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Thread: Motion inside a black hole

  1. #1 Motion inside a black hole 
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    What is there to give us the impression that matter doesn't move inside of a black hole? Why would be believe that all motion stops?

    I have my own opinions on this, but I figure I should wait for the answers, to see if any of them make sense.


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  3. #2  
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    It's because of time dilation. Something moving at the speed of light seems to freeze in time to an observer not moving at c.


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  4. #3  
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    Actually, it is at the event horizon where all time appears to stop due an outside observer. This is due to gravitational time dilation.

    Inside the event horizon the time and space dimensions swap. What happens is that, just like outside of the event horizon we can only move towards the future in the time dimension, inside the event horizon the only allowable motion in the spatial direction is towards the center of the black hole.
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  5. #4  
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    Perhaps I'm failing to grasp the notion of time dilation. Of course, if we never actually reach the singularity, then time can't ever completely stop, only get very very very slow.


    If there's still motion, then does a singularity that can fully stop time ever actually form?
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  6. #5  
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    You're thinking of time actually stopping. It doesn't actually, it only seems to do so from the viewpoint of someone outside the black hole.

    For something inside a black hole, the question is completely different. For any matter inside, time hasn't stopped. It hasn't slowed down much, unless the object is moving very closely to the speed of light. The matter might be moving either straight into the singularity at the center of the black hole, or, prevented from doing so by the rush of matter and energy.

    The mass might not stop moving at all. It merely seems to do so from our point of view.

    By the way, according to most people, a singularity is a point where time really does seem to stop. However, personally speaking, I believe that time can never really stop. If it does stop, then the object is forever frozen. It cannot move. If, for example, the object was moving in space at a velocity, then it will stop forever and never move, violating the law of conservation of energy and angular momentum.
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  7. #6  
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    I think we're on the same wavelength here.

    If an object or particle is headed toward the center of a black hole at some speed, and the "rush of matter and energy" stops it, wouldn't the momentum simply be transferred to another particle? (or particles)

    So, if we uphold conservation of momentum, then you can't ever get a singularity, because the particles will never hold still long enough to all occupy a single point in space at the same time.

    Basically, particles will always be flying away from the core at the speed of light, only to slow down, turn around, and come flying back at the speed of light. The shorter the distance it requires for gravity to make them turn around and come back, the more times per second they'll make a round trip.
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    So, if we uphold conservation of momentum, then you can't ever get a singularity, because the particles will never hold still long enough to all occupy a single point in space at the same time.
    A singularity is a point where space and time curve so much that the laws of general relativity break down. In a black hole, the singularity was caused by the attraction of all the particles of the star into a single point. This would lead to a single point in space having an extremely high amount of gravitational attraction; enough for light to be captured by its power.

    The attraction of external particles has nothing whatsoever to do with creating a singularity. Once all the particles of the parent star of a black hole are sucked in, it creates a singularity.
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  9. #8  
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    So what happens when conditions are met internally that would naturally lead to motion? Are we to assume that the collapse of the star involves everything ceasing to move?
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  10. #9  
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    Quote Originally Posted by kojax
    So what happens when conditions are met internally that would naturally lead to motion? Are we to assume that the collapse of the star involves everything ceasing to move?
    No. Everything would not cease to move. A black hole has the capacity for motion; gravitational attraction or a net initial velocity will keep it moving around for quite some time. I mean to say that the singularity will always remain, until Hawking radiation finally gets it to either explode or vanish.

    As I said before, as we can know nothing about what happens inside a black hole, we will always be uncertain as to exactly what is inside a black hole. The star's particles will be consumed into a single point, so they at least cease to move. Everything sucked in will appear to stop moving; but from the viewpoint of the object in a black hole, time is as healthy as ever. The closer it gets to the singularity, the slower time will go, as time dilation is greater nearer to a massive body, however, so close to the singularity, the object itself will start seeing things move very slowly.

    Everything in a black hole will continue to move, but to those outside the black hole, they will forever appear stuck in a single position.

    Also, I seem to remember I said
    The matter might be moving either straight into the singularity at the center of the black hole, or, prevented from doing so by the rush of matter and energy.
    In reply, you said
    Basically, particles will always be flying away from the core at the speed of light, only to slow down, turn around, and come flying back at the speed of light. The shorter the distance it requires for gravity to make them turn around and come back, the more times per second they'll make a round trip.
    They won't always move around. My answer applied only to those particles that are at the back. For example, imagine a long queue for something. There is much infighting amongst the members of the queue; some are trying to push others in, others trying to maintain their balance; one or two may even be thrown out of the line. At the front of the queue, people go in.

    It is exactly the same with the black hole. The particles nearest to the singularity will go in and become "one" with the singularity. The other particles are fighting each other for the chance to go into the singularity, and one or two particles may be rebuffed by the other parfticles enough to go temporarily into orbit around the singularity. However, all particles must eventually go in; the gravitational attraction is extraordinarily immense, enough to capture every single particle at once. The event of a particle being rebuffed then is extremely unlikely; it could happen once in, say, a lifetime.
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    Neutrons survive in a neutron star with an escape velocity of 2/3 light speed. There is no reason to believe that the gravity of a black hole which may be just a mere 50% more can crush elementary particles like electrons and quarks. At the centre of a black hole is probably a rotating sphere of such particles. A singularity does not explain how super-massive black holes can spin at near light speed.

    Time dilation is for observers and there are not many of them in black holes.
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  12. #11  
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    Quote Originally Posted by Liongold
    A singularity is a point where space and time curve so much that the laws of general relativity break down.
    Let us use the correct words. A singularity is a point where the expression is not valid. E.g., tan x has a singularity at x=90.

    At the center of the BH, the proven existence of the singularity means that the theory of GR is not valid here. This does not mean GR is not a valid theory, just not valid inside the BH.
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  13. #12  
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    Which is kind of my point. I don't see how a singularity could really exist. I mean, you can approach it, but to actually obtain it seems non-sensical.

    quote="Liongold"]
    Quote Originally Posted by kojax
    So what happens when conditions are met internally that would naturally lead to motion? Are we to assume that the collapse of the star involves everything ceasing to move?
    No. Everything would not cease to move. A black hole has the capacity for motion; gravitational attraction or a net initial velocity will keep it moving around for quite some time. I mean to say that the singularity will always remain, until Hawking radiation finally gets it to either explode or vanish.
    [/quote[

    In a way I think what I'm trying to do is rationalize Hawking Radiation into something that makes sense to me, instead of just an ad hoc adjustment to prevent us from accepting the disappearance of information.

    Without Hawking radiation, however, motion wouldn't continue for just a "little while" would it? It would go on forever, at least in the sense that particles would always be in the middle of a flight path (perhaps a mostly suspended one, but still a flight path).


    As I said before, as we can know nothing about what happens inside a black hole, we will always be uncertain as to exactly what is inside a black hole. The star's particles will be consumed into a single point, so they at least cease to move. Everything sucked in will appear to stop moving; but from the viewpoint of the object in a black hole, time is as healthy as ever. The closer it gets to the singularity, the slower time will go, as time dilation is greater nearer to a massive body, however, so close to the singularity, the object itself will start seeing things move very slowly.

    Everything in a black hole will continue to move, but to those outside the black hole, they will forever appear stuck in a single position.
    Ok, but that wouldn't lead them to compress into a single point, would it? From the particles' perspectives, they're still moving some distance from the core, turning around and coming back, and then flying away again.

    We may see them as stationary, but that means some are stationary (from our perspective) at locations outside the core, forever waiting to turn around and fall back again.


    Also, I seem to remember I said
    The matter might be moving either straight into the singularity at the center of the black hole, or, prevented from doing so by the rush of matter and energy.
    In reply, you said
    Basically, particles will always be flying away from the core at the speed of light, only to slow down, turn around, and come flying back at the speed of light. The shorter the distance it requires for gravity to make them turn around and come back, the more times per second they'll make a round trip.
    They won't always move around. My answer applied only to those particles that are at the back. For example, imagine a long queue for something. There is much infighting amongst the members of the queue; some are trying to push others in, others trying to maintain their balance; one or two may even be thrown out of the line. At the front of the queue, people go in.
    And , presumably, come back out as well.

    A net stopping force has not been described, insofar as I know. There's no mechanism present for any in-elastic collisions to occur.


    It is exactly the same with the black hole. The particles nearest to the singularity will go in and become "one" with the singularity. The other particles are fighting each other for the chance to go into the singularity, and one or two particles may be rebuffed by the other parfticles enough to go temporarily into orbit around the singularity. However, all particles must eventually go in; the gravitational attraction is extraordinarily immense, enough to capture every single particle at once. The event of a particle being rebuffed then is extremely unlikely; it could happen once in, say, a lifetime.
    Yeah, I'm kind of oversimplifying. I mean that either A: the particle itself gets rebuffed, or B: the rebuffing effect is transferred to another particle(s), most likely one on the outside.

    What I'm not getting, is what, exactly, allows us to ignore conservation of momentum? Gravity is an elastic force. Even if relativistic slowing occurs, it's still elastic.. Particles still have the same amount of momentum even if they're moving at slower speeds, and that momentum has to remain.
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  14. #13  
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    In a way I think what I'm trying to do is rationalize Hawking Radiation into something that makes sense to me, instead of just an ad hoc adjustment to prevent us from accepting the disappearance of information.

    Without Hawking radiation, however, motion wouldn't continue for just a "little while" would it? It would go on forever, at least in the sense that particles would always be in the middle of a flight path (perhaps a mostly suspended one, but still a flight path).
    It would go on forever to us, the outside observers. From the particle's perspective, everything is still the same. It still moves, except now its moving towards the core of a black hole. From our perspective, yes, it would seem motion will go on forever without Hawking radiation. From the particle's perspective, it still moves, just in the direction of the center.

    Ok, but that wouldn't lead them to compress into a single point, would it? From the particles' perspectives, they're still moving some distance from the core, turning around and coming back, and then flying away again.

    We may see them as stationary, but that means some are stationary (from our perspective) at locations outside the core, forever waiting to turn around and fall back again.
    I'm sorry, but I'm not quite sure what you mean to say here. I fail to understand, specifically, this

    From the particles' perspectives, they're still moving some distance from the core, turning around and coming back, and then flying away again.
    Why would particles come forward and fly away again? The black hole's gravity is more than capable of preventing it from ever moving away from the singularity. It would force the particles to come together in a single point.

    Also, why would some be stationary? Suppose you threw something into the black hole. Here is what would happen: the object would go flying into the black hole, and, then, impossibly, seem to vanish. It would go completely black, and you would never see it. Why? Because the object just crossed the event horizon, and the light that is supposed to bring you information of it never manages to reach your eyes.

    Objects will not appear to be stationary just outside the black hole. It's a misconception. Objects would simply vanish at the event horizon. They would imitate the darkness of space.

    And , presumably, come back out as well.

    A net stopping force has not been described, insofar as I know. There's no mechanism present for any in-elastic collisions to occur.
    Actually, no. Imagine the queue is for a Nazi death chamber, and for some insane reason, people want to go in. People will never get out. Likewise, the particles will never escape from the singularity. Hawking radiation applies to particle-antiparticle pairs that appear in a black hole.

    What I'm not getting, is what, exactly, allows us to ignore conservation of momentum? Gravity is an elastic force. Even if relativistic slowing occurs, it's still elastic.. Particles still have the same amount of momentum even if they're moving at slower speeds, and that momentum has to remain.
    Why? What's violating the law of conservation of momentum?
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  15. #14  
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    Quote Originally Posted by Liongold

    I'm sorry, but I'm not quite sure what you mean to say here. I fail to understand, specifically, this

    From the particles' perspectives, they're still moving some distance from the core, turning around and coming back, and then flying away again.
    Why would particles come forward and fly away again? The black hole's gravity is more than capable of preventing it from ever moving away from the singularity. It would force the particles to come together in a single point.
    Presumably, if an object strikes the singularity at light speed, and that momentum is... say.. transferred to another particle on the opposite side of the singularity (So, the particle strikes another particle, which in turn strikes another, which in turn strikes another, which in turn strikes another.... etc... ultimately striking one on the opposite side),

    That particle the momentum was transferred to now has a sufficient amount of momentum to be moving at light speed. It's going to move at least a little ways before gravity can turn it around.

    I know this is a very idealized example, but I'm hoping that it expresses my point. If a particle is imparted a momentum inside the singularity, it's going to travel a distance away from where it is now before gravity can stop it. It may be a very short distance, but it will travel some distance before gravity can stop it.

    Gravity is not a stopping force. It's an elastic force. There is, by necessity, a short delay between the instant an object begins moving, and the time it takes gravity to fully bring it to a stop.






    And , presumably, come back out as well.

    A net stopping force has not been described, insofar as I know. There's no mechanism present for any in-elastic collisions to occur.
    Actually, no. Imagine the queue is for a Nazi death chamber, and for some insane reason, people want to go in. People will never get out. Likewise, the particles will never escape from the singularity. Hawking radiation applies to particle-antiparticle pairs that appear in a black hole.

    What I'm not getting, is what, exactly, allows us to ignore conservation of momentum? Gravity is an elastic force. Even if relativistic slowing occurs, it's still elastic.. Particles still have the same amount of momentum even if they're moving at slower speeds, and that momentum has to remain.
    Why? What's violating the law of conservation of momentum?
    In the first place, i think i wasn't clear. I don't mean that anything gets out of the event horizon. I mean that, if something imparts a momentum to them, particles might move a short distance away from the core, and then return. (or if they have a momentum already and haven't lost it yet)


    At the core, particle momentum should prevent a singularity from ever forming. If the particles never stop moving, then there will never be enough of them gathered at any single point in space to count as a singularity. (So gravity never becomes infinite at the core, just really really strong)
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  16. #15  
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    Gravity is not a stopping force. It's an elastic force. There is, by necessity, a short delay between the instant an object begins moving, and the time it takes gravity to fully bring it to a stop.
    True. But the momentum, as compared to the gravity of a black hole, is extraordinarily small, and not strong enough to resist the gravity for longer than a second.

    Also, the law of conservation of momenetum reads: "the total linear momentum of a system is conserved".

    However, according to general relativity, objects won't move in straight lines in gravity; their paths curve and twist towards the center of mass, in this case, the singularity. The momentum will not be conserved here, unfortunately, because the momentum is no longer linear.

    I mean that, if something imparts a momentum to them, particles might move a short distance away from the core, and then return. (or if they have a momentum already and haven't lost it yet)
    They might collide, but the gravity of the black hole destroys the momentum. See above for why.

    At the core, particle momentum should prevent a singularity from ever forming. If the particles never stop moving, then there will never be enough of them gathered at any single point in space to count as a singularity. (So gravity never becomes infinite at the core, just really really strong)
    I think firstly I should clarify what I mean by gravity. I refer to the Einsteinian notion of curved space-time, and a singularity then is not necessarily a single point, merely an area where, as Sanford said, "tan x has a singularity at x=90". Or, more precisely, where we cannot predict what happens next to anything in that sphere. We consider it a point, though it may not be so.

    All that is required for a singularity to form is enough mass contained within it. This mass is the mass of the black hole.
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  17. #16  
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    Quote Originally Posted by Liongold
    Gravity is not a stopping force. It's an elastic force. There is, by necessity, a short delay between the instant an object begins moving, and the time it takes gravity to fully bring it to a stop.
    True. But the momentum, as compared to the gravity of a black hole, is extraordinarily small, and not strong enough to resist the gravity for longer than a second.
    Right, but it will move some distance in that second.

    Let's say the distance an object originally moving at light speed reaches before turning around is .000001 micron

    Well, by the time it makes it back to the core, it's traveling at light speed again, so wherever that momentum goes......... we might end up with another particle on the opposite side of the singularity launching away at light speed..... and it only makes it .000001 micron before turning back as well.

    Still, that's motion.


    Also, the law of conservation of momenetum reads: "the total linear momentum of a system is conserved".
    Well, there's another version of that law which governs conservation of angular momentum, and it reads pretty much the same way.


    I mean that, if something imparts a momentum to them, particles might move a short distance away from the core, and then return. (or if they have a momentum already and haven't lost it yet)
    They might collide, but the gravity of the black hole destroys the momentum. See above for why.
    I'm not clear on why, I guess. I know that paths curve, but curved momentum is also conserved according to the laws that govern angular momentum.
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