GMAT 考满分题库

     The general theory of relativity posits that major events in the universe, such as the creation of a black hole, create ripples in the space-time continuum, also known as gravitational waves, which were recently detected for the first time by the Laser Interferometer Gravitational Observatory (LIGO). Though the interferometer was invented in 1887, not until 2002 was one used in an attempt to detect gravitational waves. The instrument splits a laser beam so that the two resulting beams travel down perpendicular arms—perfectly identical tubes that are four kilometers long—then hit mirrors and return to their shared origin point. This allows the instrument to detect extremely minute changes in the lengths of the arms. In the absence of a gravitational wave, the wavelengths of the two beams match up perfectly. However, if something interferes with the beams' paths, their peaks and valleys no longer align, and the instrument registers the interference.

     Gravitational waves cause space to shrink in one direction and expand in a perpendicular direction. If the interferometer is hit by a gravitational wave, one arm will grow and the other will shrink by normally imperceptible amounts, but the differing distances traveled by the laser beams are registered by the instrument. The LIGO actually consists of two interferometers, thousands of kilometers apart, which compare data in order to rule out localized vibrations and flickers. In September 2015 it detected its first gravitational wave, which was determined to have been generated by the collision of two black holes between one and two billion years ago. Another gravitational wave was detected in December 2015.