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Can light be redshifted by gravitational fields?

  1. Jan 9, 2010 #1
    Can light be redshifted by gravitational fields?

    If for example a flashlight was near a black hole but just outside the event horizon. If the flashlight was directed outward (directly away from the black hole), the light would indeed escape, but would the light be redshifted?


    And a different question... Does space itself collapse (or flow) into a black hole or does it simply stay in place?

    Thank you,
    Tony
     
  2. jcsd
  3. Jan 9, 2010 #2

    Dale

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    Yes, this was shown by the famous Pound-Rebka experiment.
     
  4. Jan 9, 2010 #3

    tiny-tim

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    Welcome to PF!

    Hi Tony! Welcome to PF! :smile:
    Yes, when it reaches us, it's redshifted.

    Same for any gravitational field stronger than ours. :wink:
    I think it stays in place.

    It's not like a waterfall, carrying things along with it. It's more like a rock shaped like a waterfall, that you can't help sliding down. :smile:
     
  5. Jan 9, 2010 #4

    George Jones

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    It depends on the observer receiving the light, but, for the observers I think you have in mind, the answer is "Yes."
    I don't think this is a meaningful question. It is true that spacetime outside the black hole is static (in technical sense) and not static inside the event horizon.
     
  6. Jan 9, 2010 #5

    George Jones

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    Re: Welcome to PF!

    Not necessarily. :wink:
     
  7. Jan 9, 2010 #6
    Re: Welcome to PF!

    Under what circumstance?

    And in this situation I'm not talking about the frames in movement either. (that is unless the gravity specifically moves them)
     
  8. Jan 9, 2010 #7
    Are there any links on this subject?

    Thank you.
     
  9. Jan 9, 2010 #8
    Re: Welcome to PF!

    This is significant.

    This dictates that all emissions are redshifted to Zero frequency at the surface of the event horizon?... Zero frequency has no energy.

    What happened to the energy?
     
  10. Jan 9, 2010 #9

    George Jones

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    Consider observers A, B, and C. Observer A hovers near the event horizon, and observer B hovers farther away from the event horizon and directly above A. Light sent from A to B is red-shifted. Light sent from B to A is blue-blueshifted. Even though never made explicit, this is the standard interpretation of your original post.

    But what about non-hovering observers? Suppose that C also receives light from A at the same position as B, and that C is moving with respect B. Consequently, there is a Doppler shift between B and C, and this Doppler shift can be such that there is a blue-shift between A and C, or even no shift at all! This was how the Pound-Rebka experiment measured the gravitational shift,

    http://en.wikipedia.org/wiki/Pound–Rebka_experiment.

    I know that I didn't say anything that DaleSpam and tiny-tim didn't already know.
     
  11. Jan 9, 2010 #10
    Re: Welcome to PF!

    Thank you. The link is very help full.

    OK, so light can be red shifted by gravity and this is proven measurable through experiment. And the degree of redshift if proportional to the amount of gravity present....

    This leads me to ask a few other questions though.

    Light waves and gravitational waves are generally directional and remain that way unless they encounter some sort of interference.

    Both light waves and gravitational waves travel at the same speed in a vacuum correct? Both at the universal constant speed of light.
     
  12. Jan 9, 2010 #11

    George Jones

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    Yes.
     
  13. Jan 9, 2010 #12
    A sun will emit light waves.
    A black hole will emit gravitational waves.
    Both waves travel at the speed of light. (theoretically)

    A black holes total gravity is proportional to its mass.
    A black holes event horizon distance from the center point is proportional to its mass.
    As distance from the black hole decreases the intensity of gravity proportionally increases.
    As gravity increases the amount of redshift that can be applied to light increases to the point at which the frequency approaches zero while approaching the event horizon.

    Is it plausible to assume the frequency of the light would have some negative value beyond the event horizon?

    A negative frequency?
     
  14. Jan 10, 2010 #13

    tiny-tim

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    Any massive body, including the Sun, and any black hole, will emit gravitational waves if it changes shape.

    (i don't remember exactly :redface:, but I think it's only quadrupole and higher changes which do it)

    (Also, any body orbiting another body will emit gravitational waves (see http://en.wikipedia.org/wiki/Gravitational_wave#Power_radiated_by_orbiting_bodies") … so that includes matter orbiting a black hole.)

    An ordinary rotating black hole, behaving itself, won't emit gravitational waves.

    A black hole doesn't do anything a star of the same mass doesn't do (except, allegedly, emit Hawking radiation, which isn't gravitational), not even emit gravitational waves, nor "suck" anything in. :smile:
    Yes, its gravitational field is exactly the same as that "outside" a spherically symmetric star of the same mass and angular momentum (except, of course, that the "outside" goes further "in" and becomes more interesting there).
    No, it's proportional to d/dr √(1 - 2GM/r) (which for r >> M is GM/r2).

    For r very close to 2GM (the event horizon), with r = (1 + µ)2GM, it's approximately d/dµ √µ = 1/2√µ (not ~ 1/µ :wink:).
     
    Last edited by a moderator: Apr 24, 2017
  15. Jan 10, 2010 #14

    Dale

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    This is correct. Sometimes people will make analogies between EM waves and gravitational waves and they will make incorrect conclusions because they assume that you can have dipole gravitational waves just like you can have dipole EM waves.
     
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