Measuring with waves, seeing with fringes

Possibly the most accurate form of measure is based on the interaction of one wave with another. The principles involved are the same whether the waves are of water, light, or sound. If two waves are exactly the same, they can be adjusted so that they either cancel each other or double the combined amplitude. If the crests of one version of the wave are superimposed on the troughs of the other, the waves will cancel. If they are light waves, this will produce darkness. If the crests of one version of the wave are superimposed on the crests of another version, the waves will add. If they are light waves, the result will be brighter light (brightness indicates amplitude for light). If the waves are not exactly the same, superimposing them will produce bands of brightness and darkness, called an interference pattern. Using an interference pattern to make measurements is called interferometry.

Among the first to use interferometry in measurement was Albert Michelson in 1881. By 1887 he and Edward Morley had used the idea to conduct one of the most famous experiments in science. They established through experimental data that the velocity of light is not affected by the movement of Earth through space. This result helped make people accept Einstein's theory of relativity, which included the concept that the velocity of light in a vacuum is a universal constant.

Astronomers, starting with Martin Ryle in the 1960s, use interferometry with radio telescopes to create giant, fictitious telescopes that work better than real ones. Two or more telescopes some distance apart (called the baseline) acting together as a single telescope can be used to obtain better resolution than any one telescope by extracting data from the interference fringes between the waves that have taken two slightly different paths.

At first, Ryle used just two radio telescopes that were not very far apart. By 1980, the Very Large Array in Socorro, New Mexico, was using 27 radio telescopes linked by computer to obtain the same resolution obtainable from a single radio telescope 27.4 km (17 mi) in diameter. Today, with interferometry, a group of telescopes all over Earth can be linked by satellite and computer to give the equivalent of a single telescope with the diameter of the planet; this is the method used in the Very Long Baseline Array. Astronomers also use space-based radio telescopes that allow a baseline that exceeds the size of Earth.

Starting in 1974, astronomers began to experiment with combining light waves, which are much shorter than radio waves, in optical interferometry. The principles are the same as in radio interferometry, but the required precision is much greater and much shorter baselines are used. The first actual success came in 1995. Telescopes with giant mirrors that are close to each other, such as the Keck twins on Hawaii and the four-telescope Very Large Telescope at Cerro Paranal, Chile, can and do use optical interferometry for their best resolution. Because the atmosphere causes some distortions that cannot be eliminated, optical interferometry is planned for a two-reflector space telescope, the Space Interferometry Mission (SIM), that is scheduled for launch in 2009.

Today, interferometry is the king of measurement systems. Interferometry is so sensitive that it can detect the movement of an object that moves only a centimeter in hundreds of years. It can even be used to determine the rotation of distant stars. Using lasers as the source of light, interferometry can detect tiny movements in the crust of Earth that precede earthquakes.

The interferometer is also the instrument of choice for determining small distances. It can detect tiny variations in thickness of a lens, for example. Using lasers or radar and interferometry combined, space vehicles guide themselves with extreme precision as they cross the vast distances between planets.