I agree that size isn't the whole story. Also, all of us will be familiar with the twentieth century version of Young's double slit experiment (I think courtesy of Albert Michelson) when very little interference lost the pure quantum state. It is of note that most of the large scale examples of superposition are in the low temperature arena; SQUIDS & super-conduction (of course the S in the abbreviation SQUID does stand for that). Moreover, if there was no chance of macro scale superposition, that is bad news for quantum computing. However, as has been mentioned already in this thread, thermal agitation is a potent destroyer of the pure quantum state & the cat at 310 K has very little chance (a cat in hell’s chance even) of staying in any pure quantum state.That is still not the complete story.
First of all, the SIZE has nothing to do with all of this. There are every indication to show that "macro"-sized objects can, in fact, exhibit quantum properties. The Delft/Stony Brook experiments involved 10^11 particles exhibiting such properties. We have seen this size gets progressively bigger all the time.
The issue here is how large of a length scale and how long of a time scale can one maintains the coherence of the system. That, to me, is the first and foremost fundamental criteria of observing quantum properties. It is why superconductivity plays a central role in this because no other system can show quantum phenomena in a clearer fashion at a macroscopic scale.
Now, having said that, at what point, and why, do we lose such observation? Decoherence? Sure, but even that isn't sufficient, or at best, incomplete, and this is NOT just from the observational point. It is also from the theoretical standpoint. We have seen that even ONE, single interaction can https://www.physicsforums.com/showpost.php?p=1498616&postcount=55". So it doesn't even require a gazzillion interactions, which would make it even infinitely WORSE to try and model.
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