What makes time so hard to understand? Why is it so different from the spatial dimensions? Is time in any way connected to that most difficult of concepts, entropy? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Let me start with this. We don't understand time. We don't. We don't. We don't.
It's a mystery. So this episode is probably gonna be, wait for it, a waste of time because I'm about to do a whole lot of explaining but I promise at the end of it there will be more questions than answers. We do know some things about time and I've talked about those in other episodes like the fact that space and time appear to be linked together, woven together in a fabric of space time. It looks like travel into the past is forbidden, and it looks like our entire universe is changing with time. And lots of you have asked about time, like Michael m on Facebook, at Matt Jacobowski on Twitter, j m Autobahn on Patreon, David m on YouTube, Adam h on email, p e on email, Adiator d on email, Mihail e on email, at Emilio Mira on Twitter, and Rowan h over on Facebook, all asking about time.
All sorts of very cool questions about time. And this particular episode will only be answering a small subset of those questions and not satisfying any of you at all. And in fact, to be perfectly honest, almost this entire episode will not be about time, something you are asking about a lot, but instead be about entropy, something that two of you have asked about. But I'm sorry. This is the way it is.
You asked me about time, you're gonna get an episode about entropy because as we're about to get really thick into the weeds here very soon. And when it comes to explaining what time is, the only idea we've had for over a hundred years is that it's somehow sort of maybe connected to entropy. We think. That's the best we got. So let's begin.
The weirdest thing about time is that it has a direction. We call this the arrow of time. Contrast this with the spatial dimensions. Right? I can go left or right as easy as each other.
I can go forward as easily as I can go backwards. I can go up as easily as I can go down. You know, in general ignoring things like gravity, but just out in space, I can do whatever I want. I can go in whatever direction I please, but I can't avoid going into the future. I can't choose my direction in time like I can choose my directions in space.
It's not just that travel into the past appears to be impossible, but it looks like travel into the future is inevitable. You can't escape the future. You must go into the future. Why? Why?
Why? I literally wrote the word why three times in my notes just for emphasis. That could be the entire content contents of this episode. Just me saying why, why, why repeatedly for a half hour. Why does time have an error?
Why do we go into the future? We can affect how quickly we go into the future. That's legit, but we can't stop and we can't turn around. Why? Could it be just a trick of our brains?
Our brains do lots of little tricks like that. That in order for our conscious minds to like make sense of the world around us, it structs a constantly evolving narrative. And is this thing that we call time really just an illusion? Well, maybe. That sounds weird and wrong, but nature does a lot of weird and wrong stuff and doesn't really care about, you know, how we feel about it.
So that excuse isn't gonna fly, but I'm not gonna dig into that concept too much because this is a show about physics, and I'm gonna give you a physics answer. Plus, as far as we can tell, this arrow of time business appears to be an artifact of the natural world and not something our brains are constructing on the fly. But, hey, it's our brains that are telling us all this, so take that with a grain of salt. But like I said, this is a physics show, you get a physics answer. We're gonna assume for the sake of the next half hour or so that the arrow of time is physical.
It's a it's a thing. You you can't touch it, but it's a thing. It is is it exists. It's a part of our universe the same way. Gravity or electrons or black holes are a part of our universe.
And because it's a thing, because it's a thing, it demands explanation because physics pretends to be able to explain the existence of things. So let's look around. Are there any any any any physical laws or processes or concepts that naturally give us an arrow of time? Is there any equation we can point to and say, yeah, you see this here, you see this right over here, your equation 5.71 over here, right there? Because this physics equation is true or we have evidence to believe that this physics equation is a good model for reality, then time has an arrow.
So is there anything like that in all? In all the branches of physics that we've been studying for hundreds of years, does an arrow of time come about naturally anywhere? No, not really. Quantum mechanics? No.
Electrodynamics? Sorry. General Relativity? Got nothing. Particle physics?
Yes, kind of there's one tiny interaction involving the weak force where there is an asymmetry in time, but it hardly ever happens and never never does anything interesting ever. And the weak force is really lame and it really deserves its own podcast episodes to just ask me about it, but we don't think it's there. We don't think it's out. I'm saving that for a different episode. Just ask me about the weak nuclear force.
Almost all the equations that govern the physics of our universe don't care about time. Don't get me wrong, time appears in the equations. There's a variable usually written as t, stands for time, but the equations are perfectly happy going forwards and backwards. You just adjust the dial like you you take this t parameter, the time in the equations and you can make it run forward. You can make it go backwards.
You can make it smaller. You can make it go bigger and you can see how your system evolves, but the equations themselves don't care about the direction of time. Just like the equations, just as easily, and I'm talking about like the fundamental equations of of reality here, like the core equations of physics, you could just as easily go left or right in space. When it comes to the equations, they can just as easily go forward and backward in time. Imagine imagine looking at microscopic particle interactions where you're seeing a bunch of tiny particles bouncing against each other doing all sorts of particle y things.
Would you be able to tell? Would you be able to tell if that process was running backwards or forwards? You would see some particles approach each other, hit each other, ricochet, bounce off, go in a new direction, but if you ran in reverse, it would look exactly the same to you. Or imagine a ball being tossed in the air, like you see that it's in someone's hand, the hand rises, the let's go, the ball goes up, reaches, an apex, comes back down, touches the hand, the hand closes around it, and then returns. Okay.
Would you be able to tell if I just gave you that slice and nothing else, nothing else, if I just looked at the bare process of a ball being tossed inside of gravitational field and then being caught, would you be able to tell if that movie was being played forward or backwards? It'd look exactly the same. But this idea, this fact that almost all the fundamental equations that govern our universe do not care about time totally goes against all of our experience with the natural world because our experience of the natural world is that there is an arrow of time. Everything cares about past to future. Everything does it.
Everybody does it. It's immediately obvious when a movie is played backwards. I mean, like, you just know it in your gut. You can say that movie is being played backwards. But think about that.
Think about that. It's immediately obvious that most interactions, things that we do here in the universe are not reversible. They follow an arrow. It's easy to break an egg. It's harder to put it back together.
You can tell when a movie is playing the correct sequence of time because you'll see things like eggs being broken, if you were to see eggs reassembling themselves, you would immediately know that this movie is being played backwards. But if you zoomed in at the microscopic interactions, the tiny particles and subatomic particles wiggling and jiggling and bouncing and doing all their, you know, little particle y things, you couldn't tell if it was being backwards or forwards. The fundamental interactions at a subatomic level are totally reversible, don't care about time at all, but the macroscopic world does care. Something super stinky fishy is going on here, folks. The microscopic world of particles and forces doesn't care about time at all, but the macroscopic world of car engines and baseball games and puppies and kale salads do care about time.
There's a transition from not caring about time to caring about time when systems go from small to big. There is an arrow of time in our macroscopic world. There is no arrow of time in the microscopic world. That's a clue maybe. Perhaps with a capital perhaps, the answer to the arrow of time is something that links the microscopic and the macroscopic worlds.
If we're seeing this transition, if we're seeing that the microscopic world doesn't care about time and the macroscopic world does care about time, if microscopic processes are totally reversible, but macroscopic processes are not reversible, if there's a bridge between those worlds, if there's a set of physical laws or equations or vocabulary we'll settle for vocabulary here that take us from the microscopic to the macroscopic maybe there lies a potential answer to an explanation of why there's an arrow of time and you know what there are a set of physical laws that connect the macroscopic and the microscopic worlds it's the laws of thermodynamics you know temperature, pressure, work, heat, volume, PV equals nRT, all that stuff you learned in high school and then immediately forgot as soon as the test was over. These concepts were developed in the early and mid eighteen hundreds mostly to understand these new vengled steam engines, and they were pretty much made up. All these laws and equations and understanding were pretty much made up on the spot to describe results from experiments. That's a lovely story for another day just as the development of thermodynamics. And we had all these relationships, you know, this concept of temperature, this concept of work or volume in the relationships between them and they were begging for an explanation like what is temperature?
What is pressure? You can measure temperature. You can measure pressure, but what does it mean? What is it doing? And after decades of laborious research here, the connection was made between these macroscopic concepts like I have a cylinder of gas and it pushes on a piston.
That is macroscopic. There is a connection made between these macroscopic concepts, things that we can measure with thermometers and pressure valves to microscopic motions of particles. For example, temperature. It was realized that temperature is a connection. It's a macroscopic thing.
Stick a thermometer in there. You can measure the temperature. What you're really measuring is the motions of the subatomic particles themselves, and you're measuring their mean velocity, their average velocity. They're jiggling around. They're doing their things, and a higher temperature means these microscopic particles are moving around even faster.
They have more energy. They're really hitting each other. They're hitting your thermometer probe with more intensity. They're depositing more energy that will cause your thermometer probe to register a higher temperature. If you lower the temperature, then these particles aren't hitting each other as often.
They're not moving around as quickly. They don't have as much energy and so your thermometer will read a lower temperature. This is a macroscopic thing and I'm saying this a million times because this is like the key to the whole thing. This is a macroscopic measurement, a macroscopic property, temperature, pressure, volume, that tells us something about the microscopic world. And this is a whole field of physics.
It's not just a couple equations here and there. This is an entire industry that had already solved by the mid 1800s had already solved the challenge of connecting microscopic motions to macroscopic everyday behaviors and one of these connections already had a built in arrow. This connection, this property is called entropy. Entropy is, and I'm speaking incredibly loosely here because I'm about to get bogged down in a hilariously over extended metaphor, so I don't want to get bogged down here trying to provide completely accurate definition. Entropy is a measure of the amount of disorder in a system.
You have more disorder in a system, you have more entropy. And one of the laws of thermodynamics, one of these observations we made about how the world works when we were studying steam engines in the mid and early eighteen hundreds before we understood the microscopic interactions that was leading to all this. We had all these macroscopic measurements. Some of these measurements and rules got written down in the form of laws. A law, by the way, is something we see so much of in nature that happens all the time, and this second law of thermodynamics says that entropy goes up.
Entropy goes up, which means disorder in systems goes up. Technically, it only applies to closed systems where there's no inputs of energy, like air locked in a room or gas in a cylinder. In these closed systems, they must, must with a capital must go from order to disorder. Their entropy must increase. And even in the case of open systems where say you've got the sun shining down here on the earth, you can get lowering of entropy here on the earth.
You can get more organized systems. We tend to call that life, but at the expense of even more disorder over in the sun. So everything balances out. So if you look at a big enough picture, entropy is always going up. Disorder is always going up.
Entropy must increase with time. I'll say it again slowly because this one little statement is what can potentially make this whole thing work. Entropy must increase with time. Individual particles do whatever individual particles do. They bounce around crazily, mostly, following their little laws of physics that don't care about time.
Their little tiny interactions are totally reversible. But when we zoom out, when we look at a box of particles instead of individual ones and measure their entropy, time matters. The individual interactions may be reversible, but taken together the evolution of the system becomes irreversible. And here is an example of it happening that we already knew about, that we already have a handle on in a mathematical description. Microscopic particle interactions are totally reversible.
The macroscopic property of a system of particles known as entropy is not reversible. It does march forward right there, right there. It's like a party. You know, each individual interaction is totally reversible. John could walk towards Wendy.
John could walk away from Wendy. Tabitha can talk to Richard. Richard can talk to Tabitha. Each individual interaction is totally reversible. Doesn't care about time.
And if you looked at each individual interaction between any two people, just that snapshot, it would look totally reversible. But somehow, despite this, the whole party still has a beginning, middle, and end that cannot go backwards. Trust me. I know that this is a giant lump of physics knowledge that's hard to digest. That's why I'm gonna I'm gonna sink into this concept a little bit longer.
We're gonna we're gonna be right here. We're gonna be right here together because this is a tough thing. I get it. It's a tough thing, but this is, like, the thing. Individual particle interactions are totally reversible and don't care about time.
Macroscopically, the macroscopically, the system evolves with time. We have a way of measuring that. We have a way of quantifying that. We have a way of predicting that, and that is through this measure of entropy. Entropy measures it.
Entropy has an arrow. Entropy and time appear to be interlinked. You can't avoid entropy rising in a system. You can't avoid your future. You can slow down how quickly your entropy rises, but you can't stop it and you can't turn it around.
You can slow down how quickly you move into the future, but you can't slow down. You can't stop. You can affect how quickly you move into the future. You can slow down, but you can't stop and you can't turn around. Your entropy can't decrease.
You can't go into the past. Is this a coincidence? Is this a conspiracy? We don't know, so let's talk about entropy some more so we don't have to answer the big frightening questions. Entropy is everywhere.
It's a magical word that explains the why of almost every single physical scenario. I am not joking here. I'm not joking. Why does a hot cup of coffee cool down? Because the entropy increases.
It must go to more disorder. Why does a ball roll down a hill but eventually come to a stop at the bottom? Because the entropy increases. There's more disorder. Why will the sun burn out someday?
Because Patreon increases with time. Go to patreon.com/pmsutter to learn how you can support this show and all of my education and outreach activities. I greatly appreciate it. Also, the entropy increases leading to more disorder. Why do ripples in a pond eventually go away?
Because the entropy increases leading to more disorder. But if just about everything can be explained by, oh, you know, the entropy is increasing, why does entropy increase? Why? Why must systems go to more and more entropy? Good question.
I'm glad you asked. Like everything else in this wonderful world of thermodynamics, entropy, and I'm doing air quotes here, it's just a word. It's just a word that summarizes what's going on in the microscopic world. Remember, we have this macroscopic observable quantity that we can measure with things like thermometers and pressure valves. One of these quantities we call entropy, but really it's a summary of what's going on that we of things we can't measure at the microscopic level.
So let's say we have a box of particles like a gas, and they do whatever particles in a box do, which is bounce around. Right? We can't keep track of all the motions of all the individual particles. That's way too much work, but we have some handy devices for describing the box as a whole. If you're feeling fancy today in the physics jargon, this is known as the ensemble.
One of the devices is entropy. It's a way of measuring the disorder of a system and we note that in nature the disorder always goes up. But how do you measure disorder? How do you assign a number to it? You do it by counting.
Let's go back to our box and let's say we measure its temperature and pressure and volume and all that other high school stuff and we write it all down. This box has a temperature of 50 Celsius. Okay. A pressure of such and such Pascals. Okay.
Whatever. We write it down. Freeze frame that box. Freeze frame it. You know, it's filled with gas, filled with air.
We're gonna freeze frame it. We're gonna zoom in with a microscope. Zoom in enough that we can see all the little particles, all the little air molecules suspended on their way. Some of them are mid collision. Some are really far apart.
You know, it's just you freeze framing this. That's what we call the state of the system, the exact snapshot in that moment. What if we picked two particles, two of them, one over here and one over there, completely at random and swapped them. Just just, whoop, just plugged in and zip zip. We you know, before anyone noticed, we swapped the positions of two of those particles.
Well, would anyone notice? Those two particles are identical. An electron is electron. Helium m is a helium atom. Carbon dioxide molecule is a carbon dioxide molecule.
If you swapped their positions, it wouldn't make a difference. The temperature would be exactly the same as it was before. The pressure would be exactly the same as it was before. The volume would be exactly the same as what it was before. So really there are two states that lead to the exact same measured thing.
There are two microscopic ways of arranging the system that lead to the exact same macroscopic observation. Well, I lied. There's more than two. Imagine counting every single possible way of swapping the particles around. And once you realize that there are more air molecules in a lung lung full of air than there are stars in the observable universe, well, you're gonna be counting for a while.
Entropy is a way to measure how many different microscopic arrangements of a system lead to the same macroscopic measurements. Okay. That's nice that we've been able to put a measure on it. There's some number that we can associate with that. We do that by counting.
That's nice and nifty even. Why must it increase? How many of you have kids? Go ahead and raise your hand even though I can't see you. Okay.
Good. For those of you without kids, are you generally aware of the existence of children? Good. I want you to think of a kid. This could be a kid you know.
It could be your own kid. It could just be a hypothetical child. Doesn't matter. This kid has a room. This kid's room is probably messy.
You decide to clean the room, and I mean 100% spotless and organized. A place for everything and everything in its place. This is the most possible organized and clean room you can make. We're gonna measure the entropy of this room. Now that you've perfectly cleaned it, we're gonna go and we're gonna measure the entropy.
How many states, how many ways of arranging the room are there that lead to a perfectly clean and organized setup? There's one. There is exactly one. A place for everything and everything in its place. There is only one way for you to be satisfied that this room is clean to your standards.
Let's change something. Let's take a dirty sock and put it on the bed. Is the room still clean? No. How many ways are there for the room to be messy and disorganized?
Well, that sock can be on the bed. There's one way. It could be on the dresser. There that's another way. It can be on the floor.
That's that's another way. It could be hanging off the ceiling fan. That's another way. It could be it could be under the rug. Okay.
So there's there's five ways. There's five ways already for this room to be disorganized, so there's room to be messy, and I've only started with one sock, one dirty sock. And already, I was able to count five ways for the room to be messy compared to the one way for the room to be clean. What if there are two dirty socks out? Imagine all the places those dirty socks can go and make the room dirty.
10 ways, a hundred ways a thousand ways for the room to be messy with just two socks now really blow up your thinking There is one and only one way for the room to be perfectly clean and organized How many different ways are there for it to be not clean A million? A billion different ways? A hundred trillion different ways? Think of all the possible ways to make a room messy and you have to count all of them. Now go back to your perfectly clean room.
That clean and organized room has an incredibly low entropy. One. Just one. There's just one way to have the clean room, and so that's what we're gonna call its entropy. One.
Put the kid in the room. Close the door. Tell him to have fun. Wait fifteen minutes. Open the door.
What do you think you're gonna find? You know, because you've been here before, that you're gonna find a messy room. But why? Because even though it's possible for you to encounter a clean room after the kid has been a kid inside the room, there's only one way for it to be clean. Exactly one way.
And there's a huge number of ways for it to be messy. So with the kid in the room doing random kid things, you're simply more likely to find a messy disorganized room because there's only one shot, one shot for it to be clean, but lots of ways for it to be messy. And I'll repeat this because this is another key to the whole thing. It's not impossible. It's not impossible for the kid who's randomly doing stuff in the room to leave it in a clean and organized state, but it's overwhelmingly more likely to be in a messy state just by having more options for messiness than cleanliness.
The entropy of the room will increase. The entropy of the room will increase because there's more ways for the room to be messy than there are ways for the room to be clean. Here's another example. There is nothing, absolutely nothing stopping all the air in the room you're you're in right now from spontaneously collecting itself into a corner and leaving you in a vacuum. There is no law preventing that.
All the motions of all the particles surrounding you right now could just line up perfectly and put themselves in a corner and you would asphyxiate. But that's a low entropy state. There aren't a lot of ways for the air in the room to be compressed down in a corner. Compared to the ridiculous number of ways that the air can be spread all across the room, It's much more likely to find itself in a high entropy state because messy is more prolific than clean. It's easier to be messy than clean because you have more options.
And if all these air molecules are bouncing around each other, off each other randomly, they're gonna pick the messy way, the high entropy way, because there's so many more ways to be messy, to be high entropy than to be low entropy. The numbers here, the ways of counting the individual particle arrangements, all the possible microscopic states get way huge way fast. I mean, I can't even tell you how quickly the numbers add up. So it's not just like high entropy is a little bit more likely than low entropy, that messy is a little bit more likely than clean. Messy is overwhelmingly more likely than clean.
Ridiculously more likely than organized. High entropy dominates low entropy so much so that we only ever see entropy go up. Even though by random fluke it can go down, it is so unlikely you would have to wait multiple lifetimes of the universe just for one tiny little microscopic fluctuation to get just slightly lower entropy. Entropy always goes up. Entropy goes up across the universe from system to system through sheer force of statistics.
That is the cause behind the second law of thermodynamics. It's just numbers. It's a number. It's a it's a process. Remember, I'll say it again.
These individual particles don't care about time. Every individual interaction between air molecules in the room you're seeing right now is totally reversible and yet and yet the entropy of the system will go up. A macroscopic behavior that is irreversible caused by a bunch of reversible microscopic processes entropy must go up Disorder must increase. So is that our arrow of time? Entropy converts.
It does the job of converting random reversible motions of microscopic particles into an irreversible macroscopic process in microscopic process in nature. We know from our everyday experience this intuition is baked into our bones that disorder goes up with time. And entropy is the mathematical language that describes and explains all of it. So do we have an arrow of time? Do we hurdle inevitably into our futures because entropy always goes up?
Well, entropy is statistical. It's done by counting. Does our flow of time only exist on average? Is time travel into the past allowable, but just unlikely because of the overwhelming odds against it? But we got nothing else.
Right? We have absolutely nothing else. We have no other mathematical model, theory, set of equations, laws, whatever that naturally connects to time. Entropy is it. That is our best answer, but this leads to a big question.
If entropy always increases and increasing entropy is responsible, is causing the arrow of time, well, that means thirteen point eight billion years ago when our universe was very, very young, it was in a very, very exceedingly low entropy state. It had to be low way back when for it to increase with time. So in other words, who or what cleaned our room? Thank you so much for listening. You can go to askaspaceman.com for show notes and links to all the episodes.
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