page_title: Wheeler's delayed-choice experiment
section_title: Simple interferometer
hierarchical_section_title: Wheeler's delayed-choice experiment / Simple 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: One way to investigate the question of when a photon decides whether to act as a wave or a particle in an experiment is to use the interferometer method. Here is a simple schematic diagram of an interferometer in two configurations:
  If a single photon is emitted into the entry port of the apparatus at the lower-left corner, it immediately encounters a beam-splitter. Because of the equal probabilities for transmission or reflection the photon will either continue straight ahead, be reflected by the mirror at the lower-right corner, and be detected by the detector at the top of the apparatus, or it will be reflected by the beam-splitter, strike the mirror in the upper-left corner, and emerge into the detector at the right edge of the apparatus. Observing that photons show up in equal numbers at the two detectors, experimenters generally say that each photon has behaved as a particle from the time of its emission to the time of its detection, has traveled by either one path or the other, and further affirm that its wave nature has not been exhibited.
  If the apparatus is changed so that a second beam splitter is placed in the upper-right corner, then part of the beams from each path will travel to the right, where they will combine to exhibit interference on a detection screen. Experimenters must explain these phenomena as consequences of the wave nature of light.  Each photon must have traveled by both paths as a wave, because if each photon traveled as a particle along just one path then the many photons sent during the experiment would not produce an interference pattern.
  Since nothing else has changed from experimental configuration to experimental configuration, and since in the first case the photon is said to "decide" to travel as a particle and in the second case it is said to "decide" to travel as a wave, Wheeler wanted to know whether, experimentally, a time could be determined at which the photon made its "decision." Would it be possible to let a photon pass through the region of the first beam-splitter while there was no beam-splitter in the second position, thus causing it to "decide" to travel, and then quickly let the second beam-splitter pop up into its path? Having presumably traveled as a particle up to that moment, would the beam splitter let it pass through and manifest itself as would a particle were that second beam splitter not to be there? Or, would it behave as though the second beam-splitter had always been there? Would it manifest interference effects? And if it did manifest interference effects then to have done so it must have gone back in time and changed its decision about traveling as a particle to traveling as a wave. Note that Wheeler wanted to investigate several hypothetical statements by obtaining objective data.
  Albert Einstein did not like these possible consequences of quantum mechanics. However, when experiments were finally devised that permitted both the double-slit version and the interferometer version of the experiment, it was conclusively shown that a photon could begin its life in an experimental configuration that would call for it to demonstrate its particle nature, end up in an experimental configuration that would call for it to demonstrate its wave nature, and that in these experiments it would always show its wave characteristics by interfering with itself. Furthermore, if the experiment was begun with the second beam-splitter in place but it was removed while the photon was in flight, then the photon would inevitably show up in a detector and not show any sign of interference effects. So the presence or absence of the second beam-splitter would always determine "wave or particle" manifestation. Many experimenters reached an interpretation of the experimental results that said that the change in final conditions would retroactively determine what the photon had "decided" to be as it was entering the first beam-splitter. As mentioned above, Wheeler rejected this interpretation.
caption: Open and closed