D

  About l.3 billion years ago, two massive black holes hit each other and formed a new one. The energy set free by the collision created a ripple(波纹) in the space-time structure and spread outward in gravitational waves  (引力波)  .

  Then, on September 14, 2015, a group of scientists discovered the waves. On February 11, an announcement came from the Laser Interferometer Gravitational-Wave Observatory (LIOO)  in the US that, for the very first time, a gravitational wave was directly observed and recorded.  "We discovered gravitational waves. We did it," David Reitze, the director of LIGO, said in the

press conference on February 11. "Itˈs exactly what Einsteinˈs theory of general relativity predicted:ˈ

  Einsteinˈs 1915 theory re-imagined the framework for the universe. According to Einstein, the framework for the universe-or the space-time structure-is not fixed, but changed by  matter and energy "in a way a heavy sleeper causes a mattress to sag (床垫下凹) , producing the  effect we call gravity", explains a New York Times article.

  "A disturbance in the universe could cause space-time to become larger, break into pieces and even move up and down, like a mattress shaking when that sleeper rolls over, producing  ripples of gravity: gravitational waves," explains the article.

  Compared with the other three forces in the universe (electromagnetism, the weak nuclear  force and the strong nuclear force), gravity is relatively weak, making gravitational waves hard to discover.

  The discovery by LIGO would open a new chapter in astronomy. "Everything else in  astronomy is like the eye," Szabolcs Marka, a Columbia University professor, told The New York  Times. "Finally, astronomy grew ears. We never had ears before."