Einstein's theory of general relativity created a revolution in the field of Newton's dynamics. The theory derives from the idea that gravity is actually a distortion in space-time. The idea of gravitational wave (GW) has existed for nearly a century, but only now is evidence coming to light. Research institutes around the world anxiously await the detection of GWs, which may prove to be a key in understanding how things such as black holes or even the universe itself were created. Dr. Koji Arai, Assistant Professor of the TAMA project at the National Astronomical Observatory of Japan, took the time to explain GWs and the impact they've had on his research.

What exactly is a GW ?

GWs are a phenomenon that result in the distortion of space-time. Think about ripples on the surface of water or sound vibrations in the air, spreading out as waves. GWs, however, don't spread out through space via something. Instead, space-time themselves are distorted as the wave passes. You may find it somewhat difficult to imagine.

According to the theory of general relativity, gravity originates from this distortion of space-time. When a mass exists, space-time is distorted. This is gravity. And if a mass moves at high velocity, the distortion of space-time will change over time to become a wave. Einstein proved this mathematically in 1916, hypothesizing the existence of GWs.

If GWs occur when any mass moves, are GWs present also around us ?

The presence of a mass itself distorts space-time, and when any mass moves, GWs are generated and propagated at the speed of light. They are also emitted from us, but are very weak.

Think about the galaxy. Four fundamental forces exist in the universe. One is gravity, which exists between all bodies of matter even between the two of us. The second is electromagnetic force, which binds an atom's core to its electrons. Thirdly, there is "weak force," which enforces neutron's beta decay. Finally, there is "strong force," which stabilizes the core of the atom. Gravity is actually the weakest of all four forces, even weaker than the "weak force."

In areas of the galaxy where many stars are clustered close together, the electromagnetic, strong and weak forces (the three forces other than gravity) do not have much influence because they either cancel each other out or are only effective at extremely close distances. Among these four forces, only gravity has no positive or negative aspect, so it rules everything in the astronomical world. This also means that GWs can only be detected at an astrodynamic level of mass and movement.

I understand that GWs have been proven to exist. How was this accomplished ?

Hulse and Taylor discovered a pair of pulsars in 1974, and after 15 years of observation, indirectly confirmed the existence of GWs. Let me explain. A star bigger than a certain mass collapses under its own weight at the end of its lifetime (a phenomenon known as gravitational collapse), thereby triggering a supernova explosion. Pulsars, or neutron stars, are formed from the leftover neutrons of a supernova. The radius of a pulsar is only 10km, but its weight is 1.4 times that of our Sun, and Hulse and Taylor discovered a pair of pulsars orbiting each other at an enormous velocity. This provided an ideal opportunity to observe GWs. They continued observing the period of the two pulsars' orbits. If GWs were present, energy would be gradually radiated by GWs and orbiting would grow increasingly fast. The two would then gradually grow closer and finally, collide. Hulse and Taylor did not detect actual GWs, but observed changes in the pulsars' orbit that matched the estimated calculation of change they believed a GW would make to within 0.1%.

Are there any other methods for observing events such as the collision of astronomical bodies besides detecting GWs ?

Such collisions of astronomical bodies have not been detected, but supernovae have been observed many times. Ancient Chinese documents describe how a "star suddenly became incredibly bright." A supernova that occurred in 1987 was observed both by optical telescope and the detection of neutrinos. Supernovae were previously thought to occur in a galaxy only once every few decades. If you are observing several dozen galaxies, however, you may be able to catch a supernova as frequently as once a year. The further away the galaxies you observe, the better your chances. In fact, more than 100 supernovae a year have been observed with optical telescopes.