PDA

View Full Version : What force splits the virtual particles for Hawking Radiation?



tommac
2011-May-21, 06:25 PM
What force splits the virtual particles for Hawking Radiation? Why dont both particles just fall in?

Strange
2011-May-21, 07:07 PM
This description is only an approximation to what the math describes, so I'm not sure if the question even makes sense.


nobody has ever worked out a "local" description of Hawking radiation in terms of stuff like this happening at the horizon
http://www.desy.de/user/projects/Physics/Relativity/BlackHoles/hawking.html

Tensor
2011-May-21, 09:21 PM
In the usually given picture, which as Strange points out (with a really great link), may not even make sense in terms of the math, it is the tidal force between the two particles is the separating force.

baskerbosse
2011-May-22, 01:45 AM
What force splits the virtual particles for Hawking Radiation? Why dont both particles just fall in?

As Tensor pointed out, tidal forces would be responsible for this.

But why can't either of the two fall in?
It is usually described as a 'particle - anti particle pair' being created and the anti particle falls in.
Why doesn't the particle fall in and the anti-particle escape in half of the cases?

(Indeed, don't anti particles have mass too? How does that shrink the black hole? Or are they negative energy? )


Peter

Noclevername
2011-May-22, 01:54 AM
(Indeed, don't anti particles have mass too? How does that shrink the black hole? Or are they negative energy? )


Peter

Half the VP's mass is lost as the escaping particle, half falls back in as the antiparticle. So every time it happens, some mass is lost.

baskerbosse
2011-May-22, 02:51 AM
Half the VP's mass is lost as the escaping particle, half falls back in as the antiparticle. So every time it happens, some mass is lost.

Due to the conservation of energy, yes I can see that. Energy comes out, it must come from somewhere.

But doesn't virtual particles pop in and out of existence as quantum fluctuations of energy? They need to combine to return the energy that was 'borrowed'.

Doesn't that mean that if you snatch one particle away, you have a negative energy potential plus half of that in the form of the remaining particle?
It can obviously not be returned by recombining the pair. (Unless, it too falls in eventually, there's a lot of time going on on the outside.. ;-))

How can this low energy potential take mass from previously infallen material?

But regarding the question on splitting the pair in the first place: Looking at a supermassive black hole, -surely the tidal forces are not suficient here to pull a virtual pair apart?


The more I look at it, the less I understand how Hawking Radiation works..

Peter

Noclevername
2011-May-22, 02:58 AM
Due to the conservation of energy, yes I can see that. Energy comes out, it must come from somewhere.

But doesn't virtual particles pop in and out of existence as quantum fluctuations of energy? They need to combine to return the energy that was 'borrowed'.

Doesn't that mean that if you snatch one particle away, you have a negative energy potential plus half of that in the form of the remaining particle?
It can obviously not be returned by recombining the pair. (Unless, it too falls in eventually, there's a lot of time going on on the outside.. ;-))

How can this low energy potential take mass from previously infallen material?

But regarding the question on splitting the pair in the first place: Looking at a supermassive black hole, -surely the tidal forces are not suficient here to pull a virtual pair apart?


The more I look at it, the less I understand how Hawking Radiation works..

Peter

I'm not sure about the tidal thing (First I've ever heard of it)-- I had long assumed the particle/antiparticle pairs annihilated upon creation, with half the EM energy going above the EH, and half ending up on the wrong side. Hence the term Hawking radiation. But I'm not enough of an expert to clarify any further.

Tensor
2011-May-22, 04:41 AM
Due to the conservation of energy, yes I can see that. Energy comes out, it must come from somewhere.

But doesn't virtual particles pop in and out of existence as quantum fluctuations of energy? They need to combine to return the energy that was 'borrowed'.
Precisely.


Doesn't that mean that if you snatch one particle away, you have a negative energy potential plus half of that in the form of the remaining particle?

Ignore negative energy potential (this seems to be the thing that screws with everyone's head, so it's best, for non-mathematical explanation, if you just ignore it). You have either a particle picture, or as the link points out, Maxwell's classical EM wave picture (with all the problems of identifying exactly what or who's time you are using for the solution, which as the link points out, causes problems, because each different frame has a different time, hence a different answer).


It can obviously not be returned by recombining the pair. (Unless, it too falls in eventually, there's a lot of time going on on the outside.. ;-))

So, lets get back to the best way that I can explain it (anyone else, feel free to jump in here with your understanding). You have the two particles form, outside the horizon, both have the same mass. The tidal force separates them, one falling into the hole, one escaping from the hole. The energy that created the two particles has to come from somewhere, since they two particles can no longer recombine. The only available energy is from the black hole. Which loses the amount of energy equal to the mass of the particle that escapes.


How can this low energy potential take mass from previously infallen material?

It doesn't.


But regarding the question on splitting the pair in the first place: Looking at a supermassive black hole, -surely the tidal forces are not suficient here to pull a virtual pair apart?

Actually they are. However, you may recall that the amount of radiation escaping the black hole is dependent on their mass. The greater the mass, the less radiation escapes. This is tied up with the weaker tidal effect around a more massive black hole. When you are talking about a supermassive black hole, the weak tidal force does indeed, limit the amount of Hawking radiation. When you get to a mini black hole, the tidal effect is so great, the number of escaping particles is extremely large.


The more I look at it, the less I understand how Hawking Radiation works.

Peter

I'm not saying there's no one here, but we're talking about combining General Relativity, with classical EM theory and then converting some of it to Quantum Field Theory (or converting classical EM theory into Quantum Field Theory, and then try to force the result into GR). The number of people here (and definitely do not count me amount them) that actually understand all the technicalities of the mathematics behind Hawking radiation can probably be counted on the fingers of one foot. So consider yourself in some pretty good company. (I've been wrong before, so if there is someone here who can follow the math, by all means, please correct me, so I can apologize, for one).

Strange
2011-May-22, 05:06 AM
So, lets get back to the best way that I can explain it (anyone else, feel free to jump in here with your understanding). You have the two particles form, outside the horizon, both have the same mass. The tidal force separates them, one falling into the hole, one escaping from the hole. The energy that created the two particles has to come from somewhere, since they two particles can no longer recombine. The only available energy is from the black hole. Which loses the amount of energy equal to the mass of the particle that escapes.

The trouble with a simplified description like this, is that it beaks down when probed too far. But that is the way I think of it in these terms (be aware that I understand waaay less of the math involved than Tensor!)

One way of thinking of it (1) is that when a virtual pair pops into existence it "borrows" some energy (2). This has to be paid back (3) which is normally done by the virtual pair cancelling each other out. Therefore, over the long term, no net energy. If you do something to pull the two particles apart, such that they can no longer cancel out and pay their debt, then you will have to apply energy to do that. That energy just happens to equal the amount of energy required to create the two particles.

In the case of Hawking radiation, this energy comes from the gravitational field of the black hole (4). If one of the particles falls into the black hole, it gets half of the energy back. It has therefore lost half the energy it supplied to create two virtual particles, the remaining energy is taken away by the other particle.

I think this "accounting" approach should work as a "common sense" explanation. Until you start asking more questions ....

1) which may just confuse people more, based on past experience!
2) from Dr Heisenberg
3) the more that is "borrowed" the quicker it has to be paid back
4) don't ask how, I'm not sure the explanation can extend that far

Tensor
2011-May-22, 05:23 AM
I'm not sure about the tidal thing (First I've ever heard of it)--

I find that interesting as you usually have, at least from what I have seen, a pretty good grip the more technical analogies, and also know enough to not push them to where they will fail.


I had long assumed the particle/antiparticle pairs annihilated upon creation, with half the EM energy going above the EH, and half ending up on the wrong side.

You would still have the problem of explaining how the energy ended up above and below the horizon.


Hence the term Hawking radiation. But I'm not enough of an expert to clarify any further.

Radiation, in this case, means particles. After all, alpha radiation is, basically a helium nucleus and beta radiation is an electron or positron. So, I'm not sure why you would think that radiation can't be particles.

Tensor
2011-May-22, 05:36 AM
The trouble with a simplified description like this, is that it beaks down when probed too far. But that is the way I think of it in these terms (be aware that I understand waaay less of the math involved than Tensor!)

You do me too much credit here. If I knew it as well as you think I do, I should be able to figure out a way to explain it better.


One way of thinking of it (1) is that when a virtual pair pops into existence it

snip....

. It has therefore lost half the energy it supplied to create two virtual particles, the remaining energy is taken away by the other particle.

I don't think anyone can explain it better, unless they are able to dig deep into the math of this.


I think this "accounting" approach should work as a "common sense" explanation. Until you start asking more questions ....

heheheheh, there are a lot of "common sense" accounting approaches that work well, until you start asking questions. It's always a question of saying, well, this part A of the explanation doesn't work, if you try to include part B. But, if you try to fit part B into the explanation with part A, then Part C no longer works. Of course, since there is usually no way to explain how they are all interconnected without the math, it's easy to for those who can't follow the math to say, yeah, but what happens if you take Part B and put it into Part D....


1) which may just confuse people more, based on past experience!
2) from Dr Heisenberg
3) the more that is "borrowed" the quicker it has to be paid back
4) don't ask how, I'm not sure the explanation can extend that far

1. Yep
2. Yep
3. Yep
4. Oh yeah, that's a definite yep.

Strange
2011-May-22, 10:50 AM
Not surprisingly, Grant had a couple of insightful comments when this came up before:

One way of understanding it, within the "virtual pair" analogy for Hawking radiation, is to see that the particle is very well localized when it is produced (it's right next to the event horizon), and therefore has a correspondingly very large uncertainty in its momentum....
... and can therefore escape.


There seem to be a couple of ways to parse the energy considerations with this "virtual particle" picture. One is to look on it as a frame-dependent measurement, in which the Schwarzschild observer (distant, stationary) finds that the infalling particle has negative energy while the outgoing particle has an equivalent positive energy. Someone falling alongside the absorbed particle however sees nothing unusual about it: it has normal (positive) mass-energy in its own rest frame.

Which sounds somewhat closer to the math (in my limited understanding).

baskerbosse
2011-May-22, 01:34 PM
In the case of Hawking radiation, this energy comes from the gravitational field of the black hole (4). If one of the particles falls into the black hole, it gets half of the energy back. It has therefore lost half the energy it supplied to create two virtual particles, the remaining energy is taken away by the other particle.


4) don't ask how, I'm not sure the explanation can extend that far

Too bad, that sounds like the most interesting part.. ;-)

If the energy 'is taken' from the gravitational field, the mass of the black hole must be reduced somehow, since that is the origin of the gravitational field. How else can it evaporate?


Peter

cjameshuff
2011-May-22, 05:50 PM
But why can't either of the two fall in?
It is usually described as a 'particle - anti particle pair' being created and the anti particle falls in.
Why doesn't the particle fall in and the anti-particle escape in half of the cases?

(Indeed, don't anti particles have mass too? How does that shrink the black hole? Or are they negative energy? )

I see that description a lot, but it seems to be a common misconception. The black hole isn't eroded by antiparticles as normal matter particles escape, it loses mass by absorbing both particles and antiparticles with net negative energy.

My understanding is the massive particles don't actually escape at all, but both particles and antiparticles end up on trajectories rising a short distance out of the black hole's potential well, gaining energy in the process and becoming "real" particles. If they then annihilate with other particles/antiparticles that have done the same, the energy they gained can escape the black hole as EM radiation, redshifted as seen by outside observers to varying degrees depending on depth in the gravity well that the annihilation took place...that's Hawking radiation, not the particles themselves.

For every pair of particles that rises out of the well and gains positive energy before annihilating and allowing that energy to escape the black hole, there's another pair (a particle and antiparticle from two separate virtual particle pairs) that goes the other direction and falls into the black hole with negative energy, decreasing its mass.

caveman1917
2011-May-22, 08:52 PM
I see that description a lot, but it seems to be a common misconception. The black hole isn't eroded by antiparticles as normal matter particles escape, it loses mass by absorbing both particles and antiparticles with net negative energy.

My understanding is the massive particles don't actually escape at all, but both particles and antiparticles end up on trajectories rising a short distance out of the black hole's potential well, gaining energy in the process and becoming "real" particles. If they then annihilate with other particles/antiparticles that have done the same, the energy they gained can escape the black hole as EM radiation, redshifted as seen by outside observers to varying degrees depending on depth in the gravity well that the annihilation took place...that's Hawking radiation, not the particles themselves.

For every pair of particles that rises out of the well and gains positive energy before annihilating and allowing that energy to escape the black hole, there's another pair (a particle and antiparticle from two separate virtual particle pairs) that goes the other direction and falls into the black hole with negative energy, decreasing its mass.

I'd have to look it up, but i seem to remember that it is found that in the later stages of evaporation the black hole will also start "shooting out" heavier particles.

JohnD
2011-May-22, 09:22 PM
I'm not sure that any of the above answers deal with Tommac's Q.
"What force splits the virtual particles......"

Why do they seperate in the first place?
It must require energy to do so, that is retrieved when they recombine but entropy says, not all of that energy!

John

Cougar
2011-May-23, 04:09 PM
I'm not sure that any of the above answers deal with Tommac's Q... "What force splits the virtual particles......"

Well, just to start with, posts 2, 3, and 4 provide the answer - it's the gravitational tidal force.

grapes
2011-May-23, 04:37 PM
Radiation, in this case, means particles. After all, alpha radiation is, basically a helium nucleus and beta radiation is an electron or positron. So, I'm not sure why you would think that radiation can't be particles.Isn't radiation always particles? and always waves? :)

Grey
2011-May-23, 07:16 PM
Here (http://xxx.lanl.gov/abs/gr-qc/0304042) is a paper that I've linked to before. The author presents a case that black holes don't radiate after all. However, in order to address Hawking's arguments, he first presents them, and does a pretty good job of it. Sections 2 and 3 present first a summary of Hawking's conclusions, and then the actual messy details in terms of Bogoliubov transformations. This is the simplest treatment that I've been able to find that leaves Hawking's argument essentially intact as to why a black hole should have a temperature, and therefore radiate as a blackbody. As others have noted, the whole virtual particle picture is meant as a way to visualize what might be happening, but is only very loosely based on the actual ideas involved, and shouldn't necessarily be taken too seriously.

tommac
2011-May-23, 07:21 PM
The tidal forces would only exist in extremes for a small black hole. For SMBH there is little tidal force outside of the EH. That is why I am confused.


In the usually given picture, which as Strange points out (with a really great link), may not even make sense in terms of the math, it is the tidal force between the two particles is the separating force.

Grey
2011-May-23, 07:27 PM
The tidal forces would only exist in extremes for a small black hole. For SMBH there is little tidal force outside of the EH. That is why I am confused.1. For a very massive black hole (heck, even a typical stellar mass black hole), the expected Hawking radiation is correspondingly so small, you couldn't measure it.

2. The virtual particle picture is just that: a picture, very loosely related to the actual calculation that such radiation should exist, to help give an idea of how radiation could come from a black hole (although frankly I think it may cause more confusion than it helps). Don't take it too literally.

Tensor
2011-May-23, 07:35 PM
Isn't radiation always particles? and always waves? :)

Of course, the post I was answering however, made it sound as if particles couldn't be radiation, hence my reply.

Tensor
2011-May-23, 07:36 PM
Don't take it too literally.

This is what causes more confusion than it helps.

Cougar
2011-May-23, 07:55 PM
That is why I am confused.

Feel free to study Hawking's original paper: Particle Creation by Black Holes (1975) [pdf] (http://srv2.fis.puc.cl/~mbanados/Cursos/TopicosRelatividadAvanzada/Hawking.pdf).

tommac
2011-May-24, 01:53 PM
but it does exist. Or does it not? If it does what provides the energy?


1. For a very massive black hole (heck, even a typical stellar mass black hole), the expected Hawking radiation is correspondingly so small, you couldn't measure it.

2. The virtual particle picture is just that: a picture, very loosely related to the actual calculation that such radiation should exist, to help give an idea of how radiation could come from a black hole (although frankly I think it may cause more confusion than it helps). Don't take it too literally.

tommac
2011-May-24, 01:54 PM
This is what causes more confusion than it helps.

Yeah i guess this is one of my points ... I am not sure what benefit that analogy provides.

Grey
2011-May-24, 02:35 PM
but it does exist. Or does it not? If it does what provides the energy?By "it", do you mean Hawking radiation? As far as whether it exists or not, we're not sure. Some of the brightest physicists think it should, but as you can see by the paper I linked, not everyone does. Hawking's argument invokes quantum mechanics in a situation where general relativity also plays a crucial role, so to really be sure about it, we'd probably need a well-tested theory of quantum gravity, and we know we don't have that at this point. It's certainly never been directly detected. Indeed, for any of the black holes we know about, the Hawking radiation would be so weak that there's absolutely no way that we could detect it.

If Hawking radiation does exist, and it works like Hawking thinks it does, then the energy for it comes from the mass of the black hole, as he points out in the third sentence of the abstract of the paper that Cougar linked.


Yeah i guess this is one of my points ... I am not sure what benefit that analogy provides.Any time some way of visualising a physical process ends up making the situation more confusing, rather than less, I think it's time to just forget the analogy. Read through Hawking's paper, or the one I linked that gives a summary, or look for some other source that actually discusses the reasoning Hawking used to come to the result he did. If you can't follow the math, just look at the conclusion: that classically, nothing should escape a black hole, but if you take quantum mechanical effects into account, you find that a black hole should emit blackbody radiation at a temperature that's inversely proportional to the mass. If you don't like that conclusion, and you'd like to be able to argue against it (or even if you just want to examine it more closely to see whether it's well-founded), then no amount of examining the details of the popular way of visualizing this is going to help. If the virtual particle idea doesn't seem to make sense, it might be that there's a flaw in Hawking's reasoning, but it could equally be that the analogy isn't perfect (since no analogy ever is). And there's no way to tell which of those cases is right without looking at the actual math, rather than focusing hard on the analogy.

Strange
2011-May-24, 02:36 PM
but it does exist. Or does it not? If it does what provides the energy?

If by "it" you mean Hawking radiation, then the answer is that we don't know if it exists or not.

If it does, then the black hole provides the energy. How does the black hole provide the energy? Well, you have two choices, either:
a) accept the "virtual particle" analogy as presented - and don't ask it questions it can't answer
b) understand the full mathematical treatment involving GR, QM and Bogoliubov transformations

tommac
2011-May-24, 05:37 PM
OK let me choose b. Then what provides the energy?


If by "it" you mean Hawking radiation, then the answer is that we don't know if it exists or not.

If it does, then the black hole provides the energy. How does the black hole provide the energy? Well, you have two choices, either:
a) accept the "virtual particle" analogy as presented - and don't ask it questions it can't answer
b) understand the full mathematical treatment involving GR, QM and Bogoliubov transformations

Strange
2011-May-24, 06:31 PM
OK let me choose b. Then what provides the energy?

The black hole (via the mechanisms described in detail in Hawking's paper - hey, you chose (b) - there are no shortcuts in (b)).

csmyth3025
2011-May-24, 10:23 PM
I've read through this thread and tried to follow the linked papers (which are way over my head).

My overall impression is that Hawking radiation will have absolutely no effect on any black hole greater than about 0.8% of the mass of the Earth due to the fact that the CMB temperature of the present era is "hotter" than the "temperature" of the Hawking radiation given off by a black hole of this mass. Essentially, a black hole of this size will absorb more mass/energy in the form of CMB photons than it emits.
(ref. http://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation )

We probably should take into consideration that the CMB temperature was proportionately higher as we go back in time to the big bang. We should also probably take into consideration the probable chance encounter between such a black hole and hydrogen molecules or dust particles over eons of time since the Big Bang. In short, the 0.8% Earth mass black hole will probably gain mass/energy rather than lose it - even in the present era.

What sort of black hole would this be? Well, the mass of the Earth is about 6 x 10^24 kg. --- 0.8% of this would be about 4.8 x 10^22 kg if my math is correct. The Schwarzschild radius of a black hole of this mass is 7.129 x 10^-5 m.
(ref. http://www.wolframalpha.com/input/?i=Swarzschild+radius&f1=4.8x10%5E22+kg&f=EventHorizonRadius.M_4.8x10%5E22+kg&a=*FVarOpt.1-_**-.***EventHorizonRadius.Q--.**EventHorizonRadius.J---.*--&x=9&y=4 )

This is bit less than the average diameter of a human hair.
(ref. http://en.wikipedia.org/wiki/1_E-5_m )

As far as I know, we don't know of any physical process of stellar evolution that can produce a black hole less than about 4 solar masses (~12 x 10^30 kg).

Presumably any of these mini black holes would need to have formed at the time of the big bang.

Do we have any plausible reason to suspect that such primordial black holes were formed in the very early universe? Also, if so, do we have reason to suspect that they were formed in large numbers?

Chris

Tensor
2011-May-25, 12:33 AM
I've read through this thread and tried to follow the linked papers (which are way over my head).

snip...

This is bit less than the average diameter of a human hair.
(ref. http://en.wikipedia.org/wiki/1_E-5_m )

Very good summation.


As far as I know, we don't know of any physical process of stellar evolution that can produce a black hole less than about 4 solar masses (~12 x 10^30 kg).

Presumably any of these mini black holes would need to have formed at the time of the big bang.

Do we have any plausible reason to suspect that such primordial black holes were formed in the very early universe? Also, if so, do we have reason to suspect that they were formed in large numbers?

Try this (http://arxiv.org/pdf/astro-ph/0310838v1)paper. I found a few others, but they are not in arxiv and you need a subscription to access the papers.

csmyth3025
2011-May-25, 02:54 AM
Thanks for the link Tensor. The math in this paper is out of my league, of course. It does give me a hint of the sort of considerations that go into postulating the formation of primordial black holes, though.

I think this is a good subject for a separate thread (perhaps one already exists - I'll look around).

Chris

tommac
2011-May-25, 06:52 PM
Does this mean that any black hole greater than .8% of the mass of the Earth are eternal? In other words they will NEVER evaporate?


I've read through this thread and tried to follow the linked papers (which are way over my head).

My overall impression is that Hawking radiation will have absolutely no effect on any black hole greater than about 0.8% of the mass of the Earth due to the fact that the CMB temperature of the present era is "hotter" than the "temperature" of the Hawking radiation given off by a black hole of this mass. Essentially, a black hole of this size will absorb more mass/energy in the form of CMB photons than it emits.
(ref. http://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation )

We probably should take into consideration that the CMB temperature was proportionately higher as we go back in time to the big bang. We should also probably take into consideration the probable chance encounter between such a black hole and hydrogen molecules or dust particles over eons of time since the Big Bang. In short, the 0.8% Earth mass black hole will probably gain mass/energy rather than lose it - even in the present era.

What sort of black hole would this be? Well, the mass of the Earth is about 6 x 10^24 kg. --- 0.8% of this would be about 4.8 x 10^22 kg if my math is correct. The Schwarzschild radius of a black hole of this mass is 7.129 x 10^-5 m.
(ref. http://www.wolframalpha.com/input/?i=Swarzschild+radius&f1=4.8x10%5E22+kg&f=EventHorizonRadius.M_4.8x10%5E22+kg&a=*FVarOpt.1-_**-.***EventHorizonRadius.Q--.**EventHorizonRadius.J---.*--&x=9&y=4 )

This is bit less than the average diameter of a human hair.
(ref. http://en.wikipedia.org/wiki/1_E-5_m )

As far as I know, we don't know of any physical process of stellar evolution that can produce a black hole less than about 4 solar masses (~12 x 10^30 kg).

Presumably any of these mini black holes would need to have formed at the time of the big bang.

Do we have any plausible reason to suspect that such primordial black holes were formed in the very early universe? Also, if so, do we have reason to suspect that they were formed in large numbers?

Chris

tommac
2011-May-25, 06:54 PM
The black hole (via the mechanisms described in detail in Hawking's paper - hey, you chose (b) - there are no shortcuts in (b)).

So basically how do I go from the point I am at now ...to b) without being able to ask questions ???

Grey
2011-May-25, 06:58 PM
Does this mean that any black hole greater than .8% of the mass of the Earth are eternal? In other words they will NEVER evaporate?No. The cosmic microwave background gets steadily colder. So unless something changes to reverse the expansion of the universe (which seems pretty unlikely at this point), if you wait long enough, the background temperature will eventually drop below the effective temperature of Hawking radiation for even the largest black hole, and those black holes will begin to decrease in mass. You'll have to wait a really, really long time for this to happen, though.

Grey
2011-May-25, 07:18 PM
So basically how do I go from the point I am at now ...to b) without being able to ask questions ???You'll need to be able to understand Hawking's paper. To do that, you'll need at least a good working knowledge of general relativity, quantum mechanics, thermodynamics, and Bogoliubov transformations. But I can't easily teach you that by you asking questions on a bulletin board and me trying to answer them. You'd really need to actually take courses in those subjects, study textbooks, work out problems. Ideally, you'd actually enroll, so that you could talk to the professor regularly, have someone grading your homework, and so forth, but that requires a pretty serious commitment and may not be possible. However, there are quite a few universities that have published lectures, course notes, and all that online, in many cases freely available. So it's a lot easier to teach yourself these things today that it would have been even ten years ago, but it's still probably going to be a lot of work.

As Strange says, path b doesn't have any shortcuts.

Strange
2011-May-25, 07:24 PM
So basically how do I go from the point I am at now ...to b) without being able to ask questions ???

You can ask questions, but there is no point worrying about virtual particles and tidal forces because that isn't what (b) is about.

Grey
2011-May-25, 07:45 PM
Ah, here (http://www.staff.science.uu.nl/~hooft101/theorist.html)'s the link that I was looking for. If your goal is really to be able to understand the detail behind something like Hawking's paper, or indeed anything involving advanced modern physics, this is a very good summary of the topics you'll need to master, with links to some good online sources to help learn them and a pile of recommended texts. Certainly there would be people here on the board that could help you with questions you'd have along the way.

JohnD
2011-May-25, 08:03 PM
Well, just to start with, posts 2, 3, and 4 provide the answer - it's the gravitational tidal force.

The tide of the BH splits them and prevents them recombining, certainly.
I understand that.
But this creation of pairs of particles and antiparticles happens continuously in space, everywhere. It's only near an event horizon that they are prevented from recombining.
I was struck by the point I made originally. Something - random quantum events? - creates them and they then recombine. This happens at a very small scale, but in numbers that are unthinkable. So very many events, happening all the time, everywhere. Energy splits nothing into particles and antiparticles, that have opposign energies and that almost instantaneously recombine, leaving nothing. But entropy says (or does it?) that there is no zero sum game, and that some energy must be lost in a process.
That energy should appear somewhere and be large, as so many tiny events occur.
Or is this part of quantum strangeness?

JOhn

Strange
2011-May-25, 08:11 PM
Ah, here (http://www.staff.science.uu.nl/~hooft101/theorist.html)'s the link that I was looking for.

What an excellent resource. I have barely half the stuff in his "primary mathematics" section. Sigh.

Which makes me wonder about different people's approaches to difficult subjects. I know that my knowledge of advanced math (which I am now disappointed to find is not even advanced :)) and physics is very limited and so I understand that I can only get a blurred glimpse at what might be going on in some areas of QM and cosmology. And I accept that.

I get the impression that some other people (and, tommac, feel free to explain how you see it) think that they should be able to "fully" understand these difficult areas even if they don't have the math and physics background. They then either convince themselves they understand more than they really do (and possibly end up in ATM :)) or they persistently try to get a deeper understanding by "banging away" at the analogies. And typically just end up more confused.

It is a bit like someone trying to explain the colour "red" to an alien with a different set of senses:
"So, when you say red is a 'warm colour' do you mean the red paint has a higher temperature?" (Alien learnt his English in London)
"No, it is not actually warmer it just 'feels' warmer. Well, not actually feels; it has the sensation of... no, not a sensation ... it is just a feeling ... no ... oh, never mind"
In the end, Alien just has to accept he will never understand red but he can vaguely see it has certain connotations for these humans; repeatedly asking what "temperature" red paint is won't get him any closer.

* For certain values of "fully"

Strange
2011-May-25, 08:16 PM
Energy splits nothing into particles and antiparticles, that have opposign energies and that almost instantaneously recombine, leaving nothing. But entropy says (or does it?) that there is no zero sum game, and that some energy must be lost in a process.

There is no energy loss in this process. Perhaps because they are virtual and so they never "really" exist and never "really" recombine. But also perhaps because the interactions are quantized and so there is no way for a little bit to be left over; it is a whole particle or nothing. So, yes, I guess it is down to quantum weirdness!

tommac
2011-May-25, 08:30 PM
And Path A doesnt seem accurite in the least ...


You'll need to be able to understand Hawking's paper. To do that, you'll need at least a good working knowledge of general relativity, quantum mechanics, thermodynamics, and Bogoliubov transformations. But I can't easily teach you that by you asking questions on a bulletin board and me trying to answer them. You'd really need to actually take courses in those subjects, study textbooks, work out problems. Ideally, you'd actually enroll, so that you could talk to the professor regularly, have someone grading your homework, and so forth, but that requires a pretty serious commitment and may not be possible. However, there are quite a few universities that have published lectures, course notes, and all that online, in many cases freely available. So it's a lot easier to teach yourself these things today that it would have been even ten years ago, but it's still probably going to be a lot of work.

As Strange says, path b doesn't have any shortcuts.

tommac
2011-May-25, 08:38 PM
Or it could be like someone who isnt good at painting but is interested in it but does not really have enough time to drop everything that they are doing to go out and go to art school and live as an artist, so trys to learn to paint and although he understands he will never be dali but knows there is room for simple improvement. Maybe he first sees that his texture is off ... so goes out and reads what he can about texture and asks some questions but realizes that even just getting texture down may be more of a time commitment then he can afford. So he gets better and reaches the point where he realizes that to get much better than that will take too much time so moves on to something else. It is not like there is just kindergarden finger painting and Dali or Monet ... there are levels in between ... does that mean that one shouldnt use Monet or Dali as aspirations or direction of evolution ???




What an excellent resource. I have barely half the stuff in his "primary mathematics" section. Sigh.

Which makes me wonder about different people's approaches to difficult subjects. I know that my knowledge of advanced math (which I am now disappointed to find is not even advanced :)) and physics is very limited and so I understand that I can only get a blurred glimpse at what might be going on in some areas of QM and cosmology. And I accept that.

I get the impression that some other people (and, tommac, feel free to explain how you see it) think that they should be able to "fully" understand these difficult areas even if they don't have the math and physics background. They then either convince themselves they understand more than they really do (and possibly end up in ATM :)) or they persistently try to get a deeper understanding by "banging away" at the analogies. And typically just end up more confused.

It is a bit like someone trying to explain the colour "red" to an alien with a different set of senses:
"So, when you say red is a 'warm colour' do you mean the red paint has a higher temperature?" (Alien learnt his English in London)
"No, it is not actually warmer it just 'feels' warmer. Well, not actually feels; it has the sensation of... no, not a sensation ... it is just a feeling ... no ... oh, never mind"
In the end, Alien just has to accept he will never understand red but he can vaguely see it has certain connotations for these humans; repeatedly asking what "temperature" red paint is won't get him any closer.

* For certain values of "fully"

Strange
2011-May-25, 09:02 PM
It is not like there is just kindergarden finger painting and Dali or Monet ... there are levels in between ... does that mean that one shouldnt use Monet or Dali as aspirations or direction of evolution ???

One can aspire. But you also have to realise that aspirations (by themselves) won't take you very far!

Hmmm... I'm not sure that analogy works because art is something you can have a range of abilities in. Whereas the math of Hawking radiation, say, just is what it is - you either understand it or you don't. Maybe.

But perhaps the key point is knowing your own limits; you know the sort of thing: you see these auditions for TV talent shows with people who have no ability whatsoever but are convinced they are the next big thing. I guess they are like some of the people who end up in ATM here: convinced they are right but unable to see they have no clue!

As long as one understands that they can only understand at the level of these analogies (and that they can only be stretched so far)... Then, perhaps by considering several of those analogies, one can just begin to get a feel for what those experts are up to.

Grey
2011-May-25, 09:22 PM
The painting analogy isn't too bad. It's absolutely true that none of us here are the equivalent of Hawking or Feynman or Einstein, just like we're not Monet or Dali, even though all of us could probably learn to paint, or learn enough physics to be able to at least follow the "work of the masters". There is an important difference, that it's often possible to appreciate a painting even if you've had no formal instruction, even if you can't paint at all yourself. However, to even understand Hawking's work takes a fair amount of study. I'm not suggesting that the only hope for you to get it is to give up your job and become a student. In principle, you can learn enough to understand Hawking's argument that black holes should radiate on your own, but it will take quite a bit of work, probably on the order of years.

On the other hand, to stretch the artist analogy, saying that the mechanism for Hawking radiation doesn't make sense without taking that time to really understand Hawking's actual argument might be sort of like looking at a work by someone like Picasso, and just saying it's terrible because it looks more like a child's drawing than a picture of the subject, without taking the time to understand the background and history that led to his style.

Strange
2011-May-25, 09:44 PM
Nicely put, Mr Grey.

caveman1917
2011-May-25, 11:24 PM
you see these auditions for TV talent shows with people who have no ability whatsoever but are convinced they are the next big thing.

In modern pop culture that might not be such a contradiction :)

Tensor
2011-May-25, 11:53 PM
However, there are quite a few universities that have published lectures, course notes, and all that online, in many cases freely available. So it's a lot easier to teach yourself these things today that it would have been even ten years ago, but it's still probably going to be a lot of work.

As Strange says, path b doesn't have any shortcuts.

I'll jump in here with this, as most of my advanced knowledge has come through self study. Even with all the work you do teaching yourself, there will be a lot of holes in your knowledge. You simply cannot underestimate the give and take and the ability to ask questions of an instructor who knows this from teaching it (or working with it every day). I did most of my work before the internet so I didn't have the resources I would have now. While there are many fine and knowledgeable people on this board, there simply isn't enough people here to answer all your technical questions you will have to ask to have a complete understanding.

I spent seven years just learning the math required of General Relativity. All of it beyond Linear Algebra on my own, and there are huge holes in what I know. And in some cases, there are even holes in what I think I know. Not to mention the holes in what I don't realize I don't know, which is the most dangerous.

In addition to this, learning it on your own, you don't use it everyday and as a result, you don't remember all of it. So, you spend 4 months learning Linear Algebra, start work on the complex plane and the Cauchy Theorems and find, at the end of that, that you have to review parts of Linear Algebra or even go all the way back to Partial Differentials when you start in on the basics of tensors and differentiable manifolds.

And then, GR doesn't exist in isolation. You also have to know the physics of the various things you are going to do with GR. For instance, in the case of black holes, if you are looking at the formation of them by collapse, you have to know the physics of pressure, quantum mechanics as it applies to confinement of electrons and neutrons (not even counting quarks), thermodynamics, and more than I can't think of right now.

Of course, now, it's gotten to the point where I don't even use this every week or even every month. So, when something comes up, I may have to go back and look over my notes or textbooks, just to get a feel for what I'm looking at.

Cougar
2011-May-26, 01:26 PM
But this creation of pairs of particles and antiparticles happens continuously in space, everywhere.

I recall this was a "cover story" in Scientific American ~20 years ago, and I've read/heard it repeated a lot since then. But it is normally mentioned independently of the background spacetime - or it assumes this is going on in a flat spacetime.

It seems reasonable that a highly curved spacetime - such as that close to a black hole - would have some effect on this particle pair production. Naively, I would think the quantities and energies involved in pair production in highly curved spacetimes would be greatly increased. AFAIK, we don't know how to measure "how much" pair production is going on even in flat spacetime, so it's going to be tough comparing that to the activity in a highly curved spacetime...

Is the Heisenberg Uncertainty Principle independent of the background?

WayneFrancis
2011-May-26, 02:01 PM
Does this mean that any black hole greater than .8% of the mass of the Earth are eternal? In other words they will NEVER evaporate?

No as the universe expands the temperature will drop and eventually the black holes will be radiating more then they absorb. That will still take a long time and then it will take an unbelievable amount of time. So in a way they can pretty much be considered eternal :)

csmyth3025
2011-May-26, 02:29 PM
...But perhaps the key point is knowing your own limits...

Ahhh yes - "A man's got to know his limitations..." - Harry Callahan (aka Clint Eastwood) in Magnum Force.

A film that not only added this tidbit to the annals of American philosophical thought, but forced thousands of insecure men to focus on the size of their gun.:eek:

Chris

tommac
2011-May-26, 03:35 PM
Oh I know my ability ... I am not very good. But my question here is that nobody seems to be able to explain it at all. The one analogy that is used is nothing like what happens so why even use it? Yet this is taken as one of the fundimentals of physics at this point. The question I asked is straight forward. Where does the energy come from? Is it tidal energy? Or just energy from the black hole? I have more questions depending on the answer.


One can aspire. But you also have to realise that aspirations (by themselves) won't take you very far!

Hmmm... I'm not sure that analogy works because art is something you can have a range of abilities in. Whereas the math of Hawking radiation, say, just is what it is - you either understand it or you don't. Maybe.

But perhaps the key point is knowing your own limits; you know the sort of thing: you see these auditions for TV talent shows with people who have no ability whatsoever but are convinced they are the next big thing. I guess they are like some of the people who end up in ATM here: convinced they are right but unable to see they have no clue!

As long as one understands that they can only understand at the level of these analogies (and that they can only be stretched so far)... Then, perhaps by considering several of those analogies, one can just begin to get a feel for what those experts are up to.

Strange
2011-May-26, 03:42 PM
Oh I know my ability ... I am not very good. But my question here is that nobody seems to be able to explain it at all.

It is quite possible, as Tensor says, that there is no one here (any more) who understands it fully. If someone who does understand were to explain it, you (and I) would not understand it.


The one analogy that is used is nothing like what happens so why even use it?

Because it is better than nothing; better than just telling people "it happens but you wouldn't understand why".

Some of the links & papers provided in this thread give alternative explanations.


Yet this is taken as one of the fundimentals of physics at this point.

Note really. It is pretty speculative and many people (who do understand it) are not convinced by Hawking's argument.


The question I asked is straight forward. Where does the energy come from? Is it tidal energy? Or just energy from the black hole?

Ultimately, from the black hole (that is why it shrinks). To understand how will require a solid understanding of GR and QM. I can barely even spell "Bogoliubov".

Grey
2011-May-26, 04:19 PM
Oh I know my ability ... I am not very good. But my question here is that nobody seems to be able to explain it at all. The one analogy that is used is nothing like what happens so why even use it? Yet this is taken as one of the fundimentals of physics at this point. The question I asked is straight forward. Where does the energy come from? Is it tidal energy? Or just energy from the black hole? I have more questions depending on the answer.And we've answered several times, the energy* comes from the black hole, reducing its mass. This is because quantum mechanical effects make the black hole have an effective temperature, so that it will radiate a blackbody spectrum, rather than being perfectly "black" as we'd expect from a classical general relativistic treatment.

I was quite serious: if you really want to understand Hawking's argument, the best summary I've seen of that argument that still preserves his essential points is in this paper (http://xxx.lanl.gov/abs/gr-qc/0304042). I could quote the whole thing here, but that would be a bit silly, especially since I'd have to reformat all of the math. Read through sections 2 and 3. If there are terms you aren't familiar with, you can look them up on the Web as you go, and then ask here if something still doesn't make sense. If you can't make any progress reading through that at all, because it's all completely above your head, I'm not going to be capable of clarifying it to you until you've got a firmer grounding in advanced physics, and my guess is that anyone else trying to explain it would have the same problem.

Black holes are complicated, and it's safe to say that nobody fully understands them. If you've got other questions, certainly go ahead and ask them. We'll do our best to explain what's going on, but you do need to be prepared that the answer may sometimes be something like "complicated quantum effects that can't be explained very well without just going through the math".

* I'm not sure there is anything that I'd call "tidal energy" in any context, but even if there were, it would just be gravitational potential energy of some arrangement of objects.

trinitree88
2011-May-26, 04:56 PM
There is a priori, absolutely no reason why the antiparticle should fall into the EH and the particle escapes. It can equally well be the opposite. That scenario results in the two types emitting from the EH with equal probability and annihilating out here in the real universe as gamma rays. That is not seen, making Hawking radiation unlikely in the world of particle physics. Not impossible, unlikely.:shifty:

cjameshuff
2011-May-26, 05:11 PM
There is a priori, absolutely no reason why the antiparticle should fall into the EH and the particle escapes. It can equally well be the opposite. That scenario results in the two types emitting from the EH with equal probability and annihilating out here in the real universe as gamma rays. That is not seen, making Hawking radiation unlikely in the world of particle physics. Not impossible, unlikely.:shifty:

That gamma ray radiation is the Hawking radiation. It isn't expected to be seen, because for all black holes of stellar mass or greater, it'd be redshifted into a black body spectrum colder than the background from our frame of reference.

tommac
2011-May-26, 06:30 PM
There is a priori, absolutely no reason why the antiparticle should fall into the EH and the particle escapes. It can equally well be the opposite. That scenario results in the two types emitting from the EH with equal probability and annihilating out here in the real universe as gamma rays. That is not seen, making Hawking radiation unlikely in the world of particle physics. Not impossible, unlikely.:shifty:

From what I have heard here that is only another analogy. Not really what happens. Just a way of explaining a way that energy can escape.

JohnD
2011-May-26, 07:56 PM
I recall this was a "cover story" in Scientific American ~20 years ago, and I've read/heard it repeated a lot since then. But it is normally mentioned independently of the background spacetime - or it assumes this is going on in a flat spacetime.

It seems reasonable that a highly curved spacetime - such as that close to a black hole - would have some effect on this particle pair production. Naively, I would think the quantities and energies involved in pair production in highly curved spacetimes would be greatly increased. AFAIK, we don't know how to measure "how much" pair production is going on even in flat spacetime, so it's going to be tough comparing that to the activity in a highly curved spacetime...

Is the Heisenberg Uncertainty Principle independent of the background?

It's the source of the Casimir effect - vacuum energy - and the cosmologocal constant, Einstein's mistake, now reinstated.
Surely the uncertainty principle is universal? What background would influence it, as it operates at a quantum level, unless there is a deeper reality.

John

loglo
2011-May-27, 10:52 AM
There is a priori, absolutely no reason why the antiparticle should fall into the EH and the particle escapes. It can equally well be the opposite. That scenario results in the two types emitting from the EH with equal probability and annihilating out here in the real universe as gamma rays. That is not seen, making Hawking radiation unlikely in the world of particle physics. Not impossible, unlikely.:shifty:

That isn't how it is modelled. The Bogoliubov transform splits the wavefunction into positive and negative frequency modes. In the particle creation picture the particle that escapes has positive energy and the particle that falls in has negative energy. The particle that escapes could become an antimatter particle as far as I know. If it was a straight out matter/anti-matter particle creation with the antimatter particle falling in then it would add to the mass of the black hole not decrease it.

Having said that I think the whole particle creation picture is a poor analogy that creates more confusion than understanding. I think the answer to "how does Hawking radiation work locally" should be "we don't know yet".

trinitree88
2011-May-27, 01:38 PM
That isn't how it is modelled. The Bogoliubov transform splits the wavefunction into positive and negative frequency modes. In the particle creation picture the particle that escapes has positive energy and the particle that falls in has negative energy. The particle that escapes could become an antimatter particle as far as I know. If it was a straight out matter/anti-matter particle creation with the antimatter particle falling in then it would add to the mass of the black hole not decrease it.

Having said that I think the whole particle creation picture is a poor analogy that creates more confusion than understanding. I think the answer to "how does Hawking radiation work locally" should be "we don't know yet".

loglo. The issue I have has to do with symmetry in pair creation. In trillions of monitored events in particle detectors, no inherent asymmetry in matter production over antimatter production has ever been seen in baryons, the stuff of the universe, or leptons...electron positron pairs primarily. While it is true that a slight asymmetry has been seen in mesons (the B factory)...that in no way guarantees an asymmetry in the particles we see making up the universe.
That said, the original descriptions of Hawking radiation posited, as I recall, matter particles or antimatter particles escaping the EH and showing up in the universe.....which would mean non-conservation of baryon number or lepton number if there was a preponderance of one type (say proton or electron). That has never been seen in the trillions of events searched by algorithms at Fermilab, SLAC, DESY, Serpukhov, KEK, etc... I am unwilling to cast conservation laws aside blithely.
As for gammas of annihilation ocurring so close to the EH as causing redshifts that are undetectable, I would have to ask "What mechanism precludes sufficient energy being thrown into the creation event to allow the particle to not only escape the EH, but stream away at high velocity out to a point where the annihilations will produce visible emissions?" pete

tommac
2011-May-27, 01:45 PM
not only escape the EH, but stream away at high velocity out to a point where the annihilations will produce visible emissions?" pete



Doesnt Radiation always move at the speed of light? What do you mean by high velocity?

ShinAce
2011-May-27, 02:26 PM
Massless particles will always travel at the speed of light, but particle pair creation gives particles with mass.

So indeed we do measure a particle's energy by measuring its speed.

WayneFrancis
2011-May-27, 02:35 PM
Oh I know my ability ... I am not very good. But my question here is that nobody seems to be able to explain it at all. The one analogy that is used is nothing like what happens so why even use it? Yet this is taken as one of the fundimentals of physics at this point. The question I asked is straight forward. Where does the energy come from? Is it tidal energy? Or just energy from the black hole? I have more questions depending on the answer.

I'm sure there are people can explain it here. But in the course of explaining it if you don't have the prereqs then they'd be talking over your head. Another thing is it isn't something you explain in a paragraph. If concepts like this where that easy then scientific papers would be have to have an abstract and conclusion that is 1 line long as to not be longer then the body of the papers themselves.

trinitree88
2011-May-27, 02:37 PM
Massless particles will always travel at the speed of light, but particle pair creation gives particles with mass.

So indeed we do measure a particle's energy by measuring its speed.

ShinAce. Yep, and then the annihilation should occur far enough from the EH, so as the gravitational redshift will not diminish it beyond detection limits. It would be very odd if it came out as a diffuse background of micowaves with a blackbody temp. of ~ 2.72 K,:shifty::naughty::lol: but it should come out somewhere in the electromagnetic spectrum. The question is...where? pete

Grey
2011-May-27, 02:38 PM
That said, the original descriptions of Hawking radiation posited, as I recall, matter particles or antimatter particles escaping the EH and showing up in the universe.....which would mean non-conservation of baryon number or lepton number if there was a preponderance of one type (say proton or electron). That has never been seen in the trillions of events searched by algorithms at Fermilab, SLAC, DESY, Serpukhov, KEK, etc... I am unwilling to cast conservation laws aside blithely.I think you're making the same mistake as tommac, and taking the description that's often used to explain Hawking radiation much too seriously. It's just a visualization, to help nonscientists understand how something can escape a black hole, through quantum mechanical effects. The real original description of Hawking radiation, as stated in several of the documents have been linked to this thread, doesn't really say that. Instead, the conclusion it reaches is that a black hole should have an effective temperature, and should emit radiation in a blackbody spectrum that corresponds to that temperature. So the radiation we'd observe would be purely thermal, consisting of photons, rather than consisting of particles and/or antiparticles.

It's fine to argue against Hawking radiation if that's what you want to do. We don't have any observations confirming that it exists, and based on the predictions of Hawking's model, we don't expect to be be able to make the types of observations which would confirm or deny those predictions at any time in the forseeable future. So at this point, it remains an interesting theoretical construct that may or may not help us in developing a theory of quantum gravity. But regardless of your stance on it, if you want to criticize Hawking's ideas, you need to address what he actually proposed, not the popular visualization of what he proposed.

WayneFrancis
2011-May-27, 02:46 PM
Couldn't HR be confirmed by the LHC? Whats the time periods involved for the micro black holes that may form? How would they interpret the data when the data is highly statistical anyway?

loglo
2011-May-27, 04:19 PM
loglo. The issue I have has to do with symmetry in pair creation. In trillions of monitored events in particle detectors, no inherent asymmetry in matter production over antimatter production has ever been seen in baryons, the stuff of the universe, or leptons...electron positron pairs primarily. While it is true that a slight asymmetry has been seen in mesons (the B factory)...that in no way guarantees an asymmetry in the particles we see making up the universe.
That said, the original descriptions of Hawking radiation posited, as I recall, matter particles or antimatter particles escaping the EH and showing up in the universe.....which would mean non-conservation of baryon number or lepton number if there was a preponderance of one type (say proton or electron). That has never been seen in the trillions of events searched by algorithms at Fermilab, SLAC, DESY, Serpukhov, KEK, etc... I am unwilling to cast conservation laws aside blithely.
As for gammas of annihilation ocurring so close to the EH as causing redshifts that are undetectable, I would have to ask "What mechanism precludes sufficient energy being thrown into the creation event to allow the particle to not only escape the EH, but stream away at high velocity out to a point where the annihilations will produce visible emissions?" pete

We don't see any asymmetry in particle creation in laboratories. But we do see an asymmetry in another place where quantum gravity effects are important, in the early universe. So I don 't see how this argument necessarily holds. Perhaps QG effects breaks baryon number conversation. Who knows.

Perhaps HR is not important at all in understanding QG. We certainly haven't seen much progress with it since 1972.

David Holland
2011-May-27, 06:12 PM
Tommac,
I don't understand your problem with this analogy. As the mass of a black hole increases the tidal force at the event horizon decreases and the Hawking radiation decreases. Are you saying the two curves don't match? Do you have any math showing a mismatch? (I won't understand if you did have the math, I just want to know if it exists.)

tommac
2011-Jun-02, 01:32 PM
Lets forget for a moment that this analogy is bogus per above posts .... My problem is that if there is a minimum energy amount needed to split the virtual pair, which I believe is equal to the mass of the particles, then there would be a max size to a black hole where the tidal foces would not be able to supply enough energy to do the split. Remember for a supermassive black hole there is almost no tidal forces at the EH. If there is just minimal tidal forces then would there be enough to provide the energy equivalent of the mass of the VP? If you are saying yes then why wouldnt we see Hawking radiation coming from the surface of a neutron star or other places where you could have greater tidal forces?

Just doesnt make sense to me. In any case stated above this analogy is bogus and is just a place holder because there is no better way to describe what happens without fully undertstanding VERY complex math ( AKA smoke and mirrors ) that is behind it.


Tommac,
I don't understand your problem with this analogy. As the mass of a black hole increases the tidal force at the event horizon decreases and the Hawking radiation decreases. Are you saying the two curves don't match? Do you have any math showing a mismatch? (I won't understand if you did have the math, I just want to know if it exists.)

David Holland
2011-Jun-02, 02:10 PM
Okay, you're saying there is some minimal tidal force below which there is no Hawking radiation. The tidal force is not supplying the energy. It is separating the virtual particles. As long as the tidal force is above zero the possiblity that the two virtual particles end up on different sides of the event horizon will be above zero. I'm not a physicist, the math is way beyond my pay level. I have no idea if neutron stars produce Hawking radiation, if they don't I guess it would be due to the lack of an event horizon.

Strange
2011-Jun-02, 02:23 PM
Lets forget for a moment that this analogy is bogus per above posts ....

Bogus may be a bit strong; a crude approximation with limited applicability, might be more accurate. Which means you can't stretch it to far (or "forget" that this is what it is).


My problem is that if there is a minimum energy amount needed to split the virtual pair, which I believe is equal to the mass of the particles, then there would be a max size to a black hole where the tidal foces would not be able to supply enough energy to do the split.

Surely, you just need to invoke less and less massive particles as the black hole gets larger. Which is quantified by the black body spectrum.

tommac
2011-Jun-02, 02:28 PM
Surely, you just need to invoke less and less massive particles as the black hole gets larger. Which is quantified by the black body spectrum.

Again, forgetting for a moment that the analogy is flawed, why then wouldnt we see radiation everywhere we have tidal forces stronger than the tidal forces of a super massive black hole. From what I understand the tidal forces there are quite minimal to the point that you would not notice anything strange as you floated accross the boundary.

Strange
2011-Jun-02, 02:30 PM
Again, forgetting for a moment that the analogy is flawed, why then wouldnt we see radiation everywhere we have tidal forces stronger than the tidal forces of a super massive black hole. From what I understand the tidal forces there are quite minimal to the point that you would not notice anything strange as you floated accross the boundary.

Because you have gone beyond the area where the analogy applies. It is an analogy for what happens at the event horizon of a black hole, not for what happens in any gravitational field.

tommac
2011-Jun-02, 02:33 PM
Because you have gone beyond the area where the analogy applies. It is an analogy for what happens at the event horizon of a black hole, not for what happens in any gravitational field.

Yes this is why I say :
Again, forgetting for a moment that the analogy is flawed
I am replying to Davids question:

Tommac,
I don't understand your problem with this analogy. As the mass of a black hole increases the tidal force at the event horizon decreases and the Hawking radiation decreases. Are you saying the two curves don't match? Do you have any math showing a mismatch? (I won't understand if you did have the math, I just want to know if it exists.)

csmyth3025
2011-Jun-02, 02:36 PM
I think you're making the same mistake as tommac, and taking the description that's often used to explain Hawking radiation much too seriously. It's just a visualization, to help nonscientists understand how something can escape a black hole, through quantum mechanical effects. The real original description of Hawking radiation, as stated in several of the documents have been linked to this thread, doesn't really say that. Instead, the conclusion it reaches is that a black hole should have an effective temperature, and should emit radiation in a blackbody spectrum that corresponds to that temperature. So the radiation we'd observe would be purely thermal, consisting of photons, rather than consisting of particles and/or antiparticles.

It's fine to argue against Hawking radiation if that's what you want to do. We don't have any observations confirming that it exists, and based on the predictions of Hawking's model, we don't expect to be be able to make the types of observations which would confirm or deny those predictions at any time in the forseeable future. So at this point, it remains an interesting theoretical construct that may or may not help us in developing a theory of quantum gravity. But regardless of your stance on it, if you want to criticize Hawking's ideas, you need to address what he actually proposed, not the popular visualization of what he proposed.
As a non-scientist I must admit that the whole particle-antiparticle analogy has confused me from the beginning. My reasoning has been that whether it's the particle or the anti-particle that falls past the event horizon, the BH will gain mass either way.

I doubt that I'll ever be able to understand the mathematical details of Hawking's hypothesis. I believe the bottom line is that it's a proposed mechanism by which BH's can lose mass.

Conceptually, I could understand if the virtual particle pair were composed of a matter and a "negative matter" particle (that is, negative matter as postulated by H. Bondi in 1957). In this case, as I understand it, negative matter has the unique property of nullification when it runs into a matter particle (they both just disappear without any residual energy). Of course this type of mechanism would not only require that negative matter is real (something that I gather is very doubtful), but it would also require that there be some reason why negative matter would preferentially fall into the BH.

I suppose that, as you say, Hawking radiation will remain a possible hypothetical means of BH thermal radiation until such time as we are able to detect it by some direct or indirect observation.

Chris

Strange
2011-Jun-02, 02:40 PM
Yes this is why I say [forgetting for a moment that the analogy is flawed ]

But that is the trouble, you can't just choose to forget that. The analogy doesn't apply to the surface of the earth or a neutron star, so you can't use it to ask questions about Hawking radiation in those cases.

And David explicitly said this: "As the mass of a black hole increases the tidal force at the event horizon decreases". The analogy isn't about tidal forces in general, it is about event horizons.

You can even use the analogy to tell you why it doesn't apply in other places: one of the particles has to be disappeared by the event horizon: that can only happen at a black hole.

Strange
2011-Jun-02, 02:44 PM
As a non-scientist I must admit that the whole particle-antiparticle analogy has confused me from the beginning. My reasoning has been that whether it's the particle or the anti-particle that falls past the event horizon, the BH will gain mass either way.

Right, but it will have used twice as much energy (mass) to "de-virtualize" the two particles in the first place. It then gets half that back and the rest escapes as Hawking radiation.


I suppose that, as you say, Hawking radiation will remain a possible hypothetical means of BH thermal radiation until such time as we are able to detect it by some direct or indirect observation.

Or we have a proper theory of quantum gravity which confirms or disproves Hawking's speculations.

tommac
2011-Jun-02, 02:49 PM
And that is what I said and what I agreed to. Then I was asked what problems I had with the analogy. Those were the problems I have with the analogy.

You cant tell me that I "can't just choose to forget that" as that is the main premise of why I have problems with "that" ... I cant forget it as it is the main reason the anlogy breaks down.


But that is the trouble, you can't just choose to forget that. The analogy doesn't apply to the surface of the earth or a neutron star, so you can't use it to ask questions about Hawking radiation in those cases.

And David explicitly said this: "As the mass of a black hole increases the tidal force at the event horizon decreases". The analogy isn't about tidal forces in general, it is about event horizons.

You can even use the analogy to tell you why it doesn't apply in other places: one of the particles has to be disappeared by the event horizon: that can only happen at a black hole.

tommac
2011-Jun-02, 02:52 PM
You can even use the analogy to tell you why it doesn't apply in other places: one of the particles has to be disappeared by the event horizon: that can only happen at a black hole.

hmmm ... I disagree. If you have two virtual particles. A neutron star or other source of tidal forces provide enough energy to promote the two particles to real particles then both escape .... the neutron star would have loss energy and mass , regardless what these particles do after they are promoted.

csmyth3025
2011-Jun-02, 02:53 PM
Right, but it will have used twice as much energy (mass) to "de-virtualize" the two particles in the first place. It then gets half that back and the rest escapes as Hawking radiation...

Oh - right. I forgot about that little detail.:doh:

Chris

Strange
2011-Jun-02, 03:20 PM
hmmm ... I disagree. If you have two virtual particles. A neutron star or other source of tidal forces provide enough energy to promote the two particles to real particles then both escape .... the neutron star would have loss energy and mass , regardless what these particles do after they are promoted.

But, as far as I know, tidal forces can't do that.

tommac
2011-Jun-02, 03:28 PM
But, as far as I know, tidal forces can't do that.

OK so we agree that the analogy that the tidal forces split the v-particles is flawed.

Strange
2011-Jun-02, 03:38 PM
OK so we agree that the analogy that the tidal forces split the v-particles is flawed.

Yes.

Tensor
2011-Jun-02, 03:48 PM
OK so we agree that the analogy that the tidal forces split the v-particles is flawed.

All analogies are flawed. Some more, some less, but all area flawed.

Strange
2011-Jun-02, 03:49 PM
All analogies are flawed. Some more, some less, but all area flawed.

That is what makes them analogies...

Tensor
2011-Jun-02, 03:56 PM
That is what makes them analogies...

Yep. But how many people have you seen accept them as the actual way the physics or models work?

captain swoop
2011-Jun-02, 05:44 PM
hmmm ... I disagree. If you have two virtual particles. A neutron star or other source of tidal forces provide enough energy to promote the two particles to real particles then both escape .... the neutron star would have loss energy and mass , regardless what these particles do after they are promoted.

Tommac, this is advocating an ATM Idea. I hope you have no intentions of persuing this line of argment in the Q&A Forum

WayneFrancis
2011-Jun-03, 05:19 AM
hmmm ... I disagree. If you have two virtual particles. A neutron star or other source of tidal forces provide enough energy to promote the two particles to real particles then both escape .... the neutron star would have loss energy and mass , regardless what these particles do after they are promoted.

But you can't promote one without the other going through an event horizon. Your fixating on part of the model and disregarding other parts it seems.

WayneFrancis
2011-Jun-03, 05:22 AM
Yep. But how many people have you seen accept them as the actual way the physics or models work?

And well that's ok. Not everyone has to be an expert in everything. Many people don't understand how heat actually works but the model/analogies they have in their head is good enough for their needs.

Jeff Root
2011-Jun-03, 06:53 AM
What authoritave source claims that the particle pair
separation thing is an analogy? I see Hawking's original
work as a classical thermodynamics / relativistic theory,
and the particle pair separation is a quantum mechanical
application of the theory to actual physical objects. Not
an analogy at all.

-- Jeff, in Minneapolis

Shaula
2011-Jun-03, 07:16 AM
Hawking's paper, after describing the virtual pair splitting picture, said:
"It should be emphasized that these pictures of the mechanism
responsible for the thermal emission and area decrease are heuristic only
and should not be taken too literally"

Reasonably authoritative.

Swift
2011-Jun-03, 12:03 PM
At almost 100 posts, this thread is well beyond a simple Q&A and has been moved from Q&A to Astronomy

Tensor
2011-Jun-04, 01:59 AM
And well that's ok. Not everyone has to be an expert in everything. Many people don't understand how heat actually works but the model/analogies they have in their head is good enough for their needs.

Oh, I didn't mean that at all, Wayne. I happen to agree with you. The original post that ended with this was simply a statement that all analogies are flawed. I just happen to think that it's important to ensure that we point out that the analogies we use here on the board (and that people can see in popular science articles or books) are in some way or another flawed and not to accept that the analogy is the actual way the models work. If one wants to accept whatever analogy one has in their head to keep the universe straight, that fine (I have several analogies in my head that I use to keep the workings of the universe straight), that doesn't mean the universe actually works that way. It's also good to realize that the analogy that works for me, may not be the analogy that works for you, and the two analogies may be contradictory.

whimsyfree
2011-Jun-04, 09:48 AM
Right, but it will have used twice as much energy (mass) to "de-virtualize" the two particles in the first place. It then gets half that back and the rest escapes as Hawking radiation.


Some explanations invoke a "negative energy field". I don't understand any of the explanations but they all imply that baryon number is not conserved. Baryon number is mere hair. This implies that direct conversion of matter to energy is possible and only the problem of engineering suitable black holes needs to be solved.

A recent paper (http://arxiv.org/abs/1105.5363) by credible authors on arXiv claims that the signature of primordial black hole evaporation has already been seen. This would obviously count as a confirmation of HR, if it turns out to be true.


Or we have a proper theory of quantum gravity which confirms or disproves Hawking's speculations.

Inventing new theories cannot prove or disprove other theories. In physics you do that by confronting theories with observations.

Strange
2011-Jun-04, 10:19 AM
Some explanations invoke a "negative energy field".

That sounds as if it might be closer to what the math says (remember that energy is observer-dependent).


This implies that direct conversion of matter to energy is possible and only the problem of engineering suitable black holes needs to be solved.

I suspect that even building a working fusion reactor is slightly easier than that.


Inventing new theories cannot prove or disprove other theories. In physics you do that by confronting theories with observations.

True. But Hawking radiation is not a theory, in itself. It is a proposed consequence of combining a couple of theories in ways that we don't yet know are valid. If/when we have a proper theory of quantum gravity (which may depend on new observation, or just new insights about existing observations) then we may be able to confirm whether or not Hawking radiation is a prediction of that theory (and/or find a way of observing it directly).

Copernicus
2011-Jun-06, 06:02 PM
This can be analagous to an electron. When an electron travels it has a field around it. The closer to the electron the more dense the field. When you have different amounts of fluctuations near the electron, due to combinations of photons, neutrinos, protons, neutrons, etc the particle part of the electron can come from any part of that field with a sort of activation energy, the closer to the electron the lower the activation energy, the farther away, the higher the activation energy. Once the activation energy is achieved the electron is captured and the field collapses from its original point to the new point.

For the hawking radiation it is the same. The particle anti particle formation has an activation energy. Near the black hole more energy is available and the incidence of formation goes up.

However, I don't see why it would be more likely for the antiparticle to make it to the blackhole unless there is some asymmetry in the way that they are formed which would cause the antiparticle to have momentum in the direction of the blackhole and the particle to be ejected in the opposite direction.

Jeff Root
2011-Jun-06, 07:13 PM
Copernicus,

Do you have the impression that the antiparticle falls into the black
hole, while the ordinary particle escapes? And that it is because
the antiparticle falls in that the black hole evaporates? That is
completely wrong.

When a particle-antiparticle pair forms close to the event horizon,
most of the time the two particles will come back together again,
so no Hawking radiation results. Most of the time, both particles
either fall into the black hole together, or escape together.

Sometimes one particle of the pair falls in, while the other escapes.
It should be completely random which falls in and which escapes.
Nomatter which escapes -- the ordinary particle or the antiparticle --
the energy carried away by the escaping particle reduces the mass
of the black hole by the mass of one particle, and produces one
particle's worth of Hawking radiation.

Inside a black hole, the difference between an ordinary particle and
an antiparticle is probably of no consequence.

(The situation would be somewhat different in the very unlikely
case that antimatter has antigravity properties. I just wanted to
mention that for the record.)

-- Jeff, in Minneapolis

Copernicus
2011-Jun-06, 07:55 PM
you are right, I am misunderstanding. Although the whole idea seems less likely to ever be a real phenomenon than ever now.