# Please save me from my idiocy

1. Jun 29, 2007

### Kap160

I've recently bean reading a book about string theory (thats not important though) and I came across something that someone here may be able to help me with, I'd really appreciate it, its been bugging me for days.

I may have misinterpreted, so if I have please correct me. In a general introduction the book described the relationship between mass and energy as described by E=Mc2. It also talked about sub-atomic particles, among which there was a small passage on the Graviton (and the lack thereof). It said that it theretically has no mass. If this is so does it also mean that it has no energy? I'm probably totally wrong and will be summarily chastised for my ridiculous question, but I would be eternally greatful if someone could explain this to me. Also if you could do it as if you were talking to a rather stupid child I would appreciate it.

Thanks for any clarification that can be given.

2. Jun 29, 2007

### Staff: Mentor

Massless particles--a photon is another example--have no rest energy. They certainly have energy. (These massless particles are also never at rest--they all travel at the speed of light.)

3. Jun 29, 2007

### nrqed

This is a frequent question. The problem is that E = mc^2 is a special case applicable only to massive particle (and when those particles are at rest). The real equation is $E = \sqrt{c^2 \vec{p}^2 + m^2 c^4}$. For a massless particle, this reduces to $E = c p$ (where p here is the magnitude of the three-momentum).

4. Jun 30, 2007

### Kap160

Thank you, you have saved an idiot from himself. They are bit like me, when they are moving they have enormous energy but when stationary they simply can't be bothered to get up and put the kettle on.

Ah... that was slighty above my level of enforced stupidity, although I appreciate the effort...

My thanks to both, I am eternally grateful.