# Special relativity and rest mass

1. Sep 21, 2007

### Qyzren

What is the rest mass m of a particle traveling with the speed of light in the laboratory frame?

i believe m = (E² - (pc)²)^.5 / c² is the correct equation to use?
as velocity goes up, so does energy and momentum? so when velocity is at speed of light, E = infinity and momentum = infinity? so we have infinity - infinity on the numerator? so this gives mass = 0? is any of this correct? thanks for help.

2. Sep 21, 2007

### Gokul43201

Staff Emeritus
The answer turns out to be correct, but the method is not. For one thing, you can not simply subtract off infinities in the manner that you just did.

What is the formula for the relativistic energy of a particle of rest mass, m? Write this down. Now if v=c, this gives you the result you (incompletely) stated above, namely that E = infinity, unless _______ (fill this in).

Ask yourself: Is it physical for a particle to be given an infinite amount of energy?