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Wheeler's delayed-choice experiment
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Quantum physics thought experiment
Part of a series of articles aboutQuantum mechanics
{\displaystyle i\hbar {\frac {d}{dt}}|\Psi \rangle ={\hat {H}}|\Psi \rangle }
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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.
Concept<br>[edit]
Wheeler's delayed-choice experiment demonstrates that no particle-propagation model consistent with relativity explains quantum theory.[2]: 184 Like the double-slit experiment, Wheeler's concept has two equivalent paths between a source and detector. Like the which-way versions of the double-slit, the experiment is run in two versions: one designed to detect wave interference and one designed to detect particles. The new ingredient in Wheeler's approach is a delayed-choice between these two experiments. The decision to measure wave interference or particle path is delayed until just before the detection. The goal is to ensure that any traveling particle or wave will have passed the area of two distinct paths in the quantum system before the choice of experiment is made.[4]: 967
Cosmic interferometer<br>[edit]
Wheeler's cosmic interferometer uses a distant quasar with two paths to equipment on Earth, one direct and one by gravitational lensing.[2]<br>The Twin Quasar shown in the center of this image, is one star almost 9 billion light-years from Earth that produces two images, a result of gravitational lensing.[5]
Wheeler's cosmic scale thought experiment employs a quasar or other light source in a galaxy billions of light years away. Some of these stars are known to be located behind a massive galaxy that acts as a gravitational lens, bending light rays pointing away from Earth back towards us. The result is two images of the star, one direct and one bent. Wheeler proposed to measure the interference between these two paths. Because the light observed in such an experiment was emitted and passed through the lens billions of years ago, no choice on Earth could alter the outcome of the experiment.[1]
Wheeler then plays the devil's advocate and suggests that perhaps for those experimental results to be obtained would mean that at the instant astronomers inserted their beam-splitter, photons that had left the quasar some millions of years ago retroactively decided to travel as waves, and that when the astronomers decided to pull their beam splitter out again that decision was telegraphed back through time to photons that were leaving some millions of years...