Notices
Results 1 to 30 of 30

Thread: Hawking Radiation

  1. #1 Hawking Radiation 
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    It is radiation that gets emitted from a black hole. But I don't understand why this happens, as a black holes gravity is to strong to let light escape.


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Isotope Zelos's Avatar
    Join Date
    Jun 2006
    Posts
    2,755
    particles are constantly created from nothing, yes nothing the law of energy conservation is broken. and then destroyed. but close to the event horizont one of the particles fall into the blackhole and the other one escapes, wich is the hawking radiation.

    these creations are allways in pairs of 2 particles


    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

    On the eighth day Zelos said: 'Let there be darkness,' and the light was never again seen.

    The king of posting
    Reply With Quote  
     

  4. #3  
    Guest
    I think you will find it's the 'negative mass' of the particle-anti-particle pair that falls in, results in an internal particle being anihilated that is the cause of any reduction. Well that was my understanding from a brief read.

    I don't personally give too much weight to this theory though.
    Reply With Quote  
     

  5. #4  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    That was my understanding too but why do particle pairs just appear at the event horizon, considering they don't just appear anywhere else? Also, why is it that the antiparticle always gets pulled in by the black hole's gravity, over the regular particle?
    Reply With Quote  
     

  6. #5  
    Guest
    I'm out of my depth here, but I think these particles are supposed to appear everywhere but immediately anihilate each other when not near an event horizon.

    I would have thought mathematically either particle might be 'sucked in' and therefore is should cancel, what it is that makes that vital difference, I have either forgotten or not read of.

    Have you tried googling "Hawking Radiation Stanford" ?
    Reply With Quote  
     

  7. #6  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    hmm... Annihalating create 2 photons, correct? So If they are popping into existance all over the place would we not just see beems of energy all over the place as we carry on every day life? Also, law of conservation of mass and energy was stated, how does that law try to justify this happening? I have googled hawking radiation but not with stanford, I'll go try that now.

    *EDIT*
    So, I looked up hawking radiation stanford and one of the first results was this site: http://news-service.stanford.edu/pr/...ckholes67.html And well I found an answer. Here's a paragraph:

    Under normal circumstances, a vacuum is a space in which there is no matter. But at the quantum level, the vacuum is full of particles and antiparticles that constantly appear and disappear. The Heisenberg uncertainty principle allows these "virtual" particles and antiparticles to emerge from the vacuum for a brief moment and disappear back into the vacuum again without violating the energy conservation law. According to Hawking, if a particle/antiparticle pair is created near the event horizon of a black hole, gravity will pull one of the particles into the hole permanently, while the other particle (or antiparticle) can escape, or be "radiated," from the black hole. "In this way the black hole could radiate something from nothing,


    I just don't quite get how that doesn't violate the energy conservation law. :?

    There were also some other interesting things in that article. They said they were going to try to simulate an event horizon this decade by shooting intense lasers at each other.
    Reply With Quote  
     

  8. #7  
    Forum Isotope Zelos's Avatar
    Join Date
    Jun 2006
    Posts
    2,755
    I just don't quite get how that doesn't violate the energy conservation law.
    it do violate the conservation law, temporary
    dT * dE < h-bar / 2

    aslongest the time/energy times each other is less than that its allowed to happen, in normal scale and time we dont notice it since its so small
    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

    On the eighth day Zelos said: 'Let there be darkness,' and the light was never again seen.

    The king of posting
    Reply With Quote  
     

  9. #8  
    New Member
    Join Date
    Mar 2007
    Posts
    3
    From what I understand, one of the particles will have a negative mass, so the sum of the masses is 0 (x + -x = 0), so nothing is really created (hence "virtual" particles). However, when these virtual particles are created near the event horizon of a black hole, there is a probability (which Hawking calculated) that the negative mass would be sucked in, and the positive mass particle would escape, becoming a "real" particle, thus the mass inside the black hole would decrease (X=X-x, if X is the mass of the black hole) and the mass outside the black hole would increase (X=X+x, if X is the mass of everything not the black hole). This would appear to an outside observer as the black hole is losing mass and emitting radiation.
    Reply With Quote  
     

  10. #9  
    Guest
    What I don't understand is why is there a need to suggest that black holes ever leak? and how the hell can anyone say what happens inside a black hole when we are told ALL the laws of our universe breakdown at the event horizon.
    Reply With Quote  
     

  11. #10  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    Personally, I don't think they do break down. I think we just don't have adequite knowledge yet.

    But still, as I said before, if annihalations like that are always occuring then we don't we see them in our everyday life. I remember reading that when annihalations take place between matter and antimatter 2 photons get emitted. In which case law of conservation of energy would be broken because if this were to happen everywhere, then we would be bombarded with energy. This leads me to think it isn't matter and antimatter, but some other form of... of... particle?

    Also to me it would seem more likely that the regular particle would be pulled in, as gravity would affect it normaly, where as I would think gravity would have a negative effect on the antiparticle. In this case the positive particle would increase the mass of the black hole and the negative particle would be emitted as radiation. The opposite of Hawking's theory.

    Does anyone know what they call this phenomenon of these "particles" appearing and disappearing quickly?

    Does anyone know if Hawking radiation has been observed?

    There is no need for energy to leak but hawkings theory says that it does.
    Reply With Quote  
     

  12. #11  
    Forum Professor river_rat's Avatar
    Join Date
    Jun 2006
    Location
    South Africa
    Posts
    1,517
    Quote Originally Posted by Megabrain
    What I don't understand is why is there a need to suggest that black holes ever leak? and how the hell can anyone say what happens inside a black hole when we are told ALL the laws of our universe breakdown at the event horizon.
    Mega, the laws of physics do not break down at the event horizon - only at the singularity of a black hole (if there even is one). For a super massive black hole you could cross over the event horizon and noting untoward would happen.

    There is no need for black holes to leak, as there is no need for things to fall towards the centre of the earth - but the universe does seem strongly inclined for things to run this way.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
    Reply With Quote  
     

  13. #12  
    Forum Isotope Zelos's Avatar
    Join Date
    Jun 2006
    Posts
    2,755
    Quote Originally Posted by Megabrain
    What I don't understand is why is there a need to suggest that black holes ever leak? and how the hell can anyone say what happens inside a black hole when we are told ALL the laws of our universe breakdown at the event horizon.
    cause the blackhole gots a temperature therefor it has to leak energy as all objects, but this is more about outside the blackhole not inside.
    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

    On the eighth day Zelos said: 'Let there be darkness,' and the light was never again seen.

    The king of posting
    Reply With Quote  
     

  14. #13  
    Guest
    Quote Originally Posted by Zelos
    cause the blackhole gots a temperature therefor it has to leak energy
    Why? if 'light' photons can't escape then why should lower energy (IR) photons escape?

    And you don't know that a black hole has a temperature, if it's all packed that dense nothing has room to move, it's a black hole, no radiation escapes, maybe it's at a true 0 Kelvin, and that makes it a singularity, or maybe a million other crank ideas. You blindly support Hawking radiation as gospel, wheras the rest of the world says 'maybe, maybe not'.

    At the end of the day, a trememdous amout of 'heat' energy may be 'sucked' into a black hole BUT who really knows what happens to it, stuff the conservation of energy, all the laws break down in a singularity I'm (now) told. So for all we know this 'heat energy' when packed into a singularity might even convert back into matter. There's even a formula to suggest this, M=E/c^2 !
    Reply With Quote  
     

  15. #14  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    i don't think anyone here is saying it's fact. To me it's a theory, that is all. I'm just trying to understand it.
    Reply With Quote  
     

  16. #15  
    Guest
    Are you happy with the explanation so far?
    Reply With Quote  
     

  17. #16  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    I actually am really happy with how this has been going, I've gotten a lot of good information from it!
    But no one has said anything about this:
    Quote Originally Posted by shawngoldw
    Personally, I don't think they do break down. I think we just don't have adequite knowledge yet.

    But still, as I said before, if annihalations like that are always occuring then we don't we see them in our everyday life. I remember reading that when annihalations take place between matter and antimatter 2 photons get emitted. In which case law of conservation of energy would be broken because if this were to happen everywhere, then we would be bombarded with energy. This leads me to think it isn't matter and antimatter, but some other form of... of... particle?

    Also to me it would seem more likely that the regular particle would be pulled in, as gravity would affect it normaly, where as I would think gravity would have a negative effect on the antiparticle. In this case the positive particle would increase the mass of the black hole and the negative particle would be emitted as radiation. The opposite of Hawking's theory.

    Does anyone know what they call this phenomenon of these "particles" appearing and disappearing quickly?

    Does anyone know if Hawking radiation has been observed?

    There is no need for energy to leak but hawkings theory says that it does.
    Reply With Quote  
     

  18. #17  
    Guest
    If you find the answers - let me know - I brieflly read the theory some years ago but it's not something that interests me to the extent of investigating further.
    Reply With Quote  
     

  19. #18  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    will do!
    Reply With Quote  
     

  20. #19  
    Forum Professor wallaby's Avatar
    Join Date
    Jul 2005
    Location
    Australia
    Posts
    1,521
    Quote Originally Posted by shawngoldw
    I actually am really happy with how this has been going, I've gotten a lot of good information from it!
    But no one has said anything about this:
    i have one thing to say about this:
    Quote Originally Posted by shawngoldw
    Also to me it would seem more likely that the regular particle would be pulled in, as gravity would affect it normaly, where as I would think gravity would have a negative effect on the antiparticle. In this case the positive particle would increase the mass of the black hole and the negative particle would be emitted as radiation. The opposite of Hawking's theory.
    the nature of the anti-particle is not that of negative mass, in fact it has the same mass as the particle that it is "Anti" to, a particle and an anti-particle will be identical except that it will have an opposite charge to the 'regular' particle. if both particles have the same mass then the gravitational force between the black hole and each of the particles should be the same.

    however perhaps the opposite charges would have something to do with one of the particles flying away, its not something i could calculate so i'll say no more.
    Reply With Quote  
     

  21. #20  
    Forum Professor river_rat's Avatar
    Join Date
    Jun 2006
    Location
    South Africa
    Posts
    1,517
    When in doubt, ask the Physics FAQ

    Hawking Radiation

    In 1975 Hawking published a shocking result: if one takes quantum theory into account, it seems that black holes are not quite black! Instead, they should glow slightly with "Hawking radiation", consisting of photons, neutrinos, and to a lesser extent all sorts of massive particles. This has never been observed, since the only black holes we have evidence for are those with lots of hot gas falling into them, whose radiation would completely swamp this tiny effect. Indeed, if the mass of a black hole is M solar masses, Hawking predicted it should glow like a blackbody of temperature


    (6 x 10-8/M) kelvin,

    so only for very small black holes would this radiation be significant. Still, the effect is theoretically very interesting, and folks working on understanding how quantum theory and gravity fit together have spent a lot of energy trying to understand it and its consequences. The most drastic consequence is that a black hole, left alone and unfed, should radiate away its mass, slowly at first but then faster and faster as it shrinks, finally dying in a blaze of glory like a hydrogen bomb. However, the total lifetime of a black hole of M solar masses works out to be


    1071 M3 seconds

    so don't wait around for a big one to give up the ghost. (People have looked for the death of small ones that could have formed in the big bang, but they haven't seen any.)

    How does this work? Well, you'll find Hawking radiation explained this way in a lot of "pop-science" treatments:

    Virtual particle pairs are constantly being created near the horizon of the black hole, as they are everywhere. Normally, they are created as a particle-antiparticle pair and they quickly annihilate each other. But near the horizon of a black hole, it's possible for one to fall in before the annihilation can happen, in which case the other one escapes as Hawking radiation.

    In fact this argument also does not correspond in any clear way to the actual computation. Or at least I've never seen how the standard computation can be transmuted into one involving virtual particles sneaking over the horizon, and in the last talk I was at on this it was emphasized that nobody has ever worked out a "local" description of Hawking radiation in terms of stuff like this happening at the horizon. I'd gladly be corrected by any experts out there... Note: I wouldn't be surprised if this heuristic picture turned out to be accurate, but I don't see how you get that picture from the usual computation.

    The usual computation involves Bogoliubov transformations. The idea is that when you quantize (say) the electromagnetic field you take solutions of the classical equations (Maxwell's equations) and write them as a linear combination of positive-frequency and negative-frequency parts. Roughly speaking, one gives you particles and the other gives you antiparticles. More subtly, this splitting is implicit in the very definition of the vacuum of the quantum version of the theory! In other words, if you do the splitting one way, and I do the splitting another way, our notion of which state is the vacuum may disagree!

    This should not be utterly shocking, just pretty darn shocking. The vacuum, after all, can be thought of as the state of least energy. If we are using really different co-ordinate systems, we'll have really different notions of time, hence really different notions of energy - since energy is defined in quantum theory to be the operator H such that time evolution is given by exp(-itH). So on the one hand, it's quite conceivable that we'll have different notions of positive and negative frequency solutions in classical field theory -- a solution that's a linear combination of those with time dependence exp(-i omega t) is called positive or negative frequency depending on the sign of omega -- but of course this depends on a choice of time co-ordinate t. And on the other hand, it's quite conceivable that we'll have different notions of the lowest-energy state.

    Now when we are in good old flat Minkowski spacetime, a la special relativity, there are a bunch of "inertial frames" differing by Lorentz transformations. These give different time co-ordinates, but one can check that the difference is never so bad that different co-ordinates give different notions of positive or negative frequency solutions of Maxwell's equations. Nor will different people using these co-ordinate systems ever disagree about what's the lowest-energy state. So all inertial observers agree about what's a particle, what's an antiparticle, and what's the vacuum.

    But in curved spacetime there aren't these "best" co-ordinate systems, the inertial ones. So even very reasonable different choices of co-ordinates can give disagreements about particles vs antiparticles, or what's the vacuum. These disagreements don't mean that "everything is relative", because there are nice formulas for how to translate between the descriptions in different co-ordinate systems. These are Bogoliubov transformations.

    So if there is a black hole around...

    on the one hand we can split solutions of Maxwell's equations into positive frequency in the most blitheringly obvious way that someone far from the black hole and far in the future would do it...

    and on the other hand we can split solutions of Maxwell's equations into positive frequency in the most blitheringly obvious way that someone far in the past, before the collapse into a black hole has happened would do it.

    That'd be the heuristic explanation I'd give that most closely corresponds to the usual computation. There are additional things to say about the fact that the guy far in the future and far away from the black hole can't see what's in the hole, so he has incomplete information about the state, so he sees a state with entropy, in fact a thermal state. (Here I'm assuming the black hole was NOT eternal, so the guy way back in the past didn't have the black hole to contend with. Apparently Hawking's original computation dealt with this case, but people subsequently watered down his explanation by assuming the black hole was there eternally, to simplify the math. This is what the guy at the talk said... I'd only seen the watered-down version!)

    Now in fact when you do a Bogoliubov transformation to the vacuum you get a state in which there are pairs of particles and antiparticles, so this is possibly the link between the math and the heuristic explanation. Hopefully whoever made up the usual heuristic explanation understood the link better than I do!
    References

    * Robert M. Wald, General Relativity, Sections 14.2-14.4, University of Chicago Press, Chicago, 1984. (A good precise introduction to the subject.)
    * Stephen W. Hawking, Particle creation by black holes, Commun. Math. Phys. 43 (1975), 199-220. (The original paper.)
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
    Reply With Quote  
     

  22. #21  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    but part of the theory says that when the anti gets pulled in it causes the black hole to lose mass and the emitted regular particle makes it seem that mass has been energy has been emitted. Therefore they can't have equal mass.
    Reply With Quote  
     

  23. #22  
    Forum Professor river_rat's Avatar
    Join Date
    Jun 2006
    Location
    South Africa
    Posts
    1,517
    Quote Originally Posted by shawngoldw
    but part of the theory says that when the anti gets pulled in it causes the black hole to lose mass and the emitted regular particle makes it seem that mass has been energy has been emitted. Therefore they can't have equal mass.
    Um, you didnt read the FAQ did you?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
    Reply With Quote  
     

  24. #23  
    Guest
    Thanks RR - Yeah exactly as I said, word for word - Now I remember why I didn't take it any further
    Reply With Quote  
     

  25. #24  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    actually... I have read the forum guidelines...
    Reply With Quote  
     

  26. #25  
    Guest
    Quote Originally Posted by shawngoldw
    actually... I have read the forum guidelines...
    Don't tell me, there's nothing about Hawking radiation in there...
    Reply With Quote  
     

  27. #26  
    Forum Junior
    Join Date
    Jun 2006
    Posts
    264
    The FAQ I read talks about phpbb problems not hawking radiation...
    Reply With Quote  
     

  28. #27  
    Forum Professor wallaby's Avatar
    Join Date
    Jul 2005
    Location
    Australia
    Posts
    1,521
    maybe the hawking radiation section evaporated, in which case you'll find it several posts above mine.
    Reply With Quote  
     

  29. #28  
    Guest
    Damn! one of my post's has gone....
    Reply With Quote  
     

  30. #29  
    Forum Isotope Zelos's Avatar
    Join Date
    Jun 2006
    Posts
    2,755
    Why? if 'light' photons can't escape then why should lower energy (IR) photons escape?
    cause of what ive already told you
    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

    On the eighth day Zelos said: 'Let there be darkness,' and the light was never again seen.

    The king of posting
    Reply With Quote  
     

  31. #30  
    Guest
    Quote Originally Posted by zelos
    cause the blackhole gots a temperature therefor it has to leak energy as all objects,
    Right I see, Zelos's inverse law of radiation, low energy escapes, high energy does not have a low enough energy to escape - makes perfect sense.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •