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WaxRubiks
2010-Feb-05, 06:31 AM
One thing I keep wondering is(probably been asked before):
in the hawking radiation theory, a virtual pair of particles is produced, near the event horizon, and one of the pair is supposed to fall through then event horizon, and the other particle escapes.

But I can't understand why the other should escape the high gravitational field.

Is it just a matter of having enough energy?

gzhpcu
2010-Feb-05, 07:58 AM
I think it is often explained as a quantum tunneling effect, which allows one of the two to escape.

grant hutchison
2010-Feb-05, 11:25 AM
It's just a matter of having enough energy.

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.

Grant Hutchison

crosscountry
2010-Feb-05, 02:06 PM
IIRC, and it has been a while since I read his books, the particle made of normal matter has more energy than the antimatter particle. Some of them too fall into the black hole, which has no net effect on the mass of the black hole (matter cancels antimatter once inside) but the ones that escape effectively reduce the mass of the black hole because the antimatter reaches something inside and cancels it.

I don't recall, however, why the matter particle has more energy.

Cougar
2010-Feb-05, 02:15 PM
IIRC, and it has been a while since I read his books, the particle made of normal matter has more energy than the antimatter particle.

That would be incorrect.


...the ones that escape effectively reduce the mass of the black hole because the antimatter reaches something inside and cancels it.

Mass is reduced, but not for the reason you give. There is lots of previous BAUT discussion on this.

grant hutchison
2010-Feb-05, 03:57 PM
The matter / antimatter split is a red herring, since both have positive mass-energy in their own rest frame; either will cause a black hole to gain mass if you simply dump it across the event horizon. Any kind of annihilation inside the horizon (presumably with other infalling material) is also irrelevant to the black hole's mass, since the annihilation produces an equivalent amount of energy, which adds an equivalent amount to the black hole's mass.

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.

Grant Hutchison

tommac
2010-Feb-05, 04:45 PM
One thing I keep wondering is(probably been asked before):
in the hawking radiation theory, a virtual pair of particles is produced, near the event horizon, and one of the pair is supposed to fall through then event horizon, and the other particle escapes.

But I can't understand why the other should escape the high gravitational field.

Is it just a matter of having enough energy?

It travels at the speed of light. Light/Energy can escape as long as it is outside of the EH and has an angle that allows it to escape.

grant hutchison
2010-Feb-05, 05:38 PM
It travels at the speed of light. Light/Energy can escape as long as it is outside of the EH and has an angle that allows it to escape.Hawking radiation includes particles other than photons. Particles with rest mass can't travel at the speed of light.

Grant Hutchison

tommac
2010-Feb-05, 07:59 PM
Hawking radiation includes particles other than photons. Particles with rest mass can't travel at the speed of light.

Grant Hutchison

But wouldnt any radiation travel at least at NEAR the speed of light ... or near enough that it could escape the gravitational pull at that point?

crosscountry
2010-Feb-06, 12:45 AM
That would be incorrect.



Mass is reduced, but not for the reason you give. There is lots of previous BAUT discussion on this.


wow, very insightful.

Cougar
2010-Feb-06, 04:14 PM
IIRC, and it has been a while since I read his books...

I don't really recommend Hawking's general audience books, even to get a good idea about what's going on with Hawking radiation. Much better is Kip Thorne's Black Holes and Time Warps, Einstein's Outrageous Legacy [1994]. Thorne's explanation is more along the lines of (paraphrasing)....


The virtual particles appear, as they always do in the vacuum, but find themselves in this massively strong gravitational field of the black hole (but still outside it). This pulls the two virtual particles far apart, just like if you were to fall close to a black hole, your feet and head would be pulled apart due to the difference in gravity between them ("spaghettified"). Thereafter, unable to recombine and annihilate, the particles are converted from virtual to real particles as a result of this energy drawn from the hole's gravitational field, which is a draw of energy from the hole itself. A bit less energy means a tiny bit less mass. And I mean tiny. It takes a ridiculously long time for a hole to evaporate from this hypothesized mechanism.

tommac
2010-Feb-06, 04:43 PM
But from what I hear this is all a lie, for the sake of the layman's understanding and really is not what happens.

From what I understand ... and that is not much ... that it has much more to do with quantum fluctuations ... also I think I heard tell that the particle that falls into the BH has negative energy or something like that ...







The virtual particles appear, as they always do in the vacuum, but find themselves in this massively strong gravitational field of the black hole (but still outside it). This pulls the two virtual particles far apart, just like if you were to fall close to a black hole, your feet and head would be pulled apart due to the difference in gravity between them ("spaghettified"). Thereafter, unable to recombine and annihilate, the particles are converted from virtual to real particles as a result of this energy drawn from the hole's gravitational field, which is a draw of energy from the hole itself. A bit less energy means a tiny bit less mass. And I mean tiny. It takes a ridiculously long time for a hole to evaporate from this hypothesized mechanism.

Cougar
2010-Feb-06, 08:21 PM
...it has much more to do with quantum fluctuations...

The normally brief appearance of the particle pair is a quantum fluctuation.

ShinAce
2010-Feb-06, 08:25 PM
Quantum vacuum fluctuations themselves don't create real particles. So I'd disagree, since it's really the tidal gravity of the black hole that gives all the energy needed to make a real particle. Without gravity, vacuum fluctuations do you no good.

It takes the same amount of energy to create the particle in a lab than it does to lift that particle out of the black hole's gravity well.

Particles have energies depending on the frame of reference that you choose. So a glass of water has a certain energy. As a black of ice in my hand, it has less energy, and flying around a particle accelerator it has a lot more. Still the same amount of H20 but different energies.

From the point of view of someone far removed from the black hole, the escaping particle has positive energy and the in falling particle has negative energy. But the in falling particle is never observed, only supposed. From an observer fighting for dear life near the even horizon, both particles have positive energy.

Energy, mass, space, and time are measured differently for different observers. Translating between views is important and if we hold this concept dear, there is no negative energy particle.

tommac
2010-Feb-06, 09:42 PM
Translating between views is important and if we hold this concept dear, there is no negative energy particle.

How about the Dirac sea?

Tensor
2010-Feb-06, 09:53 PM
Quantum vacuum fluctuations themselves don't create real particles. So I'd disagree, since it's really the tidal gravity of the black hole that gives all the energy needed to make a real particle. Without gravity, vacuum fluctuations do you no good.

And without the vacuum fluctuations, the tidal gravity doesn't produce any radiation. Cougar's particle pair explanation is spot on, if all you're looking for is a quick and dirty analogy.

If you want a more precise explanation, I would suggest you look in "General Relativity" by Robert Wald. Chapter 14. Sections 2-4 to be specific. But you better be prepared. It involves separating the positive and negative parts of Maxwell's equations and Bogolyubov transformations. Which is beyond almost everyone here (me included).

ShinAce
2010-Feb-07, 12:02 AM
And without the vacuum fluctuations, the tidal gravity doesn't produce any radiation. Cougar's particle pair explanation is spot on, if all you're looking for is a quick and dirty analogy.

If you want a more precise explanation, I would suggest you look in "General Relativity" by Robert Wald. Chapter 14. Sections 2-4 to be specific. But you better be prepared. It involves separating the positive and negative parts of Maxwell's equations and Bogolyubov transformations. Which is beyond almost everyone here (me included).

You don't need to worry about me. The last time I posted on this topic, it was to provide my own derivation of Hawking radiation, in response to Grant.

I'm good with any analogy. If you want to say it's quantum tunneling, that's fine. If you want to say it's particle pair creation resulting in radiation; I'm good with that.

But when someone like tommac, who has had pages of great explanations provided to him on the topic wants to say he : thinks it has more to do with vacuum fluctuations and one particle has negative energy, or something like that : i will challenge them.

I personally love the Dirac sea, if I'm trying to get a mental atomic picture of how a semidconductor works. Otherwise, I've found no new use for it. That's what I think about the Dirac sea.

Now it's your turn. What do you think about Unruh radiation? Wouldn't a person with an insanely strong ship see a rising temperature as they approached the event horizon and accelerated more and more? Wouldn't that temperature, in theory, be infinite for someone exactly at the event horizon with an infinite acceleration? Like a photon emitted at the EH? Like your Hawking radiation? How do we explain that the same EH can, on the one hand, have a blackbody temperature for an inertial observer, and on the other hand, have an infinite temperature? All of this coming from an imaginary slice of spacetime! It's supposed to be black, right?

grant hutchison
2010-Feb-07, 12:19 AM
If you integrate out the intensity of Unruh radiation at infinity, it matches the Hawking temperature of the black hole.
No observer can encounter the infinitely hot Unruh radiation at the event horizon, since no material observer can hover there. But the nearly-infinitely-hot radiation for a stationary observer just above the horizon is nearly-infinitely red shifted in escaping from the black hole. Things cancel down.

Grant Hutchison

Tensor
2010-Feb-07, 01:32 AM
You don't need to worry about me.

snip...

The last time I posted on this topic, it was to provide my own derivation of H i will challenge them.

My apologies. I badly misunderstood your response.


Now it's your turn. What do you think about Unruh radiation? Wouldn't a

snip...

All of this coming from an imaginary slice of spacetime! It's supposed to be black, right?

Grant beat me to it.

tusenfem
2010-Feb-07, 12:44 PM
How about the Dirac sea?


tommac, can you stop please with your negative energy stuff, stop trying to put your ATM ideas into every thread.

tommac
2010-Feb-09, 04:20 PM
tommac, can you stop please with your negative energy stuff, stop trying to put your ATM ideas into every thread.


Huh? This is a mainstream description and has nothing to do with my admittedly ATM idea that dark energy is related to negative mass/energy.


From my understanding a mainstream definition of Hawking Radiation is this:

(1) Quantum fluctuations lead to the production of pairs of particles and anti-particles just outside the horizon; one of these falls into the black hole and the other escapes as radiation with positive mass-energy. The in-falling particle has negative mass-energy, so its absorption results in a reduction in the mass-energy of the black hole.

This definition was taken from:
http://www.mathpages.com/HOME/kmath591/kmath591.htm

it was listed first which would seem like the writer of the article thinks that this is the most mainstream of the descriptions.


Then from a universe today article at:
http://www.universetoday.com/2008/02/13/synthetic-black-hole-event-horizon-created-in-uk-laboratory/


However, the situation changes if this particle pair is generated at or near an event horizon of a black hole. If one of the virtual pair falls into the black hole, and its partner is ejected away from the event horizon, they cannot annihilate. Both virtual particles will become "real", allowing the escaping particle to carry energy and mass away from the black hole (the trapped particle can be considered to have negative mass, thus reducing the mass of the black hole). This is how Hawking radiation predicts "evaporating" black holes, as mass is lost to this quantum quirk at the event horizon. Hawking predicts that black holes will gradually evaporate and disappear, plus this effect will be most prominent for small black holes and MBHs.

Please retract your warning.

ShinAce
2010-Feb-09, 04:49 PM
That site you linked has 4 'summaries' of popular descriptions.

If you need 4 ways to explain one thing, obviously we don't know the exact mechanism.

Why are you holding onto this fluff so tightly? There are other views and they are all equivalent. Untestable so they become unarguable(either way)!

tommac
2010-Feb-09, 05:09 PM
That site you linked has 4 'summaries' of popular descriptions.

If you need 4 ways to explain one thing, obviously we don't know the exact mechanism.

Why are you holding onto this fluff so tightly? There are other views and they are all equivalent. Untestable so they become unarguable(either way)!

I am not ... I was just complaining about the moderators warning.

I dont care which way someone refers to it. Please read my initial post where I mention negative energy ... down to the mod warning and into my questioning it.

NEOWatcher
2010-Feb-09, 05:25 PM
I dont care which way someone refers to it. Please read my initial post where I mention negative energy ...
You mean the one that starts with calling the other possible explainations a lie, which basically excludes all possibilities other than negative energy?

But from what I hear this is all a lie, for the sake of the layman's understanding and really is not what happens.

grant hutchison
2010-Feb-09, 05:35 PM
Please retract your warning.The warning (http://www.bautforum.com/1676396-post20.html) is specifically attached to your invocation of the Dirac Sea, which is rather a long step away from the Schwarzschild observer's "negative energy" view of Hawking radiation.
Did you consider using the report button or a PM, in order to ask for clarification? That would be the usual procedure under rule 17.

Grant Hutchison

Ken G
2010-Feb-09, 06:02 PM
The Dirac sea is actually a useful notion here, because it is always a useful notion when one asks about negative energy particles. In particular, when one asks why we don't have real negative energy fermions (I don't know why we don't have real negative energy bosons, but we apparently don't). Fermions can have at most one to a state, so Dirac simply postulated that all the negative energy fermion states were already occupied! It seems to me one could view a Black Hole singularity as the one place where the Pauli exclusion principle doesn't apply, so it is the one place where negative energy particles can become "real boys." In that picture, Hawking radiation happens because there is room in the sea on the singularity side of the EH, but none on our side, so if you create a virtual pair that conserves energy (such that one has positive and one has negative energy), the negative energy particle can only fall into the hole (or its wave function gets destructively interfered, just as happens to the pairs that don't conserve energy, in the standard ways of the uncertainty principle and quantum interference). This seems like a perfectly good way to think about Hawking radiation, though I have no idea how to make a predictive theory out of it (which I also don't know how to do with any other interpretation of Hawking radiation). So I would stop short of calling any other watered-down interpretation a "lie", but this one seems pretty good to me.

What I don't understand are the ramifications for bosonic Hawking radiation. I just don't know why we don't have negative energy bosons, but I'll bet John Baez has something on it somewhere.

[Edited to actually work.]

grant hutchison
2010-Feb-09, 06:16 PM
Since much of Hawking radiation is thermal photons (at least for stellar mass black holes and greater masses), invoking the fermion Dirac Sea does seem to leave a yawning explanatory gap, which is what I was referring to in my previous post.

Grant Hutchison

astromark
2010-Feb-09, 06:34 PM
Having a less than com-pleat understanding of how anything can escape a black holes gravity...? That the gravity force exceeds c. Nothing can get past that. Could one of you please help me understand this ? Yes I have re-read 'A Brief history of time.' and still get 'confused.' by this Hawking idea.

Ken G
2010-Feb-09, 06:39 PM
Note further that this interpretation also involves tunneling-- there would presumably still be a Pauli exclusion principle interior to the EH, and that region would have to be tunneled through to get to the singularity where the negative energy particle is allowed. Perhaps because time has ended there, and so if it can make it that far, there's no more opportunity to destructively interfere its wave function (which normally happens by integrating over time). Note also that large holes would have less Hawking radiation because they require more tunneling to get to the singularity. It all seems to work out with mainstream physics, but I don't know how to do the tunneling calculation of entangled particles, especially where one of them is inside an event horizon!

Ken G
2010-Feb-09, 06:44 PM
Having a less than com-pleat understanding of how anything can escape a black holes gravity...? Nothing does-- the Hawking radiation that escapes comes from our side of the EH. It just has to come from close to the EH, because in the interpretation I just gave, the negative energy solution has to tunnel through the Dirac sea on our side of the EH as well as on the interior side. It's not so obvious the right time coordinate to integrate the destructive interference over, because it is a kind of quantum entanglement interference straddling an EH, but presumably there's a time coordinate in there that is too hard to tunnel through if the virtual particles appear well away from the EH.

Ken G
2010-Feb-09, 06:48 PM
Since much of Hawking radiation is thermal photons (at least for stellar mass black holes and greater masses), invoking the fermion Dirac Sea does seem to leave a yawning explanatory gap, which is what I was referring to in my previous post.
But note that this isn't any fundamental problem for that interpretation, because that interpretation doesn't have any particular connection to the Dirac sea, as it simply claims the Pauli exclusion principle "doesn't work" at the singularity. Hence, to extend to bosons, all we'd have to do is say that whatever makes negative energy bosons impossible also does not apply at the singularity. That probably isn't too hard to do, no matter what is our interpretation of why we don't have negative energy photons. (Though on further thought I think your remark here is specifically targeted at the notion of the Dirac sea, rather than the more general issue of when you can and cannot have negative energies.)

tommac
2010-Feb-09, 07:09 PM
You mean the one that starts with calling the other possible explainations a lie, which basically excludes all possibilities other than negative energy?


I am merely interpreting Grey from here:
http://www.bautforum.com/space-astronomy-questions-answers/99697-laser-planet-3.html#post1668393

I had a long thread discussing this and from what I understand it is not as straight forward as I posted in that thread or is being described here. I believe someone on that thread went as far as to state that it was not 100% true. So I am confused. Is it a lie or is it not a lie?

tommac
2010-Feb-09, 07:16 PM
Having a less than com-pleat understanding of how anything can escape a black holes gravity...? That the gravity force exceeds c. Nothing can get past that. Could one of you please help me understand this ? Yes I have re-read 'A Brief history of time.' and still get 'confused.' by this Hawking idea.



Hah ... let me give you my naive understanding of how energy can supposedly escape ...

The deal is that the energy in the BH tidal forces rips apart a virtual pair. the energy that is used to rip apart the pair comes from the mass/relativistic energy of the black hole. The energy needed to rip apart / promote the pair is equal to the mass of the two particles ... however one escapes , one is swallowed by the hole. So the BH nets -1 particle mass.

However ... this explination is supposedly just a simplification of what happens ... in fact the particles getting promoted happens after one particle falls into the EH ... so it is really strange. I dont understand it past this point.

grant hutchison
2010-Feb-09, 07:29 PM
I believe someone on that thread went as far as to state that it was not 100% true. So I am confused. Is it a lie or is it not a lie?It's a "lie" only in the sense used by Ian Stewart and Jack Cohen, when they gave teachers the title "liar-to-children".
Pretty much everything you learn at basic and intermediate levels of education is "not 100% true". But calling it a "lie" has benefit only if you want to make an amusing point in a hyperbolic sort of way.

Are you trying to make an amusing point in a hyperbolic sort of way?

Grant Hutchison

NEOWatcher
2010-Feb-09, 07:37 PM
I am merely interpreting Grey from here: ...
Isn't it amazing that you pick one source for your interpretations?
Besides; in the "lie" post you just say "from what I hear" which puts your statement as a claim of accumulated knowledge rather than a source of knowledge.
You may have intended to make an innocent post, but the wording you use has much to be desired.

tommac
2010-Feb-09, 07:54 PM
It's a "lie" only in the sense used by Ian Stewart and Jack Cohen, when they gave teachers the title "liar-to-children".
Pretty much everything you learn at basic and intermediate levels of education is "not 100% true". But calling it a "lie" has benefit only if you want to make an amusing point in a hyperbolic sort of way.

Are you trying to make an amusing point in a hyperbolic sort of way?

Grant Hutchison

Yes ... Actually I was sort of basing it off of somebody's quote in their signature ( I forget whos ) I think a quote from knuth ...

tommac
2010-Feb-09, 07:55 PM
Isn't it amazing that you pick one source for your interpretations?
Besides; in the "lie" post you just say "from what I hear" which puts your statement as a claim of accumulated knowledge rather than a source of knowledge.
You may have intended to make an innocent post, but the wording you use has much to be desired.

I heard it from grey ....

I have nothing to offer anyone other than my own confusion.

tommac
2010-Feb-09, 07:58 PM
Hah .... found it ....

Mr Tensor ( in sig ) :

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Was sort of what I was hinting at.

Ken G
2010-Feb-09, 08:12 PM
Everything is a lie, even that.

tommac
2010-Feb-09, 08:41 PM
Everything is a lie, even that.

Now you have me really confused ... if you are lying about everything being a lie then everything is not a lie. But any one thing can still be a lie, right? So we are back where we started from.

PetersCreek
2010-Feb-09, 08:48 PM
I am not ... I was just complaining about the moderators warning.

You have been around long enough and have a warning history rich enough to know that this thread is not the place to complain about a moderator warning. I came >this< close to infracting you into another suspension for doing so. I wouldn't expect any further slack on this point if I were you. If you want to complain, do it the right way.


I have nothing to offer anyone other than my own confusion.

If this is true, then you shouldn't be attempting to answer questions in this forum. Adding confusion to the discussion is most unhelpful and your ATM ideas invariably creep in and get you in trouble.

Now, enough of the sidetrack. Let's confine further posts to mainstream answers for the OP and discussion thereof.

DrRocket
2010-Feb-09, 09:02 PM
Rather than bantering about various "pop-sci" explanations of a complex and subtle notion, one might actually go to the original source (http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.cmp/1103899181).

tommac
2010-Feb-09, 09:19 PM
Rather than bantering about various "pop-sci" explanations of a complex and subtle notion, one might actually go to the original source (http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.cmp/1103899181).



Where are the Cliff Notes?

DrRocket
2010-Feb-09, 09:30 PM
Where are the Cliff Notes?

There are lots of Cliff notes around. That is the problem and what is creating confusion in this thread -- simple, understandable, wrong explanations. Cliff notes tend to create the illusion of understanding, which in the absence of the real thiing is usually worse than acknowledged ignorance.

Here is Baez on Hawking radiation (http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html), but at least he admits that he is not totally familiar with the hard-core calculation.

Ken G
2010-Feb-09, 09:35 PM
Rather than bantering about various "pop-sci" explanations of a complex and subtle notion, one might actually go to the original source (http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.cmp/1103899181).
Good luck! Besides, there is no "hard core" calculation here, because there is no unified theory of gravity and QM. It seems to me that Hawking is creating an effective theory, basically by horse sense, and it takes quite a thoroughbred to evaluate the likelihood that what he has done will prove correct in an actual unified theory. (And my first reaction to these thoroughbreds is that I don't think 6 X 10-8 counts as approximately 10-6.)

tommac
2010-Feb-09, 09:37 PM
Here is Baez on Hawking radiation (http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html), .

I love her rendition of Blowin' in the Wnd"

tommac
2010-Feb-09, 09:38 PM
Good luck! Besides, there is no "hard core" calculation here, because there is no unified theory of gravity and QM. It seems to me that Hawking is creating an effective theory, basically by horse sense, and it takes quite a thoroughbred to evaluate the likelihood that what he has done will prove correct in an actual unified theory.

Maybe that is the source of all of the confusion.

Strange
2010-Feb-09, 09:39 PM
Here is Baez on Hawking radiation (http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html), but at least he admits that he is not totally familiar with the hard-core calculation.

I like this explanation, not because it helps me understand Hawking radiation, but because it helps me understand how much I don't understand...

DrRocket
2010-Feb-09, 09:51 PM
Good luck! Besides, there is no "hard core" calculation here, because there is no unified theory of gravity and QM. It seems to me that Hawking is creating an effective theory, basically by horse sense, and it takes quite a thoroughbred to evaluate the likelihood that what he has done will prove correct in an actual unified theory. (And my first reaction to these thoroughbreds is that I don't think 6 X 10-8 counts as approximately 10-6.)

You are basically right. It will take a unified theory of gravity and QM to really bring understanding.

Hawking's treatment is one of the early glimpses into a theory of QM, using quantum field theory against a background of curved spacetime. There remain holes in the argument. However, this is the primary reference on Hawking radiation and it is certainly a better source than any number of, wrong, popular explanations.

I have no idea whether or not it will hold up in the future in the face of either experimental data or better theoretical constructs. Here (http://arxiv.org/PS_cache/gr-qc/pdf/0304/0304042v1.pdf) is a paper discussing potential shortcomings in the calculation.

Ken G
2010-Feb-09, 09:56 PM
However, this is the primary reference on Hawking radiation and it is certainly a better source than any number of, wrong, popular explanations. I agree that it's nice to link to the real deal to see what's really involved, but I have to agree with Henna Oji-san: it's a bit like being on a climbing wall and someone says "if you want to climb Mount Everest, these are the kinds of things you need to be able to do." Meanwhile, you are looking out the window at the actual Mount Everest in the distance, thinking, "it doesn't look like this."

DrRocket
2010-Feb-09, 10:54 PM
I agree that it's nice to link to the real deal to see what's really involved, but I have to agree with Henna Oji-san: it's a bit like being on a climbing wall and someone says "if you want to climb Mount Everest, these are the kinds of things you need to be able to do." Meanwhile, you are looking out the window at the actual Mount Everest in the distance, thinking, "it doesn't look like this."

Which is why lots of people scale climbing walls but not many summit on Everest. They are not the same thing.

Neither are trampolines and GR.

Or popularized misrepresentations of Hawking radiation, and the real deal.

Perhaps instead of pretending one is climing Everest while on the scaling wall at the local REI store, one ought to just geta pair of good boots and enjoy the local hiking trails that one can actually traverse.

A facet of modern physics is that the frontiers are fairly demanding of competence in physics and mathematics. There is no substitute, particularly if one's aspirations include understanding fundamental mechanisms or giving knowledgeable critiques of the theory, as opposed to simply understanding the most basic implications.

A person who cannot handle even basic calculus has no more chance of understanding the mechanisms or business debating the validity of explanations of quantum field theory in curved spacetime than an obese 50-year-old with emphysema has debating the merits of the route (http://www.explorersweb.com/webtv/videoconsol_everestarial.htm)up the central north face of Everest, with and without use of oxygen.

One can read the popular accounts and either be satisfied (despite the fact that they are misleading) or one can study the real thing. In between is neither fish nor fowl, and cannot lead to a satisfactory conclusion. It is not even clear if the "hard-core" calculations are correct, as you observed, and as discussed in the paper linked earlier. Hence, one ought not expect that over-simplified and wrong popularizations will lead to deep understanding. If the experts don't know then it is a dead nuts certainty that an understanding based on popularizations will not admit a reliable conclusion either.

Strange
2010-Feb-09, 11:11 PM
I am happy walking the dog and looking in awe at the mountains in the distance. I don't expect to achieve more than that.


Hence, one ought not expect that over-simplified and wrong popularizations will lead to deep understanding. If the experts don't know then it is a dead nuts certainty that an understanding based on popularizations will not admit a reliable conclusion either.

Yeah, right. Go and tell that to the ATM crowd! They seem pretty sure of themselves.

Grey
2010-Feb-09, 11:30 PM
I heard it from grey ....And I certainly described it in that thread not so much as a lie, but as an over-simplified explanation to help people unfamiliar with the detailed treatment get an idea of what's going on. Just as folks here have said.

I'll repost a link from that thread. This (http://xxx.lanl.gov/abs/gr-qc/0304042) is a pretty nice discussion of Hawking radiation. The author discusses into some of the assumptions Hawking makes in determining that black holes radiate (and why some of those assumptions might be shaky). In section three, he gives a rough outline of Hawking's reasoning. It's the simplest description I've seen that still leaves the overall argument essentially intact.

Edited to add: :doh: Now that I've finished reading the thread, I see that Dr. Rocket has already posted the very same paper that I had linked to in the other thread. Well, at least the first time I posted was before his post. :)

Ken G
2010-Feb-09, 11:48 PM
Or popularized misrepresentations of Hawking radiation, and the real deal.Nevertheless, I would not be at all surprised, if one dissects the Hawking argument, if one does not find a fruitful way to look at it in terms of negative energy virtual particle tunneling to a place (the singularity) where such a solution is allowed, consequently allowing the positive-energy entangled particle to escape to the outside world. That basic picture resonates with quite a few things in the Hawking paper (and/or the Baez characterization of it): nonlocality, Bogoliubov transformations (which mix positive and negative energy solutions in frames different from the external observer, bringing in the gravity kappa and, I wager, making contact at some point with the existence of a singularity), and basic quantum interference over time (which is how h gets into the game). The actual rate at which this occurs would be a different issue, bringing in thermodynamics and the counting of states (which is where k gets into the game), that's something different from how one can view it happening but rather how often it happens.

For example, I call your attention to the fact that the answer is, to the order of approximation that most people would be concerned with, just the temperature you can get from a characteristic gravity, the h of quantum mechanics, and the k of statistical physics. There's a reason for that, and a basic explanation should make contact with that reason without Bogoliubov transformations.


A facet of modern physics is that the frontiers are fairly demanding of competence in physics and mathematics. There is no substitute, particularly if one's aspirations include understanding fundamental mechanisms or giving knowledgeable critiques of the theory, as opposed to simply understanding the most basic implications. Nevertheless, when someone cannot explain the implications and flavor of the math in fairly basic terms, I get concerned there is not a complete understanding there. Remember Feynman's barmaid (no offense to the female members)?

A person who cannot handle even basic calculus has no more chance of understanding the mechanisms or business debating the validity of explanations of quantum field theory in curved spacetime than an obese 50-year-old with emphysema has debating the merits of the route (http://www.explorersweb.com/webtv/videoconsol_everestarial.htm)up the central north face of Everest, with and without use of oxygen.
Yet, "understanding the basic mechanisms" is a whole lot different from debating their validity. When we have learned something that only one person in a million can understand, we have not learned much at all. The challenge is to keep driving into that understanding until some key essence of it can be brought to any educated person, so that we can say we really have brought humanity into contact with some deeper truth of their reality.


If the experts don't know then it is a dead nuts certainty that an understanding based on popularizations will not admit a reliable conclusion either.It depends on what "reliable" means. Something they could use to prove theorems? That certainly would never work. Something they can put together out of fundamental physics concepts that hangs together? That's another matter.

tommac
2010-Feb-09, 11:53 PM
And I certainly described it in that thread not so much as a lie, but as an over-simplified explanation to help people unfamiliar with the detailed treatment get an idea of what's going on. Just as folks here have said.

Would you agree:
Lying deliberately to help the reader understand the ideas?

PetersCreek
2010-Feb-10, 12:16 AM
Simplified explanations are necessarily incomplete. Enough said...especially given my comment in post #41:


Now, enough of the sidetrack. Let's confine further posts to mainstream answers for the OP and discussion thereof.

Whether it's an egregious lie, a little white lie, or the Readers Digest condensed version, it's an off-topic point that doesn't need to be hammered any further in this thread.

DrRocket
2010-Feb-10, 12:47 AM
Nevertheless, I would not be at all surprised, if one dissects the Hawking argument, if one does not find a fruitful way to look at it in terms of negative energy virtual particle tunneling to a place (the singularity) where such a solution is allowed, consequently allowing the positive-energy entangled particle to escape to the outside world.

I would be surprised if one could not do something like this, and if it did not have some bit of truth. That does not necessarily make it a complete or valid explanation, but who knows ? This is an area where things are not completely settled. However, to really speculate in a meaningful way takes a real expert, and there are very few of those.

I doubt that the singularity is as important as the event horizon. The former is likely a sign of problems withe GR, but I am not sure that anybody really knows.


Nevertheless, when someone cannot explain the implications and flavor of the math in fairly basic terms, I get concerned there is not a complete understanding there. Remember Feynman's barmaid (no offense to the female members)?

I agree with this. But there are times when nobody understands the theory well enough for such a simple explanation. Remember Feynman's failure to create a freshman explanation for Fermi-Dirac statistics ?

Black hole thermodynamics and Hawking radiation are not understood well enough. I would be nice if this were not so, but that is the way it is. Perhaps when the experts understand things better, everyone else will be able to improve their comprehension also. But in the meantime uninformed speculation by the non-participants is not likely to lead to progress.



t, "understanding the basic mechanisms" is a whole lot different from debating their validity. When we have learned something that only one person in a million can understand, we have not learned much at all. The challenge is to keep driving into that understanding until some key essence of it can be brought to any educated person, so that we can say we really have brought humanity into contact with some deeper truth of their reality.

That is the objective. But that objective is not attained at the frontiers of research.


It depends on what "reliable" means. Something they could use to prove theorems? That certainly would never work. Something they can put together out of fundamental physics concepts that hangs together? That's another matter.

I have no idea what "hangs together" means in this context. It sounds rather like a descriptor of a story that is plausible but wrong. That leads to a false sense of understanding and that is, as previously noted, worse than acknowledged ignorance.

There comes a point where the frontiers of knowledge are more than a bit fuzzy and where meaningful critical discussion is possible only among experts. If we understood things better then those with less expertise might be able to participate.

I find a truly disturbing trend in recent years among research physicists, particularly theoreticians. There exist a plethora of books about speculative approaches to physics, written for the public at large. That on the surface is a good thing. What is not a good thing is the tendancy to present speculative and incompletely formulated ideas as other than just conjecture. The presentations are such that a layman could easily be led to believe that things are understood that are really a mystery and that theories exist, have been validated and are universally believed that are in fact nothing more than promising ideas still lacking in clear definition. String theory leaps to mind as an example, but it is far from unique in that regard. (As an aside Gordon Kane in his book on supersymmetry, written in 1999 is very careful to separate speculation from proven science. I alslo note that NONE of his confident predictions for events expected by about 2006, clearly labeled as predictions, have come to pass. The message being that lone should labe speculation clearly, and the record for speculation is not very good.)

Hawking radiation still remains to be proved to exist. It is the result of some brilliant, but not completely rigorous, reasoning. There is at this juncture zero experimental evidence either confirming or denying it. It is one of those specialized topics that are not sufficiently well understood for a meaningful discussion of the mechanism at a "popular" level. Valley girls need not apply. Experts have enough trouble.

The point of the "popular" explanations is to allow the discussion of implications of the existence of Hawking radiation, also at a superficial level. It is not to impart a deep understanding of the mechanisms involved. Debating the merits of a mechanism that is oversimplified and basically wrong, is futile. Most people would realize that, but they are not told that the explanation is not really correct. We have the same problem with the "trampoline analogy" in GR, and the "raisin bread" analogy in cosmology.

For a good understanding of Hawking radiation we probably do need a "theory of everything". Based on Hawking's predictioins in his 1981 book, A Brief History of Time, that should be forthcoming by about ..........now. Oops.

astromark
2010-Feb-10, 08:05 AM
I am pleased to have asked...
But remain a little confused by what is actually happening in the gravity well that has become a Black Hole. Will wait for this... "Theory of Everything" but suspect its just Jam and cheese crackers... and seems to have lost the wine. Thank you for the tutorial.:):):o

That book was 2002...

Ken G
2010-Feb-10, 10:02 AM
I would be surprised if one could not do something like this, and if it did not have some bit of truth. That does not necessarily make it a complete or valid explanation, but who knows ? Yes, I would not say complete, certainly, and "valid" is hard to define. There is such a thin line between an explanation that creates a real sense of understanding, versus just the illusion. What constitutes an explanation? That's very tough.
I doubt that the singularity is as important as the event horizon. The former is likely a sign of problems withe GR, but I am not sure that anybody really knows.
You are talking about how seriously we can take the singularity, but I don't mean that it needs to be taken that seriously, it is just a placekeeper for somewhere that physics breaks down and the time integrals end. It is normal for processes to be rendered impossible by what happens as a time integral over coherences extends to long times, so if that integral is interrupted by anything, whether it be a true singularity or not, there is the possibility of something impossible becoming possible.

I agree with this. But there are times when nobody understands the theory well enough for such a simple explanation. Remember Feynman's failure to create a freshman explanation for Fermi-Dirac statistics ?I actually don't know that anecdote, it surprises me-- that really seems like something he could do. I recall his point that spin-1/2 particles are like coffee cups held by the handle-- if you rotate them around, the cup returns to the same configuration, but the arm is all twisted-- it takes a second rotation of the cup to reset the arm! (Spin 1/2 particles suffer a change in sign of their amplitudes when you rotate them 360 degrees, which means their amplitude by itself functions the same way, but it interferes with other things differently-- much like having an invisible "arm" attached to them, it's an insanely brilliant analogy.)

Perhaps when the experts understand things better, everyone else will be able to improve their comprehension also. But in the meantime uninformed speculation by the non-participants is not likely to lead to progress. I don't see the harm in applying physics as we know it to try to bring a bizarre phenomenon into the fold of other processes we understand better. That's the explanation side of things, it's not intended as a means to achieve progress in the calculation itself.

I have no idea what "hangs together" means in this context.I'd say it means, use only physical processes that we know about pretty well to try to unify some much more bizarre process into that same milieu, perhaps by combining several processes that are not normally thought about together (like the coherences of quantum entanglement, the ambiguities of Bogoliubov transformations, and the distortions of gravity).
It sounds rather like a descriptor of a story that is plausible but wrong. That leads to a false sense of understanding and that is, as previously noted, worse than acknowledged ignorance. Although I cannot speak specifically to this example, there is certainly quite a valuable place for stories motivated by real physics that "hang together." I have to do it all the time-- I routinely tell average students, who come in not knowing why we have seasons or why the Moon goes through phases, about the reasons that some stars go supernova and some don't. I have to tell them that it takes energy to confine electrons to very small spaces, because of the uncertainty principle, and if the mass of a star isn't enough, there just isn't enough potential energy in the gravity to compress the electrons into a volume smaller than the Earth, so the core cannot collapse and blow up the star. That's a story that "hangs together"-- it's real physics, it's right, and a student can understand it without knowing quantum mechanics or general relativity, but they are never going to debate the validity of the story based on that understanding.

If we understood things better then those with less expertise might be able to participate.I have a theorem that says when a very long and complicated argument results in a very simple answer, there is always going to be a much simpler way to get that answer. Often, the complexity and difficulty comes in determining if some coefficient should be 2 or pi or 9/13, but this gets all mixed up into the basic magnitude of the answer in ways that are completely avoidable. But you do have to look for the simple explanation, and there will always be the question of what constitutes a true explanation.

What is not a good thing is the tendancy to present speculative and incompletely formulated ideas as other than just conjecture. The presentations are such that a layman could easily be led to believe that things are understood that are really a mystery and that theories exist, have been validated and are universally believed that are in fact nothing more than promising ideas still lacking in clear definition.I agree with you here, but I reach a different conclusion. To me, the validity of the point you are making here is the reason to look for the simple explanation-- when a very complicated and difficult calculation can be reduced to an essence that most can get the flavor of, they can see it much more easily for what it is. It's not the simple explanation that is the problem you refer to, it is the claim of authority. What I object to is when the experts you are talking about engage in speculation, do a very difficult calculation, and then present it to people like "here's a dumbed-down version that really isn't right but it's all you could understand, what makes my answer right is a calculation so difficult you could never tell where I pulled the rabbit out of the hat." The "dumbed-down" explanation should indicate exactly where the rabbit came out of the hat, the simple explanation could be used as easily as an opportunity to be honest as an opportunity to be dishonest.


The point of the "popular" explanations is to allow the discussion of implications of the existence of Hawking radiation, also at a superficial level. It is not to impart a deep understanding of the mechanisms involved. Actually, I don't agree there. One cannot discuss implications based on an incomplete understanding, because implications require logic, and logic cannot have holes in it. Understanding is different, it is merely an ability to fit something into one's head in a way that does not force them to take it entirely on faith, but rather to see the connections between one claim and other things that use similar physics. The understanding does not need to be "deep", that would be an unreasonable goal.

Most people would realize that, but they are not told that the explanation is not really correct. We have the same problem with the "trampoline analogy" in GR, and the "raisin bread" analogy in cosmology.Right, but the problem there is not that the explanations are simplified, it is that they aren't using the appropriate physics, they're using simulacrum physics that quacks like the duck but doesn't have any of the other properties of the duck. They are fundamentally swindles. Still, it does bring us into contact again with the question of what constitutes a true explanation, and that is not so easy, and people can disagree.


For a good understanding of Hawking radiation we probably do need a "theory of everything". But if the answer comes out like Hawking finds, then it's still going to have to represent a collision of the three basic parameters in that expression: some sort of kinematic connection with the singularity (Hawking's kappa), some sort of quantum mechanical connection with the uncertainty principle (his h), and some sort of thermodynamic interpretation relating to the information lost into the hole (his k). A TOE might package it all very differently, but those basic concepts should still be traceable, so a working explanation should still transcend the TOE.

Based on Hawking's predictioins in his 1981 book, A Brief History of Time, that should be forthcoming by about ..........now. Oops.The one thing I've often noted about futurists is that they tend to anticipate almost everything fairly well, with two notable exceptions:
1) they tend to misjudge the relative impact of various things, and
2) they invariably underestimate the timescale!

WaxRubiks
2010-Feb-10, 12:17 PM
personally I believe in incipient black holes, and that the event horizon is some sort of illusion that nothing can cross....like not being able to go further north than the north pole.

I think the mechanism for black hole evaporation is some sort of pre-Hawking radiation...which I have read that some people are trying to work out.


So Hawking radiation will just be an analogy for pre-Hawking radiation, and my guess is that the equations for HR will still apply, more or less.

Hornblower
2010-Feb-10, 01:49 PM
personally I believe in incipient black holes, and that the event horizon is some sort of illusion that nothing can cross....like not being able to go further north than the north pole.

I think the mechanism for black hole evaporation is some sort of pre-Hawking radiation...which I have read that some people are trying to work out.


So Hawking radiation will just be an analogy for pre-Hawking radiation, and my guess is that the equations for HR will still apply, more or less.

What do you mean by "incipient black holes"?

What do you mean by "pre-Hawking radiation"?

WaxRubiks
2010-Feb-10, 02:55 PM
What do you mean by "incipient black holes"?

What do you mean by "pre-Hawking radiation"?

I think incipient black holes are collapsing objects where an event horizon doesn't form, and neither does a singularity, where the matter doesn't cross the apparent boundary that would be the event horizon, but becomes more and more time dilated as it falls in, appearing to slow down.

There is some stuff on this page about it.
http://www.scienceagogo.com/news/black_hole_redux.shtml

Pre-Hawking radiation, I think, is the mechanism that would enable such an object to evaporate.

Ken G
2010-Feb-10, 03:23 PM
If my simple characterization of Hawking radiation holds any of the essence of the truth, then "pre-Hawking" radiation cannot evaporate a black hole before it forms, because if the hole never forms, there's no singularity, and there's no way to truncate the destructive interference that will make pre-Hawking radiation impossible.

WaxRubiks
2010-Feb-10, 04:22 PM
my guess is that pre-Hawking radiation may be a transformation of some of the infalling matter itself, due to the matter being very dense, rather than any quantum tunneling, or virtual particle pair separation etc.


but I'm just guessing....

tommac
2010-Feb-10, 04:32 PM
Due to GR can a star ever collapse to form an EH ( from an external observer)?

I would think that it is similar to Xeno's arrow. ( from an external point of view ) As it collapses time slows as it collapses more time slows more ... etc ... the more time you wait the more time will slow.

If you are close to the star though it can all happen in an instant right?
so I dont get how this would work ... for a local observer ( the virtual particles ) arent they close enough?

Actually ... I guess since they are still outside the SR time could never progress far enough for the start to collapse beyond the SR.




I think incipient black holes are collapsing objects where an event horizon doesn't form, and neither does a singularity, where the matter doesn't cross the apparent boundary that would be the event horizon, but becomes more and more time dilated as it falls in, appearing to slow down.

There is some stuff on this page about it.
http://www.scienceagogo.com/news/black_hole_redux.shtml

Pre-Hawking radiation, I think, is the mechanism that would enable such an object to evaporate.

Cougar
2010-Feb-10, 05:58 PM
I think incipient black holes are collapsing objects where an event horizon doesn't form, and neither does a singularity, where the matter doesn't cross the apparent boundary that would be the event horizon, but becomes more and more time dilated as it falls in, appearing to slow down.

There is some stuff on this page about it.
http://www.scienceagogo.com/news/black_hole_redux.shtml

From my reading of Kip Thorne, this is simply an illusion of an outside observer. The event horizon does form. It only appears not to - from the viewpoint of an outside observer. Anyone entering the hole will verify this. Volunteers?

But Krauss is no dummy, so I imagine there is more to this concept than meets the eye in a web article authored by a guy named "Rusty Rockets" :rolleyes: (not that the article is particularly bad or anything...)

DrRocket
2010-Feb-10, 06:24 PM
my guess is that pre-Hawking radiation may be a transformation of some of the infalling matter itself, due to the matter being very dense, rather than any quantum tunneling, or virtual particle pair separation etc.


but I'm just guessing....

You are doing an awful lot of "guessing" " and "believing" in the last several posts. Is there some basis for this ? What is your take on Santa Claus ?

WaxRubiks
2010-Feb-10, 06:32 PM
You are doing an awful lot of "guessing" " and "believing" in the last several posts. Is there some basis for this ? What is your take on Santa Claus ?

I think that Santa Claus would be considered ATM, if you want to start a thread in ATM about it...

Personally I don't believe in him anymore.

PetersCreek
2010-Feb-10, 06:45 PM
I guess my comments have fallen on deaf ears. Given that the guessing, ATMing, and now, questions on another topic have continued in spite of them, this thread is closed. Report this post if anyone has something contribute that directly addresses the OP.