330
would be the same as the speed
of light, and that the ripples in
spacetime would move outward
in all directions. The intensity of
these ripples diminishes rapidly
with distance (by a square of the
distance), so detecting a distinct
gravity wave from a known object
far out in space would require
a very powerful source of waves
and a very sensitive instrument.
Laser-guided
As its name suggests, LIGO
employs a technique called laser
interferometry. This makes use
of a property of waves called
interference. When two waves
meet, they interfere with one
another to create a single wave.
How they do this depends on
their phase—the relative timing
of their oscillations. If the waves
are exactly in phase—rising and
falling perfectly in sync—they
will interfere constructively,
merging to create a wave with
double the intensity. By contrast,
if the waves are exactly out of
phase—one rising as the other
falls—the interference will be
destructive. The two waves will
merge and cancel one another
out, disappearing completely.
LIGO’s source of waves is a laser,
which is a light beam that contains
a single color, or wavelength,
of light. In addition, the light in
a laser beam is coherent, which
means that its oscillations are all
perfectly in time. Such beams
can be made to interfere with
one another in very precise ways.
The laser beam is split in two
and the resulting beams are sent
off perpendicular to one another.
They both hit a mirror and bounce
straight back to the starting point.
The distance traveled by each
beam is very precisely controlled
so that one has to travel exactlyGRAVITATIONAL WAVES
half a wavelength farther than
the other (a difference of a few
hundred billionths of a meter).
When the beams meet each other
again, they are exactly out of phase
as they interfere, and promptly
disappear—unless a gravitational
wave has passed through space
while the beams were traveling.
If present, a gravitational wave
would stretch one of the laser
tracks and compress the other, so
the beams would end up traveling
slightly altered distances.Noise filter
The laser beams are split and sent
on a 695-mile (1,120-km) journey up
and down LIGO’s 2.5-mile (4-km)
long arms before being recombined.
This gives LIGO the sensitivity to
detect minute perturbations in space
that add up to a few thousandths
of the width of a proton. With the
distances put very slightly out of
sync, the interfering beams would
no longer cancel each other out.
Instead, they would create a
flickering pattern of light, perhaps
indicating a gravity wave passing
through LIGO’s corner of space.
The difficulty was that such
a sensitive detector was prone to
distortions from the frequent seismic
waves that run through Earth’s
surface. To be sure that a laser flickerRai Weiss and Kip Thorne
LIGO is a collaboration between
Caltech and MIT, and also
shares its data with a similar
experiment called Virgo, which
is running in France and Poland.
Hundreds of researchers have
contributed to the discovery of
gravitational waves. However,
there are two people, both
Americans, who stand out
among them all: Rainer “Rai”
Weiss (1932–) and Kip Thorne
(1940–). In 1967, while at MIT,
Weiss developed the laser
interferometry technique usedby LIGO, working from the
initial ideas of Joseph Weber,
one of the inventors of the laser.
In 1984, Weiss cofounded LIGO
with Thorne, a counterpart at
Caltech, who is a leading expert
on the theory of relativity. LIGO
is the most expensive science
project ever funded by the US
government, with a current cost
of $1.1 billion. After 32 years of
trying, in 2016 Weiss and Thorne
announced their discovery of
gravitational waves at a news
conference in Washington, D.C.No signalNormal
situationSignalGravitational
wave detectionWith no gravitational waves, LIGO’s light waves cancel one
another out when they are recombined. Gravitational waves stretch
one tube while compressing the other, so that the waves are no
longer perfectly aligned, and a signal is produced.