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Questions about Black Holes And Time

  1. Dec 2, 2008 #1
    Ok if this has been covered before if you could show me any links that would be great..

    I was watching a program about time and how it appears to slow to a stop within a black hole... yet time can only be still if an object is frozen in time, bu a black hole moves meaning it is not frozen in time.. yet time exists everywhere in the universe.. would this mean that a black hole is more like an actual hole through the universe where time can not exist meaning that a black hole conpacts matter back into a state before time ever existed?

    Also if no matter how anything moves in will be going forwards in time does this then suggest rather than moving through time objects actually absorb time, as if we was movig through time then it would suggest that it was possible to move in a way that would take a object back into past time, yet it seems no matter what everything we no of in the universe no matter how it moves will always be going forward in time just at different speeds meaning time has endless dimensions surrounding an object... But this then can not explain what has happened to the time the we have just experianced (The Past) as everything is is surrounded by furture time...

    This then suggests that time can only ever move in a single direction (Forward) meaning there is no trace of the past left after each moment passes..

    Anyway these are just random things i was thinking about.. any comments would be good if anything i have just wrote makes any sense at all.
  2. jcsd
  3. Dec 2, 2008 #2


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    An outside observer senses the slowing down of time by an object falling into a black hole. The object falling in does not have any slowing down of time.
  4. Dec 2, 2008 #3
    But say you watched a human fall into a black hole and some how you could see there watch in theory you would see it slowly stop ticking.. which would suggest time has slowed to a stand still... meaning that the object in question would have to have gone through a change in the speed of time for this to occour.. where as if there was no change in the speed time then the watch would not have stopped.
  5. Dec 2, 2008 #4
    Yes, you see an infalling watch slows down to stand still, but to the infalling person wearing that watch, he will see no such thing - the watch is ticking perfectly normally in his frame.
  6. Dec 2, 2008 #5


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    It's an interesting question about singularities. If the clock carried by an astronaut plunging into a black hole ceases ticking [from the perspective of an observer at a safe distance] at the event horizon, how is it possible for a singularity to form? Thus, it is fair to question [from an outside observer's perspective] whether infalling matter can actually achieve a state of true singularity in a finite amount of time. It does not, however, take an eternity for collapsing matter to achieve sufficient density to generate an event horizon.
  7. Dec 3, 2008 #6
    I was about to post exactly this question and then this thread got started! So if we outside observers see time slowing to a stop for an infalling object. then from our perspective, the colapsing matter of the star will colapse untill it forms an event horizon and then it won't get any smaller? at leas not from our view point outside, well, that is, if we could see it. which we can't because light can't escape. so questions:

    1, from our viewpoint, if we could see the matter of the colapsed star, would it not shrink to a singularity but stop just behind or at the event horizon?

    2, It takes time for any event or motion to occur. So light can't escape a black hole because gravity is too strong but is it also because it would take time for a photon to travel out from a black hole but time from our viewpoint is stopped so the light can not travel out because it would take time for it to do so? So the motion of a photon is affected not only by the geometry of the space it is traveling through but also by the way that geometry affects our perception of the flow of time?

    3, an infalling observer doesnt notice his own time slowing down so what does he see when he looks back at us, the outside observers and the rest of the universe? does he see us and the rest of the universe suddenly going faster and faster? does our time apear to him to be sped up?
  8. Dec 3, 2008 #7


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    1. The collapse would appear to halt at the event horizon

    2. Motion is meaningless without a time component

    3. To an observer approaching the event horizon, the outside universe would be increasingly blue shifted.

    The implications are interesting. Assuming black holes ultimately evaporate via Hawking radiation, an infalling observer can never achieve singularity.
    Last edited: Dec 3, 2008
  9. Dec 4, 2008 #8
    Apparently black holes only evaporate via Hawking radiation when there mass is 4.5e+22 kg (approx. the size of the moon) or smaller. Anything larger than this and their temperature is lower than the cosmic microwave background temperature (2.725 K) which means the black hole is 'balanced' by absorbing CMB. Spin would also reduce the Hawking radiation.
    Last edited: Dec 4, 2008
  10. Dec 4, 2008 #9

    George Jones

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