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As far as I know, we can never directly observe the superposition of states of any particle. How do we then know that it is real in the first place?

We have a theory, which isn’t fully defined in the first place, that suggests that a wave function which helps us derive probabilities “collapses” into a particular state when measured. But how can we actually know this, especially when we still haven’t figured out what a measurement consists of?

If this superposition of states is fundamentally unobservable and arguably untestable, why or how is this considered scientific in any form? Is this just a speculative narrative that we’ve conjured up in order to try to make sense of things?

Suppose I came up with a theory that said an electron is in no quantum state before observation (or maybe even doesn’t exist at all), but during observation, it magically and instantaneously turns into a particular state out of thin air for no reason. These observations of states then follow certain frequencies at different positions in relation to a wave function which evolves according to Schrödinger’s equation.

This interpretation, then, would seemingly still match all the data. How is this interpretation that I just conjured up any less magical/speculative/untestable than the superposition interpretation?

The philosophical significance arises if quantum states relate to consciousness. See Goff, Seager, Sprigge, et al.

What if the stuff of the universe is matter-consciousness? Observation then occurs at the quantum level.

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  • sometimes the interpretation is a consensus, other times there is something more profound in it. Nov 7, 2023 at 13:50
  • Apparently, you do not know very far. Superposition of states is just another state taken relative to some basis, "observing" it makes no more sense than observing coordinate axes. Should those also be considered unscientific? The collapsed "particular state" is also a superposition of states in some other basis. So how are observations of states supposed to happen without states, and follow "certain frequencies at different positions" (what positions?) to boot? It is hard to tell what this "interpretation" even means, let alone how it matches something.
    – Conifold
    Nov 7, 2023 at 14:03
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    @Conifold When I say superposition, I am talking about superposition of the basis states. Sure, each measurement can be represented as a superposition of basis states, but one and only one basis state is measured each time. So how then can we ever actually “confirm” that the particle “could have” been in one or the other basis state before measurement and then “collapses” into one. I could just as well construct an interpretation where the electron is in neither state before measured. I could even propose the electron doesn’t exist between measurements. This would still fit the data.
    – user62907
    Nov 7, 2023 at 14:21
  • To make the question more general, what exactly is happening in a physical sense to the electron in between measurements? Is it in one of those basis states? Is it in neither? Both at the same time? It seems that one cannot confirm either of this
    – user62907
    Nov 7, 2023 at 14:26
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    @thinkingman Those kind of recipes don't sit idly without analysis by philosophy. Also textbook QM does propose particles and/or fields, space, and measurement apparatuses at least. Every physics textbook I've come across on QM quickly mentions particles
    – J Kusin
    Nov 7, 2023 at 19:06

3 Answers 3

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The embarrassing truth for physics is that the ontology of quantum theory remains unclear. I might have some objections to your suggestion that electrons might 'disappear' between measurements- physically, I might object on the grounds of the requirements for conservation of mass, momentum, charge etc, which would all seem to be violated by a disappearing electron, and I might point out that you can't meaningfully define 'disappear' in this context. That sort of hair-splitting apart, I can sort of empathise with the gist of your complaint, which is that no-one can yet give an authoritative explanation of what is 'really' going on.

By the way, a superposition means that mathematically you can represent a wave-function as a blend of other wave functions. It's a bit like a vibrating string- you can represent a particular pattern of vibration as other patterns of vibration overlayed upon each other. That doesn't mean that the string 'is' a blend of strings vibrating in different ways.

The reason why the wave function idea is considered scientific is that if you model electrons etc that way and do the calculations properly you end up with results that can be in spectacular agreement with experimental measurements- to ten decimal places or better. I can't imagine there are many other theories that are so accurate.

I think I've previously answered another of your excellent questions, about how quantum mechanics came about. In summary, after finding that light, which was definitely found to behave like a wave, seemed to come in particles, Louis de Broglie had the idea that maybe particles have a wavelength. That idea has been tested again and again and again in various forms of diffraction and interference experiments, and it always seems to work- even with quite big molecules with thousands of atoms you can get interferences effects in line with the assumption that the molecule has a wavelength given by de Broglie's suggested formula.

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  • Can you explain how the 'no explanation for what is really going on' is special to quantum mechanics? Any other (good) physical theory, even something very old like Newtonion mechanics is also just a theoretical mathematical model that happens to match very well with empirical observation.
    – quarague
    Nov 7, 2023 at 20:37
  • @quarague sure, all theories have limits in that sense. In Newtonian physics you can't say what gravity 'really ' is- you can't explain how it operates at a distance. But QM has more conceptual holes in it. Take any kind of two slits experiment. The maths suggest that there is a wave function and it interferes with itself after it passes through the slits, having been blocked in some way by the barrier in which the two slits are formed. But what, actually, blocks the wave-function, since it is only a mathematical construct? Nov 7, 2023 at 21:53
  • Great answer even though it sucks that the question was closed
    – user62907
    Nov 8, 2023 at 13:20
  • Cheers TM! (By the way, I hope you are OK with TM as a contraction of Thinkingman.) I agree it sucks. the ontology of QM belongs as much to philosophy as to physics. Arguably, philosophers have done more thinking about it than some physicists. Nov 8, 2023 at 13:31
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If it is in fact the case that your alternative theory is equivalent in terms of empirical observations and predictions to the superposition theory, then it is not actually an alternative at all, but rather, just a different expression of the same theory.

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The principle of superposition of quantum states says: If a particle is prepared to be in the superposition state

alpha |lambda> + beta |mu>

with complex numbers alpha, beta satisfying |alpha|^2 + |beta|^2=1 and two states characterized by possible measurements of a real value lambda or a real value mu, then any measurement in the superposition state gives the value lambda with probability |alpha|^2 and the value mu with probability |beta|^2.

If you turn around the direction of reading, then the result of measurements like these makes it plausible to introduce the concept of superposition of the possible states |lambda> and |mu>.

Hence the concept of quantum superposition results from and is covered by experiment.

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  • I don't see the ontological aspect of the q. Nov 7, 2023 at 17:19
  • Could you please indicate where you see an ontological aspect of the question. One cannot question whether an electron exists, but the question is how to interpret the experiments.
    – Jo Wehler
    Nov 7, 2023 at 17:39
  • "the interpretation" in the q is the ontological aspect:how the interpretation-explanation is related to the phenomenon we measure, "the data". Nov 7, 2023 at 18:47