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Can Something Really Be "mass less"?

  1. Nov 24, 2014 #1
    Well if we consider that "something" can be really "mass less" , would it be correct to consider it to move at the speed of light?

    If yes, then I have a little doubt.
    Let's say it moves at the speed of light , then if we apply the mass equation (mass=[(rest mass)/squared root(1-(v^2)/(c^2))] , then we should end up at an indeterminate form , saying that the mass of that "mass less" body is indeterminate when in motion. Are we right to say this? How can mass be indeterminate ?

    If no, then what perhaps that "mass less" body would be doing around?
  2. jcsd
  3. Nov 24, 2014 #2
    I am also confused... How can mass be zero ? Where does all the atoms in our body go?
  4. Nov 24, 2014 #3
    Yes. Consider photons.
  5. Nov 24, 2014 #4
    But then what about mass being indeterminate?
  6. Nov 24, 2014 #5
    The equation you quoted is only used for particles that have rest mass.And anything with rest mass CANNOT travel at the speed of light.As you've noticed, putting v=c yields infinity so you'll need infinite energy to accelerate something with rest mass to the speed of light.For a photon, the equation's a bit different.You can use λ=h/p, where h is the Planck's constant, P momentum and λ wavelength.As can be noticed, even something with no rest mass can have momentum.Hope it answers :)
  7. Nov 24, 2014 #6
    "Indeterminate mass" in the case of photons means they don't have mass.
  8. Nov 24, 2014 #7


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    I would say that the indeterminate (relativistic) mass given by the equation means that the relavistic mass (aka total energy) of something with zero rest mass moving at light speed is not determined by its speed and rest mass alone. If you want to calculate the energy of something like a photon you would have to use some other information. Such as its frequency, wavelength or momentum.
  9. Nov 27, 2014 #8


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    When physicists (particle physicists in particular) talk about mass, we generally talk about the invariant mass (or rest mass) of a particle. Relativistic mass has largely fallen out of fashion and is interchangeable with total energy content. We have an https://www.physicsforums.com/threads/what-is-relativistic-mass-and-why-is-it-not-used-much.783220/ [Broken].
    Last edited by a moderator: May 7, 2017
  10. Nov 27, 2014 #9


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    This is not the mass equation. The correct mass equation is:

    ##m^2 c^2 = E^2/c^2-p^2##.

    For a particle with non-zero mass the correct equation is equal to the equation that you posted, but only for a particle with non-zero mass. For a particle with zero mass you instead use the general mass equation to determine the momentum: ##p=E/c## which obviously satisfies ##m^2 c^2 = p^2-p^2 = 0##.
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