Wheeler's delayed-choice experiment

Wheeler's delayed-choice experiment demonstrates that no particle-propagation model consistent with relativity explains quantum theory.

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.

A photon heading exactly towards Earth would encounter the distortion of space in the vicinity of the intervening massive galaxy.

So millions of years ago the photon decided to travel in its guise of particle and randomly chose the other path.

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.

A photon must have started at the laser, passed through one of the slits, and arrived by a single straight-line path at the corresponding telescope.

Therefore, a single photon upon coming into the double-slit diaphragm must have "decided" that it needs to go through both slits to be able to interfere with itself on the detection screen.

Some theorists argue that inserting or removing the screen in the midst of the experiment can force a photon to retroactively decide to go through the double-slits as a particle when it had previously transited it as a wave, or vice versa.

On this score everything we find was foreshadowed in that solitary and pregnant sentence of Bohr, "...it...can make no difference, as regards observable effects obtainable by a definite experimental arrangement, whether our plans for constructing or handling the instruments are fixed beforehand or whether we prefer to postpone the completion of our planning until a later moment when the particle is already on its way from one instrument to another.

Wheeler indicates that Einstein and Bohr explored the consequences of the laboratory experiment that will be discussed below, one in which light can find its way from one corner of a rectangular array of semi-silvered and fully silvered mirrors to the other corner, and then can be made to reveal itself not only as having gone halfway around the perimeter by a single path and then exited, but also as having gone both ways around the perimeter and then to have "made a choice" as to whether to exit by one port or the other.

However, it was easier to say, "We will, during random runs of the experiment, insert the second half-silvered mirror just before the photon is timed to get there," than it was to figure out a way to make such a rapid substitution.

[13][14] Their complicated experiment is based on the Mach–Zehnder interferometer, involving a triggered diamond N–V colour centre photon generator, polarization, and an electro-optical modulator acting as a switchable beam splitter.

With it switched off the path resumes its ordinary mode of action and passes through the second beam-splitter, making interference reappear.

Wheeler's interpretation of the physical results would be that in one configuration of the two experiments a single copy of the wavefunction of an entering photon is received, with 50% probability, at one or the other detectors, and that under the other configuration two copies of the wave function, traveling over different paths, arrive at both detectors, are out of phase with each other, and therefore exhibit interference.

The important thing is that by a third kind of device, a massive stellar object acting as a gravitational lens, photons from a source can arrive by two pathways.

Information from the Twin Quasars that Wheeler used as the basis of his speculation reach earth approximately 14 months apart.

The second way moves farther away from the photon source to a position where the distance between the two copies of the wavefunction is too great to show interference effects.

The cosmic experiment described by Wheeler has other problems, but directing wavefunction copies to one place or another long after the photon involved has presumably "decided" whether to be a wave or a particle requires no great speed at all.

[18]: 66 The first real experiment to follow Wheeler's intention for a double-slit apparatus to be subjected to end-game determination of detection method is the one by Walborn et al.[19] Researchers with access to radio telescopes originally designed for SETI research have explicated the practical difficulties of conducting the interstellar Wheeler experiment.

Nevertheless, it has proven very valuable over the years since it has led researchers to provide "increasingly sophisticated demonstrations of the wave–particle duality of single quanta".

[24] A macroscopic quantum delayed-choice experiment has been proposed: coherent coupling of two carbon nanotubes could be controlled by amplified single phonon events.

The fact that it is possible to decide whether a wave or particle feature manifests itself long after—and even space-like separated from—the measurement teaches us that we should not have any naive realistic picture for interpreting quantum phenomena.

[29] When this statement is applied very strictly, one could argue that by determining the detector type one could force the photon to become manifest only as a particle or only as a wave.

For example, a photon can be detected as the consequences of being absorbed by an electron in a photomultiplier that accepts its energy, which is then used to trigger the cascade of events that produces a "click" from that device.

In the case of the double-slit experiment, a photon appears as a highly localized point in space and time on a screen.