, 2014

### Scimaster

I was talking about Gamma rays where they have energy without mass

5. Aug 7, 2014

### Staff: Mentor

It's electromagnetic radiation, so my reply stands. Gamma ray photons have momentum, thus energy.

6. Aug 7, 2014

### Staff: Mentor

Gamma radiation, like all electromagnetic radiation, has energy and no rest mass - but it's not at rest. If you were to take a box with a perfectly mirrored interior, so that light would bounce back and forth inside.... Its mass will be very slightly greater if there is light bouncing back and forth inside than if there were not. As the light is absorbed its energy will be absorbed into the walls of the box as heat, and the mass of the box will not change bevause there's still the same amount of energy in the box.