How to survive in a black hole
There's no escape, but how can you maximize your remaining time?
So there you are: you discover that your spaceship has inadvertently slipped across the event horizon of a black hole — the boundary beyond which nothing, not even light, can escape the hole's fearsome gravity. The only question is how you can maximize the time you have left. What do you do?
A common idea in physics is that you shouldn't try to blast your way out of there. Black holes, it's said, are like the popular view of quicksand: the harder you struggle, the worse things become.
But Geraint Lewis and Juliana Kwan of the University of Sydney in Australia say this is a myth. Their analysis of the problem, soon to be published in the Proceedings of the Astronomical Society of Australia1, shows that in general your best bet is indeed to turn on the rocket's engine. You'll never escape, but you'll live a little longer.
Falling into a black hole is a strange affair. Because the hole's gravity distorts space-time, a far-off observer watching an object crossing the event horizon sees time for that object appear to slow down — a clock falling into a black hole would appear, from the outside, to tick ever slower. At the horizon itself, time stops, and the object stays frozen there for the remaining lifetime of the Universe.
But this isn't how things seem to the in-falling object itself. Indeed, if the black hole is big enough, nothing noticeable happens when a spaceship crosses its event horizon — you could stray inside without realizing. Yet once inside, nothing can save you from being crushed by the hole's gravity sooner or later.
Live long and prosper
Clearly, an astronaut in that situation might prefer it to be later. For a supermassive black hole such as that thought to exist at the Galactic Centre, the survival time could be hours. To stretch it out for as long as possible, the astronaut might be tempted to turn on rocket thrusters and try to head outwards, away from the hole's fatal 'singularity' at the centre.
But best not to, according to some sources. An article on black holes on the cosmology website of the University of California, Berkeley, for example, says "the harder you fire your rockets, the sooner you hit the singularity. It's best just to sit back and enjoy the ride."
Lewis and Kwan say this is mistaken. They point out that the analysis is usually done by thinking about a person who falls into the black hole starting from a state of rest at the event horizon. In that case, it's true that accelerating away from the singularity by using the rocket thrusters will only speed your demise. The longest survival time possible in that instance is free fall. Because all paths lead inevitably to the singularity, trying to travel faster - in any direction - only takes you there quicker.
Long and winding road
But in general a person falling past the horizon won't have zero velocity to begin with. Then the situation is different — in fact it's worse. So firing the rocket for a short time can push the astronaut back on to the best-case scenario: the trajectory followed by free fall from rest.
"There is one longest road - the freefall road starting from rest - as well as many shorter roads," Lewis explains. "If you cross the event horizon on one of the shorter roads, you can fire your rocket to move you on to the longest road."
But this has to be done judiciously. "If you overdo it, you will overshoot the longest road and end up on a shorter road on the other side," says Lewis. So you only want to burn your rocket for a certain amount of time, and then turn it off. "Once you know how fast you have passed through the event horizon, it is reasonably straightforward to calculate how long you need to burn your rocket to get on to the best path," he says. "The more powerful the rocket, the quicker you get on to this path." Starship captains, take note.
There's nothing particularly surprising in this analysis, but black-hole experts say that debunking this common misconception could have an educational value. "It's a misconception I had when I first did relativity many years ago, and one which I have heard in discussions with others," says Lewis. "It has generated a substantial discussion on the Wikipedia entry about black holes."
He adds that "even Einstein had a very hard time attempting to fathom just what is going on as things fall into a black hole."
- Lewis G. F. & Kwan J. et al. Publ. Astr. Soc. Australia, in press (2007).