Time is an everyday part of our lives, but do we actually know how it works? When is Now? Does Time flow? Does it even exist? We looked into it and discovered a universe where, contrary to our intuition, the past, present and future are all equally real. How could we be wrong about something so familiar? When Does Now Happen? [. . .] it is just an illusion we have here on Earth that one moment follows another one, like beads on a string, and that once a moment is gone it is gone forever. (Kurt Vonnegut, Slaughterhouse Five) At 3pm on Saturday, 6 February 2016, Marc’s shoe is screwed. At 11am that day, Marc’s shoe is spotless. At 12.30pm that day, Marc is desperately trying to save his shoe. Sometime around 12 noon that day, I drop an egg on Marc’s shoe. Take these moments in Time. Even without knowing when each event happened, it shouldn’t be hard to figure out the correct sequence. You won’t be able to make out the finer details (Marc was at my place, I was cooking my legendary shakshouka, Marc’s shoes were new) but overall, you should have a reasonably good idea of what happened when. We all understand that things occur in a certain order. Cause precedes effect. A broken egg comes after a whole one. In general, a dirty shoe will start out as clean. This is our intuitive understanding of Time. It flows in a very specific direction – from the past, to the future. It never goes backwards. You can’t unbreak an egg. Yet all it takes is to stop and consider Time for a second to realise we’re totally clueless about how it works. Try this for example: when is Now? Is it this minute? This second? Microsecond? How long does Now last? When does one Now end and a new Now begin? Let’s say that Now only lasts for a super tiny period. The smallest unit of Time we can measure is called a Planck time (that’s 10–43 seconds – have fun working that one out). This is how long it takes for a single light particle (a photon) moving at the speed of light to cover a distance we call a Planck length. To give you an idea of how tiny a Planck length is, take a look at this dot: . Now, imagine you can shrink like Ant-Man. You keep getting smaller and smaller until relative to you, that dot looks as big as the whole observable universe. If that dot is scaled to look as big as the known universe, then to you, a Planck length would look about this big: . Kinda small, right? A Planck time is how long it takes for a light particle to travel that tiny, tiny distance. So let’s take this freeze frame and say it lasts for 1 Planck time. We’ll call that moment Now1: For Marc to experience Now1, the data from that moment needs to reach his brain and be processed. Let’s make it easy and say this happens as soon as the light from the egg reaches Marc’s eyes. As you’ve probably already figured out, this doesn’t quite work. Now1 is the Time it takes for light to travel a Planck length, so by the time the light from the egg has travelled just that tiny distance, a new Now has already happened. Now2. So Now2 happens before Marc even gets the chance to experience Now1. ‘Cause with his annoying 181cm, Marc is taller than a Planck length. An online converter tells me approximately how many Planck lengths are in a centimetre: That’s 7 with 32 zeros: 7 hundred million, trillion, trillion Planck lengths, in just one centimetre. And light will have to travel 181 times that to reach Marc’s eyeholes – for you nerds, that’s means Marc is about 126,700,000,000,000,000,000,000,000,000,000,000,000 Planck lengths tall. In other words, before Marc has even begun to process Now1 as the present, about one fucktillion other moments called Now have already happened. So does Marc actually experience Now1 in the present? Looks like what we all think of as Now is actually just a memory of something that happened a little earlier. Our experience of the present is an illusion. Which is really weird. Because it would mean that no matter how smart and sure of reality we think we are, we’re not actually capable of living in reality while reality is happening. And it gets weirder. Remember our Planck time freeze frame? Well, contrary to our perception, physics suggests that Time doesn’t flow but, instead, is still. If that’s true, there is no precise Now. Just a whole lot of individual freeze frames of different Nows, all equally real. Pretty crazy. Could that mean that somewhere out there, right now, that egg is frozen mid-air, just over Marc’s shoe, permanently on the verge of going splat? That’s mind-boggling. But is it actually true? And if so, what does that mean for Marc’s shoe? For all of us? We spent the last week obsessively learning everything we could about the science and philosophy of Time. 24 physics lectures and a shitload of reading and discussing later, we found out that our experience of Time is all wrong. It turns out that, in a sense, somewhere out there, Marc’s shoe is actually still fine, as pristine as the day he bought it. So I don’t really owe him £40. Here’s why. There’s No Such Thing As Now Marc and I just happen to be extremely geeky about this topic, but the truth is everyone thinks about Time. They talk about it a bunch too – the Oxford Dictionary published a study a couple of years back showing that time is actually the most used noun in the English language. But even though we use the word a lot, when it comes to Time we’re as evolved as the people who thought Atlas was literally lifting the world, or a child who thinks he was delivered by a stork. We talk about Time in supernatural terms, like it’s the Tooth Fairy or some other magical being. We say Time flies, like Superman and heals all wounds like Jesus (unless it’s terminal, in which case it just makes things worse – thanks Time). But the only thing it has in common with those two is that it depends on faith. You can’t be absolutely sure it exists. Most of us think of Time as something that moves forward, like a thin line through space, with every new moment being brought into being there and then. There was a past, there’s the present and there will be a future. Only the present feels real to us – the past doesn’t exist anymore, the future doesn’t exist yet. This, our natural understanding of Time, is called Presentism. It goes all the way back to Heraclitus and his belief that everything flows (panta rei): the universe is all about change. The passage of Time is a real thing and the world comes into existence moment to moment. Let’s see what that looks like. We draw a square and say that it contains everything that’s happening in the universe at one single moment in Time, which we call Now. Here’s how our Now Square looks: So only the present exists. Past and future? Not there. There’s a connected theory (called Possibilism) that says that both the present and the past are real, because they’ve both already come into existence. The future, though, hasn’t happened yet, so it’s still not there. In this model, the Now Square has a fixed trail behind it: Ok, this is pretty intuitive so far. Neat. Easy to grasp. Glad we got that sorted. Except we haven’t. Because our best understanding of physics suggests that this is not how Time works at all. For one big reason: ‘Cause Einstein says so. Here’s some explanation, but if you don’t want to get into the how and why, you can skim down to the takeaway point in orange, a few paragraphs below. Quitter. Einstein’s theory of special relativity tells us that there is no absolute Now that is the same for everyone – what we call the present doesn’t actually happen everywhere at once. So pretty much, he’s saying our idea of a single Now Square is bullshit. Huh? I’m nowhere near smart enough to explain this in under eight hours so I’ll have to simplify. Basically, there’s two things you need to know: 1) Einstein figured out that space and Time are not separate. Instead they’re part of the same continuum known as spacetime. Think of it as our universe having 3 dimensions of space (length, width, depth) and 1 of Time. Time is actually part of the stuff of the universe. Don’t worry if you can’t visualise it – it’s impossible, you’re a primitive 3-D creature and so is Marc. For now just keep in mind this weird concept of a 4-D spacetime. 2) Time ticks at different rates for different observers, depending on how they’re moving relative to one another. And here’s the kicker – even though their watches won’t agree on what the Time is, they’re both 100% correct. If you want a good (and quick!) explanation, check out this video. So take Marc and me. Marc is sitting right here but I’m reeeeeally far away. Like, in another galaxy. If we’re both still relative to each other, our watches will be in synch. My Now will be Marc’s Now. What happens next is simply bananas. If I start moving towards Marc, things get messy. As we said, Time will pass differently for each of us, relative to the other – my sense of Now will start to be different from Marc’s. What happens is, my Now begins to cut through spacetime at a different angle, meaning my perceived present will include stuff that to Marc would be the future. It’s crazy, I know. Also, hard to come to grips with. But you should know this isn’t just a thought experiment. It’s a real thing. Scientists now take it as fact. If you want to watch Prof. Brian Greene do a way better job than me of explaining this brain-melting concept, check out this video: So in our universe, Now is happening at different times relative to different objects. One observer’s Now is another observer’s past, or future. The reason this is Kind Of A Big Deal is it means there is no single Now Square, no moment that is Now for the entire universe. Shit. Time Is Just Another Place So. Nerd extraordinaire Einstein called us all cretins and showed us there’s no moment that is Now across the whole universe. Where do we go from here? Well, here’s what the scientific mainstream, Eternalist view of Time says: every moment that is, was or ever will be, exists. For us who are convinced physicists, the distinction between past, present, and future is only an illusion, however persistent. (Albert Einstein) What does that even mean? Well, think about places. This is a picture of my living room I just took: It’s right there, all around me. I’m in it. How about Fiji? I’ve never even been, but I know it’s there. It’s real. When it comes to places, whether it’s a few feet away or a million miles away, we still think they’re real. We might not be able to reach the ones that are really far away, but they’re still there. What Eternalism allows us to understand is that Time works that way too. Which means it looks quite different to the Presentism and Possibilism we started out looking at. Here it is: It’s all there. Every moment ever, equally real. There’s no Now Square. Just all of Time in one complete block. Remember Einstein’s description of 4-D spacetime? Well, to locate an event in spacetime, all you have to do is give three coordinates to indicate the location in space, and then the coordinate of Time, i.e. when it happened relative to you. It’s pretty much just another place. That’s huge. And also completely contrary to what we feel as a matter of experience. How do we even picture it? Maybe this is one way: Imagine you’re on the third floor of a dark office building on a rainy day. You look around and all you see is your third floor – here, there, and just over there. Well, you also kind of see the top bit of floor 2 and you get a little of that fourth-floor upskirt, but basically all you see is the third floor. But the fifth, sixth and all the other floors, they’re there too. They exist at the same Time. You just can’t see them. Just like all the other floors in a building you’re in, the past and the future are here, even if you can’t see them. This is also called the Block Universe theory, because of how it looks: It’s pretty crazy, but it’s also commonly accepted by scientists. Here’s Prof. Sean Carroll of Cal Tech on Eternalism: This is the view, in the eyes of many, that is straightforwardly suggested by our best understanding of the laws of physics, which don’t seem to play favorites among different moments of time. It’s a popular topic and there’s some great fiction written about it. Kurt Vonnegut’s amazing Slaughterhouse Five features a protagonist, Billy Pilgrim, who has come unstuck in time, his consciousness randomly bouncing back and forth between various moments of his life. He talks about these alien creatures, the Tralfamadorians who teach him about their 4-D vision. They can see all Time, anytime because it all exists now. To them: [. . .] when a person dies he only appears to die. He is still very much in the past, so it is very silly for people to cry at his funeral. All moments, past, present and future, always have existed, always will exist. This is an incredibly comforting thought. It’s nice to think that just because we’re not experiencing something, it doesn’t mean it’s not there. If the Tralfamadorians are correct, somewhere in spacetime, my childhood cats are still having their adorable kitten fights in the basket. Also, Marc’s shoe isn’t fucked. There is nothing special about the present moment except that you are experiencing it. (Sean Carroll, Mysteries of Modern Physics: Time) So exciting! Now that we know this, I guess we’re like the Tralfamadorians. We can see how permanent all the moments are and hop back and forth in Time, just visiting the moments that interest us. I can’t wait! Say goodbye to your privacy, Caesar. Except we know we can’t do that. So one final question remains: if all of Time exists in this one permanent block, why do we experience Time sequentially? It’s the Arrow of Time’s fault. Why The Past Appears To Happen Before The Future Let me just get this off my chest: I hate the Arrow of Time. It’s a total dick. Look. It’s even shaped like one: He’s the guy that means you can’t unbreak your mum’s vase, or un-drunk call your ex. Basically, he turns up, screws shit up for everyone, is an absolute hazard to eggs and glassware, and nobody’s sure who the hell invited him. Yes, ’cause to this day, scientists still aren’t 100% sure where he came from. What we do know is the Arrow of Time is the reason we experience Time in a sequential, cause-and-effect order. In other words, it’s what gives us the illusion that Time flows and that we progress through it one moment at a time. Before we get into it, we need to keep this in mind: the Arrow of Time is not Time itself. It’s a direction, and a feature of the stuff in the universe. Ok here we go: The direction of the Arrow of Time is given by increasing entropy. We’ll quickly get to the bottom of what entropy means. And again, you can jump down a few paragraphs to the takeaway point in orange if you prefer, wimpy. All things tend to move towards a state of disorder. Or, more precisely, in any closed system (e.g. our universe), the total “entropy” always tends to increase. That movement in the direction of high entropy is what we experience as Time ticking forward. So what is high entropy? Let’s take Marc’s totally clean shoe and the unbroken egg. We call these the shoe and egg’s macro-states. These macro-states are highly ordered and very precise. At the micro-level, there are very few particle configurations that can achieve those macro-states: clean shoe, whole egg. It’s just that clean and whole are such specific macro-states for those things to be in, that there are fewer ways they could be that way. This is what we call a “low-entropy” state. On the other hand, as disorder is more probable than order, there are many, many more ways for the shoe to be dirty and the egg to be broken. Anything from a single speck of dust landing on it, to being covered in mud would change the shoe’s macro-state. There are far more configurations of particles that would allow for “not-clean” and “not-whole”. We call this “high entropy”. Stuff “happens” in a way that goes from low entropy to high entropy, which gives the Arrow of Time its specific direction. The classic example is gas molecules spreading evenly in a container – this happens because there’s billions of ways you could rearrange the molecules and still get the “gas is evenly spread” macro-state. There are very few configurations for the gas to all be packed into just a square mm. So that’s it. It’s not that Time moves forward, it’s that stuff becomes disorganised. This is the direction the universe as a whole moves in. Not past to future but organised to messy. Entropy increases because it likes states where more particle configurations are possible. Ok, fine, you get that things move towards entropy. But if past and future are equally real and the flow of Time is an illusion, why is entropy specifically increasing towards the future? Why not towards the past? In other words, why does the Arrow of Time do this: …but not this? The answer is a thing called the Past Hypothesis. This basically says that at the very beginning of the universe and Time itself, right at the Big Bang, things came out in a state of extremely low entropy. With the Big Bang acting as a backstop (i.e. no before the Big Bang) from that point on, the only thing that entropy could do was increase. And the direction of that increase is what we call the future, all the way to the eventual heat death of the universe and the end of Time itself. The main thing to take away from all this is that the Arrow of Time is not Time itself, it’s just the direction in which stuff happens. That direction is given by the universe moving from low entropy to high entropy, and it’s what we experience as the passage of Time. So… So, yeah. That’s what we’ve been thinking about lately. We’re feeling ready for a long-ass sleep. Before we go, though: why does any of this matter? Well, first of all, it makes the idea of time travel seem way less ridiculous than we thought. If points in Time are a bit like another place, then the notion of how to get there is less insane. But that’s not the main point. What we get from this is that no matter what the Arrow of Time leads us to experience, in reality, every moment is its own Now and it’s all here, in a block of which we can only see one part of at a time. Perhaps each of those moments are frozen in Time, like the egg just above Marc’s shoe. We don’t particularly like that visual. It feels a little lonely to be stuck there, like that stupid egg, just twiddling your thumbs with nowhere to go. But then we think about what Billy Pilgrim said (above) about not crying at someone’s funeral – because he’s still alive in the past – and there’s something really quite powerful about that. It makes me think that somewhere out there, in this moment, there are loads of different versions of Me, having Great times, experiencing my fondest memories. And sure, there are other versions of Me out there too, in various states of discomfort or unhappiness. Being those Me in those moments…it’s a shit task, but someone’s got to do it. And now I actually don’t mind as much. Because no matter how bad a time we might be having, all the good times we’ve ever had, all the folks we’ve ever loved and ever will love, they’re all out there, right now. Literally. And you? All the Great moments you’ll experience in the future, they’re already happening too. Bet you can’t wait to be that you. Point is, somewhere in spacetime, Marc’s shoe is doing just fine.