What is a gravitational wave? What is an interferometer?
This means that, according to this theory, when you approach to any mass, the distances you measure with a rule and the time you measure with a clock are modified. If the mass is moving and accelerating, those modifications of space-time can propagate, like a ride on the water surface: this is a gravitational wave.
When a massive star explodes (supernova), when a black hole is created, when a neutron star rotates and emits a strong electromagnetic flux (we call such a star a pulsar) or when two neutron stars orbit around each other, we suspect gravitational waves to be emitted. And they are so strong that we can detect them on the earth, hundred thousand light-years away.
First indirect evidence for existence of gravitational waves was made by Hulse and Taylor, by observing, since 1974, a binary neutron stars system, PSR1913+16. They observed over more that 20 years the decrease of the orbital period (that means that the two neutron stars move inspiralling, closer and closer to each other until final coalescence) and their measurements are in prefect agreement with the General Relativity calculation interpreting this inspiral motion as a loss of gravitational energy by emission of gravitational waves.
The best way to measure such tiny length is to use optical measurement, that means an interferometer. An infrared laser beam is splitted in two parts by a semi-mirror. In VIRGO, the two resulting beams travel along 3 km until they reach very reflective mirrors. The two reflected beams interfer and the result of this interference is an amount of light at the output of the interferometer. This amount of light depends on the path length difference between the two arms of the interferometer. When a gravitational wave passes by, the path lengths are changed and this can be seen at the output of the interferometer.
Position and orientation of the VIRGO detector (coord. WGS 84):
Where: near Cascina, 12 km from Pisa, Italy
Latitude: 43° 37' 53.0921'' North
Longitude: 10° 30' 16.1878'' East
Height: 51.884 m (arm length: 3 km)
North arm Ccw angle to geographic north: - 19° 25' 57,2"
West arm Ccw angle to geographic north: 70° 34' 2,8"
But many sources of noise inside and outside the interferometer can also be detected, preventing us to see the gravitational wave signal. The ground is always moving, thus each interferometer's mirror must be attached by wires to a seismic attenuation system. The temperature of the mirrors and their wires makes them move, thus we need to choose the best material to reduce as much as possible this thermic motion. The detection of light is submitted to a stastistical noise that can be lowered by increasing the light power, thus we will use a high power laser (20 Watts) and a recycling mirror to increase the light power inside the interferometer. The air along the trajectory of the laser light is continuously moving, thus we need to put all the interferometer inside vaccum.
The VIRGO interferometer, under planning since 1989,
financed in 1992 and built from 1997 to 2003,
reaches now a sensitivity lower than 10
Virgo-LAPP page . . .
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