The Nobel Prize in Physics this year was awarded to Roger Penrose, Andrea Ghez and Reinhard Genzel, for their discoveries related to black holes. Penrose received half of the prize for his contribution to the theoretical understanding of black hole formation, and Genzel and Ghez share the other half for the discovery of the supermassive black hole at the center of our galaxy.
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Einstein’s General Theory of Relativity predicted the existence of objects with such high density that not even light could escape them, thus earning them the name “black holes.” Einstein himself found it difficult to accept the notion that such objects could exist in nature, and regarded the idea as a mere mathematical curiosity. This year’s Nobel Prize in Physics was awarded for the theoretical and observational discoveries indicating the existence of black holes, suggesting that even Einstein could be wrong.
The initial reservations regarding the existence of black holes are not without foundation. Initially, researchers were only able to investigate these objects under the assumption of perfect spherical symmetry. This assumption stipulates that gravitationally collapsing matter retains the shape of a perfect sphere during the formation of a black hole. However, this assumption is not substantiated by empirical evidence. In the natural world, perfect spherical symmetry is exceedingly rare. The fundamental question is whether a black hole, whose core harbors a singularity (a single point of infinite density), can form in the absence of this assumption. In other words, does perfect spherical symmetry represent a prerequisite for the formation of a black hole? Consequently, the existence of black holes would be impossible in the observable universe.
In 1964, Penrose resolved the issue that had previously impeded researchers, thereby establishing a foundation for further progress in the field. In his work [2], Penrose demonstrated that even a deviation from spherical symmetry results in the formation of a black hole and subsequent collapse to a singularity. Penrose's work removed superfluous assumptions and showed that the formation of black holes is possible under very general conditions. By doing so, he transitioned the study of black holes from a theoretical framework to the empirical domain of observational astrophysics.
To substantiate his claim, Penrose developed and refined a series of innovative mathematical tools that have since been incorporated in contemporary research methodologies.
Just a few months following the publication of Penrose’s groundbreaking 1964 paper, Stephen Hawking made a substantial contribution to the research endeavor by extending Penrose’s theory. In the subsequent decade, the two scientists contributed to the maturation of the Theory of General Relativity and advanced the mathematical basis of black holes and the origin of the universe by a giant leap.
Three decades following the publication of Penrose’s seminal paper on the potential existence of black holes, two different research groups, led by Andrea Ghez and Reinhard Genzel, employed precise astronomical observations to measure the movements of stars at the center of our galaxy [3,4]
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Based in these measurements, the researchers concluded that the stars are orbiting an object of an exceptional mass relative to its size. This finding aligns with the hypothesis of a black-hole candidate. They calculated that the invisible object residing at the galactic center is four million times heavier than our Sun, yet only about as large as our solar system [5]. The presence of such an immense mass concentrated within a relatively small region at the galaxy’s center matches the anticipated characteristics of a supermassive black hole. Consequently, the research conducted by Ghez and Genzel offers the most compelling evidence to date for the existence of a supermassive black hole at the core of the Milky Way.
These groundbreaking achievements, which granted their authors the 2020 Nobel Prize in Physics, represent substantial achievements in a discipline where questions persist in outnumbering responses. Black holes will likely continue to be a focal point for the physics community for many years to come.
English editing: Gloria Volohonsky
References:
[2] The paper that earned Penrose the Nobel: gravitational collapse and singularities
[3] Measurements of stars at the galactic center
[4] Measurement of stellar accelerations at the galactic center
[5] Estimating the mass of the galactic-center black hole
Noa holds a PhD in Physics from the Technion and is currently a postdoctoral researcher at the Princeton Gravity Initiative at Princeton University. Her research focuses on quantum effects on black hole backgrounds. She also serves as a scientific editor at “Little, Big Science".
Adi is a professor of theoretical particle physics at Swansea University. His research focuses on quantum chromodynamics and string theory.
