THE NEXT BIG STEPS FOR SCIENCE

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THE EXISTENCE OF

BLACK HOLES

The idea of ‘dark stars’ that gobble up any planets in their path

dates back to the 18th century. But, as Brian Clegg explains, it

wasn’t until 1964 that hard evidence of their existence emerged

B

lack holes have escaped from

astrophysics into the everyday

imagination. But there are gaps

in our knowledge of their nature and

even, possibly, their existence.

Black holes were born from theory,

not observation. We have known about

conventional stars for as long as we’ve

been able to look at a clear night sky

but no-one ever saw a black hole.

Instead, t hey 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 in the 18th century. The

man who dreamed up what he called

‘dark stars’ was John Michell, a

scientist who 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

k new t hat when a bullet is shot

st raight up into t he air, it has just two

forces acting on it once it leaves t he

gun: air resistance and gravity. As

it gets higher, bot h of t hese forces

weaken. The air gets thinner and,

as New ton had made clea r, 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

t ravel as fast as 300 met res per second.

But despite this impressive velocity,

t he forces acting to slow it brought t he

bullet back dow n to Ea r t h. 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 t his 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.

Light conversation

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

the speed of light to be around

220,000km/s. In the following 100

yea rs, t his f igu re was measu red

more accu rately so t hat 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 sta r so vast t hat 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. Michell’s theory was 5

Ole Rømer calculated a speed for light,

settling the dispute over whether it

travelled instantly or just very quickly