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 loophole has been called a "conspiracy" model where light somehow "senses" the experimental apparatus, adjusting its behavior to particle or wave behavior. By altering the apparatus after the photon is supposed to be in "flight" the loophole is closed. Cosmic versions of the delayed-choice use photons emitted billions of years ago; the results are unchanged.^[3] The concept of delayed choice has been productive of many revealing experiments.^[4] New versions of the delayed choice concept use quantum effects to control the "choices", leading to quantum delayed-choice experiments.