The Story of Entropy and Why It Will Be the End of Everything

A visualization shows how particles will spread throughout the universe as its entropy increases, bringing about Heat Death (also known as the Big Freeze).

The idea of entropy began with a sort of bitterness. Steam power was transforming the world, its towering machines looming huge over the bustling citizens of England while in France the landscape was tumultuous as its people grew disillusioned. One man in particular — Sadi Carnot — daydreamed of England’s collapse. He mused that understanding steam power would enable him to strip England of all its advantages, turning France once more into a center of advancements and pride. Born into a military family that had instilled him with a sort of servitude towards his country, he began to pour over the engines, filling his free time with studies on heat and fire and all the inner workings of steam power — undoubtedly one of mankind’s most revolutionary steps in terms of technology. It was this desire, to some perhaps foolish devotion to his country and to others an admirable want to progress, that led him to be known as the father of thermodynamics.

Thermodynamics is one of the most fundamental theories of physics, so evident in our everyday world that great minds like Albert Einstein, Boris Pavlovich, and Arthur Eddington saw it as a certainty and not merely a possibility. At its simplest, thermodynamics is the study of the relationship of all forms of energy in the universe. It connects everything from the smallest swimming microbes to the largest structures known to man — those so big they exist somewhere out there amongst the black backdrop of dark space. Its first law says that isolated systems have a finite, set amount of energy and energy within that system can never be created nor destroyed, it can only change format. In this case, the isolated system is our universe and energy within our universe can never appear or disappear of its own accord, instead it can only be transferred into different states.

Boiler explosions, like this one from the 1940’s, were common when steam locomotives were first in use. Image by Heritage Railway Magazine.

In the case of steam engines, a hot center is surrounded by a colder environment, and it’s this difference in temperature that allows us to harness the energy of heat and turn it into mechanical energy which can then power our factories or send our boats slipping out along the silky waters. For a steam engine used by a train, for example, water surrounding a fire will heat up and produce steam which will, in turn, produce pressure that is used to power the locomotive. Carnot not only understood this but presented the idea of the perfect heat engine, which uses what we call the Carnot cycle.

However, it’s the second law of thermodynamics which describes the nature of entropy — the eventual unravelling of everything.

It’s often seen as chaos: the universe tends towards chaos. But that’s not a complete understanding of what entropy really is. It’s more statistical than philosophical in content. In a system, entropy is a measure of how evenly distributed the energy within that system is and, according to the second law, entropy must always be increasing overall. We can decrease entropy on small scales (you do this every time you heat up a pot of water, for example) but on the much grander scale of the universe, energy is becoming more evenly distributed and it’s this process that will eventually bring about the death of everything. Not just everyone, but everything.

Atoms vibrate faster as heat is applied. This vibration (or energy) is passed along to neighboring atoms as the system attempts to reach equilibrium.

If you place a hot bowl of soup on the dining table, the soup will eventually cool down to room temperature. The concentrated, ordered heat within the bowl will eventually spread out into a more disordered state where the heat has spread throughout the room. This is an example of entropy. This happens, as Ludwig Boltzmann enabled us to understand, because of the building blocks of matter that we call atoms. The more heat within the atoms, the faster they move. They then share this energy with their surroundings, transferring it from the soup to the dish to the table, and throughout the rest of the room. And while it is possible for a hot object to spontaneously grow hotter, the chances of this happening are so small that it’s never been observed. This is where the statistical nature of entropy comes into play — higher entropy is the most likely outcome for a system. Higher entropy means higher disorder because disorder has a higher chance of occurring.

Not only does Entropy cause hot things to cool down, it’s also the reason cool things, like this ice cream, heat up and melt a they approach room temperature.

It all comes down to the figures which say that disorganized states are much more likely to occur than organized ones, especially as the number of particles in a system increases. Energy is much more likely to be disordered and evenly distributed within a system than it is for it to be concentrated and orderly. Once this energy has been dispersed, the process cannot be reversed (the soup will not once again heat up unless an outside force acts upon it). This is the law that permeates the entire universe. All things with heat and energy are interconnected and this heat and energy will continue to disperse throughout the system that is space. This is why time cannot go backwards. In order for time to go backwards, entropy would have to decrease. In this way, the flow of time which is usually irrelevant in laws of motion becomes much more rigid and less malleable than we’d perhaps hope.

In terms of microstates and macrostates, the macrostate we observe will be the one predicted by most of its microstates. Macrostates are the overall properties of a system — its temperature, its volume, its pressure and so on. The microstates are then the individual particles in the system and their positions and velocities. Since most particles give you a system with more entropy and more thermal equilibrium, that’s the system which we will then observe.

As entropy in the universe continues to increase, heat will continue to spread until the system has reached maximum equilibrium, meaning that everything will degrade into sole particles and a swirl of radiation. Of all the scenarios for our universe’s death, Heat Death is the most likely to occur given what we can observe and given our laws of physics.

Boltzmann’s famous formula says that disorder will pervade the universe because it is statistically more likely than its orderly counterpart.

Entropy is defining in that way. It defines everything as mortal but, because of the inevitable outcome of the universe’s end, it also defines this moment of life and energy and vibrancy as quite a spectacular one because of its rarity, and because it will be so brief. It tells us that everything will come to a halt, yes, but also gives us the power to harness energy and improve our technology, build our cities, produce chemical reactions and even, on a more individual level, simply eat and dance and play. In much earlier years, entropy was known as a living force within the Earth which we could then extract and use to our advantage. This idea wasn’t entirely correct, but in a way it is the ability for everything to live. The only way humanity can continue to move forward is by finding new ways of harnessing this life force. At first through food, then fire, and now we look to stars to help us understand fusion and its role in our future.

There are many similarities between entropy and time. We might dread time because we cannot stop it, or reverse it, or because we’re not sure what it’ll bring. But the ongoing ticking of the clock is also a sort of relief, telling us that whatever situation we’re in is only temporary, and that life is that much more valuable because it is that much more fleeting.