Schrödinger's Cat: Explaining the Famous Quantum Paradox

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Schrödinger's cat thought experiment

Bohr's Copenhagen Interpretation

In 1927, Niels Bohr, together with ideas developed by Werner Heisenberg, presented the following basic view of quantum mechanics:

1. The wave function as information: The wave function represents what we can know about a system, rather than a physical object in itself.
2. The uncertainty principle: In general, particles do not have definite positions before they are measured.
3. Wave-function collapse: A measurement causes the wave function to collapse into a definite outcome.
4. Born's rule: The wave function gives only the probabilities of possible measurement results.
5. The need for a classical measuring device: A classical apparatus is required to record the final result of a measurement.

Bohr did not specify exactly where the boundary between the quantum world and the classical world should be drawn. In his debates with Einstein, he even treated a box containing photons as a quantum system.

This way of thinking became known as the Copenhagen interpretation, named after the Niels Bohr Institute in Copenhagen, Denmark. It is still one of the most widely taught interpretations of quantum mechanics.

The Von Neumann-Wigner Interpretation

In 1932, John von Neumann and Eugene Wigner extended the Copenhagen interpretation by adding the following ideas:

1. The wave function as information: As in the Copenhagen interpretation, the wave function represents our knowledge of a system.
2. The uncertainty principle: This is the same as in the Copenhagen interpretation.
3. Wave-function collapse: A measurement causes the wave function to collapse.
4. Born's rule: Measurement results are predicted only probabilistically.
5. The role of consciousness: A conscious observer is needed to make a measurement result definite.
6. The von Neumann chain: The boundary between the quantum system and the classical measuring apparatus can, in principle, be shifted along the measurement chain.

This interpretation is known as the Von Neumann-Wigner interpretation.

Erwin Schrödinger was dissatisfied with the idea that consciousness was necessary for measurement. In 1935, he introduced his famous thought experiment, now known as Schrödinger's cat, to point out the difficulty with this view.

The Schrödinger's Cat Paradox

Imagine a cat sealed inside a box with a device that detects radioactive decay. If the device detects decay, it releases poison gas and the cat dies. According to quantum mechanics, before the box is opened, the radioactive atom is in a superposition of the states "decayed" and "not decayed." As a result, the state of the cat also seems to remain undecided until someone observes it.

In the Von Neumann-Wigner interpretation, consciousness causes the wave function to collapse. Therefore, until someone opens the box and observes the result, the cat must be treated as being in a superposition of "alive" and "dead." This is the essence of the paradox.

Schrödinger's point was that the idea of a cat being both alive and dead at the same time is absurd from the viewpoint of ordinary experience. The thought experiment was intended to expose a problem with linking wave-function collapse to conscious observation.

Schrödinger's cat thought experiment

Possible Resolutions

Copenhagen interpretation: Since the wave function is understood as a description of information, the paradox is regarded as only apparent, not as a real contradiction.

Many-Worlds interpretation (Everett, 1957): Both outcomes occur, but in different branches of the universe: in one branch the cat is alive, and in another branch the cat is dead. So far, however, no experiment has definitively confirmed this interpretation.

Which interpretation best describes reality remains an open question. Future developments in physics may eventually settle the issue.

• The Double-Slit Experiment: An Introduction to Quantum Theory (2014/1/26)
• Thin-Film Interference and Anti-Reflective Coatings (2014/9/11)
• What Does the Wave Function Represent? (2013/6/30)

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