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 waves.

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.