On November 29, 2015, cat lovers and haters will have common cause to celebrate. That date marks the 80th anniversary of the publication of Austrian physicist Erwin Schrödinger’s extraordinary paper in which he proposed an outrageous thought experiment involving a cat, a box, a radioactive sample, a Geiger counter and a vial of poison. Schrödinger’s paper is a spacious mansion of an article, in which the kitty conundrum occupies just a windowsill-sized space. Yet, like any haughty housecat, Schrödinger’s cat has ended up dominating the whole manor.
Schrödinger loved animals and had no ill intention in crafting the bizarre imaginary experiment. Rather, he sought to offer a ludicrous example that would highlight gaps in the current theory of quantum mechanics. His intended audience included those such as Einstein who were skeptical of quantum orthodoxy, as well anyone else with an open mind about the theory’s limitations. Indeed his hypothetical situation echoed an earlier idea by Einstein about a keg of gunpowder existing in a mixed quantum state between exploded and unexploded. Any credible theory would resolve the apparent contradiction, the physicists believed.
Designed for mockery, Schrödinger’s fictional feline may then have been the first “LOLcat”—a clever parody, not a serious example. By imagining the fate of a cat (alive or dead) entangled with that of a radioactive sample (intact or decayed) through a Geiger counter set to release a vial of poison in a closed box, Schrödinger deliberately misapplied quantum mechanics rules designed for subatomic particles to a large living being. By purporting that the cat’s fate could be known only after the box was opened, he highlighted what he saw was the ridiculous notion that a mere observation could “collapse” a quantum state from a superposition (mixture) of possibilities—decayed or not, deceased or not—down to a single outcome.
Yet Schrödinger’s thought experiment had little impact during his lifetime. From the time Schrödinger proposed it, in 1935, to his death in 1961, it was scarcely mentioned in the literature. Even Schrödinger rarely brought it up. None of Schrödinger’s lengthy obituaries mentioned the cat. Today virtually any mention of Schrödinger, aside from an academic conference or classroom, elicits loud guffaws about the mixed-up feline.
For instance, I once posted a serious tweet about Schrödinger’s grave. What could be more somber than a tombstone? Yet it was met immediately with the wry comment, “Just don’t look inside.”
How did Schrödinger’s cat conundrum transform from a tiny mention in a research paper to a popular emblem for ambiguity? Originally, it was used mainly as an example in classrooms or seminars on the weirdness of quantum logic. For example, Columbia University physicist T.D. Lee would reportedly use the cat in his lectures to illustrate how quantum strangeness differed from classical expectations.
Such a sharp distinction between quantum and classical logic was first emphasized by Hungarian mathematician John von Neumann. From the 1930s until his death in the 1950s, von Neumann formalized the idea of quantum state reduction (or collapse) as a separate, sudden process—akin to the catastrophic explosion of a star after billions of years of steady evolution. The intervention of a conscious observer was the spark that caused such collapse, raising deep questions about where to draw the line between the measurer and what was being measured. Aren’t humans, after all, collections of particles? Yet von Neumann argued that such a line needed to be drawn to explain the collapse of quantum systems from superpositions to purer states.
Strongly influenced by von Neumann, in the 1960s fellow Hungarian physicist Eugene Wigner began to ask what it means to be a conscious observer. In 1963, Wigner wrote a well-regarded essay about quantum measurement, including a provocative hypothetical example in which an observer becomes the observed.
In what has been become known as the “Wigner’s friend” conundrum, Wigner imagined a researcher, Dr. Muddle, completing the Schrödinger’s cat experiment in an isolated laboratory. He opens the box, but has not yet reported the result. Presumably he is either troubled or elated depending on the outcome, but no one else knows that yet. His friend and colleague, Dr. Clarity, is waiting outside the laboratory door, and only finds out what happened after she opens it. While from Clarity’s point of view, Muddle, along with the cat and the sample, is in a superposition of quantum states until she opens the door, measures the system, and “collapses” it into a purer state of one of the two, Muddle has already done his own measurement. Therefore, his consciousness has already reduced the quantum state. Wigner’s point is that conscious observation is the precise instant in which reduction occurs. That moment of discovery creates a sharp boundary between the observer and the observed, dividing the quantum and mundane realms.
The philosophical musings of physicists such as Wigner caught the attention of renowned Harvard philosopher Hilary Putnam, who discussed the implications of Schrödinger’s cat and related conundrums in a well-noted 1965 paper “A Philosopher Looks at Quantum Mechanics,” published as a book chapter. Scientific American reviewed the book, which may have been the first mention of Schrödinger’s cat in the popular press.
Putnam recalled how he learned about the bizarre imaginary experiment:
“I met [mathematical physicist] David Finkelstein at an American Mathematical Society Summer Seminar on ‘Modern Physical Theories and Associated Mathematical Developments’ in Boulder, Colorado in 1960. It may well be that it was from him that I learned about Schrödinger’s Cat. I always assumed the physics community was familiar with the idea.”
Following the Scientific American mention, it was only a matter of time before science fiction writers latched onto the idea of an ambiguously-fated feline. Among these were notables such as Ursula K. LeGuin, Robert Anton Wilson, and Robert Heinlein, who each made use of the cat conundrum in fiction. A poem by Cecil Adams, published in 1982, became well known and later was widely distributed on the internet.
John Gribbin’s popular book about quantum physics, “The Search for Schrödinger’s Cat,” and Gary Zukav’s ode to the intersection of science and mysticism, “The Dancing Wu Li Masters,” also spread the word about the twisted tabby. Countless readers curled up with those bestsellers and sympathized with the hapless feline’s tangled yarn. Thanks to such books, readers saw how the cat paradox elucidated the fundamental weirdness of quantum physics.
Cultural references have only grown in recent years: Schrödinger’s cat earned mentions on popular television shows such as the Big Bang Theory and Futurama, won a spot on a Google doodle, and has been printed on countless tee-shirts and other geeky fashion statements.
Meanwhile, the physics community started to grapple with alternative solutions to the conundrum. In the 1970s, physicist Bryce DeWitt became deeply interested in the troubling implications of needing a conscious observer for quantum measurement. In analyzing the universe itself, who would be the observer? Clearly a quantum theory of everything could not include a separate entity to measure its properties.
Drawing upon a 1957 paper (and thesis) by Princeton PhD student Hugh Everett, who had argued that quantum states don’t really collapse but rather bifurcate along parallel universes, DeWitt become the leading proponent of what he called the Many-Worlds Interpretation of quantum mechanics. From this point of view, each time a subatomic event such as particle emission occurs, reality splits, creating multiple observers who note each possible outcome. The advantage is that the observer, who is wholly unaware of the bifurcation, plays no role in the process.
According to the Many-Worlds Interpretation, Schrödinger’s cat would survive in one branch of the universe and die in the other. The instant this split transpired, anyone observing the event would divide too, with each recording a different outcome.
Today, many quantum physicists have turned to yet another alternate to quantum state collapse—decoherence. Physicists in the decoherence camp argue that interactions between a quantum system and its environment naturally induce it to evolve toward a purer (single-valued, not a superposition) state without the need for collapse or intervention by an observer. Because decoherence applies on a microscopic level, Schrödinger’s cat, as a macrosystem, would not directly experience the decoherence process; it would either be dead or alive depending on the state into which the radioactive sample evolved.
So, happy birthday, Schrödinger’s cat! Eighty years later, you’re still thriving—dead or alive.
Author’s picks for further reading
National Geographic: The Physics Behind Schrödinger’s Cat Paradox
Discover the physics of the “pop-culture staple.”
Scientific American: The Many Worlds of Hugh Everett
Hugh Everett biographer Peter Byrne on the life of the father of the Many Worlds interpretation of quantum mechanics.
Drexel University: What is Decoherence?
A deeper look at the meaning of decoherence.