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

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Discussion Overview

The discussion revolves around whether an observer inside the event horizon of a black hole can determine their location relative to the event horizon while inside a sealed vessel. Participants explore the implications of tidal forces, the nature of the event horizon, and the observer's ability to perform experiments to ascertain their position.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that if the black hole is large, the observer may not feel any effects and thus cannot determine if they are inside the event horizon.
  • Others propose that tidal forces could be measured, and there is a question about whether these forces differ qualitatively inside versus outside the event horizon.
  • One participant mentions that an observer in free fall would not notice crossing the event horizon, as it is "invisible." However, if the observer were to remain stationary, they would experience increasing tidal forces.
  • There is a discussion about the impossibility of remaining stationary with respect to the singularity after crossing the event horizon, with some participants questioning this assertion.
  • Some participants argue that without knowledge of the black hole's mass, the observer cannot determine if they have crossed the event horizon.
  • One participant references an equation for tidal forces, suggesting that it remains the same inside and outside the event horizon, and discusses how this could be used to infer the observer's position.
  • There is confusion regarding the concept of "g-force" approaching infinity as one nears the event horizon, with participants questioning the implications of finite characteristics of black holes.
  • Another participant raises a common interpretation that time stops for an observer at the event horizon, prompting further clarification and debate on this point.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the observer's ability to determine their position relative to the event horizon, the nature of tidal forces, and the implications of remaining stationary inside the horizon. The discussion remains unresolved with no consensus reached.

Contextual Notes

Participants note that the observer's experience may depend on their motion (free fall versus stationary) and that the effects of tidal forces may vary based on the size of the black hole. There are also unresolved questions about the nature of gravity and time at the event horizon.

nickyrtr
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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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No

If BH is huge, he won't feel anything.

Note: I am ignoring the "blue sheet" problem for the rotating BH.
 
Dmitry67 said:
No

If BH is huge, he won't feel anything.

Note: I am ignoring the "blue sheet" problem for the rotating BH.

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?
 
Tidal forces
 
Dmitry67 said:
Tidal forces

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?
 
Is there any experiment he can perform that would reveal that he is inside the event horizon?

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.
 
Naty1 said:
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.
True.

Naty1 said:
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.
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).

Naty1 said:
If he accelerates to remain stationary with respect to the singularity after passing inside the horizon...
That is impossible.
 
Attached is an extract from 'Exploring Black Holes' (the full chapter is readily available http://exploringblackholes.com/" , 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).

nickyrtr said:
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?

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" (page 9) takes into account radial stretching and sideways compression.
 

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nickyrtr said:
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?
If he does not know the mass of the black hole I do not think he can determine he passed the event horizon.
 
  • #10
stevebd1 said:
The equation for tidal forces is the same inside and outside the event horizon which is [itex]\Delta g=2M/r^3[/itex].

Thank you, that is very illuminating.

DrGreg said:
If he does not know the mass of the black hole I do not think he can determine he passed the event horizon.

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.
 
  • #11
If he accelerates to remain stationary with respect to the singularity after passing inside the horizon...

Dr Greg: That is impossible.

WHY?

My understanding is reflected in post #8...
 
  • #12
Naty1 said:
stationary with respect to the singularity

Inside the event horizon, what does this mean?
 
  • #13
stationary with respect to the singularity.
Inside the event horizon, what does this mean?

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...
 
  • #14
Naty1 said:
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...

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.
 
  • #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 ...,
thanks,
 
  • #16
Naty1 said:
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...

What do you mean by "finite gravity"?
 
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  • #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?
 
  • #18
Naty1 said:
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...
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.
 
  • #19
Which kind of means our universe can be a massive singularity captured by a zillion times massive black hole. 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?
 
  • #20
FawkesCa said:
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?
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.
 
  • #21
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.

ok, I have actually read exactly that...thanks Jesse...hard to accept intuitively, but correct...

So my only question then is whether an accelerating observer just outside the horizon experiences infinite gravitational forces approaching the horizon...I am still unsure because viewed from a great distance, say a black hole as observed from earth, time seems to stop for that hovering astronut. From that observer, gravitational potential would likely appear infinite?? How can a stretched horizon exist there one Planck length outside the event horizon...

I am unsure just what a local accelerating observer would witness...they do get promptly 'thermalized' by radiation...what else??
 
  • #22
JesseM said:
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.

THANK YOU SOOOOOO MUCH! finally i understand something. damn those sci-fi shows screwing me up
 
  • #23
Naty1 said:
So my only question then is whether an accelerating observer just outside the horizon experiences infinite gravitational forces approaching the horizon...
The amount of acceleration for a stationary observer, e.g. an observer which accelerates away from the black hole so that the reduced radius stays a constant r, approaches infinity towards the event horizon.
 

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