They constructed an experiment called LIGO (Laser Interferometer Gravitational-Wave Observatory). It has two four-kilometer-long tunnels at right angles to each other, and uses a laser to measure the changes in the distance between the ends of the tunnels. When a gravitational wave comes through, it stretches space in one direction and squeezes space in the other direction. By measuring the interference of the lasers as they bounce between the different ends, physicists can measure very precisely whether the space in between has stretched or compressed. In 2016, after $620 million and decades
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