Shining a Light on Black Holes: The 2020 Nobel Prize in Physics

This year’s Nobel laureates in physics were honored for their discoveries pertaining to black holes, and indicating that there is a black hole at the center of the Milky Way, called “Sagittarius A*.” NASA/CXC/Univ. of Wisconsin/Y.Bai, et al.

Since Einstein’s seminal work in the early 20th century, black holes have become an endless source of fascination for both physicists and the public. It is easy to see why. For starters, a black hole is, well, ‘black’ in that it swallows all objects that stray into it, even light. By virtue of their extreme gravitational pull, they warp space and time in extreme ways. And the heart of a black hole is an infinitely hot and dense object known in mathematical physics as a singularity, where all known laws of physics break down. In other words, black holes are the stuff of science fiction imaginations – on a grand scale.

But if black holes exist and are truly black, how exactly would we ever be able to make an observation? This year’s Nobel Prize in Physics was awarded to three scientists who in different ways, showed that (a) black holes is a robust prediction of Einstein’s General Theory of Relativity and (b) “for the discovery of a supermassive compact object at the centre of our galaxy.”

Sir Roger Penrose is the theoretical physicist who was awarded the prize for (a). Two other scientist – Reinhard Genzel and Andrea Ghez shared one-half of the prize for (b).

Roger Penrose, Andrea Ghez and Reinhard Genzel. PAP / UCLA / MPE

Penrose’s imaginative theoretical work has influenced an entire generation of physicists through his scientific papers as well as popular books that are loaded with exquisite hand-drawn illustrations of deep physical concepts. Here is one example.

Roger Penrose was famous for his detailed illustrations. This is one of his diagrams of an empty universe. Source: Wikimedia.

This is one physicist’s recollection of his interactions with Penrose [1]

As a graduate student in the 1990s at Penn State, where Penrose holds a visiting position, I had many opportunities to interact with him. For many years I was intimidated by this giant in my field, only stealing glimpses of him working in his office, sketching strange-looking scientific drawings on his blackboard. Later, when I finally got the courage to speak with him, I quickly realized that he is among the most approachable people around.

Born in 1931, Roger Penrose, who is now at Oxford University, won half the prize for his seminal work in 1965 which proved, using a series of mathematical arguments, that under very general conditions, the astrophysical process of gravitational collapse, which occurs when a star runs out of its nuclear fuel, would lead to the formation of black holes in nature. He was also able to show that the heart of a black hole must contain a singularity – an object with infinite density, where the laws of physics simply break down. Herein lies the conundrum. Since the very conceptions of space, time and matter fall apart at a singularity, how then would it exist in the first place? Resolving this mathematical paradox is one of the biggest open problem in theoretical physics today.

Penrose’s seminal 1965 paper once demonstrated the power of mathematics in the service of mankind. His proof was possible because he invented new mathematical concepts and techniques.  The equations that Penrose derived have since been used by physicists, including the late genius Stephen Hawking at Cambridge, in studying black holes. In collaborative work, Penrose and Hawking used the former’s mathematical tools to prove that the Big Bang model of the Universe – the current best model for explaining how the Universe as we know it came into existence – had a singularity at the very start. This result is now known as the Penrose-Hawking Singularity Theorem.

Because of Penrose’s results, on and off, physicists have been on the hunt to capture images of black holes using increasingly powerful telescopes. It was for this empirical work that the remaining half of this year’s Nobel Prize for Physics was shared equally between the astronomers Reinhard Genzel and Andrea Ghez, who each lead a team that discovered the presence of a supermassive black hole, 4 million times more massive than the Sun, right at the center of our Milky Way galaxy.

Genzel is an astrophysicist at the Max Planck Institute for Extraterrestrial Physics, Germany and the University of California, Berkeley. Ghez is an astronomer at the University of California, Los Angeles.

Genzhel and Ghez used the world’s largest telescopes (Keck Observatory and the Very Large Telescope) and studied the movement of stars in a region called Sagittarius A* at the center of our galaxy. They both independently discovered that an extremely massive – 4 million times more massive than our Sun – invisible object is pulling on these stars, making them move in very unusual ways. This is considered the most convincing evidence of a black hole.

In closing, this year’s Nobel Prize for Physics 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 the 2019 image of a black hole horizon by the Event Horizon Telescope. These discoveries are a testimony to the perseverance and ingenuity of scientists at the frontier of cosmological research and also another tribute to the remarkably accurate predictions of black holes implicit in the sublime work of Albert Einstein himself.

Further reading
A fascinating account of how Roger Penrose obtained the insights that led him to derive the equations in his famous 1965 paper on the existence of black holes.

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