Originally Posted by **Ziang** I heard of something called Einstein's box which could yield the equivalence without working with the gamma factor. Is it true? |

Einstein used more than one boxes in his writing. You're thinking of the one where he deduced the equivalence of mass and energy of electromagnetic radiation. There is no gamma factor in that derivation since there's no gamma factor when E = mc^2 relates the mass and energy of EM radiation.

You can follow the derivation at my website at

Einstein?s Box
In this page I derived the equivalence of mass and energy for tardyons (particles that move at speeds v < c)

Mass Energy Equivalence
Caution: There are a few mistakes in that page which I haven't corrected yet. I referred to equation numbers which aren't in that page. I'll eventually fix hem.