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Curving light undergoing redshift

  1. Dec 5, 2009 #1
    If light is moving on a curve, for instance, if light were bent around a massive body thus curving due to gravity, does it undergo any redshift of blueshift?

    I ask this because the universe may not be expanding (what we see as redshift), but may just be affected by gravity, and curving, thus undergoing redshift?
  2. jcsd
  3. Dec 5, 2009 #2
    Good question. Light will shift red from our frame, and from the light's frame, all light sources outside the gravitational mass will shift to blue.
    Please understand, though, that light does not "curve" under the influence of gravity. In fact, the structure of space itself is determined by the presence of matter (in other words, where there's matter there's space curvature). What does that mean? That means that light is taking the shortest possible path through a gravitational field, even though to us, it appears as a curvature. It's not the light that "curves;" but it's the space the light is travelling through that's curved! The light is just taking the shortest path between point A and point B, and that path just happens to be curved.
  4. Dec 6, 2009 #3
    Curved from the Earth's frame of reference, right? So it looks like it's curved to us, because of the light's apparent change in direction? (From Earth's frame of reference, of course.) So say that a photon were orbiting a black hole on it's event horizon, would the light continuously redshift? (theoretically speaking, because we would never see it anyways)
  5. Dec 6, 2009 #4
    Yes, if the universe is rotating we would definiely observe red shifts like those observed by Hubble
  6. Dec 6, 2009 #5


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    I read recently that the event horizon of a black hole is a curtain of trapped orbiting photons that have neither the trajectory to escape or fall in. If this light is constantly red shifting where does the energy go? Hawking radiation?
  7. Dec 6, 2009 #6
    The event horizon is just an infintesmally-thin barrier whereby any EM radiation that is closer to the singularity will fall in to the BH, and any EM radiation that's farther from the singularity in relation to the event horizon will eventually make its way into interstellar space.
    Hawking radiation is nothing more than virtual particle-pair formation at the event horizon. The idea behind the Hawking Radiation is that the positron generated during particle-pair formation will be inside the event horizon (hence get pulled in by the singularity), and the electron generated during particle-pair formation will be outside the event horizon, making its way into interstellar space. Thus, we may see black holes indirectly by the positrons and electrons its event horizon emits.

    Sure, the light is constantly red-shifting (shifting farther into the red the closer it approaches the gravitational mass), but that light is not giving up any energy. The light, however, is giving up something: It's giving up its time relative to us. Let me explain. Since the light is red-shifting, its time--measured relative to us--will seem to slow down. That means that the processes occuring on that red-shifted light beam will appear to be moving in slo-mo. Since the light's time frame is slowing down, it stands to reason that a redshift should be observed (in other words, if you were to somehow slow down the frequency of a green laser beam, its wavelength would shift from green to red, and without giving up any energy in the process. Makes sense, right?).
    Likewise, if you were 'riding' on that light source that's shifted red, you'd see everything out in interstellar space shift to the blue. Of course, that also means that every processses you observe from this frame will seem to be moving in fast-forward, like playing a dvd in fast-forward.
  8. Dec 6, 2009 #7
    You are correct; we'd see the light curved from our earthly perspective. The phonomena is called, 'gravitational lensing.'
    From your theoretical consideration, you'd 'see' (or rather 'know') the light redshifting as it orbits the event horizon and orbits away from you, and as the light comes around in its orbit toward you, you'd see it shift blue.

    (Please note: in all actuality, you'd see a greater red shift as the photon orbits the black hole and moves away from you (radio frequency, perhaps?), and as it continues to orbit the black hole and comes toward you, it would be shifted closer to the microwave region of the EM spectrum.)
  9. Dec 6, 2009 #8
    Delete 'rotating' and replace it with 'expanding'.
    Believe it or not, the known Universe is not only expanding, but also accelerating!
    It's presumed this acceleration is due to stuff called, "dark matter/dark energy."
  10. Dec 7, 2009 #9
    Sorry, I recently realized that light doesn't orbit at the event horizon, it orbits a bit away from the event horizon, actually.

    Also, if black holes colliding creates a gamma ray burst, wouldn't that mean that the light is blueshifting?
  11. Dec 7, 2009 #10
    When electron and positron annihilate, gamma rays are released. Since the event horizon of both black holes are radiating these two particles, it stands to reason that gamma rays would be seen. The gamma rays will appear far less energetic at first, but as their momentum carries them farther from the event horizon, they appear to get more energetic. When the gamma rays are of sufficient distance from the black hole, a gamma ray detector will see exactly that: gamma rays.
    Since black holes are so far from us, we would indeed see gamma rays eminating from the proximity of the black hole, and not something 'blue-shifted'.
  12. Dec 9, 2009 #11
    Could it be possible that there is enough black holes/star clusters/neutron stars/dark matter to create these curves, and thus these redshifts, that would fool us into thinking that the universe is expanding (while not really expanding), because of all the redshifts we perceive?

    The universe could also be collapsing, which would also cause very severe gravity and curves for light to follow, and consequently, continuously redshifting light.

    Also, if the universe were rotating in such a way to create some pseudoforce or an inertial force, couldn't that also create curves for light to redshift on?
  13. Dec 10, 2009 #12
    Obviously, if our telescopes are seeing nothing but interstellar redshifts, then the entire Universe must be expanding. And since these redshifts are indeed getting redder, it's equally obvious that interstellar objects are accelerating.

    If our Universe is collapsing, then all interstellar objects would have to display a blueshift, as it would appear from our vantage point that these objects are coming closer to us, speeding to an unidentified 'center' of some sort. The blueshift is not observed, meaning there is no collapsing taking place. Remember, redshifts occur when things (aircraft, trains, galaxies) are moving away from us.
    In order for you to see this, you must visualize yourself as a raisin in the middle of raisin bread baking in the oven. As the bread expands and rises, all other raisins under your observation will appear to move away from you. Not a single raisin in the rising bread will appear stationary, or moving toward you. Not one. Every raisin is moving away from you! Likewise, since we live in an expanding, 'rising' Universe, all interstellar 'raisins' (galaxies; other interstellar objects not in our Milky Way galaxy) appear to move away from us.

    Objects in our Universe are not rotating around an unidentified center. If they were, we'd see some interstellar objects exhibiting a blueshift, and other objects exhibiting a redshift.
  14. Dec 11, 2009 #13
    Does gravitational lensing cause blueshifts or redshifts?
  15. Dec 11, 2009 #14
    Relative to us? Redshifts.
  16. Dec 11, 2009 #15
    Thanks for all of your help!
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