- [Voiceover] The Second
Law of Thermodynamics, one statement of it is that the entropy of the
universe only increases. And, I put an exclamation mark here, because it seems like a
very profound statement. And, on a lot of levels, it is. And, just to get us into
the right frame of mind, I have this image here
from the Hubble telescope of the night sky. And, each of these dots, these are not stars. These are galaxies. That's a galaxy. That's a galaxy there. That's a galaxy. So, hopefully this gets
you into little bit more of a cosmological scale. But, let's think about what
this is actually telling us. The entropy of the
universe only increases. So, entropy, we can define
that as the disorder of a system. And, we're really talking about the number of states that a system could take on. And then, we're saying the universe. But, we could also say the entropy of a closed system only increases. A system that is fully contained, that's not interacting
with its surroundings, because the universe is
the ultimate closed system. There's nothing for it to,
outside of it to interact with thermodynamically. And, I'll do a quick review
of open and closed systems, just so we make sure we understand that. So, if I had a campfire, so I have some logs and I had my, the flame going right over here. So, that's the campfire. If I were to just look
at the logs and the fire, that's going to be an open system. Because, it's clearly
interacting thermodynamically with its surroundings. It's releasing heat. It's warming up the air
molecules around it. It's releasing light
out into the universe. There could be interactions from the rest of the universe into the system. So, it isn't isolated from
the rest of everything else. But, a closed system, it is isolated. And, there are, it's very hard to create a true closed
system in our everyday life. But, we can approximate it. And, the one that you've
probably experienced in the not too distant
past is an ice cooler. And, an ice cooler,
we're at least attempting to thermodynamically isolate, isolate the inside of the cooler from the outside, from
the rest of the universe. So, this is, and the way we
do it is we have some type of an insulating material. Maybe some styrofoam. And, we could put, you know, we'd use it to maybe store ice. But, it's not a perfect closed system, because eventually, the heat from the rest of the universe will warm
up the walls of the cooler. And eventually, that heat will warm up, will be transferred to the ice, and it will warm it up. And, it will melt it. So, it's not a perfect closed system, but it's a good approximation, because we're at least attempting to isolate it thermodynamically from the rest of the universe. And, I can even make
a little cover of this to show that we really
wanted to isolate it. And, in research labs, you'll see things that are
much better approximations of closed systems. But, even those at some level
are, they're going to interact with the rest of the universe. The ultimate closed system, so this is a closed system, is really the universe. Nothing to interact with
outside of it thermodynamically. So, let's think a little
bit about this definition. The entropy of the
universe only increases. Why does this make intuitive sets? Well, the best example I can think of is just straight up diffusion. So, if I were to have, let's
say I have a container. So, I have a container, and I'll make it a, I'm gonna make it a closed container. We'll say this is some type of theoretical ideal closed system here. Now, let's say I had some ideal gas. So, I had some ideal gas
molecules right over here. They have some average temperature, but that means they all each
have their own individual, their own individual kinetic energy. They're all bouncing
around in different ways. What's going to happen over time? Well, over time, the
ones on the left here, they're gonna bounce off this wall. And then, they're eventually gonna go in this direction. And so, over time, you're
gonna have a situation where the system is going to
look something more like this. So, the system is going
to look more like this, where instance, let's see,
this is six particles. These six particles are gonna diffuse throughout the container. So, they're gonna diffuse
throughout the container. They're going to take up more
of the space of the container. Now, what just happened in that process? Well, when you knew that
the particles were confined to this little section of the container, there were fewer possible states. You had lower entropy
than when you are here, when you know that it's
filled up the container. There's more possible locations, more possible orientations for it. And so, you are going to have more states. You have higher entropy, higher, higher, higher, entropy. And, in general, these processes
where you have the entropy increasing, we call these
irreversible processes, irreversible, irreversible processes. And, why is it irreversible? Well, there's some probability that these molecules
might just gather back into this corner of it. But, it's very, very low probability. And, this is when we're dealing with six molecules. But, in real systems, we'd
be dealing with much larger than six molecules. We'll be dealing with millions of millions of millions of millions of molecules. So, things with, between 20
and 30 zero's of molecules. And there, it's very unlikely that
they just all bump together in the right way to start
taking a smaller volume, when they could actually
fill the container. And so, that's why you don't see, that's why you don't see
smoke just naturally turn into to some type of shaped particle, or take up less space, as opposed to filling its container. So, this is irreversible, because you went from,
you went from fewer number of potential states, as a smaller volume, to a higher number of potential states. And, the universe is
constantly doing this. That's why the entropy of the
universe is only increasing. Now, there's some processes that it feels like the entropy
isn't increasing that much. So, if you were to take one
billiard ball right over here, and you were to roll
it, you were to roll it into another billiard
ball right over here, and transfer the momentum to that one, it feels like that could
go the other way around. Like, that other billiard
ball could hit this one and go backwards. And, at a macro level, it feels like this is
a reversible process, and people will tend to
call this reversible. But, if you really were to
go on a microscopic level, and it looks like the entropy
isn't increasing that much, but if you were to look at
it on a microscopic level, and just to be clear, the entropy, you know, when this ball is moving and this is stationery, going to a state where this is moving and this is stationery, it doesn't look like the
entropy is increasing that much. And so, that's why they tend
to call this irreversible, because you tend to observe things where maybe this one,
it could go backwards. This could hit this one and then, this one could go, you can kind of run the film in rewind. But, even there, if you were
to look at a microscopic level, you would see that some
heat is being generated, and that some molecules in
the ball are getting excited as they collide, and as they
have friction with the air, and as they, roll on the ground over here. And, you're never going
to get those molecules to go back into the state
that they were before, that you actually do have
the entropy increasing in the system. So, even when in our every day lives, in thermodynamics, people talk about reversible processes. They're only approximately reversible, and that the entropy's only
increasing a little bit. It's not like there's
zero increase in entropy. Irreversible reactions,
these are the ones. Diffusion is a very obvious one, where it's very clear
that you have an increase in entropy, and it feels like it's a very, very low probability, or
almost zero probability of this thing ever going
back to where it was. And, you won't observe it, because we were talking
about that many molecules. Something with 20 or
30 zero's of molecules. The odds of all of them
just doing the right thing, you could wait around a very long time and never actually observe that happening. And so, hopefully this makes sense that the disorder in this way, the number of states only increases as you have more and more interactions. And, a lot of that is coming from heat. Everything you're doing right now, when I'm making this video, my body is generating heat. That heat is dissipating
into the universe. That is adding to the number of states that the universe can actually take on. As I move my hands up, my little digital pencil that
I'm using is causing friction. That's releasing heat into the universe. My computer is running and releasing heat into the universe. You watching this, releasing
heat into the universe. The electrons traveling on
the wire to your computer, releasing heat into the universe. And, all of that is increasing
the number of states. So, if you're thinking on a
molecular level of everything.