Hawking radiation

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

  1. hardalee Registered Senior Member

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    Hawking radiation is explained by creation of a particle and an antiparticle near the event horizon of a black hole with the positive mass going outside the black hole and the negative pass entering it, thus causing mass loss to the hole and radiation out of it. This requires negative mass which has never seen to exist.

    Recently I read of Hawking radiation described as quantum tunneling from inside the hole out, which would not require any negative mass. Observation of both theories appear to me show the same result

    Any opinions out there concerning what I see as a conflict in the theories?

    Thank you
     
    brucep likes this.
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  3. origin Trump is the best argument against a democracy. Valued Senior Member

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    I will take a stab at this...

    My understanding which is not very great, is that it is not actual mass that is being lost from the black hole but energy (which is equivalent to mass E=mc^2). It is possible for virtual particles that come into existence near the EH and to have one of the particles to 'fall' into the black hole and the other particle to escape. Virtual particle pairs are comprised of negative and positive energy. The particles are entangled so you can not say one specific particle is + energy and the other - energy. However when one of the particles falls into the black hole and the other one escapes, the escaped particle is no longer 'virtual' and is a 'real' particle, and as such is guaranteed to be + energy. Which means that there is a net loss of energy (mass) from the black hole.

    I am way out of my comfort zone on this explanation and hopefully a physicist will come and slap me down or add to what I said to give it some credibility.

    PS. I may have lost the mainsail sheet on this one!

    Please Register or Log in to view the hidden image!

     
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  5. paddoboy Valued Senior Member

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    Sorry, I aint a physicist, but what you have posted seems pretty well spot on and the way that I understand it.
    Your m'sail is set well.
     
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  7. Daecon Kiwi fruit Valued Senior Member

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    From what I gather from Wikipedia, spin-0 particles can apparently quantum tunnel faster than light, which may allow them to escape the event horizon of a black hole and escape as another form of Hawking radiation.

    I'm not too certain which particles have a spin of 0, though. Other than the Higgs boson, all other spin-0 particles seem theoretical. Hopefully someone more knowledgeable can clarify the situation.
     
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  8. KilljoyKlown Whatever Valued Senior Member

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    I'm not sure I am following your logic when you say "the escaped particle is no longer 'virtual' and is a 'real' particle, and as such is guaranteed to be + energy." Why can't the real particle be - energy, which would make it an anti-matter particle? If that were to happen on average of 50% of the time we would have no net lose from the black hole.
     
  9. Daecon Kiwi fruit Valued Senior Member

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    There's a difference between an antimatter particle such as a positron (both electrons and positrons have positive energy in accordance with E=mcc) and an antiparticle, which is literally an anti-particle.
     
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  10. KilljoyKlown Whatever Valued Senior Member

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    That didn't help much. My understanding regardless of the particle type is that when anti-particle and normal particle meet they annihilate each other. However what exactly is a "virtual particle"? Where does it come from? What is the theory behind it? If one of a pair of particles escapes from the other, what kind of real particle is it? If in fact this is what happens, why would the anti-particle of the pair be the one to trapped by the black hole in most all the cases?
     
  11. Daecon Kiwi fruit Valued Senior Member

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    I'm not qualified to answer those questions, but hopefully someone who knows more about it than I do, can explain these things.

    From my limited understanding, the virtual particle/antiparticle pairs come into existence and annihilate each other so fast, that the Universe barely has time to register their presence. They take advantage to quantum effects to "cheat" the system before the energy required to manifest these particles is allocated from, and returned to, the vacuum.

    To put it another way - They borrow and then return the energy to exist before the Universe notices the energy has gone.

    When one of the pair falls into a black hole, that pair is no longer able to annihilate and so the Universe tallies up the equations and notices that there's now a particle's worth of energy that has apparently spontaneously come into existence. Well, the first rule is that energy can't be created or destroyed, and so that new quanta of energy HAS to come from somewhere. It can't come out of the vacuum itself, so the Universe subtracts that amount of energy from wherever the other one of that particle/antiparticle pair happens to be - in this case, inside a black hole. And thus, the black hole pays off that debt by losing that same amount of energy.

    Does that help, or did I just make things more confusing?
     
    Last edited: Sep 18, 2015
  12. KilljoyKlown Whatever Valued Senior Member

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    But in order for that to be true, virtual particles would actually have to be the fabric of space. I believe the theory is every point in space is composed of virtual particles popping into existence then annihilating each other so fast that they are undetectable. Sounds like a pretty far fetched theory to me. Those virtual particles would have to be coming from some where. The theory does not cover that point very well.
     
  13. brucep Valued Senior Member

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    They're virtual because they're undetectable. If they were detectable we'd call them real particles. With respect to outside the event horizon, commonly referred to as Hawking radiation, a virtual pair can be physically separated by the tidal accelerations, delta g, in the local gravitational field. If the separation grows to a distance ~ 1/4 the circumference of the event horizon they gain enough energy from the gravitational field to become detectable and possibly escape to infinity. Per Kip Thorne.
     
  14. paddoboy Valued Senior Member

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    The Universe it appears, is a weird and wonderful place.
    I agree, it sounds far fetched on face value, but isn't this the case with most of quantum mechanics?
    I suppose also a couple of centuries ago, the non absolute nature of space and time would have also seemed far fetched.
    I've read two books on string theory and it's derivitives, and many scientists describe it as beautifully mathematically represented.
    But talking of 6, 10 or more dimensions is still hard to fathom.
    And despite a hell of a lot of anti string propaganda that's going round at the moment, the problem really is one of technology or lack thereof, and our inability to observe at such levels.
    Thinking about this stuff makes my head ache...
    Irrespective of my head ache but, we do have evidence that it certainly takes place...The Casimir effect.

    The particle pair creation scenario is not violating any conservation law, because they are only virtual, existing for less than the Planck time.
    Occurring at such levels, and within the limit of the uncertainty principal, means that no law of matter/energy of which we are more familiar is violated.
    Far fetched? hard to understand? agreed.....but the Casimir effect supports it.
     
  15. hardalee Registered Senior Member

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    Thank you all for your posts. Some were quite informative.

    But would anyone care to address the question I tried to ask, which concept is "correct"', delivering negative mass to the blank hole or quantum tunneling out of positive mass? Both would appear to me to "look" the same but are very different concepts.
     
  16. paddoboy Valued Senior Member

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    http://newt.phys.unsw.edu.au/~jkw/phys3550/Hawking_radiation/How_does_Hawking_radiation_work.pdf

    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.
     
  17. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    Where does the kinetic energy come from for the positive-energy particle (whether particle or anti-particle) to escape to infinity from a black-hole event horizon? Why doesn't it instead also fall into the event horizon?
     
  18. Q-reeus Valued Senior Member

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    For those of us who are not versed in QFT but do exercise the faculty of critical thinking, that ostensibly standard account has some evident issues.

    Maybe QFT can actually make sense of the notion that negative frequencies = negative energies, but it's use elsewhere has no such exotic inference - e.g.:
    http://www.bitweenie.com/listings/negative-frequency/
    Nothing more or less than an arbitrary coordinate dependent designation of forward vs reverse propagation direction. Actual energies always positive. Hence E = h|f| is what's physically significant, regardless of any mathematical convention where f < 0 is used. A similar thing could be said for the convention of 'complex frequencies'.

    Brings me to another troubling aspect of that piece - always referencing HR to particle/anti-particle pairs. The anti-particle always having negative energy somehow. Yet an assumed initially stellar mass BH would have an exceedingly low effective T for the vast bulk of it's slowly shrinking existence. The assumed radiation would be EM, not e.g. electron-positron and higher energy particles.
    It's well known a photon is its own anti-particle. Hence both necessarily having positive E = hf energy. By what seeming magic then does an intrinsically positive energy 'anti-particle' photon somehow 'go negative' once it crosses the dreaded EH? Oh yeah, that's right - 'space becomes time, and time becomes space' there. Really? Well gee, if that's truly the case in a physically sensible way, one has to ask why nobody suggests that dropping say a positive energy stone into a BH has that mass magically turning negative and reducing the overall exterior M value of BH. Something special about photons then?

    One could go on and e.g. question the logic behind assertion that somehow only the 'negative energy partner' is selected for swallowing. The above though is probably enough to stimulate possible doubts in those not given to unquestioning devotion to orthodoxy. Maybe a QFT expert can now step in and graciously alleviate my doubtlessly unwarranted scepticism.
     
  19. paddoboy Valued Senior Member

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    Perhaps if you read all post 13 and the links within you may get your answers.
    Although I dare say those that automatically question and deride others falsely, because they align with mainstream, and/or the majority accepted view, ought to first perhaps examine the possibility of hidden agendas that drives there own scepticism.
    And of course if that which is generally accepted by mainstream, does have problems, then one would think that one would/should raise these problems for their peers to evaluate.
     
  20. Q-reeus Valued Senior Member

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    I need feedback from knowledgeable folks versed in the topic, not vacuous drivel with a witch hunting agenda attached. If you qualify - by all means provide the relevant and technically useful info.
     
  21. The God Valued Senior Member

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    For a starter you should refer to a thread on A&AC section, on the subject


    My quote in that thread has the similar objection, but could not find any answer anywhere in the literature...In fact I am of the contrary opinion about HR, read below, it may evaporate Universe....Disclaimer:Just an observation


    http://sciforums.com/threads/is-hawking-any-closer-to-solving-the-puzzle-of-black-holes.152514/
     
  22. hardalee Registered Senior Member

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    Thank you all. This is getting better, but where might I find a particle with negative energy?
     
  23. Q-reeus Valued Senior Member

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    Standard HR narrative has that negative energy particles are always swallowed by BH hence never observed. Only inferred indirectly as a supposed BH mass loss. However, as a supplement to my last point in #15:
    , it may be more pointedly asked why on earth the selection process would not in fact exclusively select the supposed -ve energy particles for expulsion.
    Since -ve energy surely logically equates to -ve gravitating mass! Hence, one might reasonably conclude, impossible for a -ve energy particle to ever reach EH let alone sink without trace.

    Anyone else think there may be more than a whiff of insanity with the standard narrative? I do NOT apologize if this makes devoted followers of consensus position hurt in their heads.
     

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