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Physics + MathPhysics & Math

Schrödinger’s Menagerie

You already know Schrodinger’s cat. Now meet the rabbits, fleas, dinosaurs, and other animals of the famous physicist's thought experiments.

ByPaul HalpernThe Nature of RealityThe Nature of Reality
Schrödinger’s Menagerie

Erwin Schrödinger loved animals.

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That may come as a surprise to those who associate Schrödinger most closely with his cat paradox. Of all of Schrödinger’s accomplishments, including the Nobel-Prize-winning development of the quantum wave equation that bears his name, that hypothetical thought experiment, occupying just a few paragraphs in a single paper, is what sticks in the public memory. It has also earned Schrödinger an undeserved reputation for animal cruelty .

Yet his family knew better. As related by his daughter Ruth Braunizer, Schrödinger’s dog Burschie (“Laddie”) offered great comfort to the family throughout the dark days of World War II. No family scene was complete without their beloved collie.

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Why would a lover of animals imagine something so cruel? For the same reason that, 80 years later, Schrödinger is still branded a cat-hater: preposterous analogies are memorable.

Schrödinger was a master of vivid analogy. He loved literature and drama, particularly the writings of the ancient Greeks and eastern mystics, and drew upon such sources for ways of expressing his ideas. His theatrical lectures captivated audiences with references to commonplace situations with which they might identify and have a laugh.

In fact, Schrödinger’s cat is just one member of a whole zoo of (imagined) animals that Schrödinger pressed into service to bring his analogies to life: Schrödinger’s menagerie!

Schrödinger’s Cat


The epitome of ridiculousness, Schrödinger’s cat paradox explores the extreme ramifications of quantum entanglement, a term he coined to describe how a single quantum state can encompass multiple, separate objects. Although Schrödinger pioneered the wave version of quantum mechanics, he was flummoxed by how his ideas were reinterpreted by Max Born and others as a probabilistic theory that allows for sudden, discontinuous transitions from one particle state to another. As Werner Heisenberg and John von Neumann suggested, observation of a mixed quantum state—representing, for example, a range of position values–can trigger it to collapse into an unblended state, pertaining to, for instance, a single position value. If the initial state describes an entangled set of particles, then the entire state collapses at once, immediately affecting all of the particles in question.

As a believer in continuous transformations, rather than sudden changes, Schrödinger found preposterous the idea that a scientific measurement could instantly spur a cavalcade of transformations. Therefore he developed the idea of linking a cat’s fate to that of a decaying radioactive sample by making a Geiger counter the trigger for the release of poison. Just as the sample would be in a mixed quantum state before observation, so would be the cat—in a bizarre superposition of life and death.

While Schrödinger hoped that his thought experiment would stimulate a rethinking of quantum assumptions, the collapse idea has remained the mainstream approach, with only a minority of theorists supporting alternative notions such as the Many Worlds Interpretation (in which the cat would be alive in one parallel universe and dead in the other). As for the cat itself, it has become a symbol of ambiguity.

Schrödinger’s Flea


Schrödinger’s wave equation gathered acclaim because of its ability to predict the energy levels and other properties of electrons in an atom. It was more tangible than Heisenberg’s matrix mechanics approach, which achieved the same aim but was based on more abstract calculations. When Born fused the two ideas into a comprehensive quantum mechanics based on probability waves, he included the idea of instant quantum transitions between energy levels. Mortified by the transformation of his continuous theory into a discontinuous one, Schrödinger reportedly said, “If you have to have these damn quantum jumps then I wish I’d never started working on atomic theory.”

Schrödinger argued that electrons, unlike creatures of the animal kingdom, lack free will and therefore couldn’t “decide” when to jump. Therefore, they must be described through a continuous theory. According to physicist Walter Heitler, who worked for a time with Schrödinger, he was often known to say, “I can’t believe that an electron hops about like a flea.”

Schrödinger sharply distinguished animate objects, such as fleas, from inanimate particles, such as electrons. That’s because, versed in the writings of the Hindu Vedas and other eastern ideas, he firmly believed in a world of the spirit that transcended the material world. While the inanimate world behaved like clockwork, the animate world was subject to choices.

Schrödinger’s Hydra Fusca

Schrödinger had a strong interest in possible connections between physics and biology. In his influential book, “What is Life?,” he speculated that life possessed a unity that was derived from a template. His proposal that life emerged from a aperiodic crystal helped inspire James Watson and Francis Crick to search for the molecular blueprint of life—what became known as DNA.

In another book, “My View of the World,” Schrödinger further stressed the interconnectedness of all living beings through examples involving simple organisms such as planaria and Hydra fusca (a freshwater polyp.) Noting the polyp’s tendency to divide he wrote, “I want the reader to think himself into Hydra fusca. We cannot but attribute some kind of consciousness, however dim and undifferentiated, to our primitive little cousin on the ladder of life. And this consciousness will appear in both sections as the undivided continuation as what was there before.”

Interestingly, physicist Hugh Everett made a similar argument when he proposed what became known as the Many Worlds Interpretation. He suggested that human conscious awareness splits into parallel branches each time a quantum measurement is taken, like the dividing of primitive animals.

Schrödinger’s Rabbits


In the mid 1940s, Schrödinger joined Einstein in a quest to explain all of the forces of nature in a single unified theory. While Einstein felt confident that the theory would replace quantum mechanics, Schrödinger wasn’t so sure. He once asserted that Einstein was being too ambitious. “You are after big game, as they say in English,” he informed Einstein. “You are on a lion hunt and I’m talking about rabbits.”

Schrödinger’s Iguanodon


As I describe in my new book “ Einstein’s Dice and Schrödinger’s Cat ,” in January 1947 Schrödinger declared to the Royal Irish Academy that he had discovered a unified theory of physics—the goal that had eluded Einstein for decades. Privately, in a letter to Einstein, he expressed some self-doubt. Referring to a story by Kurd Lasswitz about a primitive creature vexed by climate change, Schrödinger wrote to Einstein about his theory: “There is simply nothing else sensible… If that is not right, then I’ll let myself be Iguanodon, say ‘Cold, cold cold,’ and stick my head in the snow.”

We see that cats were far from the only creatures in Schrödinger’s menagerie of animal analogies. Far from someone cruel to animals, the great Austrian physicist believed they are connected to us in the grand scheme of life. Next time you hear about Schrödinger’s cat, perhaps you can offer a witty reply about his flea or Iguanodon.

Go Deeper
Our picks for further reading

The Conversation: Schrödinger’s cat gets a reality check
Physicist Eric Cavalcanti explains a new experiment designed to probe the nature of the wavefunction and answer the question: Is Schrödinger’s cat really dead and alive?

MacTutor: Erwin Rudolf Josef Alexander Schrödinger
Discover more about Schrödinger’s life in this concise online biography.

University of Pittsburgh: The Measurement Problem
John Norton of the University of Pittsburgh provides a detailed, accessible primer on Schrödinger’s cat.

This project/research was supported by grant number FQXi-RFP-1822 from the Foundational Questions Institute and Fetzer Franklin Fund, a donor-advised fund of Silicon Valley Community Foundation.