Nobel Prize in Physics 2020
We present this Collection of research, review and comment from Nature Research to celebrate the award of the 2020 Nobel Prize in Physics to Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity” and to Reinhard Genzel and Andrea Ghez “for the discovery of a supermassive compact object at the centre of our Galaxy”. Their combined work cemented the existence of black holes and paved the way for the Event Horizon Telescope and gravitational wave detectors.
It is often said that nothing can escape from a black hole. But in 1974, Stephen Hawking realized that, owing to quantum effects, black holes should emit particles with a thermal distribution of energies — as if the black hole had a temperature inversely proportional to its mass. In addition to putting black-hole thermodynamics on a firmer footing, this discovery led Hawking to postulate 'black hole explosions', as primordial black holes end their lives in an accelerating release of energy.
Supermassive black hole observations
The radio source Sgr A* in Sagittarius is thought to be the site of a supermassive black hole lying at the centre of the Milky Way. A study of stellar orbits has identified an object moving towards Sgr A* at a speed of 1,700 kilometres per second. Its low temperature and spectral properties suggest that it is a dusty cloud of ionized gas, three times the mass of Earth, in the process of falling into the black hole. Models predict that as the cloud gets closer to the black hole, X-ray emissions will become much brighter, and a giant radiation flare may be emitted in a few years if the cloud breaks up and feeds gas into the black hole.
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.
Using Very Long Baseline Interferometry (VLBI) at the relatively short radio wavelength of 1.3 mm, a new intrinsic size estimate has been obtained for Sagittarius A*, the supermassive black hole candidate at the centre of the Milky Way. The resulting lower limit on the size of Sgr A* is less than the predicted size of the event horizon of the presumed black hole, suggesting that Sgr A* emissions centre not on the black hole itself but on the surrounding accretion flow. VLBI observations of the Galactic Centre at around 1.3 mm, less influenced by interstellar scattering than those made at longer wavelengths, open a new window onto black-hole physics that will become even more sensitive as new VLBI stations are built.
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.
Most galaxies are thought to have a supermassive black hole at their centres, but proving it is very difficult. The centre of our own Galaxy, occupied by the compact radio source Sagittarius A* (Sgr A*) is a good place to start looking as it is so close to us, just 26,000 light-years from the Sun. A new short-wavelength radio image of Sgr A* has made it possible establish the intrinsic size of Sgr A* for the first time. It is 1 astronomical unit (the Sun-Earth distance) across. This suggests that its mass density is more than 10 orders of magnitude greater than any other known cosmic object, well into supermassive black hole territory.
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.