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Inertia and Black Holes

  1. Jun 12, 2006 #1
    Can anyone answer my amateur questions, and please don't heckle if it seems like a stupid question:

    1) Do black holes have a velocity (ie. move within three dimensional space), and if so, does the law of inertia applie to a black hole?

    2) Are black holes affected by gravity? (ie. will a large mass travelling toward a black hole tend to slightly pull the black hole towards it as well?)?

    3) If the answer to question 2 is affirmative, can two black holes move toward each other and eventual collide, and if so, what would happen? Thanks
  2. jcsd
  3. Jun 12, 2006 #2


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    Yes, BHs are just objects with mass in which the mass has gravitationally collapsed in upon itself. Normally they are massive (old stars with MBH > 3 MSun), and can be very massive (MBH > 109MSun), but also it is possible that low mass BHs formed in the vary early universe.
    Yes, but what happens depends on what else the BHs bring with them.

    Two 'naked' BHs will radiate a lot of energy as gravitational waves.

    Some BHs are surrounded by an accretion disk of ordinary matter spiraling into them (the model for a quasar engine). If two of these coalesced the ordinary matter would heat up very rapidly and radiate before disappearing into one or both of the BHs event horizons. This model may be the engine for short Gamma Ray Bursts (GRBs) .

    Also some of the baryonic matter may be accelerated and thrown out of the system, this scenario may be the source of ultra high energy http://www-mariachi.physics.sunysb.edu/wiki/index.php/Cosmic_Rays, but if so then they would have to be fairly close to the Milky Way (at least <50 Mparsecs and closer for the highest energy CRs). This would require a dense population of BHs throughout the universe.

    Last edited by a moderator: Apr 22, 2017
  4. Jun 12, 2006 #3
    Thanks Garth

    That helps. The formulas are way over my head but I understand the principles of what you are saying. I figured this was the case, but the Hawking tapes I listened to were riddled with mathematical lingo on this particular issue. I assumed that there is no matter which has no mass and no matter which is unaffected by gravity, but I started to wonder after I read a discussion of whether or not some subatomic particles have no mass at all (it was an old article, and at the time there was, I believe, a general consensus that said particles simply had a mass so inifitely small that it could not be measured). Thanks again.
  5. Jun 12, 2006 #4
    Additional Question

    This discussion lead me to one further question, though it probably lies solely in the realm of the hypothetical: is there anything in the universe that cannot be moved, ie has absolutely no velocity? I suppose this is something akin to "if God can do anything, can God make something so heavy that he can't lift it". Nonetheless, any ideas are welcomed.
  6. Jun 16, 2006 #5
    That would imply a notion of absolute space, which doesn't exist. If you've got two objects, one 'fixed at the origin' and the other moving in a straight line past it (ie not accelerating), the one which is moving is perfectly allowed to say it's not moving and the one 'fixed at the origin' is moving. It's a matter of your choice of interial frame.

    Hence, everything is moving in some frame or other. A super massive black hole would be very hard to move about due to it's enormous inertia but that doesn't mean it's always at rest.
  7. Jun 16, 2006 #6


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    The question would not require absolute space if we consider it as; "is there anything in the universe that cannot be accelerated?". I think this is the idea String was trying to get at.

    If that is indeed the question, I would submit that the answer is "no". Accelerating an object becomes more difficult as the object's mass increases. To make an object impossible (or infinitely difficult) to accelerate would require that the object have infinite mass, wich no object can have.
  8. Jun 16, 2006 #7


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    No. Give me a lever long enough and a place to stand......

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