Hello again my faithful few, it is time, to discover the hidden rules that govern everything from your morning cup of tea to the edge of the universe. Are you ready? Of course you are!

Did you know, the universe has a favourite direction?

If you place a hot cup of tea on a table and it cools, if you drop an ice cube into a drink and it melts,  and if you spray air freshener into a room and it spreads out, none of these events require any effort, they just happen naturally.

Yet you’ve never seen the reverse occur on its own, have you? Cold tea becoming hot, melted ice spontaneously reforming, or air freshener spray gathering back in to the can.

There’s a reason for this, which lies in one of the deepest principles of nature, and that is, you’ve guessed it, the clue is in the title, it is entropy. A subject I love so much, it gets bolded.

Entropy is the central idea of thermodynamics, the branch of physics that governs energy, heat, and work, along with the direction of physical processes.

And what’s remarkable is, entropy doesn’t just influence everyday events, it plays a crucial role in understanding stars, galaxies, time itself, and even, as I’ve touched on in a previous article or three, black holes.

And now, my faithful readers, I am going to attempt to explain the science of energy without writing a full-blown scientific paper, or a book. Are you ready? Here goes!

Now then. Thermodynamics began during the Industrial Revolution as scientists attempted to build better steam engines, and as is usual when building these types of engineering feats, what they discovered was far more important.

The same laws governing steam engines also govern chemical reactions, batteries, human metabolism, planetary atmospheres, nuclear fusion in stars, black holes, and the evolution of the universe itself.

Thermodynamics became the universal rulebook of energy.

OK. There are four laws of thermodynamics. The first is the Zeroth Law. Now, this law was established by Robert H. Fowler in the 1930s quite some time after the other three laws were established, which is why it isn’t the first law. And it’s a simple one.

If object A is in thermal equilibrium with B, and B is in equilibrium with C, then A and C are in equilibrium with each other. Simples, right? Told you it was. And as simple as this sounds this law allows temperature to be measured. Without it thermometers couldn’t exist.

Then we get the First Law.

The First Law states that energy cannot be created or destroyed, energy only changes form, chemical energy becomes motion, motion becomes electricity, electricity becomes light. However, the total amount of energy remains unchanged.

After the first law, unsurprisingly we get the Second Law

The Second Law is the famous one, you often hear about in the movies, or albums by Muse, and is where entropy enters the story.

The Second Law states that the entropy of an isolated system never decreases. While entropy may remain constant in an ideal reversible process, it naturally tends to increase, and it is this law that explains why so many processes happen in one direction only.

Next? You’ve guessed it, we have the Third Law.

And that is, as temperature approaches absolute zero, entropy approaches its minimum possible value.

So, that takes us to the first question, what is entropy?

People often hear entropy described as a measure of disorder, and as useful as that description is, it’s incomplete.

There is a deeper meaning, and that is, entropy measures the number of possible microscopic arrangements of a system. Physicists call these arrangements microstates. Which, put simply, means the more possible ways something can exist while appearing the same overall, the higher it’s entropy.

Think of it like a jigsaw puzzle of an idyllic country cottage, surrounded by blue sky with fluffy clouds, a cute picket fence and dozens of flowers growing in the garden. One arrangement produces the completed image, whereas millions of arrangements produce a random mess. Nature overwhelmingly favours the states that are statistically more likely. And that tendency is entropy.

Still with me? Good. Let’s have a look at why ice melts. Ice contains water molecules arranged in an organised crystal structure and when ice melts, those molecules gain freedom. They can occupy vastly more possible arrangements which means the liquid state therefore possesses higher entropy than the solid state. Nature generally moves toward states with more possible configurations.

Next, we have one of physics’ greatest mysteries, the arrow of time, that is, why time only moves forward.

The fundamental equations of physics often work equally well forward or backward, yet reality doesn’t. Glass shatters, eggs break, and people age etc. We never witness these events naturally reversing themselves, and it’s entropy that provides us with the explanation.

As systems evolve, they overwhelmingly move from less probable states to more probable ones, and this increase in entropy creates the direction we perceive as the flow of time.

In the nineteenth century, Ludwig Boltzmann had an incredible insight. He had connected entropy to probability with one famous equation:

Where: S = Entropy, k = Boltzmann’s constant, and  Ω = the number of possible microstates. This equation links the visible world to the invisible world of atoms and molecules, and to many physicists (and amateurs like me), it is considered one of the most beautiful equations ever discovered.

Next, we’ll take a look at why perfect engines can’t exist

Suppose engineers build the ultimate engine, every bit of heat becomes useful work. There’s no waste, and no losses.

Thermodynamics says this is impossible. The Second Law requires that some energy inevitably becomes unavailable for useful work and is released as waste heat, so therefore, no heat engine can ever achieve 100% efficiency. Nature always demands an entropy bill.

Let’s have a look at one of my favourite examples, Black Holes, the very first article I wrote for this blog. Black holes changed everything as for much of the twentieth century, physicists believed black holes were simple objects. They appeared to have, no temperature, no entropy, and no internal complexity, and this, my friends, created an enormous problem.

Imagine throwing a book into a black hole. The book contains information and the information has entropy. If the black hole were to simply swallow everything and hide it forever, entropy would seem to disappear from the universe, and this would  violate the Second Law. That means, somewhere, something was wrong.

In the early 1970s, physicist Jacob Bekenstein had a revolutionary idea and  proposed, what at the time was, a shocking solution. He suggested that black holes themselves possess entropy.

At first, the idea seemed absurd.  How could an object that appears completely black have entropy?

Bekenstein, however, had a realisation, and that was entropy had to be related to the surface area of the black hole’s event horizon. In effect, the horizon was storing information about everything that had fallen inside. And this was one of the most important insights in modern theoretical physics.

Soon afterward, our favourite physicist, Stephen Hawking, made an even more astonishing discovery. Using quantum mechanics, Hawking showed us that black holes are not truly black. They emit tiny amounts of thermal radiation, now known as: Hawking Radiation. Regular readers will  know all about Hawking radiation, I wrote about it a couple of weeks ago, remember?

And this implied something absolutely astonishing. If black holes emit thermal radiation, they possess temperature, entropy, and thermodynamic behaviour. Black holes were no longer just gravitational objects. They were thermodynamic objects. And in some sense, they were giant cosmic heat engines.

Let’s move on to the entropy of a black hole. One of the strangest discoveries in physics is that a black hole’s entropy is proportional not to its volume, but to the area of its event horizon, and this was completely unexpected, as this meant that for ordinary objects, more volume means more storage capacity. Whereas for black holes, more surface area means more entropy. And this relationship became known as the Bekenstein-Hawking Entropy Formula, and it suggests that information about a three-dimensional region may somehow be encoded on a two-dimensional surface. This idea later inspired the famous Holographic Principle, one of the most profound concepts in modern theoretical physics, but sorry folks, once again that isn’t for now, that’ll have to be another article (which luckily for you I am working on, so it won’t be long).

What was discovered was black holes are entropy monsters. The entropy contained within a black hole is enormous. In fact, a black hole possesses vastly more entropy than an ordinary star made from the same amount of matter.

When a star collapses into a black hole, entropy increases dramatically, which means black hole formation actually satisfies the Second Law beautifully.

Far from violating entropy, black holes may represent some of the highest-entropy objects in the universe, which is why physicists sometimes describe them as the ultimate entropy engines.

And with that, it was time for another mystery to emerge from the shadows, The Information Paradox.

If Hawking Radiation causes black holes to slowly evaporate, what happens to the information that fell into them? There appears to be three possibilities to answer that question. Either information is destroyed, it escapes somehow, or, more likely, our understanding of physics is incomplete. This became known as the Black Hole Information Paradox, and it remains one of the greatest unsolved problems in physics. Entire fields of theoretical research have grown from attempts to solve it.

Today, many cosmologists believe the universe began in an extraordinarily low-entropy state and that this may govern the fate of the universe. Since then, entropy has steadily increased. Stars burn hydrogen, galaxies evolve, black holes grow, and entropy rises.

Far in the future, black holes may dominate the universe, and even they will eventually evaporate through Hawking Radiation. After unimaginable lengths of time, the cosmos may approach a state of maximum entropy known as Heat Death. This is standard cosmology, and in this state, all the stars are gone, temperature differences completely vanish, any useful work is impossible, and although energy remains, it is hopelessly spread out to be of any use to anything. The universe doesn’t run out of energy, but it does run out of opportunities to use that energy.

So, at the end of the day, thermodynamics began with steam engines, and it ended up explaining reality itself! Which again, as far as I’m concerned, absolutely boggles the mind!

To put it another way, entropy tells us why my favourite Assam tea cools, why stars shine, why time moves forwards, why engines have limits, why life can exist, and why black holes may hold the key to unifying gravity and quantum mechanics.

If the First Law says energy can never be destroyed, then the Second Law says energy becomes increasingly dispersed and increasingly difficult to use.

And nowhere is that truth more dramatic than in a black hole, where gravity, quantum physics, information, and entropy collide at the edge of the known universe.

Entropy is not merely a property of matter. It may be the accounting system for the whole of reality itself!