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Infinite mass at speed of light?

  1. Nov 5, 2012 #1
    The relativistic mass formula is m=γm, and at the speed of light, relativistic mass is infinity. But, the Lorentz factor at the speed of light is 1/0, but this is undefined, so why do physicists call this "infinity"?
  2. jcsd
  3. Nov 5, 2012 #2


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    Staff: Mentor

    They don't, and you've been victimized by the pop-sci crowd looking for easy non-rigorous gee-whiz "explanations".

    Around here, you're more likely to get responses saying that questions about the mass of an object traveling at the speed of light are meaningless, for roughly the same reason that questions about what would happen if pigs could fly are also meaningless.
  4. Nov 5, 2012 #3
    So no mass can reach the speed of light and your Lorenz factor will never produce an infinitive number.
    An example: when an observer A brings some mass M at 99% of c it's mass seems to be 7 times larger. (factor 0.7*√(c/c-v).
    For another oserver B traveling along with M the mass does not change in his reference frame. So he pushes mass M again at 99 % of c.
    Now to B mass M again seems 7 times larger, but to A it seems 70 times larger. (Also here mass is calculated as force/acceleration)
    You can try to repeat this an infinite number of times. And every time the mass M seems to grow with a factor of 10 to the original observer. But you will never reach c.
    Hope this is helpful.
  5. Nov 6, 2012 #4


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    Use of the term relativistic mass has fallen from favor among professionals. It was orginally used as a way to explain why a massive object could not be accelerated to the speed of light, but, had more confounding than explanatory power. It invited wildly erroneous conclusions - like an electron would become a black hole if sufficiently accelerated.
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