Transcription
There is a quiet assumption running beneath almost everything you do. It doesn't announce itself. It doesn't ask for your trust. It just sits there underneath your alarm clock, your appointments, your deadlines, the way you say, "See you soon." and mean it.
The assumption is this, that right now is something every one of us shares. Somewhere tonight, two people are standing in a kitchen watching a tick towards midnight. They've decided this is the moment. An anniversary, a birthday, or even a new beginning. They're holding on to the idea that when the clock strikes 12, something real will shift, that they'll cross a threshold together at the same instant. And they're not alone.
Right now, all over the planet, millions of people are counting on the same invisible agreement, that there is a now, and that it's universal. That the present moment is the one thing that we can all point to and say, "This. This is where we are. Together."
But physicists went looking for that moment, and they couldn't find it. Not in the laws of physics, not in any equation, not anywhere in the most complete description of reality that human science has ever built. The present, though, one thing you are more certain of than anything else, simply doesn't exist.
But if time doesn't have a now, then why do you feel it so completely? And if that feeling is something your mind is making up, rather than something the universe is giving you, what exactly are we all living in? These are the terrifying secrets buried in the one dimension you never I to question time.
To understand the dimension of time, we first need to understand spatial dimensions. I think my past self would explain it a lot better. Let's strip it back to something almost painfully simple, so bear with me here. A dimension is just a direction that you can move in. That's it.
If you're standing on a straight line, you can only move forward or backward in, well, one straight line. So, the world to you has one dimension. There's only one independent direction available to you.
Now, imagine you're standing on a flat sheet of paper instead. You can move forward and backwards, but you can also move left and right. That's two dimensions, two independent directions. Add one more. Suddenly, you have space. You can move forward, backward, left, right, up, down. Three independent directions, and that's the world that we live in. A dimension is simply a new way to move that isn't just a combination of the others.
Thanks, me. I'll take it from here. You know, that version of me had no idea that he was going to be showing up here, but I do remember filming it in the past, and if I remember, then he still exists somewhere in the universe, doesn't he? Which means the version of me speaking to you today isn't just here for the present moment, because even right now, I might be saying or doing things that will only make sense if you have future me giving you context.
This will make sense in the future, but for now, the three dimensions, or 3D as we know it, only focuses on the three dimensions of space. The tangible world we can point to and physically interact with. However, 3D is only useful for us if we're locating a place, not an event. And I don't mean an event like a gathering or a birthday party. I mean an event literally, which is a thing that happens.
When your mom goes out for work and reminds you to take the chicken out of the freezer, you immediately understand that there's a timeline for this event. Assuming she gets back from work by 6:00 p.m., then that means you have a 2 to 3-hour window to take that chicken out. Now, according to three dimensions, you know where the chicken is currently located. It's in the freezer. But 3D doesn't account for the intricate nature of your mom's instructions. If you take it out too early, it begins to spoil before she's come back. But if you take it out too late, you get a tongue-lashing. The specific valid moment to take it out, that is what the fourth dimension is all about. The moment. Therefore, every real event needs four numbers to make complete sense. Three for where it's happening and one for when. And that fourth number is time.
For most of human history, space was space and time was time. They were two different things running alongside each other but never touching. Space was the stage, time was the clock on the wall, completely separate. Then Einstein arrived and quietly dismantled that separation.
What Einstein showed through his theory of special relativity is that space and time don't just coexist. They trade off against each other. If you were to move faster through space, time slows down for you. Slow down in space and time speeds up. They aren't two separate measurements, they're two sides of the same measurement.
A few years later, a mathematician named Hermann Minkowski took Einstein's equations and made the implication impossible to ignore. He showed that what we experience as space and what we experience as time are both just different angles on a single four-dimensional fabric that he called space-time. If every event follows a fixed four-number address, then that means you can picture them all laid out together as one complete grid, and each event, no matter how small or insignificant, sits somewhere on its own permanent coordinate. So, yes, your birth has a four-number address. Your first day of school has one, too. Same with your graduation and every other moment that you have ever experienced.
Although those events are memorable and set in the past for you, they haven't disappeared from existence. We like to think of time like it's a movie with each new frame replacing the last, creating a clear indication of where the timeline currently sits. But, unlike movies, every point in reality sits like a point on a map. There is no current frame or current page to turn to. Many physicists favor this reading, and they even have a name for it, the block universe. Past, present, and future are all laid out at once, every moment as real and as permanent as any other.
But, you can see this creates a problem. If every moment is equally real, if if the map just sits there with no preferred direction, then where does the feeling of forward come from? Why does time only ever seem to move one way?
As unsettling as false vacuum decay is, I think it also forces you to confront just how fragile and improbable life really is. Against impossible odds, the universe has arranged itself in just such a way that matter could become alive and life could become conscious. Tiny collections of atoms began telling stories, building civilizations, and contemplating the fate of the cosmos itself. And if conscious life really is this rare and delicate thing, then maybe that fragility gives us a responsibility not just to preserve ourselves, but to reduce the suffering of every other living thing wherever we can.
Unfortunately, right now, billions of animals exist in conditions of extraordinary suffering. Hens spend their lives trapped in cages so small that they can barely move. Entire industrial systems are built around maximizing efficiency while minimizing the value of conscious experience itself. And beyond factory farms, trillions of wild animals suffer through starvation, disease, injury, and environmental collapse with almost nothing capable of helping them at all.
The problem is that most people genuinely want to help, but figuring out where to contribute can feel overwhelming. There are thousands on thousands of organizations making competing claims, and it can be difficult, really difficult, to know which ones are actually creating a meaningful impact, which is why I appreciate the sponsor of today's video, Animal Charity Evaluators, or ACE. ACE is a nonprofit that uses evidence and reason to identify the animal charities capable of helping the greatest number of animals with your donation. Rather than relying on emotional marketing or vague promises, ACE researches which interventions are actually effective and directs funding towards organizations capable of producing the most measurable impact.
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Now, back to our story. Here's what makes this strange. The laws of physics don't actually have a direction. Uh look at this video of two pool balls colliding. Now, play it forwards normally. Play it backwards. It looks normal. There's nothing in physics that objects to either version. The equations work identically in both directions. Time at that level of fundamental physics is completely reversible.
Now, picture this. That's totally different. You watched it and you already knew this can't be reversed. You will never see those shards leap off the floor and seal themselves back into a cup. Same with a scrambled egg or an ice cube melting. The direction is obvious, instinctive, and absolute.
So, here's the contradiction sitting at the heart of reality itself. The laws of the universe have no preferred direction, but every single thing that you experience does. If nothing in physics points one way, why does everything in life feel like it's moving forward and never back?
Well, the answer is hidden within something that has been in motion since the very first moment of the universe. And scientists call it entropy. The basic idea is this. At any given moment, there are vastly more ways for things to be disordered than ordered. And the universe, left to its own devices, drifts towards disorder simply because disorder is more likely.
Think about what it actually takes for something to be a cup. It needs a specific shape, a minimum volume to hold liquid, and then walls sturdy enough to carry it, a form friendly enough to drink from without spilling. The requirements of a cup are strict. The window is pretty narrow. A cup has to have a particular set of qualities. Now, think about what a shattered cup looks like. There are no requirements for the shards. They can be large or or so small they embed into your foot weeks after you've finished cleaning. They can be jagged or smooth, scattered across the floor or just piled neatly in a corner. There is no wrong way for a cup to be broken. The number of possible shattered states is almost infinite, while the number of possible cup states is vanishingly small in comparison.
This is what entropy actually means. It isn't a state of chaos for chaos's sake. It's simple math. When you have far more ways to be disordered than ordered, disorder becomes the overwhelmingly statistical favorite, not because the universe prefers it or some force is actively pushing things towards chaos, but because if you pick a random outcome from all of the possible outcomes, you are almost certainly going to land somewhere more messy. An unscrambled egg, a reassembled cup, those outcomes aren't necessarily forbidden by any laws of physics. They are just so astronomically improbable that they may never happen once in the entire lifetime of a finite universe. The universe isn't moving towards disorder on purpose. It's just that disorder has the numbers on its side, and that's where time's direction comes from.
So, thanks to entropy, we have a model for understanding the difference between the past and the future. What we call the past just means a more ordered world. Meanwhile, the future is harder to predict or pin down because it has unfathomably more possibilities. It even reflects in how we remember and imagine things. When you think of the past, you can pinpoint exactly when you ate for breakfast down to the second. You can recall lottery numbers, sports scores, election results. But the future? Well, that looks like disorder. There are no lottery numbers you can predict, no guarantees. The only thing separating the past and the future for us is an increase in entropy.
The direction of time isn't in the laws of the universe. It only feels that way because reality is a slope and we live on the downhill side of it. That rush you feel moving towards the future isn't the universe moving, it's you sliding down the slope of entropy.
But here's what that quietly implies. If the direction of time is something that emerges from probability rather than law, then it isn't woven into the fabric of the universe the way we assumed. It's a consequence, a side effect of where we happen to be on the slope. And that raises a question that we've been glossing over without realizing it. We've been talking about sliding down this slope as if everyone does it the same way, as if that slope runs at one universal speed for every person, every clock, and every corner of the universe. What if it doesn't? What if your slope runs faster than mine?
The whole concept of time as we understand it rests on one strange fact about light. And to see why it's so strange, we need to start somewhere ordinary. So picture yourself standing on a platform as a train speeds past you. Someone on board rolls a ball forward. To them, the ball moves at rolling speed. But to you, standing still on the platform, it moves at rolling speed plus the train's speed. The two speeds stack up. For the most part, that's how motion works, and it's the most intuitive thing in the world.
Swap the ball now for a beam of light. Common sense says the same rule should apply. The light should pick up the train's speed on top of its own, just like the ball did. But it doesn't. You on the platform and the passenger on board measure that beam moving at exactly the same speed, and not approximately the same, exactly the same. It doesn't matter if you're running toward it or racing away from it or chasing it on the fastest rocket ever built. Light always clocks in at around 300,000 km per second. No exceptions, no adjustments, and no stacking. How weird is that?
Speed is just distance divided by time. If light measures the same for everyone, no matter how they move, then their distances and their times can't match. Something has to give, and what gives is both space and time themselves. The universe would rather warp your ruler and slow your clock than allow light to travel at any other speed.
This reveals something deeply fascinating about the nature of the universe, that space and time are not fixed neutral containers that everything happens inside of. They They can bend. They can stretch and run at different rates depending on how you are moving through them. When you're sitting still, you're not moving through space at all. All of that motion is spent moving through time. In essence, you are racing towards the future at maximum speed while you're not moving physically at all. But when you start moving through space, some of that budget gets diverted into your space speed. You're trading speed through space for speed through time and vice versa.
Uh this isn't a weird quirk of time or anything. It's how the universe works. Space and time are interconnected in an undeniable way. Uh the best way to look at it is that time is a resource, and you are spending it just by existing. Though the metaphor has a limit worth pointing out. Simply put, you can't run out of the resources and you can't choose how to spend it. You are always spending the full amount. The only thing that changes is how much goes into moving through space and how much of that budget is left over for moving through time.
Astronaut Scott Kelly spent nearly a year aboard the International Space Station while his identical twin, Mark, stayed on Earth. When Scott returned, a landmark experiment called the NASA Twins Study measured how a year in space changed his body down to his genes. They discovered that orbiting Earth at a much greater speed than his twin left Scott roughly 5 milliseconds younger than his brother. And and sure, sure, 5 milliseconds doesn't sound as fancy as what we got to see in Christopher Nolan's Interstellar. But the point isn't in seeing a stark difference. The point is that the theory of relativity is factually proven. There are no measurement errors or illusions going on with space and time. Everyone's clock is actually running at its own rate. You're sliding down the universe's slope of entropy at a pace only you possess.
This creates a small dilemma in the way we see the world around us. Think back to the couple holding hands vowing to transition to new beginnings together at midnight. They believe that time is one thing they can count on in this chaotic world. They're both very wrong. We exalt the clock of time as something that keeps us relative to one another, yet we have lived in different motions our whole lives. Our clocks have always disagreed. And if they can't agree on how fast time runs, there's a worse question waiting for us. Can they even agree on what's happening right now.
Now, by definition, is a universal moment. But according to time, it's unique to the individual experiencing it. A slice through the grade of space-time that connects everything happening at the moment. Which slice counts as now depends on motion. If you were to move differently, your slice tilts. And because no one's motion is the true motion, no one's slice is the real now, either. Physicists call this the relativity of simultaneity.
The problem is that most people never get to acknowledge the variance in slices because differences tend to be so subtle, it's hard to feel anything is different at all. But when you include motion in the mix, the perception shifts. Astronomically, even a microscopic difference in motion creates a huge gap in which events count as now.
Imagine two people who pass each other on a sidewalk. They're strolling in opposite directions at a regular walking pace. Although they're moving differently, their slices of now seem insignificant to anyone here on Earth. But that tiny difference, when observed from the Andromeda galaxy, creates a whole new version of now for either person.
To put it into perspective, the physicist Roger Penrose poses an interesting scenario using this distant galaxy. Andromeda is 2 and 1/2 million light-years away. That single moment of two people walking across each other on the sidewalk from Andromeda actually happens days apart. Two directions. Two perspectives. Infinitely different times across the entire scale of the universe.
The weirdest part is that there's no canon event to point to, no master clock the universe keeps in a back room that settles who's right and who's wrong. And this isn't about us being too primitive to measure it yet. We are never going to invent a better instrument someday that finally syncs everyone's time up. That fix is impossible. It doesn't exist. Reality genuinely has no fact, no real element about whose now is the real one because there is no real one. Billions of versions of this moment exist, all true, and there's nothing anywhere that decides which is correct.
It's not like the universe is doing any of this on purpose to keep us confused. If If anything, it's us humans who have overrated our influence over reality. We deluded ourselves into thinking our shared experience has meant that a universal present was guiding us forward through time. We were wrong. Our universe refuses to have a present. The shared now you have always imagined, a whole planet counting down towards midnight, every living person standing in the same instant, all of that was never something the universe held for you.
It feels special living in such moments here on Earth, but the reason is completely mundane. And that reason is we are all sharing a big blue spaceship drifting through the endless cosmos. So, of course, our slices of now seem to coincide. But that slice being somewhere we can agree on is more like an accident than a cosmic fact. The universe never crowned us as anything more important than what we really are. Our shared present, the thing we stake anniversaries and countdowns and New Year's kisses on, turns out to be a local fluke, a consequence of sharing the same rock moving at the same speed through the same tiny corner of an indifferent cosmos. If you were to stretch past that bubble, the agreement dissolves. From the universe's perspective, there is no now, and there never was. We just never had a reason to look far enough to notice.
And yet, here you are, feeling it completely. That's the part physics has no answer for. Not the direction of time, entropy handles that. Not the relativity of clocks, special relativity handles that. The thing physics can't account for is the one thing that you are most certain of in this moment, the feeling that something is happening, that this is now, that you are here inside of it and not watching it all happen from outside.
Physics is the most complete description of reality that we as a species have ever built. Every moment is accounted for. Every coordinate in space-time is mapped and described. And yet, nowhere in any of these equations is there a variable for the present. No term that says, "This moment, right here, specifically this one, is the one that is real." The math works perfectly without it. The universe apparently doesn't need it.
Think about a book. Every page is already printed before you open it. The story is fixed, it's finished, and it's complete. But nothing in that book itself marks one page as the one being read right now. The book doesn't know, or frankly doesn't care, where you are in it. You know where you are, you create the bookmark, but never the book. The knowledge of the present lives in the reader and not the text.
So physics is that book. It describes every page of reality with extraordinary precision, every moment, every coordinate, every event across the entire history of space-time. But it contains no bookmark. There is no mechanism in the laws of reality for flagging one moment as current. No equation that reaches out of the mathematics and says, "Hey guys, right here. This one. This is now. We've settled it."
If physics has no arrow to direct us and no now to ground us, then where do we really belong? Here's where that question stops being abstract. If every moment is equally real, if nothing in physics gives the present any special status over the past or the future, then the implications don't stay philosophical for very long. They get very personal very quickly.
Your death already exists. It's not approaching you. It's not waiting somewhere up ahead just down the road. It's already real, already fixed, sitting at its own address on the map the same way this moment is. Think of the person you would never want to lose, whether it's a parent, a partner, whoever that is for you. The day you lose them isn't coming. It's already there, finished and permanent, as real as the moment that you are living in right now, whether you've reached it or not.
We tell ourselves that that dreaded day is coming for us and somehow, as strange as it is to admit, there's a comfort in that. Coming means maybe later, maybe never, maybe you'll find a way to manage it on your own terms. You know what? Maybe you'll be ready. But, the block universe doesn't bend like that. Death isn't on its way to you. It's not in an Uber arriving at your destination soon. It is already there, and there is no version of the timeline without it.
You're like someone tracing a route across a map with their finger. The map doesn't change when you trace it. You're moving towards things you'd give anything to avoid and yet not a single line will shift. The destination the map was always drawn. You're reading a finished book mistaking yourself for the writer and the page you're on for the page being written. But the page was already there before you ever opened it and so are all the ones after it right up to the very last.
But that same fixedness cuts the other way and this is a truth worth holding on to. If the worst day is permanent, so is the best one. The day you were happiest is still there finished and intact exactly as it was or will be and nobody that you've lost is gone. They're not erased. They're elsewhere on the map in the moments you shared in the memories of them that you keep exactly where you left them.
After the passing of his lifelong friend Michele Besso in 1955, Einstein wrote a letter of condolence to Besso's grieving family. In that letter, he said that for those who accept physics the separation between past, present, and future is only a stubbornly persistent illusion. A month after the letter, Einstein himself passed away. Although it was intended for a grieving family, that message also represents a kind of comfort that cuts deep into how time works. To Einstein, Besso was never truly gone. The moments they shared still existed. They still do. The block universe takes something from you. Yeah. But it also gives something back. And what it gives back is permanent in exactly the same way loss is.