Hard SF : Science Issues : Quantum Physics & The 2-Slit Experiment

Quantum Physics & The 2-Slit Experiment

True confessions: I can tell you what physicists report from experiments on probabilistic quantum behavior, but I can't make sense of it.

Physicists have what is referred to as a "two slit experiment". To explore the "wave / particle duality" of quantum physics, scientists send a beam towards a detector. Between the projector and the detector is a barrier with two slits. Based on our common experiences, we would expect the beam (or part of the beam) to go through one or the other slit in the barrier. Thinking of the beam as being a series of particles, we can imagine some particles going through slit A and some through slit B. This can result in an interference pattern on the detector – suggestive of waves going through the two slits then interacting. In actual experiments, scientists have found that sending a single "particle" can cause an interference pattern as if the one "particle" went through both slits. However, if you add another kind of detector at the slits to identify which slit(s) the "particle" passes through, the experiment results are different - there is no interference pattern and everything seems consistent with the idea of one "particle" going through only one slit. Somehow merely attempting to determine what happens causes the results to be different. The following discussion assumes you have some familiarity with this issue.

I could accept that quanta are fuzzy things that aren't limited to one definitive location (the way baseballs are) and therefore can go through both openings in a two-slit experiment. But if you put detectors by the slits to show where it goes I’d expect it to say either (1) something went through both slits or (2) nothing substantial enough to detect went through either slit. To one degree or another, a physicist would explain that it is a probability wave that passes through both slits, but we have no way to detect a probability wave. Yet, if nothing other than an undetectable probability wave passes through the slits, what is it that is detectable enough to cause an interference pattern when we don't have detectors on the slits themselves? And what is detectable enough to say something went through only one slit if we do have detectors on the slits? The simple fact we need to put slits in the barrier tells us we are dealing with something detectable. If a barrier without slits will stop whatever it is, the thing interacts with matter enough that it should interact with some kind of detector. If a probability wave can't be detected by a material detector, it presumably can't be blocked by a material barrier. So, if something more substantial than a probability wave goes through both slits (when we're not using detectors at the slits), why can't we detect it going through both?

There is apparently "something" - a physical wave, a particle or some phenomenon that lacks a good analogy in our sensory experience - that is capable of causing either an interference pattern or a single slit particle-like result.

One of the proposed ways of looking at quantum reality is the "many worlds" view. I could accept that branching of universes could occur. However, if in our universe the quanta go through the left slit and in the other universe it goes through the right, there is only one path taken in our universe. Therefore, I see no basis for an interference pattern. One would need a version of "many worlds" that says branching only occurs if an observation takes place, otherwise all alternatives occur in our universe. According to that version, I would think we would have to conclude that all alternatives occurred in our universe, without branching, for billions of years until the possibility of observations became available. What would it mean to have all alternatives for every quantum event piling up and compounding for billions of years? In the short run, if a single "particle" is sent out and results in one "particle" going through Slit A and another through Slit B, wouldn't that violate conservation laws? (Conservation laws can be violated for miniscule amounts of time, but I gather a two-slit experiment can be designed that would indicate two particle paths for longer than that.) Also, does an observer-dependent branching of universes really provide an advantage over observer-dependent events in a universe without “many worlds” - or is it just an additional layer that neither explains how observation affects results nor how (without observation) a single quanta takes both paths?

Research Ideas

I wish I was in a position to try these experiments (or see the results someone else got):

Are the experimental results actually human interpretations based on the mind's attempt to "pigeon hole" quanta into "particle" and "wave" concepts that aren't entirely accurate descriptions of quanta?

  1. Have a person, AI or whatever that thinks of the universe in a manner that doesn't include point-like particles carry out observations on quantum events. See if their results lack particle-like events.
  2. Use an experiment that does not "detect" which slit is used, but "tags" quanta differently depending on which slit it passes through. Have a computer randomly select what tagging will be used for which slit, have the computer store a record of which tagging it used for each experiment. Computer fires quanta at slits with a film on other side. Nobody checks film yet. Have computer delete its records of which tagging it used. Then check film. Does the deletion of the computer records result in an interference pattern?
  3. Is it possible to tag quanta in two ways that aren't mutually exclusive? For instance, is it possible to tag quanta going through the right slit as having its orientation set to a position on the X axis, and if it goes through the left slit have it set the orientation to a position on the Y axis? If so, the process of tagging it at one slit doesn't overwrite or nullify the tagging at the other slit. Does this affect the results of two-slit tagging experiments?

Further Questions

Insofar as an “observation” can effect the results of a two-slit experiment, I can’t help wondering what constitutes an “observation”. If a chimp or dog sees an indicator light respond to the experimental apparatus, is that an “observation”? What’s tricky is how do you verify whether the dog affects the results without having a human do something that would constitute a human observation? Perhaps you could have one indicator light for Slit A and one for Slit B, train a dog to step on one lever #1 if only indicator A lights up, step on lever #2 if only indicator B lights and step on lever #3 if both light up. These levers simply add one to counters; they don’t say which particular “particle” contributed to which particular counter. We would then know if the dog observations resulted in only the “both” counter being added to, or if the “only A” and “only B” get added to, or if none of the counters get increased. Of course, a human will observe the counters; can we exclude the possibility that impacts the results?

One might try a variant without the dog. Have three separate “two slit experiments” carried out simultaneously. Between them, there are a total of six slits, each with a detector that will add 1 to a commonly shared counter if a “particle” goes through the slit. A single particle is fired at each of the three experimental apparatuses. The shared counter might end up with any number between zero and six. If the number is zero, it suggested no particles passed through any slit. If the count in six, it indicates particles went through both slit on all of the three places. Perhaps being able to deduce that constitutes an “observation”, maybe not.

Suppose we have just one “two slit experiment”. One at a time we fire 100 “particles”. The detector at Slit A adds 1 to a shared counter. The detector at Slit B subtracts 1 from the shared counter. If the final count is zero, we don’t know whether each of the 100 times it registered at both slits, whether it never registered at either slit, or whether it simply registered 50 at A and 50 at B. If we repeat the process numerous times, and always get zero, it’s unlikely that every time there are exactly 50 at A and 50 at B – so it is probably not “single particle through one slit” results. But if the counter is not zero, it means at least some of the time it was registering at one slit but not the other – what is usually only seen if an observation occurs. Would it mean an observer effect or would it be an indication of what really happens observer or not?