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
Vibrations of Spacetime
The sounds of the cosmos are not the familiar sounds our ears sense,
carried by vibrations in air. Space is a near-perfect vacuum, and
ordinary sound carries only where there is matter to vibrate. That's
one reason why our immediate knowledge of the universe far away from
the solar system, from prescientific astronomy up to now, comes
almost entirely from studying one form of energy: light. As James
Clerk Maxwell showed in the 19th century, light is another name for
vibrations in electrical and magnetic fields that travel through
space—at the speed of light.
(To be fair, we should not forget here other messengers from
afar—cosmic rays, neutrinos, cosmic dust, meteorites and other
matter falling to Earth from outer space—and most of all, we
should not forget the cosmic origin of all the atoms which make up
Earth and ourselves! But those are other stories.)
In contrast to the swift-traveling vibrations in electrical and
magnetic fields that we call light, the sounds of the universe are
carried by vibrations in spacetime called gravitational
waves. Albert Einstein's general theory of relativity tells us
that all forms of matter create warps in spacetime and that motions
of matter create vibrations that travel throughout space at the
speed of light. The vibrations stretch the fabric of space itself
back and forth in a way that can be detected far away. The fastest
accelerations of the densest objects, with the strongest gravity,
presumed to be black holes (which are themselves nothing but dense
knots of spacetime curvature), create the loudest vibrations. When
we can hear them, those vibrations will let us listen to huge and
often invisible cataclysms throughout the observable universe.
Gravitational waves are emitted when big masses accelerate; light is
emitted when tiny electrical charges accelerate. That means that
gravitational waves have much lower frequencies than light, and come
from totally different kinds of happenings in the universe. For
example, normal stars sitting on their own emit lots of light, from
jiggling electrons in their hot atmospheres, but almost no
gravitational radiation. At the opposite extreme, the most powerful
energy transformations in the universe, where two black holes merge
with each other and form a larger black hole, emit almost all of
their energy as gravitational waves and almost none of it as light.
Indeed, for the brief time of the merger, up to an hour or so for
the largest holes we know about, just one such merging pair emits a
thousand times more power in gravitational waves than all the stars
in all the galaxies in the visible universe, everything combined,
emit as light. So the loudest things in the universe are not the
brightest things, and vice versa; the two kinds of energy really are
like entirely different senses of what is happening out there.
As Peter S. Shawhan explained in these pages ("Gravitational Waves and the Effort to
Detect Them," July-August 2004), Einstein's theory
allows us to calculate many properties of gravitational waves. It
tells us that gravitational waves pass through anything; they
traverse the farthest reaches of spacetime, the earliest moments of
the Big Bang, to reach us. The theory tells us that ordinary pairs
of stars orbiting each other, including binaries we know about
already, should be emitting gravitational waves, and exactly how
much energy they emit. It tells us the exact mathematical shape of
the warped spacetime in black holes, which is encoded in precisely
predictable gravitational waves emitted by any object falling in.
In short, we have a definite mathematical model for the ways space
and time around us should vibrate. By eavesdropping on gravitational
waves, we can explore the whole universe in an entirely new way, and
at the same time test our fundamental ideas about how space and time behave.
There is precise indirect evidence that gravitational waves exist.
Russell Hulse and Joseph Taylor were awarded the Nobel Prize in 1993
in part for measuring the effects of gravitational-wave energy loss
on a binary pulsar system. But up to now, nobody has detected a
gravitational wave directly. As I write this, the first major
detector is a few months into its first sustained period of listening.
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