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Q & A: E=mc^2

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Most recent answer: 06/03/2008
If E=MC2, how could a massless particle exist? Wouldn't it have 0 energy?
- Roland Dougherty (age 60)
Nice question. The formula E=mc2 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 m0, the rest mass (observer-independent) the formula for the same fact becomes E2=m02 c4+p2c2, where c is the speed of light.
You can see that even for m0=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/c2 for these massless particles travelling at c.

Mike W

(published on 06/03/2008)

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