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Thread: Hawking Radiation Constituents Questions (originally posted in general forum)

  1. #1 Hawking Radiation Constituents Questions (originally posted in general forum) 
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    Hi, I'm hoping to get some help on understanding the type of Hawking radiation emissions that would be expected from decaying micro-black holes such as they hope to see in the LHC. Also interested in the types of particles and radiation that can be absorbed by a black hole (before it decays).

    As I understand Hawking radiation it is due to the creation of particle and anti-particle pairs from QM at the event horizon of the black hole. One part of the pair ends up in the black hole but its partner escapes carrying away energy/mass thus causing the decay of the black hole. So ordinary matter and anti-matter particle pairs such as an electron and anti-electron must make up some of the decay radiation. Photon pairs (photons being their own anti-particle) will make up I think most of Hawking radiation. I would also think its possible to have the other bosons like gluons and weak force bosons (also being their own anti-particles) make up some of the Hawking radiation?

    Now all these constituents listed so far are from particles with multiple charges (for example quarks with 3 possible color charges and quarks also having fractional positive or negative electric charge) or force particles that carry multiple types of charge (whether gluons or photons etc).

    But what about gravitons? As a spin 2 particle the graviton is different from the other bosons. Gravitons carry charge too like the other bosons but in the case of gravitons you can say they carry 'mass' charge. But my sense is that the physics community does not generally support the idea of negative mass charge. So if gravity charge only comes in one polarity then I'd guess you wouldn't have graviton and anti-graviton particle pairs. (particle pairs seem to be associated with multiple polarities whether fermions or bosons). Which means you couldn't have gravitons emitted as part of Hawking radiation?

    So if a graviton was absorbed by a black hole then during the decay process you would never see a graviton emitted. In other words a graviton swallowed by a black hole would disappear but during the decay process the energy associated with that graviton would be emitted in some other form such as photons or matter or anti-matter particles.

    I'm not sure anyone has considered the possibility that gravitons could be absorbed by a black hole either. (I assume your everyday garden variety graviton is virtual)

    Sorry this was such a long post but I'd be very curious what people think. Thanks! Bob


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Robert Courtney View Post
    As I understand Hawking radiation it is due to the creation of particle and anti-particle pairs from QM at the event horizon of the black hole.
    That is only an approximate(*) description of what happens. It doesn't help understand the nature of the radiation.

    Hawking radiation is just electromagnetic radiation with a black body spectrum with a "temperature" which is inversely proportional to the mass of the black hole.

    (*) Approximate to the point, arguably, of being wrong.


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    Hi, I'm hoping to get some help on understanding the type of Hawking radiation emissions that would be expected from decaying micro-black holes such as they hope to see in the LHC. Also interested in the types of particles and radiation that can be absorbed by a black hole (before it decays).
    Hawking radiation, by its very definition, is a type of thermal radiation - its is therefore almost exclusively electromagnetic in nature. It is inversely related to the surface area of the black hole's event horizon ( which in turn is directly related to the total mass ), meaning that the smaller the black hole, the more Hawking radiation it emits, and the shorter its lifespan. A decaying micro black-hole as in your original question would appear as a very hot black body, emitting ( comparatively ) large amounts of thermal radiation.
    On the other hand, even though it is microscopic, it would still behave just like any other black hole, meaning any infalling matter or radiation would be irrevocable absorbed by it once it crosses the event horizon.

    But what about gravitons? As a spin 2 particle the graviton is different from the other bosons. Gravitons carry charge too like the other bosons but in the case of gravitons you can say they carry 'mass' charge. But my sense is that the physics community does not generally support the idea of negative mass charge. So if gravity charge only comes in one polarity then I'd guess you wouldn't have graviton and anti-graviton particle pairs. (particle pairs seem to be associated with multiple polarities whether fermions or bosons). Which means you couldn't have gravitons emitted as part of Hawking radiation?
    Ok, I see your general idea. The problem here is that at this point in time the existence of such a particle as the Graviton is mere speculation as there is no experimental evidence that it actually exists. If it did actually exist it would be a massless, uncharged spin 2 vector boson; just like is the case with photons, it would also be its own anti-particle, so a "Anti-Graviton" wouldn't exist.

    I'm not sure anyone has considered the possibility that gravitons could be absorbed by a black hole either. (I assume your everyday garden variety graviton is virtual)
    I am not sure what you mean by this.
    Last edited by Markus Hanke; January 22nd, 2012 at 03:40 AM.
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    Thanks Markus! Yup, I should have said that the graviton (if it exists) is its own anti-particle. My thinking is that gravitons may only carry one polarity of charge, i.e. positive mass charge, so maybe an 'anti-particle' (altho it is its own anti-particle) state wouldn't exist for a graviton...even being its own anti-particle. The idea is that the concept of anti-particles is associated with particles (matter or force) that have or carry multiple polarities. If the graviton has only one polarity that it carries then there may be no way for a graviton to have an 'anti-particle' state.

    Black holes presumably absorb everything. I think they can emit charged matter particles or spin-1 force particles (via QM) but most of their emissions I believe (as you mentioned) are in the EM spectrum. So I thought if its not possible to have a a graviton 'antiparticle' because the graviton carries only one polarity (and the creation of anti-particles [even if the particle is its own anti-particle like a spin-1 photon] is associated with multiple polarities) then the creation of QM graviton pairs wouldn't occur. In that case if a graviton is absorbed by a black hole then you will never see a graviton emitted by a black hole. Conservation of energy will require that something is emitted by the black hole during its decay to account for the gravitons energy. So the absorbed gravitons energy in the black hole will eventually be emitted, but in some other form, probably EM radiation.

    But of course there is no proof gravitons even exist so far. Though I did see something about 5-D Kaluza-Klein gravitons potentially being created by the LHC.
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    To be clear, a gravitons carry information about changes in gravity, not static unchanging gravitational fields. So if gravitons can't escape a black hole, that wouldn't affect their ability to influence matter around them, would it?

    Or is there something I'm missing here?
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by kojax View Post
    To be clear, a gravitons carry information about changes in gravity, not static unchanging gravitational fields. So if gravitons can't escape a black hole, that wouldn't affect their ability to influence matter around them, would it?

    Or is there something I'm missing here?
    No, I think so far as I understand it you are pretty much spot on.
    To be fair I have never really thought about this, I always picture gravity as spacetime curvature, thereby avoiding all the pitfalls of particle interactions. Seems much easier
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    Quote Originally Posted by Robert Courtney View Post
    Thanks Markus! Yup, I should have said that the graviton (if it exists) is its own anti-particle. My thinking is that gravitons may only carry one polarity of charge, i.e. positive mass charge, so maybe an 'anti-particle' (altho it is its own anti-particle) state wouldn't exist for a graviton...even being its own anti-particle. The idea is that the concept of anti-particles is associated with particles (matter or force) that have or carry multiple polarities. If the graviton has only one polarity that it carries then there may be no way for a graviton to have an 'anti-particle' state.
    Gravitons, if they exist, wouldn't carry any charge; in fact their only defining characteristics ( to the best of my knowledge ) would be their vanishing rest mass and their spin 2. Therefore there is no anti-particle to the graviton.

    Though I did see something about 5-D Kaluza-Klein gravitons potentially being created by the LHC.
    I haven't heard about this. Actually, Kaluza-Klein-Theory is a geometric attempt to unify electromagnetism and gravitation by introducing a compactified 5th dimension; Einsteins field equations can be directly derived from this. KK theory does not make any predictions as to the existence of gravitons.
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    Hi Markus and Kojax, Thanks for your input! On the 5-D KK gravitons they are predicted by some of the theories derived from M-theory. I was aware that the original Kaluza-Klein idea dates back to the early part of the twentieth century when Theodor Kaluza attempted to unify EM with GR. But if you check pages 326, 327 etc in Lisa Randall's new book "Knocking on Heaven's Door" you'll see part of her discussion on 5-D KK graviton modes. She also talks about KK gravitons in her prior book "Warped Passages". Dr. Randall is a professor of physics at Harvard.
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    When black holes where discovered to radiate why didn,t they immediately adapt the name to grey holes ? The term black hole seems misleading then.
    As grey comes in all shades there probably are darker grey holes and lighter grey holes as well ; Just as for any black (paint) we see there is always something more black. Or the darkest night is not absolutely dark.

    This brings a wondering question ; what would a more light-grey hole, substance or - objekt (of any size) look like ?
    The most obvious maybe ; As a light grey substance ? Seeing white is explained as seeing all colours reflected. Seeing something black is explained as seeing nothing reflected. So grey is seeing nothing (black) and something (all colours) same time ? So why is grey not just a pale white then ? Because in essence "seeing something black" is explained by fysics as seeing nothing (a dark hole also). So maybe this idea is not that real as assumed.
    That,s the idea I have from Hawkins and penrose work. Their work proofs fysics (before them) was wrong on a much more basic level. Keeping the name black hole hides this ; therefor it,s a misleading term.
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    Quote Originally Posted by Ghrasp View Post
    When black holes where discovered to radiate why didn,t they immediately adapt the name to grey holes ?
    Well, strictly speaking they haven't been discovered to radiate. It is a theoretical prediction by Hawking. It is not universally accepted as correct (he had to invent his own approximation to quantum gravity).

    The term black hole seems misleading then.
    It is just a name.

    As grey comes in all shades there probably are darker grey holes and lighter grey holes as well
    All realistically sized black holes radiate minute amounts of radiation with a black body temperature of a faction of a degree above absolute zero. So they are pretty black. Empty space is warmer!

    This brings a wondering question ; what would a more light-grey hole, substance or - objekt (of any size) look like ?
    Well, it would look black. But it would be highly visible for two reasons. Black holes are usually surrounded by gas and dust which is falling in (an accretion disk). as this stuff falls in it gets heated up and radiates. Also, the intense gravity around the black hole distorts the path of light from stars behind (gravitational lensing).

    What Does a Black Hole Look Like
    http://apod.nasa.gov/htmltest/gifcity/rsgrow.mpg
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  12. #11  
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    Quote Originally Posted by Robert Courtney View Post
    Hi Markus and Kojax, Thanks for your input! On the 5-D KK gravitons they are predicted by some of the theories derived from M-theory. I was aware that the original Kaluza-Klein idea dates back to the early part of the twentieth century when Theodor Kaluza attempted to unify EM with GR. But if you check pages 326, 327 etc in Lisa Randall's new book "Knocking on Heaven's Door" you'll see part of her discussion on 5-D KK graviton modes. She also talks about KK gravitons in her prior book "Warped Passages". Dr. Randall is a professor of physics at Harvard.
    Ok, I must admit I am not familiar with Prof Randall's work, nor am I an expert in the intricacies of KK theory, so I might well be wrong here. At the outset though KK theory is of a geometrical nature, so I'm not quite sure where the graviton comes in. My guess would be that Prof Randall probably examined how the standard graviton would behave in 5-dimensional space-time as described by KK theory.
    You have sparked my interest here; I shall read up some more about KK theory when I get a chance, never really got into this over and above some general texts.
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    Thanks Markus, Strange, Ghrasp and Kojax for your replies. I've been researching a little and found a discussion on Hawking radiation in "arXiv:hep-th/0512002v3 28 Apr 2006". The title is: "Black hole particle emission in higher-dimensional spacetime". The Randall-Sundrum model is included in the bibliography along with other higher dimensional models derived from M-theory. Anyway on the last page there is a table with the authors estimate of the types of emissions from a black (grey) hole. It looks like on the 4-D brane the emissions will consist of scalars (Higgs..if they exist), fermions and gauge bosons. But any graviton emissions will end up in the 5-D bulk and in the form of 5-D bulk gravitons (KK gravitons). As far as absorption of 4-D gravitons from the 4-D brane into the black hole I wasn't quite able to interpret their discussion on the 2nd and 3rd pages. It looks like ordinary 4-D gravitons are absorbed into the black hole but not emitted back onto the 4-D brane. But I need some help here from someone familiar with the string/brane theory math here. Thanks! Bob
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    But I need some help here from someone familiar with the string/brane theory math here. Thanks! Bob
    Sorry Bob, I read the article, and, while I understand the principles involved, the maths are unfortunately beyond me - String/Brane theory isn't really my area of expertise ( yet ).
    Anyone else able to help ??
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    Thanks Markus for checking that article out. (Glad you're interested in checking out string/brane math too). I found another paper that talks about black holes and graviton emission/absorption. This one is explicit in describing the absorption of (4-D) gravitons by black holes. In the introduction the first sentence states "Following the Regge-Wheeler algorithm, we derive a radial equation for the brane-localized graviton absorbed/emitted by the (4 + n)-dimensional Schwarzschild black hole." The title of this abstract is: "Hawking Radiation of the Brane-Localized Graviton from a (4 + n)-dimensional Black Hole" by D.K. Park, a South Korean physicist. I found it in: "arxiv:hep-th/0512021v5". So it looks like these micro black holes can absorb ordinary 4-D gravitons (though they mention it varies based on number of extra dimensions etc). Bob
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