A New Window In Search Of Gravitational Waves
THERE are two unusual L-shaped buildings, one in the forests of Louisiana and the other in the plains of Washington state. The skinny arm of each building stretch four kilometres across the land. Inside each arm, a flashlight-sized laser beam bounces back and forth. These buildings, also called Laser Interferometre Gravitational Wave Observatory (LIGO), are established to search for gravitational waves of cosmic origin created in supernova collapses of stellar cores (which form neutron stars and black holes). These function continually to confirm the existence of the vibrations in the composition of space-time.
LIGO, a joint project between the scientists of the California Institute of Technology (Caltech) and Massachusetts Institute of Technology (MIT), is sponsored by the National Science Foundation (NSF). A sum of $296 million was spent in the development of this effort and it stands as the first experiment to stand any chance of detecting gravitational waves.
Albert Einstein was the first scientist to predict the existence of gravitational waves. It was his theory of general relativity, in 1916, which introduced the idea that linked the perturbations of the gravitational field with the structure of the space-time. Einstein suggested that just as air can vibrate, so can space and time. He also asserted that every motion, from the moon orbiting the earth to one car bumping into another, puts the fabric of space-time into vibration, sending out gravitational waves.
To understand this concept, we can take an instance of a fishing bob in a still pond. As the bob floats on the water, it disturbs the plane of the pond surface. When a fish disturbs the bob, waves ripple out in all directions over the surface of the water, distorting a larger area of the plane.
On a larger canvas, the same analogy can be stretched to explain the phenomena behind a dying star. As the star explodes, it moves up and down on the plane of space-time causing waves resulting from the massive disturbance. These ripples travel at light-speed in all directions of space. As the waves travel through the space, they affect the path by changing it. By the time the waves reach the Earth, they would have travelled millions of light-years. They reach Earth as scattered, weak waves that interfere with the space-time fabric and everything in it, but only to a small extent. Scientists have assumed that gravitational waves can go through any materiain existencel and distort it as they are passing through. When they arrive at Earth, they will distort space along the interferometre beam paths. Theoretically, this effect can be measured by a sophisticated instruments such as the LIGO.
LIGO's strategy of hunting down gravitational waves is to split a laser beam down two long, steel vacuum tubes. It houses a laser interferometre, consisting of mirrors overhanging from each of the corners of a massive L-shaped vacuum system, measuring 4 kilometres on each side. Accurate laser beams in the interferometer will sense small motions of the mirrors, which are caused by gravitational waves.
If a gravitational wave ripples past Earth, it will warp space-time in the direction it is coming from, changing the relative lengths of the different paths and altering the interference pattern produced when the beam is recombined. Gravitational waves that originated hundreds of millions of light years from earth are expected to distort the 4 kilometre mirror spacing by about a thousandth of a Fermi (less than one trillionth of the diametre of a human hair).
A gravity wave passing over the Earth ,will slightly compress space in one tube and stretch it in the other. One mirror will move slightly forward, and the other slightly back. That tiny movement is what they're trying to measure.
The experiment ran for 17 days and found no evidence of gravitational waves. As for the team, they said they will likely have to upgrade their equipment before it is truly sensitive enough to detect the first gravity wave.
Einstein predicted, some 88years ago, that gravitational waves would be too small to measure. The unsuccessful results by the LIGO proved him right to a certain extent, but the hunt was not a total faliure. The negative results provided a platform to astronomers who wished to observe the number of violent, space-twisting events happening in our galactic backyard.
The LIGO could have effectively detected these happenings with an appropriately strong signal. Alas gravitational waves are so faint that even shattering events like black holes running ino one another would produce just the faintest whisper back here on Earth.
The first results have allowed researchers to come to the conclusion that the Milky Way must be home to less than 164 mergers per year. This shows that LIGO can do real astrophysics.
Unfortunately, those numbers don't tell astronomers what they really want to know. They only expect to see between 1 and 100 mergers per hundred thousand galaxies. The fact that LIGO didn't pick up any motions from the early universe allows researchers to predict that only the energy carried by gravitational waves must be less than 72.4 times the total amount of energy in the Universe.
In spite of the fact that the successful first run did not bring out any satisfactory result, it is still an important proof of principle, made more hopeful by the fact that the machine was running at only a hundredth of its projected sensitivity. Modifications made to the mirrors that canal the laser light in the past few months have led to an amazing increase for the second run. The team expects another tenfold increase by 2006, giving LIGO a view of thousands of galaxies. And that will finally bring them within touching distance of the range at which astronomers expect to see something of interest.
But the only way to be certain of spotting gravitational waves - if they are out there, is to put in some more cash. Another 100 million dollar upgrade to LIGO should certainly reach into the anticipated range of gravitational wave activity by 2012. That sure is a lot of time but it is worth waiting for.
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