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Q & A: E=mc^2 and conservation

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Most recent answer: 10/22/2007
Q:
Does e=mc2 defy the laws of the law of conservation of energy?
- Shona
Singapore
A:
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.

Mike W.

(published on 10/22/2007)

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