How 3 imaginary physics demons tore up the laws of nature

There is a long history of learning physics through imagination. Albert Einstein developed his special theory of relativity by imagining himself chasing a ray of light. Erwin Schrödinger gave us a cat that was both alive and dead. The German mathematician David Hilbert demonstrated the illogic of infinity by imagining a hotel with an infinite number of rooms and guests. By allowing creative freedom, physicists use thought experiments to stress test ideas and thus understand them better.

It is curious that all three of the longest and most mysterious thought experiments involve so-called “demons.” The most famous of these is Maxwell's Demon, invented in 1867, which is a tiny creature with strange but logical abilities. Along with two other similar thought experiments – Laplace's demon and Loschmidt's demon – it still leaves physicists scratching their heads. It turns out that thinking about these demons can help us understand some of the most difficult concepts in physics.

“The most exciting and amazing thing is that scientists can learn so much about reality by immersing themselves in these fictional spaces,” says Michael Stewartphilosopher of science from the University of York, UK. “And many argue that without it, science would not be possible.”

Laplace's demon

The man who summoned our first demon was a French polymath who worked in the long shadow of Isaac Newton. In 1814, Pierre-Simon Laplace asked a simple question: if Newton's laws can predict how an apple will fall, can the same logic be used to predict everything? What if you had perfect knowledge not just of one falling apple or orbiting planet, but of every particle, every object, everywhere? He asked us to imagine a demon (although he used the word “intelligence”) that could do just that. If he knew the position and momentum of every particle and understood the laws of nature, he could calculate the future of the universe. “Nothing will be uncertain,” he said. “The future, like the past, can appear before his eyes.”

We may never be able to build a machine that has the powers of Laplace's demon, but imagining it can still help us spot any logical inconsistencies in our theories. Does science mean that everything from planets to people predetermined? If the laws of physics fix every result, then free will at best it may seem like an illusion—a by-product of our ignorance.

Luckily, our first demon is relatively easy to exorcise. Physicists have reason to believe that no entity could ever possess the kind of knowledge that Laplace's demon is said to possess. To begin with, Einstein's special theory of relativity states that no information can travel faster than light. This means that although some events may affect your future, you cannot know about them at the moment. Information about those events, traveling at the speed of light, simply did not have time to reach you, which defeats the demon of Laplace.

And even if a demon could access information from all corners of the universe, quantum mechanics would create another obstacle. We've known since the 1920s that it's impossible to be sure of both a particle's position and its momentum, so a demon simply can't know exactly where every particle is and what it's doing. He could only describe the probabilities of the properties of the particle.

Laplace's clear, detailed picture of reality is replaced by a quantum universe described by a huge, changing wave function, an abstract mathematical object that encapsulates all the outcomes that could potentially occur. Even if a demon could keep track of all these results, it wouldn't know which one would end up being real.

Demon of Loschmidt

Laplace's demon seemed to have lost its teeth, but even more diabolical thought experiments lay ahead. Our second demon appeared at the peak of industrialization. Steam engines brought new urgency to issues of heat, energy and disorder. Austrian physicist Ludwig Boltzmann tried to explain entropy, a slippery concept that shows how systems become increasingly disordered over time. Sand castles collapse, ice melts, rust forms, and so on. Boltzmann believed he could explain this by zooming in on reality and looking at the tiny building blocks of large systems, such as individual gas molecules filling an entire room.

But his senior colleague, the Austrian physicist Joseph Loschmidt, doubted this approach and in 1876 posed a simple but destructive problem. Imagine the Universe frozen in time. Each molecule has a position and a direction of movement. Now, Loschmidt said, reverse the direction of motion of each particle. Loschmidt's original formulation did not include a “demon”, but later versions often add one that can somehow see and freeze all particles – mostly because of what came later.

Loschmidt's scenario has physicists so concerned because it appears to present a paradox involving time. At the particle level, there seems to be nothing particularly wrong with the directions being reversed—no laws of physics are being violated. But zoom out and the macro-scale effects are unimaginable: puddles freeze into perfect ice cubes and broken mugs reassemble as the world plays backwards. This prompts us to ask: if we can trivially turn back time in a microcosm, why does it always seem to flow? One Way for us?

Later experiments attempted to turn back time, as did Loschmidt's demon. In the 1950s Erwin Han used radio waves to briefly cause electric dipoles (such as the hydrogen atoms in a water molecule) to spin in unison, temporarily reducing the entropy of the system. Because of this, it seemed as if time had gone backwards. So, was Loschmidt's demon capable of defeating the concept of entropy?

Not really. We now understand that entropy does not mean that systems must always descend into chaos. Some systems may even briefly evolve and become more orderly. But as Khan saw, entropy ultimately conquers all. As soon as he turned off his radio beam, the dipoles went into disorder again.

So why does entropy always increase? As far as we can tell, the cosmos was originally in an extremely tidy state: low entropy, with all the parts neatly arranged. This gave him only one path – to disorder. There are many more ways to destroy a neat system than to make it even more orderly by increasing the likelihood of disorder. This means that Loschmidt's demon can theoretically reverse the trajectory of tiny particles, but it does so against all odds.

“The status of the second law is not like Newton's second law,” says philosopher Katie Robertson of the University of Stirling, UK. “It has this probabilistic nature, like, 'You probably won't be able to reduce entropy.'

The laws of probability eventually exorcised this demon, but not before it helped us deepen our understanding. Boltzmann, in response to Loschmidt, abandoned his original approach and adopted a statistical approach because it better reflected this soft logic of probability. His sophisticated thinking led to the creation of the Boltzmann equation, which is now carved on his tombstone.

Maxwell's demon

The third and most famous demon appeared in 1867, less than a decade before Loschmidt's challenge, from the Scottish physicist James Clerk Maxwell. Like Loschmidt, he was interested in the second law of thermodynamics, but criticized the idea that entropy always increases at different angles. Instead of rewinding the universe, what if you could tamper with it, molecule by molecule? Imagine an intervening creature – later described by physicists such as William Thomson as a demon – that could move gas molecules locked in a box separated by a hatch. Over time, he will be able to separate fast-moving molecules from slow ones, breaking the second law.

Various simple “solutions” come to mind. Perhaps the demon has to exert energy to open and close the door. But in principle, this “work” can be as small as desired. The demon can be as frivolous as he likes, and the paradox will remain.

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Scientists can learn so much about reality by immersing themselves in these fictional spaces.
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Instead, physicists began to suspect that the true cost lay not in the energy the demon expended, but in how much information it would have to process. Taking into account the position and momentum of each molecule seems to require some kind of memory. And it turns out that this memory is not free.

In the 1920s, Hungarian physicist Leo Szilard showed that even in a stripped-down version of Maxwell's setup, with just one molecule jumping around inside the box, a clever demon could still extract work from the system. But to do this he would need to observe the molecule and store that information, which he argued would require energy.

Eventually it gives something. In the 1960s, IBM physicist Rolf Landauer made a crucial point: for a daemon to continue functioning, it must clear space in its memory, a process that generates heat, increasing the entropy of the system. The second law has been preserved.

At the same time, however, physicists made a crucial realization: information is a physical resource, just like energy. Knowing something about a system is not just a matter of abstract accounting. Under the right conditions, information can even be thought of as fuel. After all, Maxwell's demon somehow converts information into work. Today the demon is the mascot of machines that work where information and energy intertwine. These “information machines” don’t just challenge our intuitions—they promise to turn demonic logic into working technology. In 2024, researchers created a quantum version of the Szilard engine to charge batteries inside a quantum computer. Instead of a demon, the researchers used microwave pulses to drive more energetic qubits away from less energetic ones, creating a difference in energy that can do work like battery.

They're a long way off from powering your phone, but there's hope that these new tiny quantum engines could help move particles or flip qubits using information.

From this point of view, Maxwell's demon has not been exorcised at all. It has been reborn in ways Maxwell never imagined—not as a threat to the Second Law, but as a guide to the strange and subtle ways in which nature allows us to use information as a physical resource.

Together, these demons tested the limits of theory and intuition. Although some of them have been contained, new paradoxes are emerging. But these are demons that physicists don't particularly object to. These diabolical thought experiments are scientists' favorite way to push the boundaries of what they know.

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