What Are Gravitational Waves ?
Gravitational waves (GWs) 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.
Source of GWs
GWs are generated from accelerated masses. However, only dense and massive objects can generate GWs with enough energy to be detected. Astronomical phenomena are potential sources of detectable GWs. Below are examples of GW sources:
- Orbital motion of compact binary objectsartist's renditionNAOJ
- Collision of binary starsartist's renditionNAOJ
- Spinning neutron starsartist's renditionNASA
- Super novaeartist's renditionNAOJ
- Density fluctuation in the early universeNAOJ
Detection of GWs
J.Weber started the first attempt of detecting gravitational waves with a resonant bar type detectors in the 1960's, having being followed by many similar efforts all over the world. At present, laser interferometeric detectors are considered as one of the most promising types of the detectors.
GWs change the proper distance between free masses. A laser interferometer measures the change of the proper distance between its mirrors to detect the passage of GWs. The mirrors are suspended with wires like a pendulum so that they behave like free masses at high frequencies.
The standard configuration of the interferometer for a GW detector is the Michelson interferometer. A laser beam is splitted into two orthogonal paths by a half-mirror called a beam splitter. Each of the two beams is reflected by a mirror. The reflected two beams are recombined at the beam splitter, and intefere with each other. A photodetector is placed at the output port of the intererometer to measure the light power after the interference. If GWs arrive at the detector, the optical path lenghts of the two arms are changed differentially, resulting in a change of the interference condition at the beam splitter. By monitoring the change of the light power on the photodetector, we can detect GWs.
- Principle of GW detection with a Michelson interferometer
Several large-scale laser interferometric gravitational wave detectors have been constructed around the world. TAMA300 is a large-scale prototype interferometer constructed in the Mitaka campus of NAOJ Mitaka. TAMA300 started its observation in 1999, achieving the world highest sensitivity at that time, and performed long-term (~2 months) observation runs. LIGO in the USA and Virgo in Italy (Italian-French collaboration) are km scale interferometers. GEO in Germany is a 600m interferometer focusing on the development of advanced technologies. These first generation large GW detectors started operation in the early to mid 2000. Now LIGO and Virgo are being updated to their second generation configurations. In Japan, we are promoting a km-scale detector called KAGRA at an underground site in Kamioka, Gifu prefecture, Japan. Heralded by the first detection of GW by LIGO, the network of those large-scale detectors will start the era of GW astronomy by allowing us to investigate GW signals more in detail.
- The central part of TAMA 300 gravitational wave detector
- A mirror suspended by wiresPhoto by Nikon Gijutsu Kobo
Detectors in space
Ground-based detectors have limited sensitivity at low frequencies because of the size limit as well as various low-frequency noises such as seismic motion. In order to detect low frequency GWs such as ones from the early Universe, which is expected to be strongly red-shifted, projects to construct GW detectors in space are proposed. The eLISA project promoted by the Europian Space Agency (ESA) aims to build a large (million km scale) triangular interferometer in space by launcing three satellites housing test masses. Japanese researchers are conducting R&D programs to build a 1000 km scale space GW detector called DECIGO targeting a different frequency range from eLISA. NAOJ researchers are participating in the studies of DECIGO.