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You're unlikely to ever run into a black hole, but here's what it "looks" and "sounds" like when two black holes run into each other. The movie below, produced by Frans Pretorius at Princeton University, shows a simulation of the gravitational waves generated when two black holes collide and form a third. Gravitational waves are ripples in the fabric of spacetime, resulting from events involving massive objects which distort spacetime. The waves were predicted by Einstein's theory of gravity. No-one has directly observed gravitational waves yet, but there's indirect evidence for their existence. And if you could see these ripples, this is what they would look like.
"[Collisions like this are] expected to happen in the Universe for any two black holes that are sufficiently close to form a bound, orbiting system," says Pretorius. "Their orbital motion causes the gravitational waves to be radiated outwards. However, gravitational waves carry energy, which comes at the expense of the orbital energy, which is why the black holes spiral in and merge into a single large black hole. "