Hawking radiation

Discussion in 'Physics & Math' started by hardalee, Sep 16, 2015.

  1. Schmelzer Valued Senior Member

    Try to learn the meaning of analog gravity. It is about the sound waves of classical condensed matter. The speed of sound is, in this analogy, what corresponds to the speed of light. So, this analogy is the completely classical ether - which is a material such that the speed of light is its speed of sound. And, of course, Unruh knows this very well. By the way, he cites Jacobson, who openly uses the e-word.

    By the way, here is the explicit quote where Unruh assumes a preferred frame, and tells us which it has to be:
    W.G. Unruh, R. Schutzhold, On the Universality of the Hawking Effect, Phys. Rev. D 71, 024028 (2005), arXiv:gr-qc:0408009
    And he also knows that if these conditions fail, there may be no Hawking radiation.
    This is, by the way, the main problem which prevents me from writing a paper: The claim that if the preferred frame would be the rest frame of the black hole there would be no Hawking radiation would be nothing really new. It would be only yet another illustration of a fact mentioned already in this paper.
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  3. paddoboy Valued Senior Member

    Except that there are no facts, which I have no trouble admitting or realising...Only anologues that may or may not support a particular scenario, and that which most see as more likely than other possibilities.
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  5. OnlyMe Valued Senior Member

    I don't think so.

    The first line of one of his paper's abstracts begins,

    ... But that paper goes on to describe an analogue of what he, Unruh and the coauthors of his earlier paper, interpret the process Hawking predicted, to be - in a purely classical setting. So what the paper actually argues is that how sound waves act in a moving fluid is similar to the escape process Hawking predicted for Hawking radiation. But even that is a stretch because admittedly no classical environment duplicates the conditions that are predicted to exist at an event horizon in the case of gravity... And Unruh's analogues deal only with how waves in a moving medium seem to simulate what would be expected of EM radiation or particles escaping a black hole from near its event horizon.

    The third paper involving ultrashort laser pulses (Unruh was not a contributor), again did not produce any conclusive evidence even though they did observe photon emissions unlike other photon known emissions. From that paper note the last sentence quoted below.

    Again even in this case they were not working under conditions involving the extreme gravitational fields associated with black holes.., and they don't make any explicit claims that any separation of virtual particle pairs is involved. All they are really saying is that they have observed photon emission, where the mechanism of emission cannot be readily explained, but fits with their assumptions about the process Hawking predicted.

    In the papers presented so far it has been only a simplification of the Hawking process, to classical conditions, that has been explored. Nothing that duplicates the extreme gravitational filed conditions required.

    BTW playing only the role of devil's advocate, in these papers it is unclear whether the authors are talking about an emmision process limited to massless photons or ultimately virtual particles that become real massive particles escaping.., and the bigger issue of whether the counterpart photon or particle captured by the black hole represents some negative mass in any real sense. Basically what the whole process depends on, in an over simplified way, is that the captured half of the virtual "particle" pair is always an antiparticle (or maybe some sort of unknown anti-photon?), that ultimately some how reduces the mass of the black hole..? Since both energy and mass contribute to the gravitational field in GR and neither can escape a black hole across an event horizon, even the energy of an anti-particle/photon should add to the gravitational field of the black hole. When particle/antiparticle pairs annialte, they each contribute half of the resulting released energy. Even if half of that energy cannot escape the black hole it adds to the black hole's total energy... At least as long as both mass and energy contribute to a gravitational field.
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  7. paddoboy Valued Senior Member

    I said it earlier, Hawking Radiation certainly is not on as firm ground as is SR or GR for example.
    What I'm saying is that the anologues do support the likelyhood of Hawking Radiation. Nothing more and nothing less.
    And just as obviously if the anologues did not support the concept, then its detractors would certainly be using that in their arguments.
    My opinion based on that is that Hawking Radiation appears a reasonably logical concept and for that reason, is generally accepted.

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    I'm not sure how to answer that, but I did find the following answers which may help in that regard.


    The following answer at

    There are two ways to approach your question. The first is to explain what Brian Greene means, and the second is to point out that the "particles being swallowed" explanation is a metaphor and isn't actually how the calculation is done. I'll attempt both, but I'm outside my comfort zone so if others can expand or correct what follows please jump in!

    When a pair of virtual particles are produced there isn't a negative energy particle and a positive energy particle. Instead the pair form an entangled system where it's impossible to distinguish between them. This entangled system can interact with the black hole and split, and the interaction guarantees that the emerging particle will be the positive one. NB "positive" and "negative" doesn't mean "particle" and "anti-particle" (for what it does mean see below), and the black hole will radiate equal numbers of particles and anti-particles.

    Now onto the second bit, and I approach this with trepidation. When you quantise a field you get positive frequency and negative frequency parts. You can sort of think of these as representing particles and anti-particles. How the positive and negative frequencies are defined depends on your choice of vacuum, and in quantum field theory the vacuum is unambiguously defined. The problem is that in a curved spacetime, like the region near a black hole, the vacuum changes. That means observers far from the black hole see the vacuum as different from observers near the black hole, and the two observers see different numbers of particles (and antiparticles). A vaccum near the event horizon looks like excess particles to observers far away, and this is the source of the radiation.
  8. paddoboy Valued Senior Member

    Here is another explanation by Steve Carlip:
    Hawking radiation
    There are a number of ways of describing the mechanism responsible for Hawking radiation. Here's one:
    The vacuum in quantum field theory is not really empty; it's filled with "virtual pairs" of particles and antiparticles that pop in and out of existence, with lifetimes determined by the Heisenberg uncertainty principle. When such pairs forms near the event horizon of a black hole, though, they are pulled apart by the tidal forces of gravity. Sometimes one member of a pair crosses the horizon, and can no longer recombine with its partner. The partner can then escape to infinity, and since it carries off positive energy, the energy (and thus the mass) of the black hole must decrease.

    There is something a bit mysterious about this explanation: it requires that the particle that falls into the black hole have negative energy. Here's one way to understand what's going on. (This argument is based roughly on section 11.4 of Schutz's book, A first course in general relativity.)

    To start, since we're talking about quantum field theory, let's understand what "energy" means in this context. The basic answer is that energy is determined by Planck's relation, E=hf, where f is frequency. Of course, a classical configuration of a field typically does not have a single frequency, but it can be Fourier decomposed into modes with fixed frequencies. In quantum field theory, modes with positive frequencies correspond to particles, and those with negative frequencies correspond to antiparticles.

    Now, here's the key observation: frequency depends on time, and in particular on the choice of a time coordinate. We know this from special relativity, of course -- two observers in relative motion will see different frequencies for the same source. In special relativity, though, while Lorentz transformations can change the magnitude of frequency, they can't change the sign, so observers moving relative to each other with constant velocities will at least agree on the difference between particles and antiparticles.

    For accelerated motion this is no longer true, even in a flat spacetime. A state that looks like a vacuum to an unaccelerated observer will be seen by an accelerated observer as a thermal bath of particle-antiparticle pairs. This predicted effect, the Unruh effect, is unfortunately too small to see with presently achievable accelerations, though some physicists, most notably Schwinger, have speculated that it might have something to do with thermoluminescence. (Most physicists are unconvinced.)

    The next ingredient in the mix is the observation that, as it is sometimes put, "space and time change roles inside a black hole horizon." That is, the timelike direction inside the horizon is the radial direction; motion "forward in time" is motion "radially inward" toward the singularity, and has nothing to do with what happens relative to the Schwarzschild time coordinate t.

    The final ingredient is a description of vacuum fluctuations. One useful way to look at these is to say that when a virtual particle- antiparticle pair is created in the vacuum, the total energy remains zero, but one of the particles has positive energy while the other has negative energy. (For clarity: either the particle or the antiparticle can have negative energy; there's no preference for one over the other.) Now, negative-energy particles are classically forbidden, but as long as the virtual pair annihilates in a time less than h/E, the uncertainty principle allows such fluctuations.

    Now, finally, here's a way to understand Hawking radiation. Picture a virtual pair created outside a black hole event horizon. One of the particles will have a positive energy E, the other a negative energy -E, with energy defined in terms of a time coordinate outside the horizon. As long as both particles stay outside the horizon, they have to recombine in a time less than h/E. Suppose, though, that in this time the negative-energy particle crosses the horizon. The criterion for it to continue to exist as a real particle is now that it must have positive energy relative to the timelike coordinate inside the horizon, i.e., that it must be moving radially inward. This can occur regardless of its energy relative to an external time coordinate.

    So the black hole can absorb the negative-energy particle from a vacuum fluctuation without violating the uncertainty principle, leaving its positive-energy partner free to escape to infinity. The effect on the energy of the black hole, as seen from the outside (that is, relative to an external timelike coordinate) is that it decreases by an amount equal to the energy carried off to infinity by the positive-energy particle. Total energy is conserved, because it always was, throughout the process -- the net energy of the particle-antiparticle pair was zero.

    Note that this doesn't work in the other direction -- you can't have the positive-energy particle cross the horizon and leaves the negative- energy particle stranded outside, since a negative-energy particle can't continue to exist outside the horizon for a time longer than h/E. So the black hole can lose energy to vacuum fluctuations, but it can't gain energy.
  9. brucep Valued Senior Member

    What you think is irrelevant since you're pretty much confused. Read the entire Unruh paper on the analog experiment. What he said was the original derivation that Hawking wrote down had some issues. Then in 1980 he had the idea to use the analog because the analog experiment domain resolves the issues that he had with the original derivation. From there they designed the experiment which Professor Unruh claims they measured Hawking radiation. He also made it clear that the experimental analysis doesn't include a analysis of a domain, he defines, that might be included in further experimental analysis. They measured the Hawking radiation because the experiment met the original conditions for an analog experiment. Tell us why you think you understand the experiment well enough to say all they measured was the speed of sound. Read the paper which describes the experiment in detail if you can't figure the relationship between the experiment components and why the Hawking radiation should be present. Then you can say what you really mean that analog experiments can't tell us anything about the physics.
  10. OnlyMe Valued Senior Member

    Bruce, where did you get the idea I was saying anything about the speed of sound? What I was pointing out is that the analogue experiment could be conducted specifically because it did not involve the conditions that would exist in a strong gravitational field....., a black hole event horizon....

    The reference to sound was from,

    A 20 cm wavelength is within the wavelength range of sound waves.

    Black hole event horizons as you imagine them to be remain prediction..., that is at present they are not real things known to exist. Even while many theorists believe that they do. What observational evidence we have for the existence of black holes falls far short of confirming event horizons. Which leaves the Hawking radiation as it involves black holes..., speculation about how an unconfirmed prediction of GR might interact with the unconfirmed virtual particles of quantum theory.

    None of your references, the ones you presented in response to my questioning your use of the word "fact" in reference to Hawking radiation, prove your assertion or use of the word. Stop once in a while and think! Even read your own references and quote the specific place they present evidence, with regard to black hole Hawking radiation. That's where this started!

    I don't usually get into discussions involving things like the Big Bang or Hawking radiation because they require blind acceptance of too many unproven assumptions. The intent of my posts here has been on the certainty that an obviously lay discussion has been putting on what is theoretical at best and the way I read the papers referenced mostly speculation.

    There is no scientific consensus about virtual particles as real components of the vacuum, at best you might get some consensus about an EM spectrum component.., so any speculation about how they (virtual particles) interact with anything, is as I said at best theoretical and most times speculation.
  11. OnlyMe Valued Senior Member

    I never said analogue models or experiments cannot tell us something about physics... But it is foolish to forget that they do not reproduce the conditions, in this case existing at a black hole event horizon, and so while they may provide some limited insight into the process being discussed, they cannot be expected to be taken as proof of anything that happens at or near a black hole event horizon, or even that Hawking radiation does in fact occur. And is the fact that I was questioning.
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  12. tashja Registered Senior Member

    Hi Q. Apologies for the delay. Here's Prof. Unruh's response to your #76:

    I also sent Prof. Polchinski your posts. Here's his reply:

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  13. tashja Registered Senior Member

    I'm very grateful to both Prof. Unruh and Prof. Polchinski for their informative replies, and also to Prof. Adam Helfer for addressing Q's questions/concerns. Here's his reply:

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  14. The God Valued Senior Member


    Thanks Tashja..

    Prof Adam clearly and candidly explains that things around HR are not so clear and thus questionable. Under the circumstances claiming that HR is measured in Analogue experiments etc may not be applicable in BH domain. I am not disputing the analogue experiment but its extension. I fully agree and believe that HR is extremely speculative idea, and present interpretation available from various sources which loosely talks of particles and negative/positive energy is highly simplistic and confusing...

    What surprises me the tinge of exasperation and rudeness in Unruh's response to Q-reeus argument. Q-reeus argument is valid and air of confusion gets over only with the candid response of Prof Adam. There are at least two posters here who claim HR to be a near certainty, but thats not so as clear from Prof Adam's response. Prof Unruh seems to be taking a different stand, kind of pushing HR valiantly.
  15. paddoboy Valued Senior Member

    I believe the forum as a whole is grateful to you also for your efforts tashja.
    I seem to get more "ignores" sometimes the occasional reply.
    Might be a gender/age thingy......ie, answering a pretty young lady rather then replying to some old bastard!

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    [tic mode on of course]
  16. paddoboy Valued Senior Member

    If you are referring to me [which you do fanatically] then that is entirely wrong
    In fact what I have said [if you are referring to me] is

    I believe all our professional replies have supported that concept.
    Sorry, but the need to correct you once again is for the benefit of highlighting the truth as against your misinterpretations, errors and unjust claims.
  17. The God Valued Senior Member

    Dishonesty cannot be cured without punishment. You are expert in changing track and that diameter dance proved that.

    Now let see this changing HR stand of yours...

    You were so passionately in argument (supporting HR) with at least 3 of us, schemelzer, Q-reeus and myself with a brief interlude with OnlyMe. Schmelzer declared you 'Uneducable', Q-reeus declared you 'illiterate cretan' and I have proved you a 'liar' and now your dishonesty will be further proved..

    In the course of discussion, you started playing safe and conveniently started blowing varied temperature air...the present change of stand after prof Adam's reponse is part of your usual dishonest ways without admitting.

    Now see, below post by Schmelzer..

    Correct observation, which gets confirmed by Prof Adam, if you knew anything about HR realistically then you would not have made below comment...now you are changing to align with Schmelzer without acknowledging his original stand on HR. Is it not dishonesty ?

    I know you will come up with some wierd argument to justify your change of stand without admitting it, but thats how you are scripted. I am getting convinced that rationality and reasonableness do come from education also. Lack of education may make a person irrational.
    Last edited: Oct 9, 2015
  18. Q-reeus Banned Valued Senior Member

    Many thanks again tashja for quietly getting down to business and eliciting useful feedback - from three different Profs this time.

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    There are a number of statements made by Prof Unruh that I'm inclined to question if not outright challenge, but see no point in keeping the ball rolling. He is a well recognized professional physicist convinced of the correctness of his model and has sizable support for it, which I guess explains to some extent an exasperated tone in that reply.
    Prof. Polchinski's answer seems to reflect a very formal approach relying in part on the 'weirdness' of interior Schwarzschild coordinates. For me, it's enough to note that owing to sqrt(g_tt) -> 0 at BH EH, any notion of energy -> momentum etc. or any physical 'process' occuring for r < 2M that could have real consequences for exterior r > 2M is just absurd and self-contradictory. But that's me. It was while participating in a thread elsewhere dealing with (attempted) metric matching across a spherical shell, the pathological nature of Schwarzschild metric became evident and soon after I became a fan of 'fringe' Yilmaz gravity. For which there can never be issues of HR or e.g. bizarre 'spacetime swapping'. That's enough to stir righteous indignation in some so enough of that talk.

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  19. Q-reeus Banned Valued Senior Member

    Again, not interested in keeping the ball rolling here, but will state I do have a preference for Prof Helfer's 'warts and all' explanation over that of the other two above. His response does imo acknowledge there are consistency issues, although it's also fair to say he believes that HR is more likely real than not.

    Thanks again tashja.
  20. paddoboy Valued Senior Member


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    The only thing dishonest is you and your posts as most already know, along with your other questionable "god like" qualities.

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    You do realise that people are laughing at you don't you?
    I don't need to say much more, as like I said previously you are totally and seriously unable to accept what a total fool you have made of yourself, due to your arrogant self delusional nature, and of course your peers on this forum will be the judge of who has lied in near every post, and I'll gladly stand by that judgement.
  21. The God Valued Senior Member

    So you agree that HR prevalent concept with virtual particle pair is misleading?
  22. OnlyMe Valued Senior Member

    Tashja, once again....

    I believe that Professor Helfer's response is the better of the three for a lay oriented discussion, because it clearly conveys the theoretical nature of the subject. That does not mean that I believe the earlier two responses are faulty in any way, only that in those responses, I prefer to believe, that the professors assume the theoretical character of the discussion is taken for granted.

    Even when we discuss theory or our favorite speculations, we tend to do so in a way that projects some measure of certainty. In a peer to peer professional discussion that should present little problem. In a lay discussion it can sometimes be misleading. It is that tendency for the way a discussion is phrased that often draws my attention... And one reason why I prefer Professor Helfer's response.

    Once again good work in obtaining these comments and responses.
  23. OnlyMe Valued Senior Member

    Paddoboy has been clear in his position, at least where his own words are involved. It appears to me that you confuse.., or perhaps better interpret his many quoted references, as representing his personal opinion.

    Almost all references presented in these discussions carry with them some bias of the author. As I mentioned, in different words, an earlier post.., as long as all parties understand whether they are discussing the physics of what is known to be real or the physics of theory and/or speculations, debate should be a matter of discussing differing interpretations of the implications....

    Some of your criticism and even mine, might be the result of paddoboy's habit of not putting articles and references in the QUOTE function, which would create a clear separation between what he is quoting and what he is saying hisself... But that would just make it easier for those of us reading to see the change of voice. The fact that he does not use the quote function as often as I would like does not justify misreading his own words.

    As pointed out by the responses tashja, just posted.., in lay discussion reducing the virtual particles of quantum field theory to particles as commonly defined in a lay context, is what is misleading.., and yet almost a necessity in a lay discussion.

    BTW it should be clear that there is no real prevalent or mainstream consensus, when it comes to the exact nature of virtual particles as any kind of real component of the vacuum. They are a useful part of a theoretical model that is often extended beyond any practical description of observable reality.

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