escaping a black hole?

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Vahir
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Re: escaping a black hole?

Postby Vahir » Fri Aug 31, 2012 1:46 am UTC

Since we're on the topic of black holes, once they are created, are they permanent? If not, won't an ever increasing number of black holes in the universe absorbing matter be a problem?

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Re: escaping a black hole?

Postby PM 2Ring » Fri Aug 31, 2012 8:19 am UTC

Vahir wrote:Since we're on the topic of black holes, once they are created, are they permanent?


No, they evaporate via Hawking radiation, but the process is rather slow for typical black holes. The Hawking radiation temperature of a stellar mass black hole is around a millionth of a degree above absolute zero, for larger black holes the temperature is even smaller. But the temperature of the Cosmic Microwave Background radiation is currently a few degrees above absolute zero, so it's currently too warm for any of these black holes to evaporate.

Vahir wrote:If not, won't an ever increasing number of black holes in the universe absorbing matter be a problem?


Not really, since a black hole doesn't absorb more matter than a normal star of equal mass, and in fact black holes that are in the process of absorbing matter tend to blast quite a bit of the neighbouring matter away from themselves.

Not all stars become black holes when they die: only rather massive stars do. When a star can no longer counteract the effects of gravity through normal nuclear fusion processes it undergoes gravitational collapse. The energy released in this collapse process causes various nuclear reactions which cause the star to expel a large amount of matter. If the remaining matter has a mass around 3 to 4 times that of the sun, then that remnant will collapse to form a black hole.

OTOH, in the far distant future, black holes will dominate the population of the universe...

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Re: escaping a black hole?

Postby yurell » Fri Aug 31, 2012 8:35 am UTC

PM 2Ring wrote:OTOH, in the far distant future, black holes will dominate the population of the universe...


From what I've read (below) This will 'only' last EE26 years (I hope that's parsable as 10E26).
Does anyone have any more papers on this? The only one I have is Frautschi Entropy in an Expanding Universe from Nature in 1982 (vol. 217 No. 4560 if anyone wants to look it up).
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Re: escaping a black hole?

Postby PM 2Ring » Sat Sep 01, 2012 4:49 am UTC

yurell wrote:
PM 2Ring wrote:OTOH, in the far distant future, black holes will dominate the population of the universe...


From what I've read (below) This will 'only' last EE26 years (I hope that's parsable as 10E26).
Does anyone have any more papers on this? The only one I have is Frautschi Entropy in an Expanding Universe from Nature in 1982 (vol. 217 No. 4560 if anyone wants to look it up).


Wikipedia has a Timeline of the far future, and a few related articles.
Wikipedia wrote:Black Hole Era
1040 years to 10100 years

After 1040 years, black holes will dominate the universe. They will slowly evaporate via Hawking radiation.[3] A black hole with a mass of around 1 solar mass will vanish in around 2×1066 years. However, many of these are likely to merge with supermassive black holes at the center of their galaxies through processes described above long before this happens. As the lifetime of a black hole is proportional to the cube of its mass, more massive black holes take longer to decay. A supermassive black hole with a mass of 1011 (100 billion) solar masses will evaporate in around 2×1099 years.[26]

Hawking radiation has a thermal spectrum. During most of a black hole's lifetime, the radiation has a low temperature and is mainly in the form of massless particles such as photons and hypothetical gravitons. As the black hole's mass decreases, its temperature increases, becoming comparable to the Sun's by the time the black hole mass has decreased to 1019 kilograms. The hole then provides a temporary source of light during the general darkness of the Black Hole Era. During the last stages of its evaporation, a black hole will emit not only massless particles but also heavier particles such as electrons, positrons, protons and antiprotons.


The above time estimates are made under the assumption that protons (and bound neutrons) are unstable, due to supersymmetry, with a half-life between 1034 and 1041 years. But see the link for details of an alternative scenario without supersymmetry nucleon decay, where black holes are still forming up to 1010^76 years from now.

Note that
the proton is still expected to decay, for example via processes involving virtual black holes, or other higher-order processes, with a half-life of under 10200 years
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Re: escaping a black hole?

Postby yurell » Sat Sep 01, 2012 9:19 am UTC

Yeah, the paper I mentioned had similar figures assuming baryons decay; the EE26 assumes nucleons being stable, and calculating the time it takes for all matter to quantum tunnel into black holes (the time it takes for the BH to evaporate is negligible on that timescale).
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Can you use a large mass to 'fetch' things from black holes?

Postby dudiobugtron » Sat Sep 01, 2012 8:18 pm UTC

As always there have been a few threads recently about black holes, such as the 'escaping from a black hole' thread, and this one:
viewtopic.php?f=18&t=86516
It asked about what would happen if two black holes got close enough to each other to influence each others' event horizons.

It seems quite plausible that a sufficiently large gravity-well near a black hole will influence the event horizon (the place where the gravity mathematically becomes to much for anything to escape from the black hole) in such a way that it now appears 'closer' to the black hole than it was before. So my question is this:

Can you, from outside the black hole, and by using a sufficiently large mass to change the event horizon of a black hole, 'uncover' items that were once within the event horizon, so that they are now outside the event horizon? And if so, could we then 'fetch' those items? An item can be as small as you like, even a photon would be fine.

Assumptions:
Assume we have the ability to move a sufficiently large mass (eg: a planet, a neutron star, another black hole etc...) arbitrarily close to a black hole's event horizon (without actually reaching it of course), and then either:
a) move it away again if necessary, or
b) fetch things which are also arbitrarily close to the event horizon and 'rescue' them,
at speeds arbitrarily close to the speed of light, if needed.



PS: I feel this is sufficiently different from either of those threads to warrant a new thread. But I'm sure the mods will merge it with one or the other (or both) if they disagree! ;)
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Re: Can you use a large mass to 'fetch' things from black ho

Postby Copper Bezel » Sat Sep 01, 2012 8:30 pm UTC

Well, according to the previous thread, the influence on the event horizon doesn't work that way, and the two masses would inevitably combine anyway. So.

I'm enjoying these threads. This question really belongs in the 'escaping' one, though.
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Re: Can you use a large mass to 'fetch' things from black ho

Postby dudiobugtron » Sat Sep 01, 2012 8:41 pm UTC

Copper Bezel wrote:Well, according to the previous thread, the influence on the event horizon doesn't work that way, and the two masses would inevitably combine anyway. So.

Is that the case even for non-black-hole large-mass objects?
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Re: Can you use a large mass to 'fetch' things from black ho

Postby gmalivuk » Sat Sep 01, 2012 9:04 pm UTC

Yes, because even if something (spherically symmetric) isn't a black hole, the way it affects gravity outside its surface is the same as if it were replaced by an equally massive black hole at its center.

So if the event horizon of the existing hole "reaches out" toward large incoming masses, it doesn't matter whether those masses are themselves black holes.

In addition, someone stationary at the event horizon would perceive infalling matter to be moving at the speed of light, I believe, which means no external change (such as bringing a large mass closer) will be able to affect the infalling matter thereafter.
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Re: Can you use a large mass to 'fetch' things from black ho

Postby dudiobugtron » Sat Sep 01, 2012 9:42 pm UTC

gmalivuk wrote:In addition, someone stationary at the event horizon would perceive infalling matter to be moving at the speed of light, I believe, which means no external change (such as bringing a large mass closer) will be able to affect the infalling matter thereafter.


Damn. There go my chances at running an 'extreme tourist centre' at a black hole. ;)
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Re: Can you use a large mass to 'fetch' things from black ho

Postby Drowsy Turtle » Sat Sep 01, 2012 9:48 pm UTC

gmalivuk wrote:In addition, someone stationary at the event horizon would perceive infalling matter to be moving at the speed of light, I believe, which means no external change (such as bringing a large mass closer) will be able to affect the infalling matter thereafter.


This is correct. Even if you were using velocities approaching the speed of light, the items falling into the black hole have reached the speed of light by definition - any changes to the gravitational field strength propagates outwards at the speed of light, so as soon as an object passes the event horizon in your reference frame, it is irretrievable.

Quite what happens to it inside the event horizon is a mystery though, since time is infinitely dilated and length is infinitely contracted.
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Re: Can you use a large mass to 'fetch' things from black ho

Postby Drowsy Turtle » Sat Sep 01, 2012 9:49 pm UTC

dudiobugtron wrote:Damn. There go my chances at running an 'extreme tourist centre' at a black hole. ;)


Sounds like a good idea to me - you wouldn't get many bad reviews, and wouldn't have to handle refunds.
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Re: escaping a black hole?

Postby somebody already took it » Sun Sep 02, 2012 1:18 am UTC

What if I'm in a curved region of space such that the black hole neighbors itself on both sides so its event horizon overlaps itself, would it be possible to escape from between the black holes? To put this another way, if I'm on one side of a cylinder and a black hole is on the opposite site, can I get out of its event horizon?

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Re: escaping a black hole?

Postby Drowsy Turtle » Sun Sep 02, 2012 2:15 am UTC

You mean like a Mobius geometry? Sure, there'd be an unstable equilibrium position directly 'opposite' it, but if you're inside the event horizon then you have the problem that time and space have broken down. To escape the event horizon, you'd have to travel an infinitely short distance, but time outside the event horizon is passing infinitely faster - so the point at which you finally escape, would be when the black hole has lost enough mass from hawking radiation that you're no longer inside the event horizon.

So really what would happen, is you'd skip forward a few hundred billion years.
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Re: escaping a black hole?

Postby eSOANEM » Sun Sep 02, 2012 8:54 am UTC

And, depending on what happens to the information of infalling objects (because Hawking radiation is thermal and depends only on the horizon so can carry no information about objects which have already entered), you might not even be recognisably you once you come out.
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Re: escaping a black hole?

Postby thoughtfully » Sun Sep 02, 2012 1:04 pm UTC

eSOANEM wrote:And, depending on what happens to the information of infalling objects (because Hawking radiation is thermal and depends only on the horizon so can carry no information about objects which have already entered), you might not even be recognisably you once you come out.

There's some dispute about what happens to the information. If Hawking Radiation really is "just thermal", then it's lost (or trapped inside a remnant). Lost information doesn't jibe with QM, see this thread.
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Re: escaping a black hole?

Postby Drowsy Turtle » Sun Sep 02, 2012 3:37 pm UTC

It's a bit weird to think about really, you'd expect to be pretty messed up if you eventually left the singularity, but then again no time has passed in your reference frame. Then again, along this line of thinking, everything that had ever fallen into the black hole would be just inside the event horizon (at its relative distance from the singularity at the time of its capture), so you'd probably emerge to find yourself pretty crushed.

On balance, I'll probably stay at home.
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Re: escaping a black hole?

Postby eSOANEM » Sun Sep 02, 2012 11:07 pm UTC

thoughtfully wrote:
eSOANEM wrote:And, depending on what happens to the information of infalling objects (because Hawking radiation is thermal and depends only on the horizon so can carry no information about objects which have already entered), you might not even be recognisably you once you come out.

There's some dispute about what happens to the information. If Hawking Radiation really is "just thermal", then it's lost (or trapped inside a remnant). Lost information doesn't jibe with QM, see this thread.


Yeah, I tried to work out a way to phrase this into my post but in the end opted for inaccuracy instead. Probably not the best judgement.

Hawking's personal "solution" is that whilst the radiation is just thermal, the information isn't lost because you have to sum across all histories including those without the black hole. This has, I believe, been widely criticised as being variously unsatisfying at best and handwaving at worst.
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Re: escaping a black hole?

Postby gmalivuk » Mon Sep 03, 2012 1:37 am UTC

While quantum information might be in some way preserved, I can't imagine that further scientific discovery will ever reveal that a whole person could in any sense whatsoever be said to come out of a black hole recognizably.
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Re: escaping a black hole?

Postby Drowsy Turtle » Mon Sep 03, 2012 2:13 am UTC

For a start, time would probably be passing at significantly different rates for different parts of your body when you're near the edge of the event horizon (not to mention the squishiness of length contraction)
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Re: escaping a black hole?

Postby thoughtfully » Mon Sep 03, 2012 2:26 am UTC

Tidal forces eat you up every time. You just have to get further in with a supermassive BH.
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Re: escaping a black hole?

Postby gmalivuk » Tue Sep 04, 2012 1:35 pm UTC

Yeah, differences in gravity would rip you apart long before differences in time dilation have any noticeable effect.
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