For particles with non zero mass m we know that:(adsbygoogle = window.adsbygoogle || []).push({});

E = mc^{2}γ (1)

P= mvγ (2)

with the usual meaning of the symbols.

From those two equations is easy to prove the equation:

E^{2}= (cp)^{2}+ (mc^{2})^{2}(3)

which doesn't contain the factor γ any longer and so it can even be used for particles of speed = c as photons. But is this really correct? We started from equations (1) and (2) which *are not* applicable to v = c particles.

Is there a more general way to prove (3)?

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# Question about a general way to prove E^2 = (cp)^2 + (mc^2)^2

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