S C H R O D I N G E R ' S C A T
"The task is ... not so much to see what no one has yet seen; but to think what nobody has yet thought, about that
which everybody sees."
ERWIN SCHRODINGER
which everybody sees."
ERWIN SCHRODINGER
'Schrödinger's Cat' is a paradoxical thought experiment devised by Erwin Schrödinger in 1935. It illustrates what he identified as the problem of the Copenhagen interpretation of quantum mechanics as applied to everyday objects. The experiment features a luckless cat which might be alive or dead; the outcome dependent on an earlier random event. Whilst developing this experiment Schrödinger coined the term Verschränkung (entanglement).
Schrödinger intended his thought experiment as a discussion of the EPR article, named after its authors—Einstein, Podolsky, and Rosen—in 1935. The EPR article highlighted the strange nature of quantum entanglement, which is a characteristic of a quantum state that is a combination of the states of two systems (for example, two subatomic particles), that once interacted but were then separated and are not each in a definite state.
The Copenhagen interpretation implies that the state of the two systems undergoes collapse into a definite state when one of the systems is measured. Schrödinger and Einstein exchanged letters about Einstein's EPR article, in the course of which Einstein pointed out that the state of an unstable keg of gunpowder will, after a while, contain a superposition of both exploded and unexploded states.
To further illustrate the putative incompleteness of quantum mechanics, Schrödinger describes how one could, in principle, transpose the superposition of an atom to large-scale systems of a live and dead cat by coupling cat and atom with the help of a diabolical mechanism. He proposed a scenario with a cat in a sealed box, wherein the cat's life or death depended on the state of a subatomic particle. According to Schrödinger, the Copenhagen interpretation implies that the cat remains both alive and dead (to the universe outside the box) until the box is opened. Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; quite the reverse, the paradox is a classic 'reductio ad absurdum'.
The thought experiment illustrates the bizarreness of quantum mechanics and the mathematics necessary to describe quantum states. Intended as a critique of just the Copenhagen interpretation (the prevailing orthodoxy in 1935), the Schrödinger cat thought experiment remains a typical touchstone for all interpretations of quantum mechanics. Physicists often use the way each interpretation deals with Schrödinger's cat as a way of illustrating and comparing the particular features, strengths, and weaknesses of each interpretation.
Schrödinger intended his thought experiment as a discussion of the EPR article, named after its authors—Einstein, Podolsky, and Rosen—in 1935. The EPR article highlighted the strange nature of quantum entanglement, which is a characteristic of a quantum state that is a combination of the states of two systems (for example, two subatomic particles), that once interacted but were then separated and are not each in a definite state.
The Copenhagen interpretation implies that the state of the two systems undergoes collapse into a definite state when one of the systems is measured. Schrödinger and Einstein exchanged letters about Einstein's EPR article, in the course of which Einstein pointed out that the state of an unstable keg of gunpowder will, after a while, contain a superposition of both exploded and unexploded states.
To further illustrate the putative incompleteness of quantum mechanics, Schrödinger describes how one could, in principle, transpose the superposition of an atom to large-scale systems of a live and dead cat by coupling cat and atom with the help of a diabolical mechanism. He proposed a scenario with a cat in a sealed box, wherein the cat's life or death depended on the state of a subatomic particle. According to Schrödinger, the Copenhagen interpretation implies that the cat remains both alive and dead (to the universe outside the box) until the box is opened. Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; quite the reverse, the paradox is a classic 'reductio ad absurdum'.
The thought experiment illustrates the bizarreness of quantum mechanics and the mathematics necessary to describe quantum states. Intended as a critique of just the Copenhagen interpretation (the prevailing orthodoxy in 1935), the Schrödinger cat thought experiment remains a typical touchstone for all interpretations of quantum mechanics. Physicists often use the way each interpretation deals with Schrödinger's cat as a way of illustrating and comparing the particular features, strengths, and weaknesses of each interpretation.
If a tree falls in the woods and there’s no one there to hear it, does it make a sound?' The experiment suggests that because there is no one around to witness what had occurred, the cat was alive and dead simultaneously.
It illustrates just how weird the rules of quantum mechanics really were. The thought experiment is notorious for its complexity, which has encouraged a wide variety of interpretations. One of the most bizarre is the 'many worlds' hypothesis, which states that the cat is both alive and dead, and that both cats exist in different universes that will never overlap with one another.
It illustrates just how weird the rules of quantum mechanics really were. The thought experiment is notorious for its complexity, which has encouraged a wide variety of interpretations. One of the most bizarre is the 'many worlds' hypothesis, which states that the cat is both alive and dead, and that both cats exist in different universes that will never overlap with one another.
Schrödinger wrote in the German magazine Naturwissenschaften :
"One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small that perhaps in the course of the hour, one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges, and through a relay releases a hammer that 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 psi-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.
"It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks."
Schrödinger's famous thought experiment poses the question, when does a quantum system stop existing as a superposition of states and become one or the other? If the cat survives, it remembers only being alive. But explanations of the EPR experiments that are consistent with standard microscopic quantum mechanics require that macroscopic objects, such as cats and notebooks, do not always have unique classical descriptions. The thought experiment illustrates this apparent paradox. Our intuition says that no observer can be in a mixture of states—yet the cat, it seems from the thought experiment, can be such a mixture. Is the cat required to be an observer, or does its existence in a single well-defined classical state require another external observer?
Since Schrödinger's time, other interpretations of quantum mechanics have been proposed that give different answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or if) they collapse.
In 1957, Hugh Everett formulated the many-worlds interpretation of quantum mechanics, which does not single out observation as a special process. In the many-worlds interpretation, both alive and dead states of the cat persist after the box is opened, but are decoherent from each other. In other words, when the box is opened, the observer and the already-split cat split into an observer looking at a box with a dead cat, and an observer looking at a box with a live cat. But since the dead and alive states are decoherent, there is no effective communication or interaction between them.
When opening the box, the observer becomes entangled with the cat, so 'observer states' corresponding to the cat's being alive and dead are formed; each observer state is entangled or linked with the cat so that the 'observation of the cat's state' and the 'cat's state' correspond with each other. Quantum decoherence ensures that the different outcomes have no interaction with each other. The same mechanism of quantum decoherence is also important for the interpretation in terms of consistent histories. Only the 'dead cat' or 'alive cat' can be a part of a consistent history in this interpretation.
A variant of the Schrödinger's Cat experiment, known as the quantum suicide machine, has been proposed by cosmologist Max Tegmark. It examines the Schrödinger's Cat experiment from the point of view of the cat, and argues that by using this approach, one may be able to distinguish between the Copenhagen interpretation and many-worlds.
The experiment is a purely theoretical one, and the machine proposed is not known to have been constructed. In quantum computing, however, the phrase 'cat state' often refers to the special entanglement of qubits wherein the qubits are in an equal superposition of all being 0 and all being 1.
"One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small that perhaps in the course of the hour, one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges, and through a relay releases a hammer that 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 psi-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.
"It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks."
Schrödinger's famous thought experiment poses the question, when does a quantum system stop existing as a superposition of states and become one or the other? If the cat survives, it remembers only being alive. But explanations of the EPR experiments that are consistent with standard microscopic quantum mechanics require that macroscopic objects, such as cats and notebooks, do not always have unique classical descriptions. The thought experiment illustrates this apparent paradox. Our intuition says that no observer can be in a mixture of states—yet the cat, it seems from the thought experiment, can be such a mixture. Is the cat required to be an observer, or does its existence in a single well-defined classical state require another external observer?
Since Schrödinger's time, other interpretations of quantum mechanics have been proposed that give different answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or if) they collapse.
In 1957, Hugh Everett formulated the many-worlds interpretation of quantum mechanics, which does not single out observation as a special process. In the many-worlds interpretation, both alive and dead states of the cat persist after the box is opened, but are decoherent from each other. In other words, when the box is opened, the observer and the already-split cat split into an observer looking at a box with a dead cat, and an observer looking at a box with a live cat. But since the dead and alive states are decoherent, there is no effective communication or interaction between them.
When opening the box, the observer becomes entangled with the cat, so 'observer states' corresponding to the cat's being alive and dead are formed; each observer state is entangled or linked with the cat so that the 'observation of the cat's state' and the 'cat's state' correspond with each other. Quantum decoherence ensures that the different outcomes have no interaction with each other. The same mechanism of quantum decoherence is also important for the interpretation in terms of consistent histories. Only the 'dead cat' or 'alive cat' can be a part of a consistent history in this interpretation.
A variant of the Schrödinger's Cat experiment, known as the quantum suicide machine, has been proposed by cosmologist Max Tegmark. It examines the Schrödinger's Cat experiment from the point of view of the cat, and argues that by using this approach, one may be able to distinguish between the Copenhagen interpretation and many-worlds.
The experiment is a purely theoretical one, and the machine proposed is not known to have been constructed. In quantum computing, however, the phrase 'cat state' often refers to the special entanglement of qubits wherein the qubits are in an equal superposition of all being 0 and all being 1.