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How can the results from this paper, Quantum superposition of molecules beyond 25 kDa (Fein et. al, 2019) be best understood in the context of molecules? Here's a layperson sci-pop article, Giant Molecules Exist in Two Places at Once in Unprecedented Quantum Experiment (Letzter, Scientific American, 2019), on this as well (which is what I read). From my perspective, having studied some Biochemistry, what is interesting to me is that this result would be interpreted as the molecules "being split" on some dimension (time?) and being re-created at two different places.

I was hoping someone more well-versed in Physics could shed light on how to interpret these results.

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  • $\begingroup$ You can find some resources by searching on-line for the keywords "two-slit experiment" or "double slit experiment." This has been done many times before with smaller particles, including less-complex molecules, and it is one of the iconic experiments used to introduce people to quantum physics. The basic principles are the same regardless of the particle's size or complexity, so studying the two-slit experiment with photons would be a good place to start. $\endgroup$ – Chiral Anomaly Oct 9 '19 at 0:23
  • $\begingroup$ Minor comment to the post (v1): Please consider to mention explicitly author, title, etc. of links, so it is possible to reconstruct links in case of link rot. $\endgroup$ – Qmechanic Oct 9 '19 at 6:27
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The double slit experiment, regardless of the size of the particles (electrons, neutrons, molecules) does not prove that those particles exist in two places at once, as claimed by the SciAm article. The difficulty of understanding this experiment in classical physics is caused by the use of an unsuitable classical model, rigid body Newtonian mechanics with interactions only by direct contact (bullets, biliard balls, etc.).

The right classical model to be used is a field theory, like classical electromagnetism (CE). In such a theory the trajectory of the particle does depend on the entire distribution of field sources (in CE those would be electrons and nuclei), so opening a slit will influence the particle passing through the other slit, contrary to SciAm's claims. In other words, this experiment does not represent an obvious problem for classical physics.

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One way to understand this (and here I mean just that: one way, not anything backed up by any other consideration than its intuitive plausibility, at least to me) is as follow:

Big molecules in a superpostion of different spatial positions are isolated from the rest of the world - this is an experimental requirement to get such a superposition. This means, by definition, that no other physical system is querying their position. If there was one, this would lead to decoherence.

So, we can imagine that spatial relationships are relational: if no-one is asking you where you are, you do not need to answer, and in fact you are nowhere specifically. The structural integrity of a molecule would be a consequence of the interactions between its subsystems, so the fact that the molecule itself does not have an absolute position does not means that it does not have a spatial (and dynamic) organization in itself. It would just mean that its spatial organization is not (or only loosely) connected to that of the rest of the world.

Again, this is not an accepted view in physics: I only provide this picture as a way to escape the lazy but rather meaningless idea that a superposition of states in QM means that a system is in all states at once.

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  • $\begingroup$ That's an interesting perspective, thanks for sharing. $\endgroup$ – stexacc Oct 9 '19 at 8:10
  • $\begingroup$ Isn't it just as difficult to imagine that you are nowhere as it is to imagine that you are everywhere? $\endgroup$ – D. Halsey Oct 9 '19 at 14:17

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