Derivation of Minkowski norm of the four-momentum

1. Jan 16, 2016

I have attached a derivation of the Minkowski norm of the four-momentum but just don't quite see how the writer arrived at $-m^2 c^2$ from what was given. How exactly does this quantity follow from $-\frac {E^2}{c^2} + p^2$? I feel like it might be very obvious, so any explanation would be great!

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2. Jan 16, 2016

Staff: Mentor

I would say that it is a definition rather than a derivation. We use the norm of the four momentum often enough that it deserves a name, so we define the name "mass" to refer to it.

3. Jan 19, 2016