Everything doesn’t obey it’s rules. Relativity and qm make different predictions. In big, fast, high energy stuff relativity usually is right. In small atomic level stuff qm is usually right. Neither works all the time.

In my view the “weirdness” of quantum mechanics is just people arbitrarily anthropomorphizing relativity for no reason. By “relativity” I do not mean specifically Einstein’s relativity, but relativity in general, any theory which requires you specifying a reference frame in order to describe other systems.

What the material sciences have shown us is that reality is very relative. You cannot describe anything without specifying a reference frame, a coordinate system. If we had a universe with only two objects, you could never actually describe such a universe as genuinely containing two objects, because you have to pick one of those objects as the reference frame, and then from that frame of reference you could only describe another object. So you would have a universe that could be described in two different ways, but both ways would only contain one object, just a different one. To actually include two objects in your picture, you would need at least three, to pick a “third-person” reference frame to describe the other two.

Most people have trouble grappling with this. They think in a very Newtonian way and think there should be an absolute, almost cosmic perspective, that is entirely independent of perspective. But that’s just not how reality works. All our scientific models require you to first begin with some sort of chosen reference frame in which the rest of reality will be described from.

If two objects interact, as I said before, you can only include two objects in the picture from the reference frame of a third object. If you want to deal with those objects in themselves, you have to pick one of them as the reference frame to describe the other, and at that point you will only be working with a description of one object. Since you no longer have two, you don’t have an “interaction” any more. Rather, we might say that from that context, the other system’s properties are “realized.”

For example, you might say the reason I “see” is because light interacts with my eyeballs. Yet, I do not see my own eyeball, from my own perspective it is not actually in the picture. The only thing in the picture is the light, i.e. what I am seeing, and I just describe what I see as is, as it has been realized in front of me. To say something is “interacting” you ultimately need three systems, but the simplest unit in which we can describe reality is with two systems, but doing so requires you adopting the reference frame of one of those two systems, and thus your description will no longer be of systems interacting but the realization of the properties of just one of those systems.

The issue, however, is people love to anthropomorphize relativity. They insist that this relative nature of the theory is what they call “observer-dependence” assigning it to some property of conscious observers. Sometimes it is also referred to as “subjectivity,” or even the “first-person point of view,” all implying it has some relevance to humans or conscious beings themselves. Quantum mechanics is a theory that only makes predictions as to the properties of systems that will be realized under a particular frame of reference, i.e. in a particular context. But people’s obsession with anthropomorphizing relativity causes them to instead say quantum mechanics is an observer-dependent theory that only predicts what properties of systems will be observed.

They then think it somehow is in contradiction with philosophical realism because of this supposed “observer-dependence,” but they entirely made up the observer-dependence. Quantum mechanics is a contextual theory. There is context-dependence but no “observer-dependence,” no subject-dependence, no fundamental role for measuring devices, or anything like that. If you just accept quantum mechanics is a contextual theory from the get-go, then you never actually have to introduce cats that are simulateously alive and dead, “spooky action at a distance,” some fundamental role for conscious observers or a measuring device, a multiverse, or any of that. It’s really not even that “weird” if you can get over the mental hurdle of trying to think of it in a Newtonian way and just embrace the contextual nature of the theory.

The physicist Francois-Igor Pris has a lot of good books on this topic but sadly very little in English. I’ve asked him if he plans to ever translate any to English and he told me he does have plans, but it’s not happened yet to my knowledge.

You see this in the article, for example:

If you were to only teach someone the classical laws of physics that we thought governed the universe as recently as the end of the 19th century, they would be utterly astounded by the implications of quantum mechanics. There is no such thing as a “true reality” that’s independent of the observer

This gives idealist vibes, as if quantum mechanics is not describing “true reality” but something inherent to the conscious observer. But it is describing reality, just not independent of context, because reality is contextual. The fact that there is no non-contextual reality, a reality you can describe without specifying a frame of reference, does not mean there is no reality. There is a reality, it is just one that is contextual in nature.

I am also a bit confused as to the section of the article “Entanglement can be measured, but superpositions cannot.” What really makes quantum mechanics distinctly different from a classical probability theory is interference effects. You can observe interference effects even with a single particle (which is a consequence of “superposition”). You can also observe interference effects with multiple particles that are statistically correlated with each other, that’s what we call “entanglement.” What things like Bell’s theorem shows is just interference effects as exhibited by two statistically correlated particles.

All we observe are the interference effects, which are both equally in the single particle and double particle case a consequence of the same thing, that being the fact that systems can exist in probabilistic states that are described by complex numbers. In classical probability theory, the numbers can only be between 0% and 100%, and so they can only accumulate. In quantum probability theory, they can be negative and even imaginary, which allows them to cancel each other out, giving rise to destructive interference.

This simple fact is what both gives rise to interference effects observed in the single particle case, as well as the case of two or more particles. There is not something special to entanglement that we can observe that we can’t observe in the single particle case. What we always observe is just the consequence of the way probabilities work in quantum mechanics, and thus ultimately just observe statistical effects that defy basic intuition because quantum probabilities don’t work the same as classical, which we are used to.

The distinction the author is making just doesn’t make much sense.

But whereas superposition is different effects or particles or quantum states all superimposed atop one another, entanglement is different: it’s a correlation between two or more different parts of the same system

Yes, but it’s just that, a correlation. Statistical correlations are part of classical probability theory. On their own, they are not interesting at all. What makes them interesting in quantum mechanics is interference effects. If you ignore the interference effects then nothing is particularly interesting about entanglement, it’s just a boring statistical correlation. If you include interference effects and ignore the correlation aspect, then what you’re left with that distinguishes it from classical probability theory is interference effects, which is also the same as the single particle case. They are both equally a consequence of “superposition” which is just that we predict the systems probabilistically using complex numbers.

Also, the statement “Schrodinger’s cat can be alive and dead at once” is only true if for some reason you choose to interpret something not yet being determined and thus can only be predicted probabilistically as actually physically existing as a probability distribution, as if the cat is stretched out halfway dead and halfway alive. Schrodinger actually put forward this thought experiment to mock this perspective as an appeal to absurdity, not to encourage people to think this way. It is simpler to just treat it as if it’s not yet been determined in that context, and so you have to predict it probabilistically, and that probability distribution is just that, a probability distribution. It is predicting the outcome for when it is realized. It is not describing anything.