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Are Black Holes Camouflaged

  1. Dec 17, 2011 #1
    To my understanding, as an object approaches the event horizon of a black hole, the light that is seen by a relatively static observer slows down more and more until the light is eventually "trapped" on the event horizon, making it appear to the observer as though the object is standing still. If this is wrong, then you can probably ignore the rest of my post.

    If this is true, then wouldn't black holes appear much like a desolate planet? Gasses, dust, and debris would eventually give the appearance of a surface that could cause unwary travelers to be sucked in. Is there a flaw in my reasoning somewhere? If so, where?
  2. jcsd
  3. Dec 17, 2011 #2


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    You are mixing things up a little. The distant observer, who sees everything come to a halt (getting redder and redder to the point of being invisible), cannot be an unwary traveler. Anyone getting close to the black hole will experience a very strong gravitational pull.
  4. Dec 17, 2011 #3
    Can gravity cause the Doppler Effect in addition to velocity? And I realize that the identity of a black hole could easily be determined gravimetrically, but the idea of someone being pulled in was never meant to be the main idea of the post. Anyway, if the Doppler Effect turned outgoing light infrared and lower, couldn't scientists theoretically determine the size and location of a black hole by measuring this steadily increasing wavelength assuming that they could get close enough to make these measurements?
  5. Dec 18, 2011 #4
    Here lies the problem. The thing to make a note of is that this steady flow of photon/s will take forever/ infinite amount to reach us so at no point would we have a finite value of the wavelength ( correct me if I am mistaken).

    Doppler effect is a day to day phenomena , you can observe that without the need of gravity for instance an oncoming ambulance will have a loud siren noise that will subside as it passes away (it's dependent on the velocity of the source /radial velocity)
  6. Dec 19, 2011 #5
    Your original question is somewhat similar to a post I made recently. It does seem to me that a stationary distant observer would be able to see the last reflected photons that were able to escape the BH before crossing the EH. Now, obviously this isn't the case because we in fact cannot see them.

    I'm guessing that the cause for this is the photons that are able to make it away from the BH, (read: didn't get close enough to the BH to get caught) are infinitely red-shifted out of the visible range and are also scattered in random directions.
  7. Dec 19, 2011 #6


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    There seems to be some confusion about this shifting. Something emitting photons approaching a black hole would appear to us (distant observer) to be slowing down and the photons would get redder and redder. To us the object would never get to the black hole.
    However, in the reference frame of the object itself, it would simply fall in.
  8. Dec 20, 2011 #7
    This I understand. But does this phenomenon have to occur while said distant observer is watching for it to be visible afterward? Take the same scenario and distant observer, except this time add another distant observer who does not discover the BH until after the "something emitting photons" has been sucked into the BH.

    Would the second distant observer be unable to see the photons because he didn't actually watch the object approach the EH?

    The reason I ask is that to us BH's are, unless currently devouring matter, completely invisible.
  9. Dec 20, 2011 #8
    Actually not quite true.

    Remember if BH's are not currently gaining mass then they are expected to be losing mass via Hawking radiation. This does create a slight temperature difference between the CMB and the black hole which would allow for detection.
  10. Dec 20, 2011 #9
    I should have specified; as by invisible I was implying that it would be invisible to the optical telescope aided eye. Or since this is a completely theoretical situation, the naked eye.

    I'm thinking here of a BH in empty space, we know they are out there but we can't see them because they are not currently devouring anything. This is not to say they haven't in the past, in fact we can say with almost complete certainty that they have at some point destroyed something. However since we did not directly observe that particular incident, our only means of detection would be comparing the BH temperature with CMB. This of course we would need to know the exact location of the BH anyway to even find it.
  11. Dec 20, 2011 #10


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  12. Dec 20, 2011 #11
    This is precisely what I was saying; when matter is not currently falling into the black hole the only way to detect it would involve specifically looking for the "hole" in your viewing area.

    When matter is currently falling into the BH, we observe (from a distance) the destruction / gasses heating up directly through visible as well as infrared/X-Ray/other means, although we still cannot directly observe the BH itself.

    This "matter" should, according to whether or not I'm interpreting the theory correctly, appear to freeze in time just before crossing the EH as long as we are watching it. If this matter is frozen in time, wouldn't it forever be visible from the point of the same distant observer?
  13. Dec 20, 2011 #12


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    No, I think you had it right above when you said it gets redshifted out of visibility

    EDIT: also I think (but am not positive) that it is NOT the matter that is "frozen in time", it's the photons that it emits. The photons that it emits (or that reflect off of it) as it moves towards the event horizon take longer and longer to reach us but the object goes on its merry way (well, not all that merry, since it gets crushed by the black hole)
  14. Dec 20, 2011 #13
    I feel like this conversation is getting red-shifted out of comprehension.

    Thank you for pointing out my self contradiction, I meant the former when I typed the latter. I completely agree that from the perspective of the distant observer, they are observing not the matter itself but the reflected photons.

    I think the point that's being beat around the bush on here is the following:

    Observer and Matter. Matter falls into BH and Observer is observing from a distant point.

    Matter falls through EH and is consequently destroyed.

    Observer watches matter approach EH. The closer that Matter gets to the EH the greater the effect of red-shift. I think there would be two things happening here.

    #1 Photons reflected off of Matter are red-shifted
    #2 Quantity of photons reflected in the direction of the observer would decrease. (Due to tidal forces, etc)

    The combination of these two effects would produce a red-shifted image that appears to be slowing down. Eventually it would be red-shifted to the point that it would no longer be detected by visible light.

  15. Dec 20, 2011 #14


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    Well *I* think you've got it right, but I'm no expert so take that with a grain of salt.
  16. Dec 21, 2011 #15
    This is mostly true but needs caveats. As the matter proceeds past the EH it increases the mass of the BH. By increasing the mass of the BH, the EH expands, which means the EH is now further into space then previously and now encompasses photons previously outside the EH.

    So your argument, while essentially correct does not take into account expansion of the EH due to mass increase in the BH.

    Hope this helps.
  17. Dec 21, 2011 #16
    Ah thanks I hadn't even considered that. Is there a specific formula for the increase in diameter of the EH in relation to increase in mass?

    I need a book >.<
  18. Dec 21, 2011 #17


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