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View Full Version : Black hole vs unbreakable/unellastic rope.



tommac
2008-Dec-04, 03:04 PM
If I had an unbreakable/unelastic rope ( lets assume that we could create a rope that could withstand the force of gravity at the EH ), and fed it into a black hole from a distance.


What would happen to the rope in my hands?


The way I see it ... please tell me if I am wrong. Is that the rope would be pulled out of my hands at the speed of light.

The thought would be this.

Me-----------------------------------BH

--- is the rope.

One end of the rope would be pulled to the EH and time would stop.

At first I thought that this would just be permanently attached to the BH ... however obviously the rest of the rope would still be sucked into the EH also piling up an arbirarily long rope into a flat circle on the EH.

From our perspective all of the rope ( lets assume a 5 light year long rope ) would eventually be contained in a circle of the diameter of the rope of 0 volume on the EH.

Ross PK81
2008-Dec-04, 03:11 PM
The rope would sure burn your hands.

John Mendenhall
2008-Dec-04, 03:15 PM
BONG! Three demerits for proposing physically impossible questions!

Next, it's an interesting question. Counter-proposing a regular rope, I think you're right. It's going to get yanked out of your hands at a heck of rate. My advice: let go, quick!

WaxRubiks
2008-Dec-04, 05:01 PM
but doesn't time slow down near the EH, so the rope would slow down.....

John Mendenhall
2008-Dec-04, 05:15 PM
but doesn't time slow down near the EH, so the rope would slow down.....

Not for the co-moving observer. Ah - but it might depend on how far back on the rope you are. You get into other interesting physical questions with light-years long ropes, sticks, etc. Best to keep the questions not only physically possible, but also realistically physically possible.

Despite my being chief twit about boundary conditions in thought experiments, this is a fun one. How about dropping a video camera with a long, long cable into a BH? I still don't think you can get any info back out, even if it all stayed intact, but better theorists than me will have to explain why.

mugaliens
2008-Dec-04, 05:34 PM
If you your etherskis on, enjoy the wakeboarding.

tommac
2008-Dec-04, 05:38 PM
but doesn't time slow down near the EH, so the rope would slow down.....

The rope would slows down at the EH. However the rest of the rope is being sucked in. In fact it would appear that the rope is being compressed into a flat circle.

Sort of like those fake swords that they use for acting/ magic tricks .... if you push it against a wall or a body ... it would just seem to compress at the point of the wall ... except in the case of the BH the entire rope or stick or sword could exist in a 0 volume space.

grant hutchison
2008-Dec-05, 12:44 PM
I fear you would need a very long rope.

Proper radial distances for stationary observers get a little interesting as one approaches the event horizon. A small change in Schwarzschild coordinates down near the horizon involves a large change in distance as measure by a local stationary observer.
If we imagine a set of spherical shells around the black hole, evenly spaced in the Schwarzschild metric, local observers inhabiting these shells find that they become more widely spaced the closer they get to the horizon, with the conversion factor becoming infinite at the horizon.

So if you lower a rope quasi-statically towards the horizon, you'll never have a long enough rope to get there: you can pay out any finite length, and the other end will still be a little short of the horizon. So the far end never gets to a point of infinite acceleration.
So when you let go of the rope, it will depart from your hands with some large but finite acceleration, determined by the integrated forces along its length.

Grant Hutchison

Delvo
2008-Dec-05, 04:01 PM
If you stay outside of the black hole and lower the rope toward it, an unbreakable rope would never get into the hole just because getting into the hole would mean breaking.

Things have the tensile strengths that they have because of the forces between the electrons and protons within them. Those electrons and protons interact by sending photons back and forth, so an unbreakable object is one within which the photons are guaranteed to always be able to move back and forth from one mass particle to another. Thus, no part of the unbreakable rope can ever enter a region which is defined by the inability of photons to come out from. If part of any object could enter a black hole, it would not be an unbreakable object.

grant hutchison
2008-Dec-05, 04:19 PM
There's also the interesting matter of Unruh radiation.
As the end of the rope slowly approaches the event horizon, it finds itself in a bath of radiation, the temperature of which climbs towards infinity at the event horizon. Free-fallers don't encounter this, but it's a real entity for any accelerated observers, such as those dangling from the end of a stationary rope.
So the rope needs to be infinitely long, of infinite tensile strength, and infinite refractoriness.

Tricky ...

Grant Hutchison

stutefish
2008-Dec-05, 04:49 PM
So where can I buy one?

Nick Theodorakis
2008-Dec-05, 04:53 PM
So where can I buy one?

Same place all the the first year physics students get their massless springs and frictionless inclines.

"Consider a spherical freshman of uniform density..."

Nick

mugaliens
2008-Dec-05, 05:32 PM
"Consider a spherical freshman of uniform density..."

I would consider him to be, as of yet, unschooled!

jokergirl
2008-Dec-07, 01:03 PM
So can we use that rope for a space elevator? Or maybe a slingshot around the BH?

;)

eric_marsh
2008-Dec-07, 01:46 PM
Interesting question. As for the physical impossibility aspect, isn't that where many thought experiments exist?

grant hutchison
2008-Dec-07, 02:40 PM
Interesting question. As for the physical impossibility aspect, isn't that where many thought experiments exist?Problem is, if you put in unphysical assumptions, you're in danger of getting uninformative results.
Notice that the imaginary rope is totally inelastic. That means it transmits force instantaneously, which implies an ability to transmit signals faster than light. So we're conducting a thought experiment in general relativity using an assumption that violates special relativity. There's a great danger that the violating assumption gets stirred into the mix and pops back out again in the form of a nonsensical answer. In which case our understanding is no farther forward; it has actually retreated.

Grant Hutchison

trinitree88
2008-Dec-07, 03:36 PM
Problem is, if you put in unphysical assumptions, you're in danger of getting uninformative results.
Notice that the imaginary rope is totally inelastic. That means it transmits force instantaneously, which implies an ability to transmit signals faster than light. So we're conducting a thought experiment in general relativity using an assumption that violates special relativity. There's a great danger that the violating assumption gets stirred into the mix and pops back out again in the form of a nonsensical answer. In which case our understanding is no farther forward; it has actually retreated.

Grant Hutchison

Grant. Yup. pete

Tensor
2008-Dec-08, 12:15 AM
Problem is, if you put in unphysical assumptions, you're in danger of getting uninformative results.
Notice that the imaginary rope is totally inelastic. That means it transmits force instantaneously, which implies an ability to transmit signals faster than light. So we're conducting a thought experiment in general relativity using an assumption that violates special relativity. There's a great danger that the violating assumption gets stirred into the mix and pops back out again in the form of a nonsensical answer. In which case our understanding is no farther forward; it has actually retreated.

Grant Hutchison

Grant, I just want to mention that the quality of your posts has been really impressive.

grant hutchison
2008-Dec-08, 01:07 AM
Grant, I just want to mention that the quality of your posts has been really impressive.You're kind. :)

Grant Hutchison

John Mendenhall
2008-Dec-08, 05:50 PM
Interesting question. As for the physical impossibility aspect, isn't that where many thought experiments exist?

Unfortunately, yes. Einstein's were carefully constructed to be physically possible. Since then everybody and their brother has come up with (often) ridiculous 'thought experiments' which, as Grant has explained very well, violate either physically possible or SR or GR. As the computer folks say, "Garbage in, garbage out!"

This one with the impossible rope, however, I haven't seen before, and it is a fun idea. Thanks to the good theorists, we now know why it won't work.

tommac
2008-Dec-08, 10:01 PM
The point of the OP has been totally lost in detail of the impossible rope.

The question is how would this react. Lets make it a possible rope OK ...
So it is barely elastic ( just enough so that the speed of light thing rule is not broken) and it almost unbreakable ... and VERY radiation resistant. OK ...

The question is basically a thought experiment to ask about the relativistic effects on a continuous object falling into a black hole.

m74z00219
2008-Dec-15, 10:53 AM
From outside the BH, you will never see the rope cross the event horizon (i'm speaking of non-rotating BHs, i'm not sure of the rotating kind). This is because you can never see something accelerate to the speed of light. You would see the rope get closer and closer to the EH and the light you perceive would become increasingly red-shifted, but you would absolutely never see it cross the EH.

tommac
2008-Dec-15, 10:19 PM
You would never see it cross the event horizon because it never does.

It would take infinite time relative to an external observer, to cross the event horizon.



From outside the BH, you will never see the rope cross the event horizon (i'm speaking of non-rotating BHs, i'm not sure of the rotating kind). This is because you can never see something accelerate to the speed of light. You would see the rope get closer and closer to the EH and the light you perceive would become increasingly red-shifted, but you would absolutely never see it cross the EH.

ravens_cry
2008-Dec-16, 07:00 AM
In that case, I guess a finding a black hole that had absorbed its accretion disk would be impossible?