The existence of black holes is a mind-boggling thought, especially in light of the idea that the cosmos could be populated by billions of them.
For decades in the 20th century, eminent physicists refused to believe they could be real, ignoring what mathematicians had predicted.
Even those "unbelievers" included Albert Einstein - whose own theory of general relativity made black holes possible.
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However, there was one person who demonstrated extraordinary clairvoyance regarding black holes - and he did so long before Einstein was even born.
Using only Newton's laws, a little-known British clergyman, John Mitchell, anticipated these astronomically unusual bodies in some important and surprising ways, back in the 18th century.
Who was Mitchell, what did he predict, and why are his ideas largely forgotten today?
The Reverend John Mitchell, a proponent of "Newtonian Christianity," was surprisingly insightful when it came to black holes.
Mitchell was born in 1724 in the village of Ikring, England, the son of Gilbert Mitchell, the parish priest, and his wife Obedience Gerard.
Homeschooled alongside his younger brother and sister, John early gained a reputation as a quick learner and perceptive person.
According to historian Russell McCormack, his father Gilbert liked to quote a family friend who described John as "the brightest head I ever met".
Gilbert valued independence of thought, describing himself as "unaffiliated with any body or denomination of people in this world".
The family followed latitudinarian Christianity – a tradition that elevated reason above excessive doctrine and which originated at the University of Cambridge under Isaac Newton.
And so when it came time for John to start studying, he went to Cambridge.
With an abundance of coffee shops available and an intimate community of 400 students, the university was an ideal place for intellectual discussions.
Mitchell remained there for over 20 years in various positions, studying and teaching disciplines such as Hebrew, Greek, arithmetic, theology, and geology.
He was a dedicated experimenter and, as another biographer, Archibald Gickey, puts it, "he loved to make his own devices... His rooms at Queen's College, with all his tools and machinery, sometimes seemed like workshops".
He also began to show a talent for scientific prediction during his years at Cambridge.
In 1750, he published a study on magnetism, presenting for the first time at least one law – the "inverse square law" – which advanced the use of magnets in navigation.
In 1760, he published a study on the mechanics of earthquakes, describing the nuanced layers of the Earth that are now known to make up the "crust" and showing how earthquakes move through these layers in the form of waves.
He also showed how the epicenter and emphasis of the catastrophic Lisbon earthquake of 1755 could be calculated and explored the idea that underwater earthquakes could cause tsunamis.
Mitchell also published a study on earthquake mechanics, which presented a way to calculate epicenters and explored the idea that earthquakes could trigger tsunamis.
After leaving Cambridge in 1764, he married Sarah Williamson and moved to Thornhill in Yorkshire to follow in his father's footsteps as the parish's head curate.
Sarah died the following year, and Mitchell remarried to Anne Brecknock in 1773.
In addition to his work in the church, he maintained correspondence with various other natural philosophers and intellectuals of the time, among them the American polymath Benjamin Franklin.
From a 21st century perspective, the idea that an employee of a Christian church could be at the center of scientific life may seem surprising.
But, like most 18th-century intellectuals, Mitchell did not distinguish between religion and science.
The discovery of the telescope in the early 17th century led to a great philosophical upheaval throughout Europe. upheaval
The fixed, observable hierarchy of God's creations - Earth and sky - was overthrown by what the historian of science Alexander Kojre calls an "indeterminate and even infinite Universe" that had to be understood through observation of "its fundamental components and laws".
But for thinkers like Mitchell, this revolution did not exclude God, it merely restored his mystery: the natural laws that came under investigation were still divine laws.
As Newton wrote in 1704, "our duty to God, and to one another, will appear to us in the light of Nature."
Mitchell followed precisely this Newtonian Christianity.
As McCormack says, "the truths of his faith were in harmony with the truths of nature."
And so, along with all his parochial duties, Mitchell gradually focused his attention more and more on cosmology and in particular on the nature of gravity.
It was an area in which he would produce works that were both revolutionary and visionary, even long after his death.
Like Mitchell, Isaac Newton - pictured here in the sacristy of a London church - saw no division between faith and science.
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Mitchell built his own three-meter reflecting telescope and, in 1767, was the first to apply the new mathematical methods of statistics to the study of visible stars, showing that clusters such as the Pleiades could not be explained by random arrangement and must be the result of gravitational attraction.
In 1783, Mitchell's friend Henry Cavendish wrote to him mentioning the difficulties Mitchell was having in building a new, even larger telescope.
"If your health does not permit you to continue with it," he wrote, "I hope it will at least enable you to have an easier and less strenuous job of weighing the world."
This sounds like a joke, and perhaps it was meant to be humorous, but Cavendish was talking about a real, real undertaking.
Mitchell was working on a torsion balance, a device that would allow him to measure the density of planet Earth by measuring the gravitational attraction between lead weights.
Mitchell died before he could use this apparatus, but after his death it was succeeded by Cavendish who performed the experiment in 1797.
He calculated the density of the Earth to within 1 percent of the currently accepted value.
The accuracy of Cavendish's result was not disputed until 1895, and a variation of Mitchell's apparatus is still used today to measure the gravitational constant - the strength of the gravitational force acting throughout the Universe.
Predicting black holes
In the same year as Cavendish's letter, Mitchell published a study with a hypothesis that, while it would prove scientifically less durable, was arguably brilliant in its power of observation.
Using Newtonian principles, she began by explaining how the density of stars could be determined by observing how their gravity affected other nearby bodies, for example, the orbits of other stars or comets.
Mitchell then talked about how the behavior of light can be used for similar purposes:
"Let us now suppose that the particles of light are attracted in the same way as all other bodies known to us... which there is no justifiable doubt about, since gravitation is, as far as we know, or have reason to believe, a universal law of nature."
Light would escape from such a star, but, as Mitchell explained, "it would be forced back toward it by the force of its own ordinary gravity."
Although no one had been able to prove it, the particle or "corpuscle" theory of light put forward by Isaac Newton some 80 years earlier remained the dominant belief in Mitchell's time.
Mitchell explained how the behavior of light under gravity could offer a way to calculate the density of stars, at least hypothetically, especially if the star is "large enough to affect the speed of light emitted from it."
Although the current belief is that he was wrong about the effect of gravity on the speed of light (it does not slow down), his reasoning was common sense.
By the same principles, Mitchell concluded – correctly this time – that it was also possible that the gravity of the most massive astral bodies could completely overpower their own light rays.
For a star to achieve this, it would have to be the same density as the Sun and approximately 50 times larger.
Light would initially escape from such a star, perhaps making its way to nearby orbiting planets, but, Mitchell explained, "would be forced back, by the force of its own ordinary gravity."
Since the light of such a star could not reach us, "we could have no information about it by sight," but we could still distinguish it by irregularities in the orbits of other nearby astral bodies caused by the gravity of the invisible star, "which could not be easily explained by any other hypothesis."
These speculations, Mitchell explained, were "somewhat beyond my current intent," but they contain perhaps the closest approximation to the idea of black holes possible under Newtonian physics, not to mention the outline of a functional method for their identification.
Several black holes have been discovered based on the orbits of nearby stars in exactly the way Mitchell described.
It is only in the last few years that telescopic images have confirmed this indirect evidence.
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According to McCormack, the existence of invisible stars was a relatively common idea among scientists at the time.
The same year that Mitchell published his study, several other astronomers were corresponding about stars that had gone out.
In 1805, astronomer Edward Pigott published a study suggesting the possibility of stars "that have never shone with a single gleam".
Although their true number can never be known, "would it then be too bold or visionary to assume that their numbers are equal to those stars that are endowed with light?" he wondered.
In France, the polymath Pierre-Simon Laplace promoted the idea of dark stars independently of Mitchell during the early 1790s.
Soon, however, new experiments gave credibility to the idea that light consisted of waves rather than massive particles, and suggestions that it could be deformed or trapped by gravity began to go out of fashion.
Mitchell's astronomical work fell into oblivion and was rediscovered only in the second half of the 20th century.
In his 1994 book Black Holes and Time Warps, physicist Kip Thorne bemoans the "stark contrast" between the enthusiasm with which Mitchell and his contemporaries embraced the idea of gravitationally invisible stars and the "widespread and almost unique 20th-century resistance to black holes."
The key difference, he concludes, is that Mitchell's dark stars, while exotic, "posed no threat to any accepted beliefs about nature" and did not question "the permanence and stability of matter."
As McCormack points out, modern black holes, by contrast, are just that: "a hole in space-time, an endless well from which nothing can escape."
Despite this, McCormack speculates that Mitchell, "who accepted 'the infinite variety we find in the works of creation,' would have had no problem with our black holes."
There is no way to test this claim, but given Mitchell's extraordinary scientific imagination, as well as his commitment to the Newtonian tradition of reason, it does seem appealing.
In this illustration attributed to the 19th century, a man looks from Earth at how the wider Universe really works.
Mitchell died on 21 April 1793 at the age of 68, remaining the main vicar of Thornhill until the very end.
Other intellectuals of his period were – and still are – much better known.
They published more frequently and on topics that were more popular.
Mitchell, unlike them, followed his own instincts.
According to McCormack, "he tackled scientific problems as they interested him, in whatever field, and pursued them as far as he wished and no further; and he published his papers as and when he wished, and only when he was thoroughly satisfied with them."
This goes some way to explaining his anonymity after death – he sacrificed influence and fame in the name of intellectual freedom.
As the Alexandrian astronomer Ibn al-Haytham observed 700 years before Newton, the "seeker of truth" is not one who believes authorities "but one who doubts his own belief in them... one who is guided by argument and demonstration."
Following this tradition, Mitchell, just like his father, was an autodidact, protecting his own scholarly integrity and remaining unaffiliated with any "body or denomination of men".
Mitchell's independence allowed him another freedom crucial to original thought: freedom of imagination.
According to McCormack, he chose astronomy specifically because it offered new perspectives for theories.
In his own passion for scientific imagination, Mitchell anticipated the creativity of today's theoretical physicists.
As Einstein said in 1929, "imagination makes the world go round."
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Ben Platts-Mills is a writer and illustrator whose work explores power, reasoning, and vulnerability, and the ways science is represented in popular culture. His memoir, Tell Me the Planets, was published in 2018. He is @benplattsmills on Instagram.
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