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Lisa: A spacecraft to test the nature of the universe

A joint research operation of the American and European space agencies is designed to detect gravitational waves

By: Dr. Noah Brosh

In the 19th century, physicists believed that light waves move through the world thanks to an invisible medium, which fills everything and space. Light, they believed, is the "vibration" of that medium. They called it: "The World Site".

In 1887 the two physicists Michelson and Morley showed that the ether does not exist; The results of the experiment paved the way for Einstein's special theory of relativity (1904). In the famous experiment, Michelson and Morley split a beam of light into two rays. They let one advance in one direction, and the other advanced in a perpendicular direction. At the end of the roads, and the two horns retraced their steps and were reunited into one horn. The connection created the phenomenon of "interference": the rays strengthened or weakened each other, according to the difference in the paths of one ray compared to the other.

The hope of the researchers was that the movement of one ray in the direction of the Earth's movement around the Sun would change its speed compared to the ray moving perpendicularly. It didn't happen. Hence: the speed of light is constant.

A giant version of the Michelson-Morley experiment is now being prepared, as a collaboration between NASA and ESA, the European Space Agency. The LISA - as the project will be called - is an enlargement of the device that was built more than a century ago. Its purpose is to detect gravitational waves.

According to Einstein's theory, the presence of matter in the universe causes its curvature. The rays of light, in the universe where matter exists, move in curved lines with the curvature according to the curvature of space.

Einstein calculated this theoretically and his calculations were found to be correct in the total solar eclipses seen at the end of the second decade of the 20th century.

The rays of starlight seen near the edge of the sun, which was hidden by the eclipse by the moon, twisted their paths, so that the stars closest to the sun seemed to be diverted from their expected position.

Lisa will attempt to measure the momentary curvature change of space as a gravitational wave passes through the solar system.

Gravitational waves are another result of general relativity. They are created due to the movement of matter - for example the mutual rotation of black holes - or by the fall of an ordinary star into a black hole, in a supernova. If a gravitational wave passes through the solar system, it causes at a certain moment the distance between two points to change.

Lisa is built to measure tiny distance changes, over distances of millions of kilometers. The main part of the experiment are the mirrors: gold and platinum alloy cubes, whose sides are only four cm and one wig in each cube is polished to act as a mirror.

Each cube resides in the core of the spaceship and is protected from external influences by the sides of the spaceship. The reason: not only gravitational waves can affect the position of each cube - also radiation pressure from the sun, or the impact of various particles. To prevent this, the spacecraft will move around the cube using ion engines and keep the cube always in its center.

LISA consists of three spaceships and in each one a mirror-cube. A laser beam will run between the light-reflecting wigs, which will travel a distance of five million kilometers between returns. When a gravitational wave passes through the triangle made up of the three spacecraft, a measurable change will occur through one or two of them. According to preliminary calculations, a continuous measurement of at least three years will be required to find enough cases of possible gravitational waves that will lead to a breakthrough in the understanding of the universe.

Update April 2007: The spacecraft is scheduled for launch in 2011

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