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Can an observer know he's in a black hole?

  1. Jul 20, 2010 #1
    My question is about an observer inside the event horizon of a black hole, but not yet at the singularity. Let us say the observer is inside a sealed vessel, so he can only measure local properties of space-time, and their derivatives with respect to space and time. Is there any experiment he can perform that would reveal that he is inside the event horizon?
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  3. Jul 20, 2010 #2

    If BH is huge, he wont feel anything.

    Note: I am ignoring the "blue sheet" problem for the rotating BH.
  4. Jul 20, 2010 #3
    What if the BH is not so huge? In other words, what effect is observable to first order in (1/Rs) where Rs is the Schwarzschild radius?
  5. Jul 20, 2010 #4
    Tidal forces
  6. Jul 20, 2010 #5
    Are the tidal forces inside the event horizon qualitatively different from those outside the horizon? For example, does the tidal force extend in all three spatial directions rather than just one direction? Does the tidal force inside the horizon compress rather than stretch?
  7. Jul 20, 2010 #6
    First, what he observes depends on his motion: free fall or stationary via acceleration. no question about that.

    I am not positive, but I think both these are accurate:

    Tidal forces gradually and smoothly increase for a free falling observer without respect to the horizon...the horizon is invisible. He could compute the tidal forces at the circumference of the horizon and then compare actual measures as he freely falls to determine whether he is inside or outside. He cannot stop and measure as he would be fried outside the horizon....virtual particles become thermal radiation via the UNRUH effect outside the horizon.

    When in free fall the observer passes thru the event horizon without knowing it. If he accelerates to remain stationary with respect to the singularity after passing inside the horizon, the event horizon will appear above him and I think he'll be cut off from any outside starlight. As he approaches the horizon in free fall from the outside, light arrives shifted to the vertical so that objects formerly on the horizon rise to a vertical position...a single bright spot above is observed, according to Leonard Susskind....I assume that is shut off as I described when he stops inside to make observations...that spot disappears.
  8. Jul 20, 2010 #7


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    True, but the Unruh radiation would be caused by the observer's very high (proper) acceleration, regardless of where in the Universe he is. Sealed inside his box, he has no idea whether he is stationary relative to the singularity or not (assuming he hasn't done the calculation mentioned above).

    That is impossible.
  9. Jul 21, 2010 #8
    Attached is an extract from 'Exploring Black Holes' (the full chapter is readily available http://exploringblackholes.com/" [Broken], InsideBH100607v1.pdf page 8) which could be described as a method of calculating when you cross the event horizon (for a rain observer at least, i.e. an observer who has fallen from rest at infinity).

    The equation for tidal forces is the same inside and outside the event horizon which is [itex]\Delta g=2M/r^3[/itex]. http://www.mrao.cam.ac.uk/~steve/astrophysics/handouts/overheads2.pdf" [Broken] (page 9) takes into account radial stretching and sideways compression.

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  10. Jul 21, 2010 #9
    If he does not know the mass of the black hole I do not think he can determine he passed the event horizon.
  11. Jul 21, 2010 #10
    Thank you, that is very illuminating.

    The link in steve's post show how the circumferential radius, r, could be measured by the observer. If the tidal force is also measured, the observer could solve for M using steve's formula: [itex]\Delta g=2M/r^3[/itex] -> [itex]M=r^3 \Delta g/2[/itex], thus he would know if he is inside the event horizon, unless I have misunderstood something.
  12. Jul 21, 2010 #11
    Dr Greg: That is impossible.


    My understanding is reflected in post #8....
  13. Jul 21, 2010 #12

    George Jones

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    Inside the event horizon, what does this mean?
  14. Jul 21, 2010 #13
    All I meant was that the tidal forces and gravity are only incrementally stronger just inside rather than just outside the horizon...so an incremental increase in power to accelerate is all that should be required....

    but I suspect Dr. Greg and you and may know better....
  15. Jul 21, 2010 #14

    George Jones

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    Yes, this is true for freely falling observers, but the "g-force" on an observer who hovers (i.e., remains stationary) outside the event horizon approaches infinity as the position of the hovering observer approaches the event horizon.
  16. Jul 21, 2010 #15
    Hi George:

    Can you explain why the g force approaches infinity??

    My understanding is that the black hole horizon is of finite area, finite gravity,finite entropy, and finite temperature....I did not expect a physical characteristic to approach infinity as you describe...
    BUT.....the other argument as I think about it is that once inside you can't get back out, [like a roach motel] so something else must be going on I have not recognized ...,
  17. Jul 21, 2010 #16

    George Jones

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    What do you mean by "finite gravity"?
    Last edited: Jul 24, 2010
  18. Jul 21, 2010 #17
    im a bit confused (not that suprising). the way it's always been described to me, once a person reaches the event horizon of a black hole, time stops for them. an outside observer sees them enter the black hole, but for the person entering, time stops and they are permently stuck at that moment. do i have this wrong?
  19. Jul 21, 2010 #18


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    Inside the horizon, every point in your future light cone is at a smaller Schwarzschild radius than your current position. Also, the radial Schwarzschild coordinate is actually timelike rather than spacelike inside the horizon, so just by letting your clock tick into the future you get "closer" (in terms of the Schwarzschild radial coordinate) to the singularity, you can't avoid hitting the singularity by accelerating in some direction any more than you can avoid the year 2011 be accelerating. All of this is a lot clearer if you use Kruskal-Szekeres coordinates rather than Schwarzschild coordinates to draw a spacetime diagram, see here and here.
  20. Jul 22, 2010 #19
    Which kind of means our universe can be a massive singularity captured by a zillion times massive blackhole. And we might not even know it.

    And think that our whole universe comes from a single bright spot and that spot might be the collective light and all coming from outside of the event horizon. And everything coming from there on free fall towards the singularity.

    Oh wait.. We have Big Bang Theory and a thorough mathematical model about it.

    But i was going to ask about Blackholes inside Massive Event Horizons, would they have some unique properties?
  21. Jul 22, 2010 #20


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    You've got it exactly backwards--the outside observer never sees them reach the horizon, but they experience crossing it in a finite time according to their own clock. See this entry from the Usenet Physics FAQ for more info.
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