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Hawking radiation

  1. Jun 30, 2006 #1
    I was thinking you know hawking radiation where a virtual particle can escape, is it specific as to which particle can escape as in the particle or it's anti particle? does that make a black hole like a particle 'producing' machine, can these escaped particles be a significant fraction of the missing/dark matter, i guess that depends on the number of black holes and how much matter ecapes, (how are the virtual particles treated when it comes accounting for dark matter, is there like a value of mass over a period of time which these virtual particles contribute to? or is it taken as net 0 mass, because they end up cancelling each other out?) whats the rate at which these particles escape? is it known, does it vary for different sized black holes? i guess bigger the black hole the more particles that can escape...because theres more area of the black hole where one of the virtual particles particle can be taken in and one escape.

    sorry for so many questions, its one of those days.
    I read The universe in a nutshell by Stephen Hawking, didn't really understand it, got dazzeled by the pictures lol

    o plus i was wondering do gravitational waves have a corresponding particle, is it the graviton or is that something else?
    what about the wavelengh of these waves are they known....are they long wavelengths longer than radiowaves, i heard these waves are weak...as in they dont have much energy so they're wavelengths would be longer. but if the em spec is continuous doesn't that make gravitational waves an extreme form of radiowaves?

    one more thing...what are those 5 pink boxes at the top of this text box thing next to 'warn (0%)' ??
    o erm can i change my username somehow? i don't like it anymore, it's boring.
    Last edited: Jun 30, 2006
  2. jcsd
  3. Jun 30, 2006 #2


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    In regards to your first question, I would point out that most of the "particles" being discussed (when talking about Hawking Radiation) are photons, and the antiparticle for a photon is just anotherr photon with opposite spin and impulse.
  4. Jun 30, 2006 #3
    Most normal (one solar mass or greater) black holes produce very little radiation indeed. Left to their own devices they are effectively considerably colder than the cosmic microwave background at a couple of degrees K and are therefore absorbing more radiation than they emit.

    Only very tiny black holes ( the weight of a mountain or less ) are likely to be hot and emitting significant radiation. these could only have been made during the very dense period just after the big bang.

    The radiation that they are likely to radiate is mostly photons which are part of normal matter and energy.

    Some people consider that there may be a lower limit to the size of a black hole because it gets stuck because it so small that particles can't effectiverly radiate from it a sort of ground state gravitating particle. these could also form during the early microseconds of the universe and could well be a significant part of dark matter
  5. Jun 30, 2006 #4
    Gravitational waves are nothing to do with radio waves which are electromagnetic the particle associated with these waves has been called the graviton

    The sort that people are trying to detect are in fact very long (or very low frequency) in the audio range or below brcause thats how fast normal star sized black holes or neutron stars would orbit each other before they merge. To get significant gravitational radiation at higher frequencies you would need very small black holes interacting and because everything is mush smaller and lighter you would need to be much closer to an event. As far as I know no one has tried to design a device to detect gravitational waves at radio frequencies or higher
  6. Jul 1, 2006 #5
    o i didn't know these 'particles' are photons, hmm and they have spin too, o i guess they would if theyre like particles, but protons don't spin in the nucleus do they? or is it just the fundamental particles that have spin? what's impulse, change in momentum = F.t?. I think thats strange that bigger black holes are colder...arn't bigger suns hotter, so i guess i expected the same for BHs, isn't mass directly proportional to the size of the BH. I heard that black holes can explode is that true? is it because too much matter has fallen into it? some kind of critical density?, but i heard nothing really 'falls' into a black hole ...but gets stuck on the event horizon...wel as we see it. Does a BH really reduce the disorder in the universe? (is disorder like the thermal energy that can't be transformed into other forms of energy?, o i did know once my memory isnt too great.)
    isn't that good because i heard that disorder only increases in the universe so black holes can maintain a constant disorder 'value' thing.
    Last edited: Jul 1, 2006
  7. Jul 1, 2006 #6


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    A lot of misconceptions here, alias. Yes, photons have spin (spin 1). But protons do have spin in the nucleus or anywhere else; they are fermions which means they have spin 1/2. You mustn't think of quantum spin as turning like a top; protons are thought to be bound systems of 3 quarks, and each of the 3 quarks has spin 1/2 (quarks are fermions too) and the three 1/2 spins combine to 1 1/2, but only the difference between integer and half integer matters here so the proton bound state comes out 1/2.

    It's not always true that bigger stars are hotter, though there is a strong tendency for this to be true. When the Sun finally expands into a dim red giant, it will actually be cooler than it is now. And anyway you don't figure things out in physics by vague analogies but by detailed calculations. The temperature of a black hole is the temperature of the Hawking Radiation; the fact that the radiation is thermal and has a well-defined temperature can be proved. And it is a calculated fact that the Hawking Radiation temperature is inversely related to the radius of the black hole.

    The counterintuitive behavior of something falling into a black hole horizon - although in the viewpoint frame of the falling object nothing odd happens and it falls through the horizon in finite time, while to an observer watching it from far away it seems to take literally forever to reach the horizon - is due to the severe bending of the light rays by the black hole gravity. It's like trying to watch a scene through the bottom of a bottle.
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