Who Killed Schrödinger’s Cat?
Inside a sealed box, a cat has a 50% chance of surviving. Its fate is tied to a single atom: depending on the atom’s state, a vial of poison may or may not be released. So is the culprit the atom? Or half of it? The question sounds absurd—something even Sherlock Holmes couldn’t solve.
It’s like the Snow White tale: she bites the poisoned apple and collapses, but later revives after the prince’s carriage jolts the lodged piece from her throat. Was she dead or alive before that? If she later revived, she clearly wasn’t completely dead—but quantum physicist Erwin Schrödinger might have said, “She was both dead and alive at the same time.”
The Electron That Seems to Think
To understand this, recall the famous double-slit experiment. When electrons are fired toward a barrier with two narrow slits, quantum mechanics tells us that each electron can pass through both slits simultaneously—producing an interference pattern of many stripes on the screen, as if it were a wave. Yet electrons are particles.
If we set up detectors to check which slit an electron passes through, the interference pattern disappears. The electrons now behave like classical particles, making only two stripes—one behind each slit.
It’s as if the electron “knows” when it’s being watched: under observation, it behaves like a particle; without observation, it acts like a wave. Of course, electrons have no consciousness—so what exactly does “observation” mean?
The Copenhagen Interpretation
The dominant view in early quantum theory, the Copenhagen interpretation, divides the universe into two domains:
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The macroscopic world, ruled by classical physics, where particles have definite positions and paths.
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The microscopic world, ruled by quantum physics, where particles can exist in multiple states at once—a superposition.
In this view, measurement by a macroscopic apparatus collapses a quantum superposition into a single definite outcome.
But this raises problems:
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The act of “observation” plays a crucial role, yet its physical nature is unexplained.
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Where exactly is the boundary between microscopic and macroscopic worlds? Everything macroscopic is made of atoms—so why should quantum behavior suddenly stop at some arbitrary size?
Einstein once challenged this logic by asking: Does the Moon only exist when we look at it?
Schrödinger’s Cat
In 1935, Schrödinger designed a thought experiment to expose the Copenhagen interpretation’s weakness. A single atom is prepared in a superposition of two states:
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State A triggers a detector that shatters a vial of poison.
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State B leaves the vial intact.
Because the atom is in a superposition of A and B, the vial is simultaneously broken and unbroken, and the cat is both dead and alive—until observation collapses the superposition.
The paradox: the cat is macroscopic, yet is entangled with a quantum system. Where, then, is the boundary between quantum and classical worlds?
Testing the Idea
In 1999, Austrian physicist Anton Zeilinger tested “Schrödinger’s cat” with a large molecule—fullerene (C₆₀), made of 60 carbon atoms. Shooting these molecules through a double slit produced an interference pattern, showing they, too, behaved as quantum waves—if they weren’t disturbed in transit.
If a fullerene collides with even a single air molecule, its quantum interference vanishes. This loss of coherence, called decoherence, is effectively “observation” by the environment—not by a human. Decoherence occurs whenever enough information leaks into the surroundings for the universe to “know” which path a particle took.
In principle, if we could block all decoherence—no air, no light, no stray atoms—you, as a collection of atoms, could also pass through two doors simultaneously. In practice, it’s impossible, because the environment is constantly interacting with you.
