Timeline for Why can interaction with a macroscopic apparatus, such as a Stern-Gerlach machine, sometimes not cause a measurement?
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| Oct 21, 2016 at 21:09 | comment | added | Ruth E Kastner | But re your 2nd concern- the 2 paths through the S-G are analogous to the 2 slits in the 2-slit experiment. These just prepare the system in a superposition of slits; there is no collapse or measurement at the plane of the slit (except to filter out components not compatible with the desired superposition). This is easy to understand in the TI picture; the key is that there's no real momentum transfer from the system to the slits at the point of passage through the slits (or through the S-G). If you find this paradoxical in the 'standard' approach, that would be a good reason to consider TI! | |
| Oct 20, 2016 at 23:41 | comment | added | Ruth E Kastner | I'm getting a warning to avoid extended discussions here, so for now I'll just say that it's not the measurement problem that's at issue, it's the adequacy of the whole decoherence program for addressing the kinds of questions you're asking. Appealing to environmental decoherence doesn't really work because of the circularity problem. I do think I address your 2nd concern in my 2nd linked paper re Bohr's thought experiments (analogous to the S-G situation here), but if you still have questions/concerns, feel free to contact me via my blog, rekastner.wordpress.com Thanks and best wishes, RK | |
| Oct 20, 2016 at 20:23 | comment | added | Ruben Verresen | [cont] Curiously though, it seems to me your argument would actually give the wrong conclusion: if the SG machine were to have a fixed (and measurable!) momentum, then upon the neutron having gone through it, the SG machine would have picked up a corresponding momentum shift, which would exactly tell us whether the spin went up or down, which on its turn would imply that it is impossible to ever rejoin the neutron beams again in a quantum coherent fashion, inconsistent with experiment. | |
| Oct 20, 2016 at 20:23 | comment | added | Ruben Verresen | I'm sympathetic towards quantum foundations, but I struggle to see how issues like the measurement paradox are relevant here. The OPs question can be phrased without any relevance to an interpretation of QM: "How is it possible that a neutron going through SG machines can preserve its quantum coherence despite having transferred momentum to the SG machines?" Whatever interpretation one chooses to word it in should not matter. [cont] | |
| Oct 20, 2016 at 19:27 | comment | added | Ruth E Kastner | Also, a related discussion about the uncertainties applying to macroscopic objects needing to be epistemic rather than ontological is here: arxiv.org/abs/1601.07545 | |
| Oct 20, 2016 at 19:20 | comment | added | Ruth E Kastner | Thanks Ruben, to clarify: the issue is that if one allows the SG to be in a superposition, however microscopic, given unitary-only evolution that superposition can be reversibly amplified to an arbitrary macroscopic size--which we never see (basically the Schrodinger Cat situation). Appealing to environmentally-induced decoherence to eliminate superpositions depends on a circular argument in establishing a preferred basis for diagonalization of the density matrix for the system. I discuss this here: arxiv.org/abs/1406.4126 | |
| Oct 20, 2016 at 1:58 | comment | added | Ruben Verresen | Or if your issue is with the SG machine not strictly being in a superposition: of course that is true since it interacts with an environment, but to answer the OPs question it is conceptually much more useful to work in the setting where we are in an empty universe with only a neutron and a SG machine. At least, then the whole issue can be explained, as I have tried to do in my post, and only then does it make sense to introduce additional features like an environment, which doesn't conceptually change anything in this case. | |
| Oct 20, 2016 at 1:56 | comment | added | Ruben Verresen | Why do you say the issue is not resolved in the standard picture? It seems to me you want to say the SG machine has a fixed momentum and fixed position, but there is no justification for this. (And the common sense answer 'because you see it at a particular location' doesn't cut it: the accuracy to which we 'see' is well below the accuracy relevant for the discussion at hand.) | |
| Oct 20, 2016 at 0:04 | comment | added | Ruth E Kastner | Sure, it was first proposed by John Cramer in 1986. I have 2 books out discussing TI. I've developed a relativistic version of it. For an introduction to the basic concepts you can see wordpress.com/post/transactionalinterpretation.org/372 | |
| Oct 19, 2016 at 21:33 | review | Low quality answers | |||
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| Oct 19, 2016 at 20:29 | comment | added | knzhou | Can you add some more explanation of what TI is, and what its axioms are? I've never heard of it before, and this sounds very different from the interpretations of QM that I know. | |
| Oct 19, 2016 at 20:29 | review | First posts | |||
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| Oct 19, 2016 at 20:25 | history | answered | Ruth E Kastner | CC BY-SA 3.0 |