Schrodinger's cat

Schrodinger's cat is a thought experiment devised by Erwin Schrodinger in 1935 to illustrate what he saw as an absurd consequence of applying quantum superposition to everyday objects. In the scenario a cat in a sealed box would, according to a literal reading of the formalism, end up in a superposition of alive and dead. Schrodinger used the image to dramatize the measurement problem, not to describe a real experiment.

#The scenario

A cat is placed in a closed box with a small amount of a radioactive substance, a detector, a hammer, and a flask of poison. Over some interval a single atom has a fifty percent chance of decaying. If it decays, the detector triggers the hammer, breaks the flask, and kills the cat; if it does not, the cat lives. The atom, being a quantum system, evolves into a Superposition of decayed and not decayed. Because the cat's fate is linked to the atom through the apparatus, quantum mechanics applied without interruption would place the whole system, atom and cat together, in an entangled superposition of the two outcomes (Stanford Encyclopedia of Philosophy). The cat would be neither definitely alive nor definitely dead until the box is opened and observed.

#The point Schrodinger was making

Schrodinger introduced the example in his 1935 paper on the present situation in quantum mechanics, written in dialogue with the Einstein, Podolsky, and Rosen argument about entanglement published the same year. His aim was critical. He found it unacceptable that the theory offered no clear boundary between the microscopic realm, where superposition is routine, and the macroscopic world, where cats are always either alive or dead. The cat sharpens the question of why and how a superposition of possibilities becomes a single definite fact, and where in the chain from atom to observer that transition is supposed to occur.

#Responses

Interpretations of quantum mechanics answer the puzzle differently. The Copenhagen interpretation holds that the superposition is resolved when a measurement occurs, though it does not fix a sharp definition of measurement. The many-worlds interpretation denies any collapse: both outcomes occur, in branches of the wave function that no longer interact, so there is a living cat in one branch and a dead cat in another. Objective-collapse models modify the dynamics so that superpositions of large systems break down on their own extremely quickly.

A widely accepted physical ingredient is decoherence. A macroscopic object like a cat interacts constantly with its environment, and this interaction rapidly destroys the interference between the alive and dead components, making the superposition practically impossible to observe. Decoherence explains why cat-like superpositions are never seen in the lab, though on its own it does not settle which single outcome is realized, which is the residual core of the measurement problem.

#Legacy

The cat has become the most recognizable image in the public understanding of quantum mechanics, often misread as a claim about mystical observation rather than a critique of the theory's foundations. In physics it names a real research target: so-called cat states, superpositions of distinguishable states of increasingly large systems such as trapped ions, superconducting circuits, and small mechanical resonators, are created and studied to probe how far quantum superposition extends. Erwin Schrodinger, who shared the 1933 Nobel Prize for wave mechanics, is remembered as much for this thought experiment as for the equation that bears his name (Nobel Foundation).