# Computing magnitude of any 4-vector

1. Sep 27, 2014

### Jonsson

Hello there,

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

2. Sep 27, 2014

### Staff: Mentor

It is 0. You did it correctly. The magnitude of the four momentum is the mass and light is massless.