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The following two things seem to be true:

(1) The universe is massively entangled because the wave function that describes the entire universe has measure 1 of being entangled. Further, given how entanglement spreads, it seems likely that the universe is massively entangled.

(2) On a canonical presentation, entanglement of properties implies that one cannot specify the property of one particle without referring to the properties of the others entangled within the system. the example is the EPR state. One can't talk about the spin of either particle without referring to the spin of the other.

But it seems like we are able to talk about the spin of particles independently in the lab. After a measurement we may say that one particle is x-spin up without reference to any other particles. But if that's true, how is the truth of (1) and (2) maintained? How do we manage to talk about the state of a particle as if it wasn't entangled with the rest of the universe? How is that even possible given that entanglement implies that we must refer to the rest of the universe in our discussion of that property? Thanks for any answers! I'm sure I'm just really confused about something...

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    Why do you consider the universe massively entangled, please give a reference from the physics literature. Thanks.
    – Jo Wehler
    Apr 16 at 4:40
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    To the contrary. "[A] wave function 'encompassing the whole universe' is an idealization, formalistically perhaps a convenient idealization, but an idealization so strained that it can be used only in part in any forecast of correlations that makes physical sense." (Wheeler, John. Quantum Mechanics, A Half Century Later, 1977, Volume 5)
    – g s
    Apr 16 at 4:42
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    First, the universe is not massively entangled due to decoherence, entanglement is washed out very quickly without special isolation measures. In fact, maintaining entanglement long enough even on a small scale is a major technological problem in the way of building large quantum computers. And second, even entanglement does not imply that one cannot measure individual spins, it implies instead that they are non-trivially correlated. That is what "cannot be described independently" means.
    – Conifold
    Apr 16 at 4:54
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    @JoWehler For philosophy papers see Schaffer 2010 “Monism: The Priority of the Whole” and Ismael and Schaffer 2022 “Quantum Holism: Nonseparability as Common Ground” For physics literature see Penrose 2004 “road to reality…”
    – zzz
    Apr 16 at 11:36
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    @zzz What about asking at physics.stackexchange.com about the existence of a wave function of the universe, and about the probability that it is entangled?
    – Jo Wehler
    Apr 16 at 12:38

5 Answers 5

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You can specify (make predictions about measurements pertaining to) the state of one part of an entangled system. If you know how it's entangled, measuring the state of one part lets you know the state of another part. It doesn't require you to know the state of the other part first.

Up and down spin are in reference to the local system. For any given system, the assignment of which direction is up is determined by human convention. For instance, all the first-filling electrons in all spin-orbitals of all isolated atoms are assigned spin up, even though both the atoms and the chemists are oriented every which way.

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The short answer is that the Universe isn't massively entangled with itself in the way you imagine, and particles can have measured properties such as spin or position, notwithstanding the fact that they were entangled, so the basis of your question is a misunderstanding. Leaving that aside, in QM you can model systems with different degrees of sophistication. For example, if you want to calculate the energies of electrons within an atom of iron, say, you might assume that the atom is unaffected by its surroundings and look only at the interactions between its constituent elections and its nucleus, taking the latter to be a point charge. If the result of your calculation is in good agreement with experimental results, then you were justified in treating the atom as isolated. If you want to model the energy levels of electrons in a block of iron, you can't possibly do it in the same way because there are countless trillions of electrons and nuclei to keep track of, so instead you make additional simplifying assumptions.

The trick in physics is to find assumptions that simplify your calculations without affecting the result in any significant way. In QM, interactions between certain particles in a large system can be so weak that you can ignore them completely. That is no different in Newtonian physics- in principle, an apple falling to the ground is influenced by the gravity of every particle in the Universe, but in practice you don't need to consider the effect of the motion of Saturn, say, when determining how long the apple will take to fall to the ground, because the effect of Saturn's gravity hereabouts is negligibly small.

In practice, the kind of entanglement that leads to interesting physics is hard to maintain. If you create two entangle particles and leave them alone, they will tend to interact with their environment in a way that scrambles the entanglement.

You might also be interested in the observation that when we talk about measuring an electron's spin, we are usually inferring the spin from a measurement of its position. The typical experiment fires an electron between the poles of a specially designed magnet that produces a curved field. The electron interacts with the magnetic field causing it to veer towards one of two detectors depending on its spin orientation. When the electron is caught be a detector (which is effectively a position measurement) you can work out which spin direction it had.

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I think what you might be confused about is how it stems from the fact that entanglement is a phenomena that arises from a process. The idea of quantum entanglement is just how particles act as if they are still the same system (are in superposition) when observed and measured, Einstein calls it "spooky action at a distance", but for entanglement to be measured and observed, there must be a direct association between the particles in the first place i.e., they should have originated from the one atom or at least must have become the same system at one point in time, if you see how physicists work to observe quantum entanglement they split a photon into two lower state photons and observe the properties of these photons and they have the same property and in case of bigger particles (like electrons) one must make sure that there was a superposition between these two particles at some point in time.

Quantum entanglement happens on the quantum scale hence why it is quantum, like you might say all the particles in the universe can be tangled in a very weird way (as a result of physics - nature of the universe we are yet to understand) but we can never see (and never know) this effect on a larger scale also a lot of things that happens in the quantum states do not make sense or cannot be translated to the macro world. Take wave-particle duality even if every thing has a wave nature to it, thinking of everyday physical objects as waves would slightly be non-sensical

TL;DR If you take the universe to be entangled, then this is the normal state we start with, there is no use in bringing forth the whole universe into question when observing the extra step of measuring the entanglement of two particles. Yes, it might be the case that every particle might be entangled but this is not out of ordinary and it seems like the universe still functions according to the laws of nature/intuitively , but in the case of the particles there seems to be an extra unintuitive event that occurs, the entanglement of these particles and this we measure.

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  • This doesn’t answer my question. As I said in the post, the universe is massively entangled with itself. The question is how do we talk as if it wasn’t even in the microscopic case!!!
    – zzz
    Apr 16 at 4:17
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    @zzz Because the entangled universe is a natural state and there is nothing out of the ordinary that is taking place, it is the normal state of being. More of this in my edit.
    – How why e
    Apr 16 at 4:25
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Two cents.

I would say that the answer is that both entanglement and dis-entanglement happen at various times, places and ways as the wavefunction(s) evolve. For example, decoherence might act as both disentanglement (in one sense) and entanglement (in another sense).

So these complementary functions result in there being subsystems (in the whole system) which are only loosely (minimally) coupled to each other(*) (at a given instant), so we can focus only on some subsystem to an adequate approximation.

(*) See, for example, holon

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I talked to a physicist at Columbia and it seems like the answer is this. This all depends on your preferred interpretation of quantum mechanics. In Bohmian Mechanics, we talk about particles having particular spin because of there position in the wave function on that region in configuration space. So, the wave function will still be massively entangled but particles will have definite spin depending on where they are in configuration space. In MWI, worlds cease to interfere with one another so there's a definite spin state of the particle. In GRW, and other collapse theories, states will never be truly localized... So, what's going to happen is that the particle will remain entangled with the rest of the universe. This is called the tail's problem. However, even though states are never fully localized, it seems like there are plausible ways of resolving it.

Important note: the state of the world in any of those theories after we make a "measurement" will be an eigenstate of some particle at some position having some spin– we cannot distinguish between particles!

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