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## Main Question or Discussion Point

I have been reading Sean Carroll's recent book called Something Deeply Hidden where he advocates a Many Worlds Interpretation of Quantum Mechanics. I was thinking that the most important question in all of Physics might be is everything quantum or is there a quantum/classical boundary.

If we answer this question, doesn't it explain just about everything? It would mean a mixed state is just a mixture of real probable states that just don't interfere. So classical reality would still be quantum but without interference. So you can have a superposition of earths where in 80% of the earths Hillary won the election, 20% of the earths Trump won the election and we're in one of those 20% of earths so that's our local experience.

So looking at Schrodinger's cat it's either the cat is distinctly a live cat until decay/not decay occurs if there's a quantum/classical boundary but if all is quantum then the cat would be in a mixed state of both probable states until information about the cat reached the environment because if all is quantum how can the state of the cat be known prior to a measurement of decay/not decay?

Here's some papers on this topic and they all seem to point to the all is quantum point of view.

The paper talks about some of the amazing things we could do if we could put large objects like a planet into a superposition of probable states.

Here's a recent study where Wigner's Friend was confirmed on quantum scales.

Is there any more important question in Physics than this one? Also, what's the evidence that points to a quantum/classical boundary where a cat or a baseball distinctly becomes a cat or a baseball independent of QM? Also, is a pure state and a mixed state essentially the same thing minus interference?

If we answer this question, doesn't it explain just about everything? It would mean a mixed state is just a mixture of real probable states that just don't interfere. So classical reality would still be quantum but without interference. So you can have a superposition of earths where in 80% of the earths Hillary won the election, 20% of the earths Trump won the election and we're in one of those 20% of earths so that's our local experience.

So looking at Schrodinger's cat it's either the cat is distinctly a live cat until decay/not decay occurs if there's a quantum/classical boundary but if all is quantum then the cat would be in a mixed state of both probable states until information about the cat reached the environment because if all is quantum how can the state of the cat be known prior to a measurement of decay/not decay?

Here's some papers on this topic and they all seem to point to the all is quantum point of view.

**Death by experiment for local realism**https://www.nature.com/articles/nature15631A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests.

**Quantum superposition of molecules beyond 25 kDa**https://www.nature.com/articles/s41567-019-0663-9Matter-wave interference experiments provide a direct confirmation of the quantum superposition principle, a hallmark of quantum theory, and thereby constrain possible modifications to quantum mechanics1. By increasing the mass of the interfering particles and the macroscopicity of the superposition2, more stringent bounds can be placed on modified quantum theories such as objective collapse models3. Here, we report interference of a molecular library of functionalized oligoporphyrins4 with masses beyond 25,000 Da and consisting of up to 2,000 atoms, by far the heaviest objects shown to exhibit matter-wave interference to date. We demonstrate quantum superposition of these massive particles by measuring interference fringes in a new 2-m-long Talbot–Lau interferometer that permits access to a wide range of particle masses with a large variety of internal states. The molecules in our study have de Broglie wavelengths down to 53 fm, five orders of magnitude smaller than the diameter of the molecules themselves. Our results show excellent agreement with quantum theory and cannot be explained classically. The interference fringes reach more than 90% of the expected visibility and the resulting macroscopicity value of 14.1 represents an order of magnitude increase over previous experiments2.

The paper talks about some of the amazing things we could do if we could put large objects like a planet into a superposition of probable states.

**Wheeler's delayed-choice gedanken experiment with a single atom**https://www.nature.com/articles/nphys3343The wave–particle dual nature of light and matter and the fact that the choice of measurement determines which one of these two seemingly incompatible behaviours we observe are examples of the counterintuitive features of quantum mechanics. They are illustrated by Wheeler’s famous ‘delayed-choice’ experiment1, recently demonstrated in a single-photon experiment2. Here, we use a single ultracold metastable helium atom in a Mach–Zehnder interferometer to create an atomic analogue of Wheeler’s original proposal. Our experiment confirms Bohr’s view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes place1. This result is encouraging for current work towards entanglement and Bell’s theorem tests in macroscopic systems of massive particles3.

Here's a recent study where Wigner's Friend was confirmed on quantum scales.

**Experimental rejection of observer-independence in the quantum world**https://arxiv.org/abs/1902.05080The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically exposed in Eugene Wigner's eponymous thought experiment where two observers can experience fundamentally different realities. While observer-independence has long remained inaccessible to empirical investigation, recent no-go-theorems construct an extended Wigner's friend scenario with four entangled observers that allows us to put it to the test. In a state-of-the-art 6-photon experiment, we here realise this extended Wigner's friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. This result lends considerable strength to interpretations of quantum theory already set in an observer-dependent framework and demands for revision of those which are not.

Is there any more important question in Physics than this one? Also, what's the evidence that points to a quantum/classical boundary where a cat or a baseball distinctly becomes a cat or a baseball independent of QM? Also, is a pure state and a mixed state essentially the same thing minus interference?