Schrödinger's Cat

The interpretations below are genuinely competing. Presenting one as the answer would misrepresent the state of physics.

Copenhagen

The loose family of views associated with Niels Bohr and Werner Heisenberg. The wave function is not a picture of a cat; it is a device for computing the probabilities of outcomes on classical measuring apparatus. Questions about what the cat is doing between preparation and observation are, on Bohr's account, not questions physics answers, because the description of a quantum system is inseparable from the experimental arrangement that probes it.

The strength: it never makes a wrong prediction and demands no extra structure. The complaint, pressed hard by Einstein and by Schrödinger himself: it draws a line between "quantum system" and "classical apparatus" without saying where the line is or what puts it there. The apparatus is made of atoms too.

Many-worlds (Everett)

Hugh Everett's 1957 proposal simply deletes the collapse postulate. The Schrödinger equation applies always and everywhere. When you open the box you become entangled with it, and the universal wave function now contains a branch with a live cat and a happy you, and a branch with a dead cat and a sad you. Both are real; each version of you sees one definite result.

The strength: one rule instead of two, no vague "measurement", and decoherence explains why the branches never interfere. The standing difficulty is probability. If every outcome happens, it is not obvious what the number 0.5 means, and the various attempts to derive the Born rule from decision theory inside the Everett picture — David Deutsch's and David Wallace's in particular — are not accepted by everyone.

Objective collapse

Collapse is not something observers do; it is a real physical process, and the Schrödinger equation is approximately but not exactly right. The 1986 model of GianCarlo Ghirardi, Alberto Rimini and Tullio Weber adds random spontaneous localisations at a rate so low that a single particle is essentially unaffected, but so amplified by the number of particles that any macroscopic object localises in a fraction of a microsecond. The cat is definite because collapse really happened, long before anyone looked.

The strength: it answers the question directly and it is not an interpretation at all — it is a different theory that makes different predictions, so experiments can rule it out. Interferometry with ever-larger masses and searches for the tiny anomalous heating the models predict have already excluded large regions of the parameter space. That is real scientific progress on a question usually treated as philosophical.

Bohmian mechanics

Particles always have definite positions, and the wave function is a real field that guides them via a deterministic equation of motion. The cat is alive or dead throughout; the wave function's other branch persists but becomes empty and ceases to influence the particles. All standard quantum predictions are recovered exactly.

The price is explicit and its advocates say it openly: the guiding equation is non-local. What happens at one place depends instantaneously on the configuration elsewhere. After Bell's theorem this is less of an embarrassment than it once seemed, since no local theory of the kind Einstein wanted survives anyway.

QBism

On the Quantum Bayesian view developed by Christopher Fuchs, Rüdiger Schack and Carlton Caves, the wave function is not a property of the cat at all. It encodes a particular agent's personal degrees of belief about what they will experience. Collapse is nothing more dramatic than a person updating their expectations when something happens to them. There is no paradox because there was never a claim that the box contains a smeared cat.

The complaint: critics argue this purchases dissolution of the puzzle by declining to say what the world is like when nobody is looking, which is precisely what many people wanted physics for.

What everyone agrees on

Three things are not in dispute and should not be muddled with the above:

1. The predictions of quantum mechanics are the same on every interpretation and are confirmed to extraordinary precision.
2. Decoherence is real, calculable, measurable, and explains the absence of observed macroscopic interference.
3. The cat was Schrödinger's criticism, not his creed.

1926. Schrödinger publishes the wave equation. He initially hopes the wave function describes a real, smeared-out physical density of charge. Max Born's probabilistic reading, also 1926, wins the argument, and Schrödinger never becomes comfortable with it.

1927–1930. Einstein and Bohr conduct their famous exchanges at the Solvay conferences, with Einstein proposing ingenious thought experiments and Bohr answering them.

May 1935. Einstein, Podolsky and Rosen publish their argument that the quantum description is incomplete. Schrödinger writes to Einstein, delighted; Einstein writes back with a simpler version of the argument involving two boxes and a ball, and, in a related letter, the example of an unstable gunpowder keg that the formalism would describe as both exploded and unexploded.

November–December 1935. Schrödinger publishes "Die gegenwärtige Situation in der Quantenmechanik" in Naturwissenschaften, in three instalments. The cat occupies a single paragraph of a long essay. The same essay introduces the term Verschränkung, entanglement, and calls it "the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought." Schrödinger's own summary of the cat is that it prevents us "from so naively accepting as valid a 'blurred model' for representing reality."

1957. Everett's relative-state formulation. Bryce DeWitt popularises it as "many worlds" in the 1970s and gives the cat scenario a new reading.

1970 onwards. H. Dieter Zeh, then Zurek in the 1980s, develop decoherence theory and show quantitatively why macroscopic superpositions are unobservable.

1986. GRW gives the first workable theory in which collapse is a physical process, making the cat question empirically testable rather than purely interpretive.

1996 onwards. Serge Haroche's group in Paris creates "cat states" of a few photons in a microwave cavity and watches decoherence happen in real time, tracking the loss of coherence as it occurs. Haroche shares the 2012 Nobel Prize in Physics with David Wineland for this line of work.

A note on the legend. Schrödinger is very often quoted as a mystic of indeterminacy, and the cat is routinely used to argue that observation creates reality. This inverts his position. He was among quantum theory's sharpest critics, and his cat was a weapon aimed at exactly the reading it is now used to support.

It made a vague discomfort into a specific technical problem. Before the cat, unease about measurement could be waved away as philosophy. Afterwards there was a concrete scenario that any proposed account of quantum mechanics had to address. Every serious interpretation is judged in part on what it says about the box.

It kept foundations alive during decades when they were unfashionable. From the 1930s to the 1960s, quantum foundations was close to a career-ending field; the working attitude was "shut up and calculate". The cat, and Einstein's objections alongside it, preserved the questions that John Bell picked up in 1964 and turned into the most important experimental programme in the field.

It forced the boundary question. "Where does quantum end and classical begin?" is now an experimental discipline with instruments, budgets and a moving record, not a rhetorical flourish. Groups pursue interference with larger and larger objects and place ever-tighter bounds on spontaneous-collapse models. That research programme exists because someone insisted on scaling the question up to something absurd.

It is the reason the essay it appeared in matters. Entanglement — the concept, and the word — comes from the same paper. The single most important resource in modern quantum technology was named in the act of complaining about quantum theory.

Quantum computing. A quantum computer is an engineering effort to keep a system in superposition long enough to compute with it. Decoherence is the enemy in a precise, budgeted sense: coherence times are the headline specification of every processor, and the entire discipline of quantum error correction exists to fight it. Schrödinger's thought experiment describes the central practical obstacle of a multi-billion-dollar industry.

Testing collapse models. Optomechanics experiments cool small mirrors and levitated nanoparticles towards the quantum regime, aiming to place them in spatial superpositions. Alongside underground searches for the anomalous radiation that collapse models predict, these have already ruled out sizeable parts of the GRW parameter space. This is a live experimental programme with a real chance of settling a question that began as a joke about a cat.

Cat states as a technology. "Cat state" is now standard laboratory vocabulary for a superposition of two macroscopically distinguishable states of a resonator. Some of the most promising superconducting error-correction schemes deliberately encode a logical qubit in one, because its symmetry makes certain errors easy to detect.

Public understanding. The cat is the most misused image in popular science — routinely enlisted to suggest that consciousness creates reality, or that anything ambiguous is "quantum". Correcting it is a useful exercise in a general skill: checking whether a famous example was originally offered as support for the claim it is now used to support. Here, it was offered as an attack on it.