The binary system of two neutron stars discovered by Hulse and
Taylor gives excellent indirect evidence for gravitational radiation,
but might it be possible to detect this radiation directly? Physicists
and astronomers are anxious to find out. If gravitational radiation
is detected, it will give us a new way to learn about violent events
in the universe.
Whenever a solar mass or more is accelerated to high speeds, it
is possible for large amounts of gravitational radiation to be generated.
It would happen, for example, when two neutron stars collide, as
will eventually happen in the Hulse-Taylor binary system. It could
also happen from when the core of a massive star collapses just
prior to a supernova. Events like these are largely hidden from
ordinary telescopes, because neutron stars are difficult to observe
directly and the core of a massive star concealed by the star's
opaque outer layers. If they could measure the gravitational radiation
emitted from such cataclysmic events, astronomers would be able
to prone within the cataclysm itself.
Unfortunately, even the most powerful bursts of gravitational radiation
are very weak and hard to detect. To appreciate how weak gravitational
waves are, imagine two electrons separated by a short distance.
They each possess mass and charge, so these electrons exert both
gravitational and electric forces on each other. If these two electrons
are made to wiggle back and forth, they will radiate both gravitational
and electromagnetic waves. But since the gravitational force between
the electrons is only about 10 to the power of -42 times as strong
as the electric force, the resulting gravitational waves carry only
10 to the power of -42 times as much energy as the electromagnetic
Undaunted by the challenge, in the 1960s, Joseph Weber at the University
of Maryland constructed gravitational-wave antennas by gluing sensitive
crystals onto a large aluminum cylinder. Because gravitational waves
are ripples in the geometry of space, the cylinder should vibrate
slightly when a gravitational wave passes through it. If this should
happen, the crystals would produce an electrical signal that can
be amplified and recorded. While no confirmed detections of gravitational
radiation have yet been made, several research groups around the
world continue to develop gravitational-wave detectors based on
Weber's pioneering principal.
Even more sensitive gravitational-wave antennas are now under construction.
Instead of an aluminum cylinder, these use lasers to look for tiny
vibrations of mirrors. The LIGO project (short for Laser Interferometer
Gravitational-wave Observatory) uses two antennas thousands of kilometers
apart, one near Baton Rouge, Louisiana, and the other near Richland,
Washington. Each consists of an L-shaped vacuum pipe with arms 4
km long for the laser beams. Mirrors mounted at the ends of the
pipes reflect the laser beams back and forth. If a gravitational
wave passes by, the mirrors should move by an infinitesimally small
but measurable distance, changing the length of the light path.
These changes will be detected by combining the laser beams from
the two arms of the antenna. By having two antennas located thousands
of kilometers apart, the LIGO scientists will be able to distinguish
gravitational waves coming from space (which will be detected by
both antennas) from false signals, such as seismic activity (which
will be detected by only one antenna).
When LIGO becomes fully operational after the year 2000, it should
be able to detect bursts of gravitational waves from colliding neutron
stars or the collapsing cores of supernovae as far away as 2 x 10
to the power of 7 parsecs (about 7 x 10 to the power of 7 light
years). This volume of space is so huge and contains so many galaxies
that astronomers might be able to detect a burst of gravitational
waves as frequently as once a year. A similar gravitational-wave
observatory, called VIRGO, is under construction outside Pisa, Italy,
by a French-Italian consortium.
The European Space Agency has proposed an even more ambitious gravitational-wave
antenna, called LISA (Laser Interferometer Space Antenna). Six spacecraft
would orbit the Sun together in a ring arrangement millions of kilometers
in radius. Laser beams shone from one spacecraft to the next would
continuously monitor the distances between the spacecraft. And passing
gravitational wave would disturb these distances. Thanks to its
great size, LISA would be much more sensitive than LIGO or VIRGO
and would be unaffected by seismic disturbances that plague any
Earth-based gravitational wave antenna. While LISA has not yet been
funded, it may be put into orbit by 2020.
With such remarkable new instruments becoming available, the first
decades of the twenty-first century will hopefully see the dawn
of the new field of observational gravitational-wave astronomy.