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B Why does light not have infinite mass?

  1. Mar 31, 2016 #1
    I have read in the book 'A Brief History Of Time' that other bodies can't reach the speed of light because as its speed gets near the speed of light, it gets infinite mass and requires infinite energy to reach the speed of light. So light itself should be having infinite mass and would be requiring infinite amount of energy. But it does not have these properties. Why?
  2. jcsd
  3. Mar 31, 2016 #2


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    Light has zero rest mass so that notion doesn't apply. Light does have momentum.


    They estimate that you would need 30 billion laser pointers hitting a penny to have enough force to move it.

  4. Mar 31, 2016 #3


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    Take a look at the physical properties of a photon. In particular, the full version of the relativistic equation
    As jedishrfu stated, light doesn't have mass. It's energy is just E = pc.
  5. Mar 31, 2016 #4
    So photons doesn't have mass right?
  6. Mar 31, 2016 #5


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    There's no such thing as infinite mass or infinite energy.
  7. Mar 31, 2016 #6
    So what does it actually mean or state, sir?
  8. Mar 31, 2016 #7
    Instead of thinking about motionlessness being the default state and energy being required to move, think of it more like this: light speed is the default state, and things with mass experience drag. No matter how hard you push against something, you can never remove the effects of the drag, just keep adding more power until that drag is negligible, so you can never move as easily as something that does not experience that drag.

    This is not unlike reality, in fact, there was a brief point in time where everything: photons, gravity, quarks, electrons, all moved at the speed of light. The Higgs field hadn't settled yet, so nothing had mass. Since the Higgs field turned on however, everything with mass have been dragging along through it.
  9. Mar 31, 2016 #8


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    You could say that no finite amount of energy would accelerate a particle to the speed of light. Which is equivalent to saying it is physically impossible.
  10. Mar 31, 2016 #9


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    And furthermore, the speed of anything is just:

    [tex]v = {pc^2 \over E}[/tex]

    Since [itex]E = pc[/itex] for photons, [itex]v = c[/itex]. For anything with mass, though the energy is always greater than [itex]pc[/itex], so it always has a speed less than that of light.
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