What is Gravitational Wave (GW)?

Gravitational waves (GW) was introduced from the general theory of relativity by A.Einstein, and its like a ripple of space-time. Nobody has succeeded to directly detect GW as a change of space-time. In our daily life, we never noticed that space and time as a special relation between them. If we could move at the speed of light, we will notice that time could be extended due to the action of moving itself. Unfortunately, we have no technology to carry a human-beings at near the speed of light. On the other hand we have already succeeded to accelerate an elementally particle, which had known finite life time, almost at the speed of light, and to observe its particle life time was extended ! Consequently, space ant time do have special relation between them, as it were “structure of space time”. If GWs pass, the structure of space time will change a little bit.

How to detect GW?

In principle, a michelson type laser interferometer is used. A laser beam emitted from a laser source will be divided in power at a beam-splitter at a right angle. The split laser beam will be reflected by mirrors that are set away from the beam-splitter at the same distance, and come back again to the beam-splitter. At the beam-splitter, they are recombined and interfered with each other. If GWs go through the interferometer, the each arm will be extended and shrunk in the opposite phase, and the time that the each laser beam takes to travel will be different. Consequently, the interfered fringe between the laser beams will change.

Why is interesting to detect GWs ?

Firstly, we will obtain a new essentially different method to observe the Universe, because gravitational waves’ origin is gravity, not dipole charge that is an origin of the electro-magnetic waves. Once gravitational waves are generated, they are expected not to be shielded by anything. So, the only gravitational waves and neutrinos are a method to prove the beginning of the Universe.

GWs do exist ?

The Gravitational waves have been proven to be entities by Dr. Hulse and Dr. Taylor, and they received a prize of Nobel in physics in 1993. They observed the decrease of the revolution period of a binary neutron star (PSR1913+16) including a pulsar, and found that the revolution time decrease could be explained with less than 1% error by the energy loss due gravitational waves from the binary neutron stars.

What are sources of GW?

They are binary compact star systems, such as neutron stars and black holes, coalescences of compact stars, rotating neutron stars, supernovae and the big bang.

Why are GW detectors so large?

Because the absolute displacement change due to GWs is proportional to the distance between two masses that we observe.

How much dose KAGRA cost ?

It costs about 200 million US dollars ($1 = \80). The Shinkansen (Japan Railroad) costs about 60 million US dollars per 1km、the Japan highway costs 250 million US dollars per 1km, Subway in Tokyo costs about 350 million US dollars per 1km.

Who research about GW in Japan?

Because the detection of GWs is extremely challenging task for human-beings, the only University of Tokyo, Kyoto University, National Astronomical Observatory and KEK mainly proceeded researches about GWs. At this distance of time when enough theoretical and experimental researches and developments have been verified, the KAGRA project was finally selected as one of Leading-edge Research Infrastructure Program. So, we hope many collaborators in other universities and institutes will join KAGRA project with enthusiasm.

Are there any GW detectors in the World ?

There are two GW detectors in USA, and one in Pisa, Italy, accounting for their arm length over 3km. On the other hand, GEO600 in Germany has better displacement sensitivity by using advanced sensing technique. It is of course very important to have enough sensitivity for GWs, while it is also very important to make a GW detector network with each other.