Schrödinger's cat

• 1. Introduction

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  Schrödinger's cat is a thought experiment in quantum mechanics, and its inventor described it as a "quite ridiculous case". Physicist Erwin Schrödinger devised it in 1935 as a barb, not a proposal. He aimed it at a prevailing way of thinking about the quantum world, hoping to show how absurd that thinking became when pushed far enough. The barb did not sink its target. Instead the cat became part of the foundation of quantum mechanics, and physicists still reach for it when they argue about what the theory means. That outcome leaves several questions hanging. At what moment does a cloud of possibilities turn into one definite fact? Does anyone need to be watching for that to happen? Has a laboratory ever come close to building something like the cat? And how did an animal born in a physics argument end up in novels, films and webcomics?

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• 2. Einstein's Gunpowder

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  The EPR article, named for its authors Einstein, Podolsky, and Rosen, set the whole argument in motion. It dwelt on how strange quantum superposition is. A system of two particles stays inseparable, the article observed, even when the particles are detected far from the place where they last made contact. The authors concluded that this lack of separability meant quantum mechanics was an incomplete theory of reality.

  Schrödinger wrote to Einstein about the article, and the two men traded letters. In the course of that exchange, Einstein pointed to an unstable keg of gunpowder. After a while, he noted, such a keg holds a superposition of exploded and unexploded states. Schrödinger took the idea further. He asked how a large object could be made to depend on a quantum particle sitting in superposition. His answer was a cat in a closed steel chamber, with its life or death tied to a radioactive atom that had either decayed and emitted radiation or had not.

  Niels Bohr and Werner Heisenberg were the targets. By Schrödinger's reading, their position implied that the cat stays both alive and dead until someone observes it. He had no wish to promote living-and-dead cats as a serious possibility. He wanted to expose the absurdity of that position, a strategy known as reductio ad absurdum. From the exercise he concluded that the wave function cannot represent reality.

  Einstein was impressed. In a letter to Schrödinger in 1950, he wrote: "You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest." Einstein's letter also mentions a charge of gunpowder, which Schrödinger's setup never contained. The gunpowder came from Einstein's own suggestion made 15 years earlier, and he carried it forward into the later discussion. Schrödinger's published version used a Geiger counter and poison instead, and he described it in careful detail.

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• 3. The Infernal Device

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  "Infernal device" is how Schrödinger himself labeled the contraption in the steel chamber, adding that it must be secured against direct interference by the cat. Inside a Geiger counter sits a tiny amount of radioactive substance. It is so tiny that over the course of an hour one atom will perhaps decay, with equal probability that none will. If an atom does decay, the counter tube discharges, and a relay releases a hammer. The hammer shatters a small flask of hydrocyanic acid.

  Leave the whole arrangement alone for an hour, Schrödinger wrote, and you would say the cat is still alive if no atom has decayed. A single decay would have poisoned it. The psi-function of the entire system, he said, would express this by containing the living and the dead cat "mixed or spread out in equal parts". He added the parenthetical "(pardon the expression)" to acknowledge how odd that sounds.

  The oddness is the point. An uncertainty that began among atoms turns into one a person can see at ordinary scale, and it can then be settled by looking. Schrödinger argued that this should stop us from naively accepting a "blurred model" as a picture of reality. A blurred model, taken alone, would contain nothing unclear or contradictory. He compared the difference to that between a shaky, out-of-focus photograph and a snapshot of clouds and fog banks.

  Physicists today call the underlying puzzle the measurement problem. Quantum theory describes the cat as a combination of two possible outcomes, yet only one outcome is ever seen. The question becomes when a quantum system stops being a superposition and becomes one thing or the other. Intuition insists the cat cannot be in two states at once, but the quantum description seems to demand it. The school of thought Schrödinger was attacking offered the first reply.

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• 4. What Counts as Observing

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  The Copenhagen interpretation holds that a measurement yields only one state out of a superposition. Schrödinger's cat makes visible what this view leaves unexplained, which is the condition of the cat while the box stays shut. The wavefunction describes a mixture of "decayed nucleus/dead cat" and "undecayed nucleus/living cat". Only when the box is opened and observed can anything be said about the cat.

  Niels Bohr, a leading figure behind that interpretation, appears from analysis of his work to have seen the cat's state before opening as indeterminate. The superposition itself had no physical meaning for him. The cat would be either dead or alive long before anyone looked, yet the cat and box formed an inseparable combination. Bohr saw no role for a human observer. He stressed the classical nature of measurement results, and he held that an irreversible or effectively irreversible process gives "observation" its classical character.

  In 1932, John von Neumann described a chain in his book Mathematical Foundations of Quantum Mechanics. A device observes the radioactive source, another device observes that device, and so on. Quantum predictions come out the same wherever along the chain the superposition collapses. The chain might end if the last device were replaced by a conscious observer, since an individual's consciousness cannot be multiple. Eugene Wigner asserted that an observer is needed for the collapse to one outcome, either a live cat or a dead one.

  Wigner explored this in a thought experiment called Wigner's friend. Suppose a friend opens the box and sees the cat without telling anyone. From Wigner's conscious perspective, the friend is now part of the wave function, having seen a live cat and a dead cat. Once Wigner learns the result, a third person's perspective would fold Wigner in as well. The chain could be extended indefinitely.

  A different resolution says that the Geiger counter's trigger already counts as a measurement of the radioactive substance. By the time a human looks, the outcome has been decided, and the look merely records it. Roger Carpenter and A. J. Anderson analyzed an actual experiment and found that measurement by an apparatus alone is enough to collapse a wave function before any human knows the result. In their setup the apparatus showed one of two colors. Physicists still dispute whether merely seeing the color counts as consciously observing the outcome.

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• 5. Branching Worlds

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  Hugh Everett formulated the many-worlds interpretation in 1957, and it treats observation as nothing special. After the box is opened, both the alive and dead states of the cat persist. They are decoherent from one another, which means they no longer interact. The observer and the possibly-dead cat split into one observer looking at a dead cat and another looking at a live one. Neither version can communicate with the other.

  Opening the box entangles the observer with the cat. This produces observer states matching a live cat and a dead cat, and each is linked to its own cat so that what is seen and what is real agree. Quantum decoherence ensures the different outcomes have no interaction with each other. Decoherence is generally considered to rule out observing several states at once.

  Cosmologist Max Tegmark proposed a variant called the quantum suicide machine. It looks at the cat experiment from the point of view of the cat. He argues that this approach might help tell the Copenhagen interpretation apart from many-worlds.

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• 6. Four Rival Answers

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  Ensemble interpretations treat superpositions as sub-ensembles of a larger statistical ensemble. The state vector would not describe any individual cat experiment, only the statistics of many similarly prepared ones. Supporters argue that this turns the paradox into a trivial matter or a non-issue. When the physicist opens the box, they simply find out which subensemble that particular cat belonged to.

  The relational interpretation draws no basic line between experimenter, cat and apparatus, or between living and nonliving systems. All are quantum systems obeying the same rules, and all can count as observers. The cat can be an observer of the apparatus, while the experimenter observes the cat and apparatus together. The cat has information about whether the atom decayed, but the experimenter does not. So the wavefunction appears to have collapsed for the cat and to remain in superposition for the experimenter. Only when the box opens and both share the same information do the accounts agree on a cat that is either alive or dead.

  The transactional interpretation has the apparatus send an advanced wave backward in time. It combines with the wave the source sends forward, and together they form a standing wave. These waves are regarded as physically real, and the apparatus counts as an observer. Collapse is "atemporal" and unfolds along the whole transaction between source and apparatus. The cat is never in superposition, and is in only one state at any moment, whenever the experimenter looks.

  Objective collapse theories say superpositions destroy themselves spontaneously once some physical threshold is reached, such as time, mass, temperature or irreversibility. The cat would have settled into a definite state long before the box opens. This could loosely be put as "the cat observes itself" or "the environment observes the cat". Such theories require modifying standard quantum mechanics, and they might be tested by creating mesoscopic superposition states. Energy cat states, for instance, have been proposed as a precise detector for energy decoherence models related to quantum gravity.

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• 7. Cat States in Laboratories

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  Nobody is known to have built Schrödinger's machine, but experiments built on similar principles have superposed objects that are large by the standards of quantum physics. None shows that a cat-sized object can be superposed. Even so, they have pushed upward the known limit on "cat states". In many cases the state is short-lived, even when cooled to near absolute zero.

  A cat state has been achieved with photons. A beryllium ion has been trapped in a superposed state. A superconducting quantum interference device, or SQUID, has been tied to the theme of the thought experiment. One description of it reads: "The superposition state does not correspond to a billion electrons flowing one way and a billion others flowing the other way. Superconducting electrons move en masse. All the superconducting electrons in the SQUID flow both ways around the loop at once when they are in the Schrödinger's cat state."

  A piezoelectric "tuning fork" has been constructed that can be placed in a superposition of vibrating and non-vibrating states. Its resonator contains about 10 trillion atoms. Two further experiments exist only as proposals, one involving a flu virus and another involving a bacterium coupled to an electromechanical oscillator.

  In quantum computing, the phrase "cat state" sometimes means the GHZ state. Several qubits sit in an equal superposition of all being 0 and all being 1. And according to at least one proposal, it may be possible to determine the state of the cat before observing it.

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• 8. The Cat Escapes

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  Historian of science Robert P. Crease says the thought experiment did not become widely known until the 1970s. In 1972, Ursula K. Le Guin learned of it while researching quantum mechanics for her novel The Dispossessed. Crease credits her 1974 short story "Schrödinger's Cat" with carrying the concept into popular culture. Other science-fiction writers soon picked it up, often in a humorous vein.

  Greg Bear's "Schrödinger's Plague" appeared in Analog on the 29th of March 1982. Rudy Rucker's "Schrödinger's Cat" ran in Analog on the 30th of March 1981. George Alec Effinger published "Schrödinger's Kitten" in Omni in September 1988. F. Gwynplaine MacIntyre's "Schrödinger's Cat-Sitter" followed in Analog in July and August 2001.

  Fiction has used the experiment both as a plot device and as a metaphor. It has turned up in genres from apocalyptic science fiction to young-adult drama. It has also appeared in film, poetry, theatre, live-action television, cartoons, music and webcomics. That reach made the cat more prominent in popular culture than in physics itself.

  Robert Anton Wilson's Schrödinger's Cat Trilogy, from 1988, went a step further. It used the cat to illustrate various interpretations of quantum physics, carrying the physicists' quarrel into the pages of a novel.

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