What Light Itself Will Experience at the End of the Universe

We treat light as the fastest traveler there is. A messenger that crosses billions of years of empty space to reach us, enduring the distance, carrying its message across the long ages. But here is what that picture leaves out. For light, the journey takes no time at all. The 8 minutes of sunlight that warm your face. The 2 and 1/2 million years of light coming from the Andromeda galaxy. The light from a galaxy at the very edge of what we can see. All of it from light's own side is a single instant with no middle. The moment it leaves and the moment it arrives are pressed together, touching with everything in between folded to nothing. And that means something staggering about the end of the universe. A photon that leaves a star tonight and is never caught will in its own reckoning meet the death of the cosmos in the very same instant it was born. Light is the one thing in existence for which the end has already happened. Tonight, we follow it all the way there. Get comfortable. Hit subscribe if you're new here because by the end of this, the light filling the room around you will feel less like something you simply see by and more like the one thing that touches both ends of time. Now, let's slowly ease into this. Part one, the messenger you think you understand. Light is the one thing in the universe we trust to arrive. We send it across rooms and across oceans and across the empty distance between galaxies, and it always gets where it is going. The screen you might be looking at is throwing it at your eyes right now. The sun has been pouring it onto this planet for almost 5 billion years without once running late. When astronomers want to know what a galaxy on the far side of the cosmos is made of, they do not travel there. They wait for its light to come to them and they read the message it carries. Light is the courier. The universe never fails to deliver. We tend to picture that delivery as a journey. A photon leaves a distant star and then it travels mile after mile, year after year, crossing the dark between the stars like a runner crossing a continent. We imagine it out there in the cold, moving, enduring, carrying its little parcel of information across spans of space so large that the numbers stop meaning anything. We imagine that the trip takes something out of it the way a long trip takes something out of us. And we assume without ever quite saying it out loud that the light experiences the crossing. That somewhere in all that travel, there is something it is like to be that beam of light moving through the long night toward us. Tonight I want to take that picture apart very slowly and show you what is actually underneath it. Because almost everything in that comfortable image turns out to be wrong. And the way it is wrong opens onto one of the strangest questions you can ask about the universe. Not where light comes from. We have talked before about the oldest light there is. The faint glow left over from the birth of the cosmos and where it began. This is the opposite question, the one that waits at the other end of time. What happens to light when the universe itself is ending? and stranger still what light itself would experience if it could experience anything as everything around it goes dark. To get there we have to start with something that sounds like a small technical detail and turns out to be the hinge the whole story swings on. It is this light does not travel at just any speed. It travels at the speed of light 299,792,458 m every second. And it always travels at exactly that speed for everyone, no matter how fast they are moving when they measure it. That last part is the piece that broke physics open a little over a century ago. And it is the piece that is going to matter most for us. The speed of light is not just fast. It is special in a way that changes what time and distance even mean for the thing moving at it. Hold on to one ordinary fact for a moment because we are going to need it as an anchor. When you step outside on a clear morning and feel the sun on your face, the light landing on your skin left the surface of the sun about 8 minutes and 20 seconds earlier. The sun is roughly 93 million miles away. And even at the fastest speed there is, it takes light that long to cross the gap. So the sunrise you see is always a little bit old. You are never looking at the sun as it is. You are looking at the sun as it was 8 minutes and 20 seconds ago. That much most people have heard before and it already bends the everyday idea that seeing is immediate. It is worth pausing on just how far that idea reaches because it applies to everything you have ever seen. The moon you glance at is a little over a second old by the time its light reaches you. The light from the planets wandering across the night sky is minutes to hours old. The nearest star beyond the sun is so far away that its light has been traveling for more than 4 years before it touches your eye. Which means you are seeing it not as it is but as it was when you were four years younger. And the faint smudge of the Andromeda galaxy, the farthest thing most people can see without a telescope, sends light that has been crossing space for about 2 and a half million years. When you look at it, you are looking at a time before our own species existed. Every act of seeing is an act of looking into the past. And the farther out you look, the deeper into the past you are reaching. There is no way to see anything as it is right now. There is only the light arriving late carrying an old picture. So the night sky is not a snapshot of the universe as it is. It is a layered thing, a collage of different pasts all reaching us at once. The nearby stars showing us years ago and the distant galaxies showing us ages ago, all painted together on the same dark canvas. We have built our entire understanding of the cosmos out of this delayed light. Reading the history of the universe in the ages of the messages that reach us. And all of it depends on the quiet fact that light takes time to arrive. Time we measure on our clocks. But hold on to what I said because the strangeness is hiding underneath even this. All of that lateness, all those years and millions of years of travel is something we measure. It is not something the light measures. The light that has been crossing space for 2 and a half million years to bring you the sight of Andromeda did not spend 2 and 1/2 million years doing anything. We are about to see why. But here is the part almost nobody mentions and it is where the floor begins to tilt. Those 8 minutes and 20 seconds are how long the trip takes on our clocks. The clocks here on Earth, the clock on your wall, the clock in your body. They are not how long the trip takes for the light. For the light, the answer to how long the journey lasted is not 8 minutes. It is not a few seconds. It is not even close to zero in the way a very short time is close to zero. For the light, the journey takes no time at all. The photon that warms your face left the sun and arrived at your skin in what was from its own point of view the very same instant. That probably sounds like a trick of language or like I am rounding something off. I am not. This is one of the most solid, most tested results in all of physics. And it falls straight out of the same theory that gave us everything from the glow of the atomic age to the satellites that keep your phone's map honest. The faster anything moves, the less time passes for it compared to the world it is moving through. We have measured this directly with atomic clocks flown around the world in aircraft with particles in laboratories with the timing corrections engineers have to bake into the navigation satellites overhead so their clocks stay in step with ours. Speed slows time and light moves at the one speed where that slowing goes all the way to the end of its scale. So the messenger we trust to arrive, the courier that crosses the universe is not having the long journey we imagine for it. It is not out there enduring the cold and the distance and the years. From its own side of things, there is no journey at all. There is the moment it leaves and there is the moment it arrives. And for the light, those two moments are pressed together into a single point with nothing in between. The travel we picture all that heroic crossing of the dark is something only we see from the outside on our clocks. The light keeps no such clock. The light keeps no time at all. I want you to sit with how quietly strange that is before we go any further because we are going to build everything else on top of it. The thing you rely on more than almost anything else to tell you about the world, the light that lets you see this room and the stars and the faces of the people you love, is moving through reality in a way you can never join and can barely picture. It crosses all of space and pays nothing in time. And if that is true for an 8-minute trip from the sun, then it is just as true for a trip that lasts on our clocks a billion years. We are about to follow the light out to the very end of the universe. Across spans of time so enormous they will make a billion years feel like a held breath. And the whole way out we are going to keep circling back to this one impossible fact. For us the trip is everything. For the light the trip never happens. Now hold that thought because the next thing we have to look at is what that single timeless instant really means. Once the distances stop being millions of miles and start being millions of years. Part two, the clock that light refuses to keep. We left off with a claim that deserves to be examined rather than just accepted because it is the kind of claim that should make you suspicious. I told you that for light, the journey takes no time at all, no matter how far it goes. Let me show you where that comes from gently without a single equation. Because once you see the shape of it, you will never look at a beam of light the same way again. Start with something you already know in your body. When you are sitting still, all of your motion in a sense is motion through time. You are not going anywhere in space, but you are still traveling steadily into the future. 1 second per second carried along by the simple passage of time. Now imagine you start to move through space faster and faster. What the last century of physics discovered is that you cannot add motion through space for free. You have a kind of fixed budget of motion shared between moving through space and moving through time. And as you pour more of it into crossing space, less of it is left over for moving through time. The faster you go, the slower your clock runs compared to the world around you. This is not a malfunction of the clock. It is what time itself does when you move. For ordinary speeds, the effect is so tiny we never notice it. Even a jet crossing the ocean only loses a few billionths of a second against the clocks on the ground. I want to give you the actual evidence for this because it would be fair to be skeptical of a claim this strange and the proof is wonderfully concrete. In 1971, two scientists named Joseph Haffalei and Richard Keiting did something almost comically simple. They took extremely precise atomic clocks, bought ordinary airline tickets, and flew the clocks around the world, first eastward, then westward, while identical clocks stayed behind on the ground. When the traveling clocks came home, they disagreed with the ground clocks by exactly the tiny amount the theory predicted. Billionths of a second measured and confirmed. The moving clocks had genuinely experienced a little less time. This was not a thought experiment. It was clocks on a plane. The same effect runs right now in the device in your pocket. The satellites that let your phone find its location carry atomic clocks. And those clocks are moving fast enough, high enough that they tick at a noticeably different rate than clocks on the ground by about 38 millionths of a second every single day. That sounds trivial, but if the engineers did not correct for it, the navigation system would drift off by miles within a day and become useless. Every time you follow a map on your phone, you are relying on a working correction for the slowing of time with motion. And nature shows us the same thing in the rain of particles that falls constantly through the upper air. Tiny particles called muons are created high in the atmosphere and should by their own internal clock fall apart long before they reach the ground. But because they are moving so close to the speed of light, their clocks run slow from our point of view. and they survive the trip down to be detected at the surface in numbers that only make sense if time really does slow for fastmoving things. The slowing of time with speed is one of the most thoroughly confirmed facts in all of science tested with clocks and satellites and particles again and again never once failing. So when we follow it all the way to the speed of light, we are not speculating. We are taking a well- tested rule to its honest conclusion. But the effect is real and it grows as you approach the speed of light and it grows without mercy. At 90% of light speed, your clock runs at less than half the rate of the worlds. At 99% it crawls. And in the limit, at the speed of light itself, the budget tips all the way over. Every last bit of your motion is poured into crossing space, and there is nothing left for moving through time. Your clock does not run slow. It does not run at all. Light lives at that limit. It spends its entire budget on crossing space, which is why it is the fastest thing there is. And the price of that speed is that it gets no motion through time whatsoever. This is why physicists say there is no such thing as the rest frame of light. No valid point of view that travels along with a beam. No chair you could pull up next to a photon to ride along and watch the scenery go by. The mathematics that lets us step into the point of view of any slower object simply breaks when you push it all the way to light speed. It stops giving answers. It divides by zero. There is no there there. I want to be honest with you about something because this channel does not paper over the hard parts. When I say what light experiences, I am already standing on shaky ground because strictly speaking, light has no point of view for there to be an experience in the frame does not exist. So everything we say tonight about what light would feel or see or undergo is a thought experiment, a careful walk right up to the edge of what the theory allows and a long look over the side. But here is why it is worth doing anyway. Even though the frame is forbidden, the thing it points to is exact, not vague. The amount of time that passes along the path of a beam of light is not undefined. It is precisely provably zero. We cannot ride along with the light. But we can calculate to perfect sharpness that whatever the light might experience, it does not experience the passage of time. The door is locked, but we can read the number written on it. There is a name for the path light takes through space and time. Physicists call it a null path, and the word null is doing real work there. Along any ordinary path through space and through time, there is a kind of cosmic distance you can measure. A single combined measure of how far apart two events are when you fold space and time together properly. For the path of a beam of light, that combined measure comes out to exactly zero. The two ends of a light beam's journey, the place and moment it is emitted and the place and moment it is absorbed are separated by zero of this cosmic distance. Even when they are separated by billions of miles and billions of years in the ordinary sense, in the only measure that the universe treats as fundamental, the beginning and the end of light's journey are the same point. Think about what that means for the message light carries. We said light is the courier the universe never fails to deliver. But the courier does not age on the way. It does not experience the distance it crosses or the time it spends crossing. The picture of a far galaxy that lands in a telescope tonight after traveling for hundreds of millions of years on our clocks was from the light's own side, handed over in the same instant it was created. The light did not carry that image across the gulf the way a runner carries a letter. It is more as if the two ends of the trip were stapled together, and the enormous distance and the enormous time are things only we see. Looking at the staple from the side, this is the foundation, and I have spent two parts laying it carefully, because everything strange that follows is built on it. Light crosses all of space and keeps no time. For us, watching from the outside, its journeys can last as long as the universe has existed. For the light, every journey is a single timeless instant. Emission and absorption touching. No middle, no duration, no passage at all. We have tested the slowing of clocks with speed about as thoroughly as we have tested anything in science. And this is where that road leads when you follow it to its end. So now we are ready to do the thing. this whole video is really about. We are going to stop talking about an 8-minute trip from the sun and we are going to let the distances grow and grow until a single beam of light is crossing not a room or a solar system but the entire age of the cosmos. And we are going to ask what that timeless instant looks like when the trip on our clocks outlasts the stars themselves. Part three. A billion years in a single instant. Let us make the distances enormous now and see what happens to that timeless instant when we do. Pick a galaxy far out toward the edge of what our telescopes can see. So far that its light has been traveling toward us on our clocks for a billion years. A billion years is a length of time that does not fit in a human mind. When that light set out, there was nothing on Earth you would recognize as an animal walking on land. Whole mountain ranges have risen and worn back down to plains in the time that light has been in transit. And all of that, every era of it, is something the light crossed without experiencing a single moment of it. Try to feel the size of a billion years for a moment because we are going to need a sense of it. If you counted one number every second without stopping to eat or sleep, it would take you over 30 years just to count to a billion. A billion seconds ago, no one alive had a smartphone. A billion minutes ago, the Roman Empire was still standing. A billion years then is not a long time in the human sense. It is a length of time outside human meaning altogether, longer than there have been animals on land, longer than there have been trees, and that is the span our photon crosses while on its own side nothing happens at all, not a year, not a second, not the briefest flicker of duration. The mismatch between those two pictures is so total that the mind keeps trying to reconcile them and cannot. And that failure to reconcile is exactly the thing I want you to sit inside because it is real. Here is one way to hold both pictures at once. Picture the photon's journey laid out like a long film. Every frame a year of travel, a billion frames in all, showing the galaxies drifting and the stars wheeling and the slow turning of cosmic history. That film is what we see from the outside when we trace the light's path across space. Now imagine the photon's own experience of that same film. It is not the film played fast. It is not the film played in an instant. It is the first frame and the last frame laid directly on top of each other with all the frames in between pressed to nothing. So that the departure and the arrival are not two ends of a story but a single image. The film exists. We can watch it. But for the light there was never any film at all. Only the one frame that is also the last frame. The leaving that is also the arriving. The billion years are entirely ours. The instant is entirely the lights. I want to take you inside that crossing. as far inside as the physics will let us go, knowing all the while that the door is officially locked. So let this be a thought experiment, a careful imagining, and let yourself become the light for a moment. You are a single photon and you are leaving the surface of a star in a galaxy a billion lighty years from Earth. There is no countdown, no sense of setting out. The instant you exist, you are already moving at the only speed you will ever have. Ahead of you is a billion years of empty space. The long dark between the galaxies, the slow drift of cosmic history, but you will not feel any of it. For you, there is no ahead and no long. The star you are leaving and the eye you will eventually enter are not separated by any time at all. You do not watch the galaxies slide past. You do not wait through the silent eons. There is the star and there is the eye and they are the same instant pressed together with a billion years folded invisibly between them. You are born and you arrive in a single point that has no inside. Whatever a billion years of travel is, it is not something that happens to you. It is something that happens to everyone else. while you in your own reckoning do not move at all. Come back now back to our clocks where that same crossing took a billion years and notice how completely the two stories disagree. From where we sit, that photon had one of the longest journeys imaginable, a billion years alone in the dark. From the photon's side, there was no journey, no dark, no waiting, no time. Both of these are true at once. They are not two opinions about the same event. They are two exact descriptions and the universe holds both of them without the slightest discomfort. The disagreement is not a problem to be solved. It is a feature of how reality is built. Now push it further because we are going somewhere with this. We chose a galaxy a billion light years away. So the trip took a billion years on our clocks. But there is nothing special about a billion. We could choose light that has been traveling for 10 billion years and the photon's experience would be exactly the same which is to say no experience of duration at all. We could imagine, and soon we will, a photon that is never absorbed by anything that simply keeps traveling on our clocks for a trillion years and then a trillion trillion and then spans of time so long that the universe will have changed beyond all recognition. And through every bit of that, the photon's own answer to how long have you been traveling would be the same. No time, not a moment. The instant I was born is the instant I am in. This is the thread we are going to pull on for the rest of the night. So let me name it clearly. We are used to thinking of the end of the universe as something unimaginably far away in time. An event waiting at the bottom of an almost endless future. And from our point of view it is. But light does not share our point of view. For a photon that leaves a star tonight and is never caught, the entire future of the universe, however long it runs, is folded into the same timeless instant as its birth. Whatever waits at the end of time, the light is in its own strange way already there. It left the star and it reached the end of everything in the same breath. Because for light there is only ever the one breath. I told you at the start that we would take the comfortable picture of light apart slowly. This is the piece that should unsettle you most. And I want you to feel it before we move on. The light in your eyes right now, the light from this screen, the light from the lamp in the corner, the starlight coming through your window is the one kind of thing in the universe for which there is no difference between now and the end of time. It does not travel toward the future. It does not wait for anything. If we could somehow ask it, the moment it was made and the last moment the universe will ever have would be to it the same moment. So we have a choice about how to ask our question and the choice changes the answer completely. If we ask what light experiences at the end of the universe from light's own side, the honest answer is that there is no end for it to experience because there is no time in which an end could arrive. The end and the beginning are the same instant. But there is another way to ask and it is just as real and it is the one that is going to carry us through the rest of this. We can step firmly back onto our own clocks into the world where time genuinely passes where a billion years is a billion years. And we can ask what actually becomes of light out there in that long future. As the universe we know runs down and goes dark because from our point of view something does happen to light at the end of the universe. something slow and patient and strange and to see it we have to leave the photon's frozen instant behind for a while and go look at the future the way the rest of the universe has to live it. So let us do exactly that. Let us go and watch the universe end the slow way. Part four the question that folds back on itself. Before we travel out into that long future, I want to stand for one more moment in the place where the two answers meet. Because this is the strange center of everything we are doing tonight. And if we rush past it, we will lose the thread that makes the rest matter. We have found two completely different answers to a single question. And the question is what light experiences at the end of the universe. From light's own frame, the answer is that there is nothing to experience because there is no time, no duration, no journey, only the one instant in which birth and arrival are the same. And from our frame, the answer is going to turn out to be a long and detailed story full of fading stars and dying galaxies and stretching wavelengths. A story that takes spans of time so enormous they break the meaning of the word long. Both answers are correct. Neither one is the illusion. They are two faces of the same reality and the universe simply holds them both. There is something almost funny about this in the way the deepest things sometimes are. We set out to ask a simple question and the question turned out to fold back on itself. Asking what light experiences at the end of time is a little like asking what is north of the north pole. The words are all in the right order. The sentence sounds like it should have an answer and then you go looking for the answer and find that the question dissolves in your hands because for the thing you are asking about the very idea of an end in time does not apply. A photon that is never absorbed, never reaches an end. Not because it lasts forever in the ordinary sense, but because it stands outside the kind of time in which lasting and ending happen at all. And yet here we are, and I do not want to leave it there because that would be a cheat. It would be too easy to say the question dissolves and walk away. The honest thing, the more interesting thing is to take both answers seriously at once. So this is the deal we are going to make for the rest of the video. We are going to spend most of our time in our own frame on our own clocks watching with patient attention as the universe actually winds down and seeing exactly what happens to light as it does. And then at the very end after we have followed light all the way into the final darkness the slow way we are going to come back to this spot and we are going to lay the photons frozen instant down next to the long fading we watched and we are going to look at the two of them side by side. I think when we do you will feel something that is hard to feel any other way. The end of the universe is going to turn out to be both the longest, gentlest ending imaginable and an ending that for light has already happened. Holding those two together is the whole point. Let me also say plainly why this is worth your attention tonight and not just a clever puzzle. Light is not some exotic ingredient in the universe that you can take or leave. It is how the universe shares information with itself. Every time anything anywhere sees anything else, light is the messenger. The warmth of the sun, the colors of the world, the images of galaxies that taught us the cosmos was expanding in the first place, all of it is light. So the fate of light is not a side question. It is in a real sense the fate of seeing, the fate of connection, the fate of the universe's ability to be witnessed at all. When we ask what happens to light at the end of time, we are really asking whether the universe in its final state can still be seen, still carry a message, still be known by anything. And the answer is going to be tender and unsettling in equal measure. There is a reason this particular question gets under the skin in a way that other cosmic facts do not, and it is worth naming before we go on. Most of what we learn about the end of the universe is about things out there far away other than us. Stars we will never visit. Black holes we will never meet. Spans of time we will never live to see. It is easy to hold all of that at a comfortable distance like a story about a country you will never travel to. But light is not far away. Light is the most intimate thing there is. It is touching your eyes at this very moment. It is the medium of every face you have ever loved, every sunset you have ever stopped to watch, every word on every page you have ever read. When we ask what becomes of light at the end of the universe, we are not asking about a distant object. We are asking about the very thing that is right now carrying the world to you. And so the question has a strange double character. On one hand, it is the most abstract thing imaginable, a puzzle about photons and the end of time. On the other hand, it is almost unbearably close because it is about the fate of seeing itself, the fate of the universe's ability to be witnessed at all. A universe with no light left is not just a dark universe. It is a universe in which nothing can be shown to anything. in which no message can cross from one place to another, in which the cosmos loses its last way of being known. That is what is really at stake at the end of the road we are walking. Not just whether the lights go out, but whether there is any seeing left when they do. So let us go and find out and let us start by discovering that the bright universe we live in, the universe full of light, is not the way things usually are. It is a brief exception. One more thing before we set out so that you can let yourself relax into the rest of this. I am not going to ask you to hold a single number in your head or to follow a chain of equations. The story from here is a story with a beginning and a middle and an end. The same way any story has those things, except that this one is the story of everything told in order. From the bright world we live in now, all the way down to the last cold whisper of light in an empty cosmos. You can let your eyes close if you like. You can let the numbers wash over you. The shape of it is what matters, and the shape is simple, even when the spans of time are not. We are going to watch the lights go out slowly, one kind at a time. And we are going to ask at each step what is left to shine and what is left to see it. So here is where we are. We have a universe that from our point of view has a future, a real one with real time passing in it. And we have light threaded all through that universe. Both as the timeless thing that keeps no clock and as the very real glow that fills the present sky. To find out what becomes of that glow, we have to leave the comfortable present behind and go forward far forward past anything that has ever happened into the far future of the cosmos. And the first thing we are going to discover when we get there is that the bright star-filled universe we take for granted, the only kind of universe humans have ever known, is not the normal state of things at all. It is a brief opening chapter. The age of stars is almost over. Let me show you what I mean. Part five, the slow closing of the age of stars. We think of the universe as a place full of stars. When you picture the cosmos, you picture points of light scattered across the dark, galaxies like cities of suns, the whole thing blazing. That is the universe we were born into. And it is easy to assume it has always been this way and always will be. But astronomers who study the long future of the cosmos have a humbling thing to tell us. The bright star-filled universe is not the rule. It is a phase. and on the time scales that matter to the universe as a whole. It is an early and a short one. There is a way of organizing the entire future of the cosmos that comes from two astrophysicists, Fred Adams and Gregory Laughlin, who in 1997 wrote a careful scientific paper with the quietly devastating title, A Dying Universe, and later a book that laid it out for everyone. They divided the whole life of the cosmos, past and future, into a handful of great eras. And the scale they used is worth understanding because it is the only way to keep these numbers from blurring into nonsense. Instead of counting years one at a time, they count by powers of 10. Each step is not one more year, but 10 times as many years as the last step. It is the only honest way to talk about a future this long because the future of the universe is not measured in billions of years. It is measured in numbers with the number of zeros itself being huge. Let me give you one way to feel how short our own era really is. Suppose we squeeze the entire future of the cosmos that we are going to discuss tonight. all of it out to the long darkness at the very end down into a single day 24 hours from midnight to midnight. On that scale, where would the age of stars sit? You might guess it fills the morning or at least the first hour. It does not. On that compressed day, the entire Stelliferous era, the whole age of stars, from the first sunrise anywhere in the cosmos to the fading of the very last star, would be over in the first tiny fraction of the first second after midnight. Everything else, every hour of that long day, all the way through to the following midnight is darkness. The eras after the stars, the long cooling, the final emptiness, the age of light is not the day. It is the first faint instant before the day has really begun. There is something almost vertigenous about realizing this because it inverts the picture we carry without thinking. We imagine the universe as fundamentally a place of stars with darkness as the gaps between them. But measured across the whole life of the cosmos, it is the other way around. Darkness is the fundamental state, the overwhelming majority of all the time there will ever be. And the stars are the brief exception, a short flaring at the very start. We did not arrive in the normal universe. We arrived in the rare bright opening, the only stretch of cosmic history in which there is enough light to see by, enough warmth for worlds, enough fire in the sky to call it day and night. That is not a small thing to have been born into. It means the ordinary act of looking up and seeing stars is something only possible during a sliver of time so thin that on the scale of the universe's full life, it has barely begun and is already nearly done. On that scale, the very first era, the one before there were any stars at all, lasted only until the universe was a few hundred,000 to a million years old. Then came the era we live in now, the one Adams and Laughlin call the Stelliferous era, which simply means the era filled with stars. It began when the first stars caught fire a few hundred million years after the beginning. And we are living inside it right now, almost 14 billion years in. When you look up at night and see stars, you are seeing the signature of this era, the age of starlight. It feels permanent. It feels like what the universe is. But here is the number that should stop you. The Stelliferous era, the entire age of stars is expected to end when the universe is about 100 trillion years old. Written out that is a 1 followed by 14 zeros 10th years. That sounds like forever. And compared to a human life, it may as well be. But compared to what comes after, it is nothing. After the age of stars ends, the universe keeps going dark and quiet. For spans of time that make a hundred trillion years look like the first second of a long, long day. The eras that follow are measured not in trillions of years, but in numbers like 10 to the 39th and 10 to the 100th and beyond. If you laid the whole future of the cosmos out as a single day, the entire age of stars, the only kind of universe that has ever existed for us, would be over before the first faint light of dawn. Let that resettle your sense of where we are. We do not live in the normal settled middle period of the universe. We live in its brief bright morning, in the short window when stars exist and shine and make the kind of light you can see by. The universe spends the overwhelming majority of its future in the dark, and we happen to have arrived during the small slice of time when the lights are on. That is not a depressing fact, at least. Not only that, it is also a strange piece of luck, the kind that should make the ordinary night sky feel less like a backdrop and more like something you caught at exactly the right moment before it goes. and the lights are going out. Not suddenly, not all at once, but in a long gradual fading that has, in a sense already begun. The universe is not making new stars as fast as it used to. Stars are born out of clouds of gas, and every generation of stars uses up some of that gas and locks part of it away forever in dead remnants that will never form stars again. The cosmic supply of star- makingaking material is finite and it is being slowly spent. The peak of star formation, the time when the universe was lighting new suns at the greatest rate, is already behind us. It happened billions of years ago. We are living in the long slow decline that follows the peak in a universe that is gently and steadily winding down its production of light. You will not see it happen, of course. The decline is so slow that across all of human history, the night sky has looked essentially unchanged. But the direction is set. Fewer and fewer new stars generation after generation as the gas runs low. And the stars that already exist do not last forever. Big bright stars burn through their fuel fast and die young in millions of years rather than billions, going out in the explosions that seed the next generation. The smaller, dimmer stars last far longer. And the very smallest, the faint red dwarfs, are the ones that will carry the last light of the age of stars far into the future, long after every bright sun has died. They are misers with their fuel, burning so slowly and so cool that a single one can keep shining for trillions of years. So the age of stars does not end in a blaze. It ends in a long dimming, the bright stars dying first and not being fully replaced, the gas running thinner until all that is left burning anywhere in the cosmos are the small patient red embers, the red dwarfs holding the line, and then one by one even they begin to go. That brings us to a moment I want to take you to directly because it is one of the quietest and most moving moments in the entire future of the universe. Somewhere out there in time on a scale of about a 100 trillion years, there will be a last star. Not the last star in a region or the last star in a galaxy, but the last star anywhere. The final sun still burning in the whole of the cosmos. Let us go and stand beneath it. Part six. The last star that will ever burn. The smallest stars are the longest lived things that shine. A red dwarf with a mass of only about 800ths of the mass of our sun burns its fuel so slowly and so gently that it can keep glowing for 10 trillion years. And the very faintest may last even longer than that. Our own sun, by comparison, has only about 5 billion more years before it swells, sheds its outer layers, and settles down to die. The red dwarfs will still be shining when the sun has been a cold ember for thousands of times the present age of the universe. They are the marathon runners of the cosmos, and they are the ones who will carry the last of the starlight. It is worth understanding why the smallest stars last so absurdly long. Because the reason is a kind of cosmic thrift. A star shines by fusing hydrogen into helium in its core. And a big bright star like our sun is wildly extravagant with this process, burning through its fuel at a furious rate and lasting only billions of years before the core runs dry. But it never even gets to use most of its hydrogen because only the fuel in the central core ever reaches the temperatures needed to fuse. A red dwarf is different. It is small and cool and crucially its whole body churns and mixes slowly circulating fresh hydrogen from its outer layers down into the burning core and carrying the spent helium back out. So, a red dwarf can eventually use almost all of its fuel, not just the small fraction in the center. And it burns that fuel so gently at such a low rate that the supply lasts for a length of time that beggars belief. A star with a mass of only about 800ths of the sun's mass can keep glowing steadily for 10 trillion years, which is roughly a thousand times the entire present age of the universe. So when we talk about the last stars, we are talking about these small, patient red embers, the most frugal fires in creation. The bright stars, the ones that fill our night sky and give our galaxy its glow, are the first to die, racing through their fuel and going out relatively young. The red dwarfs inherit the dark. They will still be burning dim and red and steady long after the last bright star has faded. Long after the Milky Way has merged with its neighbors and the merged galaxy has used up its gas. They are the ones who carry the final light of the age of stars across the trillions of years, holding the line against the dark until at last even they begin to run out. And when the very last of them goes, something happens that has never happened before in the history of the universe and will never happen again. But even a red dwarf runs out eventually. And so there comes a point somewhere around a 100red trillion years from now when the second to last star fades. And then there is only one, one last star somewhere in what used to be a galaxy burning low and red against a sky that has otherwise gone completely dark. I want to take you there because it is one of the most quietly extraordinary places in all of time and standing in it tells you something about light that no number can imagine you are standing on a cold world far in that future. The ground beneath you is old beyond reckoning, scoured smooth by ages no living thing remembers. You look up and the sky is almost entirely black. The other galaxies which in our own time crowd the distant sky in their billions are gone from view carried so far away by the expansion of space and their light stretched so thin that not one of them can be seen. There are no constellations. There is no band of the Milky Way arching overhead. There is only the dark and low on the horizon one dim red point of light. This is the last star, the final sun still burning anywhere in the universe. Its light is feeble and red, the color of a dying coal, and it gives almost no warmth. But it is light. It is the last of the kind of light that stars make, the light that the whole age of stars was named for. You are watching it the way you might watch the last candle in a house at the end of a long night, knowing there are no more candles, knowing that when this one gutters out, the dark will be complete. And then slowly, over a span of time longer than the entire history that came before it, the last red ember dims and cools and finally goes out. And with it, the age of starlight ends. There will never be another star. The universe has lit its last sun. After that moment, no star shines anywhere in the cosmos ever again. Take a breath with that because it is worth feeling. Everything we mean by daylight, by sunlight, by the warmth on your skin and the colors of the world and the points of light in the night sky, all of it belongs to a single chapter of the universe's life. And that chapter has an ending. There is a last sunrise somewhere in the future of the cosmos. A final time that any world is lit by the steady fire of a nearby star. And after it there is no more. The thing we have treated all our lives as the very definition of permanence. The reliable presence of the sun, the eternal sky full of stars turns out to be a temporary arrangement, beautiful and brief with a definite close. I said at the beginning that we would watch the lights go out one kind at a time. And the first kind to go is the most important to us, the kind we live by, the kind stars make. But here is the thing I do not want you to miss as we move past it. The end of the age of stars is not the end of light. It is only the end of one source of it. The universe after the last star is dark to our eyes, yes, but it is not entirely without light. Not yet. There is still the faint warmth of the dead stars themselves, the white dwarfs and the cooling embers glowing dimly for ages as they slowly release their leftover heat. There is the occasional flash when two dead stars happen to collide in the dark, a brief flare in an otherwise lightless sky. And there is the oldest light of all, the faint glow left over from the birth of the universe, still soaking all of space, though it too is fading. The cosmos after the stars is not yet a place of no light. It is a place of fading light, of embers and afterglows and rare lonely sparks. The bright sources are gone, and what remains is the long cooling of everything they left behind. And underneath all of it, something else is happening quietly the whole time to every photon already in flight. While the stars are dying and the embers are cooling, the light that is already out there crossing space is not staying the same. It is changing slowly in a way we have not talked about yet. A way that has nothing to do with the stars at all and everything to do with the space the light is moving through. To see it, we have to look not at the sources of light, but at light itself, the photons in transit. And we have to notice what the expanding universe is doing to them with every passing eon. But before we get there, we have to follow the death of light one more step down. Because the dead stars and the cooling embers do not last either. Even they come apart. And what comes apart with them is the very stuff that light is made to interact with. So let us go further into the dark, past the last star, into the long age of the dead. Part seven. When the makers of light come apart, the universe after the last star is a graveyard, but for a very long time it is a graveyard that still glows faintly. The dead remain. White dwarfs, the hot cores left behind when stars like our sun die, sit in the dark, radiating the last of their stored heat, slowly cooling from white to yellow to red to eventually a cold black. Neutron stars, the impossibly dense remnants of larger stars, spin down and fade. Brown dwarfs, objects that never quite became stars, drift cold and dim. and black holes. The collapsed remains of the most massive stars sit silent, pulling in whatever rare scrap of matter strays too close. For ages upon ages, this is the state of things. A dark universe scattered with the cooling corpses of stars, lit only by their dwindling heat and the occasional collision. This is the era astronomers call the degenerate era and it lasts an almost incomprehensible length of time from about 100 trillion years after the beginning out to something like 10 to the 39th years. A 1 followed by 39 zeros. To say that number out loud is to say nothing meaningful because there is no human experience to compare it to. The entire age of stars, the 100red trillion years we just walked through, is a vanishingly thin sliver at the very start of this era. The degenerate era is most of what we have covered so far, stretched out almost beyond imagining, a long slow cooling in the dark. But even the dead do not last forever. And this is where the story takes a turn that should genuinely surprise you. The matter that the dead stars are made of, the protons and neutrons that make up every atom in your body, and every cold ember in that far future, may not be permanent. There is a long suspected possibility in physics, predicted by some of our attempts to unify the forces of nature, that the proton, one of the basic building blocks of all ordinary matter, is very slowly unstable. that given enough time, a proton will simply decay, fall apart into lighter particles and radiation and cease to be a proton at all. We have never seen this happen. Physicists have built enormous detectors far underground, tanks holding tens of thousands of tons of water, watched over by sensitive instruments, waiting for even a single proton among all those countless trillions to fall apart. So far, none has. And that patient watching has told us that if the proton decays at all, it must do so extraordinarily slowly with a typical lifetime longer than about 10 to the 34th years. That is far longer than the present age of the universe. Longer almost beyond meaning. The grandest of these underground experiments sits beneath a mountain in Japan. A colossal tank holding 50,000 tons of ultra pure water. Its walls studded with thousands of sensitive light detectors. All of it watching for the single faint flash that a dying proton would produce. The reason such a vast tank helps is a matter of patience and numbers. Any one proton might take far longer than the age of the universe to decay. But a tank that size holds so many protons, more than you could count in a thousand lifetimes, that if the protons lifetime were short enough, you would expect to catch a few of them falling apart each year. Year after year, the watches have waited, and year after year, the flash has not come. And so, the lower limit on the proton's lifetime has been pushed higher and higher. We do not know that protons decay. We only know that if they do, they do it more slowly than 10 to the 34th years, which is exactly the kind of time scale the degenerate era runs on. And here is a fork in the story worth pausing on because it shows how genuinely open the far future is. If protons do decay, then ordinary matter has an expiration date, and the dead stars dissolve over those enormous spans, and the degenerate era ends with the matter of the cosmos gone. But if protons turn out to be perfectly stable, the story is even stranger and even slower. In that case, the matter does not dissolve, but it does not stay still either. Over time scales so long, they make 10 the 34th years look immediate. Even solid matter behaves like a slow liquid. Every object very gradually rearranging itself and the dead stars would over something like 10 to the power of 1500 years slowly collapse and transmute until they became spheres of cold iron. Either way, whether by decay or by unimaginably slow transformation, the familiar matter of the universe does not last. The only question is how it ends, not whether. And whichever path it takes, the thing that matters for our story is the same. The stuff that light interacts with is leaving the stage. But in the degenerate era, there is time. There is more than enough time. And if protons do decay, then over those enormous spans, the dead stars themselves slowly evaporate. The white dwarfs and the neutron stars and the cold planets, the last solid remnants of the age of stars, gradually dissolve particle by particle into faint radiation and drifting debris. The matter comes apart. I want you to notice what is really happening here because it is the quiet hinge of this whole part of the story. Up to now, we have watched the sources of light go dark. The stars stopped burning. The embers cooled, but now something deeper is happening. The very stuff that light interacts with is disappearing. Light does not just need something to make it. It needs something to receive it, something to absorb it, something to be warmed or seen or changed by it. A beam of light only becomes an event, only really arrives anywhere when it meets matter and is absorbed. And in the long decay of the degenerate era, the matter itself is thinning out, coming apart, dissolving into the dark. The universe is not just turning off its lamps. It is removing the things that light was for. Think of what that does to a photon. We have spent all this time imagining light crossing the cosmos. And at the end of every journey, we have quietly assumed there is something waiting to catch it. an eye, a detector, a grain of dust, a cloud of gas. But as matter decays and spreads and thins, the chance that any given photon ever meets anything to absorb it drops and keeps dropping. More and more of the light in the universe is light that will never be caught, never be seen, never complete the journey we picture for it by arriving somewhere. It just keeps going through emptier and emptier space with less and less left to ever stop it. And yet, even in this thinning, dissolving universe, light has not made its last appearance. There is one more source still to come, the strangest and faintest of all, and it does not come from stars or from their embers. It comes from the one kind of object that survives even the decay of matter. The one structure left standing when everything else has come apart. It comes from black holes. Black holes are about to become the last things in the universe that make new light. And the way they do it is one of the most beautiful and unsettling ideas in all of physics. So let us follow the dark down one more level into the age when black holes are the only lamps left. And let us watch how even they in the end give up their light. Part eight. The faint glow at the end of everything. When everything else has come apart, the black holes remain. They are the most durable structures the universe ever makes. The collapsed remnants of the largest stars and at the centers of galaxies, monsters with the mass of millions or billions of suns. After the stars have died and the dead stars have cooled and even ordinary matter has begun to dissolve, the black holes are still there, silent and patient in the dark. For a long stretch of the future, they dominate the cosmos so completely that astronomers call this span the black hole era. And it reaches from around 10 to the 39th years out to about 10 to the 100th years. That last number has a name. A one followed by 100 zeros is called a Google. And the black hole era runs nearly that long. You would think a black hole would be the end of light entirely. It is, after all, the one place in the universe from which light cannot escape. The very definition of darkness, a region where gravity is so strong that anything falling in, including light, is trapped forever. And for most of their existence, black holes are exactly that dark. But here is where physics turns the story inside out. In 1974, Steven Hawking showed through a careful argument combining gravity with the rules of the very small that black holes are not perfectly black after all. They glow very faintly. From just outside their edge, a black hole leaks a thin trickle of radiation into the surrounding dark. We call it Hawking radiation. And it means that a black hole is not truly a one-way door. It is slowly, almost imperceptibly giving itself away. The glow is unimaginably feeble, especially for the large black holes. A black hole the size of one left behind by a dying star radiates at a temperature so close to absolute zero that for most of the history of the universe it is actually colder than the faint background warmth around it. And so it grows rather than shrinks. Only in the far future, when the universe around it has cooled even further, does the black hole finally begin to lose more than it gains and start with terrible slowness to evaporate. The idea behind the glow is one of the strangest marriages in physics, a meeting of gravity and the rules of the very small. Empty space, it turns out, is not truly empty. At the tiniest scales, it sees with pairs of particles constantly flickering into being and vanishing again, borrowed from nothing and paid back almost instantly. Hawings insight was that right at the edge of a black hole, at the boundary where light can no longer escape, one member of such a pair can fall in while the other gets away, carrying off a little energy. To an outside observer, the black hole appears to be glowing very faintly with a thin stream of escaping particles and paying for that glow with its own mass. Bit by bit, the black hole gives itself away to the dark around it. What makes this so slow and so strange is that the glow gets fainter the bigger the black hole is. A giant black hole is colder and dimmer than a small one, which is the opposite of almost everything in our experience where bigger fires burn hotter. And because the evaporation time grows as the cube of the mass, the difference between large and small black holes is staggering. Double the mass and it takes 8 times as long to evaporate. The black holes left by dying stars take something like 10 to the 67th years to disappear. A number that already dwarfs the entire age of stars by an amount the mind cannot hold. The super massive giants at the centers of galaxies weighing as much as 100 billion suns take more than 10 to the 100th years. The full Google. For almost all of that time, each black hole is colder than the faint warmth of the space around it, sitting in the dark, doing almost nothing, losing mass so slowly that the process is closer to stillness than to burning. The evaporation of a black hole is the slowest fire in the universe, and it is also in the end the last one. As it does, it loses mass, and as it loses mass, it grows slightly warmer and glows slightly brighter. And so the process very gradually speeds up over spans of time that dwarf everything we have discussed so far. A black hole with the mass of our sun would take something like 10 to the 67th years to evaporate completely. The largest black holes, the giants at the centers of galaxies with the mass of 100 billion suns would take more than 10 to the 100th years, the full Google to finally disappear. But disappear they do. And the way a black hole ends is the strangest sunset in the cosmos. For almost its entire life, it has been the faintest possible glow, colder than empty space. But as it nears the very end, having shed most of its mass, it grows hotter and brighter with gathering speed, until in its final moments it flares, releasing the last of itself in a closing burst of radiation. And I want to take you to one of those moments, the very last one, because it marks a threshold the universe only crosses once. Imagine the final black hole, the last one left in all of existence. Somewhere past 10 to the 100th years from now, everything else is gone. There are no stars, no galaxies, no planets, perhaps no atoms at all. only an almost perfectly empty darkness stretching in every direction farther than light could ever cross. And in that darkness, this one last black hole reaches the end of its long evaporation. For ages, it was colder and dimmer than anything. A slow leak of the faintest radiation. Now, in its last moments, it brightens. It grows hot. And then it flares. A final burst of light and particles thrown out into the empty dark. And here is the thing I want you to hold. These are the last new photons the universe will ever make after this flash fades. There is no star left to shine, no ember left to cool, no black hole left to glow. There is no process anywhere in the cosmos that creates fresh light ever again. This is the last light the universe will ever switch on. When it goes out, the universe has made its final photon. And from then on, there is only the light that already exists, set loose long ago, traveling through the dark with nothing left to make more. So this is the threshold. Up to this point, the universe has always somewhere been making new light from stars, then from embers and collisions, then from the slow glow of evaporating black holes. After the last black hole evaporates, that ends. The cosmic factory of light shuts down for good. Everything that happens after this in the long long darkness that follows happens by the light of photons that were made before this moment and never caught. And that is where we finally turn our full attention to those photons. The ones already in flight, the ones still crossing the emptiness with nothing ahead of them. Because while all of this has been happening, while the stars died and the matter decayed and the black holes evaporated, those photons have not been holding still. They have been changing. And what has been happening to them is the real answer in our frame to the question we started with. So let us leave the last burst behind and go look at what the expanding universe has been doing all this time to light itself. Part nine. Light does not die. It only thins. Let us go back and pick up a single photon. One that was set loose long ago. Maybe by a star in the age of starlight. Maybe by the last burst of an evaporating black hole. And let us follow it through all this time and watch what happens to it. Not what happens to the stars around it. What happens to the light itself traveling through space while the universe ages around it? Because something does happen steadily the whole way and it is not the kind of ending we might expect. The space the photon is traveling through is not standing still. The universe is expanding has been expanding since the beginning. And that expansion is not slowing down, but speeding up, driven by something we call dark energy that we still do not understand. As space expands, it carries everything in it farther apart. And that includes the light traveling through it. A photon is a wave and it has a wavelength, the distance from one crest of the wave to the next. And that wavelength is what determines its color and its energy. Short wavelengths are blue and energetic. Longer wavelengths are red and gentle. Longer still and the light slips out of the visible range entirely into the infrared, the microwave, the radio, growing ever weaker. Here is what expansion does. As space stretches, it stretches the light traveling through it, lengthening the wavelength of every photon in flight. A photon that set out as visible light slowly reens as the eons pass. Its wave drawn out longer and longer by the swelling of space itself. Given enough time, visible light becomes infrared and infrared becomes microwave and microwave becomes radio and on and on the wave stretching without limit as the universe keeps expanding. We have direct proof of this happening right now in the oldest light there is. The glow left over from the birth of the universe set out as fierce hot light and across almost 14 billion years of expansion. It has been stretched so far that it now reaches us as faint microwaves. Cold radiation only a few degrees above absolute zero. The same stretching that turned that primordial fire into a cold whisper is happening to every photon everywhere all the time. The story of that oldest light is worth telling because it is the clearest proof we have of where all light is headed. About 380,000 years after the beginning, the whole universe was filled with a glow as hot as the surface of a star. A fierce light pouring out of the cooling fire of creation. If you could have been there, the entire sky in every direction would have blazed like the face of the sun. That same light is still here, still filling all of space. But in the nearly 14 billion years since, the expansion of the universe has stretched its waves longer by a factor of more than a thousand, draining its fierce heat down to a cold whisper just 2 and 3/4° above absolute zero. Invisible to the eye, detectable only as faint microwaves. The fire of creation has become a cold hum in the dark, and it is not finished cooling. As the universe keeps expanding, that ancient light keeps stretching. And in the far future, it will be drawn out so far that no instrument anyone could ever build would be able to find it. The oldest light, the light that has been with the universe since almost the beginning, is on its way to becoming undetectable. If that is what happens to the brightest, most all-encompassing light there ever was, then it is what happens to all light given enough time. The oldest light in the universe is simply farther along the same road that every photon travels. and watching it cool toward invisibility is like being shown the ending of the story in advance. Every beam of starlight, every photon from every fire the cosmos ever lit is on that same path, stretching, reening, weakening, headed toward a faintness from which there is no return. The fire of creation is already most of the way there. The rest of the light is simply behind it on the same road. And because a photon's energy is tied to its wavelength, the stretching means the photon is losing energy. The longer its wave gets, the less energy it carries. Across the spans of time we have been discussing, the spans in which stars die and matter decays and black holes evaporate. Every surviving photon is being drawn out longer and weaker, its energy bleeding away, fading towards zero. Out in the farthest future, a photon's wavelength can grow longer than the entire visible universe was in our own time. An absurd, beautiful image, a single ripple of light stretched across a length that once held everything we could see. But now I have to be very careful with my words because here is the part that matters most and it is easy to get wrong. The photon is fading. It is not dying. Those are not the same thing and the difference is the whole tender center of this story. When we say something dies, we usually mean it stops existing. It is destroyed. It comes apart. It is gone. That is not what happens to the photon. The photon is not destroyed by the expansion of the universe. It does not break. It does not decay. It does not stop being a photon. As far as we can tell, light is stable. Left to itself, a photon will travel forever. What expansion does to it is gentler and stranger than destruction. It stretches it. It thins it out. It draws the wave longer and longer and the energy lower and lower, forever approaching nothing, but never quite arriving at nothing. Never actually winking out of existence. I find this one of the most quietly moving facts in all of physics and I want to give it room to land. The universe at the end of everything does not kill the light. It cannot really. It has no mechanism to simply erase a photon. What it does instead is so much softer and in a way so much sadder. It stretches the light thinner and thinner, colder and colder, weaker and weaker until the photon is still there, still traveling, still perfectly real, but carrying so little energy that it can warm nothing, illuminate nothing, be noticed by nothing. The light is not extinguished. It is diluted past the point of mattering. It survives its own irrelevance. fading, not dying, turns out to be the fate of every photon in the universe. And there is something about that, the endless gentle thinning rather than a clean end, that is harder to sit with than destruction would be. Nothing is lost. It is only spread so thin that being lost and being saved stop being different. And that raises a question that should bother you because it bothered physicists for a long time. And the answer turned out to be stranger than anyone expected. If every photon in the universe is losing energy steadily forever, then where is all that energy going? Energy is not supposed to just vanish. It is supposed to be the one thing the universe is most careful about. The quantity that is always always conserved. So as all the light in the cosmos fades and cools and gives up its energy across the long dark, that energy has to be going somewhere or does it? The answer to that question is one of the deepest surprises in modern physics and it is where we are going next. Part 10. The energy that goes nowhere. There is a rule in physics that most of us absorb early and never question because it has never once let us down in ordinary life. Energy is conserved. It is never created and never destroyed, only moved around and changed from one form to another. The energy in your breakfast becomes the energy of your moving body. The energy in falling water becomes the energy in an electric wire. Add it all up before and after and the total is always the same. This rule is so reliable that when an experiment seems to violate it, physicists do not throw out the rule. They go looking for the missing energy and historically they have always found it. Conservation of energy is about as close to sacred as physics gets. So when we discover that every photon in the universe is steadily losing energy as its wavelength stretches with the expansion of space, the natural thing to do is ask the question conservation trains us to ask where does the energy go. When a photon reens and weakens, that lost energy must turn up somewhere else in some other form so that the books balance. That is what conservation demands. And for a long time, people tried to find the missing energy to figure out what it turned into to make the accounting work the way it always has. The answer when it came was not a place the energy goes. The answer was that on the scale of the whole expanding universe, energy is simply not conserved. The sacred rule does not apply. The energy a photon loses to the stretching of space does not go anywhere because there is no law requiring it to go anywhere. It is just gone. You will sometimes hear people offer a softer version of this, a way of keeping the books balanced after all. They will say that the energy the light loses goes into the expansion of the universe. that it is spent doing the work of stretching space, the way a gas cools when it pushes outward against a piston. It is a comforting picture and it captures something true about the feel of the process. But most physicists who study this carefully will tell you it is not really right because there is no piston, nothing for the light to push against, no container with walls. Space is not expanding into anything. So the honest statement is the stranger one. The energy is not transferred and it is not stored. It simply is not conserved at the scale of the whole universe. And the search for where it went is a search for something that was never required to exist. The physicist Sha Carol has put this about as plainly as anyone. People often want to be reassured that energy is really conserved, that the universe is keeping a careful ledger somewhere we just cannot see. And the truthful answer is that it is not. Energy conservation is a rule that holds whenever the background you are working against stays the same from moment to moment. In a laboratory, on a planet, in any small enough patch of space and time, that condition holds beautifully and energy is conserved to staggering precision. But the universe as a whole does not hold still. It grows. And the moment the stage itself is changing, the rule that depends on an unchanging stage simply does not apply. It is not that the rule is being broken by some hidden force. it is that the conditions the rule requires are not met. The strangest part is that the same lack of a conservation law that lets the light's energy drain away also lets the dark energy driving the expansion keep its grip on every new volume of space that appears. So the total energy of the cosmos is not even close to a fixed number. The universe runs no ledger. And once you accept that, the fading of all the light in the cosmos stops being a mystery of missing energy and becomes something simpler and stranger. The light just gives up its energy to nothing at all forever. Because at the largest scale, there is no rule that says it cannot. I know how that sounds. It sounds like a cheat, like physics throwing up its hands. And the first time most people hear it, they assume they have misunderstood. So let me explain why this is not a failure of physics but one of its deepest insights because the reason conservation of energy fails here is beautiful and it ties back to the very nature of time. There is a profound connection discovered by the mathematician Emmy nother early in the 20th century between conservation laws and the symmetries of nature. Energy is conserved precisely because the laws of physics do not change over time. Because an experiment you run today gives the same result as the same experiment run tomorrow. That steadiness of the laws through time is what guarantees that energy is conserved. The two are the same fact seen from two angles. But on the scale of the whole universe, that steadiness does not hold because the universe itself is changing in time. Space is expanding. The cosmos today is genuinely different from the cosmos of a billion years ago. Larger, emptier, more spread out. The stage on which everything happens is not the same from one age to the next. And when the universe itself changes through time like that, the underlying symmetry that guarantees energy conservation is simply not there. So there is no law that says the energy lost by all those fading photons has to be conserved because the very condition that would make such a law exist an unchanging universe is not met. The energy does not hide somewhere. It does not transform into something else. It thins away into the expansion and is not replaced and is not stored because the universe at large keeps no such ledger. This is the kind of thing that should genuinely unsettle you and I do not want to smooth it over because it is one of the strangest truths we have found. The single most trusted rule in all of physics, the conservation of energy. The rule we lean on so hard that we reorganize our whole understanding rather than give it up. turns out to be a local rule. A rule that holds in any one place in time, but quietly stops applying when you zoom out to the whole expanding cosmos. In your kitchen, energy is conserved to perfect accuracy. Across the universe and across the ages, it is not. The light fades, the energy drains away, and there is no missing energy to find because out there at that scale, the books were never required to balance in the first place. I should clear away one wrong idea before we go on because it is a tempting one and a lot of people reach for it. You might think that light loses energy the way a runner loses energy by friction, by effort, by wearing out as it crosses all that distance. that the long journey simply tires the light out. This is an old idea and it even has a name tired light and it was proposed seriously a long time ago as an alternative to the expanding universe. But it is wrong and we know it is wrong because it makes predictions that do not match what we see. If light simply lost energy by some kind of friction as it traveled, distant events would not appear stretched out in time the way they do, and the oldest light would not have the particular smooth character that it has. The evidence is clear. Light does not get tired. It does not lose energy by traveling. It loses energy because the space it is traveling through is stretching, carrying the wave longer with it. And that lost energy does not go anywhere because at the scale of the cosmos it does not have to. The fading of light is not exhaustion. It is geometry. The universe is not wearing the light out. It is drawing it thin. So now we have the full picture of what happens to light in our frame on our clocks across the long death of the universe. The sources of light go out one kind at a time until the last black hole gives its final flash. And the light that remains, all the photons ever made and never caught, stretches and reens and weakens forever, losing energy that goes nowhere, fading without ever quite dying. There is one last stop on this road, the coldest and emptiest place of all. The state the universe is heading toward across all these eons. We have to go and see what light becomes when the stretching has gone on almost forever and the universe has cooled almost all the way down. We have to go to the dark era and the strange cold floor the universe never quite reaches. Part 11. The cold that has a bottom. After the last black hole has evaporated, the universe enters the era astronomers call the dark era. And it is exactly what it sounds like. There are no stars. There are no dead stars. There are no black holes. Ordinary matter, if protons decay, has long since dissolved. What remains is almost nothing, spread almost infinitely thin. An emptiness so complete that the universe of stars and galaxies we live in now would seem, by comparison, impossibly crowded and warm. The dark era is where the universe spends the overwhelming majority of its future. And it begins after about 10 to the 100th years and continues, as far as we can tell, without end. What is left in that darkness is light. Not new light, for nothing is left to make it, but the old light, the photons set loose across all the previous eras and never absorbed, still traveling through the empty dark. There is the ancient glow from the birth of the universe stretched now so far beyond microwaves that no instrument could ever detect it. There are the last photons from the last stars and the final radiation from the last evaporating black holes. All of it stretching, reening, cooling as the expansion goes on and on. The dark era is not entirely without light. It is full of light in a sense, the accumulated light of the whole history of the cosmos. But it is light stretched so thin and so cold and so faint that it might as well not be there. Light with nothing left to shine on and nothing left to see it. And the universe keeps cooling because the expansion never stops stretching that light to longer and weaker wavelengths. You might expect it to cool all the way down to absolute zero, the coldest temperature there can be, the total absence of heat. But here is a final strange detail, and it is a fitting one for a universe that never seems to do anything cleanly. The expansion of the universe is driven by dark energy. And an expanding universe driven by dark energy has surrounding every observer a kind of horizon, a far boundary beyond which things are receding faster than light can cross back. And that horizon through an effect worked out by Gary Gibbons and Steven Hawking in 1977 is associated with a tiny but real temperature. For the amount of dark energy we actually measure, that temperature comes out to something like 10us30 kel. That is a number so close to absolute zero that the difference is almost meaningless, a fraction of a degree with 30 zeros after the decimal point, but it is not zero. That same horizon, the one that gives the universe its faint floor temperature, is also doing something to the contents of the cosmos that has already begun in our own time. And it is worth understanding because it makes the loneliness of the far future concrete. Because the expansion is speeding up, distant galaxies are being carried away from us faster and faster, and the farthest ones are already receding faster than their light can ever cross back to us. They are slipping over the horizon, out of reach, out of view forever. This is not a future problem. It is happening now. Over the coming spans of time, one by one, the galaxies we can currently see will reen and dim and finally vanish past that horizon until an observer in the far future, looking out from what is left of our own galaxy, would see nothing beyond it at all. only empty dark in every direction. The rest of the universe will not have been destroyed. It will simply have been carried beyond the reach of any light. Gone not because it ended, but because the light connecting us to it can no longer make the crossing. So the cold floor and the emptying sky are two faces of the same fact. The accelerating expansion that draws every photon's wavelength out toward nothing is the same expansion that pulls the galaxies apart faster than light can bridge them. And the same expansion that sets the faint temperature the universe approaches but never reaches. Everything is being separated from everything else gently and permanently. And the light that once tied it all together is being stretched too thin to tie anything anymore. The universe does not end in fire or in a final crash. It ends in an endless drifting apart, in a darkness that is not empty of light, but full of light, too faint and too far and too cold to ever be caught. So the universe has a floor it never reaches. It cools and cools and cools across the endless dark era, drawing closer and closer to that faint floor temperature, but never quite touching absolute zero and never quite touching the floor either. The end of the universe, it turns out, is not a final state that arrives. It is a destination forever approached and never reached. A cold that always has a little further to fall. Even the ending does not finish ending. It only keeps getting closer across a future with no last page. I want to take you into that emptiness now to follow one last photon as far as we can because it is the truest picture I can give you of what light becomes at the end of everything. Imagine a single photon far out in the dark era, almost unimaginably far in the future. It was made long ago, perhaps by a star that died before the universe was a fraction of its present age. And it has been traveling ever since, never once meeting anything to absorb it. Around it there is almost nothing. No stars, no galaxies, no planets, no warmth. Only the immense empty dark stretching in every direction farther than this photon could ever cross. Because space itself is expanding faster than the photon can close the distance. Its wavelength has stretched longer than the entire visible universe was in our own time. A single ripple of light drawn out across a length that once held every galaxy we could see. It carries almost no energy now. A whisper of a whisper fading still. And there is nothing anywhere to catch it. No eye, no detector, no grain of dust, nothing that could ever absorb it and turn its long crossing into an arrival. So it simply travels. It reens and cools and stretches and goes on through an emptiness with no edge and no end, carrying the faintest possible memory of a universe that was once full of light, with no one and nothing left to receive the message. It does not stop. It does not die. It just keeps going forever, growing fainter forever in the dark. That is the long answer. The answer in our frame, on our clocks, to what happens to light at the end of the universe. It is not destroyed, and it does not vanish. It is stretched past meaning, cooled toward a floor it never reaches. set a drift in an emptiness with nothing left to absorb it, fading forever without ever quite being gone. It is the gentlest, slowest, most patient ending imaginable. An ending that takes longer than the mind can hold and never actually finishes. And it would be a fitting place to stop except that we made a promise at the beginning and it is time to keep it because there is another frame, another point of view, the one we set aside all those eras ago. And from that point of view, none of this long fading happened at all. Part 12. The end that has already happened. We have spent this whole journey in our own frame on our own clocks watching the universe end the slow way and it has taken us across spans of time so enormous they stop meaning anything. But remember what we found at the very beginning before we ever set out for light itself. There is no time. The photon keeps no clock. And now that we have followed light all the way into the final dark. It is time to lay that fact down next to everything we have seen and look at the two of them together because this is where the whole thing turns over. Take that lone photon in the dark era, the one we just watched drifting through the emptiness with nothing left to catch it, fading forever in the cold. From our point of view, that photon has had the longest existence of anything in the universe. It was made in the age of stars and it is still traveling after 10 to the 100th years and beyond. An existence so long it makes the entire history of stars and planets and life look like a single spark at the very start. From our point of view, that photon has endured the whole death of the cosmos, watched the stars go out and the matter decay and the black holes evaporate and traveled on through all of it into the endless dark. But from the photon's own point of view, it has done nothing of the kind. From the photon's own point of view, it has not been traveling for 10 to the hundredth years. It has not been traveling at all. The instant it was born in the light of some long deadad star is the very same instant it finds itself in now, a drift in the final dark. Because for light there is only ever the one instant with no time between the beginning and the end. Let that settle because it is the strangest thing I know how to tell you. The photon that leaves a star tonight, the starlight coming through your window right this moment will in its own reckoning arrive at the end of the universe in the same instant it set out. Not in the future, not after a long journey, in the same instant. for that light. The death of the cosmos, the last dark, the final cold, is not something waiting unimaginably far ahead. It is happening now in the same breathless point as its birth. Because birth and end, and everything between are pressed together into the single timeless instant that is all a photon ever has. The end of the universe for light has already happened. It is happening. It will always have happened all at once with no before and no after. There is a way of thinking about time that physicists have argued over for a century and it fits this strangeness like a key in a lock. In this view, the flow of time, the sense that the present is sliding forward and the future is not yet real is something about us, about how we experience the world rather than something built into the universe itself. In the universe itself, on this view, past and future are equally real. All of it laid out together. every moment existing as surely as every other like the frames of a film that all exist whether or not anyone is watching them in sequence. For ordinary matter for you and me that is a hard and abstract claim because we feel time passed so vividly that it is difficult to believe the feeling is not the whole truth. But for light the claim is not abstract at all. It is almost a description. Light with its zero proper time does not experience any sliding present, any unfolding of one moment into the next. For light there is no flow, no passage, no before becoming after. There is the single point that holds its whole existence. If the universe really is a four-dimensional hole in which all times exist together, then light is the one thing that lives that way openly, the one thing for which the end of time is not ahead, but simply present alongside the beginning in the same eternal instant. This is why I keep returning to the photon in your eyes. You experience this moment as now with the end of the universe unimaginably far ahead in a future that has not arrived. The photon experiences no such thing for it. This moment and the last cold instant of the cosmos are not separated by time because it has no time to separate them with. So the light touching you carries folded invisibly inside it. The entire span from creation to the final dark. All of it present at once. You are in the most literal sense the universe allows. in contact with the end of all things every time you open your eyes to the light. I find this almost impossible to hold in my mind. And I have thought about it for a long time. We treat the end of the universe as the most distant thing there is. An event waiting at the very bottom of an almost endless future so far away it can barely be called real. And it is that from our side. But the light in your eyes right now does not live on our side. For the light, that unthinkably distant ending is not distant at all. It is folded into the same instant as the light's creation, the same instant in which it is touching your eye. The end of everything and the glow of this moment are for the light itself the same. Now you are in a sense looking at the end of the universe every time you see a star because the light carrying that starlight to you holds its own end in the very same instant it holds its beginning. So we have our two answers finally side by side and we can look at them honestly from our frame. Light endures the end of the universe in the slowest, gentlest, most patient way anything can end. Fading across spans of time that break the mind, never quite dying, never quite finishing. And from light's own frame, there is no ending to endure. Because there is no time in which an end could arrive, only the single instant in which birth and death are the same. Light is at once the most patient witness to the death of the cosmos and the one thing that was never present to witness it. It crosses all of time and pays nothing. It outlasts everything and experiences nothing. It is there for the very end and it left for the very end in the same breath it was born. And here is the part that I think is the real center of all of this. The thing worth carrying away. We will never be able to ask the light what the journey was like. Not because the light is gone, but because for the light there was no journey and no time and no ending at all. The one thing in the universe that touches both ends of time is the one thing for which there are no ends and no time. The messenger that crosses everything experiences nothing. So when we ask what light itself experiences at the end of the universe, the deepest honest answer is a kind of beautiful paradox we have to hold rather than solve. From outside, the end is real and slow and almost endless. From light's own side, the end has already happened and never happens and is the same as the beginning. Both are true. The universe holds them both without the slightest strain. The way it has held every strange thing we have found tonight. Part 13. What light becomes? So let us come back at the end to the question we started with and ask it one more time now that we know what is hidden inside it. What does light itself experience at the end of the universe? We have followed it the whole way from the comfortable image of the untouchable messenger through the discovery that it keeps no time at all out across the dying eras of stars and matter and black holes into the final dark and the cold floor the universe never reaches and back again to the timeless instant where the end and the beginning are the same. And we are left I think not with a single answer but with something better. A question that has opened all the way up. What does light become at the end of the universe? It does not become nothing. That is the first thing and it matters. The universe for all its winding down never manages to destroy the light. It has no way to. It cannot break a photon or burn it out or erase it. All it can do is stretch it, cool it, thin it, draw it longer and fainter and weaker, forever approaching nothing and never arriving. So light does not end. It becomes the faintest possible version of itself, a whisper stretched across distances that once held everything, traveling on through an emptiness with nothing left to catch it. It becomes in a way the last thing the universe still has. The only trace left that there was ever anything bright. When every star has died and every ember has cooled and every black hole has flared its last, the light that remains is the universe's final memory of having shone. Faint and cold and unwitnessed, but still there, still carrying the message, even with no one left to read it. There is one more idea I want to leave with you here, and I offer it gently because it is speculation rather than established fact. The kind of thing the channel explores honestly without pretending it is proven. Some physicists, the mathematician Roger Penrose chief among them, have noticed something curious about that final state, a universe of nothing but light. Think about what is left in the very far future. Once matter has decayed and the black holes have evaporated, there are no particles with mass anymore. There is only radiation, only light stretched out across an endless dark. And here is the strange part. Mass is what lets the universe keep track of size and scale and the passage of time. A universe with mass in it has rulers and clocks built into it. But a universe of pure light has neither. Light keeps no time as we have seen. And without mass there is nothing to set a scale, nothing to say what counts as big or small, near or far. And so the argument goes that cold, empty, light only ending becomes geometrically indistinguishable from a hot dense beginning because there is no longer anything present that could tell the difference between them. If that is right and it is a large if, then the end of the universe is not a wall but a doorway and light is the thing that passes through it. The final fading light of one universe with no scale and no clock left to anchor it could become the first searing light of another. A new beginning rising out of the old ending without any seam between them. In that picture, light is not just what dies last. It is the bridge, the one thing that carries across from the end of everything to the start of everything else. Precisely because it is the one thing for which end and beginning were never really different to start with. I cannot tell you that this is true. No one can yet. But I find it a fitting thought to hold beside everything else we have found, that the light which seems to fade into nothing might be in the end the thread on which a new cosmos is strung. And from its own side light becomes nothing different at all. Because from its own side it never changed and never traveled and never aged. The photon, a drift in the final dark, is to itself still in the very instant it left the star. It does not experience the fading we watched. It does not feel the long cold. For the light, there is no long anything. There is only the one instant, the same one it has always had, holding its birth and its end, together with all of time, folded invisibly between them. I do not have a tidy ending for you tonight. And I think that is exactly right because the truth here does not tidy up. We are left with a photon in the dark and two ways of seeing it and no way to choose between them because both are true. Is that lone photon drifting through the empty cosmos with its wavelength longer than the universe once was experiencing the end of everything from our side? Yes, it is living through the final state of the universe. The long, cold, dark, the slow approach to a floor it never reaches. Or has it all along been experiencing nothing born and ended in the same instant? No journey, no fading, no end. From its side also, yes. And here is the question I want to leave you with, the one I cannot answer and neither can anyone else. Is there even a difference between those two? Between a thing that experiences the entire end of the universe and a thing that experiences nothing at all for light somehow those are the same. The most patient witness and the one that was never there are the same photon, the same instant, the same light. The light coming into your eyes right now carries all of this inside it. It is fading very slowly, the way all light fades, drawn thinner by the expansion of a universe that will go on stretching it long after everything you know is gone. And it is also in its own reckoning already at the end of time holding the death of the cosmos in the same instant it holds this moment, this room, your open eyes. It is the gentlest ending there is and an ending that has already happened both at once in the same quiet glow. Light does not die at the end of the universe. It thins and stretches and cools and goes on becoming the last faint memory the cosmos keeps of ever having been bright. And from its own side, it never left the star at all. So the next time you see a star or feel the sun or simply notice the light filling the room around you, you might let yourself remember that you are looking at the one thing that touches both ends of time. The messenger that crosses everything and keeps nothing already in its own silent way. home at the end of all things.