GRAVITATIONAL WAVES: RIPPLES IN THE FABRIC OF SPACE-TIME

Albert Einstein predicted the existence of gravitational waves in 1916 as part of the theory of general relativity. In Einstein's theory, space and time are aspects of a single measurable reality called space-time. Matter and energy are two expressions of a single material. We can think of space-time as a fabric; The presence of large amounts of mass or energy distorts space-time – in essence causing the fabric to "warp" – and we observe this warpage as gravity. Freely falling objects – whether soccer balls, satellites, or beams of starlight – simply follow the most direct path in this curved space-time.

When large masses move suddenly, some of this space-time curvature ripples outward, spreading in much the way as ripples on the surface of an agitated pond. When two dense objects such as neutron stars or black holes orbit each other, space-time is stirred by their motion and gravitational energy ripples throughout the universe in a manner represented by the colorful figure at the top of the page.

In 1974 Joseph Taylor and Russell Hulse found such a pair of neutron stars in our own galaxy. One of the stars beams regular pulses of radio waves toward Earth. Taylor and his colleagues were able to use these radio pulses, like the ticks of a very precise clock, to study the orbit of neutron stars. Over two decades, these scientists watched for and found the tell-tale shift in timing of these pulses, which indicated a loss of energy from the orbiting stars – energy that had been carried away as gravitational waves. The result was just as Einstein's theory predicted. The discovery of Hulse and Taylor represents strong indirect evidence of gravitational waves. LIGO now seeks to measure gravitational waves directly. Mirrors that are suspended along 4-kilometer laser light paths in the LIGO interferometers will vibrate as gravitational waves pass by. The laser light will record these tiny fluctuations and photo detectors will convert the light signal to an electrical signal that scientists can measure.