Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black hole, time and observations

  1. Jan 11, 2011 #1
    Hi All,

    Before I state my questions, lets test my assumptions / background knowledge:
    (The classical explanation?) Aaron and Bongani (sorry Alice and Bob is so boring) sits in a spaceship near a black hole. Aaron gets out and speeds towards the black hole. Aaron will experience acceleration due to the gravitational pull of the black hole, x m/s2.

    Bongani's observation is that Aaron's time will slow down (m/s2) as he approach the event horizon (correct term?), to the degree that it seems that Aaron is frozen in time, just outside the event horizon.

    Aaron's observation is that he keeps accelerating until he pass the event horizon and the "puff!" gone - or what ever happens inside the event horizon.

    Now Bongani gets bored looking at Aaron and goes back to the space station. After a long time; Bongani's grandchildren looks at the Black hole and still sees Aaron "stuck" just before the event horizon. They wonder when he'll ever "go in".

    Correct so far or is it a http://en.wikipedia.org/wiki/Zeno%27s_paradoxes" [Broken]?

    I assume it is correct (else the rest is invalid):

    Bongani's grandchildren observers another event; a very big star flying towards the black hole, perpendicular to their line of sight of the black hole and on a collision course (straight line, although it is improbable). Because of the size of the start, the grandchildren observers that the "side" of the start closets to the event horizon starts to slow down, while on the other side, the observe deceleration is slower, thus flattening the star into a 2D disk. (But for this star's perspective, nothing but acceleration pass the event horizon happens) Also never passing the event horizon?

    A second star comes along, following an elliptic path around the gravity well of the black hole. It is going to pass very close to the event horizon. Time for the particles (photos, magnetic fields, etc?) closest to the event horizon will slow down a lot more than the particles further away from the event horizon? Will it be observed that this star seem to be smeared across the event horizon?

    If these observation, far away from the event horizon, is valid, then our observation of a black hole will be a mass of "stuff" stuck just outside the event horizon, even producing light (from the "trapped" stars)?

    Like I stated, my assumptions and logic needs to be analysed.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 16, 2011 #2
    I'm just a layman, but the same thought has crossed my mind. If everything that falls into a black hole seems to freeze at the event horizon to outside observers, then how can it still be black? Wouldn't we, as outside observers, be seeing all the stuff that has fallen in over the lifetime of the black hole frozen all over its event horizon?
  4. Jan 16, 2011 #3
    There is something called: "Gravitational Redshift" http://en.wikipedia.org/wiki/Gravitational_redshift , which explains why, even if we don't see that "stuff" if it's frozen at the event horizon FROM OUR Frame of Reference.

    See, this is part of the problem; you're all asking a question that has Relativity at its heart, but you're thinking in terms of universal standards of spacetime that don't exist. If you were falling into a black hole, you might fail to notice that you had even passed the Event Horizon (EH) of the Black Hole (BH), if the BH is massive (big) enough! From the perspective of Bongani, Aaron gets redder and redder... dimmer and dimmer, slower, and slower...

    Aaron could spend an infinite (indeterminate) amount of time watching Bongani fall, although most of this time-dilation would happen VERY close to the Event Horizon. Aaron, because the gravitational 'pull' on his feet would be FAR greater than his knees, waist, chest, and head... becomes a subatomic noodle, usually called "Spaghettification" http://en.wikipedia.org/wiki/Spaghettification .

    I hope this helps you (TungstenX, Hello!) somewhat. I want to fast-forward on your to the end: NOBODY can be sure, but remember: frames of reference... this is Relative. http://en.wikipedia.org/wiki/Frame_of_reference

    Once we get there, it isn't simple.
  5. Jan 16, 2011 #4
    Gravitational time dilation goes hand in hand with gravitational redshift (stretching) of light. Mathematically, they are just inverse of each other. Slower the time (for distant observer), greater the redshift. So stuff that is falling into black hole fades to black, thus black hole remains black.
  6. Jan 16, 2011 #5
    Ah, that makes sense, I think. So, to the outside observer, the gravitional red shift would take the light coming from the falling subject all the way to black when we see time stop for them and they are "frozen" at the event horizon?
  7. Jan 16, 2011 #6
    Well... really REALLY dim, and REALLY REALLY red, but yeah, black works. Remember that from the infalling person's perspective they are NOT frozen... they pass through without any of this hand-wringing. It's only us, watching from a different frame of reference that believes Aaron is still OUTSIDE of the EH.
  8. Jan 16, 2011 #7
    Aaron would be pulverized by the black hole and Bongani would never see it happen. To Aaron, the tripe could take seconds to minutes, then puff, pulverized. Most of Aaron’s mass would be sucked into the black hole and some of Aaron's molecules could form a really nice jet, fireworks, something like the 4th of July. But Bongani will die waiting for Aaron to fall into the black hole, and will never see the fireworks. Bongani’s great grand kids may see it, but only if they have an interest in astronomy.
    Last edited: Jan 16, 2011
  9. Jan 16, 2011 #8
  10. Jan 17, 2011 #9
    Thank you very much for the responses. I'll need to find a frame of reference that contains some spare time to read all the refs :smile:

    Thus I'll give some response after updating my background knowledge on this subject.
  11. Jan 17, 2011 #10
    Take your time, ask questions (I'm the least of this community, believe me), and keep reading. You'll get it all.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook