Discovering the black hole

Black holes have escaped from astrophysics into the everyday imagination. Yet the gaps in our knowledge of their nature and even, possibly, their existence are considerable.

Black holes were born from theory, not observation. We have known about conventional stars for as long as we’ve been able to look up at a clear night sky.

But no one ever saw a black hole. Instead, they were predicted to exist at a time when there was no way of checking whether there was any such thing out there. And that prediction happened not once, but twice.

The first inspired thinking on the matter was back in the 18th Century. The man who dreamed up what he called ‘dark stars’ was John Michell, a Cambridge scientist who later became a clergyman. It was from his rectory that he came up with the concept, combining two key ideas of the latest science at the time.

One was escape velocity. Michell knew that when a bullet is shot straight up into the air, it has just two forces acting on it once it leaves the gun – air resistance and gravity.

As it gets higher, both of these forces weaken. The air gets thinner and, as Newton had made clear, gravity’s attraction drops off with the square of the distance between the centres of the bodies involved – in this case, the bullet and the Earth.

A typical bullet from the black powder guns of Michell’s day could travel as fast as 300 metres per second. But despite this impressive velocity, the forces acting to slow it brought the bullet back down to Earth. Michell, though, knew that a bullet travelling about 37 times faster would be able to overcome the Earth’s attraction and fly off into space. It would have achieved escape velocity.

He combined this idea with a discovery from the 1670s when Danish astronomer Ole Rømer realised that an apparent variation in the timing of Jupiter’s moons was caused by the varying time that light took to reach us from the planet.

Ever since ancient times, there had been arguments over whether light travelled instantly, or just extremely quickly.

Rømer found evidence for a measurable speed, as the changing relative positions of Jupiter and Earth in their orbits varied the time that light took to reach us. He calculated light’s speed to be around 220,000km/s.

In the following 100 years, this figure was measured more accurately so that Michell was working with something closer to our current 300,000km/s. But the specific value didn’t matter – the point was that light had a speed.

Combining the two concepts of escape velocity and light having a finite speed, Michell wondered what would happen if a massive star had an escape velocity that was above the speed of light. The more mass in a body, the higher its escape velocity.

Therefore, in principle, there could be a star so massive that even light would not escape from it. Such a ‘dark star’ would have to be immense. Even though the escape velocity from the surface of the Sun, for instance, is over 600km/s, it is still far lower than the speed of light.

Science Focus