Rectifier wrote:

...
No, actually, a little fella named Albert Einstein made it up. An
electron and a positron have mass. When they come together and
annihilate, they turn into pure energy (two 511 KeV photons travelling
in opposite directions if I remember right), which has no mass. That's
what E=MC^2 predicted; and that's what happens in certain radioactive
decays all the time. Positrons get produced by the deceleration of
neutrons which come too close to the nucleus of an atom with large
mass. They then annihilate when coming close to an electron. This is
just one example.
...

Actually, we only wish things were/are that simplistic. There is then
"the other story" (hey, did I just see Paul Harvey in here?)

http://en.wikipedia.org/wiki/Mass-energy_equivalence

Time and movement are very much in play, as are "kinetic energy factors"
.... in a nutshell, mass to energy is much "easier" process than energy
to mass -- and, certainly, much easier to compute/define/determine.

Regards,
JS