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Does a black hole cause light to move faster?

  1. Mar 30, 2008 #1
    Say light was a stream of water. What would the black hole be? A bend in the river or a waterfall?
  2. jcsd
  3. Mar 31, 2008 #2
    wonderful question! i guess a bend, the point is as i know light can go faster but i can not transform information faster than c..........
  4. Mar 31, 2008 #3
    It would be a bend that bends on itself and nothing spills out of it (Except that backholes to radiate - Hawkin's radiation)
  5. Mar 31, 2008 #4


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    Welcome to PF!

    Hi Gamerex! Welcome to PF! :smile:

    I think it would be a plughole!
  6. Mar 31, 2008 #5
    A whirlpool I suppose if we're going to stick with the river theme. :smile: Only this whirlpool leads to a singularity.
  7. Mar 31, 2008 #6


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    Since light is not a stream of water, I have no answer to that.

    However, in response to your title "Does a black hole cause light to move faster? "
    the answer is "NO". Nothing can cause the speed of light to change. What happens to light as it tries to come out of a black hole is that its wavelength becomes longer and longer until, eventually, you no longer have a wave.
  8. Mar 31, 2008 #7
    Shapiro delay shows that any gravity field actually slows down the speed of light.
    So presumably, if you could watch a photon fly into a black hole, you would observe its speed to decrease

    I'd say it wouldn't behave like a plug hole at all. Not if the Shapiro delay is true. Seems to me the slowing of light would make each photon appear to be repulsed by the black hole rather than sucked into it.
  9. Mar 31, 2008 #8
    A physics professor at my university posed this same question to his students. He asked, "if a black hole sucks everything into it, does that mean light accelerates into a black hole?"
  10. Apr 1, 2008 #9
    This document might help you understand more about that:
  11. Apr 1, 2008 #10
    So it's a no. Or a yes. Unfortunatly, I know almost nothing about space-time(Nor the terminology), and like theory more than other things. I guess it's a no.
  12. Apr 2, 2008 #11
    Maybe this is completely wrong, but I think light will be bent toward the black hole for sure.

    The only thing is, that if you could track the motion of each photon it wouldn't appear to speed up as it got closer, but in fact slow down. In fact I think it stops altogether at either the Swartzchild radius or 2xSwartzchild radius. So it would look like it was attracted to the outside circumference of the hole - rather than toward the centre of it. And it gets stuck there forever. If that's wrong some expert here will correct it I hope.
  13. Dec 17, 2009 #12
    what if the black hole was directly infront of it
  14. Dec 17, 2009 #13


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    Not really, it just increases the distance over which the light has to travel. (Well... perhaps you could think about it as slowing light, but that interpretation doesn't really make sense to me)
  15. Dec 17, 2009 #14


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    Still no.
  16. Dec 17, 2009 #15
    In a system of any observer - no. Locally, passing light always have speed=c

    From perspective of a distant observer, light entering the black hole can have different speeds, based on the choice of the coordinate system. In different coordinate systems speed of light can be different, but this is pure mathematics, not physics. You can do the same by turning around: in coordinate system associated with you Adromeda galaxy made a full circle with speed >1000000c
  17. Dec 17, 2009 #16
    The photon's gravitational potential energy is converted into increased frequency?
  18. Dec 17, 2009 #17
    I hypothesis that because a photon has no mass gravity only affects it by adjusting it's trajectory.
  19. Dec 18, 2009 #18


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    First, the whole "light is a stream of water" is a really poor similie. However, the more basic question about light moving faster or slower is actually fairly complicated.

    In a curved spacetime there is simply no way to compare the velocity of two objects unless they are right next to each other. Also, in GR light always follows a null geodesic. Any observer at any event will always measure the relative speed of any local null geodesic to be c.

    In GR you can assign any coordinate system you like to any spacetime. In these coordinate systems it is common to have light travel at speeds other than c. But there is no physical significance to the coordinate system.
  20. Dec 18, 2009 #19
    Stephen Hawking said light will travel faster than c in a black hole but I think he's lost the plot a bit these days, light will travel at c in the vacuum end of story. Gravitational lensing may make it look like it is going faster or slower but speed=d/t will never be more than c.
  21. Dec 18, 2009 #20


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    As others have already indicated above, there's more than one way to measure speed in general relativity. Any observer, anywhere in the Universe, making a local measurement of light passing right by, using local rulers and local clocks, measures light to travel at c in vacuum. But if you try to measure light some distance away from you, you may well get a different answer. So if you are hovering outside a black hole, you can calculate the speed of light inside the event horizon to be greater than c, but if you fell into the hole and made a local measurement you would get c.

    It's a bit like the problem cartographers have mapping the world on a flat piece of paper. Small areas (say, less than 50 miles / 100 km across) can be mapped pretty accurately. But if you try to draw a map representing thousands of miles, although part of the map may be accurate, another part will inevitably be on the wrong scale or have the wrong angles.

    Hawking hasn't lost the plot.
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