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Easy Relativity Question

  1. Apr 6, 2005 #1
    Maybe its because my exams are close and I'm starting to panic but there is a question that has been bustin' my balls all day and was hoping any of you could help....

    A particle with mass 10eV and a photon are produced simultaneously in a galaxy 150,000 light yrs away. The energy for both particles is 10MeV. Estimate the difference in arrival time between the particles.

    My question really is: Is the energy for the photon a red herring because surely this travels at speed c and you don't need this info?!

    Thanks in advance people cleverer than me
  2. jcsd
  3. Apr 6, 2005 #2


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    So the photon has 10MeV (ignoring GR effects),and that is not a useful piece of information.It should arrive in exactly 150,000 years.

    What about the electron...?The time that is needed is in the Earth's reference frame...

  4. Apr 6, 2005 #3
    you are gonna need the relativistic formula for energy. you know, the Einstein-formula(E² =p²c² + m²c^4). Also be sure to know wether the given mass is restmass or not...

    For a photon E =pc since the restmass is ZERO

  5. Apr 6, 2005 #4


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    You needn't momentum & stuff.

    The gamma factor is 1000.That should give you the velocity,which is all u need.

  6. Apr 7, 2005 #5
    cheers for your help guys.
    Once again much appreciated
  7. Apr 7, 2005 #6
    dextercioby said: "The gamma factor is 1000."

    How do you get that?
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