Wheeler's Delayed Choice Experiment - Wheeler's Astronomical Experiment

Wheeler's Astronomical Experiment

In a response to the argument that at short distances interactions at the screen with slits in it might be compromised by "knowledge" of events that occur at the location of the detector screen, Wheeler is reported to have come up with a more elaborate thought experiment. Wheeler suggests that one may imagine a more extraordinary scenario wherein the scale of the experiment is magnified to astronomical dimensions: a photon has originated from a star or even a distant galaxy, and its path is bent by an intervening galaxy, black hole, or other massive object, so that it could arrive at a detector on earth by either of two different paths.

The thought experiment assumes that the emitter of the photon is so positioned that the two paths are equal. If experimenters observe the single photon with a detector screen, e.g., a photographic plate or other imaging device (as in the original experiment), they should see it as part of an interference pattern (to be filled out by additional incoming photons), but if they instead use two telescopes focused to either side of the black hole they may expect to observe the photon only in one of them.

Some interpretations of Wheeler's thought experiment are premised on the belief that interference will indeed occur between the two images, and the crux of the experiment lies in determining whether identifying photons as coming from one referred image or the other will make a difference in experimental outcomes. Experimenters are already gathering light from one referred image (one pathway) by means of one telescope, and they can add light that has come by the other pathway by means of the other telescope.

If experimenters keep the two telescope images separate physically, then they ought not to expect any kind of interference fringes or other "spooky" behavior. And it is known that some photons must have reached earth via each pathway.

On the other hand, if experimenters project the images from the two telescopes onto the same spot on a detection screen and they move the images with respect to each other to change their phase relationship so that they can get cancellation in some areas and reinforcement in another area, they will then get an interference pattern, and will have demonstrated that this experiment is another version of the double-slit experiment.

There appears to be a problem, however. It may be claimed that one knows which series of waves have come by path A and which photons have come by path B, and that one has that knowledge because the photons have been physically fenced in by the tubes out of which the telescopes are constructed. However, once experimenters merge the two images on the detection screen, one can no longer know that a photon that lights up a certain spot on the detection screen has come through telescope A or through telescope B. So they have abandoned that information by mixing the two streams.

There is one more possibility, as indicated in a diagram above. If interference is actually thwarted, then photons should be found only at the position of the two primary maxima. Suppose that experimenters project the images onto two separate detection screens. That should give them a situation analogous to the one where they were viewing a distant light source with only one slit open. In the physics laboratory there are some diffraction effects due to light's having been put through a narrow opening, but not the broad band that is known as the interference fringe.

If interference between the images brought in via two telescopes does appear, experimenters ought to see dimmer images at the secondary, tertiary, etc. maxima predicted for interference effects. They should not expect to see the same range and clarity of secondary images (if any at all) with one telescope capped off. What occurs in this case is again a matter for empirical study to determine.

The idea behind some interpretations of the Wheeler experiment is that it might be possible to determine which side of a double-slit experiment a photon traveled through without destroying the interference pattern that occurs when the two versions of its probability wave interact on the detector screen of the typical double-slit experiment. Another view is that whether interference fringes are noted or not depends not on anything that happened between the distant star and earth. Instead, it depends entirely on what form the observation or the measurement of the photon or photons takes. Looking for a photon on one path or the other will produce the observation of one photon at a single point by a telescope aimed in a certain direction. Looking for an interference pattern by merging the beams coming through both paths will produce interference fringes.