File photo shows a technician inspects one of LIGO's core optics (mirrors) by illuminating its surface with light at a glancing angles, prior to sealing up the chamber and pumping the vacuum system down. U.S. scientists said Thursday they have detected the existence of gravitational waves, which were predicted by Albert Einstein's theory of general relativity 100 years ago. [Photo: Xinhua/Matt Heintze/Caltech/MIT/LIGO Lab]