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
Twenty Octaves of Spacetime Sound
The reason to build interferometers both on the ground (LIGO, the
Laser Interferometer Gravitational-Wave Observatory, and others
around the globe) and in space (LISA) is that they observe very
different frequencies of gravitational waves, in the same way that
optical and radio telescopes observe different frequencies of
electromagnetic radiation. The frequencies span the same range as a
piano keyboard with 20 octaves of sound. That means that they will
detect very different kinds of things bumping around the universe.
Ground-based detectors listen to spacetime wiggles at audible
frequencies, in a broad band around 100 cycles per second, or
hertz—a bit over three octaves, or about the range of a
versatile soprano. These frequencies come screaming from neutron
stars and black holes with around the mass of single stars; that's
how fast they spin and orbit each other when they are at their
loudest, just before their catastrophic mergers. LIGO will hear
these death rattles of stars.
In space, detectors can listen to frequencies a million times lower.
Those deep rumbling noises, in a broad band around a millihertz,
come from catastrophic mergers of black holes much bigger than those
LIGO hears—millions of times the mass of a single star. They
can also come from binary stars that are not so massive and that are
more slowly and distantly orbiting each other. Indeed, binary stars
are so common that their gravitational waves pile together and are
the main source of "noise" for LISA at some frequencies.
For LISA, the universe is a bustling, noisy place. As soon as it
turns on, there will be a cacophony of sounds; the science challenge
will be to distinguish them from one another, like trying to
understand conversations at a cocktail party where everyone is
talking at once.
The LIGO and LISA styles of observing are quite different. LIGO is a
bit more like bird-watching. It lies in wait for the rare songs of
merger events which tend to be brief, high intensity flurries of
activity from the final coalescence of stars. These events are
happening all the time in the universe, but we don't know exactly
how often, or when exactly one will happen nearby enough for LIGO to
hear it. Depending on the rate of events, and on our luck,
LIGO—which began its first extended data run at full
sensitivity earlier this year—may detect gravitational waves
sometime in the next year or the next decade. When LISA flies,
perhaps a decade from now, it will detect gravitational waves from
some known sources immediately. From then on astrophysicists and
cosmologists will be occupied with sorting out a wide variety of
known and unknown cosmic noises from one another.
What will we learn from gravitational radiation when it is detected?
We know we will learn many new things about what is happening in the
universe, ushering in a new way of doing astronomy. We also know we
will study the physics of gravity and spacetime in a completely new
way; the results might either confirm what we think we
know—that is, Einstein's theory of spacetime—or they
might tell us something new about how spacetime behaves. We may also
find something radically new, such as entirely new states of mass
and energy that we have only guessed at until now. Such a discovery
could illuminate some of the deepest mysteries of physics, such as
the unification of ideas about space and time with ideas of energy
and quanta, perhaps in the form of a string theory.
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