Difference between revisions of "Schrödinger's Cat"
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| − | '''Schrödinger's cat''' is a thought experiment that illustrates | + | '''Schrödinger's cat''' is a thought experiment that illustrates a paradox associated with the [[principle of superposition]] in [[quantum mechanics]], it was proposed by [[Erwin Schrödinger]] in 1935. Schrödinger's cat serves to demonstrate the apparent conflict between what quantum theory tells us is true about the nature and behavior of matter on the microscopic level and what we observe to be true about the nature and behavior of matter on the macroscopic level. |
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| + | ==The Thought Experiment== | ||
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| + | In a paper entitled ''The Present Situation in Quantum Mechanics'', Schroedinger wrote the following: | ||
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| + | <blockquote> | ||
| + | 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 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 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.<ref>http://www.tu-harburg.de/rzt/rzt/it/QM/cat.html#sect5</ref> | ||
| + | </blockquote> | ||
Here's Schrödinger's (theoretical) experiment: We place a living [[cat]] into a [[steel]] chamber, along with a device containing a vial of hydrocyanic [[acid]]. There is, in the chamber, a very small amount of a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a [[hammer]], which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an [[atom]] of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, the cat is both dead and alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.) | Here's Schrödinger's (theoretical) experiment: We place a living [[cat]] into a [[steel]] chamber, along with a device containing a vial of hydrocyanic [[acid]]. There is, in the chamber, a very small amount of a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a [[hammer]], which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an [[atom]] of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, the cat is both dead and alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.) | ||
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| − | + | ==Explanation/Interpretations== | |
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| + | In lay terms, Schrödinger's experiment results in a paradox: According to the quantum mechanical model, because the cat's "live-ness" is not being measured directly, there is no way to know whether or not it is alive or not. As a matter of fact, quantum mechanics dictates that ''it is existing in both states simultaneously.'' Such behavior is not encountered in daily life, but is accepted as possible behavior for subatomic particles. | ||
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| + | There are two primary interpretations of the theory: the Copenhagen interpretation and the many-worlds interpretation. | ||
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| + | ===Copenhagen Interpretation=== | ||
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| + | The Copenhagen interpretation is the interpretation dealing fundamentally with [[decoherence]]. Because quantum mechanics is a statistical method of describing the physical world, there are multiple states that a system can exist in at the same time; this is the principle of [[superposition]]. A quantum system is said to always be in a state of superposition until the system is measured, at which point the system ''decoheres'' and assumes a unique value. Such behavior has been the basis of revolutionary research into computing technologies (see [[quantum computer]]). This interpretation also questions at what time the "measuring" of the system takes place; for instance, a fundamental aspect of the system is measured when a nucleus decays and the event is recorded by the Geiger counter. | ||
| − | + | ===Many-worlds Interpretation=== | |
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| − | + | The many-worlds interpretation deals with the concept that measuring the system is not a significant act. Instead, both states exist simultaneously, but are decoherent with each other, meaning that there is no communication between them. When the system is observed or measured (the box opened), the observer becomes [[quantum entanglement|entangled]] with the system, and essentially, two new observer states are formed - an observer who discovers a dead cat, and an observer who discovers a living cat. At this point, the [[universe]] "splits" into two branches or separate universes that are unable to communicate with each other. | |
| − | + | ==References== | |
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==See also== | ==See also== | ||
* [[Maxwell's demon]] | * [[Maxwell's demon]] | ||
[[category:physics]] | [[category:physics]] | ||
Revision as of 15:12, July 31, 2007
Schrödinger's cat is a thought experiment that illustrates a paradox associated with the principle of superposition in quantum mechanics, it was proposed by Erwin Schrödinger in 1935. Schrödinger's cat serves to demonstrate the apparent conflict between what quantum theory tells us is true about the nature and behavior of matter on the microscopic level and what we observe to be true about the nature and behavior of matter on the macroscopic level.
The Thought Experiment
In a paper entitled The Present Situation in Quantum Mechanics, Schroedinger wrote the following:
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 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 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.[1]
Here's Schrödinger's (theoretical) experiment: We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, the cat is both dead and alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.)
Explanation/Interpretations
In lay terms, Schrödinger's experiment results in a paradox: According to the quantum mechanical model, because the cat's "live-ness" is not being measured directly, there is no way to know whether or not it is alive or not. As a matter of fact, quantum mechanics dictates that it is existing in both states simultaneously. Such behavior is not encountered in daily life, but is accepted as possible behavior for subatomic particles.
There are two primary interpretations of the theory: the Copenhagen interpretation and the many-worlds interpretation.
Copenhagen Interpretation
The Copenhagen interpretation is the interpretation dealing fundamentally with decoherence. Because quantum mechanics is a statistical method of describing the physical world, there are multiple states that a system can exist in at the same time; this is the principle of superposition. A quantum system is said to always be in a state of superposition until the system is measured, at which point the system decoheres and assumes a unique value. Such behavior has been the basis of revolutionary research into computing technologies (see quantum computer). This interpretation also questions at what time the "measuring" of the system takes place; for instance, a fundamental aspect of the system is measured when a nucleus decays and the event is recorded by the Geiger counter.
Many-worlds Interpretation
The many-worlds interpretation deals with the concept that measuring the system is not a significant act. Instead, both states exist simultaneously, but are decoherent with each other, meaning that there is no communication between them. When the system is observed or measured (the box opened), the observer becomes entangled with the system, and essentially, two new observer states are formed - an observer who discovers a dead cat, and an observer who discovers a living cat. At this point, the universe "splits" into two branches or separate universes that are unable to communicate with each other.