VINCENT BONTEMS AND ROLAND LEHOUCQ
ILLUSTRATIONS BY SCOTT PENNOR
Black holes are neither holes, nor black, but gray around the edges.
—Michel Cassé, Les trous noirs en pleine lumière
In 1783, Reverend John Michell (1724–1793), a British physician and astronomer, wrote a letter to the Royal Society in which he theorized about the existence of bodies of such mass that even light could not escape from them. He also conceived of a principle by which such stars could be detected: “If any other heavenly bodies should happen to revolve about them, we might still perhaps from the motions of these revolving bodies infer the existence of the central one with some degree of probability (…).” A few years later, in 1796, Pierre Simon de Laplace (1749–1827), in Exposition du système du monde, independently formulated the same conjecture: “The gravitational attraction of a star of the same density as the Earth, and with a diameter 250 times that of the Sun, would be so great that theoretically no light would escape from its surface, and it would be invisible by reason of its very magnitude.” His idea was received just as coldly as his predecessor’s because in the meantime, the experiments on light interferences conducted by
PHYSICS’ DARK THOUGHTS
Thomas Young (1773–1829) and Augustin Fresnel (1788–1827) had shown that light was a wave without mass, and therefore not affected by gravity. However, the concept of an “occluded star,” as it was called in the late 18th century, was destined to become a subject of intense scrutiny in the 20th century under the name “black hole.”
What are black holes?
We all know that a ball thrown up into the air eventually falls back to the ground after reaching a maximum altitude, whose height will depend upon the force of the throw. This is one of the principles of Earth’s gravity, which pulls all objects toward its center. This height increases in proportion to the square of the initial speed: thrown 2 times as quickly, the ball will go 4 times higher. If the ball is thrown fast enough—beyond 6.7 miles per second (approximately 24,000 miles per hour), it will not fall back to Earth, and will definitively escape its gravitational pull. This escape velocity must be reached by the interplanetary space probes launched to explore our Solar System. The escape velocity is proportional to the square root of the ratio of the planet’s mass to its radius. A planet 4 times bigger, or 4 times smaller, than the Earth will have an escape velocity two times greater. Since light travels at about 180,000 miles per second, Laplace calculated what the radius of a body as dense as the Earth would have to be in order for light to be unable to escape from it: 250 times the radius of the Sun. Obviously, the Laplace formula is only valid in classical physics
for a material projectile and cannot be applied a priori to light whose corpuscles—photons—are without mass.
The rigorous study of gravitation’s impact on light must be conducted within the framework of the theory of general relativity published by Albert Einstein in 1915. This theory states that gravitational force is, in reality, the manifestation of the geometry of spacetime, itself imposed by the distribution of matter and energy; spacetime is deformed and curved by their presence. A freely moving particle can only move according to the lines of the shortest path—geodesics—of the new geometry. Thus, contrary to the classic theory, even if deprived of mass, light is affected by gravity, or rather by the curvature of spacetime. Its trajectory is thus deviated when passing near a massive body. This effect was verified for the first time by the English astronomer Arthur Eddington (1882–1944) during a total eclipse of the Sun, in 1919. The minuscule difference between the position of a star observed in a direction near to that of the Sun during the eclipse, and the position of the same star measured some time later, fully corroborated Einstein’s calculations. This was a blatant confirmation of his groundbreaking theory of gravity. Today, we know that the instruments used by Eddington were too imprecise for this observation to be truly conclusive, but since this fortunate event, the experiment has been repeated countless times, and unambiguously confirms Einstein’s predictions.
If spacetime curvature affects light, it is once again possible, within the context of general relativity, to calculate under what condition a star can prevent its light from escaping. Einstein’s theory confirms the existence of a radius below
PHYSICS’ DARK THOUGHTS
which light cannot escape from a star and, contrary to all expectations, the formula obtained is exactly identical to that deduced by Laplace, under the classic theory. The “Schwarzschild radius,” named in honor of the German physician Karl Schwarzschild (1873–1916) who calculated it in 1915, is proportional to the mass of the object. For example, to transform the Sun into a black hole, it would be necessary, according to Schwarzschild’s formula, to concentrate its entire mass in a ridiculously small radius of only 1.8 miles!
The spherical surface delineating the region from which neither light nor matter can escape is called the black hole’s “horizon.” It is a geometric surface with no material reality, thus named by analogy to the terrestrial horizon, which represents the boundary of what can be seen. While the position of the Earth’s horizon depends on the observer’s position, the black hole’s horizon is absolute. It is a spacetime boundary behind which events cannot be observed, and which splits events into two categories. Outside the horizon, it is possible to communicate at arbitrarily great distances by means of light signals. This is the ordinary Universe in which we live. Inside the horizon, light rays are no longer free to travel between any points, because they are focusing toward the center. Communication becomes subject to severe constraints. Matter and rays, for example, can pass from the external field into the internal field, but not the reverse. That is the very justification for the term “black hole,” proposed in 1967 by the American theoretician, John Archibald Wheeler (1911–2008) for what was then still only a theoretical possibility. Wheeler was a theorist with a fertile imagination and a mind open to the boldest speculations. In addition to “black holes,” he popularized numerous highly original
concepts such as “many-worlds,” “wormholes,” “back in time particles,” or the parallelism between matter and information (“it for bit”). These concepts have sometimes remained intellectual conceits, but for “black holes,” proof of the existence of these theoretical monsters came in 1971: astrophysicists detected Cygnus X-1, a binary system whose characteristics suggested that it was a pair formed by a black hole and a giant star. Since then, some 20 black holes have been identified in our Galaxy, the largest of which—its mass is equivalent to 4 million solar masses—is crouched in the center of the Milky Way. As Vincent remarks in the following chapter, the presence of a black hole in the core of our Galaxy may capture our imagination, but there is nothing malevolent about it: to the contrary, it assists in ensuring its stability, and promotes its evolution. Its presence was demonstrated in 2002 by the observation of orbiting stars near the center of our Galaxy. In 2022, the Event Horizon Telescope project plans to capture images in the vicinity of this central black hole’s horizon by combining the data from radio telescopes distributed over the entire Earth’s surface. Observations have also shown that the center of certain active galaxies, such as Messier 87, shelters a supermassive black hole which may easily exceed a billion solar masses!
Although black holes seem to unquestionably exist, a mechanism has yet to be formulated that will track their formation. Stellar black holes—those whose mass consists of at least several solar masses—are formed as a result of the gravitational collapse of the center of a massive star (about 10 or more solar masses). Indeed, when the star reaches its thermonuclear silicon burning phase, the mass of its iron core increases until it becomes unstable. The core then implodes, producing a neutron star,
PHYSICS’ DARK THOUGHTS
while the star’s envelope is blown by a titanic explosion called a “supernova.” In 1939, American physicist Robert Oppenheimer (1904–1967) showed that if the neutron star’s mass exceeds 3 solar masses (the Landau-Oppenheimer-Volkoff limit), the star’s own gravitation supersedes all other interactions, and a black hole forms. Since then, black holes have been part of the cosmic bestiary, although astrophysicists’ observations long remained confined to detecting the indirect effects of their presence. This changed in October 2015, with the first detection of gravitational waves by the American LIGO experiment team. These physicists saw, in the recorded vibrations, proof of a gigantic saraband. Two black holes 1.3 billion light-years away, respectively 36 and 29 times more massive than the Sun, which were spiraling around their shared center of gravity at the rate of 250 turns per second, eventually fused into one single and enormous black hole consisting of 62 solar masses. The difference between them—3 solar masses—was radiated in the form of gravitational waves.
THE GRAVITY OF BLACK HOLES
There we can take great baths of darkness …
—Charles Baudelaire, Anywhere out of
As Roland explained it, a “black hole” is a star so dense and massive that light rays cannot escape from it. The night sky purifies the black color of its earthly associations; the darkness of the black body transcends the color spectrum. With a black hole, another obscure gap is bridged: blackness assumes a terrible gravity. Not only can no light dissipate this darkness, but it does not survive it. The adjective “black” undergoes so much intensification that it goes beyond opposing the light: it threatens it. It no longer foretells that light will be evaded, but that the black hole will devour it (…). As Pascal Quignard wrote, “the gravitational appetite of a black hole rivals with the speed of light” (Critique du jugement, 2015, p. 92). Within the context of general relativity, this means that the black hole’s mass curves spacetime into a bottomless pit: anything can fall into it, nothing will ever reemerge from it. It is the supreme implosion. Because it collapses unto itself, this body is called a “hole.” Because it consumes matter
PHYSICS’ DARK THOUGHTS
and light, it is called “black.” The metaphorical interpretations of the two terms, however, leave something to be desired.
Isn’t it paradoxical to call a “hole” the strongest concentration of matter in the Universe? Ordinarily, a hole is a void, not an overflow. As for the blackness, Roland recalled that it was not so absolute: black holes are all slightly gray around the edges. Nonetheless, our musings should start by exploring the connotations of the adjective “black” by drawing inspiration from the ideal black hole formulas, rather than from their actual fringes. Theoretically, the blackness of a black hole is so intense that it consumes light rays and never lets them out. Transposed into the imaginary, its blackness is endowed with a dynamic in total contrast to the black body’s expansive one. Its unsurpassable gravity makes a black hole concentrate all spacetime into a singular point that blocks any communication toward the outside, thus becoming a symbol of annihilation.
A cosmic depression
In order to grasp the imaginary implications of this irresistible obscure attraction, we need to elevate ourselves to a consciousness level that is no longer merely human, but truly cosmic: black holes result from the death of the most massive stars. As a spacetime singularity, the black hole represents a traumatic event on a Universe scale. There is, in its darkness, something that affects the very structure and rhythm of the Universe, which wounds what Neoplatonists called “the Soul of the World.” This trauma leads to an eternal mourning process: black holes symbolize the irreversible capture of light,
THE GRAVITY OF BLACK HOLES
which we believed to be elusive, free by its very essence, immortal. Within the Universe’s existence, irreversible events and insurmountable disasters are taking place.
We often imagine black holes to be like siphons, sucking up what surrounds them as if the whole Universe were doomed to absorb itself. However, outside their horizon, beyond which no communication with the exterior is possible, they do not curve space any differently than other masses do. The inescapable destiny of a black hole threatens only those bodies of light and matter that imprudently venture across a fatal horizon: intense tidal forces will stretch them disproportionately before amalgamating them with its monstrously dense mass. Black holes do not seek to destroy us, they reign solitarily, autistically. Those bodies which remain beyond the “static surface” may continue their ordinary existence, while those which draw near black holes—even if they resist their gravitational pull—feel, from that point on, as though they were revolving in an orbit threatened by collapse, with the knowledge that they can do nothing for those who have been trapped.
A black hole is thus the most powerful symbol of depression: nothing can alleviate this affliction, because anything which approaches it too closely is definitively consumed. The structure of space and time seems to evade those bodies who allow themselves to be caught up in it. They no longer have the strength to move or rise again. The Universe darkens when the body collapses on itself in a motionless fall. The gravity of this blackness is terrifying and absolutely unmatched. There is something frightening in the thought that crouching in the heart of our Galaxy is a presence so hideously obscure, hungry, and heavy.
PHYSICS’ DARK THOUGHTS
The dark layers buried in the core of the black body were resistant to light, but not menacing; they even harbored a certain warmth. With black holes, however, darkness takes on a negative connotation, and acquires a capacity of troubling concentration. As for aerial imagination, Bachelard stressed the opposition between progressive and liberating reverie and the vertiginous fall into the dark depths of guilt. Darkness is what draws us into the sinkhole: what awaits us at the bottom of the abyss. Any accomplished oneiric fall ends in the heart of darkness: “It appears that the blackness of the abyss erases everything and that ultimately the fall has but one color: black” (La Terre et les Rêveries de la volonté, p. 400). The insightful reverie that takes over us at the mention of a black hole inevitably gravitates toward the semantic Universe of this “blackness of the abyss” which nothing seems able to resist. And a gruesome end awaits anyone who yields to the fascination exerted by this color. Bachelard concludes his evocation of subterranean night with these words endowed with a lugubrious undertone: “It is a spacetime of the abyss-fall. Further along, in an accomplished fall, the poet will encounter blackness. Therefore, ‘blackness and emptiness’ are inseparably united. The fall ends. Death begins” (La Terre et les Rêveries de la volonté, p. 402). The extraordinary phrase, “Death begins!” corresponds perfectly to the astrophysicists’ no less suggestive: “crossing the event horizon.” There is no going back, there is only the inexorable pull of the black star—a fascination with the inevitable destruction that many romantic poets have glorified.
THE GRAVITY OF BLACK HOLES
The star of melancholy
To precisely describe the nuance of such a terrifying blackness, some think of the “black sun” haunting romantic poetry and engravings. No one has described it better than Victor Hugo:
“There, everything floats and drifts away in a dark shipwreck;
In that abyss without margin, without skylight, without wall,
Rain down the ashes of all things that have lived;
And when one’s eye dares to go down into the very depths,
It makes out, beyond all life, all breath, all sound,
An horrific black sun whence night gleams forth!”
(Victor Hugo, Les Contemplations, VI.26, “Ce que
dit la Bouche d’Ombre”)
Each verse seems to herald the most perilous journey on which Roland invited us: there is a shipwreck as soon as we draw too close to the dark star; once captured, it is impossible to hold onto anything at all; everything is dragged toward the dark center, where the black sun dwells that destroys all of our hopes.
This fatal star could already be seen in the admirable engraving by Albrecht Dürer (1471–1528), Melencolia I (1514). It shows a beautiful woman with angel wings surrounded by symbols of power, wealth, and knowledge, yet indifferent to them, absorbed in some morbid thought. Her facial expression is more pensive than sad. Beside her, a dog is sleeping and a cherub looks bored. Meanwhile, in the night sky, a small demon flies in triumph, deploying the banner of melancholy, beneath a star that seems to attract to itself all the rays of light rather than to radiate them.
PHYSICS’ DARK THOUGHTS
The concentration dynamic is obviously dominant: as the mind focuses on some dark rumination, the light congeals in a black star.
Gerard de Nerval (1808–1855) also anticipated astrophysics by correctly attributing the birth of this dark star casting a pall over his poetry to the death of a star: “My only Star is dead—and my starred lute/Bears the black Sun of Melancholy” (Gerard de Nerval, “El Desdichado”, 1854). His sad musings ultimately drove him to commit suicide. Nowadays, the beautiful and unsettling comic book series, Black Hole, by Charles Burns (1955), revives this romantic fatality myth: a strange illness leads to horrible mutations among pubescent teenagers. The black hole symbolizes the profound discontentment of a generation, the obsessive fear of unsafe sex, and the fascination with death that young people experience who feel ill at ease with themselves. The black hole is undoubtedly the supreme test of individualization in the meaning afforded by Carl Gustav Jung (1875–1961)—namely the awareness that failure is possible, probable, and perhaps even unavoidable.
The romantic melancholy myth also lies in the premise that the creators’ personality combines inspired genius with a gloom that is all the more relentless because it stems from a lucid contemplation of the human condition. The idea is not new. Ever since Hippocratic medicine first described the atrabilious temperament—meaning one resulting from an excess of black bile, such as one attributed to gloomy and serious “humors”—philosophers and doctors have never stopped speculating about the secret ties uniting genius and melancholy. The brief treatise Problema XXX, long attributed to Aristotle, stresses the large number of epileptics among geniuses—the “falling sickness”
THE GRAVITY OF BLACK HOLES
then believed to be caused by an excess of black bile. Closer to our era, wasn’t Sir Isaac Newton, whose law of universal gravitation made possible the notion of an “occluded star” from which even light could not escape—a classic prefiguration of the relativistic black hole—an epileptic himself?
Of course, all of this was nothing but nonsense and superstition: neither John Michell, nor Pierre Simon de Laplace—the inventors of occlusion—nor Einstein, to whom we owe the theory of general relativity, nor Karl Schwarzschild (whose last name means “black shield”), who speculated on the existence of the relativistic black hole, nor John Wheeler, who finally baptized this singularity with the unsettling name of “black hole,” were subject to such epileptic seizures. Insightful musings can no longer fantasize that way about the influence of stars on the mood of geniuses. However, the vague intuition that an exceedingly great intelligence must inexorably lead to terrible discoveries is not easily banished from the collective imagination. The black legend draws us. It possesses a seductive quality that is hard to resist: since the feeble-minded are happy, shouldn’t the wise man and scholar be prone to melancholy? Their thoughts are so serious that they simply must brood.
We thus spontaneously come to suspect that those deemed to be most brilliant all conceal an unfathomable sadness. Thus a black hole is detected indirectly, like a somber madness hidden under the guise of a cold intellect. To surmise its existence, we must observe the otherwise inexplicable rotation of other stars around a seeming nothingness, just as we observe that certain individuals use repetitive meticulous gentures for no apparent reason. When
PHYSICS’ DARK THOUGHTS
far-distant rays transmit a deformed image after having passed through an apparently empty zone in space, when a rationale that seemed, at first glance, well-founded, deforms reality to the point of rendering it unrecognizable, we surmise the existence of a gigantic depression that modifies the perception of space and time. Isn’t it horrible that white galaxies sparkle around such sink-holes? Isn’t it appalling to think that certain cherished stars, for having shone too brightly, must end up collapsing upon themselves, imprisoning the light that they were meant to shed on the cosmos? Despair erases even the memory that a world outside of ourselves exists.
A black hole would be one of the most mournful symbols of all if it were this abyss in which a great mind endlessly sinks, never to emerge again. It would be simplistic, however, to mistake black holes for the glorified metaphor of mere depression. For let us recall once again that a vast and beautiful image, a true image, endowed with a genuinely psychodynamic dimension, is always ambivalent.