Many millennia ago, in a distant patch of space some 6500 lightâ''years from Earth, a hot blue giant star exploded in catastrophic but glorious stellar suicide. In one stupendous runaway thermonuclear reaction, the star blasted off 95 percent of its gaseous outer layers, and its core collapsed, blazing so fiercely that for a few magnificent days that single star rivaled the total brilliance of all the other million or so stars around it. Over the next few months, the star’s naked core cooled and faded away, leaving a dim, dense neutron star one-twentieth of its original mass surrounded by a rapidly expanding multicolored cloud of gases. Eventually, the star’s outer layers attained immortality as the gorgeous, gaseous Crab Nebula [see photo, ” ”].
The story doesn’t end there, though. For when an exploding star, or supernova, suddenly redistributes its mass, something strange also happens to its gravitational field. Thanks to Einstein’s general theory of relativity, we have a good idea what that something might be.
According to Einstein, the explosion’s powerful acceleration of star matter should generate distortions in the normal curvature of space. These hypothesized distortions, known as gravitational waves, would ripple outward into the universe at the speed of light, stretching and compressing the space around any objects they happen to pass through. Other acts of astronomical violence—galaxies colliding or black holes cannibalizing other black holes—should also emit gravitational waves.
Astronomers believe that if we could detect these waves, they would illuminate much about the universe that is now obscured. Detecting gravitational waves would also give physicists a definitive new test of general relativity. Newtonian physics doesn’t come close to explaining gravitation from black holes or other regions of strong gravity, but Einstein’s theory does; the observation of gravitational waves from such sites would serve to confirm, or possibly enhance, the theory. Gravitational waves may also give astronomers the means to look back to the earliest moments of cosmic evolution, when the universe was still small and dense. And they could provide a completely new way to survey the contents of the universe, perhaps even revealing phenomena that may not have electromagnetic signatures.
None of the current astronomical observatories—whether detecting visible light, radio waves, X-rays, or any other type of electromagnetic radiation—can peer too far below the surfaces of stars and other objects. Photons that originate deep in a star’s interior get absorbed, reemitted, or otherwise altered on their way out to the surface—it can take a million years for a photon to work its way from the core of our sun to the surface, for example. And once photons leave the surface, they may be further altered or blocked by gas and dust in space before ever arriving at a detector on Earth.
In contrast, scientists believe gravitational waves pass unaffected through all intervening matter, carrying with them intimate secrets about the universe’s most violent events that can’t be learned in any other way. It’s analogous to the way sounds detected by a stethoscope can reveal essential information about a person’s heart or lungs, details unobtainable by simply looking at the surface of the skin. Indeed, astronomers hope that gravitational waves may let us effectively listen to the very pulse of the cosmos’s most brutal and exotic events.
The hitch is that because gravitational waves travel right through matter unaltered, they are extraordinarily difficult to detect—some would even say impossible.
Undeterred, pioneering astronomers and physicists around the world have teamed with engineers to build technologically ingenious detectors to seek evidence of gravitational waves. This past November, the world’s two largest gravitational-wave detectors began their first full-scale run of observations. Like a pair of ears listening simultaneously for the same sounds, they are the twin L-shaped instruments of the Laser Interferometer Gravitational Wave Observatory, or LIGO (pronounced LYE-go). One of the observatory’s two sites, LIGO-Livingston, is located in a dense forest in Louisiana, 42 kilometers northeast of Baton Rouge in the southeastern United States [see photo, ” ”], and the other, LIGO-Hanford, is 3000 km away in the sagebrush desert of eastern Washington state. If gravity waves are to be detected anytime soon, these are probably the machines that will do it.