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Author Topic: What is Hawking radiation and how does it act with tiny black holes?  (Read 4872 times)

Offline Damien Huxley

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Damien Huxley  asked the Naked Scientists:
   
Hi Chris,
 
What is Hawking radiation and how does it act with tiny black holes?
 
Regards
 
Damien Huxley

What do you think?
« Last Edit: 11/10/2010 12:30:03 by _system »


 

Offline Soul Surfer

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The Hawking radiation becomes more intense as the black hole gets smaller black holes are also very small objects.  A one solar mass black hole which is about one mile across would have a hawking radiation temperature way below the two and a bit degrees above absolute zero of the microwave background radiation and so it would always be gaining energy from the CMB.   A high temperature black hole would be very small indeed and although the radiated energy and the life before final evaporation could be useful for powering a civilisation like ours it is very small even on the scale of even a modest star let alone a supernova and would be almost invisible in the depths of interstellar space.

You can look at this in detail using this website   http://xaonon.dyndns.org/hawking/
 

Offline Damien Huxley

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Hi Soul Surfer, thanks for the reply.

Some questions that I have is,
  • Why does Hawking radiation becomes more intense as the black hole gets smaller black holes?
  • Wouldn't black holes gain energy every time the take in matter?
  • Why does the temperature matter and how do we know small black holes have high temperature?
  • Why aren't micro size black holes created when cosmic rays bombarding the Earth?

 

Offline Soul Surfer

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The hawking radiation is due to the gradient of the gravitational field at the event horizon.  As a black hole increases in size through accreting mass this gradient become less so the radiation become less intense.

black holes do gain energy and size as they accrete more matter but as stated above this is not the feature that creates the radiation.

You are familiar with the radiation from hot objects,  warm radiators, red hot steel taken from a furnace and the sun itself even the cosmic microwave background.  These are all radiations related to the temperature of the object  a radiator may be 60 deg C  (333deg K)  red hot steel 600 deg C (873 deg K)  and the sun 6000 deg C ( 6300 deg K approx)  the CMB is 2.7 deg K  (-270 deg C)  This is a basic process of all bodies and the hawking radiation is very similar but black holes of any size are very cold and lose there energy extremely slowly.

Any micro sized black holes created by cosmic rays ( even the LHC) would evaporate extremely quickly and leave no detectable residue.  As the total energy in the interaction is not very great in comparison with the mass of an extremely tiny amount of material it is negligible.
 

Offline Bill S

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Following Damien Huxley's question: What is Hawking radiation and how does it act with tiny black holes?  It would be great to have a fuller answer to the first part of the question, "What is Hawking radiation...? I have read a bit about it, but there are a few points about which I am not clear.

Bill S.
 

Offline Damien Huxley

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This is a good video about Hawking radiation
http://www.youtube.com/watch?v=S6srN4idq1E [nofollow] A clip from a BBC documentary explaining Hawking radiation around black holes.

Not that it answered my questions

  • How do atom or a pair of particles pop up?
  • Do we see them on earth?
  • Why does the black hole get hotter as it gets smaller?

Regards Damien
 

Offline Soul Surfer

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The quantum mechanical vacuum consists of a melee of virtual particles appearing and disappearing at all times because of the action of the uncertainty principle.  The particles are pairs of particles and antiparticles.  If a pair of particles appears at either side of the event horizon for the particle pair energy (which is also of course subject to uncertainty) one will escape and one will fall into the hole.  The uncertainty in the event horizon is dependant on the gradient of gravity at the event horizon which I have already told you becomes smaller as the black hole gets bigger.  Under most circumstances the only particles that can escape are photons because they can have very low energies  (although low energy neutrinos may also be a possibility)

A naked stellar mass or greater black hole is virtually undetectable in its own right it is only detectable from its gravity like the stars we observe orbiting the one in the centre of the galaxy.   Very small black holes might be detected by their hawking radiation at close range.  The final evaporation of a black hole is virtually inconsequential in an interstellar scale it is not very violent like a supernova. this is because such a black hole would only be about the size of an atom and weigh as much as a mountain.
 

Offline Bill S

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Quote from: Soul Surfer
a pair of particles appears at either side of the event horizon....one will escape and one will fall into the hole.

Please correct my thinking here if it is off the mark.
The particle pair will be one particle and one antiparticle.
If the antiparticle falls into the black hole its mass will be reduced by one particle.
Should there not be an equal chance that the particle will fall into the B H, and if so would not the loss and gain average out over time?
 

Offline Soul Surfer

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No. Either way the black hole loses energy.
 

Offline Bill S

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Quote from: Soul Surfer
No. Either way the black hole loses energy.
I'm happy to believe that, but could you explain it so that a non-scientist can understand it?
 

Offline Soul Surfer

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Black holes are very simple things and easily understood maybe I should write a short piece explaining them so that people can understand them.  Most of the texts I have read do a terrible job of this and try to make them mysterious and somehow exciting but they are very simple and normal things and it would be far more peculiar if they DIDN'T exist.

The simple explanation of energy loss with hawking radiation is based on finance.  when a virtual pair of particles is formed it "borrows" energy from the quantum mechanical vacuum to come into existence.  This debt has to be paid back in a time defined by the uncertainty principle which says that the more you borrow the quicker you have to pay it back so you can borrow a tiny amount of energy for a "long"  time  and a large amount for a very short time.  If the particles are separated and one escapes the black hole itself has to pay the energy  debt  so one particle or photon falls into the hole but the energy debt is for two so the hole must lose energy.
 

Offline Bill S

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Thanks SS, in over 100 P S books I have yet to find an explanation that comes near yours.
If you are not already a P S author, perhaps you should be.
« Last Edit: 17/10/2010 13:17:31 by Bill S »
 

Offline Damien Huxley

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Thanks SS, one question on the "virtual pair of particles" do they pop up anywhere, inside metal or the sun?
 

Offline Soul Surfer

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Yes
 

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