Black holes are some of the most fascinating and mind-bending objects in the cosmos: gravitational bottomless pits that are predicted by Einstein’s theory of general relativity. The very thing that characterizes a black hole also makes it hard to study: its intense gravity. All the mass in a black hole is concentrated into a single point in space, surrounded by a boundary called the “event horizon,” and nothing that crosses that boundary can return to the outside universe, not even light. A black hole itself is invisible.
Nevertheless, the last decade has seen a resurgence of research into black holes and observations of their immediate surroundings. Astronomers have tracked the motion of stars around the black hole at the center of the Milky Way galaxy, made images of the glowing material falling toward the gargantuan black hole M87*, and detected the chirps of gravitational waves emanating from merging black holes billions of light-years away.
Joshua Winn, a professor of astrophysics at Princeton University, reviews the theory of black holes and these recent observational developments.
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