# Intro

Hello everyone,

Today we cover entropy and the heat death of the universe.

The previous chapters can be found here and here. While I recommend you read both, you should at least read the first part and skim the second.

A collection of all topics covered can be found here: https://tildes.net/~tildes/8al/.

# Subject

## Intro

Entropy describes how chaotic a system is. In thermodynamics, chaos is created from an irreversible process. We are all sort of familiar with this concept. A broken cup will not unshatter itself. As a consequence of how our universe works, (net) chaos can only increase. And this might have far reaching consequence, if we look at the effects of entropy on a cosmic scale.

## Entropy

Entropy describes an amount of irreversible chaos.

But first, let's cover cycles super quickly. In thermodynamics, a very important concept is a "cycle". A cycle is a repeating process, that returns to its initial condition. For instance, when we ride a bike. We're turning our feet around the crank shaft. Repeatedly returning to the same position we started from. As we push on the pedal, some of our work is lost and turned into heat. Primarily due to friction from the wheels and from the different mechanical parts.

A cycle that wastes no energy is called a reversible cycle. That would mean 100% of the work in a cycle (even the work that is turned to heat) has to be returned in some way to its original state. The most famous example of this is the Carnot heat engine.[1] But in reality, the Carnot heat engine is nothing more than a theoretical engine. As we remember from before, we cannot turn 100% of heat back into work. So any heat engine, be it a car's motor, a refrigerator, a star, or the human body, will in some way contribute to this irreversible chaos.

Now what about entropy? If we look at entropy at the molecular level, it all becomes a bit abstract. But we can think of this concept with bigger building blocks than molecules, and still be close enough. Say you have a brick house with orderly layed bricks. This house would love to come crashing down. And lets imagine it does. When the house lays in ruins, it is not likely to suddenly "fall" into the shape of the house again. So if the house has collapsed, our system is in a higher state of chaos. Our entropy has increased. And unless we supply work to the system (and waste energy trough heat), we will not get the brick house back.

So now we understand, that on the grand scale of the universe, entropy will only increase.

## The heat death of the universe

But what are the consequences of this? Imagine entropy going on for billions and billions of years. Everything in the universe slowly reaching a higher state of chaos. Everything that is orderly, turns into chaos. All high quality energy has turned into low quality energy. Everything has been wasted and turned into heat. Everything ripped apart until you are left with nothing to rip apart. At this point, there is no interactions between molecules any more. Everything has reached absolute zero temperature.

At this point, entropy is at its absolute maximum. And we have reached entropic equilibrium.

This is the heat death of the universe.

## Afterword

Of course, the heat death of the universe is just one of the many theories about the end of the universe. It assumes that thermodynamics properly describes the universe, and that there are no hidden surprises.

Frankly told, it's the best bet we have with our current knowledge. But we still know so little. So I would not panic just yet. Alternatively, this is where we could continue with "an engineer's perspective on existensial nihilism". But I think that this is something better reserved for later, and better presented by someone else.

We have covered what I consider the absolute minimum of thermodynamics, that still gives us a basic understanding of thermodynamics. There are of course a lot of other topics we could cover, but thats it for now. I will potentially write an appendix later with some questions or things that have been asked.

But for now, that's it. Questions, feedback or otherwise?

## Notes

[1] The Carnot heat cycle is a bit beyond the level of what we have discussed so far. It describes a system where heat is supplied and removed to have a piston expand and contract without any energy becoming waste heat.

1. [4]
cfabbro
This is not really related to this topic per se, as it's more of an etymological question (I guess?)... but why is entropy referred to as chaos and the opposite state "orderly"? The way I picture...

This is not really related to this topic per se, as it's more of an etymological question (I guess?)... but why is entropy referred to as chaos and the opposite state "orderly"? The way I picture things, molecules not being able to interact with each other seems the opposite of chaotic since it's devoid of action whereas molecules interacting seems the more chaotic state. Does chaos have specific meaning in physics that doesn't match the colloquial definition or am I simply looking at this all from the wrong perspective?

1. [3]
ducks
Can't speak of physics in general, since it's a pretty broad subject. But in the case of thermodynamics, there are many who consider the terms of chaos/disorder and order to be the wrong terms And...
• Exemplary

Can't speak of physics in general, since it's a pretty broad subject. But in the case of thermodynamics, there are many who consider the terms of chaos/disorder and order to be the wrong terms And in my opinion rightly so. But the names have been used so long, it's kind of a standardized thing now.

But if we look at a more "normal" example than the heat death of the universe, it makes a bit more sense. If we convert a potential source of energy into heat, we will see molecules behave a lot more erratically. For instance if we have a heater on, high quality energy is turned into heat, and we see the air molecules shake and move a lot more. So in that sense they seem more disorderly.

1. [2]
cfabbro
Ah, so it's only really when you look at the "end state" of entropy that using chaos/disorder seems incongruous. That makes sense, thanks.

Ah, so it's only really when you look at the "end state" of entropy that using chaos/disorder seems incongruous. That makes sense, thanks.

1 vote
1. ducks
Pretty much. You could have a system with an extreme that is smaller than the universe. So it doesn't necessarily have to be at the end of all. But it is in quite extreme conditions the terms...
• Exemplary

Pretty much. You could have a system with an extreme that is smaller than the universe. So it doesn't necessarily have to be at the end of all. But it is in quite extreme conditions the terms start to be wrong.

I do think some better terms should be made at some point. But I'm not qualified for that.