# Does E = Mc^{2}?

*Most recent answer: 07/10/2010*

Q:

If e=mc

- Anonymous (age 49)

Longmont, CO USA

^{2}then it must follow that e/m=c^{2}if this be so then is the speed of light not dependent on available mass and energy? How then, can it be constant? If another Universe had different quantities of mass and energy wouldn't the light speed differ as well?- Anonymous (age 49)

Longmont, CO USA

A:

Only if the mass, m, involved is at rest. The full equation is E

LeeH

^{2}= (mc^{2})^{2}+ (pc)^{2}where m is the rest mass of an object and p is the momentum of the object. When an object is at rest the momentum, p, is zero so the equation reduces to E = mc^{2}. When the mass is in motion you have to add in the kinetic energy term. So as far as we can tell, and this has been verified by many experiments, the speed of light, c, is the same wherever.LeeH

*(published on 07/10/2010)*