The Science Book

(Elle) #1

89


Matter swirls around a black hole in
a doughnut-shaped “accretion disk”
before being sucked in. Heat in the
swirling disk causes the hole to emit
energy—as narrow beams of X-rays.


See also: Henry Cavendish 78–79 ■ Isaac Newton 62–69 ■ Albert Einstein 214–21 ■ Subrahmanyan Chandrasekhar 248 ■
Stephen Hawking 314


EXPANDING HORIZONS


relativity, which described gravity
as a result of the curving of space-
time. Einstein showed how matter
can wrap space-time around itself,
making a black hole within a region
called the Schwarzschild radius,
or event horizon. Matter—and also
light—can enter it, but cannot
leave. In this picture, the speed of
light is unchanged. Rather, it is the
space the light travels through that
changes, but Michell’s intuition
now had a mechanism by which
the velocity of light would at least
appear to diminish.


From theory to reality
Einstein himself doubted whether
black holes existed in reality. It was
not until the 1960s that they began
to acquire general acceptance as
indirect evidence of their existence
grew. Today, most cosmologists
think that black holes form when


massive stars collapse under their
own gravity, and grow as they
assimilate ever more matter, and
that a giant black hole lurks at the
center of every galaxy. Black holes
pull matter in, but nothing escapes,
other than faint infrared radiation,
known as Hawking radiation after
Stephen Hawking, the physicist
who proposed it. An astronaut
falling into a black hole would
feel nothing and notice nothing

unusual on the approach to the
event horizon, but if he or she
dropped a clock toward the black
hole, the clock would appear to
slow down, and approach but
never quite reach the event horizon,
gradually fading from sight.
Problems with the theory still
exist, however. In 2012, physicist
Joseph Polchinski suggested that
effects at the quantum scale would
create a “firewall” at the event
horizon that would burn any
astronaut falling through it to a
crisp. In 2014, Hawking changed
his mind and concluded that black
holes cannot exist after all. ■

John Michell


John Michell was a true
polymath. He became professor
of geology at the University of
Cambridge in 1760, but also
taught arithmetic, geometry,
theology, philosophy, Hebrew,
and Greek. In 1767, he retired
to become a clergyman, and
focused on his science.
Michell speculated on the
properties of stars, investigated
earthquakes and magnetism,
and invented a new method for
measuring the density of Earth.
He built the apparatus for

“weighing the world”—a
delicate torsion balance—but
died in 1793 before he could use
it. He left it to his friend Henry
Cavendish, who performed
the experiment in 1798, and
obtained a value close to the
currently accepted figure.
Ever since, this has somewhat
unfairly been known as “the
Cavendish experiment.”

Key work

1767 An Inquiry into the
Probable Parallax and
Magnitude of the Fixed Stars

Black holes ain’t so black.
Stephen Hawking
Free download pdf