There has always been a strong interplay between imagination and physics. Albert Einstein crafted his theory of relativity by envisioning a scenario where he chased a beam of light. Erwin Schrödinger famously introduced the idea of cats that are both alive and dead. German mathematician David Hilbert illustrated the paradox of infinity by conceptualizing a hotel with limitless rooms and patrons. Through inventive thought experiments, physicists rigorously examine concepts and deepen their comprehension.

Interestingly, three of the most enduring thought experiments revolve around what is now known as “the devil.” The most recognized is Maxwell’s Demon, conceived in 1867, envisioning a minuscule being endowed with unusual but logical abilities. Together with Laplace’s Devil and Roschmidt’s Devil, these thought experiments continue to baffle physicists today, suggesting that pondering these devils can illuminate some of the most complex principles in physics.

“What’s refreshing and unexpected is that scientists can gain profound insights about reality by engaging in these fictional realms,” says Michael Stuart, a philosopher of science at the University of York, UK. “Many would contend that the essence of science hinges upon such imaginings.”

Laplace’s Devil

The concept of our first demon originated from the mind of French polymath Pierre-Simon Laplace, who was largely influenced by Isaac Newton. In 1814, Laplace posed a straightforward query: “If Newton’s laws can predict the fall of an apple, could we apply the same logic to predict everything?” What if we had perfect knowledge about every particle and object? He invited us to picture a devil—whom he referred to as “intelligence”—that could do exactly that. If it understood the position and momentum of all particles alongside the laws of nature, it could foresee the entirety of the universe’s future. “Nothing would remain uncertain,” he asserted. “The future could be as clear as the past.”

While we may never construct a machine endowed with Laplace’s demonic faculty, envisioning such a being assists in identifying logical inconsistencies in the theory. Does it imply that everything—from planets to humans—is predetermined? Does science assert that the laws of physics dictate all outcomes? Free will may appear to be, at best, an illusion, a mere byproduct of our ignorance.

Fortunately, the essence of the first demon is relatively straightforward to dismantle. Physicists are convinced that no entity could possess the knowledge attributed to Laplace’s demon. First, Einstein’s special theory of relativity establishes that information cannot travel faster than light. Therefore, some events can indeed influence your future, but you remain ignorant at that moment since the information must travel at light speed and lacks time to reach you, thereby nullifying Laplace’s demon.

Even in the event that this devil could access knowledge from every corner of the universe, quantum mechanics introduces another obstacle. Since the 1920s, it has been acknowledged that one cannot simultaneously ascertain both a particle’s position and momentum. Therefore, the devil cannot precisely determine where each particle is or what it is doing; it can only describe the probabilities surrounding particle properties.

Laplace’s tidy particle-by-particle depiction of reality is superseded by a quantum universe, characterized by a vast, fluctuating wavefunction—an abstract mathematical construct that encapsulates all potential outcomes. Even if the devil were able to monitor these outcomes, there remains no certainty regarding which one would ultimately manifest in reality.

The Devil of Roschmidt

Though Laplace’s devil seems to have lost its potency, even more sinister thought experiments lie ahead. The second demon emerged during a period of rapid industrialization, where the steam engine intensified inquiries about heat, energy, and disorder. Austrian physicist Ludwig Boltzmann sought an explanation for entropy—a slippery concept that explains how systems devolve into chaos over time. Sandcastles fall apart, ice melts, and rust forms. Boltzmann believed that zooming into reality and observing the minute components of a larger system, like individual gas molecules filling a room, could clarify this concept.

However, his elder colleague, Austrian physicist Josef Loschmidt, challenged this approach in 1876 by posing a simple yet devastating dilemma. Imagine a universe in which time has halted; all molecules have a defined position and direction of movement. Loschmidt suggested that if you reversed the movement of each particle, you could essentially undo entropy. Roschmidt’s original positing did not mention a “demon,” although later iterations often included a demon that could perceive and freeze all particles, largely due to subsequent developments in the field.

Loschmidt’s scenario deeply unsettled physicists as it suggested a time-related paradox. When considered at a microscopic level, reversing particle movement doesn’t seem to result in any contradictions. However, this breaks down at a macroscopic level; as the world seemingly restores itself in reverse, puddles solidify into ice, and shattered vases reassemble. This raises the question: “Why does time appear to flow in only one direction if at the microscopic level we can easily reverse it?”

Subsequent experiments attempted time reversal, much like Roschmidt’s demons. In the 1950s, Erwin Hahn utilized radio waves to temporarily synchronize electric dipoles (such as hydrogen atoms in water) to rotate uniformly, momentarily decreasing the system’s entropy. This seemingly created the illusion of time moving backward. So, did the Roschmidt demon manage to outsmart the concept of entropy?

Not entirely. It is now understood that entropy doesn’t imply that a system must always degenerate into disorder. Some systems can evolve into a more ordered state in a brief span. However, as Hahn demonstrated, entropy ultimately prevails. When the radio beam was switched off, the dipole reverted to chaos.

Why does entropy consistently rise? Scientifically speaking, we believe that the universe began in a highly ordered state with low entropy, where everything was systematically arranged. This constrains progress to one direction: toward chaos. Aside from fostering additional disorder, there are various methods to disrupt an orderly system. This suggests that in theory, Roschmidt’s demon can reverse small particles’ trajectories, albeit contrary to expectations.

“The situation with the second law differs fundamentally from Newton’s second law,” notes Katie Robertson, a philosopher at the University of Stirling in the UK. “Its probabilistic nature suggests that ‘You probably cannot reduce entropy.’”

Ultimately, the probabilities dispelled this demon, but they did little to enhance our understanding. In response to Loschmidt, Boltzmann shifted from the original approach to a more statistically oriented framework, as it succinctly captured the delicate logic of probability. His advanced thinking led to the formulation of the Boltzmann equation, now inscribed on his epitaph.

Maxwell’s Devil

The third and perhaps best-known demon was proposed by Scottish physicist James Clerk Maxwell in 1867, shortly before Roschmidt raised his concerns. Like Loschmidt, Maxwell grappled with the second law of thermodynamics, but he examined the notion of increasing entropy from a different perspective. What if, instead of rewinding the universe, we could intervene in it molecule by molecule? Envision a meddlesome being (later referred to as a demon by physicists like William Thomson) that could manipulate gas molecules trapped in a box divided by a trapdoor. Over time, this entity could violate the second law by segregating faster-moving molecules from slower-moving ones.

Various straightforward “solutions” might come to mind. Perhaps this demon expends energy opening and closing the door. However, theoretically, this “work” can be minimized infinitely. The demon could act as frivolously as desired, yet the paradox persists.

Instead, physicists began to suspect that the actual cost wasn’t the energy exerted by the demon, but the amount of information it needed to process. A certain type of memory seems mandatory to record the position and momentum of each molecule. And astonishingly, this memory is finite.

In the 1920s, Hungarian physicist Leo Szilard demonstrated that even a simplified version of Maxwell’s experiment—featuring only one molecule bouncing within a box—could enable a clever demon to extract work from the system. Nevertheless, he posited that this necessitates observing molecules and storing that information, requiring energy in the process.

Ultimately, something must yield. In the 1960s, IBM physicist Rolf Landauer made a crucial point. For the demon to remain functional, it must free up space in memory, generating heat and consequently increasing entropy within the system. The second law remains intact.

Moreover, physicists acknowledged that information, akin to energy, constitutes a tangible resource. Gaining insight into a system is not merely a matter of abstract logistics. Under appropriate conditions, information can also serve as fuel. Thus, Maxwell’s demon somehow translates information into work. Today, this demon symbolizes devices that function at the intersection of information and energy. These “information engines” not only challenge conventional wisdom but also hold the potential to convert demonic logic into practical technology. In 2024, researchers devised a quantum variant of the Szilard engine to power batteries within quantum computers. Instead of demons, microwave pulses were employed to displace higher-energy qubits from lower-energy ones, generating an energy differential capable of doing work like a battery.

While we remain distant from utilizing these innovations to charge mobile devices, the aspiration is that these miniature quantum engines will aid in manipulating particles or toggling qubits.

In this light, Maxwell’s demons have not been vanquished at all. Rather, they evolved into concepts that Maxwell could never have envisioned. Not as an infringement upon the Second Law, but as a means to explore the intricate and unexpected ways nature allows us to utilize information as a physical resource.

Collectively, these demons challenge both theoretical limits and intuitive understanding. While some have been tackled, new paradoxes continue to emerge. Yet, these are dilemmas that physicists welcome. These intriguing thought experiments provide scientists with a compelling avenue to push the boundaries of their knowledge.