# Gravitational wave detector

A gravitational wave detector (also known as an observatory) is an experimental setup with which small disturbances in space-time ( gravitational waves ) are measured, which were predicted by Albert Einstein's general theory of relativity .

On February 11, 2016, the LIGO Observatory announced that it had directly measured and thus detected gravitational waves from two colliding black holes for the first time in September 2015. In 2017, the scientists Rainer Weiss (USA, 50%), Barry C. Barish and Kip S. Thorne (USA, 25% each) were honored with the Nobel Prize in Physics for “decisive contributions to the LIGO detector and the observation of gravitational waves” .

## Proportions

The local detection of gravitational waves is made more difficult by the extremely small effect of the waves on the detector. The amplitude of a gravitational wave is inversely proportional to the distance from the source. As a result, even waves from extreme systems such as two merging black holes on their way to Earth fade to a small amplitude. Astrophysicists expect some of the waves to have a relative change in length of around h ≈ 10 −20 , but usually not greater.

## Resonance detector

A simple device for detecting wave movements is the resonance detector : a large, solid metal rod that is isolated from external vibrations. This type of instrument was the first type of gravitational wave detector. The pioneer of this development was Joseph Weber . Deformations of the room caused by a gravitational wave stimulate the resonance frequency of the rod and can thus be amplified beyond the detection limit. It is also conceivable that a nearby supernova would be strong enough to be seen without the resonance enhancement. Resonance detectors can only detect extremely strong gravitational waves.

Modern forms of resonance detectors are now cooled with cryogenics and read out by SQUID sensors. MiniGRAIL is a spherical gravitational wave antenna that uses this principle. It is located at the University of Leiden and consists of a precisely manufactured ball weighing 1150 kg, which has been cryotechnically cooled to 20 mK. The spherical shape gives the same sensitivity in all directions and is experimentally a bit simpler than the larger linear devices that require a high vacuum. Proof is provided by measuring the multipole moments . MiniGRAIL is very sensitive in the 2 to 4 kHz range and is therefore suitable for the detection of gravitational waves emanating from rotating neutron stars or when small black holes merge.

## Interferometric detector

Schematic diagram of a laser interferometer .

A more sensitive detector uses laser interferometry to measure the movement of “free” masses caused by gravitational waves. This allows a large distance between the masses. The constancy of the speed of light in a vacuum is used to measure the distance covered by the masses during the triggered movement . The light runs continuously in a vacuum tube. The two arms of the detector are at right angles to each other. Another advantage is the sensitivity in a wide frequency range (not only in the vicinity of the resonance frequency as in the case of the resonance detector).

Ground-based interferometers are now in operation. Currently the most sensitive is LIGO - the laser interferometer gravitational observatory. LIGO has three detectors: one is in Livingston, Louisiana , and the other two (in the same vacuum tube) are in Hanford Site in Richland, Washington . Each consists of two Fabry-Pérot interferometers , which used to be two kilometers long and are now four kilometers. Since a gravitational wave is a transverse wave , it stretches and compresses a space slightly when it passes through it. Thus, the simultaneous consideration of the change in length of the two arms results in different signs. The direction from which the wave came can be narrowed down by lateration .

Even with such long arms, the strongest gravitational waves change the distance between the ends of the arms by a maximum of about 10 −18  meters. LIGO should be able to measure small gravitational waves of h ≈ 5 · 10 −22 . Improvements to LIGO and other detectors such as B. Virgo , GEO600 and TAMA 300 should increase the sensitivity further. The next generation (Advanced LIGO, Advanced Virgo and KAGRA ) should be ten times as sensitive. An important point is that increasing the sensitivity by a factor of ten increases the volume of the observable space by a factor of 1000. This increases the rate of detectable signals from one within decades to dozens per year.

Interferometric detectors are limited at high frequencies by shot noise , which is caused by lasers emitting photons at random. This leads to noise in the output signal of the detector. In addition, if the laser radiation is strong enough, a random impulse is transmitted to the test masses by the photons. As a result, low frequencies are covered. After all, the detection itself has noise analogous to the source's shot noise. Thermal noise (e.g. Brownian motion ) is another limitation on sensitivity. In addition, all ground-based detectors are limited by seismic noise and other environmental vibrations at low frequencies. These include the creaking of mechanical structures, lightning strikes, or other electrical disturbances that generate noise and that mask or simulate an event. All of these factors must be considered and excluded from the analysis before an event can be considered gravitational wave detection.

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

## High frequency detectors

There are currently two detectors that focus on detecting high frequency gravitational waves from 0.1 to 10 MHz: 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 Chongqing University , China. The English detector measures the change in the polarization state of a microwave beam that circles in a closed loop of about one meter. Two rings have been built and are expected to be susceptible to spacetime distortions with a power spectral density of . The INFN detector in Genoa is a resonance antenna that consists of two coupled spherical superconductors with a diameter of a few centimeters. The resonators, when they are decoupled, should have almost the same resonance frequency. The system is said to have a sensitivity to spacetime distortions with a power spectral density of . The Chinese detector should be able to detect high-frequency gravitational waves with the predicted typical parameters f g  ~ 10 GHz and h  ~ 10 −30 to 10 −31 . ${\ displaystyle h \ sim {2 \ cdot 10 ^ {- 13} / {\ sqrt {\ mathit {Hz}}}}}$${\ displaystyle h \ sim {2 \ cdot 10 ^ {- 20} / {\ sqrt {\ mathit {Hz}}}}}$

## Pulsar timing method

Another approach to detecting gravitational waves is used by pulsar timing arrays such as the European Pulsar Timing Array , the North American Nanohertz Observatory for Gravitational Waves, and the Parkes Pulsar Timing Array . The purpose of these projects is to detect gravitational waves by observing the signals from 20 to 50 well-known millisecond pulsars. As the gravitational wave passes the earth, space contracts in one direction and expands in the other. The arrival times of the pulsar signals are shifted accordingly. By observing a fixed number of pulsars scattered across the sky, gravitational waves in the nanohertz range should be observed. Pairs of merging supermassive black holes are expected to emit such signals. Although the measurements according to current models should be sensitive enough for detection, no gravitational waves were found until 2015.

## Einstein @ Home

The most easily detectable signals should come from constant sources. Supernovae and neutron star and black hole mergers 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 tone in acoustics . The signal would hardly change in amplitude or frequency.

Einstein @ home is a project for distributed computing with the purpose of detecting these simple gravitational waves. LIGO and GEO600 data are broken down into small packages and distributed to thousands of volunteer computers who do the analysis. Einstein @ Home can screen the data much faster than otherwise possible.

## Individual evidence

1. Max Planck Institute for Gravitational Physics: Gravitational waves discovered 100 years after Einstein's prediction. February 11, 2016, accessed February 11, 2016 .
2. B. P. Abbott, R. Abbott et al. a .: Observation of Gravitational Waves from a Binary Black Hole Merger (PDF) . In: Physical Review Letters. 116, 2016, doi: 10.1103 / PhysRevLett.116.061102 .
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4. Janka, Hans-Thomas: Supernovae and cosmic gamma-ray bursts. Causes and consequences of star explosions. Heidelberg: Spektrum Akademischer Verlag, 2011; P. 170
5. For a review of early resonance detectors, see J. Levine: Early Gravity-Wave Detection Experiments, 1960–1975 . In: Physics in Perspective (Birkhäuser Basel) . 6, No. 1, April 2004, pp. 42-75. doi : 10.1007 / s00016-003-0179-6 .
6. Arlette de Waard, Luciano Gottardi, Giorgio Frossati : Spherical Gravitational Wave Detectors: cooling and quality factor of a small CuAl6% sphere . In: Marcel Grossman meeting on General Relativity . Italy.
7. The idea of ​​using laser interferometry for the detection of gravitational waves was first mentioned by ME Gertsenshtein and V. Pustovoit in 1963 Sov. Phys. – JETP, Volume 16, p. 433. Joseph Weber mentioned it in an unpublished laboratory book in the mid-1960s. Rainer Weiss first described in detail a practical solution with the analysis of realistic limits of technology in R. Weiss (1972). Electromagetically Coupled Broadband Antenna Gravitational , Quarterly Progress Report, Research Laboratory of Electronics, MIT , 105: 54 ( The electromagnetically coupled broadband gravitational antenna ). For the history, see Kip Thorne Gravitational Radiation , in Hawking, Israel (editor) 300 years of gravitation , Cambridge University Press 1987, p. 413. There is also an early essay by Felix Pirani On the physical significance of the Riemann Tensor , Acta Physica Polonica, Vol. 15, 1956, pp. 389-405.
8. ^ Matthew Pitkin, Stuart Reid, Sheila Rowan, Jim Hough : Gravitational Wave Detection by Interferometry (Ground and Space), Living Rev. Relativity 14, 2011, pp. 5 ff
9. ^ GH Janssen, BW Stappers, M. Kramer, M. Purver, A. Jessner, I. Cognard: European Pulsar Timing Array , in 40 Years of Pulsars: Millisecond Pulsars, Magnetars and More . AIP Conference Proceedings, Volume 983, 2008, pp. 633-635, bibcode : 2008AIPC..983..633J
10. North American Nanohertz Observatory for Gravitational Waves (NANOGrav) homepage
11. Parkes-Pulsar-Timing-Array-Homepage
12. GB Hobbs et al. a .: Gravitational wave detection using pulsars: status of the Parkes Pulsar Timing Array project , 2008, arxiv : 0812.2721 (English)
13. ^ John Timmer: Gravitational waves missing in action in latest test. September 27, 2015, accessed November 28, 2015 .
14. Einstein @ Home