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Can A Black hole shrink


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  • 2 weeks later...

Regarding the Hawing Radiation.... How likely is it that two particles will spontaneously appear, one of them go into the BH and the other not to cause the BH to lose angular momentum like that? Sounds a bit whacky to me...! But then that's why he is a *theoretical* physicist

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Regarding the Hawing Radiation.... How likely is it that two particles will spontaneously appear, one of them go into the BH and the other not to cause the BH to lose angular momentum like that? Sounds a bit whacky to me...! But then that's why he is a *theoretical* physicist

Hawking did all the math, and using current models of virtual particle pairs (confirmed by observation), he showed that the expected emission from a black hole based on virtual particle pairs matches black-body radiation closely. The colour temperature of that black-body radiation is such that the temperature is inversely proportional to the mass of the black hole.

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Regarding the Hawing Radiation.... How likely is it that two particles will spontaneously appear, one of them go into the BH and the other not to cause the BH to lose angular momentum like that? Sounds a bit whacky to me...! But then that's why he is a *theoretical* physicist

The Universe is whacky, at least to "common" sense. Which just goes to show how poor common sense actually is.

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Besides Hawking radiation, BH's can also lose energy by a process of 'superradiant scattering'. When waves of a suitable frequency are scattered by a rotating BH, part of the wave is absorbed, but under the right conditions the scattered part can have more energy than the incident wave.

This sounds unlikely but the details are in a famous 1973 paper here http://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.cmp/1103858973.

BH's can also theoretically lose energy via The Penrose Process.

A BH of 1 stellar mass takes about 10^67 years to evaporate. Massive BH's could take something like 10^100 years to evaporate. Don't wait up :grin:.

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dumb question...

isn't there a point where an evaporating black hole stops being a black hole?.. i.e. where gravity stops winning out against strong nuclear forces etc... so what's the smallest mass an evaporating black hole can attain before it pops back into life as a neutron star?

just pondering.

Derek

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This is a very interesting question. It turns out that a black hole doesn't stop being a black hole until it radiates away all of its mass. As it radiates away mass, a back hole's size ("radius") decreases, i.e., its event horizon moves to smaller and smaller radial positions. The event horizon, however, remains a "speed of light barrier". If I fall through the event horizon, I am still forced to remain inside the event horizon, even though it moves. It is somewhat like being locked in a shrinking room that has inwardly moving walls.

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Right.. I see what you're saying.

Surely there's a limiting case once we're down to the last few sub atomic particles.

i.e. two electrons that are gravitationally so tightly bound the are at the same position.. would this have an event horizon larger than the 'size' of a single electron?

Derek

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i.e. two electrons that are gravitationally so tightly bound the are at the same position.. would this have an event horizon larger than the 'size' of a single electron?

With BH's, more mass means bigger and therefore more area of event horizon.

Surely there's a limiting case once we're down to the last few sub atomic particles.

This is a toughie as thermodynamic treatments break down. ie. There comes a point when the emitted Hawking particle has greater mass/energy than the BH itself.

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  • 2 weeks later...

Yes, that is the process. It is extremely slow for massive black holes, and most likely accretion usually outstrips loss of mass by Hawking radiation by orders of magnitude.

Entropy, gotta love it. When the universe eventually does end, all that will be left is slowly shrinking blackholes and other dead remnants slowly evaporating until there's nothing left but an increasingly equal spread of, well, everything. Bleak, huh?

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A very interesting thread gents which I have been following for a while. Decided it’s time i threw my very limited ideas in and then wait for the usual wave of corrections to my thoughts. Hopefully you guys can put me straight.

First off are BH's actually a part on Entropy? We assume entropy involves order descending into chaos, with that in mind if we then follow back the current chaos in time to a time of order we arrive at the moment before the big bang!. Current theories would seem to suggest that at this point a singularity existed that was order. BH's the maths seem to indicate are also singularities and therefore surely represent pockets of order in our chaotic universe?. I guess this does not mean they are not part on the entropy process but it would seem to me they are a reversal of the process.

With regards to Hawking radiation I don’t quite get how this process actually results in the BH radiating anything. My very limited understanding is that two particles pop into existence at the point of the event horizon. They rotate each other until one is flung clear of the horizon, the other is caught and trapped via Newton’s third law of motion. Surely in this process (if my understanding is correct) the BH acquires mass without ever losing any as the escape particle was never part of the BH in the first place.

Thirdly unfortunately many of the other methods for BH's radiating mass are a little beyond my current understanding. However I can’t see how a BH could ever pop back into existence as a Neutron star or anything else for that matter. If we are to believe our current maths on the subject both classical and even more so quantum maths they would suggest that BH's are infinitely small and infinitely massive (yes I know they can’t be otherwise they would produce infinite gravity). However we are quick to promote mathematics for solutions to many problem beyond our current theories yet are quick to dismiss it when it suggests an outcome we don’t like. With that in mind and going with the maths for now even if a BH radiates mass down to one or two remaining particles it would still presumably be infinitely small and massive and therefore even when reduce to single particle would not be able to pop back into anything?

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A very interesting thread gents which I have been following for a while. Decided it’s time i threw my very limited ideas in and then wait for the usual wave of corrections to my thoughts. Hopefully you guys can put me straight.

First off are BH's actually a part on Entropy? We assume entropy involves order descending into chaos, with that in mind if we then follow back the current chaos in time to a time of order we arrive at the moment before the big bang!. Current theories would seem to suggest that at this point a singularity existed that was order. BH's the maths seem to indicate are also singularities and therefore surely represent pockets of order in our chaotic universe?. I guess this does not mean they are not part on the entropy process but it would seem to me they are a reversal of the process.

With regards to Hawking radiation I don’t quite get how this process actually results in the BH radiating anything. My very limited understanding is that two particles pop into existence at the point of the event horizon. They rotate each other until one is flung clear of the horizon, the other is caught and trapped via Newton’s third law of motion. Surely in this process (if my understanding is correct) the BH acquires mass without ever losing any as the escape particle was never part of the BH in the first place.

Thirdly unfortunately many of the other methods for BH's radiating mass are a little beyond my current understanding. However I can’t see how a BH could ever pop back into existence as a Neutron star or anything else for that matter. If we are to believe our current maths on the subject both classical and even more so quantum maths they would suggest that BH's are infinitely small and infinitely massive (yes I know they can’t be otherwise they would produce infinite gravity). However we are quick to promote mathematics for solutions to many problem beyond our current theories yet are quick to dismiss it when it suggests an outcome we don’t like. With that in mind and going with the maths for now even if a BH radiates mass down to one or two remaining particles it would still presumably be infinitely small and massive and therefore even when reduce to single particle would not be able to pop back into anything?

Ok, it's been a long time since I did the maths at uni on this so hopefully someone will correct me where I stray from the path.

black holes are subject to entropy - but your instincts were right. check this out: http://en.wikipedia.org/wiki/Black_hole#Entropy_and_thermodynamics And your right, a black hole would be more ordered and hence Hawking radiation is reducing that order. As the black holes get slowly smaller and smaller they wouldn't regress - the schwartzchild radius is proportional to mass so shrinks at the same rate. Hence no problems with regression (progression?) to a neutron star.

But I discovered something interesting, if Wikipedia is to be believed. http://en.wikipedia.org/wiki/Black_hole#Evaporation. It turns out that the cosmic microwave background is enough to compensate for Hawking Radiation and keep a black hole expanding for stellar mass black holes and up.

It scares me how much has changed since I was an undergrad :)

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With regards to Hawking radiation I don’t quite get how this process actually results in the BH radiating anything. My very limited understanding is that two particles pop into existence at the point of the event horizon. They rotate each other until one is flung clear of the horizon, the other is caught and trapped....

Conservation of energy. If we were observing this event from a distance we would "see" a BH emit a solitary particle. The BH would also be a tinsy winsy bit lighter.

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Thanks Matt an interesting reply. Its good to know i was on roughly the right tracks with some of my ideas. Found the thermodynamic Black hole wiki link a bit hard to get my head round if im honest, and just outright painfull when it began to talk about the Holographic principle :shocked: I find the concept of information loss and retention quite difficult, although i think i get the general idea i do get totally lost when the idea of 2d and 3d projections of information being trapped on event horizons or the boundaries of the cosmos are mentioned.

The background radiation thing is very interesting, seems logical to me that BH's would just constantly be hoovering up background radiation. I wonder if thats how SMBH are created? Maybe early BH's were able to feed on higher levels of the stuff. Not enough to exceed the Eddington limit but just a constant feast which over the billions of years has created these monsters?

Conservation of energy. If we were observing this event from a distance we would "see" a BH emit a solitary particle. The BH would also be a tinsy winsy bit lighter.

Sorry Tiki still dont get it. If the two particles were never a part of the BH then how can the escape of one of the particles result in the BH being a tinsy winsy bit lighter? Seems to me at best it would neither lose or gain mass as it would of captured one particle and lost another. However what it really seems to me is that the BH would of gained a particle of mass and lost none. But from our relative view it would appear as if the BH had lost a particle of mass as we would only see the one particle escaping, which would appear of course as a loss of mass as we would not be able to determine that it was never a part of the BH in the first place. Combined with that is the fact that presumably we would never be aware of the existance of the particle trapped within the event horizon.

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