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Dead AND Alive
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Dead AND Alive

Credit: Laura Guerin
Source: CK-12 Foundation
License: CC BY-NC 3.0

Imagine a sealed box with a cat and a flask of poison inside. The flask has an equal chance to either remain intact or break, which would save or kill the cat, respectively. Until we open the box, we won't actually know if the cat is dead or alive. At this point, the cat is both dead and alive. Only after opening the box will we find out for certain whether or not the cat survived. 

Amazing But True

Credit: Nobel Foundation
Source: http://commons.wikimedia.org/wiki/File:Schrodinger.jpg
License: CC BY-NC 3.0

Erwin Schr?dinger won a Nobel Prize for his contributions to quantum mechanics [Figure2]

  • This thought experiment, known as Schr?dinger's cat, was devised by Erwin Schr?dinger to model the nature of quantum mechanics. Before opening the box, the cat is in what is called a superposition of states; it's both dead and alive. This concept of a dual state defines quantum mechanics. Only the act of observation reveals whether or not the cat is alive or dead.
  • Erwin Schr?dinger's described this thought experiment as follows: 

"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."

?Erwin Schr?dinger, Die gegenw?rtige Situation in der Quantenmechanik (The present situation in quantum mechanics), Naturwissenschaften (translated by John D. Trimmer in Proceedings of the American Philosophical Society)

Explore More

Using the information provided above, answer the following question.

  1. How does the act of the observer looking in the box with the cat affect the probability wave functions?

Image Attributions

  1. [1]^ Credit: Laura Guerin; Source: CK-12 Foundation; License: CC BY-NC 3.0
  2. [2]^ Credit: Nobel Foundation; Source: http://commons.wikimedia.org/wiki/File:Schrodinger.jpg; License: CC BY-NC 3.0

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