With black holes, what you see is not what you get. The ring of light visible around a black hole’s silhouette originates from a radius of about 5GM/c2, where G is Newton’s constant, M is the black hole mass and c is the speed of light. This ring is larger than the event horizon of a nonspinning black hole by a factor of 2.5—or up to five with the addition of spin. And so, truth in advertising requires me to tell you the Event Horizon Telescope did not actually image the event horizon of the supermassive black hole in the galaxy M87, but rather the light from much farther out.
For a distant observer, the difference between the light ring and the horizon is academic, but for an astronaut en route into the black hole, the difference is existential. Entering the ultimate prison walls associated with the horizon implies a death sentence, with no opportunity for sharing the experience with the outside world. After less than a day, the astronaut’s body will reach the singularity and be torn apart by gravitational tidal force.
In 1939, Albert Einstein wrote a paper in Annals of Mathematics doubting that black holes exist in nature. Now, black holes are in vogue—so much so that the 2020 Nobel prize in physics was awarded to three scientists who have studied them. This gave me reason to celebrate, as the founding director of Harvard’s Black Hole Initiative, which brings together astronomers, physicists, mathematicians and philosophers, all dedicated to research on black holes.
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