Talk:Wheeler's delayed choice experiment/Archive 1

message: There must be something wrong the article, or with its source(s). The photon that strikes the detector screen is either reflected back toward the emitter (but scattered unless the detector screen is a mirror), or it is absorbed -- which means that it disappears and an electron in orbit around the nucleus of some atom is bumped into higher orbit.  Anything that goes through the detector screen and is observed by a telescope is a different photon. It's like two bullets going through the window of a car. One bullet hits a passenger and the other bullet goes through the car and is observed when it goes right down the barrel of somebody's telephoto lens on a very fast movie camera that is capable of photographing its own imminent destruction. Even in that case one would not know whether the bullet that hit the passenger came from the same assault rifle that hit the camera. If there were two shooters standing side by side the camera could be aimed at one and so get a direct hit from its bullet, but the passenger could have been in direct line with the shot fired by the adjacent shooter. P0M (talk) 09:29, 23 November 2007 (UTC)

message: The article gets confused between one photon and many photons. The reason for the confusion may be that the article on which it depends is not much better. I have difficulty believing that a physicist as eminent as Wheeler could have made the elementary mistakes of reasoning displayed in this article. The cited article is not much better than the Wikipedia article.
Part of the problem may be that if one assumes the conclusion that one purports to prove the whole exercise is compromised. One of the questions that seems to be implied in the Wikipedia article and in the article behind it is whether light is a wave or light is a particle, and the writers seem to assume that light is really a particle. 
The description of the large-scale "experiment" is absurd as it is presented in the article because it assumes that one could isolate one photon and determine which of two telescopes that one photon comes through. The reality, of course, is that there is a star somewhere far away that is sending off a high flux of photons from points all over its surface, and many of those photons are going to reach the observer simultaneously -- and even more are going to arrive within so short a time interval that it would be impossible to separate them in temporal sequence in our experience. 
To cut to the quick, Imagine that one has a detector screen in place and observes the expected interference pattern. Now walk out in front of that screen with telescope in hand and aim at either slit (or either side of the distant black hole or galaxy. Or bore a couple of holes through the detector screen and point your telescope through those holes.  Now dissolve the detector screen. Does anything change? P0M (talk) 18:18, 23 November 2007 (UTC)
thread_title:  First attempt at analysis 

message: I found a quotation from one of Wheeler's interviews (?) in Scientific American that shows that he was clearly aware that the wave/particle dichotomy has everything to do with what is happening at the time that the photon is "observed" (or put into a state where it could be observed).  
If we start thinking about rifle bullets  or other such "particles," the whole two-slit experiment doesn't make sense. In the beginning, when people thought of light as a continuous disturbance in the aether, it was easy to see light waves moving out from the source like water waves moving out from a bobber jiggling on the surface of a pond. When a wave front encounters a barrier wall it is reflected back unless there is a hole or two in the wall, and in that case a portion of the wave goes through each hole. The two holes behave much like two small bobbers replicating the motion of the original bobber. Ripples spread out from each hole onto the water surface beyond. Their interference (reinforcement and cancellation) is manifest on the surface of the pond. Eventually they reach a barrier and manifest their energies in another way by sloshing water onto the barrier wall where there are maxima.
Wave motion spreads out from a "point" (actually a small volume) so it is easy to see how it can go through two holes in a barrier wall. Rifle bullets will, in reality, reach different points even when fired from rifles fixed firmly in vices. The reasons have to do with the machining of the rifle barrel, the differences in individual loads of power, the differences in individual slugs, etc. One can work toward the perfect rifle, the perfect rifle cartridges, and also arrange to shoot the rifle through a vacuum to avoid differences in air pressure, unlucky sparrows, etc. One can arrange for a tighter and tighter spread in the pattern of bullet holes in a target. If a well designed rifle were shot at a double-slit device constructed to scale, it seems likely that many of the shots would hit the part of the wall between slits, many of the shots would hit the parts of the wall outside the slits, and many of them would go through one or the other slit.  The bullets that happen to go through the slits would not lose any energy in the process. The bullets that hit the barrier wall would lose energy to the wall. With water waves, a huge storm outside a barier reef will produce small waves in the protected cove.
Now go back to imagine (on the basis of what we have learned by observations) what happens when a single photon is emitted from some emitter, e.g., a laser.  We can't even do the equivalent of bouncing bullets off flying bullets to try to "see" bullets. We have no way of observing whatever is going on between the time the photon is presumably emitted and the time when its presence is manifested by a flash on the detection screen. But it is clear that putting a wall up in the middle of things makes a big difference. My understanding of the work of the major physicists working on quantum mechanics is that there is some, very low, probability that the photon will "tunnel" right through the wall. There are some spooky things about how long it takes to transverse distance in such cases. But in the vast majority of cases the "spook" hits the wall and is either reflected or is absorbed.  That is certainly what we expect from looking what happens when flashlights are shone on black walls, white walls, mirrors, etc.
If the "spook" does advance forward, whatever it is seems to act in some ways analogous to the behavior of the water wave. When it reaches a wall with slits in it, something happens -- especially if it is the part of the "spook" that is in a direct line from the emitter. We do not know what happens to the "ghost," but we know from long experience that we can very reliably make predictions by following the methods of Huygens (drawing a series of circles along the gap, joining them in a smooth curve, drawing another series of circles along that curve, and so on). From a single slit we will see a diffraction pattern develop, and from a double slit we will see an interference pattern develop. These patterns move out from the second surface of the barrier wall. What happens next seems to depend on what is in the way of the expanding "ghost".  It has a certain probability of appearing as a "flash of light" at different places -- but it is not only the place that is important but what is located at that place. A vacuum will rarely if ever produce anything. (Maybe part of the explanation for the spontaneous appearance of matter-energy in pure vacuums has something to do with the occasional light photon passing by?)  If there is a half-reflective mirror in the path of its expanding wave front then it may pass through or it may be reflected. If it encounters a solid wall, e.g. a white plaster surface, it will almost surely be manifested as a flash of light at one of its maxima.  Walls of different composition do not seem to alter the interference patterns manifested on them extect to the extent that black patches absorb light better than do white patches.  The amount of energy, i.e., the number of photons, that gets manifested seems to be invariant.
The operational definition of the word "to observe" seems always to boil down to our doing something to a "ghost" that will make it manifest in a highly localized thing that we call a photon.  If we have one slit, we have a slightly different kind of "localization" -- a localization by elimination. If we have two slits and attempt to determine which slit a given photon has gone through, most if not all attempts have involved detectors that cause the "ghost" to manifest as a photon in the detector so that we can observe it. But at that point the trajectory of the photon emitted by the laser has terminated, and if any photon goes on from that point it is a photon that is emitted because of the delivery of the first photon.  Moving that detector farther and farther away changes the size of the mouth of the detector that is needed to ensure trapping the "ghost," but if the "ghost" is trapped it is no long a "ghost" but has manifested as a photon.  Even if the detector is moved to just in front of the target wall, it will still cause decoherence because it will change "ghost A" into a detected photon and then into "ghost B" and "ghost B" will be what hits the detection screen.P0M (talk) 20:29, 27 November 2007 (UTC)
thread_title:  Further research 

message: The association of Wheeler's thought experiment with distant sources of light sent on two paths by gravitational lensing seems not to be in the article by Wheeler that has been cited. What is the source, if any, for the binocular telescope stuff in Walker's own writing? P0M (talk) 19:48, 27 December 2007 (UTC)
thread_title:  Ungrounded assertions