Antonio Sortino

There is a long history of doing physics by imagination. Albert Einstein built his special theory of relativity after imagining himself chasing a beam of light. Erwin Schrödinger gave us a cat that was both alive and dead. The German mathematician David Hilbert demonstrated the counterintuitiveness of infinity by imagining a hotel with an infinite number of rooms and guests. By taking creative liberties, physicists use thought experiments to stress-test ideas and so better understand them.

Curiously, three of the most enduring and perplexing thought experiments all involve what have come to be known as “demons”. The most famous is Maxwell’s demon, devised in 1867, which imagines a tiny being with strange but logical powers. Along with two other similar thought experiments – Laplace’s demon and Loschmidt’s demon – it still gets physicists scratching their heads today. Thinking about these demons, it turns out, can help us come to grips with some of the trickiest concepts in physics.

“The exciting and amazing thing is that scientists are able to learn so much about reality by going into these fictional spaces,” says Michael Stuart, a philosopher of science at the University of York, UK. “And many would argue that science would be impossible without it.”

Laplace’s demon

The man who conjured up our first demon was a French polymath working in the long shadow of Isaac Newton. In 1814, Pierre-Simon Laplace asked a simple question: if Newton’s laws could predict how an apple would fall, could 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 – though the word he used was “intellect” – capable of just that. If it knew the position and momentum of every particle and understood the laws of nature, then it could calculate the entire future of the universe. “Nothing would be uncertain,” he said. “The future, just like the past, could be present before its eyes.”

We might never be able to build a machine with the powers of Laplace’s demon, but imagining it can still help us pick out any logical inconsistencies in our theories. Does science actually mean that everything, from planets to people, is pre-determined? If the laws of physics fix every outcome, then free will would seem to be, at best, an illusion – a byproduct of our ignorance.

Happily, our first demon is relatively easy to exorcise. Physicists have reasons to believe that no entity could ever have the knowledge that Laplace’s demon is said to have. For starters, Einstein’s theory of special relativity says that no information can travel faster than light. That means that although some events may affect your future, you can’t know about them in the present moment. Information about those events, travelling at the speed of light, simply hasn’t had time to reach you, which defeats Laplace’s demon.

And even if the demon could access information from all edges of the universe, quantum mechanics throws up another hurdle. Since the 1920s, we have known that there is no way of being certain of a particle’s position and its momentum at the same time, so the demon simply cannot know exactly where each particle is and what it is doing. It could describe only the probabilities of a particle’s properties.

Laplace’s neat particle-by-particle picture of reality is replaced by a quantum universe described by a vast, shifting wavefunction, an abstract mathematical object that encapsulates all the outcomes that could potentially happen. Even if a demon could keep track of all these outcomes, it wouldn’t know which one would eventually become real.

Loschmidt’s demon

Laplace’s demon seems to lose its teeth, then, but there were more devilish thought experiments lying ahead. Our second demon cropped up at the height of industralisation. Steam engines had given new urgency to questions about heat, energy and disorder. Austrian physicist Ludwig Boltzmann tried to explain entropy, a slippery concept that captures how systems tend to grow more disordered over time. Sandcastles disintegrate, ice melts, rust forms, and so on. Boltzmann believed he could explain it by zooming in on reality and looking at the tiny building blocks of big systems, like individual molecules of gas filling an entire room.

But his older colleague, the Austrian physicist Josef Loschmidt, had doubts about this approach and posed a simple but devastating challenge in 1876. Imagine the universe frozen in time. Every molecule has a position and direction of motion. Now, Loschmidt said, reverse the direction in which each particle is travelling. Loschmidt’s original formulation didn’t involve a “demon,” but later versions often add one that can somehow see and freeze all the particles – mostly because of what came later.

The development of steam engines raised questions about heat, energy and entropy

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Loschmidt’s scenario troubled physicists so much because it seems to present a time-related paradox. At the level of particles, nothing seems to be particularly wrong when the directions are reversed – no laws of physics are broken. But zoom out and the macroscale effects would be unthinkable: puddles would freeze into perfect ice cubes and broken mugs would reassemble themselves as the world starts playing backwards. It prompts us to ask: if we can reverse time trivially in the micro-world, why does it only ever seem to run one way for us?

Later experiments would try to reverse time, just like Loschmidt’s demon. In the 1950s, Erwin Hahn used radio waves to briefly nudge electric dipoles (such as the hydrogen atoms in a water molecule) into spinning in unison, temporarily lowering the system’s entropy. This made it look as if time were running backwards. So, was Loschmidt’s demon capable of defeating the concept of entropy?

Not quite. We now understand that entropy doesn’t mean that systems must always slide into chaos. Some systems can even evolve to be more ordered very briefly. But entropy does conquer all eventually, as Hahn saw. Once he switched his radio beam off, the dipoles fell back into disorder.

So, why does entropy always increase? From what we can tell, the cosmos started in an extraordinarily tidy state: low entropy, with all the pieces neatly arranged. That gave it only one way to go – towards messiness. There are just many more ways to ruin a neat system than to make it even more ordered, making disorder more likely. This means that Loschmidt’s demon can, in theory, reverse the trajectory of tiny particles, but it is doing so against the odds.

“The status of the second law isn’t like Newton’s second law,” says philosopher Katie Robertson at the University of Stirling, UK. “It’s got this probabilistic nature, like ‘you probably won’t manage to reduce entropy.’”

Ultimately, then, the laws of probability exorcised this demon, but not before it helped us deepen our understanding. Boltzmann, in response to Loschmidt, abandoned his original approach and adopted one based on statistics because it better captured this soft logic of probability. His refined thinking led to the Boltzmann equation, which is now carved on his tombstone.

Maxwell’s demon

The third and most famous demon came in 1867, less than a decade before Loschmidt raised his challenge, from Scottish physicist James Clerk Maxwell. Like Loschmidt, he was interested in the second law of thermodynamics but attacked the idea that entropy always increases from a different angle. Instead of rewinding the universe, what if you could interfere with it, molecule by molecule? Picture a meddling being – which was later described as a demon by physicists like William Thomson – that could push around gas molecules trapped in a box, partitioned by a trap door. Over time, it could separate fast-moving molecules from slow ones, violating the second law.

Various simple “solutions” spring to mind. Perhaps the demon has to exert energy to open and close the door. But, in principle, this “work” could be arbitrarily small. The demon could be as light-fingered as you like, and the paradox would remain.

“ Scientists are able to learn so much about reality by going into these fictional spaces “

Instead, physicists began to suspect the true cost wasn’t in the energy the demon expends, but in how much information it would have to process. Keeping a tally of each molecule’s position and momentum would seem to require a memory of some sort. And it turns out, this memory isn’t free.

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

Eventually, something gives. In the 1960s, IBM physicist Rolf Landauer made the crucial point: for the demon to keep functioning, it must clear space in its memory, and that process generates heat, raising the entropy of the system. The second law is saved.

Laplace’s demon could calculate the future of the entire universe

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However, physicists made a crucial realisation at the same time: information was a physical resource, just like energy. Knowing something about a system isn’t just a matter of abstract bookkeeping. Under the right conditions, information can even be treated as fuel. After all, Maxwell’s demon somehow converts information into work. Today, the demon is a mascot for machines that operate where information and energy intertwine. These “information engines” don’t just challenge our intuitions – they promise to turn the demon’s logic into working technology. In 2024, researchers built a quantum version of Szilárd’s engine to charge batteries inside a quantum computer. Instead of a demon, researchers used microwave pulses to corral more energetic qubits away from less energetic ones, creating an energy differential that can do work, like a battery.

They’re still far from powering your phone, but the hope is that these new tiny quantum engines can help move particles or flip qubits using information.

Seen this way, Maxwell’s demon hasn’t been exorcised at all. It’s been reborn in ways Maxwell could have never imagined – not as a threat to the second law, but as a guide to the strange and subtle ways nature permits us to exploit information as a physical resource.

Together, these demons have tested the limits of theory and intuition. While a few have been kept at bay, new paradoxes creep in. But these are demons that physicists don’t really mind. These devilish thought experiments are a beloved way that scientists push the envelope of what they know.

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