What happens when a black hole is swallowed by another black hole?
• When two black holes approach one another, they usually follow hyperbolic paths around a common focal point, so they end up galloping off in different directions.
If there are stars around, the situation becomes more complicated and the black holes can end up orbiting one another after tossing some stars away. Such an orbit will continue almost forever, unless there are still more stars nearby that can be thrown off, removing some energy and angular momentum.
A black hole may also head straight towards another, so that it would hit its event horizon. In that case, an observer would see the two black holes getting closer and closer (assuming they’re visible), but would never see them coalesce. Instead, they would seem to slow down, get dimmer and the light from them would get redder. This is because their time slows down, so we might never see them reach the point at which one crosses the other’s event horizon, even though that happens in due course.
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Eric Kvaalen, Les Essarts-le-Roi, France
• It is not quite as exciting as you might think. The two black holes will fuse, creating a new one with a mass equal to the two constituent masses. The surface area of a black hole is related to its mass, so this will increase in proportion.
Because black holes are generally accompanied by an accretion disc of material that rotates around them, moving faster and faster as it spirals inwards, we can assume that the two discs will crash into one another before the holes meet. The end result will be a bigger disc, but when the two very hot discs hit each other, there will be an increase in the radiation given off until the new system settles down.
“We can assume their two accretion discs will crash into one another before the black holes meet”
John Anderson, Warsaw, Poland
This article appeared in print under the headline “Hole number”
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