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.\u00a0<\/p>\n
What\u2019s more, many Enlightenment thinkers also suspected that the center of the cosmos was not even the Milky Way \u2014 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, \u201c[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.\u201d<\/p>\n
It was as though humanity\u2019s 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 \u201cthe infinite variety which we find in the works of the creation.\u201d 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\u2019Alembert wrote that the enthusiasm of this new era was \u201clike a river which has burst its dams.\u201d<\/p>\n
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.<\/p>\n<\/p>\n
Imagining the Universe\u2019s \u201cdark stars\u201d<\/p>\n
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.\u00a0<\/p>\n
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.<\/p>\n
In a comparable way, a star\u2019s 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\u2019s 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\u2019s telescope is sharp enough. The further away a star is, the harder it is to observe the change in the star\u2019s position.<\/p>\n
Though many had tried, no astronomer in Michell\u2019s 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.\u00a0<\/p>\n
Through a complex rationale, he realized that if it were possible to determine a star\u2019s 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\u2019 mass by studying the light they emitted. His starting point was the force that operates throughout space: gravity.<\/p>\n
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\u2019s gravity. But gravity is not just pulling the object. It also grounds our bodies on the Earth\u2019s 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.<\/p>\n
In 1687 Isaac Newton published a book that profoundly contributed to our understanding of the characteristics of gravity. It was called Philosophi\u00e6 Naturalis Principia Mathematica<\/a>, and in it he demonstrated that the gravitational force between two bodies \u2014 such as the Moon and the Earth, or the Earth and the Sun \u2014 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\u2019 orbits, how the Moon causes the Earth\u2019s tides, and why comets appear and disappear from the sky.<\/p>\n Michell realized that he could use Newton\u2019s results to weigh the stars, by studying their light. He reasoned that the larger the star\u2019s mass, the greater its gravity, and the greater a star\u2019s gravity, the more influence this force would have on the light it sends out. Michell made use of another of Newton\u2019s theories that stated that light was like small particles, or \u201ccorpuscles,\u201d and he imagined that the light of the stars was affected by gravity like an object tossed in the air here on Earth.\u00a0<\/p>\n When we throw an object up, the force of gravity makes the object\u2019s speed decrease. In the end, it slows down so much it returns to the Earth\u2019s 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\u2019s gravity. Because gravitational force is dependent on a star\u2019s mass and size, it should be possible to deduce information about the star\u2019s characteristics by measuring how fast its light traveled.<\/p>\n To use modern terminology, Michell was talking about the concept of escape velocity, which can be explained by means of a simple experiment.\u00a0<\/p>\n 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\u2019s thinking \u2014 and, ultimately, how black holes work.<\/p>\n 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\u2019s 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\u2019s impossible to throw an object so fast. Not even spaceships taking off from the Earth\u2019s 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\u2019s gravity.<\/p>\n The gravitational force of objects in space varies, and therefore so does their escape velocity. From the Moon, the escape velocity is \u201conly\u201d 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.<\/p>\n 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.\u00a0<\/p>\n 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.<\/p>\n 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. \u201cAll light emitted from such a body,\u201d wrote Michell, \u201cwould be made to return towards it, by its own proper gravity.\u201d Such a star would be completely dark.<\/p>\n 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 \u2014 125 million times that of the Sun, and its diameter would be larger than the orbit of Mars.<\/p>\n Michell\u2019s 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.<\/p>\n 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\u2019s 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.<\/p>\n A very curious paper<\/p>\n When I read Michell\u2019s letters and try to imagine his life, I\u2019m 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\u2019s 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.\u00a0<\/p>\n 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. \u201cThe expense of such a journey is more than I can afford every year,\u201d he complained in a letter to a friend.<\/p>\n To counteract his loneliness, Michell invited his friends to Thornhill. Many of England\u2019s 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.<\/p>\n 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, \u201cI am sorry however that you wish to have the principle kept secret.\u201d He kept on at Michell to publish his results, and Michell reluctantly conceded that he had given \u201chints\u201d of his findings when he met scientists in London, but that his indications had probably been \u201ctoo obscure to have the drift of them fully understood.\u201d In short: He wished to keep his discoveries to himself.<\/p>\n 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 \u201cit 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.\u201d In order to reassure himself that he would receive proper credit for his results, he added of his method that \u201cas far as I know, [it] has not been suggested by any one else.\u201d\u00a0<\/p>\n Cavendish read the paper, \u201cOn the Means of Discovering the Distance, Magnitude, &c. of the Fixed Star<\/a>,\u201d 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\u2019s Great Mass in C Minor<\/a> had its premiere in Vienna, as the Montgolfier brothers<\/a> undertook the first manned balloon trip in Paris, and as the Continental Army fought British soldiers in North America.<\/p>\n 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 \u201ca very curious paper.\u201d Franklin was thus one of the first people outside Britain to hear about Michell\u2019s new ideas. We don\u2019t know whether Franklin discussed Michell\u2019s vision among Paris\u2019s scientific circles. But a decade later, Michell\u2019s ideas turned up once again in the French capital.\u00a0<\/p>\n![\"An](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/Royal_Society_-_Isaac_Newtons_Philosophiae_Naturalis_Principia_Mathematica_manuscript_1.jpg\")
A manuscript of Isaac Newton\u2019s Philosophiae Naturalis Principia Mathematica at the Royal Society Library. Newton\u2019s work would have a profound influence on Michell\u2019s thinking. (Credit<\/a>: Mike Peel \/ Wikimedia Commons)
\nA boundary of knowledge<\/p>\n
![\"A](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/Pierre-Simon_marquis_de_Laplace_1745-1827_-_Guerin.jpg\")
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\u2019s paper or not. (![\"A](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/Andrea_Ghez.jpg\")
Andrea Ghez giving a presentation. A U.S. astrophysicist, Ghez shared the 2020 Nobel Prize for physics \u201cfor the discovery of a supermassive compact object at the center of our galaxy\u201d with Reinhard Genzel and Roger Penrose. (