I What Are the Official Terms for the Two Types of Quantum Superposition?

[Secan]

There seems to be two kinds of superposition. One refers to superposition within one branch/world. Here it is difficult to maintain superposition because you have to shield it from the environment. The second kind of superposition involves branches/worlds, this is where you have Schrodinger cat that is alive or dead. Or Covid positive or negative where you have both status and leads separate lives, one life intubated or in the miami beach. I just want to know the official terms for this two kinds of superposition. Or maybe it is simply referred to as Copenhagen superposition and Everettian superposition? There must be more specific terms. What could they be?

 

[Nugatory]

There’s only one kind of superposition, the kind that is inherent in the vector space structure of a Hilbert space. The difference that you’re seeing is just a matter of how we choose to represent state function reduction after a thermodynamically irreversible interaction - do we discard the irrelevant terms, or do we keep but ignore them?

 

[Secan]

There’s only one kind of superposition, the kind that is inherent in the vector space structure of a Hilbert space. The difference that you’re seeing is just a matter of how we choose to represent state function reduction after a thermodynamically irreversible interaction - do we discard the irrelevant terms, or do we keep but ignore them?

If we keep and ignore them. And the universe is a close system. Then we have a universal wavefunction. In it. I win and don't win the lottery tomorrow. In my branch now. The fate whether i win or not is already written. But is there possibility unitarity in QM can be violated. Meaning i can escape my branch and fate and jump to the branch where i won the lottery tomorrow? Or is unitarity in QM as solid and proven by all known experiments already?

 

[Secan]

What i mean is, today or right now we are in one branch. But how about an hour or 12 hours or one day from now. Are we forever trapped in the same branches or can we jump to another branch?

What are the strings that bind the branches in the temporal sequence, or are there no links between the temporal aspects like Julian Barbour ever present moments?

 

[StevieTNZ]

Branches are a consequence of the Many Worlds interpretation. It doesn't mean it actually applies to how we should interpret quantum theory. As Nugatory said, there is only one kind of superposition.

 

[Secan]

Branches are a consequence of the Many Worlds interpretation. It doesn't mean it actually applies to how we should interpret quantum theory. As Nugatory said, there is only one kind of superposition.

I was discussing with a college professor about the measurement problem and mentioned many worlds. He seemed not to be familiar with it and said there was no mystery. He just said the secret was about eigenvalues. Have you talked to a professor personally who didnt believe there was a measurement problem? What was his reasoning?

Sometimes I tend to think to just ignore the so called measurement problem and just focus on quantum gravity and dark matter. But Lee Smolin wrote quantum gravity could only find solution by also finding solution to the measurement problem. This made me go back to it from time to time.

Background:

http://backreaction.blogspot.com/2020/08/understanding-quantum-mechanics-5.html?m=1

"I know I promised I would tell you what it takes to solve the measurement problem in quantum mechanics. But then I remembered that almost one of two physicists believes that the problem does not exist to begin with. So, I figured I should first make sure everyone – even the physicists – understand why the measurement problem has remained unsolved, despite a century of effort. This also means that if you watch this video to the end, you will understand what half of physicists do not understand.

That about half of physicists do not understand the measurement problem is not just anecdotal evidence, that’s poll results from 2016.This questionnaire was sent to a little more than one thousand two hundred physicists, from which about twelve percent responded. That’s a decent response rate for a survey, but note that the sample may not be representative for the global community. While the questionnaire was sent to physicists of all research areas, forty-four percent of them were Danish."

 

[PeterDonis]

is there possibility unitarity in QM can be violated

We have no evidence that unitarity is ever violated in experiments that specifically test quantum mechanics. But there are a lot of experiments--not to mention our ordinary observations of a classical world--that don't directly test QM, and which, on certain interpretations of QM, at least strongly suggest that unitarity can be violated. So I would say this is an open question.

Meaning i can escape my branch and fate and jump to the branch where i won the lottery tomorrow?

That is not what unitarity violation would mean. Unitarity violation would mean the MWI is an invalid interpretation to begin with, and there are no branches; all events have single, unique outcomes, so you either win the lottery or you don't; there is no alternate branch where the other outcome exists.

Are we forever trapped in the same branches

Yes. There is no way to switch branches, because what defines a "branch" is a particular outcome of some irreversible operation, such as making a measurement and recording a particular result. In more technical language, branches are always decohered, meaning there is no quantum interference between them, and hence zero probability of switching from one to another.

What are the strings that bind the branches in the temporal sequence

The particular outcomes of irreversible operations in each branch are what define the branch. See above.

 

[PeterDonis]

Background

This is a blog post by Sabine Hossenfelder, and doesn't mention Lee Smolin at all. Did you mean to give a different reference?

 

[Secan]

This is a blog post by Sabine Hossenfelder, and doesn't mention Lee Smolin at all. Did you mean to give a different reference?

https://www.google.com/amp/s/www.fo...s-lead-to-our-next-scientific-revolution/amp/

Ill look for his direct quote or statement that quantum measurement problem is directly related to problem in quantum gravity. I think i read this in one of his books.

 

[PeterDonis]

https://www.google.com/amp/s/www.fo...s-lead-to-our-next-scientific-revolution/amp/

Ill look for his direct quote or statement that quantum measurement problem is directly related to problem in quantum gravity. I think i read this in one of his books.

Direct quotes would be nice, yes. The argument given in this article (to be specific, the one at 3:54 PM in the "live blog" part of the article), as far as it goes, seems to only be an argument for why GR and QM are incompatible, not an argument for why quantum gravity is needed to solve the measurement problem.

And it's really not even an argument for why GR specifically is incompatible with QM, it's an argument for why any classical theory is incompatible with QM. For example, we could ask classical Maxwell electrodynamics what result we will get if we measure the electric field of the electron in the double slit experiment, and classical Maxwell electrodynamics won't be able to give us an answer. So classical Maxwell electrodynamics is also incompatible with QM by this argument. And we could run a similar argument for any classical theory. All this is true, but it's hardly news to anyone.

 

[Secan]

Direct quotes would be nice, yes. The argument given in this article (to be specific, the one at 3:54 PM in the "live blog" part of the article), as far as it goes, seems to only be an argument for why GR and QM are incompatible, not an argument for why quantum gravity is needed to solve the measurement problem.

And it's really not even an argument for why GR specifically is incompatible with QM, it's an argument for why any classical theory is incompatible with QM. For example, we could ask classical Maxwell electrodynamics what result we will get if we measure the electric field of the electron in the double slit experiment, and classical Maxwell electrodynamics won't be able to give us an answer. So classical Maxwell electrodynamics is also incompatible with QM by this argument. And we could run a similar argument for any classical theory. All this is true, but it's hardly news to anyone.

Isn't it this is what Lee Smolin is famous for. Quantum foundations and quantum gravity related. Here is sample of a previous $50,000 grant exploring it:

https://www.templeton.org/grant/the...oblem-from-the-perspective-of-quantum-gravity

"The quantum measurement problem would be solved were quantum mechanics to be discovered to be an approximation to a theory with a precise description of individual systems. Quantum mechanics would arise as the statistical mechanics of that theory. The challenge is that this theory must be non-local to satisfy the tests of the Bell inequalities. But remarkably, there is evidence from research in quantum gravity that space and locality are emergent. This suggests that the ultimate reality might be described by a non-local theory from which space and the quantum both emerge. The goal of this project is to construct such a theory. Earlier work has already shown that theories that are both pre-quantum and pre-geometric can be expressed in a common mathematical language, which is the dynamics of matrices and graphs. This project will also attempt to develop experimental signatures of the emergence of space and the quantum. If space is emergent then there is a scale, usually taken to be the Planck scale, below which lorentz invariance breaks down or is modified. This is expected to lead to effects detectible in present observations by astrophysical observatories such as AUGER and Fermi. This is a high risk/high payoff project. If fully successful, the enduring impact will be the replacement of the current formulation of a quantum mechanics, with a deeper theory; this will also lead to the exploration of novel experimental phenomena."

In the book Einstein's Unfinished Revolution (when I searched for "gravity"). He mentioned about relational quantum mechanics too.

Also I think Smolin's concept was not about exploring how quantum gravity is needed to solve the measurement problem. But the measurement problem is needed to solve quantum gravity.

For folks familiar with Smolin. Is it about quantum gravity needed to solve the measurement problem. Or solution to the measurement problem is needed to solve quantum gravity?

Is it not the latter?

 

[PeterDonis]

Isn't it this is what Lee Smolin is famous for

We can only figure that out by seeing some direct quotes from his actual papers.

 

[Secan]

We can only figure that out by seeing some direct quotes from his actual papers.

https://arxiv.org/abs/1805.12468

"Because of the non-locality of quantum entanglement, realist approaches to completing quantum mechanics have implications for our conception of space. Quantum gravity also is expected to predict phenomena in which the locality of classical spacetime is modified or disordered. It is then possible that the right quantum theory of gravity will also be a completion of quantum mechanics in which the foundational puzzles in both are addressed together. I review here the results of a program, developed with Roberto Mangabeira Unger, Marina Cortes and other collaborators, which aims to do just that. The results so far include energetic causal set models, time asymmetric extensions of general relativity and relational hidden variables theories, including real ensemble approaches to quantum mechanics. These models share two assumptions: that physics is relational and that time and causality are fundamental. "

I wonder if there are versions of it where time is still relational.

Also in your blog https://www.physicsforums.com/insights/fundamental-difference-interpretations-quantum-mechanics/ you mentioned either quantum theory needs more complete description or there are actual superpositions like many worlds.

It's more sensible that the quantum state is subjective and not in the sense of Bohmian Mechanics or Many worlds. And I like Smolin and Rovelli idea of Relational quantum mechanics.

https://en.wikipedia.org/wiki/Relational_quantum_mechanics

"Relational quantum mechanics (RQM) is an interpretation of quantum mechanics which treats the state of a quantum system as being observer-dependent, that is, the state is the relation between the observer and the system. This interpretation was first delineated by Carlo Rovelli in a 1994 preprint,[1] and has since been expanded upon by a number of theorists. It is inspired by the key idea behind special relativity, that the details of an observation depend on the reference frame of the observer, and uses some ideas from Wheeler on quantum information.[2]

The physical content of the theory has not to do with objects themselves, but the relations between them. As Rovelli puts it:

"Quantum mechanics is a theory about the physical description of physical systems relative to other systems, and this is a complete description of the world".[3]

The essential idea behind RQM is that different observers may give different accurate accounts of the same system. For example, to one observer, a system is in a single, "collapsed" eigenstate. To a second observer, the same system is in a superposition of two or more states and the first observer is in a correlated superposition of two or more states. RQM argues that this is a complete picture of the world because the notion of "state" is always relative to some observer. There is no privileged, "real" account. The state vector of conventional quantum mechanics becomes a description of the correlation of some degrees of freedom in the observer, with respect to the observed system. The terms "observer" and "observed" apply to any arbitrary system, microscopic or macroscopic. The classical limit is a consequence of aggregate systems of very highly correlated subsystems. A "measurement event" is thus described as an ordinary physical interaction where two systems become correlated to some degree with respect to each other."

This is related to loop quantum gravity. So if this idea is good. Then only valid in loop quantum gravity. But can't this be made compatible to the gravitons based Quantum gravity concept too?

 

[Secan]

Branches are a consequence of the Many Worlds interpretation. It doesn't mean it actually applies to how we should interpret quantum theory. As Nugatory said, there is only one kind of superposition.

In your opinion reading all these. What you treat as more fundamental, states or observations? I think these are better descriptions of the distinction between interpretations instead of realism and non realism?

Many are developing theories that can test it. I read in Nathan Siegal mulling in one of the articles in this thread "After listening to Lee Smolin’s talk, I’m more convinced than ever that, until you have a theory that makes different predictions from either one (Bohr’s and de Broglie’s theories give identical prediction), you can either try to develop one, like Smolin does, or you can waste your time thinking about it."