BBC_Knowledge_2014-06_Asia_100p

to everyone’s surprise, it turned out there was

a slumbering supermassive black hole in the

heart of pretty much every galaxy. The Milky

Way, for instance, contains a moderate version,

Sagittarius A*, which is a mere 4.3 million

times the mass of the Sun.

Something from nothing

In 1974, Hawking discovered something

amazing about the horizon surrounding

a black hole. Black holes, in addition to

powering some of the most energetic objects

in the Universe, also provide theorists with

a unique window on physics in the most

extreme conditions imaginable. Quantum

theory, our very best description of the

microscopic world of atoms and their

constituents, permits a submicroscopic

particle and its ‘antiparticle’ to pop into

existence, literally out of nothing – just

as long as the pair ‘annihilate’ each other

and vanish within a split-second. Hawking

found that, just outside the horizon, it is

possible for one particle of a pair to fall

into the hole while the other escapes. With

nothing to annihilate with, the left-behind

particle is endowed with a permanent

existence. Such particles, streaming away

from the horizon in all directions, comprise

the Hawking radiation.

The energy to create such particles

must come from somewhere. In fact, it

comes from the gravitational energy of the

black hole, causing the hole to shrink and

eventually vanish altogether. This poses

a big problem because a cornerstone

of physics is that information cannot

black hole, Cygnus X-1, was discovered
in 1971 by the Uhuru x-ray satellite. Now
about a dozen black holes are known in the
Milky Way, though there are believed to be
millions more.
In addition to stellar-mass black holes,
however, nature boasts another, even
more dramatic type. The first hint of
their existence came with the discovery of
‘quasars’ by Dutch-American astronomer
Maarten Schmidt in 1963. Quasars typically
pump out the energy of a hundred normal
galaxies from a volume smaller than our
Solar System. The only plausible source
of their prodigious light output is matter
heated to incandescence as it is sucked down
into a black hole. But this is no normal

“If Hawking is right,

black holes are not

what we thought

they were; they may

not even exist”

Three light rays from three torches come close to a black hole. The furthest one escapes the hole,

the nearest passes the event horizon and is sucked in, while the middle ray orbits indefinitely

The centre of our Galaxy glows with x-rays

produced by a supermassive black hole known

as Sagittarius A*

ANATOMY OF A

BLACK HOLE

Singularity

Event horizon

Light is captured

Light orbits

indefinitely

Light escapes

Schwarzschild

radius

What can happen to light if it approaches

one of these cosmic sinkholes

black hole – it would be a black hole up to

30 billion times the mass of the Sun.

Initially, such ‘supermassive’ black holes

were thought to power only the unruly 1 per

cent of galaxies known as ‘active galaxies’. But,