powehi. The word is Hawaiian, taken from the kumilipo, an ancient chant describing the creation of the universe, the ’embellished dark source of unending creation’. In Maori, it simply means horror. Pōwehi is a monster, a terrifying behemoth, lurking at the core of Messier 87, a supergiant galaxy in the constellation of Virgo. In April 2019, those of us on Earth beheld it for the first time.
The startling image of Pōwehi was captured by the Event Horizon telescope, an array of eight ground-based radio observatories strategically positioned across the globe. It was an extraordinary accomplishment, given the size and distance to the source. Imagine yourself sitting in a Parisian café and peering through your telescope to read a newspaper in New York. That is what it took to capture this astonishing image in such magnificent detail.
But what is it, this horror, this dark source? Pōwehi is a black hole of gargantuan proportions, billions of times more massive than the Sun. It is gravity taken to its terrifying limit. We have already seen how light is bent by gravity. What happens as you ramp up the gravitational field, as you curve the spacetime more and more? You create a prison. Light is bent to such an extent that it becomes trapped, it cannot escape, and if light cannot escape, nothing can. Pōwehi is a cosmic oubliette, an unforgiving hell, a gaol for the forgotten.
It was an English clergyman who first conceived of such horrors. In November 1783 the Revd John Michell proposed the existence of dark stars, huge astrophysical objects five hundred times larger than the Sun whose gravitational pull was so strong that light itself could not escape. It was an exciting idea at the time, invisible giants hiding in plain sight, although it would soon be forgotten. The reason for this was that it was based on the corpuscular theory, where light is made up of particles, a theory that ultimately gave way to a wave-like model following the experiments of Thomas Young at the turn of the 19th century.
Light is bent to such an extent that it becomes trapped, it cannot escape, and if light cannot escape, nothing can.
Although Michell’s work on black holes would be ignored for almost two centuries, he would be heralded in science as the father of seismology. His work on the devastating earthquake and tsunami that struck Lisbon in 1755 included the idea that it originated from faults in the Earth’s crust rather than from atmospheric disturbances.
Today most scientists are confident that black holes really do exist. Typically, they form when a fairly large star—at least twenty times heavier than the Sun—runs out of fuel. Stars power themselves with nuclear fusion, squashing and squeezing atomic nuclei together in their core, a furnace of thermonuclear bombs exploding continuously. This power prevents the star from collapsing under its own weight, exerting outward thermal pressure to counter the effects of gravity. But it doesn’t last forever.
Once the star has produced too much iron in its core, the fusion processes become inefficient and it can no longer support its own weight. Star death. Gravity quickly begins to overwhelm the star, crushing it inwards, a garrote that gets tighter and tighter. And then bang! The star fights back, a dramatic counterpunch to gravity’s relentless attack. It is the neutrons that carry the fight, subatomic particles in the stellar core, violently repelling one another through a strong nuclear force whenever they are pushed too close together. Outer layers of material fall inwards, strike the immovable core of neutrons and rebound. In an instant, a pressure wave powers its way to the surface of the star and it explodes. A supernova, a cataclysmic event, briefly outshining an entire galaxy.
What is left behind? More than likely a neutron star, an object of tremendous density, so much so that a mere teaspoonful of its matter would weigh as much as a mountain here on Earth. If its total mass can stay below that of about three Suns, the neutron star has a chance of survival. Any heavier, and the gravitational garrote will begin to tighten once more. There will be nothing the neutrons can do. There will be nothing anything can do. The collapse becomes unstoppable. Eventually, the star becomes so dense that light can no longer escape. Everything that was once the star is hidden behind an event horizonthe trapdoor to the cosmic oubliette, a spheroidal surface beyond which there is no return.
At the core of the black hole is a singularitya place where spacetime touches infinity, where the gravitational field grows without bound.
About one in every thousand stars is heavy enough to end its life consumed by gravity. These stellar mass black holes are everywhere, scattered across the galaxy, shadowy remnants of the largest and most powerful stars ever to have existed. But Pōwehi is so much more. Black holes born from star death typically weigh between five and ten Suns and yet Pōwehi has a mass of six and a half billion Suns. A leviathan, a supermassive black hole, the anchor at the core of an enormous galaxy more than 50 million light years away. Pōwehi dwarfs our own leviathan, Sagittarius A, a black hole of 4 million solar masses at the center of the Milky Way.
Most galaxies are thought to be anchored around a supermassive black hole. Galaxy 0402+379 contains two such leviathans, probably as a result of two daughter galaxies colliding. The core of 0402+379 must be a raging tsunami of gravitational waves, tearing through spacetime as the two leviathans wrestle for supremacy. The truth is that we don’t fully understand how Pōwehi or any of these other monsters came to be. It is possible they are the greedy remnants of giant stars, once stellar mass black holes that grew to gargantuan sizes after millions of years of feeding on any material that dared to stray too close.
The existence of the event horizon defines the black hole. Just to stay still on its surface you would need to travel at the speed of light. For a stellar mass black hole, edging close to the horizon would be fatal. In a way, this is weird; gravity is fake, remember, and we can always eliminate it by climbing inside the blacked-out telephone box and falling, be it from the Burj Khalifa or towards the event horizon of a black hole. The trouble is that the region over which we can eliminate it—the size of the telephone box—gets smaller and smaller as the gravitational field grows stronger, as the spacetime becomes more strongly curved. Beyond the box there are dangerously large gradients in the gravitational stress, tides of gravity that cannot be ignored. For a stellar mass black hole, the horizon is too close to the bottom of the well and the tides of gravity would tear you apart as soon as you got too close.
On the other hand, for a supergiant black hole like Pōwehi, the bottom of the well is further away so passing through the horizon is unremarkable. Once you have crossed this threshold, however, your days are numbered. Literally. Time will end. At the core of the black hole is a singularity, a place where spacetime touches infinity, where the gravitational field grows without bound. The singularity is not an end of space but an end of time. Once you cross the event horizon, your trajectory through spacetime will take you there, to a place where there is literally no tomorrow, where the future does not exist—not even in principle.
As you approach this Armageddon, the gravitational stresses, those monstrous tides, stretch you out like a string of spaghetti, the atoms in your body torn apart, the nuclei ripped into protons and neutrons, the protons and neutrons ripped into their constituent quarks and gluons . Whatever consciousness is left will seek the end, and the end will come at the singularity, a merciful inevitability.
However, if others were to watch you fall into the black hole from afar, they would see a very different picture. At first, they would see you accelerate towards oblivion, and if they could somehow see your subjective clock, the watch on your wrist, they would see it slow more and more as you plunged deeper and deeper into the gravitational well. As you approached the threshold, it—and you—would appear to slow a complete halt. It would be as if you were frozen in time and space, decorating the horizon with a permanent reminder of what can happen when you stray too close. It is not that you didn’t cross into the black hole; you did, it’s just that those outside could never see you do it because every second you experienced at the horizon would be an eternity to them.
Once you cross the event horizon, your trajectory through spacetime will take you there, to a place where there is literally no tomorrow, where the future does not exist
For objects away from the horizon, time will not stop, but it will slow down considerably if they get too close. If the black hole has enough spin, there can be stable planetary orbits that veer very close to the horizon and, in principle, you could visit these for a while, slow down time and then return home catapulted years into the future. in the film interstellarthe crew of the endurance experience the full force of gravitational time dilation by visiting Miller’s planet, orbiting a supermassive black hole called Gargantua. Gargantua is assumed to be spinning so fast—within a trillionth of a percent of the theoretical maximum—that Miller’s planet can orbit within a few thousandths of a percent of the horizon radius.
The reconnaissance crew visit the planet for a little over three hours, yet they return to find their colleague, who had stayed aboard endurance, aged by a staggering twenty-three years. That said, black holes with this amount of spin will be incredibly rare, if they exist at all, since there are natural mechanisms to prevent the spin from increasing beyond 99.8 per cent of the maximum. This means the planetary orbits cannot edge quite so close to the horizon and the dilation effects are weaker. The spin of Pōwehi could well be around this 99.8 per cent mark. Three hours or so on an innermost planet orbiting this real-life leviathan would then equate to thirty-two hours and twenty-four minutes for those waiting on the mothership. Although this is not quite Hollywood, we should remember that Pōwehi is realwe have seen it, and perhaps some of its planets are inhabited by beings whose lives tick along almost eleven times more slowly, in comparison to our frenzied existence here on Earth.
The image of Pōwehi is compelling evidence for the existence of black holes in Nature—make no mistake about that—but it is not conclusive. After all, we don’t see the event horizon itself, but a shadow that is two and a half times larger. Despite the remarkable and inspiring imagery offered by the Event Horizon telescope, the strongest evidence for black holes comes from gravitational waves.
On 14 September 2015 the team at LIGO, the Laser Interferometer Gravitational Wave Observatory, detected these tiny ripples in the fabric of spacetime for the very first time. LIGO operates across two sites: one in Hanford, Washington—a decommissioned nuclear production complex—and the other in the alligator-infested swamps of Livingston, Louisiana. These ripples were tiny, stretching and squeezing the 4-kilometer arms of the detectors by less than the width of a proton, betraying their violent beginnings from the merger of two black holes, the mass of thirty-six and twenty-nine Suns respectively, in the furthest reaches of the observable universe. The energy carried by the wave at the source was spectacular, equivalent to the mass of three Suns, or 1034 Hiroshima bombs, an explosive spacetime tsunami crushing space one way and stretching it the other.
But could it have been something else that generated the wave, a coming together of some other exotic compact object different from a black hole? At the point of their merger, the two objects were just 350 kilometers apart, a combined mass of sixty-five Suns crammed into a region less than twice the size of the would-be event horizon. It’s hard to imagine it was anything other than a pair of black holes spiraling towards the ultimate embrace.
Excerpted from Fantastic Numbers and Where to Find Them: A Cosmic Quest from Zero to Infinity by Antonio Padilla. Copyright © 2022. Available from Farrar, Straus and Giroux, an imprint of Macmillan.