GRAVITATIONAL WAVE DETECTORS upper limits on the ˚ux of gravitational waves. The Advanced LIGO...
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{ LISTENING TO THE UNIVERSE } GRAVITATIONAL WAVE DETECTORS Interferometer Detector Interferometric detectors are designed to compare the time it takes light to travel in two orthogonal directions. The interferometer works by dividing a laser beam at the beam splitter so that each beam travels along one arm. When the beams return to the beam splitter they are redirected to the photodetector. If the two beams have taken equal times (or time differences that are multiples of the period of optical oscillation), there is destructive interference of the light (dark fringe) at the photodetector. A gravitational wave propagating through the detector causes the time difference to vary by stretching spacetime along one arm of the interferometer and compressing spacetime along the other, causing an amount of light proportional to the time difference to appear at the photodetector. This signal is proportional to the gravitational wave strain h. The optical components of the detector are suspended by pendula mounted on vibration isolation systems to reduce the disturbance from seismic noise. Within the interferomenter, two classes of noise interfere with the measurements: photon counting noise limits the ability to determine the time difference (the ability to split the fringe), and radiation pressure noise moves the mirrors. The noise is reduced by regulating the amplitude, frequency and beam jitter of the laser source. However, laser adjustments which reduce the photon noise also increase the pressure noise and vice versa. The standard quantum limit is the lowest noise level achieved by balancing these effects. The next generation of interferometric detectors is expected to be limited by the standard quantum limit. Squeezing the state of light is a promising new technique which has been found to reduce noise caused by quantum effects. The injection of squeezed states of light into the output port of the interferometer will reduce photon and pressure noise, improving sensitivity beyond quantum limits. LIGO - a long baseline interferometer In recent decades, large baseline interferometers have begun operations in the United States , Italy (VIRGO), Germany (GEO) and in Japan (KAGRA). Such an international network provides detection confidence and information to determine source position and wave polarization, and thereby a means to attach gravitational wave observations to electromagnetic and neutrino astronomy. (LIGO) Long baselines gain sensitivity because gravitational wave displacements grow in proportion to the arm length while most random force noise is independent of the arm length. LIGO (Laser Interferometer Gravitational-wave Observatory) is a joint project of the California Institute of Technology and the Massachusetts Institute of Technology sponsored by the National Science Foundation. Between 2002 and 2010, LIGO operated three detectors that achieved sensitivities of h = 10 near 100 Hz. The 4-kilometer L1 detector is located near Livingston, Louisiana, and the 4km H1 detector and a companion 2km H2 detector shared a facility near Hanford, Washington. All three LIGO interferometers were upgraded in the Advanced LIGO program, and the upgraded L1 and H1 interferometers are now completely installed. The subsystems for the second Hanford instrument currently rest in storage, waiting for transport to a new detector location in India rather than undergoing installation at the location of H1 in Washington. -21 All three interferometers were run in coincidence to search for gravitational-wave bursts as well as in correlation to search for a stochastic background of gravitational waves and periodic gravitational wave sources. The detection of gravitational waves by LIGO would have required an observation at both sites, a 2/1 ratio (within statistics) in the 4 km/2 km interferometers and no detection in a host of environmental and apparatus monitors. 1962 A paper published by Gertsenshtein and Pustovoit proposing the use of interferometers for the detection of gravitational waves. 1971 Moss, Miller and Forward construct the first prototype interferometric detector with an arm length of 2m. 1980’s Interferometer detectors become operational at Caltech (40m), MIT (30m), and in Munich (30m), Glasgow (10m), and Japan (100m). 2010 LIGO achieves 'first lock' of its three first-generation detectors. 2015 The Advanced LIGO detectors begin operations. LIGO's advanced instruments should achieve a factor of ten improvement in sensitivity over their first-generation predecessors, eventually making gravitational wave detections a routine occurence. 1992 A NSF Cooperative Agreement initiates the construction of LIGO and funds related research and development. 1964 1972 1995 TAMA, Japan (300m). 2000 The first-generation LIGO detectors cease operations. No gravitational wave detections are made with these interferometers, however data from Initial LIGO establishes upper limits on the flux of gravitational waves. 1989 Proposals are submitted for LIGO in the USA (2 x 4km, 1 x 2km), VIRGO in Italy (3km), and GEO in Germany (600m). Interferometers in space Operation of a very long baseline interferometer (5 x 10 km) in solar orbit would make the low frequency (10 to 1 Hz) gravitational wave band accessible. The gravitational field gradients from seismic compression of the ground and density fluctuations in the atmosphere limit the sensitivity of terrestrial detectors below frequencies of a few Hz. The Laser Interferometer Space Antenna (LISA) is a proposed ESA mission for a space-based gravitational-wave detector. 6 -4 This material is based on work supported by the National Science Foundation under Grant Nos. 1104371 and 1307429. Leonard E Parker Center for Gravitation, Cosmology & Astrophysics [email protected] The initial interferometers did not detect any gravitational waves. However, they established upper limits on the flux of gravitational waves. The Advanced LIGO detector is designed to achieve more than a factor of 10 improvement in strain sensitivity over the initial interferometers, resulting in sensitivities which should make the detection of gravitational waves a routine occurence. The mission will place three identical spacecraft in a triangular configuration. The spacecraft consist of an outer shield and an inner proof mass. The shield will follow the proof mass by means of a servo system that uses ion engines as controllers and that protects the proof mass from random forces due to fluctuations in the solar wind and radiation pressure. The motions of the proof masses relative to each other are measured by heterodyne interferometry which can detect the gravitational wave signal. Weber, unaware of the previous suggestion, proposes detecting gravitational radiation using a laser to measure differential motion of two isolated masses in phone conversation to Forward. Weiss published detailed report on the design of gravitational wave antennae. 10 2 10 3 10 −24 10 −23 10 −22 10 −21 10 −20 10 −19 10 −18 10 −17 10 −16 Frequency [Hz] h(f), 1/sqrt[Hz] Comparison of all iLIGO Science Runs LLO 4km − S1 (2002.09.07) LLO 4km − S2 (2003.03.01) LHO 4km − S3 (2004.01.04) LHO 4km − S4 (2005.02.26) LHO 4km − S5 (2007.03.18) LHO 4km − S6 (2010.05.15) LIGO I SRD Goal, 4km Frequency [Hz] Comparison of all iLIGO Science Runs h(f), 1/sqrt[Hz] 10 2 10 3 10 -16 10 -17 10 -18 10 -19 10 -20 10 -21 10 -22 10 -23 10 -24 LIGO Scientific Collaboration The evolution of LIGO strain sensitivity vs. frequency from 2002 to 2010 due to detector improvements.
Transcript of GRAVITATIONAL WAVE DETECTORS upper limits on the ˚ux of gravitational waves. The Advanced LIGO...
{ LISTENING TO THE UNIVERSE }GRAVITATIONAL WAVE DETECTORS
Interferometer DetectorInterferometric detectors are designed to compare the time it takes light to travel in two orthogonal directions. Theinterferometer works by dividing a laser beam at the beam splitter so that each beam travels along one arm. Whenthe beams return to the beam splitter they are redirected to the photodetector. If the two beams have taken equaltimes (or time differences that are multiples of the period of optical oscillation), there is destructive interference ofthe light (dark fringe) at the photodetector. A gravitational wave propagating through the detector causes the timedifference to vary by stretching spacetime along one arm of the interferometer and compressing spacetime along the other, causing an amount of light proportional to the time difference to appear at the photodetector. This signalis proportional to the gravitational wave strain h.
The optical components of the detector are suspended by pendula mounted on vibration isolation systems to reducethe disturbance from seismic noise. Within the interferomenter, two classes of noise interfere with the measurements:photon counting noise limits the ability to determine the time difference (the ability to split the fringe), and radiationpressure noise moves the mirrors. The noise is reduced by regulating the amplitude, frequency and beam jitter of thelaser source. However, laser adjustments which reduce the photon noise also increase the pressure noise and viceversa. The standard quantum limit is the lowest noise level achieved by balancing these effects.
The next generation of interferometric detectors is expected to be limited by the standard quantum limit. Squeezing the state of light is a promising new technique which has been found to reduce noise caused by quantum effects. Theinjection of squeezed states of light into the output port of the interferometer will reduce photon and pressure noise,improving sensitivity beyond quantum limits.
LIGO - a long baseline interferometerIn recent decades, large baseline interferometers have begun operations in the United States , Italy (VIRGO),Germany (GEO) and in Japan (KAGRA). Such an international network provides detection con�dence and informationto determine source position and wave polarization, and thereby a means to attach gravitational wave observations to electromagnetic and neutrino astronomy.
(LIGO)
Long baselines gain sensitivity because gravitational wave displacements grow in proportion to the arm length while most random force noise is independent of the arm length. LIGO (Laser Interferometer Gravitational-wave Observatory) is a joint project of the California Institute of Technology and the Massachusetts Institute of Technology sponsored by the National Science Foundation.
Between 2002 and 2010, LIGO operated three detectors that achieved sensitivities of h = 10 near 100 Hz. The 4-kilometer L1 detector is located near Livingston, Louisiana, and the 4km H1 detector and a companion 2km H2 detector shared a facility near Hanford, Washington. All three LIGO interferometers were upgraded in the Advanced LIGO program, and the upgraded L1 and H1 interferometers are now completely installed. The subsystems for the second Hanford instrument currently rest in storage, waiting for transport to a new detector location in India rather than undergoing installation at the location of H1 in Washington.
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All three interferometers were run in coincidence to search for gravitational-wave bursts as well as in correlation to search for a stochastic background of gravitational waves and periodic gravitational wave sources. The detection of gravitational waves by LIGO would have required an observation at both sites, a 2/1 ratio (within statistics) in the 4 km/2 km interferometers and no detection in a host of environmental and apparatus monitors.
1962A paper publishedby Gertsenshteinand Pustovoitproposing the useof interferometersfor the detectionof gravitationalwaves.
1971Moss, Miller and Forward construct thefirst prototypeinterferometricdetector with an armlength of 2m.
1980’sInterferometer detectorsbecome operational atCaltech (40m), MIT (30m),and in Munich (30m),Glasgow (10m), and Japan(100m).
2010
LIGO achieves 'first lock' of its threefirst-generation detectors.
2015The Advanced LIGO
detectors begin operations.
LIGO's advanced instrumentsshould achieve a factor of ten
improvement in sensitivity overtheir first-generation
predecessors, eventuallymaking gravitational wave
detections a routineoccurence.
1992A NSF CooperativeAgreement initiates theconstruction of LIGO andfunds related research anddevelopment.
1964 1972 1995TAMA, Japan (300m).
2000
The first-generation LIGO detectorscease operations. No gravitational wavedetections are made with these interferometers, however data fromInitial LIGO establishes upper limits onthe flux of gravitational waves.
1989Proposals are submitted for LIGO in the USA
(2 x 4km, 1 x 2km), VIRGO in Italy (3km), andGEO in Germany (600m).
Interferometers in spaceOperation of a very long baseline interferometer (5 x 10 km) in solar orbit would make the low frequency (10 to1 Hz) gravitational wave band accessible. The gravitational �eld gradients from seismic compression of the groundand density �uctuations in the atmosphere limit the sensitivity of terrestrial detectors below frequencies of a few Hz.The Laser Interferometer Space Antenna (LISA) is a proposed ESA mission for a space-based gravitational-wavedetector.
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-4
This material is based on work supported by the National Science Foundation under Grant Nos. 1104371 and 1307429. Leonard E ParkerCenter for Gravitation, Cosmology & [email protected]
The initial interferometers did not detect any gravitational waves. However, they established upper limits on the �ux of gravitational waves. The Advanced LIGO detector is designed to achieve more than a factor of 10 improvement in strain sensitivity over the initial interferometers, resulting in sensitivities which should make the detection of gravitational waves a routine occurence.
The mission will place three identical spacecraft in a triangular con�guration. The spacecraft consist of an outer shield and an inner proof mass. The shield will follow the proof mass by means of a servo system that uses ion engines as controllers and that protects the proof mass from random forces due to �uctuations in the solar wind and radiation pressure. The motions of the proof masses relative to each other are measured by heterodyne interferometry which can detect the gravitational wave signal.
Weber, unaware of the previoussuggestion, proposes detecting
gravitational radiation using a laserto measure differential motion of
two isolated masses in phoneconversation to Forward.
Weiss published detailed report onthe design of gravitational wave
antennae.
102
103
10−24
10−23
10−22
10−21
10−20
10−19
10−18
10−17
10−16
Frequency [Hz]
h(f)
, 1/s
qrt[H
z]
Comparison of all iLIGO Science Runs
LLO 4km − S1 (2002.09.07)LLO 4km − S2 (2003.03.01)LHO 4km − S3 (2004.01.04)LHO 4km − S4 (2005.02.26)LHO 4km − S5 (2007.03.18)LHO 4km − S6 (2010.05.15)LIGO I SRD Goal, 4km
Frequency [Hz]
Comparison of all iLIGO Science Runs
h(f
), 1/
sqrt
[Hz]
102103
10-16
10-17
10-18
10-19
10-20
10-21
10-22
10-23
10-24
LIGOScientificCollaboration
The evolution of LIGO strain sensitivity vs. frequency from 2002 to 2010 due to detector improvements.
