Interferometers
When waves interact, specifically when they interfere, the effect is called, in physics, interference. Instruments which rely upon the interference of electromagnetic waves are called interferometers.
Some of the most sophisticated interferometers are to be found in astronomical observatories, because they can be used to reveal far more about stars, galaxies, planets etc than simple telescopes can.
Aperture synthesis is perhaps the most common form of astronomical interferometry. This involves a number (at least two!) of separate telescopes working together to act like a single telescope the size of the distance between them. Electromagnetic (EM) radiation – light, infrared, submillimeter radiation, microwaves, radio – received by each unit (telescope) is combined (it interferes), and information – often an image – about the source extracted. Sometimes this is done in real time – by piping the EM radiation to a device (the actual interferometer) near the center – and sometimes by recording the signal and combining it electronically later; the former is common in optical astronomy, the later in radio astronomy. An array of telescopes can be used – with different combinations giving different possible baselines – or the Earth's rotation may supply the changing viewing angle and distance needed to reconstruct an image ('synthesize the aperture'). In VLBI (very long baseline interferometry, an important part of radio astronomy), the most distant telescopes may be separated by tens or thousands of km (i.e. at least one is in space).
The first interferometer used in astronomy was in 1920, when Michelson and Pease used one to measure the diameter of Betelgeuse (Michelson was very familiar with interferometers, having built one – with Morley – that formed the heart of an experiment which showed that the aether does not exist; oh, and he also won the Nobel Prize for his work on interferometers). However it was in radio astronomy that interferometry really made its mark; in the radio part of the EM spectrum, the waves are so long that a single radio dish has a very low angular resolution (often much worse than that of the human eye), so to get images even close to those which optical astronomers get, even with atmospheric seeing, interferometry is essential.
The most sensitive interferometer used in astronomy today does not observe the universe through EM radiation, but looks for gravitational wave radiation. In LIGO, VIRGO, and other gravitational wave observatories interferometers constantly measure the distance between two masses suspended in vacuum chambers, several km apart. Changes in that distance may be caused by passing gravitational waves (not to be confused with gravity waves!); the changes will be tiny – some fraction of the size of a proton! – but detectable.
NASA's Ask an Astrophysicist has a good short article on interferometers; the ESO's VLTI (Very Large Telescope Interferometer) explained; CHARA (Center for High Angular Resolution Astronomy), ALMA (Atacama Large Millimeter/Submillimeter Array), and LIGO (Laser Interferometer Gravitational-wave Observatory) … just some of the hundreds of astronomical interferometers.
Universe Today stories which rely upon interferometers include A Glimpse at the Future of Our Sun, Squadrons of Planet Hunters Could Find Life, European Astronomers: 'Era of Stellar Imaging' Has Begun, and New Limits on Gravitational Waves From Big Bang.
Interferometry, an Astronomy Cast episode, explores this topic in more detail; and Gravitational Waves discusses how gravitational wave radiation may be detected by interferometers.
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