LISA and the Search for Elusive Gravity Waves
RAS — For almost 100 years, scientists have been searching for direct evidence of the existence of gravity waves ““ faint ripples in the fabric of spacetime predicted in Albert Einstein’s theory of General Relativity.
Today, the hunt for gravity waves has become a worldwide effort involving hundreds of scientists. A number of large, ground-based facilities have been developed in Europe, the United States and Japan, but the most sophisticated search of all will soon take place in space.
Speaking on Tuesday 5 April at the RAS National Astronomy Meeting in Birmingham, Professor Mike Cruise will describe a joint ESA-NASA project called LISA (Laser Interferometric Space Antenna),
Scheduled for launch in 2012, LISA will comprise three spacecraft flying in formation around the Sun, making it the largest scientific instrument ever placed in orbit.
“LISA is expected to provide the best chance of success in the search for the exciting, low frequency gravity waves,” said Professor Cruise. “However, the mission is one of the most complex, technological challenges ever undertaken.”
According to Einstein’s theory, gravity waves are caused by the motion of large masses (e.g. neutron stars or black holes) in the Universe. The gravitational influence between distant objects changes as the masses move, in the same way that moving electric charges create the “electromagnetic waves” that radio sets and TV’s can detect.
In the case of a very light atomic particle such as the electron, the motion can be very fast, so generating waves at a wide range of frequencies, including the effects we call light and X-rays. Since the objects which generate gravity waves are much larger and more massive than electrons, scientists expect to detect much lower frequency waves with periods ranging from fractions of a second to several hours.
The waves are very weak indeed. They reveal themselves as an alternating stretching and contracting of the distance between test masses which are suspended in a way that allows them to move.
If two such test masses were one metre apart, then the gravity waves of the strength currently being sought would change their separation by only 10(-22) of a metre, or one ten thousandth of a millionth of a millionth of a millionth of a metre.
This change in separation is so small that preventing the test masses being disturbed by the gravitational effect of local objects, and the seismic “Ëœnoise’ or trembling of the Earth itself, is a real problem that limits the sensitivity of the detectors.
Since each metre length in the distance between the test masses gives rise separately to the tiny changes being searched for, increasing the length of the separation between the masses gives rise to a greater overall change that could be detected. As a consequence, gravity wave detectors are made as large as possible.
Current ground-based detectors cover distances of a few kilometres and should be able to measure the millisecond periods of fast-rotating objects such as neutron stars left over from stellar explosions, or the collisions between objects in our local galactic neighbourhood.
There is, however, a strong interest in building detectors to search for the collisions between massive black holes that take place during mergers of complete galaxies. These violent events would generate signals with very low frequencies- too low to be observed above the random seismic noise of the Earth. The answer is to go into space, away from such disturbances.
In the case of LISA, the three spacecraft will fly in formation, 5 million kilometres apart. Laser beams travelling between them will measure the changes in separation caused by gravity waves with a precision of about 10 picometres (one hundred thousandth of a millionth of a metre).
Since the test masses on each spacecraft will have to be protected from various disturbances that are caused by charged particles in space, they must be housed in a vacuum chamber in the spacecraft. The precision required is 1,000 times more demanding than has ever been achieved in space before and so ESA is preparing a test flight of the laser measurement system in a mission called LISA Pathfinder, due for launch in 2008.
Scientists from the University of Birmingham, the University of Glasgow and Imperial College London are currently preparing the instrumentation for LISA Pathfinder in collaboration with ESA and colleagues in Germany, Italy, Holland, France, Spain and Switzerland.
“When LISA is operating in orbit, we expect to observe the Universe through the new window offered by gravity waves,” said Cruise. “In addition to neutron stars and massive black holes, we may be able to detect the echoes of the Big Bang from gravity waves emitted tiny fractions of a second after the event that started our Universe on its current evolution.”
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