Strange beasts stalk a history of thought experiments.

Bedeviled: A Shadow History of Demons in Science Jimena Canales Princeton Univ. Press (2020)

The workings of powerful computers, the processes of evolution, the market forces that drive the global economy. To conceptualize such unseen forces, researchers have long invoked thought experiments involving demons, devils, golems or genies.

These strange beasts aren’t creatures of superstition and pseudoscience. They are useful ideas that have had an important role in the advancement of science, argues historian of science Jimena Canales. Her latest book, Bedeviled, sizes up imagined imps over the centuries and follows their impacts.

The seventeenth-century French philosopher René Descartes conceived of the disturbing possibility that some devious spirit could hijack our sense of reality. This ‘malicious demon’ would affect what we think we see, hear, smell and touch — presaging how virtual reality challenges us today. Descartes’s idea caused him to question his senses, and even his existence. He found his way back to reality by asserting that cogito ergo sum — he thought, therefore he was. A reasoning human being foiled the deceptive demon.

The spectres of physics

In the early nineteenth century, scientists built on the physics of Isaac Newton to understand the forces of nature, making it possible to calculate with precision the motions of atoms and planets or the forces of a steam engine. Taking those ideas to their logical conclusion, French mathematician Pierre-Simon Laplace proposed that some demonic intellect would be able to calculate the past and future of anything if it knew the precise location and trajectory of all particles and all forces acting on it. British mathematician Ada Lovelace was aware of Laplace’s work, and in 1842 she was arguably the first to speculate about whether computing programs could be considered thinking beings. Laplace’s demon thus seeded a debate that continues 180 years later.Maxwell’s demon and the hunt for alien life

In 1867, Scottish physicist James Clerk Maxwell summoned an even more powerful demon while trying to understand the statistical behaviour of gas particles. Maxwell’s tiny demon operates a door between compartments in a gas-filled vessel, choosing when to allow molecules to pass from one side to the other. The demon can open the door for faster-moving molecules but leave the slower ones on the other side, thus heating up one compartment and cooling the other — decreasing entropy and thwarting the second law of thermodynamics. To some, Maxwell’s ideas suggested the possibility of a perpetual-motion machine, or even of reversing time.

In practice, Maxwell’s research informed improvements in the efficiency of engines and refrigerators. His demon also demonstrates the surprises that can arise from probabilities, because every once in a while, the rarest events do occur — such as only fast gas molecules spontaneously slipping through a hole.

Canales also surveys thought experiments on the uncertainties in quantum mechanics. Unlike in classical physics, for example, particles in the quantum world can seem to go through two doors at once. But German mathematician Grete Hermann and, later, US physicist David Bohm suggested that such paradoxes could be resolved if “hidden variables” or unknown mechanisms determine whether a particle travels through one door or another. A few physicists dubbed this “Bohm’s demon”.

Nanoscale demons

Canales explores so many fields and societal implications of scientific debates, from atomic bombs to stock-market fluctuations, that she seems to weave in nearly every demon reference of the past four centuries, however tangential. Some meandering historical asides stray from her solid survey of seminal demonic invocations.Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists

Today’s demons dwell in genetics, economics and artificial intelligence (AI). Searle’s demon is named after the US philosopher John Searle, who in the 1980s pointed out that a powerful nanobot — or some nanoscale demon — could control which neurons in a person’s brain get stimulated and which don’t, thus making the brain’s base operations similar to a computer program. If such a demon were possible, then one could imagine AI mechanisms almost indistinguishable from human intelligence. Taking this debate about consciousness versus machine learning further, Searle criticized the idea of “strong AI” — that machines could think as well as or better than humans, operating not merely as tools but as minds of their own. (The University of California, Berkeley, stripped Searle of emeritus status in 2019 after finding that he had violated its sexual-harassment policies.)

Canales highlights some women, including Marie Curie, who envisaged quantum-level demons acting on radiation, and mentions others in passing, such as Rosalind Franklin and Lise Meitner, who did groundbreaking work on DNA and nuclear fission, respectively. But she mostly takes in demons imagined by male scientists in Europe since the Renaissance. Her history would have benefited from an exploration of the disputes between astrology and science in the medieval period, such as those between figures such as Abu Rayhan al-Biruni and Ibn Sina (Avicenna) in Persia. These scholars, too, probed the limits of theory, observation and experiment, and their demarcation from pseudoscience.

In his classic 1995 book The Demon-Haunted World, astronomer Carl Sagan wrote that because scientists frequently use their imagination in their work, they don’t know what to expect as they push against the boundaries of knowledge. Canales has given us a glimpse into this haunted realm.

Nature 588, 27-28 (2020)