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spratleyj
2009-Jun-06, 11:45 PM
I'm re-reading Kip Thorne's Black Holes and Time Warps and I'm a little confused by the way he explains black hole radiation on pages 440-441. I understand how the virtual particles are created by vacuum fluctuations and how the black hole's tidal gravity turns them into permanent particles, but I'm confessed about the position of the virtual particles. Are they both outside the horizon? Also is black hole radiation limited to virtual particles? Would "normal" particles falling into the black hole also emit radiation?

Cougar
2009-Jun-07, 01:08 AM
I'm re-reading Kip Thorne's Black Holes and Time Warps...

That's a good one.


I'm conf[u]sed about the position of the virtual particles. Are they both outside the horizon?

Yes, to begin with.


Also is black hole radiation limited to virtual particles? Would "normal" particles falling into the black hole also emit radiation?

Well, as they fall in, before they reach the horizon, they are being accelerated to near the speed of light, and the tidal differential is going to be doing its best to rip apart whatever is falling in, a process that might tend to "emit radiation." But once inside the certain radius, that radiation can't make it out.

spratleyj
2009-Jun-07, 01:26 AM
Good response Cougar. Okay, so what I'm ultimately trying to understand is how the "information" is preserved. If a particle fails into a black hole, then wouldn't that "information" be lost forever - resulting in the information paradox? I have done some reading about Susskind's Holographic Principle but I didn't really understand it.

robross
2009-Jun-08, 06:27 AM
What I don't understand is how they determine that energy from the *black hole* is lost when this happens. I don't understand that part.

I understand the virtual particles forming near the event horizon. They form from vacuum energy. Normally they'd immediately annihilate and return that energy to the vacuum, but in this case, one of them is too close to the event horizon and falls in, leaving behind a now real particle.

So the way I see it:

1. The black hole's mass increases by the mass of the virtual particle that fell in
2. The universe now has a new particle that didn't exist before
3. The vacuum energy has been reduced by the energy of those two virtual particles.

In what way does all this work to have the black hole lose mass or energy?

Rob

Cougar
2009-Jun-08, 01:32 PM
In what way does all this work to have the black hole lose mass or energy?

The way Kip Thorne explains it, the tidal gravity near the horizon pulls the photons apart with a huge force, feeding great energy into them. The increase in photon energy is sufficient to convert them into real photons and have enough energy left over to give back to the neighboring, negative energy regions of space (where the virtual photons borrowed from). The photons, now real, are liberated from each other. One coincidentally falls in the hole, the other does not. The one that escapes carries away the energy (that is, the mass) that the hole's tidal gravity gave to it. The hole, with its mass reduced, shrinks a bit. He also says there are different ways to picture evaporation. I picture them all as theoretical.

Cougar
2009-Jun-08, 01:59 PM
Okay, so what I'm ultimately trying to understand is how the "information" is preserved.

The planet's top physicists have been trying to understand that for decades. For a book-length discussion on this, check out Leonard Susskind's The Black Hole War, My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics [2008]. "Making the world safe for quantum mechanics" refers to this preservation of information question. Susskind considered Hawking's early, initial claim that information is lost as it falls into a black hole as a dire prediction of the crumbling of the very foundations of quantum mechanics, since, for example, baryon number (information) would not be maintained.

I'm just a piano player, but this all seems pretty far out there - theoretical explanations piled atop an as-yet unobserved theoretical phenomenon. Of course, the question is very real, but I doubt Susskind's explanation is going to be the last word on the subject....

Spaceman Spiff
2009-Jun-08, 02:34 PM
In what way does all this work to have the black hole lose mass or energy?

Rob

Wording it slightly different from Cougar.... Think of the two virtual particles borrowing enough potential energy of gravity (or the GR equivalent) to become real particle/anti-particle pairs (this can also be done in a very strong electric field, for example). If both particles fall in, then in effect nothing happened as far as distant observers are concerned. However, if only 1 of the 2 particles falls in, then the black hole has lost an equivalent amount of mass equal to the particle's rest mass (plus whatever KE it might have had).

mugaliens
2009-Jun-08, 03:11 PM
So the way I see it:

1. The black hole's mass increases by the mass of the virtual particle that fell in
2. The universe now has a new particle that didn't exist before
3. The vacuum energy has been reduced by the energy of those two virtual particles.

In what way does all this work to have the black hole lose mass or energy?

Rob

Spaceman Spiff's version is fine. I'd add that your 1, 2, 3 layout provides the answer, robross, except that 1 isn't quite right. The vacuum fluctuation provides for the pair of virtual particles, but it's the BH's gravitationally-imparted energy which turns them into real particles. Thus, 1 should read "by the mass of the real particle that fell in."

Since both particiles given energy/mass by the BH, but only one fell in, the one that doesn't fall in escapes with the energy/mass imparted by the BH, thereby reducing the BH's energy/mass.

trinitree88
2009-Jun-08, 03:13 PM
Wording it slightly different from Cougar.... Think of the two virtual particles borrowing enough potential energy of gravity (or the GR equivalent) to become real particle/anti-particle pairs (this can also be done in a very strong electric field, for example). If both particles fall in, then in effect nothing happened as far as distant observers are concerned. However, if only 1 of the 2 particles falls in, then the black hole has lost an equivalent amount of mass equal to the particle's rest mass (plus whatever KE it might have had).

Spaceman Spiff. Agreed. The issue I've always had with the phenomenon is that there seems to be no a priori reason for either particles or antiparticles to be preferentially emitted by the EH. That would mean a net conservation of baryon number, and lepton number from the pairs seen...along with all the other conserved quantities. From the outside, the net effect ought to be pair annihilation, with electron/positrons emitting gammas, and the baryons, meson, hyperons, doing the same. If there were to be an asymmetry in the pairs entering the BH, then we ought to see an equally asymmetric excess of matter or antimatter, along with energetic gammas, emanating from some region of space. This is not seen. The recent results suggesting that it was (Pamela/ATIC) could not be confirmed, leaving us Hawking Radiation-less.

Spaceman Spiff
2009-Jun-08, 03:29 PM
Right. We've haven't yet observed/identified an evaporating black hole.

gzhpcu
2009-Jun-08, 05:47 PM
What I don't understand is how they determine that energy from the *black hole* is lost when this happens. I don't understand that part.

I understand the virtual particles forming near the event horizon. They form from vacuum energy. Normally they'd immediately annihilate and return that energy to the vacuum, but in this case, one of them is too close to the event horizon and falls in, leaving behind a now real particle.

So the way I see it:

1. The black hole's mass increases by the mass of the virtual particle that fell in
2. The universe now has a new particle that didn't exist before
3. The vacuum energy has been reduced by the energy of those two virtual particles.

In what way does all this work to have the black hole lose mass or energy?

Rob


Thus, even though the two particles are virtual from the standpoint of the free-falling system of reference at the point of origin, they are real (one with positive and one with negative mass-energy) from the standpoint of a nearby system of coordinates. The particle with positive energy radiates away (although it might fall back if it is a massive particle with insufficient velocity to escape), and the other particle has the effect of conveying negative energy to the black hole from the standpoint of the external spacetime.

source: http://www.mathpages.com/home/kmath592/kmath592.htm

The negative energy makes the black hole lose mass and slowly evaporate.

tommac
2009-Jun-08, 06:03 PM
What I don't understand is how they determine that energy from the *black hole* is lost when this happens. I don't understand that part.

I understand the virtual particles forming near the event horizon. They form from vacuum energy. Normally they'd immediately annihilate and return that energy to the vacuum, but in this case, one of them is too close to the event horizon and falls in, leaving behind a now real particle.

So the way I see it:

1. The black hole's mass increases by the mass of the virtual particle that fell in
2. The universe now has a new particle that didn't exist before
3. The vacuum energy has been reduced by the energy of those two virtual particles.

In what way does all this work to have the black hole lose mass or energy?

Rob

Great questions!

tommac
2009-Jun-08, 06:09 PM
source: http://www.mathpages.com/home/kmath592/kmath592.htm

The negative energy makes the black hole lose mass and slowly evaporate.


The question I have with this is supposedly the energy from the "system" of the BH is used to split the pair. However wouldnt this suggest that gravity has a cost? If you use up gravitational energy the system ( mass? ) gets used up?

gzhpcu
2009-Jun-10, 05:08 AM
The question I have with this is supposedly the energy from the "system" of the BH is used to split the pair. However wouldnt this suggest that gravity has a cost? If you use up gravitational energy the system ( mass? ) gets used up?
Don't quite follow you here. The force of gravity of the black hole sucks in one of the virtual particles (the one with negative energy), and the remaining particle becomes a "real" particle and escapes. The total mass of the black hole has diminished minutely (Hawking radiation) since it has absorbed negative energy.

Now this is all pretty theoretical. Am not sure if negative energy actually exists or is just a mathematical construct...

Cougar
2009-Jun-10, 01:59 PM
If you use up gravitational energy...

This apparently doesn't happen. Consider the Earth-Sun relationship....

phunk
2009-Jun-10, 04:17 PM
This apparently doesn't happen. Consider the Earth-Sun relationship....

But there are ways that gravity can remove energy from a system. For example binary pulsars losing orbital energy as gravitational waves.

I don't understand the mechanism, but tidal forces separating a pair of virtual particles and making them 'real' has a similar affect of removing energy from the black hole via gravity.

tommac
2009-Jun-10, 05:00 PM
This apparently doesn't happen. Consider the Earth-Sun relationship....

That is the point. From my understanding it is the tidal gravity that does the work to split the particles.

How does that lower the mass of the BH?

tommac
2009-Jun-10, 05:01 PM
But there are ways that gravity can remove energy from a system. For example binary pulsars losing orbital energy as gravitational waves.

I don't understand the mechanism, but tidal forces separating a pair of virtual particles and making them 'real' has a similar affect of removing energy from the black hole via gravity.


I can see this slow the rotation of a system or something like that ... I just dont see it reducing the mass of the system. Can someone please clarify?

alainprice
2009-Jun-10, 05:15 PM
tommac:
What's the difference between losing mass and losing energy?

tommac
2009-Jun-10, 06:05 PM
tommac:
What's the difference between losing mass and losing energy?

Yeah I get that. They are the same. E=MC^2 and all that nonsense. :whistle:

However ... If I took the earth and slowed its rotation ... would we say that the earth loss mass?

phunk
2009-Jun-10, 06:15 PM
Yes actually. And if you took the amount of rotational energy that was removed, plug it into e=mc2, you can figure out the equivalent amount of mass. The gravitational pull of the earth should be reduced by the same amount as it would if you physically removed that much mass.

tommac
2009-Jun-10, 06:38 PM
Yes actually. And if you took the amount of rotational energy that was removed, plug it into e=mc2, you can figure out the equivalent amount of mass. The gravitational pull of the earth should be reduced by the same amount as it would if you physically removed that much mass.


Cool ... so the moon constantly is losing mass by exerting energy on the oceanic tides on earth?

phunk
2009-Jun-10, 06:56 PM
No, the moon is gaining energy, it's the earth that's slowing down.

tommac
2009-Jun-10, 09:12 PM
No, the moon is gaining energy, it's the earth that's slowing down.

OK ... But is either the earth or the moon losing rest mass?

Does a BH have rest mass?

Is rest mass preserved?

alainprice
2009-Jun-10, 09:47 PM
When you involve gravitation and energy, you should be using General Relativity and the tensor.

It doesn't make much sense to talk about the rest mass of the earth diminishing because of the torque the tides generate. Rather, you should be looking at the earth and moon as a system of two objects. The energy of the system evolves in a way that makes sense.

The earth has energy from a few things. Rest mass is but one of them. The others include kinetic energy of the particles(heat), angular momentum, etc...

The earth loses angular momentum from the tidal locking that's ongoing. This energy has to count for something, but I wouldn't say it counts directly towards rest mass.

At any rate, good luck.

p.s. We are moving further and further away from Hawking Radiation. Maybe we should let the thread get back on track.

tommac
2009-Jun-11, 12:15 AM
p.s. We are moving further and further away from Hawking Radiation. Maybe we should let the thread get back on track.

Agreed and I apologize to spratleyj for hijacking. However one more quick one ...

Is the rest mass of the BH reduced when it exerts energy to split the VPs?
Does a BH have a rest mass?

alainprice
2009-Jun-11, 01:02 AM
Yes, the mass of a BH is reduced by exactly the same mass as the escaping particle. The velocity of the escaping particle should result in the BH recoiling by the same amount of kinetic energy. I say this because I refuse to believe that a BH can exist in absolute space. For the BH to be absolutely stationary while it emits radiation is a fallacy.

Personally, I prefer to think of the energy used to create both particles as taken from the BH and half of it gets reimbursed. I'm fine with negative mass when it comes to virtual particles, but the fact that one gets promoted to real status makes me want to ditch the negative mass idea.

Tobin Dax
2009-Jun-11, 01:38 AM
Yes, the mass of a BH is reduced by exactly the same mass as the escaping particle. The velocity of the escaping particle should result in the BH recoiling by the same amount of kinetic energy. I say this because I refuse to believe that a BH can exist in absolute space. For the BH to be absolutely stationary while it emits radiation is a fallacy.
That sounds reasonable to me, but a Kerr BH would likeyly end up with zero net acceleration due to recoil in many different directions.

spratleyj
2009-Jun-11, 03:42 AM
So most physicists are unsatisfied with Susskind's solution? I read his book The Black Hole War about a year ago, but I never really understood the holographic principle. At first I thought Hawking Radiation was the perfect way for the "information" to be conserved, but now (still from my extremely limited knowledge) I see Hawking Radiation as something that uses up a black hole's energy. Not something that allows information to escape. If anybody would like to take a crack at explaining the holographic principle that would be great!

alainprice
2009-Jun-11, 02:01 PM
That sounds reasonable to me, but a Kerr BH would likeyly end up with zero net acceleration due to recoil in many different directions.

I concur.

I'm not touching the information paradox. All I am willing to say is that the overall Hawking radiation should respect the charge and angular momentum of the BH. When it comes to baryon number, no comment.

robross
2009-Jun-11, 04:28 PM
The term "negative energy" has been used a few times in this thread to describe how the black hole loses energy. Is this "negative energy" simply an energy *debt* owed to the vacuum that the black hole supplies via tidal gravity, or is this an actual energy with a negative sign? This would seem to imply in this case

-E = mc^2, => m = -E/c^2 => -m = E/c^2. This implies you would be able to create particles with negative mass from this negative energy.

This is problematic, since the behavior of objects with negative mass are, well, weird, and have not been observed in our universe.

Rob

tommac
2009-Jun-11, 06:39 PM
Personally, I prefer to think of the energy used to create both particles as taken from the BH and half of it gets reimbursed.

I have a problem with this statement. "taken from the BH" part. Taken from its rest mass or taken from its relativistic mass or both?



I'm fine with negative mass when it comes to virtual particles, but the fact that one gets promoted to real status makes me want to ditch the negative mass idea.

Wouldnt negative mass mean anti-matter? Or are you saying that 2 VP are created as well as 2 anti-mass virtual particles.

I agree the negative mass idea is confusing ... I believe that what is really being questioned is the energy that is used to split the VP. Where does that energy come from? And how does it result in a loss of rest mass of a BH?

grant hutchison
2009-Jun-11, 07:03 PM
The term "negative energy" has been used a few times in this thread to describe how the black hole loses energy. Is this "negative energy" simply an energy *debt* owed to the vacuum that the black hole supplies via tidal gravity, or is this an actual energy with a negative sign? The negative energy appears in the book-keeping of an observer far from the black hole. Energy is always frame-dependent: if you were falling into the black hole along with the absorbed particle, you'd see nothing unusual about it. But from far outside the black hole, the particle has negative energy: from a state of zero energy in the vacuum close to the event horizon, negative energy goes in and positive energy comes out.
One gets even more extreme states around rotating black holes, in which real objects passing through the ergosphere can have negative energy as measured by a distant observer.

Grant Hutchison

robross
2009-Jun-11, 07:34 PM
I have a problem with this statement. "taken from the BH" part. Taken from its rest mass or taken from its relativistic mass or both?


Wouldnt negative mass mean anti-matter?

No. From what we have observed, mass is an intrinsic scalar property of matter. It does not have poles or opposites, like electric charge for example. Matter and Anti-matter both have the same kind of "mass" property. An anti-matter version of a matter particle has all of its quantum numbers reversed, but this does not include mass, and therefore, gravity. Matter and anti-matter behave exactly the same with regards to mass and gravity.

Negative mass would imply negative energy, and would also imply anti-*gravity*, not anti-matter.

Rob

alainprice
2009-Jun-11, 07:35 PM
First, let us assume that Hawking radiation is real. Secondly, let us assume that the energy of the escaping particle is borrowed from the BH and never reimbursed.

We now have two choices:
1) The energy for both particles is borrowed from the black hole, is positive, therefore the BH shrinks by the amount of energy lost.
2) The energy of the particle that doesn't escape is negative energy. Therefore, space decided to create a particle with negative energy that makes the BH shrink by that amount.

Which of these options makes sense to you? If you don't like option 1, then you're stuck with 2. If you do agree that 1 is possible, then where did the energy used for the creation of the particles come from? Even if it came from the empty space right outside the EH, we can safely say that loss is reimbursed by the BH. This reimbursement process is not something that would be classical.

Like Grant said, the negative energy is bookkeeping and means nothing about the particle itself. It can't be measured within our universe, so we don't worry about it.

tommac
2009-Jun-12, 03:29 AM
No. From what we have observed, mass is an intrinsic scalar property of matter. It does not have poles or opposites, like electric charge for example. Matter and Anti-matter both have the same kind of "mass" property. An anti-matter version of a matter particle has all of its quantum numbers reversed, but this does not include mass, and therefore, gravity. Matter and anti-matter behave exactly the same with regards to mass and gravity.

Negative mass would imply negative energy, and would also imply anti-*gravity*, not anti-matter.

Rob

Dont want to make an ATM statement here but ... would this be equal to the destruction/creation of a gravitron?

tommac
2009-Jun-12, 03:44 AM
First, let us assume that Hawking radiation is real. Secondly, let us assume that the energy of the escaping particle is borrowed from the BH and never reimbursed.

We now have two choices:
1) The energy for both particles is borrowed from the black hole, is positive, therefore the BH shrinks by the amount of energy lost.
2) The energy of the particle that doesn't escape is negative energy. Therefore, space decided to create a particle with negative energy that makes the BH shrink by that amount.

Which of these options makes sense to you? If you don't like option 1, then you're stuck with 2. If you do agree that 1 is possible, then where did the energy used for the creation of the particles come from? Even if it came from the empty space right outside the EH, we can safely say that loss is reimbursed by the BH. This reimbursement process is not something that would be classical.

Like Grant said, the negative energy is bookkeeping and means nothing about the particle itself. It can't be measured within our universe, so we don't worry about it.

The problem with either A or B is that information is being converted / used to split the pair. So information is being lost or it is escaping from the BH ...

What I am saying is that I dont think information can get lost per no-hair.

So the mass ( rest mass / komar mass ) has to be preserved. I can see all other energy from the system being lost ...


Now what may be interesting to think about is the speed of gravity.

Lets say that all of the momentum of a BH was lost. Could the gravity that is held within the EH escape?

In this case ... Wouldnt Hawking radiation strip all of the momentum from the black hole and the black hole would just cease to exist.

Could it just be mass that escapes space-time as we know it?

Would it then take Hawking radiation much less time ( although still significant ) to evaporate a BH? ( It wouldnt need to evaporate the information ... just the momentum)

Kebsis
2009-Aug-21, 01:25 AM
I came here to post the same question, and found this thread in search. Very interesting discussion here, but I still can't really make sense of it.

It just seems that no matter how you explain it, if a blackhole loses mass, then either that mass just disappeared into the nothingness within the BH, or it managed to get from inside the event horizon to the outside of it, which is supposed to be impossible. And if that mass can get into, then back out of the BH, why can't other things as well?

DrWho
2009-Aug-21, 02:23 AM
It just seems that no matter how you explain it, if a blackhole loses mass, then either that mass just disappeared into the nothingness within the BH, or it managed to get from inside the event horizon to the outside of it, which is supposed to be impossible. And if that mass can get into, then back out of the BH, why can't other things as well?

Have a look at post #7 by Spaceman Spiff again - he explains it well. The BH loses mass by contributing energy (mass and energy are equivalent) to two virtual particles which form outside the event horizon. If one of those particles remains outside the event horizon but the other one falls in, then the BH reclaims only half of the energy it previously lost, or in other words, it loses half of the energy. Over time, this energy loss leads to the 'evaporation' of the BH.

Tobin Dax
2009-Aug-21, 03:19 AM
And if that mass can get into, then back out of the BH, why can't other things as well?

That matter doesn't get back out. It enters the black hole, gets converted into part of the mass-energy of the black hole, and then some of the energy of the black hole gets converted into mass outside of the event horizon. But the exact same bit of matter doesn't enter and leave in this process.

Kebsis
2009-Aug-21, 03:39 AM
Have a look at post #7 by Spaceman Spiff again - he explains it well. The BH loses mass by contributing energy (mass and energy are equivalent) to two virtual particles which form outside the event horizon. If one of those particles remains outside the event horizon but the other one falls in, then the BH reclaims only half of the energy it previously lost, or in other words, it loses half of the energy. Over time, this energy loss leads to the 'evaporation' of the BH.


Here's the part that's confusing me:


The BH loses mass by contributing energy (mass and energy are equivalent) to two virtual particles which form outside the event horizon.

But how can the BH contribute energy to something outside the event horizon, when energy, including light, cannot escape past the EH?

DrWho
2009-Aug-21, 04:36 AM
But how can the BH contribute energy to something outside the event horizon, when energy, including light, cannot escape past the EH?
The energy boost is supplied by the BH's gravity.

Kebsis
2009-Aug-22, 01:18 AM
I went to Wikipedia and found the following explanation that seems to be satisfactory:


A slightly more precise, but still much simplified, view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). By this process, the black hole loses mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In reality, the process is a quantum tunneling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon.

So, if this explanation is accurate, then the particle that falls into the BH will 'quantum tunnel', sort of teleport, from within the blackhole to outside of it. Is that correct?

grant hutchison
2009-Aug-22, 01:28 AM
So, if this explanation is accurate, then the particle that falls into the BH will 'quantum tunnel', sort of teleport, from within the blackhole to outside of it. Is that correct?Only if you imagine the particle moving backwards in time from inside the event horizon to outside, and then "turning around" to move forwards in time. Hawking has used this description as an alternative to the "pair production" description.
Since the two descriptions are perfectly equivalent, it's difficult to know why Wikipedia should choose one as "in reality" and discard the other. (Well, apart from the usual reservations about the information content of Wikipedia.)

Grant Hutchison

robross
2009-Aug-22, 06:23 PM
Only if you imagine the particle moving backwards in time from inside the event horizon to outside, and then "turning around" to move forwards in time. Hawking has used this description as an alternative to the "pair production" description.
Since the two descriptions are perfectly equivalent, it's difficult to know why Wikipedia should choose one as "in reality" and discard the other. (Well, apart from the usual reservations about the information content of Wikipedia.)

Grant Hutchison

Although I only have a lay-person understanding of quantum tunneling, isn't it an example of a violation of some energy conservation law, but that yet does occur? The way it's usually explained is that "sometimes" a particle will spontaneously find itself on the other side of a barrier, which it ordinarily would not be able to cross.

If that is the case, and I have no reason to doubt it, then I would have to also be prepared to say that a particle within the event horizon of a black hole might be able to tunnel out as well, where the force of gravity would represent the barrier it could not ordinarily surmount.

Unless gravity is so special it doesn't count and particles still aren't allowed to tunnel out of it?

Rob

Kebsis
2009-Aug-25, 04:54 AM
But, even if the particle does get out, how is that making the BH less massive? The particle appeared and then disappeared...seems like the BH should have the same mass as it did before the particle appeared.

grant hutchison
2009-Aug-25, 09:22 AM
But, even if the particle does get out, how is that making the BH less massive? The particle appeared and then disappeared...seems like the BH should have the same mass as it did before the particle appeared.A particle appears and doesn't disappear. It is radiated as Hawking radiation. Its energy comes from the gravitational field of the black hole. That is the mass-energy that is removed from the black hole.

Grant Hutchison