FEATURE: Explainer: black holes
A simulation of a large black hole with the Milky Way in the background. Image: Ute Kraus/Wikimedia The concept of a “black hole” is one of the most curious in astrophysics. It’s the answer to the question: “What happens if the density of matter in a region becomes so high that not even light can escape?” The reason this question even arose dates back to Einstein’s prediction of 1916, in his Foundations of General Relativity, that the direction light travels will be bent in the direction of any nearby mass. That prediction has been spectacularly confirmed in recent years by the discovery of “gravitational lenses” – where a background source and a foreground mass are so closely aligned that the light from the background source is highly distorted, even to the point of forming an almost complete arc around the foreground mass. Before 1916, we had not considered that this might be possible. After all,...