It seems to be a well-known fact that the Schroedinger equation doesn't hold for macroscopic systems (e.g. an animal), except maybe for very short time durations until decoherence effects take place. On the other hand I haven't found this anywhere explicitly stated and haven't found any explanation/reasons for this well-known fact. Is there a "simple" explanation why the Schroedinger equation can't describe correctly the behaviour of a macroscopic system for e.g. 1 second?
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1$\begingroup$ Schrodinger equation is valid for closed system. When you are looking at a macroscopic system (a cat surrounded by an environment: air molecules for example), you have to find which system is isolated. In principle the only system you could consider as totally isolated would be the entire universe. For practical purpose, if system are well enough isolated, Schrodinger equation works well to describe them. But macroscopic systems are poorly isolated (they always interact with their surroundings). Because of that Schrodinger equation cannot describe them properly. $\endgroup$– StarBucKCommented Jun 11, 2020 at 15:01
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This popular review reminds you that even a small bug (or a paperclip!) of about 1 g and 1 cm has a wavefunction decohering in $10^{-23}$ seconds. The TDSE works fine, it is just not informative for it for longer.
That review will remind you there are, nevertheless, "large" systems, cryogenic Weber bars of over a ton, superconducting Josephson junctions, ..., which checkmate decoherence by suitably isolating themselves from their surroundings, and not transferring quantum information to them. One might call such systems "macroscopic", but with wide-eyed qualifications.
In view of your comment, I'd strongly recommend moving it to HSMSE.
answered Jun 11, 2020 at 13:37
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$\begingroup$ Einstein, Bohr etc. didn't know about decoherence afaik. So how did they conclude at the Copenhague conference that the SE describes a cat only for a fraction of a second? Why didn't they say: "Well, we can't evaluate the SE for the cat in the box, way too many variables, but if we could, we suppose it would describe everything perfectly => the world is deterministic!!" As we know, they didn't say that, but somehow knew (without knowing decoherence), that the SE can't describe the killing of the cat by the decay of the radioactive atom. So my question is really a bit about history of physics. $\endgroup$– ausidCommented Jun 12, 2020 at 14:50
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1$\begingroup$ There is a dedicated SE for the history of physics. You are very much in the wrong place here. Bohr and Einstein knew the dead cat shows no signs of QM interference with the live cat, but argued in generalities, pushing the limits, searching for experimental handles, only available in the last 20 years. The decoherence arguments quantify the difficulty of QM interference of macroscopic objects, is all. But, regardless of what daffy philosophers and historians enjoy claiming, all working physicists knew where to draw the line by 1930. $\endgroup$ Commented Jun 12, 2020 at 15:01