The black hole information paradox is a puzzle resulting from the combination of quantum mechanics and general relativity. Calculations suggest that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state. This is controversial because it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time. A fundamental postulate of quantum mechanics is that complete information about a system is encoded in its wave function up to when the wave function collapses. The evolution of the wave function is determined by a unitary operator, and unitarity implies that information is conserved in the quantum sense.
The first direct gravitational wave observation was made on 14 September 2015 and was announced by the LIGO and Virgo interferometer collaborations on 11 February 2016. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes and subsequent “ringdown” of the single resulting black hole. The signal was named GW150914 (i.e., “Gravitational Wave 2015–09–14“). This was also the first observation of a binary black hole merger, demonstrating the existence of binary stellar-mass black hole systems, and that such mergers could occur within the current age of the universe.
This first observation was reported around the world as a remarkable accomplishment for many reasons. Efforts to prove the existence of such waves had been ongoing for over fifty years, and the waves are so minuscule that Einstein doubted they could ever be detected. The waves given off by the cataclysmic merger of GW150914 reached Earth as a ripple in space-time that changed the length of a 4-km LIGO arm by a tiny fraction of the width of a proton, proportionally equivalent to changing the distance to the nearest star by one hair’s width. The energy released during the brief climax of the event was immense, with about three solar masses converted to gravitational waves and radiated away at a peak rate of about 3.6×1049 watts — more than the combined power of all light radiated by all the stars in the observable universe. The observation was also heralded as confirming the last remaining unproven prediction of general relativity, and validating its predictions of space-time distortion in the context of large scale cosmic events, as well as inaugurating a new era of gravitational-wave astronomy, allowing probing of violent astrophysical events unobservable until now.