Schrödinger's Cat

[List]

Schrödinger’s cat thought experiment

Bohr’s Copenhagen Interpretation

In 1927, Niels Bohr (with contributions from Werner Heisenberg) outlined these key principles of quantum mechanics:

1. Wave-function as information: The wave function represents our knowledge of a system, not a physical entity.
2. Uncertainty principle: Particles generally do not have definite positions until measured.
3. Wave-function collapse: Measurement causes the wave function to collapse to a definite outcome.
4. Born’s rule: The wave function provides only probabilistic predictions for measurement results.
5. Classical apparatus requirement: A classical device is needed to record final measurement outcomes.

Bohr never specified exactly where the quantum-classical boundary lies; in debates with Einstein, he even suggested that a box of photons remains a quantum system.

This framework became known as the Copenhagen Interpretation—named after the Niels Bohr Institute in Copenhagen, Denmark—and remains the dominant view today.

Von Neumann-Wigner Interpretation

In 1932, John von Neumann and Eugene Wigner extended the Copenhagen view with these refinements:

1. Wave-function as information: As above, the wave function reflects our knowledge.
2. Uncertainty principle: Same as in Copenhagen.
3. Wave-function collapse: Measurement induces collapse.
4. Born’s rule: Outcomes are probabilistic.
5. Role of consciousness: A conscious observer is required to finalize measurement results.
6. Von Neumann chain: The cut between quantum and classical systems can be moved arbitrarily along the measurement chain.

This extension is called the Von Neumann-Wigner Interpretation.

Dissatisfied with the necessity of consciousness, Erwin Schrödinger introduced in 1935 a famous thought experiment—Schrödinger’s cat—to expose potential flaws.

Schrödinger’s Cat Paradox

A cat is placed in a sealed box alongside a detector for radioactive decay that, if triggered, releases poison gas. According to quantum mechanics, the radioactive isotope exists in a superposition of “decayed” and “undecayed” states, so its actual state—and thus whether the cat is alive or dead—remains undefined until observed.

Under the Von Neumann-Wigner view, consciousness itself collapses the wave function. Therefore, until someone opens the box and checks, the cat exists in a superposition of “alive” and “dead”—the essence of the paradox.

Schrödinger’s point was that imagining a cat both alive and dead simultaneously contradicts common sense, highlighting a potential issue with attributing collapse to consciousness.

Schrödinger’s cat thought experiment

Proposed Resolutions

Copenhagen Interpretation: Since the wave function is purely informational, the paradox is merely apparent and poses no true conflict.

Many-Worlds Interpretation (Everett, 1957): Each outcome—alive or dead—occurs in its own branch of the universe. To date, no experiment has definitively confirmed this picture.

Which interpretation best describes reality remains an open question—future advances may ultimately resolve the debate.

• The Double-Slit Experiment in Quantum Mechanics: Guiding You into the Mysterious World of Quantum Theory (2014/1/26)
• Thin-Film Interference in Quantum Mechanics: Explaining the Mysterious Properties of Anti-Reflective Coatings (2014/9/11)
• The Mystery of Wave Functions: Examining the Physical Meaning of Wave Functions (2013/6/30)

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