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Acceleration & Mass relationship

  1. Jul 28, 2009 #1
    Apologies in advance if this question is either stupid or has been asked frequently. I have googled and googled but cannot find a answer.

    Special relativity says that when a body accelerates its mass increases and its length decreases. This continues until the body approaches the speed of light, once there its mass has become infinite and its length has become zero.

    So then, if a hypothetical spacecraft was travelling at the speed of light would it not become a singularity? If so, would this just be a singularity for the occupants of the spacecraft or for the whole Universe?

    If not, would somebody be able to explain why not or direct me to a resource for further reading.

    Many thanks
  2. jcsd
  3. Jul 28, 2009 #2
    Spacecraft cannot travel at the speed of light. Spacecraft traveling at nearly c do not become black holes either.

    YOU are currently traveling at nearly c in some reference frames, and yet you are not a singularity.
  4. Jul 28, 2009 #3

    Yes, I'm fully aware that spacecraft, or for that matter, anything with positive mass cannot travel at anything like c. It's why I used the word "hypothetical" I also have a reasonable understanding of the reasons why not too!

    I would be very interested to learn the reasons why a hypothetical positive mass object travelling at c would not become a singularity. Perhaps I am missing something obvious here or misunderstanding what relativistic mass is all about. However at the moment the logic behind this question eludes me.

    Many thanks
  5. Jul 28, 2009 #4


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    Homework Helper

    Using the word "hypothetical" doesn't allow you to describe what would happen in an impossible situation. Hypothetically or not, it's meaningless to talk about a massive object traveling at the speed of light. Physics has nothing to say on the matter, except that it couldn't happen.
  6. Jul 28, 2009 #5


    Staff: Mentor


    Regarding what happens when a massive object travels at c. As you know, physics says it cannot happen. So hypothetically you would be talking about science fiction rather than science. Since it is fiction feel free to describe it however you wish, the more fantastic the better.
  7. Jul 28, 2009 #6
    You are asking a theory which postulates that the speed of light is c in all reference frames what would happen in a frame in which the speed of light is not c. Obviously, you cannot get any answer, "hypothetical" or otherwise.

    And actually, I suspect that you may be a bit fuzzy on why c cannot be attained (not your fault), since your teachers apparently abused you with the semi-Newtonian kludge of Relativistic Mass. c cannot be attained because the speed of light is c in all reference frames, and no matter how fast you travel, light flashes continue to pass you at c. Since you can never speed up enough to catch a light ray, no one can observe you ever to do so, and therefore there are no reference frames in which your speed is or exceeds c. It really has nothing to do with mass.

    You sitting in your chair are going at nearly c in some reference frames. There aren't any reference frames in which your speed, sitting in your chair, exceeds c. Those frames don't exist, even in principle. Since those frames don't exist to begin with, no amount of acceleration will get you into one of them. Accelerate all you like for as long as you like, and you will pass through inertial frame after inertial frame, constantly, forever, none of which exceed c in any other frame. It's about geometry, not mass.

    In any case, the question cannot be answered for a spacecraft traveling at c, since there is no such thing. For a spacecraft at nearly c, the answer is simply the principle of Relativity: that there is no experiment that can be performed aboard the spacecraft that depends upon its speed alone. If the spacecraft turns into a black hole or a singularity at some speed, then there would be a possible experiment on board ship ("am I in a black hole or not") that would reveal that speed, implying the existence of an absolute rest frame against which all motion could be measured, and that cannot happen. Since you are not a singularity now, you will never be a singularity simply on account of changing your speed.
    Last edited: Jul 28, 2009
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