Serious science and mathematics readings discussion

It appears that the entire previous discussion thread was deleted by Goodreads, presumably because I had included links to external websites such as Wikipedia, Youtube and Physics today.
So I am posting the references again just as text. Readers have to look them up themselves.

Other notable books on philosophical explorations of QM : Speakable and Unspeakable in Quantum Mechanics (J. Bell) , Quantum Mechanics and Experience (D. Albert), What is Real (Adam Becker).

Important (real and thought) experiments and discussions : Double Slit, Stern-Gerlach, Mach-Zehnder, Einstein-Podolsky-Rosen (EPR), Bell's inequalities and the tests that demonstrate that QM violates them, the Bohr-Einstein debates and Born-Einstein correspondences. (These are where the now famous "spooky action at a distance" and other quips were originally made)

N. David Mermin has published great articles in Nature, Physics Today etc. In particular, look up "Is the moon there when nobody looks?"

MIT 8.04 Quantum Physics I, Spring 2013 - First lecture from here, "Introduction to Superposition", is a wonderful introduction to the central paradox of QM.

Tim Maudlin and David Albert make multiple appearances, solo and with others, in the podcasts by Robinson Erhardt, Curt Jaimungal and Sean Carrol. Searching their names along with "Quantum Mechanics" will link to the relevant episodes on Youtube / Spotify.

The single most popular and authoritative blog for Quantum Mechanics and Computing is Scott Aaronson's Shtetl-Optimized

An important clarification by Maudlin that the claim that "observation" magically changes the result of the experiment (in particular, destroying the double slit interference pattern) does not appear necessary if one follows the logic of wavefunction evolution and entanglement.

Which is to say - if one has already accepted that an electron is well described by modelling it as a wavefunction (i.e. an evolution of probability amplitudes which yields measurable outcomes using the Born's rule), and further that the wavefunctions of two independent particles will become entangled after they interact with one another and thereafter evolve as a unified state, completely differently to how they did when initially separated, then the fact that the interference pattern of the electron disappears upon trying to ascertain which of the two slits it went through does not require any additional explanations. Decoherence is thus predicted by the conceptual framework we are already working under, there is no extra secret sauce to layer on top.

He says :

"The more a given system interacts with other systems, the more entangled it becomes, and the more it tends to decohere. Experiments done on such a decohered system exhibit no interference. So if one takes interference to be the calling card of quantum theory, entanglement and decoherence make the world appear less quantum mechanical. But since the cause of the decoherence is entanglement, by Schrödinger’s lights, the observable interference disappears because the world is more quantum mechanical!
Entanglement and the consequent decoherence explain why we do not encounter quantum interference effects in everyday life. Avoiding decoherence requires severely limiting the interactions a system has with its environment (and even with parts of itself). Such isolation usually requires carefully prepared laboratory conditions."

Not surprisingly, he has a short temper for the standard idea of "a measurement collapsing the wavefunction into one of the eigenstates of the Hermitian operator", since a measurement is ordinarily supposed to be an interaction with a system that yields information about features that existed prior to the interaction. Thus, I don't "collapse my weight" and produce it as a result of the "measurement" when I climb on a scale - I use the correlation of the behavior of the apparatus with whatever was already present to find it. However, the recipe to use the Born's rule to calculate probabilities of observable outcomes when the "measurement" happens leaves one in a conundrum on how to make sense of it in terms of how one usually understands the word.

Let p denote the wavefunction as it evolves through Schrodinger. A contrast Maudlin draws attention to is between p-ontic and p-epistemic interpretations of QM (due to Harrigan and Spekken). A p-ontic view sees the mathematical wavefunction as reflective of some real physical aspect of the system, while p-epistemic instead sees it as encoding degrees of belief in facts about the world on part of the observer. In some variations, the wavefunction is a statistical characteristic of a collection and not even mapped to a unique object.
[Compare E.T.Jaynes : "our present QM formalism is not purely epistemological; it is a peculiar mixture describing in part realities of Nature, in part incomplete human information about Nature — all scrambled up by Heisenberg and Bohr into an omelette that nobody has seen how to unscramble."]

Thus, a p-epistemic theory would try to circumvent paradoxes such as "But where is the real physical object present under superposition? Is the cat dead or alive or both or neither?" by positing that the wavefunction only represents our information about where the object is, and thus if the wavefunction is spread out, it doesn't correspond to a point particle being mysteriously "smeared across reality" but that our knowledge regarding its whereabouts has become diffuse.

Most interestingly, the PBR theorem from 2012 tightens the screws in favor of the p-ontic theories. Their claim is "any model in which a quantum state represents mere information about an underlying physical state of the system, and in which systems that are prepared independently have independent physical states, must make predictions which contradict those of quantum theory. The result is in the same spirit as Bell’s theorem, which states that no local theory can reproduce the predictions of quantum theory."

Their idea is to take two collections of electrons prepared in different (non-orthogonal pure) quantum states (and hence described by different wavefunctions) and ask : Is every single electron in the first physically different from every electron in the second?
By the p-epistemic approach, it need not be, since a physically unique electron doesn't need an associated unique wavefunction, which is "merely" a representation of our ignorance. Per p-ontic approach, they are necessarily different.

So given a pair of electrons, each in one of the two possible states (X/Y) , we have four combinations (XX, XY, YX, YY) which all might correspond to the same underlying state (SS) as per a p-epistemic theory, and thus all predictions of QM on SS must be compatible with all quantum states assigned to it.
PBR comes up with an experimental procedure that can produce four possible outcomes for a given input pair such that each outcome is incompatible (i.e. QM gives probability 0) with one of the four combinations. In other words, the pair can't behave in a way consistent with all possible results and there must be an underlying physical deviation for the quantum state that produces the experimental difference.
This argument can be extended for any preparations with different wavefunctions, with appropriate experimental setups.

p-epistemic theorists accept PBR while denying that one or more of its crucial assumptions holds true in their formulation.
e.g. In the paper "FAQBism", defenders of QBism draw the difference between PBR's notion of "epistemic belief in an ontic variable" and "belief assigned to one's personal experiences upon interactions with the world". Thus PBR applies when one associates the quantum state with a real physical property, while QBism is safe since it interprets quantum states as representing probabilities about the measuring agent's own future observations.
Their motivation for their own approach is Bell's experimental invalidation of "local realism", which would either imply a change of stance on locality, which is deeply problematic, or a revision of realism, which they choose to do by denying objective physical reality to a quantum state in the first place.