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Adapted from Facing Infinity: Black Holes and Our Place on Earth by Jonas Enander. Published by The Experiment. Copyright © 2025. All rights reserved.
As geologists were realizing that earthly timescales were vast, astronomers began to discover that the same applied to the distances of the cosmos. In the 16th century, the Polish astronomer Copernicus had argued that it was not the Earth, but the Sun that was at the center of the universe. In the 18th century, astronomers realized that this too was wrong. The Sun was but one star among many moving through that great assembly of stars we call the Milky Way.
What’s more, many Enlightenment thinkers also suspected that the center of the cosmos was not even the Milky Way — they thought there were other galaxies in the depths of space. In the late-17th century, the French physicist and mathematician Pierre-Simon Laplace summarized this new cosmic understanding when he wrote, “[M]an now appears, upon on a small planet, almost imperceptible in the vast extent of the solar system, itself only an insensible point in the immensity of space.”
It was as though humanity’s place in the cosmos had shrunk. But John Michell was not daunted by this. Throughout life, his scientific credo was ambitious. He wrote that he wanted to explore “the infinite variety which we find in the works of the creation.” He belonged to the Enlightenment era. It was a time characterized by the critique of dogmatic ideas and the enthusiastic exploration of the world through rational argument and experimental methods. The French Enlightenment philosopher and mathematician Jean le Rond d’Alembert wrote that the enthusiasm of this new era was “like a river which has burst its dams.”
As a follower of Enlightenment ideas, Michell participated in this wide-eyed exploration of the world, even after becoming a priest. He wanted to be a botanist of stars, mapping their distance, size and mass, and it was when he began to do so that he had a surprising realization: that there could be a limit to human knowledge, an impediment to the ambition of the Enlightenment. He realized that space might contain objects that were gigantic, dark, and impossible to see.
Imagining the Universe’s “dark stars”
Michell made his discovery when he figured out a new method for measuring the distance to the stars. For centuries, astronomers had tried without success to determine how far away the closest stars were. They were applying a method known as parallax measurement, which can be illustrated with a simple example.
Look at a nearby object, then hold out your arm and place your thumb in front of the object. If you close your left eye, your thumb will appear to be to the left of the object in front of you. If you now close your right eye, your thumb will be to the right of the object. The reason for this is that your eyes see your thumb from different perspectives, so the position of your thumb relative to the object appears to shift.
In a comparable way, a star’s position in the sky can change as the Earth travels around the Sun. In my analogy, your left and right eye represent two observation points along the Earth’s orbit, your thumb represents a nearby star you want to measure the distance to, and the distant object represents a star that is further away. As the Earth orbits the Sun, an astronomer observing a nearby star in the sky can see its position change relative to distant stars. With the aid of geometric analysis, it is then possible to figure out how far away the star is, if the astronomer’s telescope is sharp enough. The further away a star is, the harder it is to observe the change in the star’s position.
Though many had tried, no astronomer in Michell’s time had successfully used parallax measurement to determine the distance to our nearest stars. This meant that they must be an extremely long way away. Since the parallax method had failed, Michell wanted to try to find a new way of determining the distance to the stars.
Through a complex rationale, he realized that if it were possible to determine a star’s mass, it would also be possible to determine how far away it was. But in order to do so, Michell needed to weigh the stars in the heavens. As he was unable to travel to the stars to study them, he set out to determine the stars’ mass by studying the light they emitted. His starting point was the force that operates throughout space: gravity.
Gravity is a constant phenomenon in our lives. If we drop something, it falls towards the ground. If we walk up a hill, we feel tired. If we want to lift a heavy object, we undertake a tug-of-war between the strength of our own muscles and the strength of the Earth’s gravity. But gravity is not just pulling the object. It also grounds our bodies on the Earth’s surface. It ensures that the Moon orbits the Earth, and that the Earth and the other planets travel around the Sun. Gravity governs not only our lives, but also the fate of the whole solar system and, by extension, the whole universe. Therefore, if we understand how gravity works, we can understand how the entire universe developed.
In 1687 Isaac Newton published a book that profoundly contributed to our understanding of the characteristics of gravity. It was called Philosophiæ Naturalis Principia Mathematica, and in it he demonstrated that the gravitational force between two bodies — such as the Moon and the Earth, or the Earth and the Sun — is dependent on three things: the mass of one of the bodies, the mass of the other, and the distance between them. The greater the mass, the greater the force, and the greater the distance, the smaller the force. Thanks to this description of the relationships between these elements, Newton was able to explain a number of phenomena, such as the shape of the planets’ orbits, how the Moon causes the Earth’s tides, and why comets appear and disappear from the sky.
A manuscript of Isaac Newton’s Philosophiae Naturalis Principia Mathematica at the Royal Society Library. Newton’s work would have a profound influence on Michell’s thinking. (Credit: Mike Peel / Wikimedia Commons)
A boundary of knowledge
Michell realized that he could use Newton’s results to weigh the stars, by studying their light. He reasoned that the larger the star’s mass, the greater its gravity, and the greater a star’s gravity, the more influence this force would have on the light it sends out. Michell made use of another of Newton’s theories that stated that light was like small particles, or “corpuscles,” and he imagined that the light of the stars was affected by gravity like an object tossed in the air here on Earth.
When we throw an object up, the force of gravity makes the object’s speed decrease. In the end, it slows down so much it returns to the Earth’s surface. Perhaps, Michell thought, the speed of light is affected in a similar way. He assumed that light was a particle traveling at immense speed, and imagined that when a star sent out these little particles, their speed would decrease depending on the strength of the star’s gravity. Because gravitational force is dependent on a star’s mass and size, it should be possible to deduce information about the star’s characteristics by measuring how fast its light traveled.
To use modern terminology, Michell was talking about the concept of escape velocity, which can be explained by means of a simple experiment.
Take an object you can hold in your hand. It could be a coin or a matchbox. Throw the object a few centimeters in the air. The object will travel upwards, before falling back into your hand, or to the ground. Now throw the object into the air again, moving your hand and arm faster this time. The object will reach a point higher in the air before it falls back down. After just two throws, you can surmise that the higher the speed at which you throw the object, the higher it will go before falling. This conclusion provides an important insight to help you understand Michell’s thinking — and, ultimately, how black holes work.
If you were capable of throwing the object towards the sky with a velocity of 11 kilometers per second, it would never come back to Earth. It would travel through the air and eventually leave the Earth’s atmosphere, continuing into space. The initial velocity an object would have to travel at to avoid falling back to the ground is called the escape velocity. Of course, it’s impossible to throw an object so fast. Not even spaceships taking off from the Earth’s surface need to reach that speed, because they accelerate using their rockets; furthermore, the escape velocity decreases the further they get from the Earth. But, despite this, the escape velocity is a good gauge of what it takes to overcome the Earth’s gravity.
The gravitational force of objects in space varies, and therefore so does their escape velocity. From the Moon, the escape velocity is “only” two kilometers per second. If you happened to be on an asteroid, the escape velocity could be as little as a few meters per second, which means you could leave the asteroid with a mere jump.
On more massive bodies the escape velocity can be much higher. At the surface of the Sun, it is 615 kilometers per second. The stronger the gravitational force of a celestial body, the higher its escape velocity will be.
Gravity was suddenly placing a boundary on knowledge. These dark objects presented a challenge to science in its ambition to survey and understand the world.
On the basis of this realization, Michell came to a decisive conclusion: If a star has sufficient gravity, not even light will be able to leave it. “All light emitted from such a body,” wrote Michell, “would be made to return towards it, by its own proper gravity.” Such a star would be completely dark.
Michell calculated how large such a dark star might be, taking the Sun as his starting point. What it consisted of and why it shone were not known at the time, though astronomers estimated that it was more than 100 thousand times the size of Earth. Moreover, physicists had succeeded in measuring the speed of light, confirming that it was astonishingly fast, close to 300,000 kilometers per second. Michell calculated that a star with the same composition as the Sun, but with 500 times the diameter, would have such strong gravity that no light would be able to escape it. A star of this size would have an enormous mass — 125 million times that of the Sun, and its diameter would be larger than the orbit of Mars.
Michell’s discovery indicated that there was more to gravity than objects falling to the ground or planets orbiting the Sun. Gravity was suddenly placing a boundary on knowledge. These dark objects presented a challenge to science in its ambition to survey and understand the world.
However, Michell had no proof that these dark stars existed. They were merely an idea, a product of theoretical deduction and mathematical reasoning. But the intellectually restless Michell came up with a method of identifying these dark stars: observing other stars moving around them. Just as a lighthouse’s beam testifies to the presence of otherwise-invisible cliffs on a dark night, the stars that move around these dark celestial bodies would reveal their existence.
A very curious paper
When I read Michell’s letters and try to imagine his life, I’m struck by how multifaceted he was. He was a rector and a businessman, he planned and planted a botanic garden, he mapped out his region’s coal stocks, he invented new navigational techniques for the British Navy, he mastered Ancient Greek and Hebrew, and he made contributions to geology, astronomy and physics. He seems to have possessed an unquenchable thirst for knowledge, but he also had a serious problem: He struggled to make his results known.
He was isolated in Thornhill and missed his London friends. His research could be conducted in solitude, of course, but in Thornhill he hardly had anyone with whom to discuss his findings. He dearly wanted to travel to the capital to meet with other scientists, but was hindered by the expensive tolls on the roads. “The expense of such a journey is more than I can afford every year,” he complained in a letter to a friend.
To counteract his loneliness, Michell invited his friends to Thornhill. Many of England’s foremost scientists traveled to the Yorkshire village and stayed at the rectory. Even the prolific scientist and inventor Benjamin Franklin, who would play a key role in both the American Revolution and the foundation of the new republic, took time out of a diplomatic trip to England to visit the intelligent rector.
But in spite of the visits, Michell hesitated to share his findings. His isolation in the countryside seems to have made him anxious that others would take credit for his work. When the physicist Henry Cavendish wrote to ask him to share his discoveries, Michell refused. Cavendish wrote, “I am sorry however that you wish to have the principle kept secret.” He kept on at Michell to publish his results, and Michell reluctantly conceded that he had given “hints” of his findings when he met scientists in London, but that his indications had probably been “too obscure to have the drift of them fully understood.” In short: He wished to keep his discoveries to himself.
Then at last, on May 26, 1783, Michell sent an article to Cavendish, asking for it to be read at one of the meetings of the Royal Society. In the accompanying letter, Michell wrote that “it might perhaps be possible to find the distance, magnitude, and weight of some of the fixed stars, by means of the diminution of the velocity of their light.” In order to reassure himself that he would receive proper credit for his results, he added of his method that “as far as I know, [it] has not been suggested by any one else.”
Cavendish read the paper, “On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Star,” to the members of the Royal Society. It was the first official introduction of the idea that dark objects can exist in space without emitting any light. It took place as Mozart’s Great Mass in C Minor had its premiere in Vienna, as the Montgolfier brothers undertook the first manned balloon trip in Paris, and as the Continental Army fought British soldiers in North America.
In Paris, Benjamin Franklin, who had visited Michell in Thornhill, was preparing to negotiate with Great Britain for U.S. independence. Around the same time, Franklin received a letter from the President of the Royal Society, stating that Michell had written “a very curious paper.” Franklin was thus one of the first people outside Britain to hear about Michell’s new ideas. We don’t know whether Franklin discussed Michell’s vision among Paris’s scientific circles. But a decade later, Michell’s ideas turned up once again in the French capital.
Pierre-Simon Laplace also wrote about astronomical bodies so massive that they would be rendered invisible, though he called them corps obscurs. It is unknown whether he was inspired by Michell’s paper or not. (Credit: Palace of Versailles collection / Wikimedia Commons)
Corps obscurs
Pierre-Simon Laplace, the leading physicist and mathematician in France at the time, had survived the terrors of the French Revolution with no more than a scare, though some of his colleagues were not so lucky: The chemist Antoine Lavoisier was beheaded on the guillotine. In 1796 Laplace published a multi-volume work in which he summarized all the astronomical knowledge of his time. He wrote that “there exist then in space obscure bodies as considerable, and perhaps as numerous as the stars,” and that “it is therefore possible that the largest luminous bodies in the universe, may… be invisible.” He called these bodies corps obscurs (dark bodies).
Perhaps Laplace was inspired by Michell, since the President of the Royal Society had sent Michell’s article to the Frenchman, too. Regardless of the source of Laplace’s inspiration, it is fascinating that he laid out a whole cosmic vision in which these dark celestial bodies played a central role.
However, confirming the existence of these dark objects required observations. At the beginning of the 19th century Johann Georg von Soldner, a self-taught astronomer at the Berlin Observatory, read about Laplace’s and Michell’s deductions. He wrote that “there may be heavenly bodies that, due to their size and the strong attraction associated with it, do not emit any light, or at least not to any distance; and that the largest bodies in our heavenly system must therefore remain invisible.”
Von Soldner wondered whether there might be a gigantic, completely dark object in the center of our galaxy. Just as the planets orbit the Sun, so too might the Sun be orbiting a central object in the middle of the Milky Way. Since no one had ever seen such a central object, reasoned von Soldner, it could conceivably be completely dark.
In 20 years, the idea of these dark celestial bodies had spread from Thornhill Rectory to the Royal Society’s assembly rooms in London, then to the global metropolis of Paris, and finally to Berlin. These cities were key centers of scientific thought at the time. Three of the foremost physicians and astronomers of their day were discussing the existence of these objects and how they might be studied.
But just as quickly as the idea had arisen, it disappeared. “It is very possible there may be no stars large enough to produce any sensible effect,” Michell wrote of the stars’ ability to influence the light they emitted. He doubted too whether gravity really affected light in the same way as other matter. He died in 1793 after a long illness, and was buried in the churchyard at Thornhill.
Laplace crossed out the passage on these “invisible bodies” when the time came to print a new edition of his volumes, and in Berlin, von Soldner dismissed the idea of a dark object at the center of the Milky Way, concluding that no one had seen stars moving around such an object, even with the best telescope.”
And so dark celestial bodies vanished from the scientific imagination. After all, they were merely a speculation based on certain assumptions: If gravity works as Newton had described, and if gravity affects light in the same way as particles, well, then a star of the same density as the Sun but 500 times the diameter would be completely dark.
But what if gravity didn’t work that way out in space? Perhaps light didn’t have the characteristics Michell and Laplace were assuming? At the beginning of the 19th century, light began to be viewed as a wave, rather than a particle, and it became unclear how such light waves might be affected by the gravity of the stars. Attempts to measure the speed of light coming from different stars hadn’t produced any results either. Michell’s dark stars and Laplace’s corps obscurs faded from the world of ideas.
Andrea Ghez giving a presentation. A U.S. astrophysicist, Ghez shared the 2020 Nobel Prize for physics “for the discovery of a supermassive compact object at the center of our galaxy” with Reinhard Genzel and Roger Penrose. (Credit: BorderlineRebel / Wikimedia Commons)
The dark discovery
In October 2020, more than two hundred years after the speculations of Michell, Laplace and von Soldner, a band of journalists gathered at the Royal Swedish Academy of Sciences in Stockholm to hear the Secretary General of the Academy announce the recipient of the year’s Nobel Prize in Physics.
He began by saying that the prize “is about the darkest secrets of the universe.” The British mathematician Roger Penrose received half the prize for his theoretical investigations into how black holes are created, while the German astrophysicist Reinhard Genzel and the U.S. astrophysicist Andrea Ghez were awarded their share of the prize “for the discovery of a supermassive compact object at the center of our galaxy.”
The rector of Thornhill had been proved right. Gigantic, dark objects really do exist in space. Genzel and Ghez used Michell’s method to identify the black hole in the center of our galaxy: They observed how stars move around it. But what they have discovered is something much stranger than anything Michell could have imagined.
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