Stephen Hawking said there’s a way out. Stephen Hawking lied.
“Not only does God play dice, but… he sometimes throws them where they cannot be seen.” –Stephen Hawking
By definition, black holes are objects that have so much matter concentrated in a single point that nothing — not even light — can escape from within a certain region of space around them. This was a feature of Newtonian gravity; it’s a feature of Einstein’s General Relativity; and when we have a full quantum theory of gravitation, we fully expect this will be a feature of that, too. When a massive enough star dies and its core collapses down, past the stage of atoms, past nuclei, past neutrons and past individual, free quarks, a funny catastrophe happens: even something moving at the speed of light can’t escape from this region of space. And when that threshold is crossed, everything collapses down to a singularity at the center. You’ve created a black hole, and over the next trillions upon trillions of years, all it will do is grow, eating whatever falls inside its event horizon.
You might think some of these things are wrong:
- Perhaps a singularity isn’t inevitable.
- Perhaps it won’t eat everything that dips inside its event horizon.
- And perhaps, if you were clever enough, you could even escape from inside.
But all of these are true, for reasons you might not expect. And despite Stephen Hawking’s assertions that there’s a possible exit (scientific paper here), no amount of cleverness will allow something that falls in to escape.
Escape velocity is a funny thing: it says that there’s a certain amount of gravitational “pull” in a certain region of space, and if your speed exceeds a certain amount — the escape speed — you can get out. At some lower value than that, there’s the speed you need for a stable orbit, where you won’t escape, but where you can avoid falling in forever and ever. And if your speed isn’t great enough, you fall down onto the gravitational source you’d like to escape from.
What does that mean if your escape velocity is bigger than the speed of light? Since nothing can exceed the speed of light, it means that there’s no possibility of getting out. But it means more than that, too! If you’re inside the event horizon, and you want to exert a force to “push outwards” on something trying to collapse down onto you, it means that you can’t. To exert a force, to push outwards, means to transmit a particle outwards. But all particles are limited by the speed of light! And if the escape velocity is bigger than that, you can’t move away from the point you’re collapsing towards; you can’t move farther away from the singularity. It means, oddly enough, that once you fall inside a black hole’s event horizon, you see a singularity in all directions. That singularity is inevitable.
That also means that if you’re riding by the edge of the event horizon, and you try to do something clever like just dip your toe in, you’re in for a world of hurt. The part of your body outside the event horizon will still be pulled in, but the part inside won’t successfully be able to be pulled out; there’s no way to pull on it since all of the material inside — even the force-carrying particles — get sucked into the singularity. If you want to escape, your only hope is to be the kind of person who would gnaw off your own leg to save your life. Whatever dips inside the event horizon is a goner. And even for a supermassive black hole millions or billions of times the mass of our Sun, the amount of time it takes something at the edge of the event horizon to reach the singularity is measured in mere seconds. However you slice it, falling into a black hole is a terrible idea with a catastrophic fate.
So why, then, did Stephen Hawking announce that it might be possible to escape from a black hole? Why did he announce, on stage at Harvard:
They are not the eternal prisons they were once thought. If you feel you are trapped in a black hole, don’t give up. There is a way out.
Because he wasn’t talking about you. He was talking about the information that makes you up at the moment you fell in: when you crossed the event horizon. When objects fall into a black hole, they possess all sorts of information: was it matter or antimatter that fell in; baryons or leptons; electrons or muons; neutrinos or neutrons. Was the book that fell in The Grapes Of Wrath or The Count Of Monte Cristo? And what of the human who fell in; who was it and what were they like? But under General Relativity, at least, under classical General Relativity, a black hole only contains three pieces of information: mass, electric charge, and angular momentum. (If magnetic charges existed, it would contain that information, too.)
The famous “black hole information paradox” is about how, when a black hole eventually decays (via Hawking radiation), only the total energy, electric charge and angular momentum make it out; everything else is random. In other words, all that extra information gets destroyed. What Hawking’s latest paper actually says is much less fantastic than escape from a black hole; it says that whenever a new particle falls into the black hole, it changes the information encoded on the black hole’s surface. And since a black hole surface (and surface area, for that matter) is dependent on the mass/energy inside of it, as it decays, that information should make it out, scrambled but encoded in the outgoing Hawking radiation. Just as a burned book contains the information in the pages (albeit in a not-very-useful format), the outgoing Hawking radiation might encode the information about what fell in after all.
That’s a far cry from escaping from a black hole, but it does represent a potential, possible resolution to the information paradox after all. Still, if you’re absolutely determined to escape from a black hole, there’s only one way to do it and still obey the laws of physics as we know them: don’t fall in in the first place.