FEATURE ARTICLE
The Sounds of Spacetime
In the biggest events in the universe, massive black holes collide with a chirp and a ring. Physicists are finding ways to listen in
Craig Hogan
Superbly Sensitive Microphones
Astronomers would love to tune in to the many-voiced soundtrack of
the cosmos and listen to what is going on everywhere. The problem
is, these vibrations, although they carry a lot of energy, are very
hard to detect. (This is related to the fact that they penetrate
anything!) Close to black holes, spacetime is highly warped, so much
so that escape is impossible if you get too close. However, the
gravitational waves that reach us from great distances away distort
space by only a tiny amount, causing a fractional stretching less
than the ratio of the black hole size to the distance away. Another
way of saying the same thing is that spacetime is the stiffest
medium there is, so even a huge amount of energy creates only tiny
vibrations. How then can we listen to them?
When a gravitational wave passes, it stretches space back and forth.
That means the distance between objects changes. For a given amount
of fractional stretching, the change in distance is bigger the
farther apart the objects are, so we want to measure tiny variations
in the distance between objects that are far apart. One exquisitely
sensitive way to detect the minuscule stretching over big distances
is with laser interferometry, the technology at the heart of the
most sensitive gravitational-wave detectors.
Laser light is a "pure color," made of waves of just one
wavelength. In an interferometer, some light from a laser is bounced
off a mirror. Any stretch in the distance to the mirror changes the
wavelength of the light. (Since it is a stretching of spacetime, it
is also okay to think of this change as due to the Doppler shift
from the motion of the mirror.) The reflected light is then combined
with some unreflected original laser light so that the two sets of
waves can interfere with each other. The light changes brightness
depending on exactly where the two sets of laser waves are in their
relative vibrations. By measuring variations in the intensity of the
light, tiny motions of the mirror can be measured to very high
accuracy, even if it is very far away. The interferometric
gravitational-wave detectors now deployed on Earth (described in
detail in Shawhan's article) can measure motions much smaller than
an atomic nucleus, over a distance of several kilometers; the future
detector planned for space, the Laser Interferometer Space Antenna
(LISA), will measure motions much smaller than an atom, over a
distance of 5 million kilometers—about 13 times the distance
to the Moon.
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