First published in 1686, Newton’s Law of Gravitation laid the foundation for modern physics: that every particle in the universe is attracted to and attracted by every other particle changed the way scientists and mathematicians viewed the world. But could gravity hold onto light?
The speed of light is finite and represents an upper limit to velocity in our present understanding. The idea, then, that a star could be so massive that its escape velocity would be greater than the speed of light first occurred to the English astronomer and priest John Michell in 1783. Michell calculated that a star 500 times larger than the Sun but with the same density would have a gravitational pull so large its light would be trapped. In 1796, the French polymath Pierre-Simon Laplace came to the same conclusion but for a star only 250 times larger than the sun.
Both scientists had essentially outlined the object we now call a black hole but it really wasn’t until Einstein published his General Theory of Relativity in 1915 that the idea took hold. Within two months of its publication, the German astrophysicists Karl Schwarzschild was able to wade through the complicated mathematical equations and provide the first theoretical description of a black hole using general relativity. He provided a solution to Einstein’s equation describing the curvature of space-time around a spherically symmetric, non-rotating mass.
Schwarzschild’s metric provided a practical approach for tests of general relativity, such as the precession of Mercury’s perihelion, the gravitational bending of light, or the confirmation of gravitational time dilation. But the equation also demanded the existence of two extraordinary points. The first being at a radius of zero (r = 0) which corresponds to a true singularity while the second so-called Schwarzschild singularity corresponded to a distance, R, much further from the origin (R = 2GM/c^2). We now know the latter as the event horizon. It is the sphere surrounding a black hole representing the point-of-no-return. It defines the region of space-time isolated from the rest of the universe.
In 1939, the American physicist Robert Oppenheimer, with his student Hartland Snyder, were studying the collapse of a spherical cloud of matter and realized the importance of the Schwarzschild radius: “The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists.” But their results were predicated on spherical symmetry and for many years arguments were presented refuting the premise. Indeed, American physicist John Wheeler speculated quantum mechanics would prevent the collapse of space-time to a singularity.
In the late 1950s, compact and powerful radio sources were identified in all-sky surveys with no detectable visible counterpart. These objects were labeled quasars – short for quasi-stellar radio objects. In the early 1960s, optical astronomers were finally able to identify extragalactic visible objects associated with quasars. Because of the distances involved, their luminosity would need to be 1,000 times greater than the output from all the stars in our entire galaxy. Quasars were originally postulated as supermassive stars but their size meant they would be extremely unstable. The question became could they be black holes?
The discovery of quasars prompted Wheeler to reconsider the notion of gravitational collapse and the formation of singularities. He discussed his ideas with Roger Penrose who set out to analyze what would happened without the assumption of spherical symmetry. He only needed to assume the collapsing matter had a positive energy density.
But to do this, he need to invent a new mathematics built on the concept of trapped surfaces – two-dimensional surfaces with the property that all light rays orthogonal to the surface converge when tracked toward the future, regardless of the curvature of the surface. Schwarzschild’s spherical symmetry is just a special case of Penrose’s mathematics. Penrose had provided the mathematics for describing black holes and so “for his discovery that black hole formation is a robust prediction of the general theory of relativity,” Roger Penrose has been awarded half of the 2020 Nobel Prize in Physics.
The other half goes to two astronomers Reinhard Genzel and Andrea Ghez, who followed a prediction made by John Michell in 1783. Michell wrote: “If any other luminous bodies would happen to revolve around them [super-massive stars] we might still perhaps from the motions of these revolving bodies infer the existence of the central ones with some degree of probability.”
Michell realized a super-massive star – a black hole – might be invisible but its effect on any surrounding bodies might give it away. Genzel and Ghez have each spent the last 30 years examining the core of our galaxy and for their work plotting stellar orbits in the core, they have been awarded the Nobel Prize ‘for the discovery of a supermassive compact object at the centre of our galaxy.”
We are orbiting a massive black hole that may one day consume us all.