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Black Holes: Observation of an astronaut falling

  1. Feb 1, 2012 #1
    Hey guys,
    I'm a new guy in this forum. I've been visiting it for 1 year actually but never decided to register and so I did.
    I am having a presentation on black holes at school and I want some clariffication of what's going on when falling into a black hole.
    Well let's assume that an astronaut is falling into a black hole.
    I read that he will seem to go slower and slower ( I think it said gravitational time dilation) and that the light will be also red shifted.
    Can someone make me some bullet points showing clearly what's going on and explaining briefly what happens?

    P.S English is not my native language so ignore any mistakes...
  2. jcsd
  3. Feb 1, 2012 #2


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    To an OUTSIDE observer, he appears to go slower. To himself there is no such effect.

    It sounds as though you are asking people here to do your work for you. You need to do your own research and write your own bullet points. If you have questions, people here will be happy to answer them (just NOT the question "would you please do my work for me?")
  4. Feb 1, 2012 #3
    No offense butI have devoted a lot of time and did a lot of research
    I just want some clarification on why light closer to the Event Horizon takes more time to travel to the observer and whether the astronaut will become freaking "RED" due to the redshifting. I know the general idea of general relativity, and thought that it was due to the curvature that the light takes more time to travel to the observer. But as I am going to be presenting this to high school students it's not a good idea to tell them about GR as I don't know much either.
    Last edited: Feb 1, 2012
  5. Feb 1, 2012 #4


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    Light near the event horizon takes longer to reach a far-off observer because of the distortion of spacetime due to the extreme gravity of a black hole. The closer to the EH the longer it takes, until finally, AT the EH, it never escapes. So yes, to a remote observer, infalling stuff appears to slow down and get redshifted, but to the infalling stuff, nothing happens (except sphagettification, which isn't exactly "nothing" but that can happen inside or outside the EH, depending on the size of the BH)

    If you've done so much research, why don't you post YOUR set of bullet points, and ask if anyone thinks you have any mistakes or serious omissions?
  6. Feb 1, 2012 #5
    So, The project is in greek so I will have to translate it. That is why I need that extra help.
    Observer=Ob, Astronaut=As, event horizon=EH

    1) The Ob will watch the As getting closer to the EH without any problems
    2) The closer the As gets to the EH the slower it seems to be moving In the eyes of the Ob
    3) As he gets very close to the EH he seems to be stopping and finally he seems to be stationary after passing the EH (Though it will seem to the Ob that he hasn't passed it)
    Then here comes they questions and not verified ones
    4) Due to redshifting, the light will be extended to higher frequencies until it passes the Visible spectrum and becomes radiowaves and etc.
    5) Will he emmit a final photon in a finitive/infinitive time and then dissapear after becoming redder due to redshifting
  7. Feb 1, 2012 #6


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    redshifting causes LOWER frequencies, not higher. As something recedes from you the wavelength increases, which is to say, the frequency decreases. Strong gravity has the same effect as if the object were receding from you.

    When the infalling object reaches the EH, there WILL be a "last photon" emitted which has any chance of ever being detected by a remote observed and all photons emitted thereafter will be trapped inside the EH.
  8. Feb 1, 2012 #7
    Oops I said the frequency increases and then said that this light will become radio waves! Doesn't make sense. oh by the way is there any way that the light becomes radio waves due to redshifting
    So after the last photon is received the As will be like invisible right?
  9. Feb 1, 2012 #8
    Yes once the As passes the EH no more photons will be visible to the Ob and the As will cease to have any causal relationship to the rest of the Universe except for the effects of increase in mass of the BH.

    The photons get redshifted to infinity due to the extreme gravitational driven curvature(essentially redshifting until their frequency would be too long to be detectable)

    From the point of view of the As all world lines and possible futures point inexplicably towards the singularity, whatever that may be.
  10. Feb 2, 2012 #9


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    All the relativistic affects you are worried about are largely irrelevant from the perspective of an observer in free fall. A stationary external observer will think you slow down as you approach the event horizon. You will also think the stationary external observer slows down as you approach the event horizon. Photons struggle just as much to reach you as they do the stationary EO.
  11. Feb 8, 2012 #10
    According to the following article the appearance of a person who is falling into a black hole remaining stationary at the event horizon is just an optical illusion caused by the light being trapped there. Nothing more.

    It is of little comfort that a glorious image of myself remains behind for all to see when in reality I have long since been unceremoniously spaghettified and devoured.

    Last edited by a moderator: Sep 25, 2014
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