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Given any 4-vector ##x = (x_0,x_1,x_2,x_3)##, hvor do I compute its magnitude? I couldnt find a simple explaination online.

For example I want to compute the magnitute of the momenergy 4-vector of a photon traveling in the positive x-direction where ##E## is the energy of the photon and ##P_\mu = (E,E,0,0)##. What is incorrect in the following computation of its magnitude?

$$

(P_\mu\,P^\mu)^{1/2} = (E^2-E^2)^{1/2} = 0

$$

Surely the magnitude of the vector should not be ##0##? Can you help? Thank you for your time.

Kind regards,

Marius

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# Computing magnitude of any 4-vector

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