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Scientific history was made with the first observation of gravitational waves in 2015. The signal recorded by the Laser Interferometer Gravitational-Wave Observatory (LIGO) came from 1.3 billion light-years away and was generated by the merging of two black holes.1 (For more on LIGO and gravitational-wave detection, see Physics Today, April 2016, page 14, and the article by Barry Barish and Rainer Weiss, Physics Today, October 1999, page 44.) The data were published the following year, 100 years after Albert Einstein’s prediction of gravitational waves.2
nterestingly, during his later life, Einstein was no longer convinced that gravitational waves existed. In 1936 he prepared a manuscript that claimed he had mathematically proven their nonexistence. But after Howard Percy Robertson convinced him that the proof was flawed, Einstein completely rewrote it and subsequently said that he did not know whether there were gravitational waves (see the article by Daniel Kennefick, Physics Today, September 2005, page 43). Since the existence of gravitational waves was only rigorously deduced from the general theory of relativity after Einstein’s death, it can be assumed that his thoughts remained inconclusive on the question.
Einstein was also concerned with a particular consequence of quantum physics, namely quantum entanglement, or more generally quantum correlation. The effect was first mentioned as a thought experiment, now known as the EPR paradox, in a 1935 paper written by Einstein and his colleagues Boris Podolsky and Nathan Rosen.3 The authors hypothesized that quantum correlations proved the incompleteness of quantum theory. Today, however, we know that Einstein and his colleagues were wrong, and it has since been proven that quantum theory is complete within its scope. Still, physicists struggle to make the physics of quantum-correlated systems understandable without adding assumptions that go beyond quantum theory.4
Without question, Einstein would be amazed that we are now using gravitational waves to understand the universe, that the first observations have already discovered a larger number of black holes in the universe than previously assumed, and that future observatories are expected to probe the first fractions of a second of the Big Bang. But I suspect that he would be doubly amazed to know that the quantum correlations that he and his two colleagues described in 1935, which still elude a self-evident physical understanding, are now being used as a tool to improve observations of gravitational wave

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