These Objects Are So Massive They Stop Time! | Relaxing Star Science For Sleep

Picture a single spoonful of matter suddenly appearing above your table. It looks like nothing much, a small lump about the size of a sugar cube, and then it falls. Straight through the table, through the floor, through the rock beneath the house, down into the planet, because that little lump weighs as much as an entire mountain range. This is real material, scooped from a real object circling somewhere in the sky above you tonight, and it carries a secret that runs even deeper than its impossible weight. Where matter is crushed together this tightly, time itself begins to slow. A clock sitting on the surface of that lump would tick more lazily than the watch on your wrist, falling further and further behind with every passing second. Tonight we're going hunting for the heaviest objects in the entire universe, and we're going to follow what their crushing weight quietly does to time. If the idea of drifting out through the cosmos in search of its most extreme places sounds like a pleasant way to spend the next while, then a quiet like or a subscribe genuinely helps this small channel keep making these journeys. Settle in, get comfortable, let your shoulders drop and your breathing slow. We have a long and gentle climb ahead of us, and we're going to start somewhere reassuringly familiar before things become strange. Let us begin with the word heavy, because it is going to mislead you almost immediately if we are not careful. In everyday life, heavy means hard to lift, a bag of shopping, a sleeping cat, a large sack of rice. But out in space there is a deeper quality at work, and astronomers call it mass. Mass is the amount of stuff a thing is actually made of, the sheer quantity of matter gathered into one place. And mass does something remarkable, it reaches out into the space around it and pulls. We call that pull gravity, and you have felt it every single day of your life without ever once being able to see it. For a long time people thought of gravity as a kind of invisible rope, a force tugging objects toward each other across empty space. Then, a little over a hundred years ago, a patent clerk with wild hair and a quiet stubbornness named Albert Einstein worked out that the truth was far stranger and far more beautiful. Gravity, he realised, is not a rope at all, it is the bending of space and time. A heavy object sinks into the fabric of reality, the way a bowling ball sinks into a soft mattress, and everything nearby rolls inward along the slope it makes. The heavier and the more tightly packed the object is, the deeper the dent and the steeper the slope. And here is the part that will quietly haunt the rest of our night together. That dent does not only bend space, it bends time. Down inside a deep gravitational well, close to a great mass, time genuinely runs slower than it does far away in open space. This is not a trick of perception, or a feeling, or a metaphor. It is measurable, and it has been measured over and over again. The closer you sit to a concentration of mass, the more slowly your own clock ticks compared to a clock left behind out in the emptiness. The whole journey tonight is really one long climb up a ladder, and on each rung we will find a heavier, denser, more crushing object than the one before. And on each rung, as the weight grows, time will lean a little further out of step, until at the very top we reach a place where it nearly stops altogether. So let us put our first foot on the bottom rung, and the bottom rung is our own sun. By the standards of your kitchen, the sun is unthinkably heavy. You could pour roughly a third of a million earths into it and still have room to spare. It holds more than 99% of all the mass in the entire solar system, which means that everything else, all the planets, every moon, every asteroid and comet and wandering speck of dust, is really just crumbs left over around the edge of the table. The sun is so massive that its gravity keeps the earth swinging around it from a distance of 93 million miles, and keeps distant Neptune leashed to it from nearly 3 billion miles away. And yet, for all that, the sun barely touches time at all. If you could stand on its blazing surface, holding a stopwatch and compare it later with one left floating far out in deep space, you would find that your solar stopwatch had lost only a couple of seconds across an entire year, a tiny stumble, a few heartbeats lost in 12 months. The sun is heavy, but it is also enormous and puffy and spread out, a vast loose ball of glowing gas more than 800,000 miles across. Its mass is smeared over such a huge volume that the dent it makes in time, while real, is shallow. You do not even have to go to the sun to find this effect, by the way. It is happening to you, right now, in your own home, in a way so small you will never feel it. Because the earth has mass too, and so the closer you are to the centre of the planet, the slower your time runs. Your feet, resting on the floor, are very slightly deeper in the earth's gravity than your head is, which means, strictly speaking, that your head is ageing a hair faster than your feet. Over a lifetime it adds up to a few billionths of a second, which is a wonderfully useless fact to carry around, and I am delighted to be the one to give it to you. The satellites that run the navigation in your phone take this far more seriously than your head and feet ever will. They circle high above us, where the earth's gravity is weaker and time runs a touch faster, and engineers have to correct their clocks for it constantly. If they did not, the maps in your pocket would drift wrong by several miles every single day. So that is the gentle truth we are carrying up the ladder. Mass bends time, and we have already felt the very first, faintest whisper of it. Now let us leave the warm and the familiar behind, and climb to the second rung, where a star like the sun goes to die. Every star is a slow and patient war between two enormous forces. Gravity is forever trying to crush the star inward, to collapse all that gas down into the centre, and the nuclear fire burning in the core is forever pushing outward, holding the star up, keeping it round and bright and stable. For billions of years these two forces stay almost perfectly balanced, but the fire needs fuel, and the fuel does not last forever. One day, far in the future, our own sun will burn through the last of the hydrogen at its heart, and the long war will start to turn. When that happens, the sun will not explode. It is not heavy enough for the truly violent ending. Instead, it will swell up into a vast, bloated red giant, large enough to swallow the closest planets, and then it will gently shrug off its outer layers into space in glowing rings. What it leaves behind, sitting alone in the dark, is the bare, exposed core, and that core is the second rung of our ladder. It is called a White Dwarf. A White Dwarf is the burnt out ember of a star, and it is one of the strangest objects you will ever try to picture. Take the mass of something like the sun, hundreds of thousands of times the Earth, and crush it down until it is no bigger than the Earth itself. That is a White Dwarf. Roughly a sun's worth of material, squeezed into a ball the size of our little planet. The matter inside is so compressed that a single spoonful of it would weigh several tons, as much as an elephant balanced on a teaspoon. The gravity at its surface is around a hundred thousand times stronger than the gravity holding you in your chair right now. What holds a White Dwarf up now that its fire has gone out is one of the deep and quiet rules of the quantum world. The tiny particles called electrons that make up part of all ordinary matter simply refuse to be packed too closely together. They push back, not with heat, not with fire, but with a stubborn quantum pressure that comes from the rules of reality themselves. This stubbornness is strong, but it is not infinite. There is a line it cannot hold beyond. An Indian physicist named Subrahmanyan Chandrasekhar worked out exactly where that line falls back in the 1930s, when he was barely 20 years old, doing the calculations on a long sea voyage. He found that if a White Dwarf carries more than about one and four-tenths times the mass of the Sun, the electrons lose their fight and the object can no longer hold itself up. That limit still carries his name today. So a White Dwarf sits there, Earth-sized, Sun-heavy, cooling slowly over billions and billions of years, fading from white to yellow to a dull dim red, and finally, in a future further away than the present age of the universe, to cold and dark. The densest White Dwarfs we have found are even smaller and fiercer than the average, packing all that mass into a sphere closer to the size of our Moon. And now, because we have climbed a rung, time has shifted again. Stand on a White Dwarf with a stopwatch, and you would lose far more than a couple of seconds a year. You would now lose minutes. The dent in time has deepened from a shallow scoop into something you could genuinely notice if you could survive the visit. It is still a whisper rather than a shout, but it is a louder whisper than the Sun could ever manage, because the same amount of mass is now crushed into a space ten thousand times smaller, and the slope down into its gravity is correspondingly steeper. Remember that idea, because it is the secret engine of this whole night. It is never simply how much mass an object has, it is how tightly that mass is squeezed. Hold on to that, because the next rung is going to take it and run. For the third rung, we need a star that was born much heavier than the Sun, a star perhaps eight or ten or twenty times the Sun's mass. These giants live fast and die young. Where our Sun will burn quietly for ten billion years, a massive star like this races through its fuel in just a few million, blazing with the light of a hundred thousand Suns, and then it comes to an ending so violent it briefly outshines an entire galaxy. When a massive star finally runs out of fuel, the long war ends all at once. There is no slow swelling, no gentle puff of rings. The core, no longer held up by fire, collapses in on itself in less than a second. The outer layers come crashing down onto that collapsing core, rebound and tear outward in a colossal explosion called a supernova. For a few weeks, that single dying star can shine brighter than all the billions of other stars in its galaxy put together. And in the ruins at the centre of that explosion, something new is born. Something that should not, by any everyday intuition, be able to exist at all. It is called a neutron star, and it is the fourth thing on our list and the third real rung of our climb. When the core of that massive star collapses, gravity becomes so overwhelming that it does something almost unbelievable. It crushes the atoms themselves. Normally an atom is mostly empty space, a tiny dense centre with even tinier particles whirling far out around it, like a single fly in the middle of a cathedral. But in the collapsing core, gravity slams everything together so hard that the empty space is simply squeezed out. The particles are jammed into contact. The protons and electrons are crushed together into neutrons, and what remains is a ball of pure crushed matter packed as tightly as the centre of an atomic nucleus. Now hold the scale of this in your mind gently, because it is genuinely difficult for a human brain to hold. A neutron star contains more mass than the entire sun, and it is squeezed into a sphere only about 12 to 15 miles across. You could lay a neutron star down across a single city, and it would not reach the suburbs. A mass greater than our star, packed into something, you could circle on a bicycle in an afternoon, if the gravity and the heat and the radiation did not vaporise you in the first fraction of an instant. The density is the thing that breaks the imagination. Remember the spoonful of matter that fell through the table at the start of the night? This is where it came from. One sugar cube of neutron star material weighs about a billion tons. If you could somehow bring a single thimble of the stuff back to Earth, it would punch straight down through the ground and fall toward the centre of the planet, because nothing in our ordinary world is remotely strong enough to hold it up. The gravity at the surface of a neutron star is around 200 billion times stronger than Earth's. A marshmallow dropped onto its surface would hit with the energy of an atomic bomb, and these objects spin, often terrifyingly fast. As the great core collapsed, it pulled its rotation inward and sped up, the same way a spinning figure skater whips faster when they pull in their arms. Some neutron stars turn hundreds of times every second, a city-sized ball of crushed matter whirling so fast that its surface moves at a sizeable fraction of the speed of light. As they spin, many of them sweep beams of radiation across space like a lighthouse, and each time the beam crosses the Earth, our instruments catch a pulse, steady and precise, ticking out across the galaxy. We call those ones pulses, and the first time one was discovered, the signal was so regular that astronomers half-jokingly labelled it LGM, for little green men before they understood they were hearing the heartbeat of a dead star. And what about time here on the third rung? This is where the whisper finally becomes something you could never ignore. On the surface of a neutron star, time does not stumble by a few seconds, or fall behind by a few minutes. It runs slow by something like 20-30%. Think carefully about what that means. If you could stand on a neutron star and survive for what felt to you like ten years, you would return to find that far out in the safety of open space. Thirteen years had gone by. Friends would have aged faster than you. The calendar would have run ahead. The crushing weight beneath your feet would have genuinely stolen time out from under you and handed it to the rest of the universe. We have climbed only three rungs, and already we have reached a place where a clock loses nearly a third of its life to gravity. We do not fully understand what lies at the very centre of a neutron star, and I want to be honest with you about that, because the honest mysteries are some of the most beautiful parts of this. The pressure deep inside is so far beyond anything we can make in a laboratory that the matter there may take forms we've never seen. Some scientists suspect the neutrons themselves may break down into an even stranger soup of the tinier particles called quarks. Others propose still odder possibilities with names like strange matter. These ideas are educated guesses clearly marked as guesses, sitting at the frontier of what physics can currently reach. What is certain is the weight and the smallness and the way time bows down before them. Now, a neutron star is already one of the most extreme things in creation, but the universe, it turns out, was not finished. Because some neutron stars carry one more property, dialed up to a level so absurd that for years physicists were not sure such an object could be real. And that brings us to the fourth rung, and to a kind of object whose name sounds like it came straight out of a comic book, the magnetar. A magnetar is a neutron star, with all the crushing weight and slowed time we've just described, but wrapped in a magnetic field so powerful it almost defies belief. You know magnetism, the little magnet on your fridge, the compass needle swinging gently to point north. The Earth itself is a giant weak magnet, which is what that compass is feeling. A magnetar's magnetic field is around a thousand trillion times stronger than the Earth's. Not a thousand times, not a million, a thousand trillion. It is, as far as we know, the strongest magnetic force anywhere in the entire universe. A field that strong does not just move compass needles, it warps reality. Get within a few thousand miles of a magnetar and its magnetism would reach into your body and pull you apart at the level of your atoms, because the very shape of the particles in your cells cannot survive a field like that. The empty space around a magnetar becomes unstable, almost boiling, with particles flickering in and out of existence under the sheer strain. These are objects so extreme that they bend not only space and time, but the rules of matter itself. And now and then, a magnetar does something that reaches all the way across the galaxy and touches us. On the 27th of December, 2004, an alarm of sorts went off all over the Earth. Satellites flinched, instruments designed to study the heavens momentarily saturated and went blind. For two-tenths of one second, a wave of radiation washed over our planet, and it had come from a magnetar called SGR 1806-20, sitting roughly 50,000 light years away on the far side of the Milky Way from us. Let me try to make the scale of that flash real, because the numbers alone slide off the mind. In those two-tenths of a second, that single object released more energy than our sun pours out in 250,000 years, a flicker shorter than a blink carrying a quarter of a million years of sunshine. It had travelled 50,000 years across the galaxy to reach us. And even after all that distance, even from the far side of the Milky Way, it was strong enough to measurably disturb the upper atmosphere of the Earth and ionise the air high above our heads. Astronomers who had spent their entire careers studying violent events in space said they had simply never seen anything like it. Had that magnetar been sitting a few light years away instead of 50,000, the burst would have stripped away our planet's protective ozone layer in an instant. The thing released all that fury during a kind of starquake, a sudden cracking and resettling of its impossibly rigid crust, letting its monstrous magnetic field snap and reconnect like a coiled spring finally let go. We know of only a few dozen magnetars in our galaxy, and they do not announce themselves. They sit silent in the dark for years, decades, centuries, bleeding their magnetic energy quietly into space. And then, without warning, one of them fires. They are perhaps the most genuinely frightening objects we have met so far tonight, and yet they sit on the same rung as the ordinary neutron star when it comes to time, because their weight and their size are much the same. Their horror is magnetic, not gravitational. To deepen the dent in time any further, we cannot just add magnetism. We have to add mass. We have to find a neutron star pushed right to the very edge of what it can possibly hold, which is exactly the fifth rung. Because not all neutron stars are equal. Most carry somewhere between one and a half and two times the mass of the sun. But astronomers have been searching for the heaviest ones they can find, the record holders, the neutron star sitting closest to the breaking point, and the current champion is an object with the unlovely catalogue name of PSRJ0952-0607. It is what astronomers affectionately call a black widow. It earned that grim nickname, honestly. This neutron star is locked in a close embrace with a companion star, and over millions of years it has been doing something rather monstrous to its partner. Its ferocious gravity and radiation have been stripping the companion apart, pulling its material away and devouring it, the way a black widow spider consumes its mate. By feeding on its companion in this way, the neutron star has grown unusually fat and unusually fast. It now spins 707 times every single second, which is so fast it is almost impossible to picture. A city-sized ball of densest matter in the cosmos, whirling more than 700 times around in the time it takes you to say the word once, and by eating it has grown heavy. As of the most recent and careful measurements, finished only this year, this black widow weighs about two and a third times the mass of our sun. That may not sound like a dramatic figure after all the billions and trillions we have been throwing around, but in the world of neutron stars it is enormous. It sits right at the theoretical edge, the absolute limit of how much mass-crushed matter can possibly bear before it loses the fight entirely. One more decent meal, one more substantial mouthful of its companion, and the electrons and neutrons and whatever else holds it up would finally surrender and it would collapse into something else. Something from which nothing returns. It is matter holding itself open at the very last possible instant before the floor gives way. And so, on this rung, time bends about as far as crushed matter can make it bend while still remaining matter at all. On the surface of this overfed neutron star, balanced at the brink, clocks run slow by something approaching a third, deeper than any ordinary neutron star as deep as solid stuff can manage. To go any further down into the well of time, the object can no longer stay an object, it has to fall over the edge. But before we follow it over that edge, I want to pause and clear up a misunderstanding that has probably been quietly building in your mind, because we are about to take a sharp and surprising turn. All night we've been climbing toward heavier and heavier things, and you might reasonably assume that the heaviest single object we could find, the most massive star in the sky, would be the deepest dent of all in time. So let us go and visit it. It is called R13601, and it lives about 160,000 light years away, inside a glittering nursery of newborn stars called the Tarantula Nebula, in a small galaxy that drifts alongside our own Milky Way. R13601 is a true monster of a star. For years astronomers argued about exactly how heavy it is, with early estimates running wildly high, and the most careful recent measurements settling it at around 196 times the mass of our Sun. Let that sit for a moment. Almost 200 Suns gathered into one single blazing star. It is one of the most massive stars known to exist anywhere. It burns at a temperature seven times hotter than the surface of our Sun, and it shines with something close to the light of 10 million Suns. It is so violently bright that it is tearing itself apart, flinging its own outer layers off into space in a furious wind, shedding the weight of an entire Earth every few days. So here is the most massive star we know. Almost 200 Suns of mass. Surely, surely, this is where time slows the most. And the answer, the lovely and instructive answer, is no. Not even close. Stand near R13601 with your stopwatch and time would barely flinch. It would lose perhaps a little more than the Sun does, and nothing remotely like a neutron star. This enormous star with a mass of 200 Suns bends time less than a dead cinder one ten thousandth its weight, and that is the whole secret of the night laid bare in a single comparison. R13601 has staggering mass, but it is also vast and bloated and spread thin. All those 200 Suns of material are smeared across a star so wide that if you placed it where our Sun sits, its surface would reach out a good fraction of the way toward the inner planets. The mass is there, but it is diffuse, scattered across an enormous volume, so the slope down into its gravity is gentle. The neutron star, by contrast, took less mass and crushed it into a ball the size of a city, and that fierce concentration is what dug the deep well in time. It was never about how much. It was always about how tightly. The most massive star in the sky is a gentle giant, in the literal sense that its grip on time is gentle. To truly stop time, you do not need the most mass. You need mass that has finally lost its battle and collapsed all the way down. You need the thing that waits for us on the sixth rung. You need a black hole. When a star far heavier than the Sun dies, or when an overfed neutron star takes that one last fatal mouthful, gravity wins the war completely and forever. There is no quantum stubbornness left that can hold the matter up. The core collapses and it does not stop at the size of the Earth like a white dwarf, and it does not stop at the size of a city like a neutron star. It keeps falling inward, smaller and smaller and denser and denser, and as far as our current physics can tell, it does not stop at all. The matter collapses down toward a single point of infinite density, a place where our equations themselves throw up their hands and stop giving sensible answers. We call that point the singularity, and the honest truth is that we do not understand it. It is one of the great unsolved questions in all of science, but the singularity is not the part you would actually meet. Around it, at a certain distance, there is a boundary. It is not a surface, not a wall, not anything you could touch. It is a line drawn in space, a point of no return, and it is called the event horizon. The event horizon is the place where the slope into the black hole's gravity becomes so steep that the speed you would need to climb back out exceeds the speed of light. And since nothing in the universe can travel faster than light, nothing that crosses that line can ever come back. Not a spaceship, not a beam of light, not a cry for help. The event horizon is the edge of a one-way door, and it is the single most absolute boundary that exists anywhere in nature. A black hole is the densest thing the universe can make. It is the final rung in a very real sense, because it is mass that has won mass with no size left at all, mass that has folded space and time around itself so tightly that it has sealed itself off from the rest of creation. And at the event horizon, the slowing of time that we've been chasing all night long finally reaches its destination. It does not slow by a third, as on the neutron star, it slows toward infinity. Let me show you what that strange phrase actually means, because it is one of the most haunting predictions in all of physics. Imagine you watched a friend, very brave and very foolish, fall slowly toward a black hole with a bright clock strapped to their suit and a torch in their hand. As they drifted down toward the event horizon, you, watching safely from a distance, would see their clock begin to tick slower, and slower, and slower still. Their movements would seem to drag, as though the whole scene were being played back at falling speed. The light from their torch would stretch and redden and dim, and as they approached the horizon itself, from your point of view, they would never quite reach it. They would appear to slow to a halt, hanging frozen just above the edge, the light from them fading until they simply faded from sight, suspended there, it would seem, forever. From where you float, your friend never actually crosses the line, they just freeze at the threshold and gently vanish into red. And yet, and this is the part that turns the whole thing into poetry, from your friend's own point of view, nothing strange happens at the boundary at all, their clock ticks along quite normally, they sail across the event horizon in a perfectly ordinary moment, perhaps without even noticing the exact instant they cross the most important line in the universe. Two completely different stories, both entirely true, separated only by where each person stands in the slope of time, the faller crosses in a heartbeat, the watcher waits forever. That single difference, that gap between the one falling and the one watching, is the deepest thing that gravity does to time, and it sits at the heart of every black hole in the sky. Now, because human beings cannot resist a good rescue story, this is exactly the sort of place where our imaginations start to rebel. Surely you think, there must be some way to pull the falling friend back, if you had a strong enough cable, a powerful enough engine, a bright enough tractor beam out of the science fiction films, surely you could reach in and haul them out before they are lost, and the question of whether that is possible turns out to be one of the most clarifying things you can ask about a black hole, so let us ask it properly. With two famous stories to guide us, the first you may know. It is the film Interstellar, in which a crew flies close to an enormous black hole, and a single hour spent on a planet near its edge costs them seven years of time back home. That is gravitational time dilation, the very effect we have been climbing toward all night, dramatized on the cinema screen. And the wonderful thing is that the filmmakers work closely with a real physicist, and the science underneath it is broadly sound. A single hour next to that black hole really could equal seven years far away, a stretching of time by a factor of around 60,000. The only way that particular extreme is even theoretically possible is if the black hole is spinning very fast, which the one in the film is, and which let the writers get away with a number that would otherwise be impossible. So far, so faithful to reality. The second story is older and dustier, and far less famous, and it is the one that lets us answer the rescue question head on. There was a science fiction television series around the turn of the millennium called Andromeda, and in its very first episode a great starship and its captain stray too close to a black hole and become trapped right at the edge, held there in what the show calls the time dilation field. To the captain, only a few seconds pass, but out in the wider universe, 300 years roll by while he hangs there, frozen at the threshold, until at last a passing salvage finds him, throws out cables and tows him back out to safety, where he wakes barely older while everyone he ever knew has been dead for centuries. It is a wonderful premise, and it is also a perfect little laboratory for the physics, because the answer splits cleanly in two, and the difference between the two halves is the most important idea in this entire subject. Everything depends on one question. Had the ship truly crossed the event horizon, or was it merely sitting just outside it? If the ship had genuinely crossed the horizon, then no rescue is possible. None. Not with the strongest cable that could ever be woven, not with the most powerful engine that could ever be built, not with a tractor beam carrying all the energy in a galaxy, and not, as some people like to ask, by bringing a second black hole alongside to pull it back out. And the reason is the single most important thing to understand about a black hole, so let me say it as plainly as I can. Pulling something out from inside the horizon is impossible. It is not a question of needing more power. It is simply forbidden by the laws of the universe, in the same deep way that travelling into yesterday is forbidden. Once you are inside the event horizon, the singularity lies in your future, the way tomorrow lies in yours, right now as you listen. You cannot steer away from tomorrow no matter how hard your engines burn, because it is not a place in space, it is a direction in time. Inside a black hole, every possible path you could take, every direction you could thrust, leads inward to the centre. There is no road out. Not because the road is long or steep, but because the road simply does not exist. A cable trying to haul you free would itself have to be pulled faster than light to do any good, and nothing can be pulled that fast. So the failure here is not one of strength. It is a failure of the very shape of reality. No amount of power turns a forbidden thing into a permitted one. But here is the lovely detail. The Andromeda story, perhaps by accident, actually describes the only version that could possibly be rescued at all. The ship is shown sitting on the edge in the time dilation field, and it is pulled out with cables. That is the picture of an object hovering just outside the horizon, not one that has fallen through. And for something that has not yet crossed the line, rescue is, in principle, allowed by the laws of physics. But, and this is a glorious but, the energy it would take is almost as absurd as the impossible case, and for a reason that is genuinely delicious once you see it. Remember that for 300 years to pass outside while only seconds pass on the ship, time on that ship has to be slowed by a factor of around 3 billion, and the only way to slow time that drastically without crossing the horizon is to hover unimaginably close to it. So close that the ship would have to hang at a height above the horizon thinner, in proportion, than the width of a single atom set against the whole black hole. And to hold a ship steady at a spot like that, fighting against a gravity that fierce, the force on the cable would be something like a hundred billion billion times the pull of Earth's gravity. No cable made of any material, real or imagined, survives that for even an instant. It would not snap so much as evaporate. And the closer you crept to the horizon to get that dramatic time-freezing slowdown, the closer the energy required would climb toward infinity. Which is the beautiful joke buried in the whole idea. The very thing that makes the story magical, the three centuries passing in a blink, is the exact same thing that makes the rescue impossible in practice. The drama and the impossibility are one and the same fact, wearing two different faces. And what of the idea of bringing a second black hole to pull the first one's prisoner free? It does not help in the slightest. A second black hole only adds more gravity to the situation nevertheless. Bring two black holes close together and their horizons do not cancel out or tug against each other in some helpful tug-of-war. They distort, they stretch, and eventually they merge into one larger black hole, and anything inside either of them stays inside the new combined one. The only thing that genuinely escapes a black hole, as far as we know, is a faint and ghostly trickle of radiation predicted by Stephen Hawking, leaking out so slowly that a single black hole would take vastly longer than the present age of the universe to fade away by it. That is not a toe line. That is a black hole quietly evaporating over a near eternity, so the rescue stays a dream and the heaviest objects keep, forever, whatever falls past their edge. Which brings us at last to the very top of the ladder and to the largest and heaviest black holes of all. Because the ones we've been picturing, born from single, dying stars, weigh only a handful of suns. The lightest black hole ever found is just under four times the mass of the sun. Those are the small ones, the local ones. But sitting at the centre of almost every large galaxy, including our own, is a black hole of a completely different order of magnitude. And these are the true giants of creation. At the heart of our own Milky Way, 27,000 light-years away through the crowded fields of stars, sits a black hole called Sagittarius A, Star. It weighs about four million times the mass of the sun. Four million suns, collapsed into a single point, anchoring the slow majestic turning of our entire galaxy. For most of the night we've measured things against the sun, but now even the sun has become a small coin and we are counting them out by the millions. And Sagittarius A, Star is, as these things go, a fairly modest example. In a galaxy called M87, some 55 million light-years away, there is a black hole weighing around six and a half billion suns. That one made history a few years ago, when astronomers linked telescopes all across the Earth into a single planet-sized eye and captured the very first photograph ever taken of a black hole's shadow. A dark circle ringed by a halo of glowing doomed light. We have an actual picture now. We looked into the abyss and we photographed the edge of it. But even six and a half billion suns is not the end. There is an object called TON618, a fierce and ancient beacon shining from more than 10 billion light-years away. So far that its light set out toward us when the universe itself was young. At its heart lies a black hole of around 66 billion times the mass of the sun. 66 billion. To gather that much material, you could take every single star in our entire Milky Way galaxy, hundreds of billions of suns, crush them all together into one black hole, and you would still fall a few billion suns short of TON618. It sits close to the very limit of how large astronomers think a black hole can possibly grow, and the discoveries keep coming. Only this past year, in the summer just gone, astronomers studying a distant galaxy known as the Cosmic Horseshoe, a galaxy so massive it visibly bends the light of things behind it into a glowing ring, announced a black hole of around 36 billion times the mass of the sun. It may be the single most massive black hole ever measured, and it too sits near the theoretical ceiling of what the universe seems able to build. These are the ultramassive black holes, the heaviest individual objects in all of creation, the final and highest rung of our long climb. Now you might think that something this vast and heavy would tear any visitor to shreds long before they reached the edge. And for the small black holes, that is exactly true. Near a black hole only a few times the mass of the sun, the gravity changes so sharply across even a short distance that anything falling in is stretched out into a long thin thread, an unpleasant fate that physicists with their gift for grim poetry actually call spaghettification. Your feet being closer would be pulled so much harder than your head that you would be drawn out into a strand of yourself. But here is the strange and counterintuitive gift of the giants. The bigger the black hole, the gentler the edge. A black hole of billions of suns is so enormous that its event horizon is a vast and gently curving boundary, larger across than our entire solar system. And near a horizon that huge, the gravity changes so smoothly from your head to your feet that you would feel almost nothing as you crossed it. You could drift over the event horizon of an ultra-massive black hole completely intact, calm, unbroken, perhaps not even certain of the precise moment you had passed the point of no return. Which means that the place where time slows toward infinity, the place we have been climbing toward all night long is around these largest of all black holes, a place you could actually reach in one piece. You could sail right up to the edge of forever and step gently over it and never feel the door close behind you. From the outside you would simply slow and redden and fade and hang there at the threshold, while the rest of the universe ran on without you, for as long as the universe lasts. So let us stand here for a moment, at the top of the ladder we have climbed together, and look back down the whole length of it. We began with the sun, heavy beyond imagining and yet barely able to bend time at all, losing only a few seconds a year because all its mass was spread so wide and thin. We climbed the white dwarf, a sun crushed into an earth, losing minutes. We reached the neutron star, a sun crushed into a city where time falls behind by nearly a third and a spoonful weighs a mountain. We met the magnetar, the same crushing weight wrapped in the strongest magnetism in the cosmos. We found the overfed black widow, holding itself open at the very last instant before collapse. We paused at the enormous gentle giant of a star, 200 suns wide, and learned that it bends time hardly at all, because mass alone is never the point. And we arrived, finally at the black holes, from the small and savage ones to the ultramassive lords at the centres of galaxies, where time at the edge slows not by a third, but toward forever. And running through all of it, from the first faint stumble of a clock on the sun to the frozen edge of a 66 billion sun abyss, there was only ever one quiet idea. Take matter and crush it. The tighter you squeeze, the deeper the dent, and the deeper the dent, the more slowly time itself is willing to flow. Weight, it turns out, is really a kind of patience. The heaviest places in the universe are the places where time has almost stopped being in a hurry at all. There is something strangely comforting in that thought, when you let it settle over you in the dark. Out there, around those silent and titanic objects, are pockets of the cosmos where a single calm moment can outlast empires, where a heartbeat can stretch across the death of a star. The universe contains places where, if you could only reach them and survive, you could rest at the edge of one quiet instant while 300 years, or 300 million, washed by unnoticed. The heaviest things in all of creation are not really destroyers of time, they are its keepers. They gather it up, and they slow it down, and they hold it close, and they never let it go. The astronomers tell us, too, that this will not last forever, that even the ultra-massive black holes are creeping toward a kind of limit, and that in the unimaginably distant future, the great glowing quasars, like T.O.N. 618, will quietly run out of material to feed on and wink out one by one into darkness and silence. The universe is still young enough that its heaviest hearts still blaze. We happen to be alive in the long bright afternoon of all of this, able to look up and wonder and climb ladders like the one we climbed tonight, all the way from a spoonful of fallen starlight to the slowed and frozen edge of the deepest wells in existence. So let the weight of all of it rest gently on you now, the way sleep rests, slow and patient and unhurried. Somewhere far above the roof, a neutron star is spinning 700 times a second, and on its surface a clock is running a little behind ours, and it does not mind in the least. Somewhere a black hole the mass of 66 billion suns is holding a single instant open at its edge, and it will go on holding it long after this night, and you and I and everything we know have faded gently into the past. There is no hurry out there. There is all the time in the world and more pooled and slowed and kept safe in the heaviest places of the sky. You can let yourself drift now, somewhere in among them. Rest well, take care of yourself, and good night.