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Relativic black hole question

  1. Jun 8, 2009 #1
    I've been studying up on physics, and a question occured to me. According to the Theory of Relativity, the faster objects go, the more mass they have. Now, if we have a stellar object (a star for instance) moving towards the speed of light, at some point , would it have gained enough mass that gravity pulls it into a black hole? Also, what will happen if it stops moving suddenly after that point? Will it continue to be a black hole or will it have contracted itself enough that it stays that way? Personally, I think it IS possible for it to become a black hole like this, and that if it stopped moving, magnetic fields would push the thing apart, with the loss of major gravity due to loss mass, but i'm not an expert on this stuff, so I would like you're opinion. What do you guys think?
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  3. Jun 8, 2009 #2
    How would it be pulled into a BH? You lost me here.
  4. Jun 8, 2009 #3

    If you are somehow pumping energy into it and you are able to accelerate the star that fast, then you must have put a lot of energy into it.. so my guess is that yes, it would create a black hole. If you are talking more about energy from angular momentum, then I'm not so sure.

    "Will it stay a black hole?" If it forms a black hole, then you can't undo that.
  5. Jun 11, 2009 #4
    It will not become a black hole. Black holes can only come form colappsed stars and a handfull of other things, but not a star going at the speed of light. Most likely, it will just keep on ganing mass as it travels faster. By the way, how did it accelerate to the speed of light in the first place?
  6. Jun 11, 2009 #5
    No it wont. No matter how much energy is applied to the outside of a star, it will not create a black hole, at least not instantly. The only way to create a black hole in this case, is to apply energy to the core of the star.
  7. Jun 11, 2009 #6


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    Remember, also according to Relativity, the star is not moving in its own frame and thus gains no mass in its own frame. Ergo, in its own frame, it has no reason to collapse into a black hole. If it does not collapse into black hole in its own frame, it cannot collapse into one according to any other frame.

    Or put another way, our own Sun is traveling at near light speed relative to some reference frame, yet has shown no tendency to form a black hole.
  8. Jun 11, 2009 #7
    Does the converse hold as well? If it cannot collapse into a black hole in any other frame, it cannot collapse into one in it's own frame. Would that also apply to crossing the event horizon?
  9. Jun 11, 2009 #8
    I was thinking that if one were to apply that much energy to a star somehow, the temperature of the star would rise (not from the acceleration, but from the energy itself). This would, I think, heat up the core, and that would speed up the life cycle of the star. You're right, it would not be instantaneous or something. I just thought that the life cycle would eventually lead to a black hole, since the star has gained all of that energy.
  10. Jun 17, 2009 #9


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    But that energy is kinetic energy, and fast-moving objects do not stronger gravity. Kinetic energy cannot heat, squish, or crush; it can only move objects as a whole.
  11. Jun 17, 2009 #10
    I think I know where you're coming from. It depends on how you move it. If you were to swing a giant bat at the sun, and somehow make it start moving, then yes, you're correct (I know how silly that sounds, but I'm not trying to be accurate at the moment). However, I was thinking that the way you would move a giant object would be to shine a powerful laser or strike it with high velocity ions over an extended period of time, in order to gradually move it. So it's not the fact that its moving, it's what makes it move.
  12. Jun 18, 2009 #11
    That's because objects don't gain mass by accelerating. That might still be taught in high school, but nowadays, when we say "mass", we mean the invariant rest mass. Einstein's theory is more complex than just multiplying the Lorentz-factor onto everything.
    The topic mentioned in this thread is a good example why the interpretation of a dynamic mass is nonsense.
  13. Jul 26, 2009 #12
    This was more of a theoretical physics question than anything. And if this isn't true, why do they still keep teaching it?
  14. Jul 26, 2009 #13
    Well this isn't really a matter of truth to be honest, it's a matter of definition and interpretation. You could interpret the formulas, like for example the relativistic momentum p=gamma*m0*v or the total energy E=gamma*m0*c^2, as there was a relativistic mass and write p=m(v)*v or E=m(v)*c^2. This may be sufficient and easy to memorize if you don't go beyond high school topics (which might be the reason why it is still being tought), but as I pointed out and you found out by yourself, this can lead to misunderstandings when you are thinking about more sophisticated problems.
    In the big picture it just makes more sense to discard the interpretation with the relativistic mass.
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