Nice question. The formula E=mc
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
02 c
4+p
2c
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)
