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
Of all the habits shared by ancient and modern people, stargazing may
be the most serene. When we look up at a clear night sky, or view
the fabulously beautiful pictures of stars and galaxies coming from
the Hubble Space Telescope, we enter awestruck and humble into a
magical realm that has the sacred hush of an ancient cathedral or a
great art museum. We almost feel we should keep our voices down and
turn off our cell phones out of respect.
So how would you feel if suddenly, as you quietly admired a dark and
starry sky, you heard the stars making all kinds of crazy noises?
After the initial shock of being jolted out of your poetic reverie,
I think you would find that the universe felt much more immediate,
present, real and alive. It is one thing to see flashes of lightning
in the distance, quite another to be shaken by the sound of rolling
thunder. Hearing the universe is more like touching than looking.
Happily, astronomers are finding ways to do that—to feel as
well as see the active universe around us.
Einstein's theory of spacetime tells us that the real universe is
not silent, but is actually alive with vibrating energy. Space and
time carry a cacophony of vibrations with textures and timbres as
rich and varied as the din of sounds in a tropical rain forest or
the finale of a Wagner opera. It's just that we haven't heard those
sounds yet. The universe is a musical that we've been watching all
this time as a silent movie.
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.
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.
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.
Chirps, Rings, Black Holes and Binaries
The most spectacular LISA events will be huge, roaring events when
two very big black holes, somewhere in the universe, spiral together
and merge into a single bigger black hole. The final hole weighs a
lot less than did the original two, and the difference in mass is
radiated as gravitational waves. As I mentioned above, in terms of
radiated power, just one of these mergers far outshines everything
else in the universe combined.
These events will be fun to listen to. They sing a song called a
"chirp." For a long while the orbiting black holes emit a
nearly constant set of tones, like a single note on a violin that
gets higher only very slowly. Then, just before the holes merge, the
note quickly gets higher and louder at the same time, like a
virtuoso's flourish. Finally, after the merger, there is a
"ringdown" when the sound rapidly goes away, like the
reverberations in a vast concert hall.
We think mergers happen pretty frequently somewhere in the universe.
Most galaxies have a massive black hole right in the middle, and
every galaxy has swallowed or merged with another galaxy more than
once in the past; that is how galaxies grow. When two galaxies
merge, their two massive black holes sink to the middle of the new
galaxy because they lose energy to stars and gas by gravitational
interactions. Finally the holes find each other and merge together.
There are roughly ten billion galaxies to listen to, and if each of
them does this just once during the ten billion years of active
galaxy assembly, that's about one event every year, on average.
But most massive black holes don't have to wait so long to swallow
something; they are snacking all the time on the smaller occupants
of the galaxies around them. The big holes live in dense swarms of
stars in the centers of galaxies, and every now and then one of the
stars gets too close to its neighbor for its own good.
Sometimes a very compact stellar remnant—a neutron star or a
stellar-mass black hole—finds itself in a death dance, where
it whirls in and out and around a massive black hole many times
until it finally plunges into the oblivion of the event horizon and
disappears from view. All the time it is doing that dance, it emits
gravitational radiation. The gravitational radiation records a
history of the orbit and makes a detailed map of the spacetime
around the massive black hole. Remember that the black hole is made
of gravity alone, and Einstein's theory tells us what the structure
of black holes ought to be. This kind of event will tell us a lot
about the structure of black holes themselves—how spacetime
ties itself into the stable spinning knots we call black holes.
LISA also has some sure targets. Our galaxy is full of stars. Stars
have a life cycle—they only last as normal stars until their
hydrogen fuel runs out—and many of them have burned out and
died. Most of the time the remnant is a very small and dense ember
such as a white dwarf or a neutron star, and much of the time,
because stars tend to form in binaries, the remnant is in a binary
system with a similar companion. Those remnants that orbit each
other once every few minutes to an hour radiate at frequencies that
LISA can hear.
In fact we already know of a few nearby binaries, discovered by
astronomers using normal telescopes, that LISA will be able to hear.
We call these "calibration binaries" because we already
have a pretty good idea of many of their properties, such as their
frequency and distance. After LISA, we will know a lot
more—the gravitational waveform will tell us their inclination
and much about their detailed masses and other properties. The
nearby binaries will also reassure us that LISA is actually working
and detecting gravitational waves. Thousands of more distant
binaries blend into a noisy backup chorus that will also be heard as
soon as LISA turns on.
Mapping the Distant Universe
Using the known physics of gravitational-wave emission, LISA will
let us use gravitational waves as a tool to map distances to
galaxies in the distant universe. By measuring the chirp of a
distant binary black-hole merger—how long it lasts until its
tone changes—we can tell how massive the merging black holes
are. By measuring the loudness, we can then tell how far away the
holes are. This is a completely new way to map the cosmic expansion
that can be more precise and direct than other techniques we have,
insofar as the physics of black-hole mergers is completely understood.
A tricky aspect of this project is that astronomers need to actually
identify the host galaxy with visible light (because we need an
independent measure of the redshift, the amount by which wavelengths
have been stretched by the expanding universe). We don't know for
sure that this will be possible. For loud binaries, LISA will
sometimes let us measure the direction the sound is coming from, by
combining the data from different times of year to act as a stereo
microphone. The best precision is about one degree of arc, narrowing
it down to a patch of sky which includes tens of thousands of galaxy
images. It is reasonable to hope that the very special galaxy among
these with a merging pair of black holes will look different enough
for us to identify it—perhaps by time-varying nuclear activity
in optical light, perhaps by changes in shape in response to the
disturbance created by a recent galaxy merger.
Accurate distance mapping with supernovae led to the discovery of
the cosmic dark energy that is accelerating the expansion of the
universe; measuring distances better with gravitational waves will
be a way to learn more details about that new force of nature.
The String Section
Although they sound exotic, all of the sources just discussed, even
the huge binary black-hole mergers, are actually expected to happen
in the normal course of events according to our current
understanding of the universe. But what about really weird stuff?
What new, unexpected things might really knock our socks off?
Physics now extends its reach back to the early moments of the Big
Bang, to incredibly high temperatures, even to the inflation epoch
when the cosmic expansion got the kick that made it as big as it is
today. If you go back far enough, even space and time were not like
they are today. A still-untested quantum version of Einstein's
theory, string theory, suggests that space has 10 dimensions, many
of which are highly curved or compact, and that all the particles of
matter, and maybe even spacetime and gravitational waves, are
ultimately composed of tiny quantum strings. The problem with string
theory is that despite its seemingly miraculous ability to tie ideas
from different parts of physics and mathematics together, nobody has
yet found any real-world evidence for it. Might LISA hear any
whispers from that new physics?
There is at least one kind of new, truly "stringy" object
that, if it exists at all, fills the universe with gravitational
radiation that LISA might hear. The tiny quantum strings might also
form into cosmic superstrings, which are microscopically
thin but astronomically long.
In the very early universe, a dense network of them forms by rapid
quenching as the universe expands. This formation process resembles
the cracking of cold ice cubes when you drop them suddenly into
water, the mottled patterning of alloy domains in a finely forged
Samurai sword or the trapped vortex lines that sometimes form in
sudden cooling of superconductors, superfluids or liquid crystals.
As the universe expands further, the strings unravel and rush around
at almost the speed of light; when they cross they can exchange
partners, spawning closed loops of string. A large population of
these loops accumulates and doesn't easily disappear. The loops
thrash around but are almost stable and remain around for a long
time, shrinking only slowly. Indeed the main way the loops lose
energy is by gravitational waves! If we estimate the strength of
gravitational waves, they turn out to be easily detectable by LISA
for some scenarios suggested by string-theory inflation.
The most interesting stringy events from these loops are rather rare
occasions when an unusually nearby loop beams gravitational waves in
our direction by a sort of whipping action, or cusp catastrophe. The
motion of the string for an instant, in one place, formally
approaches the speed of light, and as this moment approaches, the
gravitational waves are beamed and amplified. Such bursts, if
detected, would be a rich source of data and a completely new window
on how string theory works in the real world.
It's also possible that we might see gravitational waves directly
from the early universe, possibly from the end of inflation when the
fields driving the Big Bang converted their energy into normal
light, matter and antimatter, or at a later phase transition when
light and matter spawned the excess of matter over antimatter that
became our atoms. Gravitational waves are so penetrating that they
reach us from the entire history of the universe, right back to the
start of the Big Bang.
Picometers over Gigameters
When will LISA fly; when can we don our earbuds and listen to what's
happening out there? The instrument is challenging to build, but a
team of scientists and engineers from the United States and Europe
think they can do it.
The basic LISA concept is simple. The heart of the system, a
gold/platinum cube, floats freely within each spacecraft, not
touching anything. The cube is protected from all forces except
gravity; the spacecraft very gently senses its position and
maneuvers with tiny thrusters to avoid running into it. Laser light
reflects off the cubes and is sent by telescopes, over the 5 million
kilometers between the multiple LISA spacecraft, to measure the tiny
changes in distance between the cubes caused by gravitational waves.
The measured changes in distance are given by the fractional stretch
in spacetime, 10-23 times smaller than the distance
between them, or around 0.05 picometers. That distance is much
smaller than an atom—it is almost as small as the
nucleus of an atom.
It seems incredible to contemplate building an instrument that will
measure distances far larger than the distance to the Moon, to an
accuracy far smaller than a single atom. Among the many technical
challenges in making this work, a major one is to create an
environment for the cubes that is free of all but gravitational
forces. The spacecraft surrounding the mass must somehow sense its
position, without disturbing it, and follow it around as it follows
the wiggles of spacetime alone. The most sensitive accelerometers on
the planet—torsion balances that have also been used to search
for tiny forces from extra dimensions and new shapes to
gravity—are helping to find ways of minimizing forces.
Of course, one reason LISA goes into space is because of all the
gravitational noise on Earth. To test technology to the exquisite
precision required, especially after undergoing the rigors of rocket
launch, we must send machines into space. A satellite called LISA
Pathfinder will launch in a few years, to check the most sensitive
LISA technologies that can't be tested on Earth. It is just one
satellite, so it won't be able to detect gravitational waves, but
the proof masses and sensors on board, and the tiny micro-newton
thrusters that allow the spacecraft to maneuver delicately, will
have the same design as LISA. Engineering prototypes of these
systems already exist. As far as we know, no fundamental technology
hurdles exist to building LISA.
The actual launch of LISA itself is still many years away and will
take substantial and sustained commitments from the science and
engineering communities and from the agencies and taxpayers that
fund them from both sides of the Atlantic. At over a billion
dollars, it is a major undertaking, although it is not unprecedented
for an important science project: For example, this budget is still
much smaller than that of the largest particle accelerators, such as
the new Large Hadron Collider at the European particle-physics
laboratory CERN, or space telescopes, such as the Hubble Space
Telescope. It is unusual for the first step in such a new area to be
such a big one, but then it's also unusual for a science project to
probe the universe in such an entirely new way.
Bibliography