B Lowering a rope through the event horizon

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Here's another theoretical question. If you had a rope sufficiently strong and long enough, what would happen if you "lowered" the rope from a distance into a black hole, then tried to pull it back out? Lets say we go just past the event horizon with the end of the rope, how would this play out?
 
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GR86 said:
Here's another theoretical question. If you had a rope sufficiently strong and long enough, what would happen if you "lowered" the rope from a distance into a black hole, then tried to pull it back out? Lets say we go just past the event horizon with the end of the rope, how would this play out?
The required tension is infinite. The rope is sure to break.

The required acceleration for a material object to hover in place anywhere outside the horizon increases without bound as the horizon is approached. At the horizon, only a light pulse directed exactly outward can remain in place. Inside the horizon, all trajectories end at the singularity, regardless of any acceleration provided by a rope.
 
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GR86 said:
Here's another theoretical question. If you had a rope sufficiently strong and long enough, what would happen if you "lowered" the rope from a distance into a black hole, then tried to pull it back out? Lets say we go just past the event horizon with the end of the rope, how would this play out?
If you pull on a rope, then it takes a finite time for the force to be transmitted from one end of the rope to the other. It makes no sense to imagine a rope of infinite rigidity.

Such thought experiments entirely miss the point of the theory of relativity. You're trying to pretend that we have classical mechanics with a bit of added strangeness called a black hole. You can't learn relativity that way. By proposing impossible scenarios that don't respect the laws of physics, you end up in a logical mess.

It's like asking what happens if the Sun sudddenly vanished from the universe. The laws of physics forbid that, so physics has nothing to say about such impossible scenarios.
 
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GR86 said:
Here's another theoretical question. If you had a rope sufficiently strong and long enough, what would happen if you "lowered" the rope from a distance into a black hole, then tried to pull it back out? Lets say we go just past the event horizon with the end of the rope, how would this play out?
The rope would break at some point above the horizon. How far above would depend on the rope's breaking strength, which must be finite, and how much force you could exert on the upper end of the rope, which must also be finite.
 
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PeroK said:
If you pull on a rope, then it takes a finite time for the force to be transmitted from one end of the rope to the other. It makes no sense to imagine a rope of infinite rigidity.

Such thought experiments entirely miss the point of the theory of relativity. You're trying to pretend that we have classical mechanics with a bit of added strangeness called a black hole. You can't learn relativity that way. By proposing impossible scenarios that don't respect the laws of physics, you end up in a logical mess.

It's like asking what happens if the Sun sudddenly vanished from the universe. The laws of physics forbid that, so physics has nothing to say about such impossible scenarios.
You are making a lot of assumptions when humans likely know very little about reality. Isn't that the point of theoretical discussion, to explore concepts that we can't actually physically test in real world? Not a single person KNOWS what is really going on in the depths of a black hole yet they are a hot topic of discussion in physics. If somebody thinks these concepts are not worth discussing and are too far fetched, nobody is forcing them to speak.
 
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GR86 said:
You are making a lot of assumptions when humans likely know very little about reality. Isn't that the point of theoretical discussion, to explore concepts that we can't actually physically test in real world? Not a single person KNOWS what is really going on in the depths of a black hole yet they are a hot topic of discussion in physics. If somebody thinks these concepts are not worth discussing and are too far fetched, nobody is forcing them to speak.
If you haven't studied physics, you're hardly in a position to push its frontiers. You wouldn't claim to do that any other academic discipline.

I suggest you get a textbook on special relativity and study it. Like the rest of us have done.
 
PeterDonis said:
The rope would break at some point above the horizon. How far above would depend on the rope's breaking strength, which must be finite, and how much force you could exert on the upper end of the rope, which must also be finite.
So there must be a reason you said above the event horizon, not at or below it. We know gravity bends space, so objects in free fall will fall in a straight line but through curved space. But a rope being pulled is not free falling. And space and time are not separable, so as the gravity of the black hole distorts space it also distorts time. So are you saying the rope breaks due to the sheer pulling of gravity, or bc of the differences of "time" from outside the horizon vs inside?

This brings me back to a question i asked recently, how much is time distorted at an event horizon vs here on earth. A slight tug on the rope from a distance, would be a massive yank on the rope within the horizon, due to time variance correct?
 
PeroK said:
If you haven't studied physics, you're hardly in a position to push its frontiers. You wouldn't claim to do that any other academic discipline.

I suggest you get a textbook on special relativity and study it. Like the rest of us have done.
Im not interested in the politics of pecking order. You are basically saying i shouldn't have the right to ask physics questions bc i haven't studied enough. I disagree so here i am.
 
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GR86 said:
You are making a lot of assumptions
No, we are using our best current theory of physics to make predictions.

I strongly suggest that you take a step back and think carefully about what you are saying. You are getting good responses from experts in the field who know a lot more about this area of physics than you do.
 
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GR86 said:
Im not interested in the politics of pecking order. You are basically saying i shouldn't have the right to ask physics questions bc i haven't studied enough. I disagree so here i am.
I even more strongly suggest that you take a step back after reading this. This attitude will get you a warning if it continues. This is not about politics. This about the fact that we understand the physics we are talking about, and you don't. Sorry to be blunt, but that's the way it is.
 
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GR86 said:
Im not interested in the politics of pecking order. You are basically saying i shouldn't have the right to ask physics questions bc i haven't studied enough. I disagree so here i am.
You're not asking questions. You are pushing your own ideas.
 
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GR86 said:
there must be a reason you said above the event horizon, not at or below it.
Yes. The reason is that, as I said, the rope has a finite breaking strength, and the force that can be exerted at its top end is finite. That means that, at the point on the rope where it breaks, there will be a finite proper acceleration at the instant of breakage. That requires the break to happen above the horizon; there is no point at or below the horizon where the proper acceleration would be finite with an unbroken rope. See further comments on that below.

GR86 said:
We know gravity bends space
No. We know gravity is spacetime curvature. More precisely, tidal gravity is spacetime curvature.

GR86 said:
objects in free fall will fall in a straight line but through curved space.
No, objects in free fall will travel along worldlines that are geodesics of curved spacetime.

I am making this key distinction because it is not trivial. It makes a huge difference in the predictions that your model makes.

GR86 said:
a rope being pulled is not free falling.
That's true. But it doesn't mean spacetime curvature doesn't affect the rope. Spacetime curvature doesn't just affect geodesic (free-falling) worldlines. It affects all worldlines.

GR86 said:
space and time are not separable, so as the gravity of the black hole distorts space it also distorts time.
No. The black hole's spacetime geometry is spacetime curvature. It's not that "gravity distorts spacetime". "Gravity" is not a cause. Gravity is spacetime curvature. They're the same thing.

GR86 said:
are you saying the rope breaks due to the sheer pulling of gravity, or bc of the differences of "time" from outside the horizon vs inside?
Neither. The rope breaks when the tension in the rope exceeds its finite breaking strength. The tension in the rope is ultimately caused by the spacetime curvature at and near the horizon, combined with the pull being exerted on the top end of the rope that prevents that end from freely falling. But the effect of spacetime curvature can't be usefully described as "pulling of gravity" or "differences of time".

GR86 said:
This brings me back to a question i asked recently, how much is time distorted at an event horizon vs here on earth.
I already answered this question: it's meaningless. The concept of "time dilation" has no meaning at or below the event horizon.

GR86 said:
A slight tug on the rope from a distance, would be a massive yank on the rope within the horizon, due to time variance correct?
Not quite. There is a germ of truth here, but it has to be stated carefully.

Consider first a static rope, entirely outside the horizon, and not falling or rising, just hovering, suspended from its upper end. If the upper end is very far away from the horizon, and the lower end is very close, then yes, the tension in the rope, or more precisely the upward proper acceleration at a given point on the rope, will be much, much larger at the lower end than at the upper end. But this is not due to "time variance"; at least, that's not a useful way of looking at it. A better way to look at it would be to say that force "redshifts" as you gain altitude just as the frequency of light signals does.

However, note that I just said "entirely outside the horizon". It is impossible for even an infinitesimal piece of the rope to be static (hovering at constant altitude) at or inside the horizon; it must be falling inward. In other words, there is no force that you can apply to the rope, no matter how large, at or inside the horizon that will make it hover, even for an instant.

To put this another way: if you look at the force that has to be applied to an infinitesimal piece of the rope to make it hover, as a function of altitude (more precisely, of radial coordinate), that force increases without bound ("goes to infinity" in the more usual phrasing, but that phrasing is really not quite correct) as you approach the altitude of the horizon. But, as I've said, the breaking strength of the rope is finite: so there will be some altitude above the horizon at which the force that is required to make the rope hover exceeds the rope's breaking strength. At that point, the rope has to break. And that point will always be above the horizon. It doesn't matter that the force applied at the top of the rope can be smaller (a lot smaller, if the top of the rope is at a high altitude), and gets increased as you go down the rope. The point is that the force has to exceed the rope's breaking strength at some point above the horizon.
 
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@GR86 I am closing this thread because your questions have been answered. As I've said in a couple of previous posts now, I strongly suggest that you take a step back and think carefully about what you are saying, and read carefully the responses you have received. The bottom line is that you are currently working with the wrong set of concepts to understand the physics you are trying to understand. The responses you have gotten here can give you a start at a better set of concepts, but that will only help if you accept the responses and stop trying to question the physics based on your incorrect concepts. Please think carefully about that before starting any more threads.
 

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