Discovery of gravitational waves

Four days ago, scientists who work in LIGO(Laser Interferometer Gravitational-Wave Observatory ) announced that they found gravitational waves through LIGO experiment. Almost one century ago, Albert Einstein had predicted the existence of gravitational waves in his famous general relativity, but no evidence have been found to prove this theory until now.

The source of gravitational waves that detected by scientists in LIGO is the collision of  two black holes. Scientists discovered that there were two black holes colliding about 1.3 billion years ago, and the gravitational waves emitted in the process of collide was detected not long ago. The energy produced by the collision was even greater than that of all the shining stars and galaxies in the observable universe. Such an event is considered to be common in the universe, but this collision was the first one that has ever been detected.

When the gravitational waves arrive a planet, the planet will be contracted and expanded. Scientist tried to use this portrait to detect the existence of gravitational waves. To observe the contraction and expansion of Earth, scientists designed an observatory which has two detectors. One detector is in Washington, the other is in Louisiana. Each detector is constructed in the shape of L, with legs four kilometers long. Each detector bounces laser light back and forth. Normally,  the light bounced back from the end of left arm and that of right arm cancel each other when arriving the detector because light beam in left arm is out of phase with that in the right arm. However, if the gravitational waves arrive Earth, the whole planet will be contracted and expanded, causing the length differences of both arms. Consequently, the laser light in left arm and right arm will not always cancel each other out but produce wave signals. Since those signals and changes are tiny, LIGO must to be extremely sensitive to detect them.  

After discovering the gravitational waves, scientific community can claims that black holes are truly exist. Before that, the evidence of existence of black holes were indirect because they can not be observed directly with light.(no light can escape from black holes) By observing gravitational waves, scientists can also observe black holes in a whole new way.