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In this review article on objective collapse theories, which is also linked from this Phys.SE post, at least in the part I've read so far, a deal of fuss seems to be made about the fact that we don't observe macroscopic objects in superpositions of position states, for example, we don't see a chair as being both here and there simultaneously -- and that this is a key motivation for adopting the objective collapse paradigm.

But I'm wondering just how significant this concern really is. In particular, even if we start with a mere hydrogen atom, then isn't it correct to say that, according to any established quantum theory up to and including the standard model, the atom exists in a bound state in which, despite the proton and electron being entangled, the vast majority of the probability over their joint positions is concentrated in an extremely small region of space? So shouldn't we expect any macroscopic object made of atoms to follow this same pattern? In fact, since the size of the object is now many orders of magnitude greater than the characteristic decay scale of a single atomic wavefunction, I should think the approximation to an entity with infinitely sharp boundaries would be even more appropriate.

That is, the macroscopic object is in superposition, but its wavefunction is so extremely concentrated in one demarcated region that we aren't sensitive enough to perceive any activity outside that region. Or, is it true that the wavefunction of a macroscopic object would tend to disperse over time like that of a single free electron, in which case the argument in the article is valid?

I'm actually not at all opposed to the notion of objective collapse in general, though perhaps I just don't know enough to refute it, but I was just skeptical whether the superposition line of reasoning was really justified.

Adam Herbst
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The article makes some verbal shortcuts which may cause further confusion.

It's not that we observe the electron at two different places at the same time, and we don't when it comes to tables. Actually, we do not observe even the former; instead, single electron at a single time is detected at most at a single point of detector medium (or in a little 3D region of it, implied by the accuracy of the measurement). The belief in existence of the state of "superposition of many positions of electron" (a psi function non-zero in a large, macroscopic region of space) is motivated by the fact this state can be used to predict results of measurements, e.g. the interference pattern made of recorded positions of many electrons in a double slit experiment, and this prediction agrees with observation.

We can formally describe tables the same way, using a plane or spherical psi wave non-zero in macroscopic regions, but this has not been successful in the same way; we have never observed an interference pattern which would vindicate this kind of delocalized description of the table.

But you are right, maybe it's there, and then maybe some day, someone will figure out how to do an experiment with tables in which an interference pattern of recorded positions will manifest and then, that will support the idea that the table is described by a psi function with macroscopic extent.

There is a deeper problem though, which the article touches on. Quantum theory works in a strange way: we describe the state of the microscopic system such as electron or atom (prior to measurement of some its property) using $\psi$ and its evolution in time, by Schroedinger's equation, but we do not describe the state of the measurement device this way. If we try, we know how to calculate the future quantum state of the combined system, but this is a dubious exercise, because then we don't get predictions of probability of measurement results and we can't connect to data obtained from experiments.

Bohr's view on these questions (the Copenhagen interpretation) is the obvious, if theoretically unsatisfactory, practical attitude that seems to be demanded by experience (macroscopic facts we collect): $\psi$ applies to microsystems which manifest wave-like behaviour (interference), but it can't be applied to measurement devices, with which we check and verify manifestations of the former. The classical description must be retained to describe the experiment and the recorded results. The quantum-theoretical description thus does not replace the classical description everywhere; instead, it has limited applicability.

Of course, this bring problems, such as the problem of how the definite outcome comes about, and what exactly happens to $\psi$ during the measurement. Instant collapse is a rough kludge and everybody wants a more detailed account. This is still, after 100 years, not agreed upon. Objective collapse (GRW theories) is one direction of how to alter quantum theory in a minimal way so that it can describe, with internal consistency, what happens to $\psi$ during measurements, and it seems it may be experimentally testable.

Ján Lalinský
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Imagine a particle in free space whose wavefunction is a Gaussian wavepacket. The wavepacket may spread over time and could undergo interference in suitable circumstances. In general something like this is true even for macroscopic objects that don't interact with their environment according to the equations of motion of quantum theory

https://arxiv.org/abs/quant-ph/9612037

In reality, systems do interact with their environment, information is copied out of them and this causes decoherence:

https://arxiv.org/abs/1911.06282

As a result of decoherence interference between macroscopically different versions of objects you see in everyday life is suppressed. There is a superposition spread out over a macroscopic region, but it isn't coherent. The claim made by some that we don't know how to calculate what happens to a measurement device with quantum theory is contradicted by decades of literature on decoherence.

As such in quantum theory without collapse there are multiple non-interfering versions of all the systems you see around you: the many worlds interpretation

https://arxiv.org/abs/1111.2189

https://arxiv.org/abs/quant-ph/0104033

Advocates of spontaneous collapse theories don't like this for reasons they explain in papers in which they also outline potential experimental tests such as

https://arxiv.org/abs/2310.14969

Such theories can't currently reproduce most experimentally tested predictions of quantum theory

https://arxiv.org/abs/2205.00568

This seems like a major problem to me but YMMV.

alanf
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