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Einstein's General Theory of Relativity predicts that cataclysmic events in our Universe—such as exploding stars or colliding black holes—will produce ripples in the very fabric of spacetime. These ripples, or gravitational waves, will radiate outward at light speed and cause all matter to bob in spacetime, like buoys on the ocean.
Unlike light waves, these subtle gravitational waves barely interact with matter at all, so they can provide scientists with an "unfiltered" view of the energetic events that generated them. Detecting these ripples, however, is a daunting technological challenge. Near merging black holes, these waves would alternately stretch and compress a human body so violently that they would rip it apart. But after traveling across the immense distances of space, they weaken to an almost imperceptible level by the time they reach Earth. So far, astronomers have only detected them indirectly by watching the orbital decay of neutron stars in binary systems. But we will soon be ready to go Beyond Einstein and make direct measurements from space.
The Laser Interferometer Space Antenna (LISA) is a Beyond Einstein mission designed to detect gravitational waves. A partnership between NASA and the European Space Agency, LISA will use lasers to measure the change in separation between three spacecraft "buoys" as they bob in response to passing gravitational waves. The three spacecraft will be separated from each other by over five million kilometers (three million miles). Passing gravitational waves alter the distance between them by far less than the width of an atom.
LISA will complement the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO), which has facilities in Louisiana and Washington. LIGO is sensitive to very-high-frequency gravitational waves and is thus only capable of detecting mergers of stellar-mass black holes or neutron stars, or nearby supernovae. But mergers of supermassive black holes in distant galaxies emit much lower frequency waves than LIGO can detect. LISA can pick up these low frequencies and will thus complement the ground-based LIGO project by detecting different classes of objects.
Detecting gravitational waves from space is a challenging engineering project, but a vigorous and innovative technology development program will allow us to meet this challenge within the next decade. Much of the technology will be developed and checked in the LISA Pathfinder, a European Space Agency test flight scheduled for 2010.
LISA will reveal a universe not seen by traditional telescopes, providing an unprecedented view of colliding black holes and binary compact objects in our galaxy. The history of astronomy shows that whenever scientists open a new window on the universe, remarkable discoveries are bound to follow.
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