Black holes can also be studied by listening for the "sounds" they create, a novel form of energy called gravitational waves.
calculation of the gravitational waves produced in the merger of two supermassive
black holes (Courtesy E. Seidel and W. Benger)|
Since ancient times, astronomers have used one form of energy to study the Universe. Called simply "light," it includes X rays and radio waves and all the colors of the rainbow in between. Light is made of vibrating waves of electric and magnetic fields traveling through space.
In Einstein's theory of gravity, energy can also be carried by vibrating waves of space and time, which travel at the speed of light. In the same way that black holes
are made just of space and time, gravitational waves are also "pure" space
and time. They interact very weakly with matter and penetrate anything without
losing strength. While this makes them powerful probes of extreme conditions,
it also makes them hard to detect. They interact so weakly with measuring
apparatus that only in the past few years has technology advanced to the
point that we are confident we can build equipment to detect them.
The most powerful outflows of energy in the Universe are not carried by
light but by gravitational waves emitted when two black holes orbit, collide,
and merge. In the final minutes or hours before the merging of a single
pair of supermassive black holes, a gravitational power of about 1052
watts is radiated. This is a million times more power than all the light
from all the stars in all the galaxies in the visible Universe put together,
and millions of times more powerful than the most powerful single sources
of light: gamma-ray bursts. It is possible that the Universe contains more
of this gravitational radiation than it does light.
Detecting gravitational waves will give Einstein's theory a workout it
has never had before. We know that it works pretty well in normal circumstances---without
"spacetime curvature technology" in their software, airplanes using GPS navigation
would miss their runways by miles---but gravitational waves offer much more
profound potential. They will let us listen carefully to the most violent
events in the Universe, the collision and mergers of black holes. What goes
on there is a swirling knot of spacetime interacting mostly with itself.
A black hole merger can also briefly expose to observation the singularity
at the heart of the black hole, where Einstein's theory must fail. The sounds
of the Universe will tell us how well Einstein's ideas still work in these
extreme conditions. They will also allow us to penetrate times and places
impossible to see with ordinary light, such as the birth of our Universe.
They might reveal startlingly violent events, such as the formation of our
three-dimensional space from an original space with more dimensions.
Gravitational waves produce tiny jiggles between masses that are floating
freely in space, isolated from all forces other than gravity. The distances
between the masses can be monitored using laser interferometry. An early generation of such systems has now been deployed on the ground---the NSF-funded Laser Interferometer Gravity-wave Observatory (LIGO)
in the US and similar systems worldwide. It is hoped that these systems
will make the first detection of gravitational waves from some sources of
high-frequency waves. The Beyond Einstein Great Observatory LISA
will operate in a broad band at much lower frequency. It will detect entirely
different sources in great numbers and with exquisite precision.
The most powerful gravitational waves come from quickly-changing systems
with very strong gravity, so LISA's strongest signals will probably be tones
from black holes spiraling into other supermassive black holes. LISA will
also detect for the first time gravitational waves from calibrator sources
(such as orbiting pairs of white dwarf stars) that have been studied by optical
LISA will break ground for the new science of gravitational wave astronomy. The vision mission Big Bang Observer will extend the reach of gravitational wave astronomy towards its ultimate limit---detecting the quantum noise from the inflationary Universe.