Escape the Horizon: Can I Pull My Friend Back?

[RandyD123]

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?

 

[paisiello2]

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.

 

[Dale]

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?

No, but this is an interesting question. Here is a fairly detailed examination of it:
http://gregegan.customer.netspace.net.au/SCIENCE/Rindler/RindlerHorizon.html

 

[PAllen]

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.

 

[stedwards]

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.

 

[PAllen]

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.

 

[RandyD123]

Well beyond my understanding but it is fascinating to think of it!

 

[stedwards]

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?

 

[PAllen]

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 accleration 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.]

 

[RandyD123]

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.

 

[wabbit]

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.

 

[paisiello2]

But if light cannot escape from the event horizon then you would never see it cross to begin with.

 

[rootone]

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.

 

[wabbit]

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.

 

[wabbit]

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.

 

[rootone]

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.