A gravitational-wave observatory (or gravitational-wave
detector) is any device designed to measure
gravitational waves, tiny distortions of spacetime
that were first predicted by Einstein in 1916.
Gravitational waves are perturbations in the
theoretical curvature of spacetime caused
by accelerated masses.
The existence of gravitational radiation is
a specific prediction of general relativity,
but is a feature of all theories of gravity
that obey special relativity.
Since the 1960s, gravitational-wave detectors
have been built and constantly improved.
The present-day generation of resonant mass
antennas and laser interferometers has reached
the necessary sensitivity to detect gravitational
waves from sources in the Milky Way.
Gravitational-wave observatories are the primary
tool of gravitational-wave astronomy.
A number of experiments have provided indirect
evidence, notably the observation of binary
pulsars, the orbits of which evolve precisely
matching the predictions of energy loss through
general relativistic gravitational-wave emission.
The 1993 Nobel Prize in Physics was awarded
for this work.In February 2016, the Advanced
LIGO team announced that they had detected
gravitational waves from a black hole merger.
The 2017 Nobel Prize in Physics was awarded
for this work.
== Complications ==
The direct detection of gravitational waves
is complicated by the extraordinarily small
effect the waves produce on a detector.
The amplitude of a spherical wave falls off
as the inverse of the distance from the source.
Thus, even waves from extreme systems such
as merging binary black holes die out to a
very small amplitude by the time they reach
the Earth.
Astrophysicists predicted that some gravitational
waves passing the Earth might produce differential
motion on the order 10−18 m in a LIGO-size
instrument.
== Resonant mass antennas ==
A simple device to detect the expected wave
motion is called a resonant mass antenna – a
large, solid body of metal isolated from outside
vibrations.
This type of instrument was the first type
of gravitational-wave detector.
Strains in space due to an incident gravitational
wave excite the body's resonant frequency
and could thus be amplified to detectable
levels.
Conceivably, a nearby supernova might be strong
enough to be seen without resonant amplification.
However, up to 2018, no gravitational wave
observation that would have been widely accepted
by the research community has been made on
any type of resonant mass antenna, despite
certain claims of observation by researchers
operating the antennas.
There are 3 types of resonant mass antenna
that have been built: the room-temperature
bar antennas, the cryogenically cooled bar
antennas and cryogenically cooled spherical
antennas.
The earliest types of antennas were the room-temperature
bar-shaped antennas called Weber bar; these
were dominant in 1960s and 1970s and many
were built around the world.
It was claimed by Weber and some others in
the late 1960s and early 1970s that these
devices did observe gravitational waves; however,
other experimenters failed to detect gravitational
waves with these devices and thus it became
consensus that these devices could not detect
gravitational waves.
The second generation of resonant mass antennas,
developed in the 1980s and 1990s, were the
cryogenic bar antennas which are also sometimes
called Weber bars.
There were in the 1990s 5 major cryogenic
bar antennas: AURIGA (Padua, Italy), NAUTILUS
(Rome, Italy), EXPLORER (CERN, Switzerland),
ALLEGRO (Louisiana, USA), NIOBE (Perth, Australia).
In 1997, these 5 antennas run by 4 research
groups formed the International Gravitational
Event Collaboration (IGEC) for collaboration.
Over the years, many claims of detection of
gravitational waves have been made by scientist
using cryogenic bar antennas but none of these
was accepted by the larger community.
In 1980s there was also a cryogenic bar antenna
called ALTAIR, which along with a room-temperature
bar antenna called GEOGRAV was built in Italy
as a prototype for later bar antennas.
GEOGRAV-detector was claimed by its operators
to have seen gravitational waves coming from
the supernova SN1987A (along with another
room-temperature bar of Weber), but these
claims were also dismissed by the wider community.
These modern cryogenic forms of the Weber
bar operated with superconducting quantum
interference devices to detect vibration (see
for example, ALLEGRO).
Some of them are still in operation, for example
AURIGA, an ultracryogenic resonant cylindrical
bar gravitational wave detector based at INFN
in Italy.
The AURIGA and LIGO teams have collaborated
in joint observations.It is the current consensus
that current cryogenic Weber bars are not
sensitive enough to detect anything but extremely
powerful gravitational waves.
As of 2018, no observation of gravitational
waves by cryogenic Weber bars has occurred.
In the 2000s, the third generation of resonant
mass antennas, the spherical cryogenic antennas,
emerged.
4 spherical antennas were proposed around
year 2000 and 2 of them ended up being built
(others were cancelled) as downsized versions.
The proposed antennas were GRAIL (Netherlands,
proposal that when downsized became MiniGRAIL),
TIGA (USA, small prototypes made), SFERA (Italy),
Graviton (Brasil, proposal that when downsized
became Mario Schenberg).
Currently there are 2 cryogenic spherical
gravitational wave antennas in the world,
the MiniGRAIL and the Mario Schenberg.
These antennas are actually a collaborative
effort, having much in common.
MiniGRAIL is based at Leiden University, consisting
of an exactingly machined 1150 kg sphere cryogenically
cooled to 20 mK.
The spherical configuration allows for equal
sensitivity in all directions, and is somewhat
experimentally simpler than larger linear
devices requiring high vacuum.
Events are detected by measuring deformation
of the detector sphere.
MiniGRAIL is highly sensitive in the 2–4
kHz range, suitable for detecting gravitational
waves from rotating neutron star instabilities
or small black hole mergers.The Mario Schenberg
antenna is located in Sao Paulo, Brazil.
== Interferometers ==
A more sensitive detector uses laser interferometry
to measure gravitational-wave induced motion
between separated 'free' masses.
This allows the masses to be separated by
large distances (increasing the signal size);
a further advantage is that it is sensitive
to a wide range of frequencies (not just those
near a resonance as is the case for Weber
bars).
Ground-based interferometers are now operational.
Currently, the most sensitive is LIGO – the
Laser Interferometer Gravitational Wave Observatory.
LIGO has three detectors: one in Livingston,
Louisiana; the other two (in the same vacuum
tubes) at the Hanford site in Richland, Washington.
Each consists of two light storage arms which
are 2 to 4 kilometers in length.
These are at 90 degree angles to each other,
with the light passing through 1m diameter
vacuum tubes running the entire 4 kilometers.
A passing gravitational wave will slightly
stretch one arm as it shortens the other.
This is precisely the motion to which an interferometer
is most sensitive.
Even with such long arms, the strongest gravitational
waves will only change the distance between
the ends of the arms by at most roughly 10−18
meters.
LIGO should be able to detect gravitational
waves as small as
h
≈
5
×
10
−
22
{\displaystyle h\approx 5\times 10^{-22}}
. Upgrades to LIGO and other detectors such
as VIRGO, GEO 600, and TAMA 300 should increase
the sensitivity still further; the next generation
of instruments (Advanced LIGO and Advanced
Virgo) will be more than ten times more sensitive.
Another highly sensitive interferometer (KAGRA)
is currently in the design phase.
A key point is that a ten-times increase in
sensitivity (radius of "reach") increases
the volume of space accessible to the instrument
by one thousand.
This increases the rate at which detectable
signals should be seen from one per tens of
years of observation, to tens per year.
Interferometric detectors are limited at high
frequencies by shot noise, which occurs because
the lasers produce photons randomly; one analogy
is to rainfall – the rate of rainfall, like
the laser intensity, is measurable, but the
raindrops, like photons, fall at random times,
causing fluctuations around the average value.
This leads to noise at the output of the detector,
much like radio static.
In addition, for sufficiently high laser power,
the random momentum transferred to the test
masses by the laser photons shakes the mirrors,
masking signals at low frequencies.
Thermal noise (e.g., Brownian motion) is another
limit to sensitivity.
In addition to these "stationary" (constant)
noise sources, all ground-based detectors
are also limited at low frequencies by seismic
noise and other forms of environmental vibration,
and other "non-stationary" noise sources;
creaks in mechanical structures, lightning
or other large electrical disturbances, etc.
may also create noise masking an event or
may even imitate an event.
All these must be taken into account and excluded
by analysis before a detection may be considered
a true gravitational-wave event.
Space-based interferometers, such as LISA
and DECIGO, are also being developed.
LISA's design calls for three test masses
forming an equilateral triangle, with lasers
from each spacecraft to each other spacecraft
forming two independent interferometers.
LISA is planned to occupy a solar orbit trailing
the Earth, with each arm of the triangle being
five million kilometers.
This puts the detector in an excellent vacuum
far from Earth-based sources of noise, though
it will still be susceptible to shot noise,
as well as artifacts caused by cosmic rays
and solar wind.
An atomic gravitational-wave interferometric
sensor (AGIS) is an alternative means to detect
gravitational waves, proposed 
in 2008.
=== Einstein@Home ===
In some sense, the easiest signals to detect
should be constant sources.
Supernovae and neutron star or black hole
mergers should have larger amplitudes and
be more interesting, but the waves generated
will be more complicated.
The waves given off by a spinning, bumpy neutron
star would be "monochromatic" – like a pure
tone in acoustics.
It would not change very much in amplitude
or frequency.
The Einstein@Home project is a distributed
computing project similar to SETI@home intended
to detect this type of simple gravitational
wave.
By taking data from LIGO and GEO, and sending
it out in little pieces to thousands of volunteers
for parallel analysis on their home computers,
Einstein@Home can sift through the data far
more quickly than would be possible otherwise.
== High frequency detectors ==
There are currently two detectors focusing
on detections at the higher end of the gravitational-wave
spectrum (10−7 to 105 Hz): one at University
of Birmingham, England, and the other at INFN
Genoa, Italy.
A third is under development at Chongqing
University, China.
The Birmingham detector measures changes in
the polarization state of a microwave beam
circulating in a closed loop about one meter
across.
Two have been fabricated and they are currently
expected to be sensitive to periodic spacetime
strains of
h
∼
2
×
10
−
13
/
H
z
{\displaystyle h\sim {2\times 10^{-13}/{\sqrt
{\mathit {Hz}}}}}
, given as an amplitude spectral density.
The INFN Genoa detector is a resonant antenna
consisting of two coupled spherical superconducting
harmonic oscillators a few centimeters in
diameter.
The oscillators are designed to have (when
uncoupled) almost equal resonant frequencies.
The system is currently expected to have a
sensitivity to periodic spacetime strains
of
h
∼
2
×
10
−
17
/
H
z
{\displaystyle h\sim {2\times 10^{-17}/{\sqrt
{\mathit {Hz}}}}}
, with an expectation to reach a sensitivity
of
h
∼
2
×
10
−
20
/
H
z
{\displaystyle h\sim {2\times 10^{-20}/{\sqrt
{\mathit {Hz}}}}}
. The Chongqing University detector is planned
to detect relic high-frequency gravitational
waves with the predicted typical parameters
~ 1010 Hz (10 GHz) and h ~ 10−30 to 10−31.
== Pulsar timing arrays ==
A different 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.
These projects propose to detect gravitational
waves by looking at the effect these waves
have on the incoming signals from an array
of 20–50 well-known millisecond pulsars.
As a gravitational wave passing through the
Earth contracts space in one direction and
expands space in another, the times of arrival
of pulsar signals from those directions are
shifted correspondingly.
By studying a fixed set of pulsars across
the sky, these arrays should be able to detect
gravitational waves in the nanohertz range.
Such signals are expected to be emitted by
pairs of merging supermassive black holes.
== Cosmic microwave background polarization
==
The cosmic microwave background, radiation
left over from when the Universe cooled sufficiently
for the first atoms to form, can contain the
imprint of gravitational waves from the very
early Universe.
The microwave radiation is polarized.
The pattern of polarization can be split into
two classes called E-modes and B-modes.
This is in analogy to electrostatics where
the electric field (E-field) has a vanishing
curl and the magnetic field (B-field) has
a vanishing divergence.
The E-modes can be created by a variety of
processes, but the B-modes can only be produced
by gravitational lensing, gravitational waves,
or scattering from dust.
On 17 March 2014, astronomers at the Harvard-Smithsonian
Center for Astrophysics announced the apparent
detection of the imprint gravitational waves
in the cosmic microwave background, which,
if confirmed, would provide strong evidence
for inflation and the Big Bang.
However, on 19 June 2014, lowered confidence
in confirming the findings was reported; and
on 19 September 2014, even more lowered confidence.
Finally, on January 30, 2015, the European
Space Agency announced that the signal can
be entirely attributed to dust in the Milky
Way.
== Operational and planned gravitational-wave
detectors ==
(1995) TAMA 300
(1995) GEO 600
(2002) LIGO
(2003) Mario_Schenberg_(Gravitational_Wave_Detector)
(2003) MiniGrail
(2005) Pulsar timing array (for Parkes radio-telescope)
(2006) CLIO
(2007) Virgo interferometer
(2015) Advanced LIGO
(2016) Advanced Virgo
(2018) KAGRA (LCGT)
(2023) IndIGO (LIGO-India)
(2025) TianQin
(2027) Deci-hertz Interferometer Gravitational
wave Observatory (DECIGO)
(2034) Laser Interferometer Space Antenna
(Lisa Pathfinder, a development mission was
launched December 2015)
(2030s) Einstein Telescope
== See also ==
Gravitational-wave astronomy
Detection theory
Matched filter
== References ==
== External links ==
Video (04:36) – Detecting a gravitational
wave, Dennis Overbye, NYT (11 February 2016).
Video (71:29) – Press Conference announcing
discovery: "LIGO detects gravitational waves",
National Science Foundation (11 February 2016).
