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Effects of a black-hole on Time

  1. Nov 10, 2011 #1
    Hi there.
    I've been watching some documentaries on the Big bang and the laws of nature.
    One particular interesting bit of information I gathered was on the effects of a black-hole on light and time.

    It was explained that the gravitational pull captures light, which is why there are completely black. Okay fine I understand that.

    They also said that the closer you get to the surface of a black hole, Time begins to slow down and eventually stop. I think I understand the idea, but they didn't explain why. All they said was that the laws of nature dictate that this happens. Why?
     
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  3. Nov 11, 2011 #2

    Chalnoth

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    This is known as gravitational time dilation. This can perhaps be best-explained with energy conservation of light.

    Basically, when matter leaves a gravitational potential well, it has to lose energy. Light is no different: when you shine a light from the Earth, that light loses a little bit of energy escaping from the Earth's gravitational field. And when light loses energy, it gets redshifted. That is, light with less energy is light with a longer wavelength.

    But if the light has a longer wavelength, and therefore a lower frequency, that means that an observer sitting outside the Earth will see more time pass between the peaks of this incoming light wave. This means that to an observer outside, if they look at a clock sitting on the surface of the Earth, they will see that clock tick more slowly just because the light coming up from the Earth is losing energy.

    A black hole is the same, but the effect is more extreme. In fact, light that is emitted right at the event horizon loses all of its energy. This means that infalling objects appear to freeze in time, according to an observer sitting outside. The object does pass right on through and enter the black hole, but because the outgoing light loses so much of its energy, it leaves an image behind that can be seen forever after (though that image will be exceedingly faint, because the light has to lose so much of its energy to escape).
     
  4. Nov 11, 2011 #3
    So lets suppose a star gets captured by black hole . What do we see ? Normally one would assume we see nothing as light cant escape. But if its sat frozen in time at the event horizon then should we still see it? If so shouldnt we see the light of every object thats ever been captured by the balck hole and for super massive ones that would be a lot?
     
  5. Nov 11, 2011 #4

    Chalnoth

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    Well, basically, we see a massively redshifted image of the star. The longer we wait, the more redshifted that image has to be. And it doesn't take long for the image to be redshifted to tremendously-long wavelengths, so unless we're explicitly looking for these very long wavelengths, in practical fact it will seem to disappear.
     
  6. Nov 11, 2011 #5
    So even with ultra long base line radio we still wouldnt see anyhting?
     
  7. Nov 11, 2011 #6

    Chalnoth

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    It all just depends upon how long ago the object fell into the black hole. The longer you wait, the longer the wavelength. So no matter your instrument, wait long enough and you won't be able to see it any longer.
     
  8. Nov 11, 2011 #7
    I have thought about this before. After an infinite amount of time the wavelength would become infinitely long and take an infinite amount of time to propogate 1 wavelength. :smile:

    All very interesting!
     
  9. Nov 21, 2011 #8
    So time doesn't slow down just our perception of time?
     
  10. Nov 22, 2011 #9
    No it is not just an illusion. The two timeframes are actually different timeframes and the "tick" rates would be different. They are both equally valid.
     
  11. Nov 22, 2011 #10
    So you’re saying that time dilations are a cause for red shift (which I agree with). If expansion of the universe is based on red shift and the red shift may be caused from something other than expansion then why is expansion so dogmatically accepted?
     
  12. Nov 22, 2011 #11

    Chronos

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    No reasonable explanation for cosmological redshift has been proposed that competes with exapansion.
     
  13. Nov 22, 2011 #12
    I think the explanation Chalnoth suggested (one I too have heard before) is a good one. It even makes more sense where as expansion has other problems.
     
    Last edited: Nov 22, 2011
  14. Nov 22, 2011 #13

    Chalnoth

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    There's no conceivable way for galaxies further away to have larger redshifts simply because they sit in larger local gravitational potential wells.
     
  15. Nov 22, 2011 #14
    Why not?

    In post #4 you said " Well, basically, we see a massively red shifted image of the star. The longer we wait, the more redshifted that image has to be."

    Besides the light has to travel halfway between galaxies for it even climbs out of the well and then it’s downhill the other half. That has got to have a major effect on time dilations.

    Maybe up until the halfway point the light is affected one way and then as it drops into another well with changing time dilations (caused from the increasing density) would cause the red shift. The concept is just as easy to except as expansion and its problems.
     
    Last edited: Nov 22, 2011
  16. Nov 23, 2011 #15

    Chronos

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    Well, for one thing galaxies at great distances are also much younger than us. How does that fit into your model? Perhaps you have failed to properly define your objections to expansion.
     
  17. Nov 23, 2011 #16
    I'm not sure of your point about the galaxies age or what that would have to do with time dilations. Please explain thanks

    My objection is that expansion seems to make no sense and is not proven other than the red shift which can be caused by other sources. Everything we see is accumulating, why should the universe with some mystical dark energy be doing the opposite?

    Time dilation which have been proven seem to fit well, especially with a collapsing universe.
     
  18. Nov 23, 2011 #17

    Chalnoth

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    What I said, specifically, is that it can't be a local effect. Redshifts of 1-2 would require extreme gravitational potentials for them to be local effects. And we see individual objects out to redshifts around 7-10, and the CMB is at a redshift of around 1090. There's just no way for that to happen.

    As for gravitational redshift in between us and the galaxy, yes, that is entirely reasonable. But that's just another way of describing the expansion of the universe. You can look at the redshift from the expansion as being due either to redshift from recession velocity or due to redshift from gravitational curvature in between, or some mixture of the two. It all depends upon what you are talking about. But digging deep in the math, the redshift is still evidence for expansion, no matter whether you choose to say that redshift is due to velocity or gravitation.
     
  19. Nov 23, 2011 #18
    Can you describe this in a different way or elaborate more?

    So you don't see anyway that the diferences in time dilations could cause this?
     
  20. Nov 24, 2011 #19

    Chalnoth

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    The way a local gravitational redshift works is that you have, for example, a star sitting inside a large potential well. Imagine, for example, a star orbiting very close to a large black hole, or a star in the center of a huge quantity of dark matter. Because the photons from that star have to travel out of such a deep potential well, they get redshifted.

    It is just not at all reasonable for galaxies further away from us to be in progressively deeper potential wells.

    In effect, the redshift that happens as a result of the expansion is a time dilation.
     
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