Albert Einstein questioned the existence of black holes in nature in a 1939 study published in Annals of Mathematics. Black holes are now fashionable, to the point where three scientists who have researched them will share the Nobel Prize in Physics in 2020. A Nobel Prize is the latest evidence that an idea Einstein dismissed in 1939 has resurfaced as one of the most hotly debated subjects in physics.
What you see isn’t always what you get when it comes to black holes. The ring of light visible around the silhouette of a black hole is caused by a radius of roughly 5GM/c2, where G is Newton’s constant, M is the black hole mass, and c is the speed of light. This ring is 2.5 times bigger than the perimeter of a non-spinning black hole, and up to five times larger with spin. The Event Horizon Telescope did not picture the supermassive black hole’s event horizon in the galaxy M87, but rather light from afar further distance.
The distinction between the light ring and the horizon is academic for a distant spectator, but it is existential for an astronaut en route to the black hole. Entering the final prison walls connected with the horizon is a death sentence with no way of sharing the experience with others. The astronaut’s body will approach the singularity in less than a day and be ripped apart by gravitational tidal force.
If Stephen Hawking had been living, he would have been a strong contender for the Nobel Prize this year, as his work on classical general relativity matched that of Roger Penrose, with the addition of the quantum mechanical element of black hole evaporation.