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How do we know superposition exists? Has it been observed, or has it been deduced, and how certain are we?

The Copenhagen Interpretation seems to imply that superposition collapses into one state once measurement has occurred, so I don't understand how we can observe it.

The reason I ask is I'm totally bewildered by the mainstream interpretations of Quantum Mechanics. I understand its probabilistic; I'm very accepting of the idea of there existing fundamentally stochastic scenarios where the initial state does not entirely determine the next state. What I don't understand is why superposition is so necessary to the theory. The two mainstream explanations are the Copenhagen Interpretation and the Many Worlds Theory, which both seem so incredibly farfetched and counterintuitive.

Thanks for any help or insight.

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I posted something similar earlier today asking for a better insight into Schrodinger's Cat, but it was pointed out to me that it was similar to other questions, so I deleted it. This is essentially the same problem, but I just wanted to focus on the part that really confused me. –  James Sydow May 13 '13 at 1:08
    
If you want to add a later clarification to your own question, you don't have to do it as a comment on your own question. You can just edit it by clicking where it says "edit." –  Ben Crowell May 13 '13 at 1:49
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1 Answer 1

The Copenhagen Interpretation seems to imply that superposition collapses into one state once measurement has occurred, so I don't understand how we can observe it.

In general, you should not expect interpretations of quantum mechanics to tell you anything about what you can observe. They're not physical theories. What makes them fall short of being physical theories is that they don't make predictions about observations.

Superposition, however, is a feature of quantum mechanics that is independent of any specific interpretation, and that does make definite predictions about observations. For example, we can do double-slit diffraction with particles. For an example with photons, see the photo at the beginning of section 34.4 here http://www.lightandmatter.com/html_books/lm/ch34/ch34.html#Section34.3 . For an example with neutrons, see Zeilinger et al., Rev Mod Phys 60 (1988), 1067.

Without superposition, it's hard to imagine how we could get areas of high probility and areas of low probability. We can observe these effects even under conditions in which it's only possible for a single particle to have been present at any given time.

Either the Copenhagen interpretation or the many-worlds interpretation can tell a satisfying psychological fable about why we observe these things. If the interpretations seem farfetched to you, then you're free to dispense with the interpretations, which are philosophy, not science; you will then find youself initiated into the "shut up and calculate" school. But I don't think that suffices to render the observed phenomena less counterintuitive.

I'm very accepting of the idea of there existing fundamentally stochastic scenarios where the initial state does not entirely determine the next state.

I'm not so sure you should accept that. Whether this is true or false probably depends on your definition of "state."

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Another experiment that shows superposition and the collapse of the wave function would be the Stern-Gerlach experiment –  Neuneck May 14 '13 at 6:41
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