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B The End... Of Redshift?

  1. Jun 18, 2016 #1
    The expansion of our universe stretches out light causing the phenomenon known as "redshift."

    When light falls into a black hole... Its wavelength gets stretched out infinitely? Light with a flat wavelength? Or is there some limit to how much a wave of light can be stretched out... There's a limit to how fast light can move... so why not a limit on redshift... and would a black hole's gravity be enough to reach it?

    If so, what would this light look like? Could we even detect it? Could black holes be shining with light that we don't understand?
  2. jcsd
  3. Jun 18, 2016 #2


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    Actually I think it blueshifts as it falls in, just like how it behaves when it falls to Earth. But this may depend on your choice of coordinate systems and observers, so I'm not sure.

    There is no limit. A light wave can be stretched or compressed to any arbitrary wavelength.

    No. Light that falls into a black hole stays there. It can never escape from beyond the event horizon.
  4. Jun 18, 2016 #3
    "There is no limit." ... Well that was quick : ) Somebody delete this thread! lol. Thanks Drakkith for the shared knowledge.
  5. Jun 19, 2016 #4


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    To a distant observer the light is red shifted.
  6. Jun 20, 2016 #5


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    Can you elaborate on that?
  7. Jun 20, 2016 #6


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    Let me jump in.
    If you're an observer deep in a gravitational well, you see light from a source at higher potential, falling in, as blueshifted. If you're an observer high in a gravitational well (i.e., distant observer), you see light coming from a source deep in the well as redshifted.
  8. Jun 20, 2016 #7


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    Light coming toward you if you are 'near' the event horizon will be red shifted if rising from deeper in the potential well and blue shifted if coming from 'outside' the well. If you're not 'in' the well, then light falling into it will be red shifted. (keeping in mind that the 'well' is theoretically infinite in size, but practically limited by the 1/r² nature of gravity.). Light will cross the event horizon at time ∞, meaning the event horizon isn't a surface in space, it is a surface in space-time. Also note that while theoretically light can have any wave-length, it isn't meaningful (since we can't measure it, ever) to consider light with wave-lengths longer that the diameter of the Observable Universe. IOW, there is a real limit on the longest wave-lengths (that being 13.8 billion light years). Beyond that, there be dragons.
  9. Jun 20, 2016 #8


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    Oh I understand that. I just didn't know what exactly mathman was getting at since the OP was talking about light falling into the black hole, which I took to be light falling into the event horizon. If this is all he meant then it makes perfect sense.
  10. Jun 21, 2016 #9
    I think you don't understand the concept of rates of change and limits yet. I didn't really grasp it until I took calculus.
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