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Homework Help: Proton passes the Milky Way galaxy, find its energy?

  1. Oct 11, 2005 #1
    for the following question:
    In its own frame of reference, a proton takes 5min to cross the Milky Way galaxy, which is about 10^5 light-years in diameter.
    What is the approximate energy of the proton in electronvolts?

    my problem:
    however, i can't calulate v on the calculator, because v is too small...
    i was trying to use E=(1/2)mv^2, where m is the photon rest mass...
    is that right?

  2. jcsd
  3. Oct 11, 2005 #2

    Tom Mattson

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    This looks like algebraic soup! Why, oh why, are you combining these quantities together in this way? From what equation are you working?

    It looks like:

    [tex]vt=\frac{Dc}{\gamma ^2}[/tex]

    where [itex]v[/itex] is the proton speed in the galaxy frame, [itex]D[/itex] is the galactic radius in the galaxy frame, and [itex]t[/itex] is the travel time in the proton frame.

    No, it's not right. The photon rest mass is zero.
    Last edited: Oct 11, 2005
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