LASER INTERFEROMETER
GRAVITATIONAL WAVE
OBSERVATORY

 

Albert Einstein predicated the existence of gravitational waves in his 1916 General Theory of Relativity. According to the theory, gravitational waves should be emitted by moving masses. They are, however, very weak and only when huge masses are converted into energy (like when black holes or neutron stars collide) are measurable amounts of gravitational waves produced.

Joseph Weber of the University of Maryland is credited with being the pioneer of the field. In 1969, he suspended a massive bar, weighing over a ton in a vacuum chamber and insulated it from external forces by placing it deep within a salt mine. He compared the bar with vibrations in similar bars separated by hundreds or thousands of miles. When the separated bars vibrated in unison, it was taken to be evidence for gravitational waves.

The LIGO detectors use beams of laser light as a Michalson interferometer. Light is directed along 4 kilometer (2.5 mile) arms at 90 degrees to each other and reflected back. The way the light beams from each arm interfere with each other reveals any comparative changes in arm length. (As in the famous Michaelson-Morley experiment of 1887 to prove the non-existence of ether as a medium for light propagation)

The LIGO project is the most expensive project ever funded by the National Science Foundation (NSF). Two Michaelson Interferometers have been built. One in Livingston, Louisiana and one in Hanford, Washington. The two facilities operate in unison as a single observatory. It is estimated the cost of the two sites is in excess of $300,000,000.

 

The LIGO observatory in Livingston, Louisana showing both arms. Rotate the right picture 90 degrees to the left to align.
 

Gravitational waves as we understand them from Einstein's General Theory of Relativity are ripples in the fabric of space-time produced by violent events in the universe. Gravitational waves are emitted by accelerating mass just as electromagnetic waves are emitted by accelerating energy.

This, however is a new concept. Before Einstein, it was believed that gravitation were an instantaneous force as described by Sir Isaac Newton and published in 1687. Newton's Law of Gravity states that every body attracts every other body with a force that is proportional to the mass of each body. Newton's law of gravity also tells us that the further apart the bodies, the smaller the force.

The problem with Newton's laws of motion and gravity is that they do not establish an absolute standard of rest. The force does not have a speed limit. According to Einstein gravity, like light and radio travels at 186,000 miles per second.

It was suggested that the absolute was a substance called the "ether" that was present everywhere, even in empty space. As water is the medium in which ripples propagate across a pond, ether was the medium in which light waves propagated. Different observers, moving relative to the ether would see light coming toward them at different speeds, but light's speed relative to the ether would remain fixed.

In 1887 Albert Michelson and Edward Morley carried out a very careful experiment at the Case School of Applied Science in Cleveland to measure the speed of light at different angles to the ether. They compared the speed of light in the direction of the Earth's motion with that at right angles to the Earth's motion. To their surprise, both were the same.

The Michelson Interferometer passed a beam of light at 90-degree angles to itself so that it reflects off two mirrors. If the length of both beams is the same, the light appears as a single beam. There is no interference. If, however either of the arms differ in length, as the one going against the ether would have to be, the beams cause an interference pattern.

 

The Michaelson Interferometers at LIGO

The signals from interferometers at Livingston, Louisiana and Hanford, Washing are combined to produce a single signal from an interferometer with an effective arm length of 2500 miles. This will eliminate the large set of external noise sources like seismic and acoustic noise. Gravitational wave signals, by contrast, would be correlated.

The choice of interferometer arm length is a complex tradeoff between sensitivity and cost. In the frequency range of approximately f < 100 Hz, where the anticipated signal-to-noise ratios are expected to be the largest and the only fairly reliable source (coalescing binary neutron stars) is strongest, the dominant noise is likely to be random forces on the test masses (seismic, acoustic, thermal and local gravity gradients).

It is expected a gravitational event like the collision of two black holes or a black hole and a neutron star will be detected within the first seven years of operation. Detection of a gravitational anomaly coincident with a GRB (Gamma Ray Burst) is deemed proof of gravitational wave detection.

 
Vacuum pumps in the main building.   Laser assembly and beam splitter.

 

© 1999, 2000 Michael Conti