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Mass, masslessness and the speed of light

  1. May 7, 2014 #1
    As far as I understood the relativity:
    1. Massless particles travel at the speed of light.
    2. As the speed increases, so does the mass of the moving body. It goes to infinite as the speed gets close to that of light.

    My questions: Why would particles with infinite mass (theoretically) move at the same speed as the massless particles? What's the connection between these two kinds?

    You may say, it is impossible to reach infinite mass, but anyway, why does mass of let's say electron moving at 90% speed of light gets very big, while the mass of the photon which travels at 100% of the speed of light is zero?
    Last edited: May 7, 2014
  2. jcsd
  3. May 7, 2014 #2
    It will never reach the speed of light.

    They don't because there is no such thing as infinite mass and particles with mass will never reach the speed of light.
  4. May 7, 2014 #3
    ok, it was wrong choice of words on my side. edited now.
  5. May 7, 2014 #4


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    Staff: Mentor

    You're talking about two different kinds of mass here.

    1. refers to what is often called "rest mass" (which sounds silly for a particle that can only travel at the speed of light, but that's the way it is, for historical reasons) or "invariant mass". Most physicists call it simply "mass."

    2. refers to what is often called "relativistic mass." Most physicists don't use it, except for a while during the early history of relativity. Nevertheless, it still appears in many books for laymen, and in some low-level textbooks.
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