Einstein and his peers were 'irrationally resistant' to black holes. This illustrated story explores why

All stars must eventually die, but when they do, they enter an afterlife.

A smaller body, like our Sun, will likely shrink and stabilise as a super-dense "dwarf". A larger star might collapse into a neutron star, a ball of subatomic particles only a few miles across yet with gravity strong enough to slow time and shred nearby bodies into "spaghettified" streams of matter. But for the most massive stars, death means transformation into something that defies not only the power of description but that of thought. Overcome by their own gravity, they collapse beyond any sustainable condition of matter to form a sudden, exponential crunch: an implosion so absolute that it punctures spacetime and unstrings in its vicinity everything we know as physics.

The resulting gravitational vortex – a black hole – overpowers all matter, all energy, all radiance, including that of the star that created it. Even light cannot escape once it strays within reach. Like an invisible stitch in a garment, a black hole creates a pinch in the Universe, folding the fabric tightly around itself as an envelope of impenetrable darkness. It can be known only by its effects: its extraordinary gravitational field and the distortion, or "lensing", of spacetime it creates.

The existence of black holes began to find acceptance in the 1960s and has recently been proven by images captured by telescopes such as the Event Horizon. It's now believed that they sit at the heart of most galaxies, which means there could be billions scattered across the cosmos. Yet their theoretical underpinnings were established more than a hundred years ago. As described by the physicist Werner Israel, the evidence was "already non-ignorable by 1916" but was denied for decades by the most eminent scientists in the world, including Einstein, whose own work led to their discovery. Black holes were too strange, too unnerving for the physicists of the early 20th Century, provoking what Israel called a "a resistance bordering on the irrational". 

While some scientists had speculated about the existence of similar objects called "dark stars" more than 200 years ago, it was Einstein's theory of general relativity that laid the ground for understanding how black holes could be created. First published in October 1915, this was a profound revision of the classical physics that had dominated science and philosophy for centuries. As Einstein explained in 1921, "It was formerly believed that if all material things disappeared out of the Universe, time and space would be left. According to relativity theory, however...

Einstein showed that space and time were continually stretched and distorted by masses such as stars and planets, and that this accounted for gravity. The way bodies are drawn together is not due to a "force" attracting them, he argued, but to the "curvature" of the Universe caused by mass. The greater the mass, the more curvature it causes and so the greater the gravitational effect.

When Einstein first published his theory, he hadn't pinned down the solutions to his own equations, which would have revealed to him the full implications of his discovery. It was another scientist who made this step.

In November 1915, Karl Schwarzschild was an artillery lieutenant in the German army, on the Eastern Front. He read Einstein's new theory while working at a weather station close to the front line and wrote a letter in response.

His letter supplied the missing solutions and showed how they could be used to model a star's gravity. One feature of the model, Schwarzschild noted later, was a radius of compression below which a star – or any other spherical mass – would begin to implode indefinitely under its own gravity. If applied to the physics of the real world, this had horrifying implications. It meant that a star would continue collapsing forever, its mass being crushed ever smaller. Its gravitation would become ever more powerful as it insatiably devoured surrounding masses until, finally, it reached the point of "singularity", a moment where the laws of physics break down, and time and space cease to exist.

Decades later, the Schwarzschild singularity would be recognised as a turning point in theoretical physics – the first time black holes had been hinted at. Schwarzschild himself, however, dismissed the idea as a mathematical artefact.

The critical radius was, Schwarzschild concluded, simply the limit for a star's compression – the point it would stop collapsing. Instead of discovering black holes, then, he became the first person to reject the evidence for them on principle. We will never know if he might have revised his ideas because he died of an autoimmune disease in 1916. 

Einstein's reaction to Schwarzschild's solutions was mixed. On one hand, we know he was pleased because he helped Schwarzschild publish them in a scientific journal before he died. On the other hand, it seems he was troubled by the singularities they contained. In 1935, with his colleague Nathan Rosen, he introduced a new concept explicitly designed to do away with them. By connecting one extreme gravitational event with another, the Einstein-Rosen "bridge" – later popularised as a "wormhole" – formed a kind of tunnel between two zones of space-time, replacing singularities of collapsing matter with a fleeting, empty tunnel. Einstein and Rosen were clear about their motives in publishing this idea: singularities, they wrote, "we cannot accept at all. For a singularity brings so much arbitrariness into the theory that it actually nullifies its laws."

In 1939, Einstein again revisited the problem, demonstrating that a collapsing star could not become stable at the critical radius identified by Schwarzschild, and concluding that singularities "do not exist in physical reality". It seems as though the greatest physicist of his age was somehow unwilling – even incapable – of thinking about singularities and the infinities they contained.

He wasn't the only one. Sir Arthur Eddington was the secretary of Britain's Royal Astronomical Society and Einstein's main champion in the English-speaking world. He translated general relativity for English journals and then, in 1919, mounted an expedition to test one of its main predictions. Travelling to the West African island of Principe, he took a series of photographs during a solar eclipse. Made visible by the eclipse's darkness, the photographs showed that stars in the same sight-line as the Sun appeared in slightly different positions from when they were viewed at night, in the absence of the Sun. This proved that star light was deflected by the Sun's gravity – by the curvature of space-time – just as general relativity predicted.

It is ironic that, having gone to such lengths to demonstrate the effects of relativity, Eddington was, like  Einstein, unable to accept its most significant implications. In his seminal book, The Internal Constitution of Stars, published in 1926, he made clear his assumption that an astral body massive enough to trap light was impossible. The giant star Betelgeuse, for example, "could not possibly have so high a density as the Sun" because "the force of gravitation would be so great that light would be unable to escape from it," and because the mass would produce so much space-time curvature that "space would close up round the star, leaving us outside (i.e. nowhere)".

But mathematics was not on his side. In July 1930, a 19-year-old student named Subrahmanyan Chandrasekhar was on a steam boat travelling from his home in India to England to begin his graduate studies at the University of Cambridge. While still on the boat, he ran some calculations and found that the fate of a dying star unavoidably depended on its mass. A star around the size of the Sun would gradually shrink and stabilise as a super-dense "white dwarf" but one only a little larger would create too much gravitation to follow this steady decline. "I didn’t understand at the time what this limit meant," he wrote later, "and I didn't know how it would end."

Eddington didn't like the sound of these "other possibilities" and at a conference in 1935 he made a startling public attack, laying out his objection to the idea of stellar collapse. "Various accidents may intervene to save the star," he said, "but I want more protection than that. I think there should be a law of Nature to prevent a star from behaving in this absurd way!" Chandrasekhar would eventually be awarded a Nobel Prize for his work on stars but at the time Eddington's attack was effective. Chandrasekhar, still a young academic lacking professional standing, retreated from the argument.

In 1931, both Einstein and Eddington had responded in a similar way when the Belgian priest and physicist Georges Lemaître proposed that the Universe itself had started from a singularity. The idea would eventually evolve into the Big Bang theory but at the time Eddington described it as "repugnant"  while Einstein told Lemaître:

This was what Werner Israel was referring to, decades later, when he described Einstein and Eddington's  "resistance bordering on the irrational". But perhaps they were resistant precisely because the singularity represented to them a descent into unreason, an attack on their understanding of what "rational" could mean. They both understood the difference between scientific evidence and personal conviction. In Einstein's words, science had "the sole purpose of determining what is" and "the determining of what ought to be, is unrelated to it." But, according to physicist Kip Thorne's account in the 1990s, it was precisely the question of what "ought to be" that governed their reception of black holes. In his words, black holes "violated Einstein's and Eddington's intuitions about how our Universe ought to behave".

In the early 20th Century, black holes were inherently inaccessible to observation – if no light could escape them, there could be no way to observe them or find out what they contained. The kinds of telescopes needed to observe these otherwise invisible phenomena were still many decades in the future. Confronted with this fact, Einstein and Eddington fell back on pre-existing philosophical – even spiritual – assumptions about what counted as rational and about how the Universe worked.

In a letter written in 1951, Einstein referred to an "emotional or psychological attitude" on his own part, of "confidence in the rational nature of reality." On many occasions he stated his belief in the philosopher Spinoza's idea of God: a divine being who exists in everything. Einstein credited this God with the beauty and "logical simplicity" of the Universe and who revealed himself in its "lawful harmony". Eddington, a life-long Quaker, believed in God as an "all-pervading force" identifiable with "nature". He wrote of "mystical experiences" that unified the mind with the "harmony and beauty" of the wider Universe. But if nature was beautiful, at times he communicated a feeling of disgust towards some of its expressions, not least human beings. "By a trifling hitch of machinery," he wrote in 1934, "some lumps of matter of the wrong size have occasionally been formed. These lack the purifying protection of intense heat or the equally efficacious absolute cold of space..."

Perhaps this was intended as a joke but Eddington's reactions to singularities – his wanting "more protection" from these "repugnant" ideas – seems to communicate a similar sense of disgust, as though they were somehow existentially ugly or dirty. For both Einstein and Eddington, singularities were incommensurable with the beauty and harmony their worldviews were founded on, and with the rational god they saw behind it.

Looking back on his career in later life, Chandrasekhar communicated a similarly mystical – if emotionally different – response to black holes. "In my entire scientific life," he wrote in 1975, "the most shattering experience has been the realisation that an exact solution of Einstein's equations of general relativity... provides the absolutely exact representation of untold numbers of massive black holes that populate the Universe." In contrast to Einstein and Eddington's apparent disgust, Chandrasekhar described his experience as a "shuddering before the beautiful".

The phrase is taken from Plato's Phaedrus, a dialogue composed by the philosopher more than 2,000 years ago:

In the same lecture, Chandrasekhar quoted the physicist Werner Heisenberg talking to Einstein about the experience of scientific revelation: "You must have felt it too: the almost frightening simplicity and wholeness of the relationships which nature suddenly spreads out before us and for which none of us was in the least prepared."

Terror and awe at the infinite; the conflict between beauty and disgust: although these impulses might appear highly personal, they are in fact linked to a long cultural history. In Europe, a repugnance towards the infinite dates back at least to the Hellenistic Greeks, for whom it was something that, according to historian Tobias Dantzig, "had to be kept out, at any cost". The development of telescopes in the 17th Century brought the fearful infinity of the Universe into plain view. "The eternal silence of these infinite spaces fills me with dread," wrote the French polymath Blaise Pascale in the 1650s, describing humans as "a nothing compared to the infinite".

In the 1750s the Anglo-Irish aristocrat Edmund Burke formalised this spiritual anxiety in the concept of the "sublime". This "tranquillity tinged with terror" was exemplified nowhere better than in the contemplation of the infinite, which filled the mind with a "delightful horror". In the 1780s, the German philosopher Immanuel Kant – also fascinated by the sublime – wrote of how "the starry heavens... broaden the connection in which I stand into an unbounded magnitude of worlds beyond worlds and systems of systems" and described the Earth as "a mere speck in the Universe". Reflecting on the infinities of the cosmos, Kant wrote:

Another writer whose words foreshadowed the impact of black holes – in an uncanny way – was the American Edgar Allan Poe. In his 1841 short story, A Descent into the Maelstrom, he contemplates the transformative power of sublime infinities, as its unnamed narrator looks upon a terrifying whirlpool – the "maelstrom" – and listens to an account of a fisherman's struggle with the same vortex some years before. The story pits the narrator's sense of reason against the fear that threatens to overwhelm him.

"I struggled in vain," the narrator says during a violent storm, "to divest myself of the idea that the very foundations of the mountain were in danger from the fury of the winds." Of the whirlpool he says: "The ordinary accounts of this vortex had by no means prepared me for what I saw... [they] cannot impart even the faintest conception either of the magnificence, or of the horror of the scene...

These descriptions echo both Chandrasekhar's "shuddering" and the discomfort expressed by Einstein and Eddington. In its imagery, Poe's story also prefigures Einstein's idea of wormholes. The story's narrator mentions that the local Norwegians "almost universally entertained" the notion "that in the centre of the channel of the Maelström is an abyss penetrating the globe, and issuing in some very remote part".

Poe's source for this idea was the German priest and polymath Athanasius Kircher, whose 1664 book Mundus Subterraneus included maps of the tunnel beneath Norway that connected the whirlpool described by Poe with one in the Gulf of Bothnia, on the other side of Sweden, as well as one showing the influence of a similar subterranean canal on the whirlpool of Charybdis on the coast of Sicily. The idea has a close resemblance to the space-time tunnels imagined by Einstein as an escape from the bottomless existential vortex of the singularity.

There was, however, at least one early 20th Century scientist who was undaunted by singularities. In September 1939, Robert Oppenheimer and a colleague published the first paper to describe black holes not merely as theoretical artefacts but as real stellar phenomena. "When all thermonuclear sources of energy are exhausted," they wrote, "a sufficiently heavy star will collapse." With the biggest stars, and where no other factors intervened, the collapse would continue indefinitely and the star would close itself off from the rest of the Universe. The article has been described by some as Oppenheimer's greatest contribution to science and yet at the time it barely registered. The same month it was published, the Nazis invaded Poland and in October 1941 Oppenheimer was recruited to lead the development of the atomic bomb. He never returned to the subject of gravitational collapse. (Read more: Who was the real Robert Oppenheimer?)

Although he did not link it explicitly with black holes, Oppenheimer often spoke of a sense of awe and disorientation in his work. In one characteristic speech in 1960, he said, in words echoing Heisenberg's: "Terror attaches to new knowledge. It has an unmooring quality; it finds men unprepared to deal with it." In 1965 he described the "destitution in being cast loose in a new unknown", proposing fear almost as a signal of the greatness of a discovery. "I have heard from some of the great men of our time," he wrote, "that when they found something startling…

Though they responded in different ways to the evidence before them, what united these scientists – from Einstein to Oppenheimer – was that in the most important questions they were guided by their feelings. But's those feelings were also what separated them. When unsettled, Einstein became cautious; Eddington was repulsed; Chandrasekhar had more of a taste for the sublime. Oppenheimer, finally, could welcome that which frightened him, could understand it as a portent of transcendence. As the world soon learned through the bomb he created, fear did not delay his pursuit of discovery. 

Even today, black holes continue to evoke conflicting emotions. Earlier this year a team of Australian astronomers identified the brightest known object in the Universe – a massive black hole surrounded by a gaseous "accretion disc" seven light years across. Around 500 trillion times brighter than the Sun, the disc contains an interstellar lightning storm, with temperatures of 10,000C (18,000F) and winds blowing so fast they would go around Earth in a second. The hole at its centre consumes the equivalent of a solar mass every day. Interviewed in February, the team leader, Christian Wolf, echoed his forebears in his reaction: joy at the new discovery but also "a shock and awe moment, imagining this hellish place… imagining these conditions, and that nature does produce something even more extreme than we’ve contemplated previously."

A black hole is an impenetrable enigma. No light can escape it, no energy in any form: no sound, no image, no signal, nothing by which the inside might be examined or understood. In fact, even with the real astronomical black holes that have now been observed it's still possible that Einstein was right – that there is no singularity inside them – because there's currently no way of finding out directly. But if black holes are what most physicists believe, then they are bottomless, leading only inwards, forever. By definition, they cannot be reasoned with, about or into. They offer an infinity of darkness and destruction: the opposite of enlightenment. They are a test of both courage and reason, a dark pool in which we see the cosmos – and ourselves – reflected.

-bbc