Schrödinger's Cat
A thought experiment that illustrates the relationship between quantum superposition and observation in quantum mechanics
The Thought Experiment
In 1935, Austrian physicist Erwin Schrödinger proposed a famous thought experiment to illustrate the problems with the Copenhagen interpretation of quantum mechanics.
Scenario: A cat is placed in a sealed box with a radioactive atom, a Geiger counter, a hammer, and a vial of poison. If the Geiger counter detects radiation (indicating the atom has decayed), the hammer breaks the vial, releasing the poison and killing the cat.
According to quantum mechanics, the radioactive atom exists in a superposition of both "decayed" and "not decayed" states until it is observed. By extension, the cat would be both alive and dead until the box is opened and the system is observed.
Purpose of the Experiment
Schrödinger didn't actually believe a cat could be both alive and dead. He created this thought experiment to highlight what he saw as absurd implications of the Copenhagen interpretation.
Quantum Superposition
Quantum particles can exist in multiple states simultaneously until measured.
Wave Function Collapse
Observation forces a quantum system to "choose" one definite state.
Macroscopic Implications
If quantum rules apply to macroscopic objects, absurd situations like a cat being both alive and dead would be possible.
Schrödinger's Cat Setup
The experiment links a quantum event (radioactive decay) to a macroscopic outcome (cat's state)
Radioactive Atom
50% chance of decay in one hour
Poison Vial
Releases if atom decays
Cat in Superposition
Alive AND Dead simultaneously
Cat Alive
If no decay occurs
Cat Dead
If decay occurs
Before observation, the entire system exists in a superposition of both possible states.
Interactive Observation
In quantum mechanics, observation collapses the wave function from superposition to a definite state. Try opening the box to observe the cat's state:
Key Insight: Schrödinger's cat highlights the measurement problem in quantum mechanics - when does the wave function collapse? At the moment of radioactive decay, Geiger counter detection, or when a human observes the cat?
Historical Context
Schrödinger was responding to Niels Bohr's Copenhagen interpretation, which states:
- Quantum systems exist in superpositions until measured
- Measurement causes wave function collapse to a definite state
- We can only predict probabilities of outcomes
Schrödinger found it absurd that this could apply to everyday objects. His cat experiment was meant to show that quantum mechanics, as interpreted by Bohr, led to paradoxical conclusions when scaled up.
Modern Interpretations
Today, Schrödinger's cat remains important for understanding different interpretations of quantum mechanics:
Copenhagen Interpretation
The cat is both alive and dead until observed. Observation collapses the wave function.
Many-Worlds Interpretation
Both outcomes occur in separate, non-communicating branches of reality. The cat is alive in one universe and dead in another.
Objective Collapse Theories
Wave functions collapse spontaneously when systems reach a certain size, so the cat is never truly both alive and dead.
Quantum Decoherence
The cat interacts with its environment, causing apparent collapse without needing an observer.
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