# E=mc^2

*Most recent answer: 06/03/2008*

Q:

If E=MC2, how could a massless particle exist? Wouldn't it have 0 energy?

- Roland Dougherty (age 60)

- Roland Dougherty (age 60)

A:

Nice question. The formula E=mc

If instead you want to describe things in terms of m

You can see that even for m

Mike W

^{2}is correct if we use Einstein's definition of m, namely the thing which you have to multiply velocity (v) by to get momentum (p=mv). This m is not the same for observers in different reference frames. An oberver who sees a larger v sees a larger m.If instead you want to describe things in terms of m

_{0}, the rest mass (observer-independent) the formula for the same fact becomes E^{2}=m_{0}^{2}_{ }c^{4}+p^{2}c^{2}, where c is the speed of light.You can see that even for m

_{0}=0 (zero rest mass), you can still have energy E=pc. There's stll momentum, p, and thus still the other type of m: m=p/c=E/c^{2}for these massless particles travelling at c.Mike W

*(published on 06/03/2008)*