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Homework Help: Relativity Questions

  1. Oct 24, 2008 #1
    For some reason (most likely lack of coffee and sleep), I can't figure out these questions. I assume they all have to do with conservation of energy and/or momentum, but I'm lame and need help.

    Explain clearly why the following processes are impossible.
    1) A single photon strikes a stationary electron and gives up all its energy to the electron.

    2)A single photon in free space is transformed into an electron and a positron.

    3) A fast positron and a stationary electron annihilate, producing only one photon.

    I also have a million other questions, but I'll keep my first post short.
  2. jcsd
  3. Oct 24, 2008 #2


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    You should keep the post about the same size, but ask only one question and give us an attempt at trying to solve it (as per the forum rules). Hint: yes, they have to do with conservation of energy/momentum. Try looking up 'four momentum'.
  4. Oct 24, 2008 #3
    Work out for each of those three what conservation of momentum AND conservation of energy [or equivalently, conservation of 4-momentum] would imply. You should find there is no way to satisfy both.
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