Gravitational Wave Observatories - How they work
Gravitational waves change the shape of everything.
Circles are distorted into ellipses back and forth like the patterns below.
This means that everything is quivering in the passing gravitational waves. These vibrations, however, are infinitesimal. Laser beams are used to detect the tiny quivering of the spacing between mirrors down two long vacuum arms. Detectors can only be sensitive enough if they combine the world’s best mirrors, the world’s most precise high power lasers, the most transparent materials, the highest vacuums and the best vibration isolation ever achieved.
Laser Beams and Mirrors
Detectors consist of two long vacuum pipes constructed at right angles to each other. A laser beam is split into two identical beams which travel down each pipe arm and are reflected back precisely by supermirrors delicately suspended at the ends of the arms, as shown in the diagram opposite. The two beams come back together in such a way that they would cancel out if the two arms were identical. A gravitational wave that passes through the detector simultaneously shrinks one arm while stretching the other. This stretching and shrinking alternates between the arms. These differences in the length of the arms change the perfect cancellation of the beams, causing a brightness variation that is proportional to the strength of the wave. These tiny variations in brightness are picked up by light sensors and amplified. The entire detector is like a supersensitive microphone - one that is about one billion times more sensitive than a human ear! The longer the arms, the bigger the changes caused by gravitational waves, so it is essential that the detectors are several kilometres in length.
Vibration Isolators
Because gravitational waves are tiny vibrations, the first task is to prevent any sound and ground vibrations from the outside world from reaching the mirrors. This is achieved with vacuum, and vibration isolators which consist of chains of masses, springs and pendulums designed to absorb all vibrations that could otherwise reach the mirrors. The picture below shows a section of a vibration isolator. Vibration isolators developed for gravitational wave detection are the most advanced in the world and have many wider applications.
Supermirrors and Vacuum Chambers
Gravitational wave detectors use the most perfect mirrors ever created in order to measure the tiny vibrations. These mirrors are called ‘test masses’ and they are made out of artificial sapphire and fused silica. The mirrors must be of a large size - about 30 cm in diameter, and heavy - about 40 kg. Seven such mirrors are required in a detector as shown in the picture opposite. The laser beams must be very intense - about as bright as a billion laser pointers! To prevent any disturbance, they must travel in a nearly perfect vacuum - the pressure must be less than one thousandth of a billionth of atmospheric pressure! Very carefully prepared stainless steel is required in order to achieve such a vacuum.
Optical Cavities
The laser beam is built up by resonance inside an optical cavity. Optical cavities reflect light back and forth between mirrors, building up the light intensity in the same way that repeated pushing builds up the swinging of a playground swing. First, intense laser light enters the detector through a partially transmitting mirror, called the power recycling mirror, and into a ‘beam splitter’ which separates the light into two beams at right angles to each other. The beams then enter the long vacuum pipe through the input test masses, which are partially transmitting mirrors delicately suspended at each end. They then bounce back and forth between the input test mass and the end test mass. After several hundred kilometres of travelling back and forth between the mirrors, the beams become very intense. They return to the beam splitter where they are recombined. Most of the light passes back towards the laser, only to be reflected back on itself by the power recycling mirror. This process repeats itself many times. Overall, the incoming light combines with light already in the optical cavities, until the light power inside the instrument has built up by resonance to be thousands of times brighter than the original beam. A tiny bit of this light is modified by the passage of gravity waves. This modified light, which carries the gravity wave signal, is directed towards the signal recycling mirror, which again reflects most of it back into the instrument. This process also repeats itself so that the light containing the signal is built up by resonance. Finally, it is detected by a light sensor, called a photodetector, at the output where it is measured and processed by a computer.
Earthquakes and Lightning
Earthquakes, lightning and other disturbances are measured separately to ensure that false signals are recognised. Accurate timing information is coded with the data so that the arrival times of signals can be compared with those from other observatories, giving a fuller ‘picture’ of the wave. The further apart detectors are on the globe, the better their ability to map a wave.
Testing a prototype isolator at the ACIGA High Optical Power Test Facility (HOPTF) at Gingin
Part of a prototype vibration isolator.
ACIGA researchers assembling the prototype isolator for testing at Gingin.
Researchers working on an isolator.
Lifting an isolator into a vacume tank.
Seating the isolator into the lower section of the vacume tank.

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