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Computing magnitude of any 4-vector

  1. Sep 27, 2014 #1
    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. jcsd
  3. Sep 27, 2014 #2

    Dale

    Staff: Mentor

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