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Schrodinger's cat

Schrödinger's cat is a famous hypothetical experiment designed to point out a flaw in the Copenhagen interpretation of superposition as it applies to quantum theory.

This is a somewhat simplified version of the virtual experiment:

A living cat is placed into a steel chamber along with a hammer, a vial of hydrocyanic acid and a very small amount of radioactive substance. If even a single atom of the radioactive substance decays during the test period, a relay mechanism will trip the hammer, which will in turn, break the vial of poisonous gas and cause the cat to die.

Nobel Prize-winning Austrian physicist Erwin Schrödinger created this mental experiment in 1935 to point out the paradox between what quantum theorists held to be true about the nature and behavior of matter on the microscopic level and what the average person observes to be true on the macroscopic level with the unaided human eye.

The role of the observer in quantum mechanics

The Copenhagen interpretation of quantum physics, which was the prevailing theory at the time, proposed that atoms or photons exist in multiple states that correspond with different possible outcomes and the possibilites , called superpositions, do not commit to a definite state until they are observed.

Schrödinger's thought experiment was designed to show what the Copenhagen interpretation would look like if the mathematical terminology used to explain superposition in the microscopic world was replaced by macroscopic terms the average person could visualize and understand. In the experiment, the observer cannot know whether or not an atom of the substance has decayed, and consequently, does not know whether the vial has broken and the cat has been killed.

According to quantum law under the Copenhagen interpretation, the cat will be both dead and alive until someone looks in the box. In quantum mechanics lingo, the cat's ability to be both alive and dead until it is observed is referred to as quantum indeterminacy or the observer's paradox. The logic behind the observer's paradox is the proven ability of observation to influence outcomes.

Schrödinger accepted that superposition exists; during his lifetime, scientists were able to prove its existence by studying interference in light waves. Schrödinger wondered, however, about when the resolution of possibilities actually occurs. His thought experiment was intended to make people ask themselves if it was logical for observation to be the trigger. Wouldn't the cat be either dead or alive, even if not observed?

Throughout the years, Schrödinger's cat analogy has been used to illustrate emerging theories of how quantum mechanics works. In the Many Worlds interpretation of quantum law, for example, the cat is both alive and dead. In this interpretation, the observer and the cat simply exist in two realities -- one in which the cat is dead, and one in which the cat is alive.

What scientists have learned about the nature of matter at the microscopic level and its relationship to what humans observe at the macroscopic level has not yet been fully explored. The role of the observer remains an important question in the study of quantum physics and is an endless source of speculation and conjecture in quantum computing and pop culture. Schrödinger himself is rumored to have said, later in life, that he wished he had never met that cat.

This was last updated in April 2020

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