The Science Book

314

See also: John Michell 88–89 ■ Albert Einstein 214–21 ■

Subrahmanyan Chandrasekhar 248

I

n the 1960s, British physicist

Stephen Hawking was one

among several brilliant

researchers who became interested

in the behavior of black holes.

He wrote his doctoral thesis

on the cosmological aspects

of a singularity (the point in

space-time at which all of a black

hole’s mass is concentrated),

and drew parallels between the

singularities of stellar-mass black

holes and the initial state of the

universe during the Big Bang.

Around 1973, Hawking became

interested in quantum mechanics

and the behavior of gravity on a

subatomic scale. He made an

important discovery—that despite

their name, black holes do not just

swallow up matter and energy but

emit radiation. So-called Hawking

radiation is emitted at the black

hole’s event horizon—the outer

boundary at which the black hole’s

gravity becomes so strong that not

even light can escape. Hawking

showed that in the case of a

rotating black hole, the intense

gravity would give rise to the

production of virtual, subatomic

particle-antiparticle pairs. On the

event horizon, it would be possible

for one element of each pair to

be pulled into the black hole,

effectively boosting the survivor

into a sustained existence as a real

particle. The result of this to a

distant observer is that the event

horizon emits low-temperature

thermal radiation. Over time,

the energy carried away by this

radiation causes the black hole to

lose mass and evaporate away. ■

BLACK HOLES

EVAPORATE

STEPHEN HAWKING (1942–)

IN CONTEXT

BRANCH

Cosmology

BEFORE

1783 John Michell theorizes

objects whose gravity is so

great that they trap light.

1930 Subrahmanyan

Chandrasekhar proposes

that a collapsing stellar core

above a certain mass would

give rise to a black hole.

1971 The first likely black hole

is identified—Cygnus X-1.

AFTER

2002 Observations of stars

orbiting close to the center

of our galaxy suggest the

presence of a giant black hole.

2012 American string theorist

Joseph Polchinski suggests

that quantum entanglement

produces a super-hot “firewall”

at a black hole’s event horizon.

2014 Hawking announces

that he no longer thinks

black holes can exist.

My goal is simple. It is a

complete understanding of the

universe, why it is as it is and

why it exists at all.

Stephen Hawking