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I am not completely sure that this is the right place to ask this question, but it is clearly not a normal question about quantum mechanics.

I am considering the quantum state of the universe. I have seen the concept mentioned in connection with the question regarding the observer in a quantum physics experiment. By definition the quantum state of the universe cannot be observed by anyone or anything, and therefore seem to be part of a different context. For example, someone might say that the characteristics of an observer – e.g. a machine vs a person - matters because they would belong to different quantum states of the universe.

My question is therefore whether a universe quantum state can have the same characteristics as an “observer-observed” quantum state, such as we usually consider in an experiment.

It seems to me that the unobserved quantum state are different concepts and to invoke them is invalid in the context of traditional quantum mechanics.

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    "The wave function of the Universe" seems to be non-sense on a grand scale; Lee Smolin commented recently (2019) How to Understand the Universe When You’re Stuck Inside of It quantamagazine.org/…
    – sand1
    Sep 5, 2021 at 19:56
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    The insides of a black hole, or even of an ordinary star, cannot be observed either, it does not stop us from modeling them theoretically. Physics is full of idealizations that take us beyond narrow observational capabilities but are invoked whenever convenient. And quantum space of the universe is described by the same mathematics as quantum states of smaller systems, including those we are part of, so why should we limit ourselves artificially when exploring our models.
    – Conifold
    Sep 5, 2021 at 21:03
  • The notion of an "observer" in QM is a misunderstanding. It is might be act of observing that induces the transition of a superposed quantum state to a classic state, but the interaction with other matter. The quantum state of the universe is not a subject because every part of it interact with each other.
    – armand
    Sep 5, 2021 at 22:38

5 Answers 5

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This whole confusion stems from the standard, Copenhagen interpretation of quantum mechanics. Strange things like a consciousness induced collapse, the many worlds interpretation of Everett, different solutions to the measurement problem really solving nothing, etc. are caused by that interpretation.
According to that interpretation the whole universe, past and present, stays always in superposition. Everett would have lived longer. He drank a bottle a day, smoke three packets of cigarettes, ate junk, while he believed in a parallel world where he was okay... His daughter later killed herself to join him in that world.

Only way out: non-local hidden variables. Bohm was declared a mad Trotskyist because of it. He was ridiculed by contempory physicists.

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You need to bear in mind that QM is a model of reality- it isn't reality. When we perform calculations in QM we typically make grossly simplifying approximations, otherwise the calculations would become impossibly complicated. They idea of observers causing the 'collapse' of the wave function is an artefact of the model.

Take the famous two slits experiment. Here we model a free electron as having a particular sort of wave function, we treat the two slits as a classical barrier, and we treat the detection of the electron at a screen beyond the slits as a 'measurement' that 'collapses' the wave function. That's all approximation. What is really happening is that the electron, as it approaches the screen, is moving out of an area which can be considered as free space and into a region in which there is a complicated potential caused by the presence of countless trillions of other electrons. The inbound electron interacts with other electrons and ions at the surface of the screen. If we were able to, we could solve the Schrodinger equation for the electron in that complicated environment to determine its wave function. No collapse there- just the wave function changing as a natural consequence of the electron moving into a region with a different potential. And even that would be an approximation, since we would be adopting the 'one electron model' in which we are calculating a wave function for the single inbound electron, and treating all the other electrons and ions that from the detector screen as if they were a classical potential. A more realistic model would consider a multi-particle wave function for all of the particles in the detection screen and the inbound electron combined, but that would be unimaginably complicated. And even that would be an approximation, since really we should be treating the detection screen as being part of the wider laboratory, and so on and so on. So, in principle, we should be considering the wave function of the Universe, which would evolve over time, without any need for all of the nonsense about observers etc. However, in real life we have to be less ambitious and make drastically simplifying assumptions.

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  • I like yout treatment of the quantum mechanics but the part about the observer's roll is treated rather lightly. Oct 11, 2023 at 7:19
  • @MikaelJensen many thanks. I will add a little more about observers as a coda to my answer, just as soon as I have finished eating my roll. Oct 11, 2023 at 8:15
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You make an observation. Then the wavefunction continues, but you have a constraint, a point of data. Has the quantum system become classical? No. What has happened is a correlation has spread, a particular case of a larger phase space has been chosen.

Chiribella's Purification Principle seems a good way of conceptualising what an observer is. This helps us understand how the thermodynamic arrow of time & movement through phase can be different but point in the same direction, as per say Loop Quantum Gravity (information spreads out forwards in time, because when it concentrates that would be literally a movement backwards in time, erasing records). It's crucial to think hard about what information is. It's also very important to understand how big phase space is even for simple-seeming systems.

The uncertainty principle 'hides' a certain amount of information, a precise measurement with an observation, maximises the uncertainty in a canonically conjugate variable. From there, uncertainty can grow again until another observation, until information about the quantum subsystem leaks out, constraining the phase space again.

So the appropriate picture is not a wavefunction collapsing once and for all, but rippling uncertainties. This is captured by the metaphor of Indra's Net. Instead of a fixed external unified reality being updated by events, instead there is information rippling around, reflecting through pictures, subjectivities, which have given constraints, given data points, about all the other subjectivities (see this discussion on relating subjectivity to physics Is the idea of a causal chain physical (or even scientific)?).

Deutsch & Marletto's Umiversal Constructor Theory helps us to understand that 'things', systems, aren't fixed outputs, defined states, but sets of possibilities. They have a topological existence, within the limits of relevant uncertainties, in which the 'truth' of a particular iteration is literally non-existent, the system isn't the 'reduced' uncertainty particular, but exists as a set of counterfactuals as viewed from after taking an observation. Before the observation the shape in phase space is what is real, the topology of the different possible outcomes.

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Let me consider the background one more: I had contemplated the issue from Michael Audi: “Interpretation of quantum mechanics” that there is no mind-dependence involved in the scenario of observing in a quantum mechanical experiment.

I asked about this question before and got the answer – the Counter Argument - that the nature of the observer matters, since the whole experiment was part of different QM states of the Universe.

After having looked at some reactions to the question and thought about it little bit more the folly of the Counter Argument is clear to me: Of course it “matters” in the sense that different scenarios would amount to different QM Universe states, but it doesn’t matter in any other less-than-the whole-Universe ways. If Jane Bohr and Judy Heisenberg had constructed the Copenhagen interpretation it would have “mattered” in one sense, and several others, but not as far as our view of QM are concerned.

If commentator @Conifold considered the QM state of a black hole of if another physicist considered the same thing, it would also matter as far as the Counter Argument is concerned but not in any other way.

I hesitated to even ask the question because of the concept of the Universe’s QM state, which must involve the in- and outside of both black holes and the observable Universe with the obvious temporospatial challenges.

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The Wheeler-de Witt equation has been called the wave-function of the universe. It is an equation that arises in one approach to quantum gravity, the canonical approach. The equation itsrlf arises from quantising the so-called Hamiltonian constraint.

This equation requires boundary conditions. These can be considered as the initial observations from which the universe evolves. As there is nothing external to the universe, the initial observations cannot be determined from quantum gravity/cosmology itself.

They have to be introduced as new laws of physics.

However, Hartle & Hawking have claimed that the boundary conditions is that there are no boundary conditions. Thus they say, we need no initial observations and no new laws need be introduced.

Another another position is that of Vilenkin who has introduced a tunnelling boundary condition by drawing an analogy between quantum tunnelling and the quantum creation of the universe from 'nothing'.

These are the two main proposals and despite many years of heated debate the two camps have not resolved their differences and no consensus as of 2020, and most likely up to now, as to which is the more likely physical explanation.

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