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Objects with no mass

  1. Jun 7, 2010 #1
    Light has very little mass and always travels at the same speed. If I could adjust a body so that it had no mass would it remain stationary as the planet Earth flew off on its orbit around the Sun, or would it spread out evenly in all directions at the speed of light?

    Let's put aside the matter of the massive explosion :-)
  2. jcsd
  3. Jun 7, 2010 #2
    I am curious how you would "adjust" a body to change its mass at all. An object with zero mass must move at the speed of light or it wouldn't exist at all.
  4. Jun 7, 2010 #3
    That's an avenue you really don't want to go down :) I just wanted to know if objects without mass will always be travelling at the speed of light.
  5. Jun 7, 2010 #4


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  6. Jun 7, 2010 #5


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    Why not faster? I was told that the photon travelled at the speed of light but it has a very, very small mass that is almost negligible (but still exists). So why wouldn't a smaller mass go faster?
  7. Jun 7, 2010 #6


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    There is more than one sort of mass in relativity, e.g. "invariant mass" and "relativistic mass". Most physicists nowadays mean "invariant mass" when they say "mass". "Relativistic mass" is really just another name for "energy" (divided by c2) and the term is deprecated.

    Photons do have relativistic mass (as they have energy) but their invariant mass is always zero.
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