In 1935, the Austrian physicist Erwin Schrödinger published a rather critical three-part review of what he called the ‘present situation’ in the relatively new theory of quantum mechanics. For the most part, Schrödinger’s review, written in German, is dry and technical, and not the kind of thing that would detain anyone outside the narrow academic world of quantum physics. But in one short paragraph, written with his tongue firmly in his cheek, he gave flight to a fancy that, 90 years later, continues to resonate in popular culture. The paragraph concerned Schrödinger’s eponymous cat. How did an obscure argument about a mathematically complex and rather baffling theory of physics become embedded in public consciousness as an extraordinary exploration of the human psyche? This essay tells the story.
Here’s what Schrödinger wrote (in the English translation by John D Trimmer):
One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference from the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the [radioactive] atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The [quantum wavefunction] 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.
Although this was only a ‘thought’ experiment, the paradox of Schrödinger’s cat was destined to join Pavlov’s dog in science’s bestiary of the bizarre.
To understand the point Schrödinger was making, we need to do a little unpacking. The nature of Schrödinger’s ‘diabolical device’ is not actually important to his argument. Its purpose is simply to amplify an atomic-scale event – the decay of a radioactive atom – and bring it up to the more familiar scale of a living cat, trapped inside a steel box. The theory that describes objects and events taking place at the scale of atoms and subatomic particles like electrons is quantum mechanics. But in this theory, atoms and subatomic particles are described not as tiny, self-contained objects moving through space. They are instead described in terms of quantum wavefunctions, which capture an utterly weird aspect of their observed behaviour. Under certain circumstances, these particles may also behave like waves.
These contrasting behaviours could not be starker, or more seemingly incompatible. Particles have mass. By their nature, they are ‘here’: they are localised in space and remain localised as they move from here to there. Throw many particles into a small space and, like marbles, they will collide, bouncing off each other in different directions. Waves, on the other hand, are spread out through space – they are ‘non-local’. Squeeze them through a narrow slit and, like waves in the sea passing through a gap in a harbour wall, they will spread out beyond. Physicists call this diffraction. Push a bunch of different waves together and they will merge to form what physicists call a superposition. The peaks and troughs of all the different waves add together. Where peak meets peak, the result is a larger peak. Where trough meets trough, the result is a deeper trough. Where peak meets trough, they are both reduced and, if they happened to be of equal height and depth, they will completely cancel each other out. Physicists call this interference.
By 1935, the mathematical formulation of quantum mechanics was relatively mature, and acknowledged by most physicists as complete. But the theory does not say where all the quantum weirdness is supposed to stop. When applied to the radioactive atom, quantum theory says that, after an hour, the wavefunction of the atom is represented as an equal mix – a superposition – of decayed and undecayed atoms.
So, is the cat dead or alive? We have no way of knowing until we lift the lid of the box, and look
We could choose to stop there, but we know that the interaction between the atom and the diabolical device should also be described by quantum mechanics, at least in its early stages. If we choose to take the theory literally, then extending its equations to include the entire device means that the wavefunction evolves into a superposition of decayed atom and triggered device, and undecayed atom and untriggered device. In his review, Schrödinger coined the term ‘entanglement’ to describe this situation. The radioactive atom becomes entangled with the device.
If we logically extend this entanglement beyond the device to include the cat then, as Schrödinger explained, we arrive at a wavefunction, which is a superposition of decayed atom, triggered device and dead cat, and undecayed atom, untriggered device and live cat. The living cat and the dead cat thus appear ‘mixed or smeared out in equal parts’.
So, is the cat dead or alive? We have no way of knowing until we lift the lid of the box, and look. If we stick with quantum theory as extended to the cat, it is supposedly at the point we lift the lid that the wavefunction ‘collapses’, and we find that the cat is alive, or dead. But there is a small problem with this that has huge consequences. Nowhere in the mathematical formulation of quantum mechanics will you find an equation describing this collapse. We are left to assume it happens.
OK, but can we at least predict the fate of the cat before we lift the lid? Quantum theory says: no, we can’t. According to the accepted interpretation, the superposition of the two possibilities reflects the relative probabilities of getting one or the other. But these probabilities translate into actual outcomes only when the wavefunction is assumed to have collapsed, when the superposition of one possibility and the other transforms into one actuality or the other. It would seem that the act of looking literally kills the cat, or doesn’t.
This is a big deal. This is not the same as tossing a ‘fair’ coin, and getting heads or tails with equal probability. We wouldn’t normally choose to describe the coin as being in a superposition of heads and tails as it spins through the air, though in principle there’s nothing to stop us from doing this. We don’t do this because of course we know that both sides of the coin continue to exist unchanged as we toss it in the air, and as it spins while it falls to the ground. But this is not the way quantum mechanics works. There are now many quantum experiments that demonstrate that assuming objects like atoms or electrons exist in some state before they are observed can give predictions that conflict both with quantum theory and with the results of experiments. We simply can’t do without the superposition, or the probabilities. We need the weirdness.
Although the majority of physicists appeared to have accepted the argument that quantum mechanics provides a complete theory of individual quantum objects and events, there were some notable dissenters. Albert Einstein was never comfortable with quantum theory’s implications for the law of cause and effect, and the resort to probabilities, famously declaring that God ‘does not play dice’. Earlier in 1935, Einstein and his Princeton colleagues Boris Podolsky and Nathan Rosen had published a landmark paper arguing that quantum mechanics could not be considered a complete theory. Something profound was missing. Although they disagreed on the details, Einstein and Schrödinger shared common cause, and their correspondence through the summer of 1935 inspired Schrödinger to develop his cat paradox.
Schrödinger understood that under no circumstances could his cat be considered to be both alive and dead at the same time. As far as he was concerned, his paradox exposed the apparent absurdity of quantum theory, not by suggesting that ‘quantum theory says’ that a superposition consisting of a live and dead cat is a real possibility, but by suggesting that what quantum theory doesn’t say can lead to a logical absurdity. Einstein replied: ‘… your cat shows that we are in complete agreement.’
And there the matter rested, for a time. The cat paradox was limited to one paragraph in a lengthy review article, and Schrödinger’s dissent cut little ice with the majority of physicists, including those who spent time pondering on the meaning of quantum theory. It survived in correspondence between Einstein and Schrödinger through to the early 1950s, and resurfaced in 1957, during a conference of physicists and philosophers held in Bristol, England.
The diabolical mechanism now involved electrocuting the cat (or not)
In a discussion featured in the conference proceedings, the American physicist David Bohm resurrected Schrödinger’s cat. By this time, the paradox had evolved and was based on a single photon (a ‘particle’ of light) passing (or not passing) through a half-silvered (or ‘one-way’) mirror. Like the radioactive atom, the photon has a 50/50 chance of passing through the mirror or being reflected by it. Passage of the photon triggers a diabolical mechanism in which the cat is killed with a gun.
The paradox reappeared again in 1965, in an essay by the American philosopher Hilary Putnam titled ‘A Philosopher Looks at Quantum Mechanics’. The photon and half-silvered mirror remain, but the diabolical mechanism now involved electrocuting the cat (or not). Putnam concluded that: ‘no satisfactory interpretation of quantum mechanics exists today.’
What happened next is rather fascinating. While researching Einstein’s special theory of relativity for a book she was writing sometime in 1972, the American science fiction author Ursula Le Guin came across a reference to Schrödinger’s cat. As the philosopher Robert Crease put it in a 2024 article, she was instantly ‘entranced by the implied uncertainties and appreciated the fantastic nature of Schrödinger’s image.’ We can’t be sure of precise events and timings, as Le Guin read extremely widely but didn’t systematically take notes, but this is the ‘best guess’ of Julie Phillips, who is busy writing an authorised biography of Le Guin to be published in April 2026. Having been asked by her subject to ‘rescue me from the vultures’, Phillips conducted many in-depth interviews with Le Guin before the author died in 2018. They had agreed that the biography would be published posthumously.
In her short story ‘Schrödinger’s Cat’ (1974), Le Guin presents Bohm’s version of the paradox involving the photon, half-silvered mirror and gun. In a dialogue between the nameless narrator and a dog called Rover, Le Guin wrote:
‘… We cannot predict the behaviour of the photon, and thus, once it has behaved, we cannot predict the state of the system it has determined. We cannot predict it! God plays dice with the world! So it is beautifully demonstrated that if you desire certainty, any certainty, you must create it yourself!’
‘How?’
‘By lifting the lid of the box, of course,’ Rover said …
The floodgates opened. From this point onwards, Schrödinger’s cat makes regular appearances in fiction. Not just science fiction, but a broad range of short stories and novels, films, plays, television shows, poems, and music. Developments taking place in physics in the early 1980s simultaneously drove burgeoning interest in popular non-fiction, such as John Gribbin’s In Search of Schrödinger’s Cat (1984).
The cat’s cultural appeal lies in the ‘what if’ questions it provokes. It encourages us to ponder the consequences of our very human choices. What if we choose not to look? If we don’t look, can the cat really be said to exist at all? Our decision to lift the lid is much like encountering a fork in the road. We choose a path. Like the American poet Robert Frost, we may choose the path less travelled by. But what if we had taken the other path? The movie Sliding Doors (1998) delivers two parallel stories, one that unfolds when Helen Quilley (Gwyneth Paltrow) misses her train on the London Underground, and a second that unfolds when she manages to board it. Quilley’s life turns out very differently, depending on whether or not she beats the sliding doors and gets on the train. That such a trivial ‘sliding doors moment’ might profoundly alter the course of our future is deeply unsettling.
There’s more. As Le Guin herself observed in her short story, there appears to be nothing special about the act of lifting the lid, and quantum mechanics is silent on the question of where in the chain of events the weirdness stops. She wrote: ‘But why does opening the box and looking reduce the system back to one probability, either live cat or dead cat? Why don’t we get included in the system when we lift the lid of the box?’ Could we be just like the cat, but trapped in a much bigger box that we call reality? If we are, who is doing the looking? And what will happen when they lift the lid?
If the act of looking inside the box doesn’t collapse the wavefunction of the system, then logically the observer must in turn become entangled in the superposition. ‘[T]here we would be,’ Le Guin wrote, ‘looking at a live cat, and … looking at a dead cat.’ If you are the one doing the looking, there would now be another superposition involving two versions of you.
At this point, we might be tempted to reach for an altogether different interpretation of quantum mechanics. If the mathematics doesn’t account for the collapse of the wavefunction, why assume it happens at all? Why not suppose that, as Le Guin suggested, you become entangled with the system when you lift the lid? As nobody has ever experienced the eerie sensation of co-existing with multiple versions of themselves, all witnessing different events, we could further suppose that the act of lifting the lid ‘splits’ the Universe into two parallel versions. In one universe, one version of you observed a dead cat. In another universe, another version of you observed a live cat. There is no eerie sensation because these different universes have diverged, and you are completely unaware of the other parallel versions of yourself.
This is the so-called ‘Many Worlds’ interpretation, proposed in 1957 by the American physicist Hugh Everett III. It offers us a multiverse of parallel possibilities. The multiverse allows a much broader and more sophisticated range of ‘what ifs’ beyond the binary alive/dead-type questions relating to a sliding doors moment. What if the effects of your choices accumulate over time and conspire to change not just your future circumstances, but your entire personality?
In a multiverse of possibilities, is there a multiplicity of very different versions of you all behaving very differently and living different lives? Perhaps in one of these universes you are humane and kindly but homeless, reduced to begging on street corners. But in another you are an unempathetic tech billionaire, threatening to undermine the accepted world order. Such questions are explored to great effect in Blake Crouch’s novel Dark Matter (2016), which was adapted for television and broadcast last year on Apple TV+.
The Schrödinger’s cat of popular culture feeds our innately human desire for mystery and gives licence for daring flights of imagination that help us to explore what makes us ‘us’. And, remarkably, it claims to do these things in the name of science, because this is what ‘quantum theory says’. Who thought physics could be so much fun?
This doesn’t mean that the cat is literally alive and dead at the same time
Alas, most physicists adopt a more sober perspective. In the 1920s and ’30s, the founders of quantum mechanics nagged away at these problems of interpretation and arrived at solutions that many deemed satisfactory, though some (like Einstein and Schrödinger) deemed them deeply unsatisfactory.
What does the superposition and, more broadly, the quantum wavefunction, actually represent? One view, most closely associated with the Danish physicist Niels Bohr and known generally as the ‘Copenhagen interpretation’, is that these are merely calculation devices not to be taken literally. They are not real: they are purely symbolic. The superposition simply represents what we know about the cat-in-a-box system, and we use the equations of quantum mechanics to calculate the probabilities for various expected outcomes.
So, when we talk about the cat being in a superposition of life and death, this doesn’t mean that the cat is literally alive and dead at the same time. In truth, we don’t know what the state of the cat really is, nor how to describe its real physical situation, because we can’t say for certain when the radioactive atom will decay, or whether the photon will be transmitted or reflected. But if we represent this system as a superposition, we know we will make predictions that will prove to be consistent with experiment. Most physicists, at least those who can be bothered to think about these things, adopt this view. This might be why they’re not often invited to parties.
It follows from this that it doesn’t matter precisely where in the chain of events we declare that the weirdness stops. It doesn’t matter where we place a ‘Heisenberg cut’, named for the German physicist Werner Heisenberg (of uncertainty principle fame), the point at which we stop using quantum mechanics and switch to more familiar theories of physics published more than 300 years ago by Isaac Newton. This is the point at which we assume the wavefunction collapses, and we replace the and of the quantum superposition with the or of actual outcomes.
Heisenberg was a member of Bohr’s group, although in some of his pronouncements he diverged substantially from Bohr’s philosophy. As far as Heisenberg was concerned, it does not matter where we choose to place the cut. But a cat is not a radioactive atom, nor a photon. It clearly does not belong in the quantum realm, and the equations of quantum mechanics, even interpreted symbolically, should not apply to it. Bohr preferred to place the cut at the point of the ‘irreversible act of amplification’ associated with the early stages of the diabolical device. That we can’t be specific about precisely where or when in this process the weirdness stops doesn’t invalidate the conclusion that this happens long before we get to the cat.
Schrödinger ends his 1935 review with the observation:
The simple procedure provided for this … is perhaps after all only a convenient calculational trick, but one that today, as we have seen, has attained influence of unprecedented scope over our basic attitude toward nature.
We are faced with a choice. We can recognise that quantum mechanics – with all its weirdness – is a purely symbolic framework for predicting the probabilistic outcomes of our experiments. It is indeed a calculational trick, not to be taken literally, which allows us some ability to get a handle on an otherwise unfathomable atomic and subatomic world.
Or we can recognise (with Einstein and Schrödinger) that quantum theory is at the very least incomplete, and deeply unsatisfactory. A theory capable of fathoming the atomic and subatomic world ought to be possible, if only we have the will to look for it, and the wit to find it.
This is the fork in the road. Which path will you take?
