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Does e=mc2 defy the laws of the law of conservation of energy?
The famous formula E=mc^2 really says that mass and energy are the same
thing, but measured in different units. It's as if you'd been going
around thinking that everything had two properties, 'feet' and
'inches', and then noticed that 'inches' was just always 12 inches/foot
times 'feet'. So the separate laws of conservation- one for mass and
the other for energy- are now merged into one law. One form of energy
(rest mass) had once seemed to have separate conservation law from the
others, but now it is known that rest mass can be converted back and
forth to other forms of energy.
If you happen to read modern descriptions, the word 'mass' is often
used to refer only to rest mass, unlike the way Einstein used the
words. That makes this discussion a little more complicated, but the
same idea remains true.
(republished on 07/23/06)
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