Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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...


    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

    User Avatar
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook