Big Bad Black Holes

I was sitting with some fellow tourists on a three-day coach tour of Ireland yesterday when the topic of what I do for a living came up. After being briefly mistaken for an astrologer (I really should start charging money for my services), my mother-in-law Christine mentioned my interest in black holes.

(Aside: black holes are something of an in-joke for my in-laws. Christine was asked by an work mate what her son-in-law does, and the conversation went something like this:

Christine: He’s measuring the black hole, or something.

Office mate: Well it’s about time someone did that!)

Returning to Ireland, a fellow tourist remarked that black holes “are bad things, because they suck everything up”. I’ve encountered this opinion a number of times before. The mental image of a black hole as a giant cosmic vacuum cleaner, destroyer of worlds, is surprisingly common. But the image, as any astronomer will tell you, is wrong.

Black holes should not be thought of as vacuum cleaners, sucking things in, but instead as prisons – you can’t escape if you are inside, but so long as you are outside the event horizon you are OK. If the sun were to collapse into a black hole right now, the Earth would not be sucked into oblivion – it would continue orbiting as before, around a distant black void. Because black holes are extremely compact, they are small targets: if you find yourself in empty space falling directly toward its centre, you’re in big trouble, but a comparatively small blast to the left or right from your rocket pack will cause you to swing past the black hole like a comet swooping past the sun. So black holes don’t entirely deserve their reputation as “cosmic baddies”. Just stay in a nice, safe orbit and no-one gets hurt.

Slightly disturbing, then, is a paper appearing recently on astro-ph. Laura Blecha and Abraham Loeb report on a study of black hole mergers, and mention a few interesting facts. When two black holes collide, they radiate gravitational waves. These gravitational waves are usually emitted asymmetrically, and since the waves carry momentum, the final, larger black hole can recoil in the opposite direction, like a rocket expelling fuel in one direction and consequently being propelled in the other direction. The recoil speed can be up to 4000km/s, which is about 14 million km/h. Fast enough to escape their host galaxy.

The bad news is: there may be a couple of billion billion billion billion tonne black holes roaming through the universe at over 10 million km/h. The good news is: even if one were to pass through our galaxy, the probability of it passing through the solar system is about one in a billion billion. Still, I wouldn’t mention it to someone who asks you for their horoscope.

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