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Thread: Can we ever observe an object fall into a black hole?

  1. #1 Can we ever observe an object fall into a black hole? 
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    Ok so in my astronomy lecture today we were told that if we were to observe an object falling onto a black hole, its 'clock' would slow down due to gravitational time dilation - according to the formula

    where =the time we would observe on the clock falling in.
    =Proper time on the clock
    =the schwarzschild radius
    r=displacement.

    Using this equation, it would seem that at the point the displacement of the object is equal to the schwarzchild radius, we would observe an infinite time. Does this mean we could never see an object truly enter a black hole? Or have I made a stupid mistake?


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  3. #2 Re: Can we ever observe an object fall into a black hole? 
    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by Stuart Thomson
    Does this mean we could never see an object truly enter a black hole? Or have I made a stupid mistake?
    This is correct. We would never see anything crossing the Schwarzschild radius. But additionally, general relativity also predicts that the light emitted by such an object would be redshifted. It becomes infinity at the Schwarzschild radius. This means, we would not be able to see it at all.


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  4. #3  
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    But does this imply that the black hole can never gain any mass, or that mass which is incredibly close to the black hole would effectively increase the mass, without crossing the schwarzchild radius?
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    From the point of view of the object, things look different. It's fall into the black hole passes normally (ignoring gravitational shearing), and you can see out of the event horizon (though that's ignoring the blue shift). From its point of view the outside universe passes by in an instant. What exactly happens from there starts to become speculation.
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  6. #5 Re: Can we ever observe an object fall into a black hole? 
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    Quote Originally Posted by Stuart Thomson
    where =the time we would observe on the clock falling in.
    =Proper time on the clock
    =the schwarzschild radius
    r=displacement.
    t_o would be the time on a clock at asymtotic infinite r.
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  7. #6 Re: Can we ever observe an object fall into a black hole? 
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    Quote Originally Posted by Dishmaster
    Quote Originally Posted by Stuart Thomson
    Does this mean we could never see an object truly enter a black hole? Or have I made a stupid mistake?
    This is correct. We would never see anything crossing the Schwarzschild radius. But additionally, general relativity also predicts that the light emitted by such an object would be redshifted. It becomes infinity at the Schwarzschild radius. This means, we would not be able to see it at all.
    You will never see a black hole because there are not any. But if you guys like spending your time like this hey, I cannot stop you.



    Sincerely,


    William McCormick
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  8. #7 Re: Can we ever observe an object fall into a black hole? 
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    Quote Originally Posted by ought
    Quote Originally Posted by Stuart Thomson
    where =the time we would observe on the clock falling in.
    =Proper time on the clock
    =the schwarzschild radius
    r=displacement.
    t_o would be the time on a clock at asymtotic infinite r.
    I apologise, but I don't understand what you mean.
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