Hi, hi, hi. Welcome, time crunchers. What's up? Where were we? Block universe. That's right. It's the model that describes everything in the universe. This is a Minkowski Spacetime light cone diagram. Kind of looks like an hourglass-- pretty funny. The hypersurface is the imaginary now, and that event is at the middle, where the hypersurface meets the light cone. Think of it like that flash of light that emitted from a flashlight or a star or whatever-- the cone is where we can assume causality, where we can assume that time mattered. Because light emitted can reach the location of that event on the hypersurface at that given time. That is the light cone. Can you imagine even knowing what any of this means even just a little bit ago? Can you imagine? If you are still watching this series, and you haven't subscribed yet, what are you doing? Think about all the stuff that you've learned so far. Amazing. So what about stuff that happens outside of the light cone? Nope. Doesn't matter, because it cannot influence the event. It's almost as if it doesn't exist. The speed of light is the limiting factor in the Minkowski light cone, obviously, because light cannot get there. Special thanks to Tio "Albie", and of course, Minkowski, for the approximation of doing all of this. It's a 4D thing in 2D space, which is pretty crazy to think about. But where do all the other Ds go when you do something like that? Anyway, I think my brain is broken right now. So let me just take a break. OK. Let's kick into it.

[intro music]

(Describer) Title: Uno Dos of Trace. Science Explainers from a Professional Nerd

Welcome to Uno Dos of Trace, everyone. Every month I pick a topic, and I dig deeply into it, breaking it into chunks so that we all understand it a bit better, myself included. And this month is time. This is episode four. So we've talked about what time is and isn't, the psychology of time. We've talked about how to measure time because it's based on observers. And we've even talked a little bit about the crazy bits of time. So now, I hope your brain is limber. Do some stretches or something, you know? Like, get up in there, because time is relative everywhere in the universe. Every bit of time is individual based on circumstance. But time is also relative to location and speed and mass of the observer and mass of nearby objects. But after all of this, there's a problem with talking about time, and that is that it's really hard. It might just be simpler to call it a dimension. So they invented the block universe. It's a way of talking about time as a dimension. Think of it this way. x, y, z-- those are all coordinates that help you find your way into a 3D universe. And then, t, a fourth dimension. Now we can move back and forth-- x, y and z, just like we can in a regular three-dimensional space. And then mathematically, we should be able to move back and forth in time as well. So why not just make it another coordinate? This comes from block universe theory, and it's like all time, all space, everything. Think of it like a big box, and the length of the box is the beginning of time to the end of time. And we are moving through the box. Everything that ever existed, and everything that ever will exist, is all inside of the block universe. And it is all describable in this four-dimensional model. And I say that again. Everything is in this atemporal block. And I say atemporal, or without time, because physicists are trying to use time as just a coordinate within this box. You get all that? Great. We experience time, and it feels real to us. But that doesn't mean that it's unfolding now. We could just be experiencing it that way. Like, imagine a supernova explodes a million light years away. That explosion happens today-- right now. Now imagine that the Klingons and humans both exist. And the first humans to see it, see it about a million years after it happens. And they write down, today I saw a supernova. Then 112 years later, it gets to Klingon. And the Klingon astronomers are all like [non-English speech].

(Describer) Subtitle: Today a star explode.

Because Earth is like 112 years from Cronos by light year followings. Everybody knows this already. So they mark that supernova is happening today in two different time frames. When did it actually happen? When was the real now? As we learned from Sarafina months ago, cosmologists use observation dates as opposed to original dates based on the speed of light, because before then it couldn't have affected us. No one would know it happened. It wasn't in our Minkowski light cone. If you can't plan for it, and you can't revise all of history every time we see a new thing, it's easier to just imagine now is whenever you saw it. Now imagine two supernovas. Humans see the first one first. Klingons see the second one first. Then we both see the second 122 years later than the other. And then we have to say, oh, yeah. There was that year there were two supernovas. There was one, then another. And the Klingons are like, yeah. There was one and then there was another. And then we're like, yeah, but the one was the one, and the one was the other one, and it was really confusing. And now you have to do that with literally everything that's ever happened. Ugh. The block universe lets physicists say that the past and future are relative to us, and all other points do not matter, ignoring that time is based on the observer, and instead, that all time is just there. Special relativity works better within a block universe, because there is a way to describe time that fits with relativity. And that is every event has its own light cone, just like Hawking was saying. The light cone coming in, event hypersurfaces now, and then the past, or where the light is going after you. And just like Hawking was saying, all of those cones would be there in a block universe. Literally, every light cone would be there already. We don't have to worry about calculating it as things change. It's all calculable. All times are known within the block universe, which is confusing, but also kind of makes sense. It's just hard to imagine, because how do you make choices then? How do you decide to turn left or right if all time is known and understood already? If the block universe is how the universe actually is, and based on the theory of relativity-- special relativity, I mean-- it could be, do we have free will? Do we? If the future is already there, and we're just moving through this universe that exists, experiencing the arrow of time as it happens in something that's already there, like a system of tunnels, like a hamster cage. Think of it this way. I freely time travel to the past. I take my mom to a school dance. I tell someone who wins a truly arbitrary sporting event, and then I change the future. The block universe already accounted for all of that. You didn't change anything. You existed before you left, and then you moved into what was your future, very similarly to what you would find in Avengers Endgame. There's no such thing as changing the past. Everything already existed. You just moved into the next phase of your timeline, and it's just yours because you are the center of your own block universe. I think, commonly, we think of us moving through time. We are on a boat on a river. Time is the river. But it's more accurate to think of time moving through you. You're not a boat on a river. You're a loop, and time is the rope. Or you're a tunnel, and time is the river flowing through you, and you're experiencing it that way. You're not a car shining your light cones into the future and the past. Instead, all the decisions you've ever made have already happened. And this was argued by Carl Hoefer. He studies philosophy of science and philosophy of physics at Stanford, and he argues that everything has happened altogether already inside of the block universe, which means it's already happened. Whereas, Soler Gil and Alfonseca write their support in Philosophia, saying, quote, "Every free decision "codetermines or entails both past and future states of the universe." What they say is that free will isn't really a thing because of inconsistencies not being allowed within the block universe. Hoefer says the block universes don't care about any particular event as determinant of other events. But it's also argued by Brennan in Philosophia that we're too small to affect anything in the universe anyway.

[sighs]

This is all confusing, isn't it? Because it's not really about time anymore. It's about how we think about time. Part of how we feel about time is we do something, and that causes repercussions, like a rock in a puddle. It causes ripples, and those ripples affect the timeline. But perhaps it's causation that's the issue, not free will. Maybe that was already going to happen. Those ripples aren't somehow shocking to the molecules that they're running through. And instead, that's just part of the block universe. Either way, no one knows. More research is needed. This is where we run up against the edge of what we think is real. Physicists don't think all the verse is absolutely a block universe. I just want to be clear about that. They don't think that we live inside of a block universe where everything has happened already. It is just one simplified model of the universe that we have now. And it doesn't apply to everything. Because what if the block universe doesn't work? What if there is more to the universe? When Newton came up with the laws of motion, it worked for coffee mugs, but it didn't work for stars. When Einstein came up with special relativity, it works for stars, but it doesn't work for quantum mechanics. So there's still lots to learn. Let's say the Second Law of Thermodynamics,

(Describer) 2nd Law of Thermodynamics

which increases disorder, chaos, and entropy, does not equal the arrow of time. So as of now, we know that the Second Law of Thermodynamics

(Describer) More flames glow.

is causing the arrow of time, at least as we understand it. It's an increase in disorder, chaos, and entropy. And we are measuring that with our perceptions and our changes in the universe that we see over time. But that's not the only thing that could cause time. That's just one hypothesis, one theory based on everything we know. There's also the evolving block universe. There's gravity theory, just to name a few. There's the causal set theory. The evolving block universe is essentially the same thing we've been talking about-- the block universe. But because space time is expanding and universe inflation is a thing, the universe's future and present are ever-changing, like a progress bar. We live on the cutting edge of this never-ending loading screen. The expansion is moving into the future only. And everything else, we already know. It's already there. It's working normally, as you would think and perceive, per the standard models of physics that we understand, except causality. And maybe quantum uncertainty explains that because special relativity can explain quantum mechanics. More quantum research is needed. There's gravity theory, which looks like the Second Law of Thermodynamics hypothesis

(Describer) Fire burns.

where gravity causes things to coalesce and drives time. In a computer model experiment of 1,000 particles in an imaginary universe, with only Newton's gravity to play with, it made an interesting model of the universe that looks a lot like the one we inhabit. And there was maximum density with minimum complexity. And it caused a localized flow of time. And that's something to think about. Because we know time is only working for the observer, perhaps it also moves differently elsewhere. Like, time within our solar system behaves this way, and time in that galaxy behaves differently. More gravity research is needed to know if any of that matters. Then there's the causal set theory, which is that space-time is fixed, defined. It is set. It is not continuous. And it appears continuous on a macro level where we live, but on a quantum level-- nah, bro. And instead, at the smallest scales, the universe would be like little space-time atoms, like little teeny chunks like-- I don't know-- like Minecraft, but for the universe. And we're just experiencing it because we're so big, unlike atoms, energy, and quantum particles, which don't experience time at all. New space-time atoms would be consistently coming into existence using the causal set theory, like lava spreading the ocean floor a few centimeters at a time. And new space-time atoms would be driving the arrow of time, making time a feature of the cosmos, not a bug or a fluke. Now, I want to point out that these are all hypotheses. They're just ideas of how the universe could work, assuming that the second law of thermodynamics isn't the only one.

(Describer) Flames flicker.

And I called the cosmologist astrophysicist for this series, and she was like, I haven't heard of these, but OK. The block universe is the generally accepted model. And most physicists believe we need physical evidence of time in order to confirm it. But for that, we need to look to quantum physics, the science of very, very, very small things. Let's say you wanted to subdivide time as small as possible, right? You got nanoseconds. You got picoseconds. You got zeptoseconds, which is 8.5 times 10 to the negative 19th. That's the smallest thing we can measure. But there are still smaller things, like yoctoseconds, seconds, which is 10 to the negative 24th. So how small could things get? Well, in the 1890s, Max Planck came up with the Planck scale. And after that, physicists who work in relativity throw their hands up in the air, and feverishly go play Animal Crossing to [inaudible].. Because Max Planck basically defined the lower limit of the universe, where the relativity rules meet the quantum rules. Relativity has time. Quantum-- no time. Relativity has time. Quantum-- no time. Relativity, quantum, relativity, quantum-- One sec. Band name idea-- Relatively Quantum. Cool. Anyway, Planck came up with this idea of something very small being the smallest thing in the universe. Essentially, it's 1.6 times 10 to the negative 35 meters. It's very, very, very small. Planck time would be Planck length divided by the speed of light, or 5.4 times 10 to the negative 44 seconds-- an indescribable, unfathomably short amount of time. Planck time is defined also as when the universe was one Planck length across, and the energy was at Planck energy. It's weird to think about, but it would be something so tiny and so early after the big bang's event that it'd be kind of tough to imagine. The simplest way I can put it is that a Planck unit describes the smallest chunks that we can break the universe into as far as we know now. Again, think of it like the universe made of Minecraft blocks, but each block is Planck scale. Quantum scale-- there's no time at all, just to reiterate. So that's it. Newton's static universe-- time explains medium-sized things like apples. Einstein-- no static universe. Time connected to space explains weird big things like planets and black holes and galaxies. Quantum-- no static anything. No time at all. No space, because space is questionable and explains very small things, but not very big things, and so explains particles and little Planck chunks. So at the end of all this, perhaps time is only relevant to big things, like putting ice in a drink. You don't really need ice for a teeny tiny bottle. But for a big one or a giant one, you need to put ice in it. Well, some people do. You can debate how much ice, kind of ice is the best way. But small stuff doesn't need it. Doesn't matter. Maybe that's what time is like, is that no matter how you slice time, and we do slice it, time is going to end. The universe will end. Everything will end. Finito. Next time. I love Marvel movies. Their science fiction is-- nothing is perfect, but it's super fun. So an example would be the Ant-Man and the Wasp introduced quantum time distortion. And as I mentioned before, time on the Planck scale is questionable. We don't really know if time exists there at all. Maybe you got that streaming service where you can watch all of those Marvel movies. And if you did that already like I did, because it's been a pandemic, maybe you're looking for something else to do. I think you should try Nebula. You can watch nerdy people like me talk about Marvel movies, or learn about the physics of Marvel movies, or actually, the physics of everything. We've got metaphysics out there. You can look to think like a lawyer, or you can learn about music with Polyphonic. Nebula is a streaming service that I helped start with some of the best educationalist creators out on the internet. We put our content on Nebula, too. But the difference is it's pretty much without ads. And by joining, you support us directly. No worrying about algorithms or subscriber counts or click-through rates-- just awesome creators, all in one place. Simple. Because we own the service, we also get to mess around and do cool original programs like Title Sequences or The History of Synths, or reaction shows, or whatever we want to do. And we're, of course, always taking suggestions. We even got an original with Tom Scott. You'll have to join Nebula in order to see it. To join Nebula is easy. Just join Curiosity Stream, and you're going to get Nebula for free. I know that might sound complicated, but it's actually way simpler. See, Curiosity Stream loves Nebula. They love it so much that we were like, yo, if you want to do this, let's bundle, and give us a better deal than you give anyone else. And they said, OK. So Curiosity Stream offered us 26% off all of their annual plans, and we fact checked it. It is the best deal anywhere. So for less than $15 a year, about $1 a month, you get both CS and Nebula, and you're supporting creators you love. And you can listen to David Attenborough or Chris Hadfield or Derek Muller in these big-budget documentaries, and then go and watch Nebula, and get Alex and me and Vanessa and Jade and all these other amazing creators. It's 26% off of a year of both services. And you can feel good, because clicking that link directly helps me keep this channel going. On top of that, I know this is episode four-- we've only got one episode left. But you could have been done already if you had Nebula. Because on day one of the very first episode of "Uno Dos of Trace," I put them all on Nebula exclusively until they've all aired here on YouTube. You can see all five of the episodes of "Uno Dos of Trace" on Nebula from day one. All you gotta do is join with my offer code, and you could be watching them already. But let's go back so we can get to episode five, all right? Quantum stuff is so hard, y'all. We don't know, because we can't see things that small. And we don't know because we can't see things past a certain distance in the universe, and thus, back in time because of the universal speed limit of light. So what else can we learn about time? We've hit our edge. We can't learn any more than what we can see, and we can't learn any more than what we can see. So we know what time is a bit, but we also know that it ends. For the last episode of this series, I call Dr. Katie Mack, a theoretical astrophysicist who talks about exactly that. Thank you. Please subscribe. Grab a cold drink and put it against your forehead because, trust me, it helps. And I will see you in the future.