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## E = mc2 Probably the most famous equation in all of physics. The...

20 Apr 2012, 19:15 UTC
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E = mc2
Probably the most famous equation in all of physics. The energy of a particle is given by it’s mass times the square of the speed of light.
But is it correct?
Actually, it isn’t! The full equation, that comes out naturally from the postulates of special relativity, is:

The p term is called momentum. It is proportional to the velocity in space, so that the greater the speed of an object the greater it’s momentum. Also we can think about energy as the time counterpart of the momentum.
If we allow the particle to be at rest it will have zero speed, so zero momentum, and we can see that the full equation turns back into the more familiar one, that is in fact only valid for particles at rest.
The equation helped explain many mysteries at the time. When Ernest Rutherford discovered the radioactive decay, for example, where the energy of the radioactive process came from was an open question. Using Einstein’s equation we see that for radioactivity to take place the object must drop in mass.
The mass-energy equivalence, as the equation is called, has remained incredibly successful in explaining our universe since Einstein’s discovered ...