This is the first Nobel Prize awarded for black holes and indeed the first ever, in part, for theoretical advances in general relativity. Einstein's own prize in 1921 highlighted the photo-electric effect. Back then the first observational confirmation of Einstein's theory, by the bending of light, was only two years old. General relativity was still controversial.
Black holes are regions from which light cannot escape, and our notions of space and time break down. Time ends inside a black hole in a literal sense.
Einstein did not believe that black holes existed. Since the only black hole solution known in his lifetime did not rotate, that was reasonable. Every compact object in the Universe rotates, and rotation throws matter outwards, so researchers guessed this would prevent black holes forming.
All that changed in 1963 when a 29-year old New Zealander, Roy Kerr, discovered the rotating black hole solution of Einstein's equations, establishing black holes as likely objects of the physical Universe. Then 18 months later Roger Penrose established the first "singularity theorem".
Penrose considered the non-equilibrium situation of collapsing matter obeying general conditions consistent with known physics: its energy density is positive and the speed of sound does not exceed the speed of light. With those minimal assumptions, Penrose showed that "closed trapped regions" must form, from which light cannot escape. This showed black holes are inevitable, earning Penrose the Nobel Prize now, 55 years later.
The 1960s became a Golden Age of general relativity to which Penrose made major contributions, including the Penrose process by which the rotational energy of Kerr black holes can be transferred to energetic particles. Such processes are now understood to be involved in accretion disks around supermassive black holes that sit in the centres of all galaxies.
When the Universe was young such supermassive black holes grew, feeding on copious gas. Magnetic fields form during the accretion of matter, and as a byproduct some particles escape falling in. Magnetic fields accelerate charged particles to near the speed of light in jets along the rotation axis of the black hole. This is the power source of quasars and other active galactic nuclei: the most energetic phenomena in the Universe after the Big Bang.
Our own Milky Way galaxy houses a supermassive black hole, Sagittarius A*. It no longer has an active accretion disk and is invisible to telescopes. However, Andrea Ghez and Rheinhardt Genzel have made observations of stars orbiting around the galactic centre over a period of 28 years. From the orbits they deduced that the invisible central object has a mass 4.6 million times that of the sun: a supermassive black hole! This earned them the other half of this year's Nobel Prize.
Although black holes entered the public imagination some 50 years ago it took decades for technology to catch up and definitively prove their existence, with observations such as those of astronomers Ghez and Genzel.
Roy Kerr's solution is now the work horse for physicists and astronomers studying the weird and wonderful properties of black holes.
The discovery of gravitational waves from the merger of black holes in 2015, awarded the 2017 Nobel Prize, has opened up a completely new field of gravitational wave astronomy. Our understanding of black holes has changed from being theoretical curiosities to central objects in the evolution and "ecology" of galaxies.
There is so much we still do not understand! Gravitational wave astronomy will probe the secrets of black holes, making the next few decades ones of further epic discoveries.
Professor David Wiltshire
School of Physical and Chemical Sciences
University of Canterbury
Homepage:
http://www2.phys.canterbury.ac.nz/~dlw24/