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Exactly what is momentum?

  1. Jan 16, 2012 #1
    Hi there!

    Question on momentum in SR.
    I'm trying to understand this holistically...

    From what I understand... p=mv is an approximation.
    when we look at the energy equation for a photon E = pc.
    Since a photo is massless, this momentum p is an intrinsic feature of a particle apart from its mass...

    So...

    1. Why is it that we can use the mass of electrons/chairs/planets to calculate their momentum? Is there an a prior reason that the two are equivalent?

    2. How does this momentum in SR relate to the operator momentum in QM?

    Thank you!
     
  2. jcsd
  3. Jan 16, 2012 #2

    Simon Bridge

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    Because that makes the math easiest - these objects are not normally relativistic. Momentum is otherwise calculated from the total energy - [itex]E^2 = m^2c^4 + p^2c^2[/itex] which works for massive and massless particles.
    No. Photon momentum was a later discovery. However, iirc, it turns out to be related to fundamental symmetries in space-time.
    The momentum in SR is the expectation value of the QM momentum just like normal.
     
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