A doubt!

[chinmayrshah]

Recently, I came upon some statement like this: "Stephen Hakwing pulls back the Black Hole theory for the Universe"

 

How much is it true? Please elaborate, anyone!

 

Thank You!

 

[ajb]

Are you talking about the Thorne–Hawking–Preskill bet.

 

As an aside I met Kip Thorne a couple of years back when he visited Cardiff, he seems a very friendly guy. I tool my niece who was about 10 to his talk that was open to the public. She thought it was very funny that you get stretched and pulled apart if you fall into a black hole.

 

I was once in the same lecture hall as Hawking...

[chinmayrshah]

Ohh...That's great! But what does it mean that Hawkings pulls back his theory."? I mean what was Hawkings stand regarding role of black hole in generation or development of Universe, which he had to pull back?

[ajb]

The whole bet is to do with the information loss paradox.

 

If you recall black holes radiate and this radiation is thermal. This means that it is a perfect black body.

 

One might have expected the Hawking radiation to some how depend on the make up of the black hole. In particular it does not depend on what the black hole as "eaten". Thus all information about the black holes diet appears lost.

 

Now imagine the black hole has almost evaporated. Now as I said the information about the black holes diet has not leaked out of the black hole. So what happens when the black hole completely disappears?

 

This we do not know. The information could be lost, that is removed from the Universe completely. Black holes may not fully evaporate and leave behind some remnant that contains all the missing information. Just before the black hole disappears the information maybe given back in some kind of "supernova-like" event. Or something else.

[tomgwyther]

When it's said that the black hole radiates heat, what exactly is meant by this? More specifically, is the heat/thermal energy being radiated from within the event horizon or from outside the event horizon?

If the radiation is coming from matter before it crosses the event horizon the it's just a simple case of a fast moving dense swirling disc of stuff getting hot, noting particularly paradoxical about that.

If the radiation is coming from within the event horizon; how is it doing it since the horizon is defined as the boundary across which nothing can escape, not even thermal radiation.

[ajb]

When it's said that the black hole radiates heat, what exactly is meant by this? More specifically, is the heat/thermal energy being radiated from within the event horizon or from outside the event horizon?

 

Hawking radiation. This originates just on the horizon and so can escape. The point is that this is pure thermal, it contains no information about the make up of the black hole at all.

 

This is not to be confused with, say X-rays from an accretion disk around a black hole.

Edited August 4, 2011 by ajb

[Airbrush]

The whole bet is to do with the information loss paradox.

 

If you recall black holes radiate and this radiation is thermal. This means that it is a perfect black body.

 

One might have expected the Hawking radiation to some how depend on the make up of the black hole. In particular it does not depend on what the black hole as "eaten". Thus all information about the black holes diet appears lost.

 

Now imagine the black hole has almost evaporated. Now as I said the information about the black holes diet has not leaked out of the black hole. So what happens when the black hole completely disappears?

 

This we do not know. The information could be lost, that is removed from the Universe completely. Black holes may not fully evaporate and leave behind some remnant that contains all the missing information. Just before the black hole disappears the information maybe given back in some kind of "supernova-like" event. Or something else.

 

Thanks for the info. So what is the answer to the original question?

 

"Stephen Hawking pulls back the Black Hole theory for the Universe." What is the black hole theory for the universe?

 

Hawking admitted he was wrong about something? What does that have to do with how black holes evaporate?

 

"...As an example, a black hole of one solar mass has a temperature of only 60 nanokelvin; in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5 × 1022 kg (about the mass of the Moon) would be in equilibrium at 2.7 kelvin, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb, and thereby lose mass."

http://en.wikipedia.org/wiki/Hawking_radiation

 

When it's said that the black hole radiates heat, what exactly is meant by this? More specifically, is the heat/thermal energy being radiated from within the event horizon or from outside the event horizon?

If the radiation is coming from matter before it crosses the event horizon the it's just a simple case of a fast moving dense swirling disc of stuff getting hot, noting particularly paradoxical about that.

If the radiation is coming from within the event horizon; how is it doing it since the horizon is defined as the boundary across which nothing can escape, not even thermal radiation.

 

Seems strange to me. How 2 virtual particles, positive and negative, pop into existence just outside the event horizon, and one particle escapes and the other falls into the black hole as negative energy which will reduce the mass of the black hole slightly. The two particles are so close together, how does one go one way and the other 180 degrees the other way?

 

Also I'd like to know, does the vacuum around an event horizon create conditions that stimulate particle-antiparticles virtual particle production?

Edited August 4, 2011 by Airbrush

[pantheory]

Airbrush,

 

.....Also I'd like to know, does the vacuum around an event horizon create conditions that stimulate particle-antiparticles virtual particle production?

Of course black hole theory is just that, theory. We have plenty of evidence to support that there is something invisible creating a vast amount of gravity. Whether this something is a vacuous single point or something else, is still a matter of conjecture. If it were a presently unknown type of matter, it must be much denser than a neutron star because their known event horizons are relatively very small. So accordingly there may not necessarily be any vacuum surrounding a black hole.

 

As to virtual particle production, most models that I've seen assert that such production is plentiful. Hawking himself wrote a paper in 1975 concerning "Hawking radiation," where he proposed that both electrons and positrons were also produced as permanent particles emanating from black holes.

 

 

chinmayrshah,

 

I believe the article you were referring to stems from this reversing of theoretical positions by Hawking.

http://www.msnbc.msn.com/id/5452537/ns/technology_and_science-space/t/hawking-changes-his-mind-black-holes/

 

Edited August 8, 2011 by pantheory

[ajb]

Thanks for the info. So what is the answer to the original question?

 

Hawking and Thorne initially both thought that, due to arguments based on general relativity black holes cannot lose information. That is we truly do lose everything we could know about what the black hole has eaten. However, this contradicts what we know about quantum mechanics and unitary. So quantum mechanics needs modification.

 

Preskill argued that information does leak out and that somehow the radiation of a black hole must contain information about what it has eaten. This would be consistent with quantum mechanics but not general relativity. General relativity needs modification.

 

In 2004 Hawking said that black hole horizons can fluctuate and then leak information. He changed his position. Thorne, as far as I know has not fully accepted what Hawking has said and has not changed his position.

 

The full answer will require knowledge of quantum gravity. It seems reasonable to me that we would expect the horizon to fluctuate, but I have no idea if this really solves the information paradox.

 

 

 

 

Also I'd like to know, does the vacuum around an event horizon create conditions that stimulate particle-antiparticles virtual particle production?

 

Pair production happens all the time everywhere. I do not think we need anything special to happen near the event horizon.

 

Though I should say that the notion of the vacuum is not not observer independent on curved space-times. This is really the best way to understand Hawking radiation and similar things.

[IM Egdall]

Though I should say that the notion of the vacuum is not not observer independent on curved space-times. This is really the best way to understand Hawking radiation and similar things.

 

 

Huh?

[ajb]

Huh?

 

In short, in general one does not have symmetries of the metric and so no notion of energy. Thus one cannot identify a distinguished vacuum state. Generally different observers will see different vacua.

[IM Egdall]

In short, in general one does not have symmetries of the metric and so no notion of energy. Thus one cannot identify a distinguished vacuum state. Generally different observers will see different vacua.

 

 

depending on their relative motion. Correct?

Edited August 9, 2011 by I ME

[DrRocket]

depending on their relative motion. Correct?

 

What ajb is alluding to are difficulties with both general relativity and quantum field theories.

 

In general relativity the usual notion of conservation of energy has problems. It can be shown that mass/energy is conserved at a point, but unlike the classical Newtonian case the reasoning does not extend to a volume of finite size. The conservation of energy that one gets in classical and quantum theories from Noether's theorem from time translation symmetry does not work in GR since there is no meaning to time translation. Spacetime is not a vector space.

 

On the other hand quantum field theories on curved spacetime have all sorts of problems. Among them, particles tend to lose individual identity. So you are trying to describe vacuum energy using a theory that is ill-formulated and even more poorly understood. It is a halting and imperfect step toward a theory of quantum gravity, which as ajb notes, is what is really needed.

 

Hawking radiation, BTW, is a result of predictions made using quantum field theory on curved spacetime. It is therefore on somewhat shaky ground (though no one has a better approach at the moment and the result is probably correct), hence the issue over the potential loss of information, or lack of unitarity in the evolution of the state function. The concession of Hawking to Preskill, that information is not lost, is based in large part on an application of the AdS/CFT correspondence from string theory. Unfortunately the Ads/CFT correspondence is itself an unproved conjecture of Maldecena, dating from 1997. Thus there is doubt remaining, and Kip Thorne, also a party to the bet, has not conceded.

 

The message here is that what has been elucidated is not a theorem, but rather an opinion, and there continue to exist differing opinions held by well-qualified people.

Edited August 9, 2011 by DrRocket

[pantheory]

What ajb is alluding to are difficulties with both general relativity and quantum field theories.

 

In general relativity the usual notion of conservation of energy has problems. It can be shown that mass/energy is conserved at a point, but unlike the classical Newtonian case the reasoning does not extend to a volume of finite size. The conservation of energy that one gets in classical and quantum theories from Noether's theorem from time translation symmetry does not work in GR since there is no meaning to time translation. Spacetime is not a vector space.

 

On the other hand quantum field theories on curved spacetime have all sorts of problems. Among them, particles tend to lose individual identity. So you are trying to describe vacuum energy using a theory that is ill-formulated and even more poorly understood. It is a halting and imperfect step toward a theory of quantum gravity, which as ajb notes, is what is really needed.

 

Hawking radiation, BTW, is a result of predictions made using quantum field theory on curved spacetime. It is therefore on somewhat shaky ground (though no one has a better approach at the moment and the result is probably correct), hence the issue over the potential loss of information, or lack of unitarity in the evolution of the state function. The concession of Hawking to Preskill, that information is not lost, is based in large part on an application of the AdS/CFT correspondence from string theory. Unfortunately the Ads/CFT correspondence is itself an unproved conjecture of Maldecena, dating from 1997. Thus there is doubt remaining, and Kip Thorne, also a party to the bet, has not conceded.

 

The message here is that what has been elucidated is not a theorem, but rather an opinion, and there continue to exist differing opinions held by well-qualified people.

I very much like your summary but still am not sure I fully understand the argument. Let's say instead of a black holes we use a neutron star for example. Everything falling into it will be expressed by the increased mass of the neutron star, decreased mass (maybe antiparticles), the number of neutrons, its relative spin, internal energies, particles in a major or minor plasma other than neutrons, its magnetic characteristics, its radiation, its gravity, etc. Is this somehow a means of "saving all the information" of in-falling matter? If so then for a black holes, there is mass increases, decreases, changes of spin characteristics, a changing magnetic field, changing gravity waves of some kind, externally expressed energy levels of some kind, changes of gravitational influences, a changing relative position, changing relative motion, etc. Are these means of "saving all the information" of in-falling matter?

 

//

Edited August 10, 2011 by pantheory

[ajb]

What ajb is alluding to are difficulties with both general relativity and quantum field theories.

 

It might be rather subtle, but the usual formulation of quantum field theory on Lorentz space-time relies heavily on the Poncare group. This allows us to identify a distinguished vacua and allows one to identify the particle nature of the theory. It also allows a nice decomposition of the fields into positive and negative frequencies and other things we just take for granted.

 

Without the Poncare invariance things get much harder.

 

In usual text books on QFT the essential role of the Poincare group is generally understated. It is of course used and examined etc, but not usually stated to be essential in the theory.