Escape the Horizon: Can I Pull My Friend Back?

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Discussion Overview

The discussion revolves around the hypothetical scenario of extending a tether into a black hole's event horizon to retrieve a friend who has crossed it. Participants explore the implications of this scenario, including the physical limitations and theoretical considerations surrounding black holes, event horizons, and the nature of gravity and light in such extreme environments.

Discussion Character

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

Main Points Raised

  • Some participants argue that it is impossible to pull someone back from beyond the event horizon due to the infinite force required and the inability of outside observers to see someone cross the horizon.
  • Others propose that the interpretation of "pulling back" could vary, suggesting that if the spaceship were below the event horizon, it might be possible to exert some influence on the tether.
  • There is a discussion about the propagation of tension through the tether, with some questioning whether this implies a maximum tension due to the speed of light limit.
  • Some participants express that once any part of a person crosses the event horizon, they are effectively lost, emphasizing the strict rules governing black hole horizons.
  • Several participants highlight the paradox of attempting to retrieve someone who has crossed the event horizon, suggesting that the only outcomes are to cut the tether or follow the friend in.
  • There are considerations about the implications of being in a free-fall frame and how that might affect the ability to manipulate the tether without breaking it.

Areas of Agreement / Disagreement

Participants generally agree that once something crosses the event horizon, it cannot be retrieved. However, there are competing views regarding the nuances of how one might interact with the tether and the implications of being in different gravitational frames.

Contextual Notes

Participants note that the discussion involves complex theoretical concepts that may not have definitive answers, particularly regarding the behavior of objects near black holes and the nature of event horizons.

Who May Find This Useful

This discussion may be of interest to those exploring theoretical physics, particularly in the context of black holes, general relativity, and the implications of extreme gravitational environments.

RandyD123
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Suppose I extend a tether into a BH horizon. My spaceship is on the outside and my friend is on the end of a very long tether and crosses the event horizon...can I pull him back?
 
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No, for two reasons I can think of:
1) The force required to accelerate them back past the event horizon would be infinite.
2) You as outside observer actually would never be able see them cross the event horizon in the first place; they would seem to get closer and closer to the event horizon but never actually reach it because even light from them would have a difficult time reaching any outside observers.
 
Just to add to Dalespam's link, which discusses the Rindler horizon, it is worth noting that for a small region, a BH horizon is mathematically and physically equivalent to Rindler horizon. This makes Dalespam's link precisely on target for the OP.
 
To add to the above, it may depend on what you mean by "pull him back". You could pull him back to the ship, but by the time this happens, it would necessitate that the ship is below the event horizon.
 
stedwards said:
To add to the above, it may depend on what you mean by "pull him back". You could pull him back to the ship, but by the time this happens, it would necessitate that the ship is below the event horizon.
Yes, that is a valid possibility, if you assure that the rocket does not accelerate so as to break the rope.
 
Well beyond my understanding but it is fascinating to think of it!
 
PAllen said:
Yes, that is a valid possibility, if you assure that the rocket does not accelerate so as to break the rope.

That does bring up an interesting point. Tugging on one end of the rope cannot propagate to the other end faster than c. I'm curious if this implies a maximum tension of some sort?
 
stedwards said:
That does bring up an interesting point. Tugging on one end of the rope cannot propagate to the other end faster than c. I'm curious if this implies a maximum tension of some sort?
As with many of these types of thought experiments, it implies the rocket has to know how to move to avoid breaking the rope. Similar to Born rigid acceleration.

[edit: actually, I think it is not so hard. If the rocket adjusts its acceleration and pull on the rope so as never to approach the local breaking point near the rocket, while the other end is in free fall, I believe that will suffice. However, I have not done any math to validate this.The idea is simply that local physics in a free fall frame must be the same as SR. Thus, if the rocket acts consistent with pulling in a hanging object in an empty region, then expressing this relative to this local free fall frame should produce the same effect. As you noted, a consequence is that the the rocket must allow itself to cross the horizon to pull up the object.]
 
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  • #10
I guess that I'm thinking that obviously there is a point of no return. But that to me suggest a "hard" line. If I just put a finger over that line, does that mean I can't ever come back? The rest of me is on the "good" side and only a finger has crossed over.
 
  • #11
Well you've got a short time to decide - cut the finger off or follow it. A very short time - actually I suspect you must decide before the finger has crossed, and certainly before you see it crossing - by that time it is too late, you are inside already.
 
  • #12
But if light cannot escape from the event horizon then you would never see it cross to begin with.
 
  • #13
RandyD123 said:
Suppose I extend a tether into a BH horizon. My spaceship is on the outside and my friend is on the end of a very long tether and crosses the event horizon...can I pull him back?
It's far more likely that instead of the tether attached to your friend pulling him out, it would result in you being pulled in.
 
  • #14
It is not just more likely - there are only two possible outcomes : you can cut your friend loose, or follow him in. There is no scenario where you can pull him out. Black holes have strict rules about horizon trespassers, and they never joke about applying the rules.
 
  • #15
paisiello2 said:
But if light cannot escape from the event horizon then you would never see it cross to begin with.
You can't both see it and stay outside. You must cut your finger off before you see it cross the horizon - if you don't you will see it. But you will be inside then.

Or better, don't let your finger go past the horizon.
 
  • #16
wabbit said:
It is not just more likely - there are only two possible outcomes : you can cut your friend loose, or follow him in. There is no scenario where you can pull him out. Black holes have strict rules about horizon trespassers, and they never joke about applying the rules.
True, but if you are yourself falling into a different black hole it gets interesting.
That tether has to be made of pretty strong stuff though.
 
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