Nobel Prize in Physics 2020: Black Holes and the Milky Way's Hidden Giant

This note covers the Nobel Prize in Physics 2020: who won it, why the prize was split between a mathematician who proved that black holes must form and two astronomers who found a supermassive black hole at the centre of our own galaxy, how their work was done step by step, how the discovery unfolded over the decades, why it matters for physics today, and a quick-facts summary for exams.

What was the Nobel Prize in Physics 2020 awarded for?

The Royal Swedish Academy of Sciences split the prize into two parts. One half went to Roger Penrose "for the discovery that black hole formation is a robust prediction of the general theory of relativity".

The other half was shared by Reinhard Genzel and Andrea Ghez "for the discovery of a supermassive compact object at the centre of our galaxy".

In plain words, Penrose used mathematics, not telescopes, to show that black holes are not just a strange possibility but an unavoidable outcome whenever enough matter collapses under its own gravity, according to Einstein's general theory of relativity.

Genzel and Ghez, working with separate teams of astronomers, spent decades tracking stars near the centre of the Milky Way and showed that something invisible and extremely heavy, best explained as a black hole, controls their orbits.

The official name of this award is the Nobel Prize in Physics, given each year by the Royal Swedish Academy of Sciences. The 2020 prize carried a total amount of 10,000,000 Swedish kronor, split as one half and two quarter-shares.

Who are the laureates?

Three scientists shared the 2020 prize: one theoretical physicist whose tool was mathematics, and two observational astronomers whose tools were giant telescopes.

Roger Penrose

Roger Penrose was born on 8 August 1931 in Colchester, United Kingdom. At the time of the award he was affiliated with the University of Oxford, United Kingdom, and he received one half of the prize.

In January 1965, Penrose proved mathematically that once enough matter collapses, nothing in general relativity can stop it from forming a black hole, hiding inside it a point called a singularity where the known laws of nature break down.

His 1965 paper is still regarded as the most important contribution to general relativity since Einstein himself.

Reinhard Genzel

Reinhard Genzel was born on 24 March 1952 in Bad Homburg vor der Höhe, Germany.

At the time of the award he was affiliated with the Max Planck Institute for Extraterrestrial Physics, Garching, Germany, and with the University of California, Berkeley, USA. He received one quarter of the prize.

From the early 1990s he led a team that tracked stars near the Milky Way's centre using telescopes in Chile, eventually pinning down an invisible mass of about four million times the Sun's mass.

Andrea Ghez

Andrea Ghez was born on 16 June 1965 in New York, NY, USA. At the time of the award she was affiliated with the University of California, Los Angeles, USA, and she received one quarter of the prize.

Her team used the Keck Observatory in Hawaii to follow the same stars independently, and her results agreed closely with Genzel's, strengthening the case for a supermassive black hole.

What question had puzzled physicists for over two centuries?

The idea of an object so heavy that even light could not escape it is surprisingly old.

In 1783, the English scientist and priest John Michell calculated that a star with the Sun's density but a radius 500 times as large would trap light. The French scientist Pierre-Simon Laplace made a similar suggestion independently a little later.

These "dark stars" were pure speculation, with no theory of gravity strong enough to describe them properly.

That changed when Albert Einstein published his general theory of relativity in November 1915. Within weeks, the German astrophysicist Karl Schwarzschild found a mathematical solution to Einstein's equations describing the warped space and time around a heavy, non-rotating mass.

His solution contained a strange feature, a radius now called the Schwarzschild radius, where the mathematics seemed to misbehave.

For decades nobody was sure whether this meant a real physical object could form there, or whether it was just a mathematical curiosity.

Einstein himself doubted that such an object could really exist in nature. Many physicists, including the Soviet physicists Evgeny Lifshitz and Isaak Khalatnikov, argued that a realistic, imperfect star could never collapse all the way to a true singularity, because the calculations assumed a perfectly symmetric, round star.

The question of whether black holes were a genuine prediction of relativity, or only an idealised mathematical trick, remained open until Penrose's 1965 proof.

A second mystery emerged in the 1960s: astronomers discovered extremely bright, distant objects called quasars, radiating more energy than hundreds of galaxies from a tiny region of space.

Explaining this enormous energy output pointed towards matter falling into a very massive black hole, which in turn raised the question of whether every large galaxy, including our own Milky Way, might hide one at its centre.

How did Penrose prove that black holes are an inevitable outcome?

Penrose's breakthrough was to stop assuming that a collapsing star had to be perfectly round. Real stars have bumps, dents and uneven matter, so a proof that worked only for a perfect sphere said nothing about reality.

The idea came to him in autumn 1964 during a walk in London, when the concept of a trapped surface suddenly occurred to him.

A trapped surface is a closed surface from which every outward-travelling ray of light is forced, by gravity, to bend inward instead of spreading away.

Using this idea, Penrose showed that once such a surface forms, the collapse towards a singularity cannot be reversed or stopped, whatever the shape of the collapsing matter.

This is why the Nobel Committee described black hole formation as a "robust" prediction of relativity rather than a special case.

1. A large mass of matter (far more than needed to form a star) begins to collapse under its own gravity.
2. As the matter squeezes into a smaller volume, its gravity grows strong enough that a trapped surface forms, a boundary from which outgoing light rays cannot spread outward.
3. Once a trapped surface exists, Penrose's mathematics shows the collapse cannot reverse itself, regardless of the exact shape or symmetry of the infalling matter.
4. The collapse continues until all the matter is compressed into a singularity, a point where the known laws of physics cease to apply.
5. Already by this point, an event horizon surrounds the collapsing matter, the point of no return beyond which nothing, not even light, can ever escape.

Draw a star shrinking inward through several stages, with a dashed circle marking the trapped surface forming partway through, then a solid circle marking the event horizon forming as the star crosses inside it, and finally a single point at the centre labelled "singularity" where time and space come to an end.

This was the first major advance in general relativity to go beyond Einstein's own work, because it proved black holes form under realistic, not just idealised, conditions.

How did Genzel and Ghez find a supermassive object at the Milky Way's centre?

Proving black holes could exist mathematically was one challenge; finding one in nature was another. From the early 1990s, Reinhard Genzel and Andrea Ghez each led a team of astronomers studying a region called Sagittarius A* at the centre of the Milky Way, our home galaxy.

The centre of the galaxy is hidden from ordinary telescopes because thick clouds of interstellar gas and dust block almost all visible light along the line of sight.

Both teams worked instead in the infrared part of the spectrum, where the dust is far more transparent, allowing them to see stars that are completely invisible in normal light.

By tracking the exact positions of about thirty of the brightest stars near the galactic centre, night after night and year after year, both teams mapped out their orbits.

One star, known as S2, completes a full orbit around the centre in under sixteen years, a strikingly short time that let astronomers trace its entire path.

For comparison, our Sun takes more than 200 million years to complete one orbit of the galaxy's centre.

The orbits revealed that an invisible, extremely heavy object, with a mass of around four million times the Sun's mass, was pulling on these stars from a region no bigger than our own solar system.

Both teams' measurements agreed closely, and the only object consistent with current theories of gravity that could pack so much mass into so small a space is a supermassive black hole.

What techniques let the astronomers see through dust and atmosphere?

Seeing stars clearly enough to measure their orbits required overcoming two separate problems: dust blocking the view, and the Earth's own atmosphere blurring the image.

The dust problem was solved by observing in infrared light, which passes through interstellar clouds far more easily than visible light.

The atmosphere problem was harder: pockets of air at different temperatures act like wobbling lenses above the telescope, distorting incoming light and making stars twinkle and blur, exactly as they do to the naked eye.

1. Early observations used speckle imaging, taking many very short exposures (a fraction of a second each) so the atmosphere had no time to blur each individual frame.
2. The short, sharp frames were then shifted so that the stars lined up, and stacked together to build a single, much clearer composite image.
3. Later, both teams adopted adaptive optics, where a thin, flexible mirror in the telescope rapidly changes shape many times a second to cancel out the atmosphere's distortion in real time.
4. A bright reference point, either a nearby star or an artificial "star" made by shining a laser into the upper atmosphere, let the system measure and correct the blurring continuously.
5. With adaptive optics, the telescopes could take much longer exposures without blurring, sharpening the image resolution by more than a thousandfold over the original speckle method.

Draw a star's light passing through wavy lines representing turbulent air, reaching a flexible mirror that bends to cancel the distortion, and finally reaching a sharp, undistorted image at the telescope's detector.

These improvements let the astronomers measure not just where stars appeared on the sky but, with added instruments called spectrographs, how fast they were moving directly towards or away from Earth, giving a complete three-dimensional picture of each star's orbit around the galactic centre.

How did the discovery unfold?

This timeline shows how a purely theoretical question from the eighteenth century was transformed, through Einstein's theory, Schwarzschild's mathematics, Penrose's proof and decades of patient observation, into solid evidence for a real astronomical object.

Why does it matter?

Penrose's proof settled a question that had troubled physicists since Einstein's own time: whether black holes were a genuine feature of nature or only a mathematical artefact of idealised, perfectly symmetric models.

His trapped-surface method became a standard mathematical tool, later extended with Stephen Hawking to study the very beginning of the universe itself.

Genzel's and Ghez's work gave astronomers their most convincing evidence yet that a supermassive black hole sits at the heart of our own galaxy, confirming decades of suspicion that such objects power the intense radiation seen from distant quasars.

Their techniques, adaptive optics and careful long-term tracking of stars, are now standard tools across astronomy.

The Nobel Committee's chair, David Haviland, stated that the laureates' discoveries "have broken new ground in the study of compact and supermassive objects" while leaving many questions unanswered, including how to test theories of gravity in the extreme conditions near a black hole's event horizon.

What happens exactly inside a black hole's singularity, where general relativity itself breaks down, remains an open problem for future physics, closely tied to ongoing efforts to build a theory of quantum gravity.

How does this connect to what you study?

This prize links directly to topics in school physics such as gravity, light and the structure of the universe.

The idea that an extremely massive object bends space and traps light builds on the simpler idea of escape velocity learned in mechanics: if a planet or star is heavy and compact enough, the speed needed to escape its gravity can exceed the speed of light, which is exactly the situation inside a black hole's event horizon.

The astronomers' use of infrared light instead of visible light also connects to the electromagnetic spectrum taught in physics, since infrared waves have longer wavelengths than visible light and pass more easily through dust clouds.

The orbital motion of stars around a hidden mass, following the same basic rule that planets follow around the Sun, is a direct application of the laws of gravitation to a much larger and stranger scale than the solar system.

Quick facts for exams

The Nobel Prize in Physics 2020 was announced on 6 October 2020 by the Royal Swedish Academy of Sciences.

It was split between Roger Penrose, who received one half for proving mathematically that black holes are a robust prediction of general relativity, and Reinhard Genzel and Andrea Ghez, who shared the other half for discovering a supermassive compact object, best explained as a black hole, at the centre of the Milky Way.

Penrose worked at the University of Oxford; Genzel at the Max Planck Institute for Extraterrestrial Physics and the University of California, Berkeley; and Ghez at the University of California, Los Angeles. The total prize amount was 10,000,000 Swedish kronor.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Black hole — an object with gravity so strong that nothing, not even light, can escape from within a certain boundary around it.
• General theory of relativity — Einstein's 1915 theory describing gravity as the bending of space and time by mass and energy.
• Singularity — the point at the centre of a black hole where matter is compressed infinitely and known physical laws stop working.
• Event horizon — the boundary around a black hole beyond which nothing can escape, the point of no return.
• Trapped surface — a closed surface from which even outward-travelling light rays are forced inward by gravity, a concept introduced by Penrose.
• Schwarzschild radius — the radius defining a black hole's event horizon for a non-rotating mass, found by Karl Schwarzschild in 1916.
• Quasar — an extremely bright, distant object thought to be powered by matter falling into a supermassive black hole.
• Sagittarius A* — the compact radio source at the exact centre of the Milky Way galaxy, now linked to a supermassive black hole.
• Supermassive black hole — a black hole with a mass of millions to billions of times the Sun's mass, found at the centres of large galaxies.
• Speckle imaging — a technique combining many very short telescope exposures to reduce the blurring caused by Earth's atmosphere.
• Adaptive optics — technology using a flexible mirror to correct, in real time, the blurring of starlight caused by atmospheric turbulence.
• Infrared light — light with a wavelength longer than visible light, able to pass more easily through clouds of interstellar dust.

Common errors and misconceptions

• Misconception: Penrose used a telescope to discover black holes. Correct: Penrose's contribution was purely mathematical, proving in 1965 that black holes must form as a consequence of general relativity.
• Misconception: Genzel and Ghez directly photographed the black hole at the Milky Way's centre. Correct: They inferred its existence from the orbits of visible stars being pulled by an invisible, massive object.
• Misconception: Einstein predicted and believed in black holes from the start. Correct: The sources state Einstein doubted black holes could really exist, even though his own theory implied them.
• Misconception: The whole prize was shared equally among all three laureates. Correct: Penrose received one half, while Genzel and Ghez each received one quarter.
• Misconception: The astronomers observed the galactic centre in ordinary visible light. Correct: They used infrared light, because dust clouds block most visible light from that region.
• Misconception: A trapped surface is the same thing as the singularity. Correct: A trapped surface is the mathematical condition that guarantees collapse will continue; the singularity is the final, infinitely dense point it leads to.
• Misconception: Black holes were first seriously discussed only after Einstein's theory. Correct: Dark, light-trapping objects were already proposed by John Michell in 1783 and Pierre-Simon Laplace shortly after, using Newtonian ideas.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Physics 2020 announced? [1 mark]

1. It was announced on 6 October 2020 by the Royal Swedish Academy of Sciences.

Q2. State the exact share of the prize received by each laureate. [2 marks]

1. Roger Penrose received one half of the prize, while Reinhard Genzel and Andrea Ghez each received one quarter of the prize.

Q3. Explain what a trapped surface is and why Penrose's idea of it was important. [4 marks]

1. A trapped surface is a closed surface around a collapsing mass from which even light rays travelling outward are bent inward by gravity instead of escaping. Penrose showed that once such a surface forms, nothing within general relativity can stop the matter from continuing to collapse into a singularity, regardless of the shape of the original matter. This mattered because earlier proofs of black hole formation had only worked for perfectly symmetric, round collapsing stars, leaving doubt about whether real, imperfect stars would behave the same way. By removing the need for symmetry, Penrose proved that black hole formation is a robust, general outcome of relativity rather than a special case, which the Nobel Committee recognised as the most important advance in the field since Einstein.

Q4. Why did Genzel and Ghez observe the Milky Way's centre in infrared light rather than visible light? [3 marks]

1. Thick clouds of interstellar gas and dust lie between Earth and the centre of the Milky Way, and these clouds block almost all visible light from reaching telescopes. Infrared light has a longer wavelength than visible light and passes through such dust far more easily, allowing the astronomers to see stars near the galactic centre that would otherwise be completely hidden.

Q5. Describe the evidence that led Genzel's and Ghez's teams to conclude a supermassive black hole exists at the centre of the Milky Way. [6 marks]

1. From the early 1990s, both teams tracked the precise positions and motions of around thirty bright stars near the galactic centre, using large telescopes in Chile and Hawaii respectively, working in infrared light to see through dust. One star, S2, completes an orbit in under sixteen years, allowing its full path to be mapped in a human timescale. The stars were found to move in orbits consistent with being pulled by a single, very heavy, invisible object rather than by a spread-out cluster of ordinary stars. Calculations based on these orbits showed the hidden mass to be around four million times the mass of the Sun, packed into a region no larger than our own solar system. Both teams' independent measurements, made with different telescopes and techniques, agreed closely with each other. The only object consistent with current theories of gravity that could be this heavy and this compact is a supermassive black hole, giving the strongest evidence yet for one existing at the Milky Way's centre.

Q6. What technological challenge did adaptive optics solve for the astronomers, and how? [4 marks]

1. Earth's atmosphere contains pockets of air at different temperatures that act like uneven lenses, bending and blurring starlight before it reaches a telescope, which limits how sharply stars can be seen. Adaptive optics solves this using a thin, flexible mirror that rapidly changes shape, many times each second, to cancel out this distortion in real time. The system uses a bright reference point, either a nearby star or an artificial laser-created "star," to measure the blurring continuously and correct for it, allowing much sharper and longer-exposure images of stars near the galactic centre.

Q7. Who was Karl Schwarzschild and what did he contribute to this area of physics? [2 marks]

1. Karl Schwarzschild was a German astrophysicist who, within weeks of Einstein publishing general relativity in 1915, found a mathematical solution to Einstein's equations describing space-time around a heavy, non-rotating mass.

Q8. Why is Penrose's contribution to the 2020 prize described as "robust"? [3 marks]

1. Penrose proved that black hole formation follows from general relativity for any collapsing mass with enough density, not only for perfectly symmetric, idealised spheres as earlier work had assumed. Because his trapped-surface method did not depend on special symmetry, it showed that black hole formation is a general, unavoidable consequence of the theory rather than a rare special case, which is why the citation calls it a "robust prediction".

Key takeaways

• The Nobel Prize in Physics 2020 was split between Roger Penrose and the pair Reinhard Genzel and Andrea Ghez.
• Penrose proved mathematically, in 1965, that black holes must form as a consequence of general relativity.
• Genzel and Ghez independently tracked stars near Sagittarius A* and found evidence for a supermassive black hole.
• The hidden object at the Milky Way's centre has a mass of about four million times the Sun's mass.
• Both astronomy teams used infrared light and adaptive optics to see through dust and atmospheric blurring.
• Penrose's trapped surface idea removed the need for perfect symmetry in earlier black hole proofs.
• Einstein himself doubted that black holes could really exist in nature.
• The discoveries still leave open questions about gravity's behaviour near a black hole's event horizon.

Test yourself

Where was Roger Penrose affiliated at the time of the award?

Roger Penrose was affiliated with the University of Oxford in the United Kingdom when he received the prize.

What two institutions was Reinhard Genzel affiliated with at the time of the award?

Reinhard Genzel was affiliated with the Max Planck Institute for Extraterrestrial Physics in Germany and the University of California, Berkeley, in the USA.

What is a trapped surface?

A trapped surface is a closed surface around collapsing matter from which even outward light rays are bent inward by gravity, forcing continued collapse.

Approximately how massive is the object at the centre of the Milky Way?

The invisible object at the Milky Way's centre has a mass of around four million times the mass of the Sun.

Why did the astronomers choose infrared light for their observations?

Dust clouds near the galactic centre block most visible light, but infrared light, with its longer wavelength, passes through the dust much more easily.

Which star's short orbital period helped confirm the mass at the galactic centre?

The star S2 completes an orbit of the galactic centre in under sixteen years, letting astronomers map its full path and estimate the central mass.

What historical idea from the 1780s anticipated black holes?

In 1783, John Michell calculated that a star dense and large enough could trap even light, an early idea of what we now call a black hole.

Did Einstein believe black holes really existed?

No, the sources state that Einstein doubted black holes could really exist, even though his own general theory of relativity implied they could form.