swift

Ah no, I have no idea myself. I've just been trying to make sense of the wiki article itself. It definitely asserts that wheeler rejected the interpretation of 'many experimenters', that interpretation being wave/particle motion decision making and retro activity. I think I found a way of making sense of it, as per my last post, but I'm not sure if my explanation works. Although, to be clear, perhaps I shouldn't have asserted that the article isn't wrong, rather that the story it tells can make sense. There doesn't seem to be a decent resource for understanding the finer points of these experiments on the net, as far as I have found, unfortunately. And, Enthalpy, wow, ok. That's kind of shaken things up for me. I'm getting to the more recent experiments but I'm trying to work my way through history to make understanding the more recent experiments easier. I don't quite get the explanations you give but you're saying that it would be incorrect to consider the photon as being in a quasi state of potential that materialises (perhaps not in terms of it's location, but forms some properties at least) upon observation?

Ok, I've worked it out. The wiki article wasn't wrong. Some experimenters did consider the photon to have travelled in a particle like way in set up 1 and wave like in set up 2. The reason for this is as follows: Set up 1 necessitates particle like behaviour because only one detector at a time makes a record. If the photon travelled as a wave, it would follow both paths and make an impression on both detectors. That's what I thought but then my thinking was that set up 2 should also make an impression on both detectors if it is considered as wave like, which does not follow from the results. However, in set up 2, the wave would destructively interfere with itself on one path thus producing only one record at a given detector at a time, which is consistent the results. Hence, the results in set up 2 are coherent with a wave like propagation of light. The interference fringes exclude particle descriptions. Of course, the idea that the photon 'decides' how to propogate based on the experimental set up and the need for retro causality to explain delayed choice results makes this theory less attractive than the 'wave potentia materialises into a particle upon observation' interpretation, in my view. But at least, I can see where these scientists were coming from now...

Yes. For me, considering the photon as a wave like 'potentia' (i.e. not real, in a state of nascent potential) until 'observation' presents itself as the most obvious interpretation. But I'm just trying to understand the line of scientists who resisted this idea of a 'potential wave' by asserting the photon is always real but travels as either a particle (as in set up 1, without second beam splitter) or wave (as in set up 2, with second beam splitter). You both suggest scientists didn't really have this line, that it was just journalism and lay man misunderstanding? If that's the case, then the wikipedia article is wholly wrong as it presents a completely different story. It essentially suggests that set up 1 demonstrates particle behaviour only and set up 2 wave behaviour only and that scientists (or experimenters here) confirmed this. Quoted from the article: "experimenters generally say that each photon has behaved as a particle from the time of its emission to the time of its detection" (on set up 1) "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" (when starting with set up 1 and changing it to set up 2 after the photon had already been emitted)

But even when you introduce the second beam splitter and witness interference, the photon does not behave completely as a wave. If it did, it would also be split at the second beam splitter and end up at both detectors at the same time. As we know, this doesn't happen, only one detector makes a detection. I want to know why scientists considered the first set up as exhibiting particle like behaviour and second as wave like?

Yes, that page. Consider this, written further down: "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." The article goes on to say some scientists believed in 'retro-causality' to explain the delayed choice experiment observations. They apparently felt that the method of travel that the photon takes is distinctly and unavoidably different in the two cases, inducing the need for such a peculiar theory as retro-causality. But if it's only an option to consider the photon as having travelled as a particle and not a necessity, then why the need to explain how the photon sometimes travels as a particle, sometimes as a wave? This suggests there is something about the first set up (the one without the second beam splitter) that excludes describing the method of travel as wave like but not in the second set up (the one with the second beam splitter). I can't think what. I mean, in the first set up, assuming they slow the photon beam down so they receive one detection at a time, the fact that only one of the detectors records a result at a time and not both of them suggests a singular particle mode of being; a wave would hit both detectors at the same time. But then in the second set up, again, only one detector would record a result at any given time, suggesting a wave like description does not work here either. So why then the insistence that the photon travels like a particle in the first case but a wave in the second?

In the wikipedia entry on wheeler's delayed choice experiment, a description is given of a simple interferometer set up. The article says (if a second beam splitter is not present): "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." What about the experimental results rules out interpreting that the photon has travelled in a wave like way as opposed to particle like way?

Ah, so what I gather from that is its not the fact that you actually measure and find out the polarisation of the entangled pair that collapses the wave function, it's the fact that you could in principle. So due to the mere fact that you have placed a polarisation filter on each slit, the wave collapses and you will never see an interference pattern, whether you measure the entangled twin or not. So delaying measurement of the entangled twin is ineffectual. True.

See edited post above..

Yes, well, theoretically, you could have dp sufficiently far that many electrons have reached ds before any of their entangled twins have reached dp. Hence, you can observe an interference pattern or not. At this point scientist at ds sends a light speed signal to scientist at dp, letting him know whether he observed an interference pattern. Let's say he did, so scientist at dp receives this message and deliberately measures the entangled twins to determine which slit they went through via their polarisation. Or if ds scientist doesn't see an interference pattern, he mesaages this and scientist at dp deliberately removes the detector dp, meaning the electron wave function collapsed despite no observation.

non- local as in the decision made by the scientist at dp affects the outcome at ds? so effect (@ds) occurs before the cause (@dp)? That creates paradoxes. For instance, if electrons were used, which travel slower than light, once the result at ds is known, the scientist at ds could send a signal to the one at dp before the entangled electron has reached dp. Think I'm very confused.

Great. Ok, so here's my real question/ thought experiment. What if dp is placed a great distance (many light seconds) away such that an interference pattern is allowed to be displayed at ds before the first entangled photon is allowed to hit dp. Dp could be inserted or left out at random by a scientist next to it in fact. The photon going through the slits would have to display an interference effect but then after, it's entangled twin could be measured. What would happen here?

To collapse the wave function there is no polariser at pol1, just a detector, right? The entangled photon hits the detector, having not passed through a filter, it's polarisation detected and there is no interference pattern seen at ds. Have I got that right? If yes, then I'm asking what would happen if in the same situation you remove the detector dp, so there is no filter or detector. I'm pretty sure the answer is that there would be an interference pattern right?

If the detector, dp was removed, would there still be an interference pattern?

Appreciate your teachings. I'm still unclear though. What does it mean to 'remove polarisation information'? I thought a filter merely blocks components of the photon that are not aligned with it. Ie a vertical filter will absorb horizontal components. As I currently understand it, if two photons are entangled they will already be polarised and have opposite polarisations. Passing it through a diagonal filter will reduce the amplitude of both a horizontally polarised photon and a vertically polarised one but the polarisation of the photon is still there- how would information regarding it be 'removed'? Secondly, what thought exp gave them the revolutionary conclusion that observation causes wave collapse? It was such an unexpected result that it seems odd they'd reach that conclusion unless forced to by experimental observation.

"But then, if you erase the "which slit" information by putting a diagonal polariser in the path of the detector photon (i.e. the one that doesn't go through the slits) then you cannot determine which slit the other photon went through and the pattern re-appears." I assume a diagonal polarising filter will stop any photon regardless of its polarisation. Where do they place this filter so as to ensure it is in the path of just the detector photon? If they place it over one slit, it is known the other photon has gone through the other slit. If they place it nearer the photon beam, how do they ensure capturing just one of the entangled photons? Plus using entangled photons sounds like the kind of advancement that occurred long after they determined observation of the photon/electron causes wave collapse. What was their method of observation that caused collapse prior to this? Also, how do two photons/electrons become entangled? Thanks in advance.

In the double slit experiment, it is said scientists wanted to observe which slit a singularly fired photon went through once they discovered even this produces an interference pattern. On an atomic level of description, how exactly do they observe a singular photon? Ie what is used to extract information about the photons position at the slit without stopping the photon altogether? And what physical effect does this have on the photon ie does it affect its momentum/energy? To screen used to see the interference pattern actually absorbs the photon and essentially destroys it, so that can't be used to watch the photon pass through a given slit. Same question for an electron.

Lol. Apologies for that. I did get it, I was just paraphrasing Danijel's answer but didn't want to write the whole thing out: 'If' my theory was correct then "the pattern would emerge with one slit", but it doesn't. Hence, I understand how whatever it is interacts with both slits yet when observed has a singular defined position. Wave to particle.

"The pattern would emerge even if one slit were covered" That's the answer I was looking for. Thanks!

I question the conclusion gleaned from the double slit experiment that an electron manifests from a wave like potentia into a real particle. Not that I'm arrogant enough to think I'm right, I question it so that someone may elucidate for me. What is to rule out the possibility that the electron is always a particle which produces a wave like effect because it goes through some kind of deflection (the mechanism of which is currently unknown) which causes it to shoot off at an angle between slit and screen. Perhaps the angle is related to some quantum property of the electron, for instance its momentum. The fact that the momentum can exist in only discrete amounts means the angle exists only in discrete amounts, giving way to areas of darkness on the screen. This makes it look like a wave. Perhaps the greater the momentum, the wider the angle. Perhaps then, the act of detecting the electron by allowing it to pass through a phosphorescent screen (or however it is done) generally reduces the energy and momentum of the electron such that it can no longer be deflected at great angles. Hence, once detection of the electron is made, it is only deflected the minimum amount, ie it appears to go straight on the screen behind, as one would expect a particle to based on our current understanding of particles. How and why the electron is deflected due to it's momentum (or whatever, I simply use that as the first thing that popped into my head) is irrelevant. The point is, what is the argument to logically rule out the idea that there are simply more forces at play that we simply do not know about? I assume there is some way to logically rule out my 'devils advocate' argument above, else physicists jumping to the notion of indeterminate, unreal waves of potentia seems like a bit of a premature leap! Somebody who does physics please help me understand! Also, how exactly do detectors work and how do they physically affect the electron being fired at the slit?