Don’t know what a black hole is? Don’t worry: not even Hawking does. Not completely at least. Of course he’s among the best positioned to have a quite detailed picture but, as you might have noticed from recent press, the (in)famous black hole monster is still an open research problem for the whole scientific community who’s been working on it for the past 40 years! Yeah: that’s how difficult the problem is, so don’t feel too small.
Black holes stare at us with their load of secrets as they sit at the crossroad of physics’ most comprehensive theories. The first one is Einstein’s theory of gravity, called General Relativity, which accounts for the movement of very large objects in the universe and even the history of the cosmos itself; the second theory is called Quantum Mechanics and describes the very small and quirky realm of infinitesimal particles. Both theories were heavily influenced by Einstein, who helped jumpstart the quantum revolution and completely molded General Relativity. Hawking & Co. are right in Einstein’s tracks, as black holes find themselves at the convergence of Quantum Mechanics and General Relativity: understanding them fully is poised to bring a revolution in physics.
The problem physicists have been working on since the 60’s is called the black hole information loss paradox and can be encapsulated in one very simple question “what happens if you pour a cup of tea on a black hole“? This is an example of how physicists work: they ask “what if … ?” and embark in the ensuing adventure.
Just so I clear the doubt once and for all: most of the times these questions are explored by scientists in collaboration and, even if a single one makes great strides and becomes famous, like Einstein and Hawking, at least they rely on previous hard work by other smart people. So the reason why I take Hawking here as representative of the endeavor is because, as Einstein, he’s an iconic character.
Back to business, what happens if you pour a cup of tea on a black hole? The tea itself would disappear because black holes suck everything, as you might already know, even though this only happens in their proximity, so, unless you go poke the bear, a black hole is not dangerous.
What about the heat of the tea? is it gone, too? is there a way from the outside to reconstruct the characteristics of what went in the black hole? had it been a cup of equally hot milk, would we be able to tell the difference by inspecting the black hole? or, in current physics parlance, can we retrieve information from a black hole?
Notice that I said that, in order to know what happens to the cup of tea, we would “inspect” the black hole, I didn’t say we’d ”look at it” for a very precise reason: black holes do not reflect the light impinging on them, as do objects we can see (if you had never thought about the ability of seeing in these terms, try for a moment to picture yourself at night, shining a torch ON objects so that they can bounce light back at your eyes).
Hawking has already contributed a smart idea to the problem of how reticent black holes can be about what they have hidden in their interiors. Forty years ago, he figured black holes can sweat! He did not use these exact words and so far I haven’t heard anyone else using them but that’s a way I think can ease the understanding of Hawking radiation, as his proposal came to be known.
It turns out that calling space empty is a misnomer: even when devoid of matter particles, space is full of other stuff we do not see other than through its effects. Imagine you are on the top of a mountain, contemplating the panorama around you, especially the mountains summits framing your horizon; between the mountain you’re sitting on and the mountains at the horizon there’s no other mountain: there’s an emptiness of mountains. However, there might be fields in between, covering the valleys hidden by the clouds: lavender fields, corn fields, etc. That’s the closest I can go to the situation in space: between Earth and the Moon, and between us and the Sun, there does not seem to be anything but in reality there are so-called fields, such as the electromagnetic field and the gravitational field.
A field is like The Force, not the physics concept but the Star Wars one, an entity that permeates space and that you do not see directly with your eyes but it has visible effects, such as Yoda lifting a spaceship. In the case of physics the electromagnetic field is responsible for magnets feeling each other’s presence and for connecting you with the world through a mobile phone.
As much as flower fields can wave in the wind, physical fields oscillate, too: instead of setting daisies free in the air, they liberate particles, sometimes in entangled couples, letting them exist for a blink of an eye before they disappear again in the expanse of the field. Hawking understood that for fields around black holes the reabsorption of the particle pairs would not go as unnoticed as the emission: if one of them were sucked by the black hole, the other would be able to fly away, free from its former binding companion.
The particle ending up inside the black hole is such that, by swallowing it, the black hole takes a loss in energy, thus reducing its mass. The particle escaping to infinity instead has a temperature, so from afar it looks like the black hole emitted heat, which cost it work: in due time (a very large one) the black hole would sweat itself away or, as Hawking and Co say, it would evaporate.
Unfortunately, the energy regurgitated in this way by the black hole carries no information about what its meal has been. It would seem there’s no way to know what made the black hole sweat, hence the information loss paradox: information should not be destroyed but just transformed, like energy (if you understand French there’s a science poem of mine about energy I’d like to suggest).
If information is not destroyed, where does the black hole hide it then? Hawking has just proposed something in accordance to others before him, that information is stored on the black hole horizon, the surface of no return if you incidentally crossed it. This is a big deal for a very good reason that we can all understand. Imagine you were at the library and you were wondering how many interesting stories and theories and facts were written inside those thick books, on their millions of pages. You’d be surprised if the librarian, some Jacob Bekenstein, came to you and said that, in his library, books wear their stories on their jackets only: they do not need pages for storing their tales. That’s exactly what the late Prof. Bekenstein had found about black holes’ storage habits in the ’60s, before Hawking contributed his pieces to the puzzle.
I think this account is enough to give you an idea of what is keeping Hawking and colleagues’ mind in check; if you want to deal with the matter further see the “Backreaction” blog. Last but not least, in case you had other metaphors useful to convey a sense of the many weird features of black holes, do not hesitate to let me know, either in private or through the comments.