page_title: Wheeler's delayed-choice experiment
section_title: Cosmic interferometer
hierarchical_section_title: Wheeler's delayed-choice experiment / Cosmic interferometer
context_page_description: Wheeler's delayed-choice experiment is actually several thought experiments in quantum physics, proposed by John Archibald Wheeler, with the most prominent among them appearing in 1978 and 1984. These experiments are attempts to decide whether light somehow "senses" the experimental apparatus in the double-slit experiment it will travel through and adjusts its behavior to fit by assuming the appropriate determinate state for it, or whether light remains in an indeterminate state, neither wave nor particle until measured.
  The common intention of these several types of experiments is to first do something that, some interpretations of theory say, would make each photon "decide" whether it was going to behave as a particle or behave as a wave, and then, before the photon had time to reach the detection device, create another change in the system that would make it seem that the photon had "chosen" to behave in the opposite way. Some interpreters of these experiments contend that a photon either is a wave or is a particle, and that it cannot be both at the same time.
context_section_description: In an attempt to avoid destroying normal ideas of cause and effect, some theoreticians suggested that information about whether there was or was not a second beam-splitter installed could somehow be transmitted from the end point of the experimental device back to the photon as it was just entering that experimental device, thus permitting it to make the proper "decision." So Wheeler proposed a cosmic version of his experiment. In that thought experiment he asks what would happen if a quasar or other galaxy millions or billions of light years away from Earth passes its light around an intervening galaxy or cluster of galaxies that would act as a gravitational lens. A photon heading exactly towards Earth would encounter the distortion of space in the vicinity of the intervening massive galaxy. At that point it would have to "decide" whether to go by one way around the lensing galaxy, traveling as a particle, or go both ways around by traveling as a wave. When the photon arrived at an astronomical observatory at Earth, what would happen? Due to the gravitational lensing, telescopes in the observatory see two images of the same quasar, one to the left of the lensing galaxy and one to the right of it. If the photon has traveled as a particle and comes into the barrel of a telescope aimed at the left quasar image it must have decided to travel as a particle all those millions of years, or so say some experimenters. That telescope is pointing the wrong way to pick up anything from the other quasar image. If the photon traveled as a particle and went the other way around, then it will only be picked up by the telescope pointing at the right "quasar." So millions of years ago the photon decided to travel in its guise of particle and randomly chose the other path. But the experimenters now decide to try something else. They direct the output of the two telescopes into a beam-splitter, as diagrammed, and discover that one output is very bright (indicating positive interference) and that the other output is essentially zero, indicating that the incoming wavefunction pairs have self-cancelled.
  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 plus some minutes in the past, so that photons retroactively decided to travel as particles.
  Several ways of implementing Wheeler's basic idea have been made into real experiments and they support the conclusion that Wheeler anticipated — that what is done at the exit port of the experimental device before the photon is detected will determine whether it displays interference phenomena or not. Retrocausality is a mirage.
caption: Wheeler's plan