Scientific Controversies: Black Holes
The most enigmatic astronomical objects in the universe, so dense that not even light can escape, black holes inspire terror, tapping into the primal fear of darkness deeply embedded in our psyches. The color that conceals all and the color of night is associated with evil in practically every culture and the origin is likely hard-wired by evolution—the loss of our visual sense made us vulnerable to predators in the darkness. Not surprisingly, these seemingly unfathomable voids appear in stories, film, art, and manga.
In Rumiko Takahashi’s wildly popular manga series Inuyasha, serialized and published in the Weekly Shonen Sunday from 1996-2008, the not so ascetic monk Miroku carries an ancestral curse, a whirlpool in his right hand. Miroku’s grandfather was cursed by the demon Naraku who ordained that it would carry over to all future generations and destroy them. This “wind tunnel” void—a black hole—had swallowed his father and was growing to be more and more powerful in his own hand, threatening to suck him in too. The tale of time travel that unfolds is Miroku’s quest to find and destroy Naraku to free himself and his descendants of this obliterating black hole. Set in Japan’s Sengoku Period (1450-1615), he is joined in this journey by Kagome, a 15-year old modern day school girl, who time-travels with him and the half-dog half-demon mythical beast Inuyasha. Takahashi deploys many of the properties of black holes that we have come to understand scientifically, exploiting the warping of time and space deftly and evocatively into her narrative arc.
The terror of a whirlpool that captures all into oblivion, makes an appearance even earlier in fiction, in Edgar Allen Poe’s 1841 short story “A Descent into the Maelstrom.” Set in the backdrop of the Lofoten archipelago in Norway, here too a deadly vortex, this time in the ocean, inspired by real local tidal features and strong eddies—Moskstraumen—is at the heart of the story. The hero witnesses and survives the ghastly spectacle of “the most terrible hurricane that ever came out of the heavens” that attracted and captured into its eye bodies and whole ships. He describes in great detail how objects are dragged into this swirling malevolent vortex without nary a trace—utter and complete annihilation. Perhaps, for our imaginations, it is the allure of disappearing into nothingness beyond a point of no return that is captivating. This, it turns out, is one of the remarkable properties of astrophysical black holes.
The origin of the phrase black hole itself has an odd history and, like Poe’s maelstrom, pre-dates its scientific use. The term was used to describe an infamous prison in Calcutta’s Fort William, in which the local ruler, the Nawab Siraj ud-Daulah of Bengal, confined prisoners of war. On the night of June 20, 1756, more than a hundred captured British East India Company soldiers were interned there. It is claimed that while enclosed in this claustrophobic dungeon, roughly 4 meters by 5 meters, most died of suffocation or heat stroke. Henceforth, as a place of no return, this cell was known as the Black Hole of Calcutta in soldier slang. News of this gruesome incident catalyzed retaliation from British troops stationed almost a thousand miles away in Madras. At the avenging Battle of Plassey in January 1757, the British successfully retook Calcutta, capturing and killing the Nawab. It was this decisive battle that solidified the British colonial conquest of India. An obelisk marking the spot was erected in 1901 in memoriam, which still stands in post-Independence India.
It is not just writers of fiction and science fiction who have been captivated by this bizarre property of halting light. Artists like Shea Hembrey have experimented and explored ways of visualizing what this impenetrable darkness and the inexorable suction might look and feel like.
In a sculpture, titled Radius, that was part of his 2012 "Dark Matters" exhibition, he uses wheat straw to depict the bottomless pit as black holes guzzle anything and everything that strays close.
Black holes made their scientific debut in 1916 as a peculiar and exact mathematical solution to Einstein’s formidable field equations that underpin his general theory of relativity. It took several decades for black holes to transmute from a mere mathematical oddity to real objects in the universe that could be studied and understood. Einstein’s general theory of relativity reformulated gravity as the interplay between matter and the shape of space. The existence of matter alters the geometry of the space it inhabits. The elegant black hole solution describes the intense warping of space, nearly a puncture, that an extremely compact mass would generate. The pull of gravity in the vicinity of this puncture would be relentless and inescapable. With their intense gravitational pull, black holes represent nature’s point of no return. A fall into a black hole signifies the end of everything—material existence, space, and time.
The sacred boundary around a black hole that demarcates this point of no return is called the event horizon. The size of this region is proportional to the mass of the black hole. For the black hole at the center of our own galaxy, the Milky Way, that weighs in at about four million times the mass of the Sun, the event horizon has a radius of roughly 8 million miles. Although these big numbers seem impressive, on the cosmic scale the event horizon is really tiny: less than twenty times the width of the Sun. In comparison, the stars that orbit the supermassive black hole stably lie around 20 trillion miles away, about as far from the black hole as our Sun's nearest neighboring star is from us.
Within the event horizon, black holes enclose a singularity, a point where all our known laws of physics breakdown. Therefore, in a deeply profound way, black holes also represent the limits of knowledge—and mark the cusp between what is knowable and what may remain unknowable. It is this tantalizing aspect that has seduced me to work on black holes, to try to understand how they form and grow and the stealthy imprint they leave on their surroundings.
In 1930, astrophysicist Subrahmanyan Chandrasekhar showed that the endpoint of stellar evolution—stellar death of the most massive stars—would leave behind black holes as corpses. He showed that the birth masses of stars determined if their death state would be a dense remnant of degenerate matter, a white dwarf; an even denser remnant of crushed neutrons, a neutron star; or the ultimate catastrophe, a black hole. White dwarfs were known to exist from 1910, but it was another forty-seven years before the discovery of neutron stars by Jocelyn Bell Burnell and Antony Hewish. Finally, in 1970, with the discovery of a flickering X-ray emission from matter falling into the event horizon detected by the Uhuru satellite in the source Cygnus X-1, black holes made the leap to reality.
Over the past two decades, there has been a rapid growth in our understanding of the properties of black holes. Per our current inventory, we know that there are at least four classes of detected black holes, classified on the basis of their masses: stellar mass black holes (with masses a few times to about a hundred times the mass of the Sun); intermediate mass black holes (with masses around hundreds to hundreds of thousands of times the mass of the Sun); supermassive black holes (millions to trillions of times the mass of the Sun), and ultra-massive black holes (masses greater than 5 billion times the mass of the Sun). The universe is littered with black holes, big and small.
Intriguingly, not only are all black holes small for their weight, they don’t amount to much of the mass of a galaxy. Even supermassive black holes are less than a thousandth of the mass of the stars in their galaxy. Given how small they are and how little they contribute to the overall mass (a millionth in the case of our Milky Way), we were misled into believing that black holes may be inconsequential and may not exert a significant influence on their host galaxies.
Twenty years ago, working on my doctoral thesis, I was convinced that black holes punched beyond their weight. As a young student in Cambridge at Trinity College, I wrote a speculative paper that was counter to the then prevalent understanding of black holes, making a claim that black holes could drive detectable outflows of gas. Contrary to their image as gluttonous gorgers, I realized that a small amount of the energy released by the gas while swirling and falling into a black hole, if tapped, could actually push some gas out too (black holes might well dribble when feeding) to huge distances, well beyond the visible edge of their host galaxies. This was a testable prediction: ejected blobs of gas could reveal how black holes impact their larger-scale neighborhoods. In 2019, with the Atacama Large Millimeter Array, such ejected blobs of gas were detected for the first time, confirming my twenty-year old theory.
We are still actively learning remarkable facets of black holes. It is an exciting time for new discoveries and there have been many since our conversation at Pioneer Works (watch the full event above), including the first up close and personal image of the shadow of a black hole (see Scientific Controversies: Event Horizon). We are nowhere near finished with probing these dark monsters that seem to be lurking at the centers of most galaxies and are peppered everywhere else in the universe. Black holes, both those captive in galaxies and wandering around, seem ubiquitous, waiting to be uncovered. Once believed to play a marginal role in the formation of galaxies, black holes are now believed to play a fundamental role in shaping the galaxies they inhabit and to play a starring role in structuring the universe. Black holes, it seems, are fundamental to our entire history—and might well be responsible for our being here and now in our wondrous universe.
This event was supported by Science Sandbox, a Simons Foundation initiative dedicated to engaging everyone with the process of science. The Broadcast is supported in part by the Alfred P. Sloan Foundation.
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