Black Holes

A Solution Nobody Wanted

Within weeks of the publication of general relativity in late 1915, the astronomer Karl Schwarzschild found the first exact solution of the field equations. It described the space around a single spherical mass. It also contained something strange: if the mass were squeezed inside a certain radius, nothing, not even light, could get out.

For decades almost everyone, Einstein included, took this for a mathematical curiosity that nature would somehow avoid. In 1939 he published a paper arguing that such objects could not form. In the same year Robert Oppenheimer and Hartland Snyder showed that a sufficiently massive star, once its fuel is spent, would collapse in exactly this way.

From Curiosity to Certainty

The change came in the 1960s. In 1965 Roger Penrose proved that the formation of a black hole is not a fluke of perfect symmetry. Once collapse has gone far enough, general relativity makes it unavoidable. Half of the 2020 Nobel Prize in Physics went to Penrose for that proof.

The other half went to Reinhard Genzel and Andrea Ghez, who spent decades tracking stars at the center of our galaxy. The stars whip around an invisible object with about four million times the mass of the Sun, packed into a region no larger than our solar system.

The Photographs

In April 2019 the Event Horizon Telescope released the first image of a black hole. Eight radio observatories, from Hawaii to Spain to the South Pole, had observed together as a single telescope the size of the Earth.

The target was the object at the center of the galaxy M87, 55 million light-years away, with the mass of 6.5 billion Suns. The image shows a ring of glowing gas around a dark center: the shadow of the black hole, cast on the light behind it. Its size matched the prediction of Einstein's theory.

In 2022 the same collaboration imaged the black hole at the center of our own galaxy.

How Direct Is the Link?

The equations are Einstein's, and black holes follow from them under very general conditions. The conclusion belongs to others: Schwarzschild, Oppenheimer, Penrose, and the astronomers who found the objects. Einstein never accepted that they were real. It is one of several cases where his theory turned out to be bolder than he was.