Nobel Prize in Physics 2020

QUANTUM gravitational effects are usually ignored in calculations of the formation and evolution of black holes. The justification for this is that the radius of curvature of space-time outside the event horizon is very large compared to the Planck length (Għ/c³)^1/2 ≈ 10⁻³³ cm, the length scale on which quantum fluctuations of the metric are expected to be of order unity. This means that the energy density of particles created by the gravitational field is small compared to the space-time curvature. Even though quantum effects may be small locally, they may still, however, add up to produce a significant effect over the lifetime of the Universe ≈ 10¹⁷ s which is very long compared to the Planck time ≈ 10⁻⁴³ s. The purpose of this letter is to show that this indeed may be the case: it seems that any black hole will create and emit particles such as neutrinos or photons at just the rate that one would expect if the black hole was a body with a temperature of (κ/2π) (ħ/2k) ≈ 10⁻⁶ (M/M)K where κ is the surface gravity of the black hole¹. As a black hole emits this thermal radiation one would expect it to lose mass. This in turn would increase the surface gravity and so increase the rate of emission. The black hole would therefore have a finite life of the order of 10⁷¹ (M/M)⁻³ s. For a black hole of solar mass this is much longer than the age of the Universe. There might, however, be much smaller black holes which were formed by fluctuations in the early Universe². Any such black hole of mass less than 10¹⁵ g would have evaporated by now. Near the end of its life the rate of emission would be very high and about 10³⁰ erg would be released in the last 0.1 s. This is a fairly small explosion by astronomical standards but it is equivalent to about 1 million 1 Mton hydrogen bombs.

A gas cloud three times the mass of Earth is observed falling towards Sagittarius A*, the supermassive black hole at the centre of our Galaxy.

There is probably a massive black hole at the centre of our Galaxy, and perhaps at the centre of most other ordinary galaxies. Their dimness surprises astrophysicists, but a possible explanation has been found in the behavior of the plasma they consume.

The Milky Way, like other galaxies, is thought to harbour a black hole at its centre. The remarkable observation of a star in close orbit around the Galactic Centre is the first firm evidence that this is so.

Astronomers suspect that there is a black hole at the centre of the Milky Way, but the evidence is all indirect. Measurements of the acceleration of stars close to the Galactic centre provide stronger confirmation of the location and size of this dark mass.

The cores of most large galaxies are thought to harbour super massive black holes. Sagittarius A*, the compact source of radio, infrared and x-ray emission at the centre of the Milky Way, is the closest example of this phenomenon. This paper reports observations that set a limit less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.

The Event Horizon Telescope, an Earth-sized interferometer, aims to capture an image of a black hole’s event horizon to test the theory of general relativity and probe accretion processes, explains project director Shep Doeleman.

Huge X-ray structures, termed Galactic Centre ‘chimneys’, extending hundreds of parsecs above and below the Galactic plane, appear to be exhaust channels connecting the Galactic Centre region to the Fermi bubbles.

Current black hole spin measurements, in X-rays, radio and gravitational waves, are already constraining models for the growth of black holes, the dynamics of stellar core-collapse and the physics of relativistic jet production.

The Galactic Centre is orbited by two objects that look like gas and dust clouds but behave more like stars, and now four additional similar objects are reported.

Acoustic waves that carry orbital angular momentum are amplified as they pass through an absorbing disk when the rotation rate exceeds the frequency of the incident wave, thus providing an experimental demonstration of Zel’dovich amplification.

An elegant experiment showing that acoustic waves are amplified after scattering by a rotating body demonstrates an effect predicted in 1971 by Yakov Zel’dovich. This result has implications for the understanding of scattering from black holes.

General relativity predicts that some black holes rotate. It is now shown that these Kerr black holes imprint their signature on light emitting from nearby sources: twisting it in a way that might be detected by modern telescopes.

Examining and comparing many of the definitions of a black hole, it is concluded that the profusion of different definitions is a virtue that makes the investigation of black holes possible and fruitful in many different kinds of problems.

Black holes and spacetime singularities are fundamental in science. While observational proof for black holes is hard to come by, alternatives can be ruled out or confirmed to exist through precision gravitational wave observations.

The authors predict the ability of the Event Horizon Telescope (in its 2017 campaign) to distinguish between different theories of gravity based on images of Sagittarius A*; they suggest that it will not be possible.

The latest accurate measurements of the size of the galactic core at centimetre radio-wavelengths leave no reasonable escape from the conclusion that there is a black hole at the centre.

Recent observations have uncovered a cloud of ionized gas falling into the supermassive black hole at the centre of our galaxy. Murray-Clay and Loeb present a model that may explain these observations, in which the cloud is produced from the proto-planetary disc around a low-mass star orbiting the black hole.

Rotating black holes twist photons emitted nearby, a peculiar effect in general relativity that is now demonstrated by numerical experiments. This twisted light and its orbital angular momentum could reveal the physics of black holes in more detail than deemed possible before.

General relativity was first experimentally verified in 1919. On the centennial of this occasion, we celebrate the scientific progress fuelled by subsequent efforts at verifying its predictions, from time dilation to the observation of the shadow of a black hole.

Holographic duality establishes a connection between quantum gravity and strongly correlated many-body systems, providing a unique opportunity to study quantum black holes in the laboratory. In this Review, Sachdev–Ye–Kitaev models, which illustrate this duality, are discussed, along with their potential realization in ultracold gases, graphene, semiconducting nanowires and 3D topological insulators.