Black Holes have a certain aura about them. They are associated, in the minds of the general populace, with a certain mystique or ultra-mystery about them – terrifying objects that gobble up everything within range – the ultimate devourer, doomsday machine, berserker and weapon of mass destruction (if you could figure out how to manipulate one of course) all rolled into one. But Black Holes have other aspects about them that are equally fascinating, and not really all that weird, though some bits are weirder than others. But you don’t have to be a geek to come to terms with these concepts.
Now the common perception about Black Holes is that nothing gets out past the event horizon once it finds itself beneath it. That’s not quite the case. In theory, as discovered by cosmologist/physicist Stephen Hawking, radiation can escape – sort of – and this radiation is now called Hawking radiation. Macro objects, objects we associate with classical physics, can not get from inside an event horizon to outside an event horizon without traveling faster than the speed of light, which unfortunately, should you find yourself below and event horizon, is the ultimate cosmic speed limit. There’s no ‘get out of jail’ card. Traveling faster than light speed is not allowed.
But, any elementary particles, in the micro size realm and subject to quantum phenomena, can escape – again in theory; this hasn’t be verified by direct observation (which is currently in the too hard basket). It you are a fundamental particle, just below the event horizon, you might, just might, due to quantum fluctuations or jitters / the vacuum energy / the Heisenberg Uncertainty Principle, quantum tunnel your way, the tiniest fraction of a distance imaginable, past the mathematical event horizon boundary, to outside and potential freedom. Of course most particles might get sucked right back in again, but a tiny fraction gets away, carrying with it energy (thus the Black Hole has a temperature) and therefore mass, so the Black Hole loses a bit of mass and shrinks a bit. This quantum tunneling, crossing an energy barrier without having in theory sufficient energy to do so, is sort of like how a radioactive atom goes ‘poof’ and decays to a more stable state. Something in the nucleus, not having enough energy to break out, nevertheless quantum tunnels its way out – ‘poof’.
Very much like a human being, from the very moment a Black Hole is born, say out of the gravitational collapse of a super-massive star that’s run out of nuclear fuel and stellar puff, it will start to die, to evaporate via Hawking radiation. However, in a Universe still very much dominated by matter and energy (including the all pervasive cosmic microwave background radiation), way more stuff finds its way into a Black Hole than gets out – by many orders of magnitude. For every bit (particle) that escapes, millions of bits (particles) get trapped inside. But (and here I assume an ever expanding Universe that never results in a Big Crunch), what happens when all the available matter and energy (all those particle bits) has been consumed and Black Holes can’t grow anymore (and here I assume that individual Black Holes are so far apart and expanding away from each other that they don’t consume each other). Then, evaporation – Hawking radiation output – exceeds input, and slowly, ever so slowly, and I do mean extremely slowly (as in measured over trillions of years), Black Holes get smaller and smaller until there’s nothing left. But our now ever more vastly expanded and immensely larger than it currently is Universe is filled (albeit to a much rarified extent) with just particles – particles adrift in the eternal cold of near absolute zero temperature (zero degrees Kelvin, the absolute theoretical minimum temperature possible).
However, the ultimate death of Black Holes has posed a significant problem to some physicists, causing quite a bit of controversy in the process.
What happens to the information content that a Black Hole can gobble up? Say you toss a book, or a CD, or a fully loaded human brain into a Black Hole. Is the information contained in that book (or whatever) lost to the Universe forever? [Perhaps given the state of information overload we suffer from that might be a blessing!]
You can not have macro stuff spew out of a Black Hole without violating basic physics. Macro stuff, say in the form of a book or a CD or a human, stuff full of information, falls in – that identical macro stuff, stuff full of information, does not, can not, come back out again. It is not only an improbable event, but an impossible one and a violation of the law of physics. But we have seen that in theory at least, Hawking radiation can get back out, because radiation isn’t macro, its micro, or in the realm of the quantum.
Note that it wasn’t Hawking radiation that was tossed into the Black Hole in the first place, but a book or CD or a human being or a whatever macro object, so escaping Hawking radiation isn’t that book or that CD or that whatever, but a bit of this and a bit of that and there’s no way of distinguishing the this from the that. Though there is apparently no way to reassemble the bits into all its separate meaningful messages; one-on-one, all the bits are nevertheless there.
If you were somehow able to reassemble bits of Hawking radiation emitted from all the bits and pieces which the Black Hole swallowed – which can escape – into a meaningful message(s), how would you know that message was something part and parcel of some information that went down the Black Hole gurgler in the first place? You’re more likely to have assembled one letter from one book, another letter from another book, yet a third letter from a third book, etc. The information (say sentence) you have assembled never entered the Black Hole in that form at all!
Still, a Black Hole, in theory, eventually spews out all the information it absorbed over its existence, ultimately via Hawking radiation. Some scientists insist there is, there must be, a way to reassemble the bits into all its separate meaningful messages; one-on-one.
So therein lies the controversy – macro stuff does go in; macro stuff does come out. Macro stuff ultimately escapes as micro stuff – Hawking radiation. Some scientists will say you can’t in theory reassemble and separate out the signal from the noise; others say you can, in fact it must be possible.
As indicated above, some physicists make a big deal over the loss of information via a Black Hole relative to any other way – probably because of the non-reversibility factor already described. Methinks personally it’s a non-event. Why? The fundamental question this all boils down to be that information – in any form – is a composite of elementary particles. A book, or a CD, or Morse code ink drops, or a human brain is a composite of particles. An electron, all on its own, isn’t telling you very much (for that matter, either is any individual letter in a book – by itself). Loss of information seems to be another example of dust-to-dust, ashes-to-ashes; only it’s a more fundamental case of elementary particles to elementary particles. It’s how the Universe began and its how the Universe will end up if the current observational astronomical trends continue into the indefinite future.
There’s one other solution to the ‘is information lost forever or is it not’ paradox. It’s considered a possibility that a Black Hole, because is so distorts time and space – in the extreme - ultimately buds off from our Universe and starts or enters another universe, or a baby universe (part of a Multiverse). In such a case, any information is budded off with it and lost to our Universe forever. Of course our loss is the other universe’s gain; maybe a Black Hole(s) in some other universe has dumped its information load (or overload) onto our Universe!
There’s one further spin-off from the Black Holes make baby universes idea. In a Multiverse, different universes may have different laws of physics. There’s no reason why the laws of physics in our Universe need be identical in another universe. Thus, there might be some universes where the local physics favor the formation of Black Holes, and some universes where local physics can’t make Black Holes. Those universes that can easily make Black Holes will ‘breed’ and produce baby universes. Those universes that can’t readily make Black Holes will ‘breed’ less. Those universes that can’t produce Black Holes will be sterile. Do you see the connection with Darwinian ideas? Some universes are more ‘fit’ to reproduce than others!
Now that’s weird! There’s one other bit of weirdness I like about Black Holes, and that is that what’s inside them may well be a new form of matter. Ordinary matter goes into a Black Hole, but the conditions inside them are so extreme that there’s some sort of phase transition (like when ice goes to water goes to steam or vice-versa) and while it’s still matter, it’s matter but not as we know it. The theoretical evidence for that idea is that if you have a matter star, and an antimatter star, and you introduce them to each other, what you get is one almighty Ka-Boom! But, if your matter star compresses into a Black Hole, and your antimatter star compresses into a Black Hole, and you combine the two, what you get is just a larger Black Hole!
Some more weirdness: It’s suggested that information going into a Black Hole is actually ‘stored’ in the event horizon, that two dimensional ‘surface’ marking the point of no return that surrounds the Black Hole’s singularity – whatever that actually is. The event horizon concept isn’t difficult to envision – Earth’s crust and oceans are a two dimensional surface surrounding the spherical three dimensional planet.
Now as more and more stuff enters a Black Hole, the event horizon expands accordingly – obviously - just like our crust (area) would get bigger if Earth’s volume increased. The event horizon is also the area where Hawking radiation is emitted from.
Now say you are inside a Black Hole’s event horizon – that’s the wrong side to be on, but this is just a thought experiment and I’ll assume you haven’t been crushed into a tiny pinprick of stuff, stuff that could equally be rusted automobiles or stuff formally made from gold, silver and diamonds. There’s lots of trapped radiation (photons) in there with you because light can enter a Black Hole. Those photons can struggle up, losing energy with each unit of distance gained, to reach the event horizon, but no farther. Their energy has exhausted itself. I gather they can just barely touch and ‘reflect’ off the underside of the event horizon and come back down again (in a direction towards the singularity), picking up the energy again that they expended in their futile gesture of escape. So, you, being also beneath the event horizon can see the event horizon from the inside via these trapped photons. You can also see beyond the event horizon via new photons entering the Black Hole from outside the event horizon – photons that will join their trapped or prisoner kin. It’s like a half-way mirror. If you are inside a Black Hole, you can see out, because light can pass through the Black Hole’s event horizon to you, but people on the good side or outside of the event horizon can’t see you because light reflecting off you can’t make it past that event horizon barrier.
One further question, could we actually be living within a Black Hole, or translated, is our Universe actually a Black Hole? Now one could (and people have) suggested that one could consider the entire Universe as being the inside of a Black Hole – after all, nothing can escape from the Universe. Well, if you can’t escape from inside a Black Hole, and assuming there’s no escape from our Universe (you are trapped in this Universe, like it or lump it), then a rose by any other name…
However, our Universe doesn’t exactly mirror a real Black Hole unless there is an outside to our Universe – a beyond the boundary or horizon that allows stuff to get into our Universe, our Universe ultimately trapping it.
So, Black Holes residing inside a Black Hole Universe, which maybe residing inside…
Russian dolls within Russian dolls within Russian dolls within Russian dolls.
Saving the best for last, could you become a Black Hole? Well, the short answer is presumably, ‘yes’. The reasoning goes as follows. If you travel at ever increasing velocities, under special relativity, your mass gets correspondingly greater and greater, and your length gets shorter and shorter. Translated, your density gets greater and greater; your own gravity gets higher and higher. At light speed (impossible to achieve), your mass would be infinite; your volume zero; your density and gravity infinite. Well, that’s not on. But, before even approaching that limit, your mass would be theoretically great enough; your volume low enough, your density and gravity great enough, that you’d warp space-time sufficiently enough to turn into a Black Hole! As noted above, what actually comprises a Black Hole is irrelevant. Any stuff will do – gold, silver and diamonds; rusted automobiles; or flesh-and-blood (i.e. – you).
Here are a few further recommended readings:
Begelman, Mitchell & Rees, Martin; Gravity’s Fatal Attraction: Black Holes in the Universe; [2nd Edition]; Cambridge University Press, Cambridge
Susskind, Leonard; The Black Hole War: My Battle With Stephen Hawking to Make the World Safe for Quantum Mechanics; Back Bay Books, New York
Thorne, Kip S.; Black Holes & Time Warps: Einstein’s Outrageous Legacy; W.W. Norton & Company, New York
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