Gravitational-wave detector

A gravitational wave detector (even Observatory ) is an experimental setup, which attempts, with minimum disruption of the space-time measure ( gravitational waves ), which are predicted by Albert Einstein's general theory of relativity. So far (as of December 2013) There are no reports of success.

Complications

The direct detection of gravitational waves is complicated by the very small effect of the waves on the detector. The amplitude of a spherical wave is incident to the inverse square of the distance from the source. This even sound waves from extreme systems like merging two black holes from on the way to earth to a small amplitude. Astrophysics expected that some of the waves have a relative length change of about, but generally not more.

Resonance detector

A simple device for the detection of wave movements of the resonance detector: a large, solid metal rod, which is vibration isolated from the outside. This type of instrument was the first type of gravitational wave detectors. The pioneer of this was Joseph Weber. Deformations of the space, resulting from the gravitational wave excite the resonant frequency of the rod and can be enhanced through the detection limit. It is conceivable that a nearby supernova is strong enough to be seen without the resonant gain. Modern forms of resonance detectors are now being cooled with cryogenics and read out by SQUID sensors. Resonance detectors are not sensitive enough to detect other than extremely strong gravitational waves.

MiniGRAIL is a spherical gravitational wave antenna that uses this principle. It is located at the University of Leiden and consists of 1150 kg heavy, precisely manufactured ball, which has been cryogenically cooled to 20 mK. The spherical arrangement allows the same sensitivity in all directions and is experimentally somewhat simpler than the larger linear devices that require a high vacuum. The proof is done by measuring the multipole moments. MiniGRAIL is very sensitive in the 2-4 kHz range. This makes it suitable for the detection of gravitational waves emanating from rotating neutron stars or created in the merging of small black holes.

Interferometric detector

A more sensitive detector uses laser interferometry to measure the movement of "free" masses that were triggered by gravitational waves. This allows a large distance from the crowds. Another advantage is the sensitivity in a large frequency region (not only in the vicinity of the resonance frequency as in the case of the resonance detector ). Meanwhile, ground-based interferometers in operation. Currently, the most sensitive LIGO - the Laser Interferometer Gravitational Observatory. LIGO has three detectors: one is located in Livingston (Louisiana ), the other two ( in the same vacuum tube ) in Hanford Site in Richland (Washington). Each consisting of two Fabry-Perot interferometers, the two to four miles. They relate to each other at a 90 ° angle. The light runs continuously in a vacuum tube with a meter in diameter. A passing gravitational wave stretches one arm and the other arm slightly compresses.

Even with such long arms changes the strongest gravitational wave the distance between the ends of the arms at most by about meters. LIGO should be able to measure small gravitational waves from. Improvements to LIGO and other detectors, such as VIRGO, GEO600 and TAMA 300 should further increase the sensitivity. The next generation ( Advanced LIGO and Advanced Virgo) should be ten times as sensitive. Another sensitive interferometer ( LCGT ) is currently in the design phase. An important point is that the increase in sensitivity by a factor of ten increases the volume of observable space by a factor of 1000. Thus, the rate of detectable signals increased by a. Within decades at dozens per year

Interferometric detectors at high frequencies is limited by shot noise, caused by the fact that laser photons radiate random. That leads to noise on the output signal of the detector. In addition, at sufficiently high laser radiation, a random pulse is transmitted by the photons to the test compounds. This low frequencies are covered. Thermal noise (eg Brownian motion ) is another limitation of the sensitivity. In addition, all ground-based detectors are limited by seismic noise and other environmental vibration at low frequencies. This includes the creaking of mechanical structures, lightning or other electrical disturbances, etc., which produce noise, which cover an event or fake. All these factors must be considered and excluded in the analysis before a proof can be considered as gravitational wave event.

Space-based interferometers such as the Laser Interferometer Space Antenna and DECIGO are in development. LISA will consist of three test masses that form an equilateral triangle. With lasers between two space probes two independent interferometers are formed. The detector should follow the earth in its solar orbit. Each arm of the triangle is said to have five million km edge length. Thus, the detector is located far away from noise sources on Earth. However, it is still susceptible to shot noise and artifacts, caused by cosmic rays and solar wind.

High frequency detectors

There are currently two detectors which concentrate on the detection of gravitational waves at the upper end of the wave spectrum ( 107-105 Hz): one at the University of Birmingham, England, and the other at the Istituto Nazionale di Fisica Nucleare Genoa, Italy. A third is being developed at the Chongqing University, China. English detector measures the change in polarization state of a microwave beam, which revolves in a closed loop of approximately one meter. Two rings were built and it is expected that they are susceptible to space-time distortion with a power spectral density. The INFN Genoa detector is a resonant antenna consisting of two coupled spherical superconductors with a few centimeters diameters. The resonators are when they are decoupled, have almost the same resonance frequency. The system should have a Empflindlichkeit for space-time distortion with a power spectral density. The Chinese detector should be able to detect high-frequency gravitational waves ~ with the predicted typical parameters fg ~ 1010 Hz ( 10 GHz ) and h 10-30-10-31.

Pulsar timing method

Another approach for the detection of gravitational waves is used by pulsar timing arrays such as the European Pulsar Timing Array, the North American Observatory for Gravitational Waves Nano Hertz and the Parkes Pulsar Timing Array. The purpose of these projects is the detection of gravitational waves by observing the signals of 20 to 50 well-known millisecond pulsars. While the gravitational wave passed through the Earth, the space shrinks in one direction and expands in the other. The arrival times of pulsar signals are thereby shifted accordingly. By observing a fixed set of distributed across the sky pulsars gravitational waves should be observed in the nano- Hertz range. It is expected that pairs of merging supermassive black holes emit such signals.

Einstein @ Home

The easiest detectable signals should come from constant sources. Supernovae and mergers of neutron stars and black holes should have larger amplitudes and be more interesting. The waves generated are more complicated. The waves of a rotating deformed neutron star would be " monochromatic " like a sine wave in acoustics. The signal would change in the amplitude or frequency hardly.

Einstein @ home is a distributed computing project for the purpose to demonstrate these simple gravitational waves. Data from LIGO and GEO600 are divided into small packets and distributed to thousands of computers of volunteers who carry out the analysis. Einstein @ Home, the data much faster than otherwise possible seven.

Specific gravitational wave detectors

• CLIO
• GEO600
• LCGT
• LIGO
• Laser Interferometer Space Antenna
• MiniGrail
• Pulsar Timing Array
• TAMA 300
• Virgo ( gravitational wave detector )