Wheeler's delayed-choice experiment

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${\displaystyle i\hbar {\frac {d}{dt}}|\Psi \rangle ={\hat {H}}|\Psi \rangle }$ Schrödinger equation
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Wheeler's delayed-choice experiment describes a family of thought experiments in quantum physics proposed by John Archibald Wheeler, with the most prominent among them appearing in 1978 and 1984.^[1] These experiments illustrate the central point of quantum theory: "It is wrong to attribute a tangibility to the photon in all its travel from the point of entry to its last instant of flight."^[2]: 184

These experiments close a loophole in the traditional double-slit experiment demonstration that quantum behavior depends on the experimental arrangement. The experiment closes the loophole that a photon might adjust its behavior from particle to wave behavior or vice versa. By altering the apparatus after the photon is supposed to be in "flight", the loophole is closed.^[1]

Cosmic versions of the delayed-choice experiment use photons emitted billions of years ago; the results are unchanged.^[3] The concept of delayed choice has been productive of many revealing experiments.^[1] New versions of the delayed choice concept use quantum effects to control the "choices", leading to the delayed-choice quantum eraser.