How does objective reality emerge from the palette of possibilities supplied by quantum mechanics? That question — the deepest and most vexed issue posed by the theory — is still the subject of arguments a century old. Possible explanations for how observations of the world yield definite, “classical” results, drawing on different interpretations of what quantum mechanics means, have only multiplied over those hundred or so years.
But now we may be ready to eliminate at least one set of proposals. Recent experiments have mobilized the extreme sensitivity of particle physics instruments to test the idea that the “collapse” of quantum possibilities into a single classical reality is not just a mathematical convenience but a real physical process — an idea called “physical collapse.” The experiments find no evidence of the effects predicted by at least the simplest varieties of these collapse models.
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