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The "L" and "I" in LISA stand for the method the array will use to detect gravitational waves: "Laser Interferometer." This technique is patterned after ground-based observatories such as LIGO (with facilities in Louisiana and Washington), VIRGO (in Italy), and GEO600 (in Germany). But LISA will be the first attempt to build a laser interferometer in space.
Lasers are well known to just about everyone: they are tightly collimated beams of light with a narrow range of colors. But interferometry is not a concept we encounter in everyday life, even though scientists have been using this technique for more than a century.
An interferometer is an instrument that exploits the wave nature of light. Like any type of wave, light waves have crests and troughs. Scientists can add two light waves together so their crests line up. This "constructive interference" brings the waves together "in phase," which amplifies the wave's strength. But if scientists align the crest of one wave with the trough of another, the two waves are out of phase. This "destructive interference" cancels out both waves.
By comparing the two original light waves with the combined wave, scientists can make precise measurements in a wide range of fields. For example, in 1887 Albert Michelson and Edward Morley used an interferometer to show that the popular idea of an all-pervasive universal "aether" was false. For this experiment and others, Michelson would later become the first American to win the Nobel Prize in Physics.
Astronomers are currently using interferometers to make extremely high-resolution observations. They have constructed telescope arrays to measure the diameters of stars such as Vega (made famous in the movie Contact as the source of the aliens' signal), even though they appear as nothing more than points of light in the sky. The Very Large Array, which detected the fictional E.T. signal, is an interferometer that collects radio waves.
But unlike astronomical interferometers, which passively detect starlight, LISA supplies its own light sources: lasers. The array consists of three spacecraft in a formation shaped like an equilateral triangle. Each spacecraft will fire lasers at the other two. LISA's spacecraft will be separated by 3 million miles (5 million kilometers) — a million times longer than the beam lengths of LIGO, VIRGO, and GEO600.
Spacecraft A will shoot a laser at spacecraft B and spacecraft C. When the beam from A reaches B, spacecraft B will receive the signal, and then amplify it and beam it back in phase to spacecraft A. The returning beam will interfere with spacecraft A's outgoing beam. The two intermingling beams will form an interference pattern of alternating bright and dark bands in A's detector, which measures the relative phases of the incoming and outgoing beams. [diagrams or animations here or above would be very helpful]
By stretching or compressing space-time, gravitational waves passing through the inner Solar System will make one beam longer than the other. The tiny changes between the distances of spacecraft A and B will cause the returning beams to arrive slightly in phase or out of phase. By studying the changing interference pattern, scientists will be able to infer the properties of the gravitational waves.
Since each spacecraft serves as both a light source and mirror, the array consists of three independent interferometers: A-B, A-C, and B-C. This allows the mission to measure the polarization of gravitational waves passing through the inner solar system. All of this information will reveal precious insights into the nature of sources such as colliding black holes, and may reveal deviations from predictions made by general relativity—truly taking us beyond Einstein.
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