Tuesday, April 24, 2012

Can Black Holes Evaporate?

"Can Black Holes Evaporate?"," Since nothing can travel faster than the speed of light, nothing (matter and/or energy) once inside a Black Hole can ever get out again - or so the seemingly ironclad logic went.
 A physicist by the name of Jacob Bekenstein came up with the idea of applying quantum physics to these objects (upon a suggestion by his mentor John Wheeler - who incidentally coined the phrase ""Black Hole""), and once that was done, well lo and behold, these objects apparently exhibited entropy, and therefore had a temperature and therefore must radiate and therefore can vomit out stuff.
 That stuff that a Black Hole can regurgitate now goes under the name of Hawking radiation, or to give credit where credit is due it is technically Bekenstein-Hawking radiation.

Of course if Black Holes have a temperature, then they must follow the same laws of thermodynamics as any other object with temperature.
 The temperature of a hot cup of coffee will stay hot longer the higher the temperature of the environment that surrounds that hot cup of coffee.
 So how does a Black Hole get temperature?

In retrospect, how this happens is obvious (as are all great ideas when applying hindsight).
 That could only be achieved at a temperature of absolute zero where and when everything is 100% frozen stiff.
 If something were at absolute zero, frozen stiff and standing still, you'd know both the momentum (which would be zero) and position (at a standstill) of that something with absolute precision.
 However, the particles come in matter-antimatter pairs, which usually immediately annihilate and return to their former pure energy state.

The vacuum energy, that which can generate particle-antiparticle pairs, exists everywhere where existence has any meaning.
 These cosmic objects all have an event horizon which surrounds them.
 I say its ""fuzzy"" since it's not razor sharp, albeit nearly so.
 Now, what if that vacuum energy generates a pair of virtual particles, one each popping into existence above the event horizon; one below the event horizon.
 One will stay within the Black Hole.
 And thus, slowly, ever so slowly, but ever so surely, these cosmic sinks loses mass, thus energy, and they evaporate.
 Black Holes can only radiate from the event horizon region which, in a very large cosmic sink is going to be very cold because it's not radiating very much, so initially only things like the mass-less photon escapes.
 When the cosmic sink is tiny, it's very warm, in a relative sense, and it can go out with a 'bang', maybe emitting an electron or positron which is way more massive.
 That's where the popular accounts end.
 The ultimate fate of Black Holes will be to evaporate via Hawking radiation, even if it does take trillions of years.
 Black Holes can acquire stuff, as well as radiate stuff.
 Now this is perhaps why Hawking radiation hasn't been observed.

Forget these universal sinks (and their massive gravity) for a moment and concentrate on Planet Earth.
 You see them because they are radiating photons - particles of electromagnetic energy of which visible light is a small part.
 Optical telescopes pick up a lot more of them, but they're still just as real.
 Though Earth's atmosphere shields us from some of these photons (ultraviolet photons are far greater in number at the top of our atmosphere than at the bottom), you still get impacted by multi-billions of them; Planet Earth many orders of magnitude more.
 Overall, there are roughly one billion photons for each and every fundamental particle with mass, like electrons and neutrinos.
 Even if you luck out, Planet Earth gets impacted by meteors and other outer space debris, sometimes debris large enough to not only hit the surface but do considerable damage.
 The trillions of neutrinos that hit us are so ghostly that nearly all pass right through you and the entire planet as well despite them having a tiny amount of mass, so as far as our planet is concerned, they are of little significance.
 If you were just outside the event horizon you'd 'see' photons (of all wavelengths) because you'd see stars and galaxies, etc.
 Neutrinos would still pass right through you on their way to their doom once passing through the event horizon.
 Black Holes will sweep up stuff just like Earth does, only more so since it has more gravity with which to grab hold of stuff with, and also because once caught there's no escape for the cosmic fish.
 Neutrinos that can pass through light-years worth of solid lead without even 'breathing hard' will be imprisoned when they try that trick in a Black Hole's inner sanctum.

But we can imagine an idealized cosmos where all Black Holes have swallowed up all existing radiated particles (photons), all the atoms, molecules, the dust and all the bigger stuff - all those stars and planets; asteroids and comets; even all that mysterious 'dark matter'.
 Of course there is one further logical extension.
 Black Holes can merge to form bigger Black Holes.

Okay, so the only scenario now possible is that this Mother of all Black Holes evaporates via Hawking radiation.
 Since matter and energy can neither be created nor destroyed, once the Mother of Black Holes has finally gone 'poof', the Universe is right back where it started from - full of stuff from photons to fundamental particles which them undergo chemistry to form atoms and molecules and stars and planets and perhaps life - and new Black Holes!

Perhaps this is a new and improved version of a cyclic/oscillating universe! - But then again, maybe not.
 That ""idealized cosmos"" was only a 'what if' thought experiment.
 Since the galaxies are getting farther and farther away from each other due to that expansion, the collection of Black Holes contained within each galaxy keep getting further and further apart from other clusters of Black Holes contained within other galaxies.

Now the collection of all Black Holes in any one galaxy could well coalesce into one super Black Hole galaxy.
 You have a pure Black Hole galaxy, or a galactic sized Black Hole.

But secondly, there's another fly in the ointment.
 All the radiating stars and stuff may have been gobbled up within each galaxy, but all of interplanetary space, all of interstellar space, and all of intergalactic space, isn't pure vacuum.

So what's this CMBR? If you have a massive hot explosion (like the Big Bang event is alleged to have been), and all that heat energy expands and expands, then you'd expect the temperature of the area occupied by that energy to drop, the temperature ever decreasing as the volume that finite amount of energy occupies increases.
 And that's just what we find on a universal scale.
 That's the diluted heat energy of the very hot Big Bang - well it has been a long time since the Big Bang event (13.
 That microwave ""hiss"", called the CMBR, was predicted way before it was discovered.

Since the CMBR is just photons with very long wavelengths, Black Holes could suck up the CMBR photons as easily as light photons.
 Combining the two effects and the Universe is a chilly place indeed and will get even colder.
 What happens when the temperature of Black Holes equals the temperature of the Universe at large - the CMBR? The answer is thermal equilibrium like when your hot cup of coffee cools off to room temperature.
 For every photon emitted via Hawking radiation, a CMBR photon gets sucked in.

What about very tiny (micro) Black Holes that are relatively 'hot'? Might they go 'poof' before thermal equilibrium is achieved? Will the contents of the Black Hole evaporate into the surrounding cosmos before they can equate to the surrounding temperature? The analogy might be like a hot drop of water could evaporate into the cold atmosphere before the liquid water drop can attain the temperature of its surrounding environment.

Of course if you could take a Black Hole, isolate and shield it from the rest of the cosmos and all that it contains, so all you have is the Black Hole and its internal energy (including the all pervading vacuum energy therein).
 If that's the case then outgoing would exceed incoming since there could be no incoming, and therefore that Black Hole would then radiate and slowly evaporate and eventually go 'poof'.
 So, Professor Hawking is quite correct - in theory.

*If it helps to conceive of the concept of the vacuum energy, here's an analogy.
 Part of that atmosphere consists of invisible water vapour.
 You get mist/fog (clouds), rain drops, snow, sleet, hail, etc.
 And so you have the invisible vacuum energy that generates particle-antiparticle pairs which annihilate back into the vacuum energy.

Spray Paint Secrets Video Banner

No comments:

Post a Comment