Gravitational waves (GW) are ripples in spacetime. The existence of the GW was predicted by Einstein's theory of general relativity in 1916. Its existence was indirectly proved by R.Hulse and J.Taylor in the 1980's. They have obseved a neutron star binary PSR1913+16 for more than 10 years, and then it is found that the distance of the binary is gradually reduced owing to GW emission. They were awarded the Nobel prize for this work.

On September 14 2015, the LIGO detectors directly observed gravitational waves from the merger of two black holes. This is an enormous achievement, but only the beginning of the entirely new field of gravitational wave astronomy.

GWs are generated from an accelerated mass. However, only dense and massive objects can generate GWs with observable amplitudes. Astronomical motions and events are likely to be potential sources of GWs. Some examples of sources are:

• binary compact stars (neutron stars, black holes and white dwarfs): their inspiral and mergers
• spinning neutron stars
• density fluctuation in the early universe
• super novae

Detection of these waveforms will allow us to verify general relativity and will contiribute to the creation of new astronomy.

J.Weber started a first attempt of detecting gravitational waves with so-called bar detectors in the 1960's, having being followed by many similar efforts all over the world. Then, at present, laser interferometeric detectors are considered as one of the most promising types of the detectors.

GWs can change a proper distance between free masses. Accordingly, a laser interferometer uses this feature and measures the change of the proper distance between mirrors. It is necessary for the mirrors to move as free masses, the mirrors are to be suspended with wires like a pendulum.

A Michelson interferometer is the standard type of the laser interferometer GW detectors. A laser beam is splitted into two paths with an "L"-shape. Each of the two beams are reflected by a mirror. The reflected two beams are recombined at the beam splitter, and inteferes. A photodetector is placed in order to receive this interference pattern. If GWs arrive at the detector, the optical paths of the arms shrinks and expands in a differential way, resulting in changes of the interference condition. By monitoring this contrast change, we can detect GWs.

It should be noted that there are the other detection method; pulsar timing method which uses the accurate clock signal from cosmic pulsars.

Laser interferometers are the most popular type of the ground-based detectors. Since the longer arm length yields the higher response against GWs, large interferometer detectors with arm length of hundreds-meters to kilo-meters have been constructed.

We have constructed a detector with 300-meter arms, TAMA300, at NAOJ Mitaka campus in Japan. TAMA300 started its observation in 1999, achieving the world highest sensitivity at that time, and performed long-term (~2 months) observation runs. In addition, we are cooperating with the Institute for Cosmic Ray Research for the development of the 100-m CLIO interferometer at Kamioka mine, Gifu, Japan. CLIO is a prototype for the Japanese future 3-km detector plan LCGT. There are the detectors, GEO600, VIRGO and LIGO, all over the world.
=>TAMA300　=>CLIO　=>LCGT
Link to GEO600 / VIRGO / LIGO

Ground-based laser interferometers are not suitable for the detection of low frequency gravitational waves because the suspended mirrors do not behave as free masses. Therefore, the space-borne detectors are proposed. The mirrors are launched on a satellite orbit to make them free masses even at low frequency. Currently our NAOJ group and collaborators started the consideration of space-craft detector called DECIGO. In abroad, LISA project is conducted by ESA and NASA.
=>DECIGO
Link to LISA@NASA / LISA@ESA