The Astronomy Book

331

was not an earth tremor, two
identical detectors were built at
opposite ends of the United States:
one in Louisiana, the other in
Washington state. Only signals
registering on both detectors were
gravitational waves (the signals
are in fact 10 milliseconds apart—
the time it takes for light, and
gravitational waves, to travel from
Louisiana to Washington). Ligo
operated from 2002–2010 with no
success, then started up again in
2015 with enhanced sensitivity.

THE TRIUMPH OF TECHNOLOGY

Colliding black holes

On September 14, 2015, at 9:50:45

GMT, two black holes a billion light-

years away collided and unleashed

huge warps in the fabric of space.

In fact, this event occurred a billion

years ago but it had taken that long

for the ripples they had released

to reach Earth—where they were

detected by both LIGO detectors.

The researchers took another few

months to check their result and

went public in February 2016.

The search is now on for more

gravitational waves, and the best

place to do it is from space. In

December 2015, the spacecraft

LISA Pathfinder was launched.

It is headed to an orbit at L1, which

is a gravitationally stable position

between the sun and Earth.

There, the spacecraft will test

laser interferometry instruments

in space, in the hope that they

can be used in an ambitious

experiment called eLISA (evolved

Laser Interferometer Space

Antenna). Provisionally scheduled

for 2034, eLISA will use three

spacecraft triangulated around

the sun. Lasers will be fired

between the spacecraft, making

a laser track 2 million miles

(3 million km) long that is many

times more sensitive to

gravitational waves than LIGO.

The discovery of gravitational

waves has the potential to transform

astronomers’ view of the universe.

The patterns in the fluctuations

in the light signals from LIGO and

future projects will produce new

information, providing a detailed

map of mass across the universe. ■

Gravitational waves will

bring us exquisitely accurate

maps of black holes—maps

of their spacetime.

Kip Thorne

LIGO’s precision instruments must
be kept completely clean. Maintaining
the purity of the laser beams is one
of the project’s biggest challenges.

LIGO splits one beam of laser light and sends beams

down two tubes at 90° to each other. To prevent unwanted

interference, the tubes are vacuums at one trillionth of the

pressure of Earth’s atmosphere. LIGO also has to make

adjustments to allow for the tidal pull of the sun and the moon.

Mirror Mirror

Beam splitter

Light detector

Laser

source

2.5-mile (4-km) tube 2.5-mile (4-km) tube