Gravitational waves are formed when objects with a large mass such as neutron stars or black holes are accelerated and orbit closely around each other. They propagate as waves at the speed of light causing extremely small deformations of space-time. Since Einstein's theory of relativity, many scientists have been convinced that there must be a method to detect gravitational waves. The first considerations come from the 1960s.The ideas were based on an interferometer, which is said to detect the interaction of gravitational waves on laser light. The challenges posed by such an instrument were difficult to grasp at that time. The strength of the gravitational wave is: h = ΔL / L. With a 4 km long interferometer on Earth, ΔL corresponds to 10-18 meters. An extreme measuring instrument is required to detect this shift.
The first two such instruments were built in the United States, one in the states of Washington and the second in Louisiana and are called LIGO (Laser Interferometer Gravitational-wave Observatory). Large number of international universities and institutes are involved.
How does it work?
The detectors work with an interferometric method according to Michelson. Inside the device, a laser beam that has been split by a beam splitter passes along two paths which are as long as possible, through an optical mirror system. The laser beams are then joined in the detector. In this way, the smallest “time-of-flight” differences of the laser beams can be measured, which are created by gravitational waves.
As far as the vacuum technology challenges are concerned, the following aspects should be highlighted:
- The mirrors installed in the vacuum chambers are sensitive to any kind of contamination.
- UHV in the beam tubes is required in order to minimize light scattering phase noise from residual gas.
- The complete structure including vacuum pumps must be extremely low in vibration
Scheme of the LIGO interferometer for the detection of gravitational waves
The LIGO actually consists of two vacuum systems: Beamline and End/Corner stations.
The beamline tubes have a volume of V = 20 million liters (per site).The inner surfaces of these tubes are 600 million cm² (per site). This vacuum system is evacuated and will be not vented again. Vacuum at levels lower than 10-7 Pa (10-9 mbar) is needed to avoid forward scattering from residual gas molecules which would cause phase noise in the interferometer output.
The chambers at the end and corner station, where the mirrors and detectors are located, extreme cleanness is of crucial importance. The mirror absorption ratio should be less than 0.1 ppm. The contamination of optics must be less than one monolayer of hydrocarbons in 10 years. As opposed to the Beam tubes the corner and end stations could be vented and the vacuum in the chambers should be also at levels lower than 10-7 Pa (10-9 mbar).
LIGO is one of the biggest vacuum technology constructions in the world. Unlike particle accelerators and fusion reactors there is no radioactive environment, no high temperatures and no ions are generated. Pfeiffer Vacuum provided vacuum equipment for many experiments at the LIGO observatories. HiPace turbopumps together with dry backing pumps evacuate the chambers. Numerous mass spectrometers for residual gas analysis and leak detectors from Pfeiffer Vacuum are used in different parts of the LIGO setup.