Last year, astronomers were fascinated by a runaway asteroid passing through our Solar System from somewhere far beyond. It was moving at around 68 kilometres per second, just over double Earth’s speed around the Sun.

Imagine if it had been something much bigger and faster: a black hole travelling at more like 3,000km per second. We wouldn’t see it coming until its intense gravitational forces started knocking around the orbits of the outer planets.

This may sound a bit ridiculous – but in the past year several lines of evidence have come together to show such a visitor is not impossible. Astronomers have seen clear signs of runaway supermassive black holes tearing through other galaxies, and have uncovered evidence that smaller, undetectable runaways are probably out there too.

The story begins in the 1960s, when New Zealand mathematician Roy Kerr found a solution of Einstein’s general relativity equations that described spinning black holes. This led to two crucial discoveries about black holes.

First, the “no-hair theorem”, which tells us black holes can be distinguished only by three properties: their mass, their spin and their electric charge.

For the second we need to think about Einstein’s famous formula E = mc ² which says that energy has mass. In the case of a black hole, Kerr’s solution........

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