Nobel Prize 2020 In Physics: Nature of Blackholes And Discovery of Centre of Milky Way Galaxy

The Nobel Prize in Physics this year honours pioneering studies about the nature of black holes, including the discovery of the gigantic one lurking in the heart of our Milky Way Galaxy.

Half of the prize goes to Roger Penrose, a mathematician at the University of Oxford, for his work in the 1960s on the formation and stability of black holes, while the other half is shared by two astronomers: Reinhard Genzel of the Max Planck Institute for Extra-terrestrial Physics and Andrea Ghez of the University of California, Los Angeles. Since the 1990s, they led rival research groups that tracked stars at the centre of the Milky Way and showed that their orbits were bent by what’s known as a supermassive black hole (SMBH).

The concept of a black hole - an object so massive that its gravity prevents light from escaping - emerged in pieces over the course of decades. Albert Einstein published his theory of gravity, the general theory of relativity, in 1915. It states that gravity arises when mass and energy warp the fabric of space and time, causing the trajectories of freely falling objects to curve like Earth’s elliptical orbit around the Sun. Only 1 year later, German physicist Karl Schwarzschild worked out the shape of the pit in spacetime that a point mass would create and showed that it predicts an event horizon. That marks the edge of a sphere around the point mass from which light can still escape.

However, the whole notion that burned-out stars could lead to these bizarre voids in space didn’t arrive until 1939. That is when physicists J. Robert Oppenheimer and George Volkoff calculated that, if a neutron star grew too massive, it should collapse under its own weight to an infinitesimal point, leaving behind only its ultra-intense gravitational field. Their work foreshadowed astrophysicists’ current understanding of stellar-mass black holes, which form when sufficiently massive stars burn out and their cores collapse.

Oppenheimer and his colleagues did not prove the imploding star had to form an event horizon. It was conceivable that the matter could somehow swirl away or that the dead star’s gravitational field might not stick around. In the 1960s, Penrose showed with extreme mathematical rigour that the formation of a black hole was essentially inevitable and that it would be indestructible, growing as it devoured more mass. “It didn’t matter what you did, the horizon was always there,” says Clifford Will, a general relativity expert at the University of Florida. “It wouldn’t break apart; it would only grow.”

Will suggests the award could be considered a prize of sorts for Stephen Hawking, who died in 2018 and with whom Penrose collaborated. In fact, Penrose’s key predictions are framed in the so-called Hawking Penrose theorems. Penrose notes that Hawking took his ideas regarding the formation of horizons around black holes and applied them to cosmology and the birth of the universe. “They were clearly advances on what I had done,” Penrose says.

In short, Penrose showed general relativity implied that black hole would be a real, stable astrophysical object, says Ulf Danielsson, a theoretical physicist at Uppsala University and a member of the Nobel physics committee. “Penrose laid a theoretical foundation so that we could say, ‘Yes, these objects exist, we can expect to find them if we go out and look for them.”

Since Penrose’s advances, astronomers have found a wealth of evidence for black holes. They found stars orbiting invisible companions, and they could see superheated gases glowing hot as they disappeared into putative black holes. Gravitational-wave detectors provided the clincher for such stellar size black holes, but not the galactic giants.

General relativity is associated with the thermodynamics and quantum effect which are strongly supportive of each other. A black hole (BH) is a compact object whose gravitational pull is so intense that cannot escape the light. It was proved by Hawking that a BH has an additional property of emitting radiation. Since Hawking’s great contribution on BH thermodynamics, the radiation from the BH has attained the attention of many researchers. There are various processes to obtain the Hawking radiation by applying the quantum field equations or the semi-classical phenomena. Different accesses to quantum gravity, as well as BH physics predict a minimum measure length or a maximum evident momentum and associated modifications of the principle of the Heisenberg uncertainty, which is called the generalized uncertainty principle (GUP).

The thermal radiation coming from any stationary metric is calculated. The physical image is that the radiation develops in the quasiclassical tunnelling of particles from a gravitational barrier. They obtained a thermal spectrum and twice the temperature for Hawking radiation of non-rotating BH. The expression exp(−2Im(∫pdr)) is not invariant under canonical transformation in general and expressed that this implies half the correct temperature for BH. In the setting of black rings significance, the radiation of the Dirac particles can be calculated by applying the Dirac wave equation in both the charged and uncharged case. The formulate of the field equations of uncharged and charged Dirac particles by using the covariant Dirac wave equation.

E. T. Akhmedov et al. calculated Hawking radiation by using the quasi-classical phenomenon. The authors analysed that the quasiclassical method for gravitational backgrounds contains subtleties not found in the usual quantum mechanical tunnelling problem.

V. Akhmedova et al. compared the anomaly method and the WKB/tunnelling method for finding radiation through non-trivial space-time. They conclude that these both methods are not valid for all types of metrics. The discreteness space effect of the GUP is investigated in space. Corda analysed interferometric detection of gravitational waves: the definitive test for general relativity.

He concludes that accurate angular and frequency-dependent response functions of interferometers for gravitational waves arising from various theories of gravity will be the definitive test for general relativity. The authors investigated insights and possible resolution to the information loss paradox via the tunnelling picture. They observe that the quantum correction give zero temperature for the radiation as the mass of the BH is zero.

The authors analysed the problems of gravitational waves and neutrino oscillations through extended gravity theory. The authors examined the rule to all alternative gravities, a particular significance of scalar-tensor and f(R) theories. Yale analysed the exact Hawking radiation of scalars, fermions and bosons 1-spin particles applying quantum tunnelling phenomena without back reaction. The different dark energy models like Λ cold dark matter, Pseudo-Rip and Little Rip universes, non-singular dark energy universes, the quintessence and phantom cosmologies with different types are analysed.

Sharif and Javed analysed the Hawking radiation of fermion particles applying quantum tunnelling phenomena from traversable wormholes. Corda studied the important issue that the non-strictly continuous character of the Hawking radiation spectrum generates a natural correspondence between Hawking radiation and quasi-normal modes BH. Jan and Gohar examined the Hawking temperature by quantum tunnelling of scalars particles applying Klein-Gordon equation in WKB approximation.

Kruglov calculated the Hawking radiation by quantum tunnelling of vector particles of BHs in 2 dimensions applying Proca equation in WKB approximation. Matsumoto et al. analysed the time evolution of a thin black ring via Hawking radiation.

The different writers determined the Hawking temperature by Hamilton-Jacobi equation of vector particles of Kerr and Kerr-Newman BHs by applying Proca and Lagrangian equations in WKB approximation. Corda analysed a precise model of Hawking radiation from the tunnelling mechanism and he found that pre-factor of the Parikh and Wilczek probability of emission depends on the BH quantum level.

Anacleto analysed the GUP in the tunnelling phenomena through the Hamilton–Jacobi process to find the corrected temperature and entropy for three-dimensional noncommutative acoustic BHs. Anacleto et al. studied the Hawking temperature by the Hamilton–Jacobi equation of spin 32-particles of accelerating BHs, applying the Rarita–Schwinger equation in the WKB approximation.

Chen and Huang determined the Hawking temperature by quantum tunnelling phenomena of vector particles of Vaidya BHs in applying the Proca equation in WKB approximation. Anacleto et al. examined the quantum-corrected of self-dual BH entropy in tunnelling phenomena with GUP.

Li and Zu analysed the tunnelling phenomena by the Hamilton–Jacobi equation of scalar particles of Gibbons–Maeda–Dilation BHs, applying the Klein–Gordon equation in the WKB approximation. Feng et al. calculated the tunnelling phenomena by the Hamilton–Jacobi equation of scalar particles of 4D and 5D BHs, applying the Proca equation in the WKB approximation. Saleh et al. studied the Hawking radiation of 5D Lovelock BH with the Hamilton–Jacobi equation by using the Klein–Gordon equation.

The one at the centre of the Milky Way, known as Sagittarius A* (Sgr A*), weighs millions of solar masses and is only 26,000 light-years away. But in addition to being black, it is quite small: Its event horizon would fit within Mercury’s orbit. On top of that, the galactic centre is cloaked from prying telescopes by gas and dust.

By pushing observing techniques to their limits, the sparring teams of Ghez and Genzel carried out a very simple study: They mapped the progress of a single star as it orbited close to Sgr A* and showed, via simple Newtonian mechanics, that the object they were orbiting had to have a colossal mass. “With high school physics, you can get a long way to understanding that there must be something supermassive there that we can’t see,” says Selma de Mink, a theoretical astrophysicist at Harvard University.

Their studies were enabled by infrared detectors. Wavelengths of about 2 micrometres proved to be a sweet spot: Those infrared photons could penetrate the haze and weren’t too disturbed by turbulence in Earth’s atmosphere. The infrared wavelengths were also small enough to locate stars relatively precisely.

In the 1990s, Genzel and Ghez’s groups both latched onto a single star, known as S2 or S0-2 by the two teams, which is the closest star to the galactic centre yet detected. “Andrea and Reinhard have had a legendary competition over the years which has kept the field moving,” says astrophysicist Heino Falcke of Radboud University. To get an accurate fix on S2, the teams needed the largest telescopes available: the four 8-meter telescopes of Europe’s Very Large Telescope in Genzel’s case, and the twin 10-meter Keck telescopes for Ghez.

In 2002, S2’s elliptical orbit appeared to reach its closest point to Sgr A*. It came within 20 billion kilometres or 17 light-hours, and travelled at 5000 kilometres per second, 3% of the speed of light. The teams then had enough of an orbit to draw conclusions about the invisible object. They calculated it must weigh the equivalent of 4 million Suns and be a concentrated object: It could only be a black hole. “They proved through observation what Penrose had predicted with theory, that black holes actually do exist,” says Gerry Gilmore of the University of Cambridge.

The teams have continued to follow S2 through its first full orbit in 2008 and its second close approach in 2018. They have used those data to subject general relativity to ever more stringent tests. “They laid the foundations for supermassive black holes,” Falcke says.

As good as the S2 results were, researchers want even more direct evidence for the existence of SMBHs. And in 2019, the Event Horizon Telescope (EHT) succeeded in revealing the shadow of an even bigger monster at the centre of M87, one of the Milky Way’s neighbouring galaxies. That black hole holds billions of solar masses. The EHT collaboration has tried to image Sgr A* but so far has been thwarted in presenting conclusive results.

Ghez is just the fourth woman ever to win a Nobel Prize in Physics, and the second in the past 3 years. “That means a lot to me,” de Mink says. In recent years, the Nobel science prizes have been criticized for their lack of diversity.

At 55, Ghez is also a relatively young laureate. Penrose, 89, is among the oldest. But Penrose says he has no regrets about waiting so long to get the prize. “I know some people who got a Nobel too early, and it ruined their science,” he says. “I think I’m about old enough.”

This 2020 Nobel Prize, which follows on the heels of the 2017 Nobel Prize for the discovery of gravitational waves from black holes, and other recent stunning discoveries in the field, such as the 2019 image of a black hole horizon by the Event Horizon Telescope serve as great recognition and inspiration for all humankind, especially for those of us in the relativity and gravitation community who follow in the footsteps of Albert Einstein himself.