- [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.