IF THERE鈥檚 a rogue black hole coming our way, we might never know about it till it hit. It鈥檚 hard to spot something just a few kilometres across and perfectly black when it鈥檚 many billions of kilometres away in space. But maybe there鈥檚 a chance if we use the Sun as a searchlight.
Although black holes are rare in our Galaxy, there鈥檚 a slim chance that a rogue black hole could come out of the depths of interstellar space and catch us unawares. 鈥淏y the time you notice, you鈥檙e sort of out of luck,鈥� says Daniel Holz at the University of California at Santa Barbara.
There is even a theory that the dinosaurs were wiped out by a black hole companion to our Sun. If such an object exists, it would approach the planets every 30 million years or so, disturbing the cloud of comets beyond Pluto and sending enough of them raining down on Earth to wreak havoc 鈥� easily enough to cause a mass extinction.
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Now Holz and veteran physicist John Wheeler of Princeton University have devised an early warning system. 鈥淲e realised that black holes produce a very distinctive signal,鈥� says Holz. Light doesn鈥檛 bounce off a black hole, but it can be whipped around by the hole鈥檚 powerful gravity and slung back the way it came. So if you were to shine a powerful flashlight at the hole, you鈥檇 see it surrounded by tiny rings of light.
The brightest light we have is the Sun, and Holz and Wheeler have worked out that modern telescopes could see the glint of returning sunlight from a black hole up to a hundred times as far away as Pluto ().
Because black holes are so rare, there is probably none close enough right now to spot with the telescopes we have. 鈥淭hat is, unless we get lucky 鈥� or unlucky 鈥� enough to have a black hole come unusually nearby,鈥� says Holz. If so, he says, this could be our first means of detecting it.
But there鈥檚 a snag. The method works best only if Earth, the black hole and the light source are all perfectly aligned. So as the Earth moves round its orbit, our solar searchlight only sweeps out a narrow slice of the sky. If the hole were coming at us from the wrong direction, we鈥檇 miss it.
A way round that problem would be to use a billion-watt laser instead of the Sun. But we are not likely to have one of them, either, until the very distant future. Perhaps that鈥檚 for the best. If we saw a marauding black hole coming at us any time soon, what would we be able to do about it?