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Waves in spacetime
 In 1915, Albert Einstein portrayed a completely new picture of our world in his theory of general relativity. In contrast to Newton, gravitation is not a force, but a consequence of the geometry of space and time: Big masses such as stars and galaxies deform spacetime around them. If other objects move through such areas, they are diverted from their original path, apparently attracted by the big mass. According to Einstein, what in fact happens is that the objects just follow the path mapped out for them by the deformation of spacetime around them. Moving masses give rise to perturbations in the spacetime continuum that propagate in all directions with the velocity of light. These moving space-time disturbances are called gravitational waves. They alternately stretch and compress space so that the distances between the objects in space are changed. However, these changes in distance are tiny: even the gravitational wave produced by a powerful event in our vicinity, like a supernova explosion within the Milky Way, changes the total distance between earth and sun only by the diameter of a hydrogen atom, merely for several thousandths of a second. This is the effect that the gravitational wave detectors attempt to measure. The great challenge is to get rid of the many disturbances, like air pressure and temperature fluctuations as well as seismic vibrations of all sorts, that would conceal a signal.
Einstein himself did not believe that gravitational waves could ever be detected, because the distortions of spacetime are so tiny. The American astronomers Russell Hulse and Joseph Taylor succeeded in the first, but indirect, proof of gravitational waves: for many years they observed a binary system consisting of a neutron star and a pulsar circling each other. They showed that the system loses energy by emitting gravitational waves and that the observed loss of energy equals exactly the value predicted by the theory of general relativity. For this achievement Hulse and Taylor were awarded the Nobel Prize in physics in 1993. Now, nearly one hundred years after Einstein's prediction, the direct detection of gravitational waves is at last within our reach due to advances in detector technology and data analysis methods.
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