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Saturday, 24 April 2010

Hawking Radiation and Black Hole Evaporation.

Bekenstein-Hawking radiation questions.

'There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature...'  "The philosophy of Niels Bohr" by Aage Petersen.

How does Hawking Radiation work?

Stephen Hawking
Stephen Hawking
In 1974, Stephen Hawking set forth a theory that due to quantum effects black holes should emit a thermal radiation with a black body spectrum. Actually, he drew on two soviet scientists’ work who claimed that rotating black holes should create and emit particles according to the quantum mechanical uncertainty principle. That means that black holes can lose mass and, if they spew out more matter than they gain, they might shrink, evaporate and eventually vanish.

This analysis is allegedly the first serious shot at a possible theory of quantum gravity, but there’s a catch — the existence of Hawking radiation hasn’t been proved so far. NASA launched a satellite to search for the expected gamma-ray flashes from the evaporating primordial black holes, and CERN’s Large Hadron Collider is bent on creating micro black holes in order to observe their evaporation, or that’s the part they let us in on.

They might as well be trying to create a wormhole and thus open portals to parallel dimensions — if what they discover turns out to be good and profitable, the ruling class will be flocking to stake out claims on it, if it doesn’t, they’ll jettison the hoi-polloi there.

Whatever it is, I highly doubt, though, it’s worth the taxpayers’ money, not to mention the possibility of a black hole going out of control (wouldn’t be the first time scientists screw up) and sucking up entire Europe and probably the rest of the world.

Anyway, I have some doubts about the whole thing. First and foremost, black holes’ gravitational attraction, according to classical theories, is so powerful that absolutely nothing, including electromagnetic radiation, can escape from its event horizon. No-one has managed to merge gravitation into quantum mechanics as of yet, however far enough from the black hole, where gravitation is weak enough, quantum field theory equations for the curved space-time (like any space-time around ordinary massive objects) can be applied.

Physical meaning of the process is somewhat vague, to say the least.

One way of breaking it down is imagining this radiation, which supposedly comes from beyond the event horizon, as a result of virtual particles being made into real ones by the black hole’s gravitation — then what makes or lets them escape?  
Black Hole
Black Hole

To put it another way, vacuum fluctuations cause a particle-antiparticle pair appear close to the event horizon, one of the pair falls into the black hole, while the other, I guess the luckier one, escapes. Virtual particles (or vacuum fluctuations) are created and destroyed out of the vacuum and thus constitute vacuum energy — an underlying background energy that exists in the space devoid of matter (free space). The effects of vacuum energy show in various phenomena such as, for example, the Casimir effect.

The picture makes a skein of mind-boggling questions grow thicker in my brain.

How come vacuum fluctuations are so strong as to override the black hole gravitation and push one of the particles outside the event horizon?

Why would only one particle escape or fall inside, and not both of them?

Since total energy should be preserved, the particle that fell into the black hole must have had negative energy (so negative energy was its undoing as a particle?) with respect to a distant observer.

Does this imply it wouldn’t have had negative energy for a local observer?

It will seem to an outside observer that the black hole emits particles as particle-antiparticle pairs come out of the blue, that is, vacuum, and some of them will tunnel outside the event horizon.

All that poses a question — if the particles don’t come from the inside of the black hole itself (the radiation is a sort of mirage seen by a far-away observer), the black hole isn’t actually losing any mass/matter and shouldn't dissolve at all, should it?

Another snag is that Hawking radiation doesn’t contain information about the body that emitted it, if we stick to the "no-hair" theorem, it only depends on the mass, angular momentum, and charge of the black hole (“hair” is a metaphor for all other information), which leads to the black hole information paradox. Any information about the matter that formed a black hole or is being swallowed by it is inaccessible to external observers (an internal observer, though, would have an edge and be in the swim of things on the both sides of the event horizon).

A controversial topic, since this seems to breach an apparently unshakeable tenet of science: complete information about a physical system at one point should determine its state at any other time.

This statement makes me (and not only me) feel particularly uncomfortable — in other words, putting aside gobbledygook, our future at any point is predetermined by our previous state or information we carry (personality, soul?). So if we manage to calculate all the variables of the system -- that's all it takes to predict future with 100 % certainty!
Then what about Free Will? Yet another illusion, I guess.

Hawking’s theory looks nice in equations, but just because the equations seem elegant it doesn’t mean they make physical sense.
Some boffins say this radiation is thermal, others claim it’s a purely quantum state.
So what do we hold to?

In my humble opinion, Hawking radiation either doesn’t exist at all, or isn’t what we think it is; on the other hand scientists have to produce some output (that is, theories) to justify their salaries, so they cook up all sorts of surrealistic theories.
But then again, it’s just my personal view.


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