It was in 1935 that the Austrian physicist Erwin Schrödinger proposed his now-famous cat image to comment on what he thought was the irresponsible failure of his colleagues to think through quantum mechanics. He could hardly have imagined that the cat, which he introduced half-jokingly, would still be discussed almost 80 years later – nor that it would have become permanently ingrained in popular culture. So why does the image still seem as packed with creative force as ever?

One recent example crops up in Will Grayson, Will Grayson, a young-adult novel published by John Green and David Levithan in 2010. In the book, Will asks Jane – a girl for whom he has mixed and unexpressed feelings – about Schrödinger’s cat. Jane describes the physicist’s famous thought-experiment, before adding that Schrödinger “was not endorsing cat-killing or anything…just saying that it seemed a little improbable that a cat could be simultaneously alive and dead”.

Will ponders that for a moment. Thinking of his own mixed emotions – though attracted to Jane, he once declined her offer of a kiss – he doesn’t think it strange that something can be real and not real at the same time. “[A]ll the things we keep in sealed boxes are both alive and dead until we open the box,” he broods to himself. “[T]he unobserved is both there and not.”

A completely different Schrödinger’s cat image is found in Blueprints of the Afterlife, an apocalyptic science-fiction novel by Ryan Boudinot, published this year. It features a character named Abby Fogg, who shows up both dead and alive at the same time after being programmed to infiltrate another reality. In a morgue one day, she creepily stares at two naked and dead identical versions of herself. “[Y]our selfhood, Abby, has gone into superposition,” the forensics director tells her. “It’s as if you are both alive and dead simultaneously, and this simultaneity is a self-replicating system in which there are various ‘snapshots’ of your dead self. Which makes an autopsy pretty dang hard, let me tell you.”

Weird stuff

Quantum mechanics describes the world as the product of two ingredients. The first is an information function, the ψ-function described by Schrödinger’s equation, which is a classical wave that expands outwards and overlays, or “superimposes”, many possibilities. The second ingredient is something that befalls this function, causing it to disappear and one of its possibilities to appear. If this sounds odd to you, you are not alone: even the pioneers of quantum mechanics struggled to connect this strange picture with the familiar world.

Niels Bohr and Werner Heisenberg said the world is divided into two separate domains: quantum and classical. The quantum domain is governed by the unobservable ψ-field and when this encounters something in the classical domain, through measurement or other interactions, the encounter evaporates, or “collapses”, the function. One hitherto only probable state becomes “real” and all other possibilities are eliminated.

This idea was sufficiently weird that it sparked opposition. Einstein led the attacks, which culminated in the famous “EPR” paper of 1935 co-authored with Boris Podolsky and Nathan Rosen, entitled “Can quantum-mechanical description of physical reality be considered complete?”. Published in May of that year (Phys. Rev. 47 777), the paper’s answer to the rhetorical title question was a clear “no”. There must be elements independent of processes of measurement, the EPR trio argued. Our common-sense experience – and the very definition of reality – depends on elements the existence of which is independent of observation and measurement.

Schrödinger was thrilled, and wrote to his friend Einstein expressing his delight. “You have evidently caught dogmatic q.m. by the coat-tails,” he declared. By “dogmatic q.m.”, Schrödinger meant quantum mechanics as espoused by Bohr and Heisenberg that denied the reality of certain properties such as position and momentum apart from in measurement situations.

Einstein replied equally enthusiastically, and elaborated on his intuitions: physics describes reality, but not all descriptions are complete. He imagined having two boxes with lids you can open to peer inside, and there’s a ball in one. Before you “make an observation” by looking inside the first box, how do you describe the situation? We say, quite correctly, that the probability that the ball is in the first box is ½, or 50%. But is that a complete description? Of course not, Einstein answers. It characterizes only our knowledge of the situation, not reality itself. Really, the ball is in the first box or it isn’t. Yet according to “dogmatic q.m.”, it can in principle be a complete description to say the chance of it being in that box is 50%. So quantum mechanics seems to be saying that the ball is not in one or the other box, but first exists in a box only when you peer inside. In the Bohr–Heisenberg account, Einstein wrote incredulously, “The state before the box is opened is completely described by the number ½.”

The year of the cat

Two months later, Einstein sent Schrödinger another analogy. Suppose a pile of gunpowder has a probability of exploding in a year, he mused. Its ψ-function is therefore a superposition of exploded and unexploded gunpowder. In Einstein’s view, this was nonsense. He felt that quantum mechanics, thanks to its ψ-function, is an incomplete and inadequate description of reality. Einstein’s letters inspired Schrödinger to set down an informal account of his own views, which he published in October 1935 as “The present situation in quantum mechanics” (Naturwissenschaften 23 807). This was the first appearance of Schrödinger’s cat.

Schrödinger began the paper by saying that the classical world bequeathed us the idea that nature can be exactly described. Sure, experimental data may not – in practice – allow this to be carried out in complete detail, but they do let us model phenomena that we can compare with reality and modify when necessary. These models describe states, which are specified by what Schrödinger calls “determining parts” or variables. A small set of variables uniquely determines all others in a state, though different sets can be used.

Yet this is impossible in quantum mechanics, which says that not all variables can be “co-determined”. The obstacle is not any practical limitation but Heisenberg’s uncertainty principle; when you measure some variables, others become uncertain. What about those other variables? “Have they then no reality, perhaps (pardon the expression) a blurred reality; or are all of them always real and is it merely…that simultaneous knowledge of them is ruled out?” puzzles Schrödinger.

To be and not to be

In philosophical language, Schrödinger is asking whether the probabilities affect the “ontology” of the variables – whether the quantities to which they refer exist or not – or merely their “epistemology”, that is, our ability to know what they are.

In thermodynamics, Schrödinger continues, probabilities affect only epistemology. Scientists model systems containing billions of billions of molecules by treating them as if they involve single states arbitrarily chosen from ensembles of many possible states. This is convenient but not strictly correct. In thermodynamics you don’t care how a system behaves exactly – indeed, you aren’t even interested – only how it behaves for the most part.

But in the quantum domain, some variables remain indeterminate or blurred when others are exact. Perhaps if we knew more about the underlying situation, Schrödinger said, we would find it more complex than we thought, and causality might reappear. Still, as long as the ψ-function is confined to the subatomic domain, the indeterminacy is harmless. “Inside the nucleus,” argued Schrödinger, “blurring doesn’t bother us [but] serious misgivings arise if one notices that the uncertainty affects macroscopically tangible and visible things, for which the term ‘blurring’ seems simply wrong.”

Schrödinger now conjures his famous image. In his words:

“One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with a Geiger counter, which must be secured against direct interference by the cat. The Geiger counter contains a tiny bit of radioactive substance, so small that perhaps in the course of an hour one of the atoms decays, but also, with equal probability, perhaps none. If an atom does decay, the counter tube discharges and – through a relay – releases a hammer that shatters a small flask of hydrocyanic acid. But if no atom decays after an hour, the cat still lives. The ψ-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.”

Later in the article Schrödinger describes the implication – the nonseparability of previously interacting quantum states even after the interaction – with the now-famous neologism “entanglement”.

Border lines

Science historian Stephen Brush has remarked that the cat image “captures the spirit of Einstein’s critique better than the published EPR paper”. Yet when Schrödinger’s paper came out, the cat provoked little discussion. For Bohr, Heisenberg and company, cats are too complicated to have ψ-functions – they inhabit classical territory. For Einstein and Schrödinger, the image showed that, just as we are not content to accept a “blurred model” to represent reality in the macroworld, we should not in the microworld. Things are different on the other side of the boundary, but not that different. For both sides, the cat symbolized nonsense.

The boundary dispute, however, did not vanish. It got worse. To the consternation of Einstein and Schrödinger, no way was found to reformulate the rules of the quantum domain so that “determining parts” could all co-exist. Entanglement did not go away, and remained an ontological, not just epistemological, disturbance. Bohr and Heisenberg were disappointed that no way was found to pin down the classical–quantum boundary, which implied that entanglement reached into more territory than they imagined. The price of eliminating superposition collapse is alternate worlds, which does not eliminate but multiplies the cat.

By the time popular-science writing took off in the 1970s, the cat image had become an accessible and accurate illustration that vividly captured the weirdness of entanglement, superposition, the measurement problem and the ψ-function – the cat was the symbol of the challenge posed to conventional realism by quantum mechanics. In The Dancing Wu Li Masters (1979), an over-the-top book about alleged connections between quantum mechanics and Eastern mysticism, Gary Zukav declared that “Schrödinger’s cat has long illustrated to physics students the psychedelic aspects of quantum mechanics”. The cat appeared increasingly often, not only in science fiction, but also in fringe and mystical fiction, amateur philosophy and self-help literature. There is even a Wikipedia page about “Schrödinger’s cat in popular culture”.

The critical point

Physicists do not seem to care much about Schrödinger’s cat any more except as a label: “cat-states” is sometimes used to refer to large coherent quantum systems, though nothing near as complex as a cat.

The rest of the world, however, seems to care a lot, for different reasons. For teenagers such as Will and Jane, the “entanglement” issue surrounding the cat is a great metaphor to express the reality of their mixed feelings, conflicting identities and unexpressed passions. “[K]eeping the box closed doesn’t actually keep the cat alive-and-dead,” Jane tells Will later in Will Grayson, Will Grayson. “Even if you don’t observe the cat in whatever state it’s in, the air in the box does. So keeping the box closed just keeps you in the dark, not the universe.” Will gets that – and gets as well that they aren’t talking about physics, but about their relationship.

For science-fiction writers such as Boudinot, the cat story makes weird and otherwise magical plots plausible, however remotely. For science writers, it symbolizes what is wrong with common-sense realism. For philosophers (amateur and professional) who seek to understand how quantum mechanics connects with the everyday world, it captures the idea of an “intermediate level of reality” that Heisenberg said was the price we had to pay for quantum mechanics.

Technically speaking, the application of the image of Schrödinger’s cat outside the quantum domain is a “fail”, to use the slang of Will and Jane and their friends. But in the real world, its persistence demonstrates how a tool developed in one human domain can, in unpredictable ways, be meaningful and useful in others.