[gpsx]

People have an adverse reaction to the "many worlds" theory. Maybe they don't understand it? Or maybe what I think is the many world interpretation is really something else. Back when I was in theoretical physics we weren't careful about defining terms like this because, as the original comment says, many worlds vs copenhagen was not really an issue of interest, other than in a philosophical sense for people like me. And I didn't take Copenhagen seriously.

If you take away any sensient beings, or whatever it is that is required to make one of these "measurements", then copoenhagen is the same as the many world interpretation. Wave functions go on evolving and there is no collapse. For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down. That is strange enough for all of us. But for some reason people have trouble extending this idea to people, so that a person can be in mulitiple states at the same time (in the different "worlds", in the same sense as the electron being in different "worlds".)

What made this strike home to me was when I was in graduate school and my advisor told me "There are no magic external observers. The observer is subject to quantum mechanics too. He is part of the experiment."

To describe the correspondance between copenhagen and "many worlds", suppose an observer measures if an electron is spin up or spin down. In "many worlds" case his memory of the outcome is correlated with the measured state of the electron. So in the "world" where the electron is spin up (that portion of the wave function) the observer also thinks the electron was measured as spin up. And in the "world" where the electron is spin down, the observer thinks the electron was measured as spin down.

In this "many worlds" case, The observer who measures the electron as spin up will not interact with the observer that measured it as spin down. For all intents in purposes, it is as if that other observer never existed. In the copenhagen case, that other observer does _not_ exist. In this interpretation, the wave function collapsed to only include the part with a single observer.

In effect the observed outcome of the two interpretations is the same. The difference being one of them, the copenhagen interpretation, postulates a magical change in the wave function of the universe.

(Aside - The fact that those observers will not interact is just in a practical sense, to my knowledge. I don't know if it is impossible for them to interact in theory. I don't think it is. Maybe someone else knows the answer to that.)

[monktastic1]

> For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down.

I don't think most MWIers would agree with this. Normally they consider worlds to have split only once (irreversible, or approximately irreversible) decoherence has set in. An electron in the coherent state |z+> + |z-> = |x+> wouldn't qualify.

In fact, this seems to be one of the biggest difficulties of the interpretation. Nobody knows whether there even is such a thing as in-principle irreversible decoherence, and if there's not, then the point at which it is "approximately irreversible" is arbitrary.

[gpsx]

Are you a many worlds person, Or is this your interpretation of what they believe? There is no need for an irreversible decoherence. Any real situation where there is a question of copenhagen versus many worlds is a pretty decoherent problem to begin with, since you are dealing with macroscopic beings.

Edit: Add the state change in the measurement

(|z+> + |z->)|obs> => |z+,obs+> + |z-,obs->

First there is an electron in one of two states, and the observer is uncorrelated. After the measurement, the observer becomes correlated with the electron.

[monktastic1]

The only explanation I've ever seen of when worlds split is when decoherence has become "effectively irreversible." That's not a well-defined physical event, and so it's hard to say that worlds "actually" split.

For your unentangled state on the left, Sean Carroll explicitly describes it as a state that doesn't have two worlds yet. I can find the post if you like, or maybe we already agree and I'm misunderstanding.

[gpsx]

If you have can find a post I'd like to see it. I don't quite follow.

In what I am describing, the two "worlds" don't really separate. It is possible that they can interact, theoretically. However, you can't construct an experiment to detect the different parts of the wavefunction interacting because of decoherence. You just can not make a coherent quantum system that invovles real people (to my knowledge). So in practice you can not do the experiment.In anything we observe, the two resulting observers (in a measurement with two choices) are effectively isolated.

In other words, decoherence is automatic. Also, it is inherently irreversible.

But, I am sure he (or whoever wrote that post) is saying something sensible so I would be interested in seeing it.

[monktastic1]

I just mean like this: http://www.preposterousuniverse.com/blog/2014/06/30/why-the-...

"We wouldn’t think of our pre-measurement state (1) as describing two different worlds; it’s just one world, in which the particle is in a superposition. But (2) has two worlds in it. The difference is that we can imagine undoing the superposition in (1) by carefully manipulating the particle, but in (2) the difference between the two branches has diffused into the environment and is lost there forever."

(State 1 is when the particle is in a superposition by itself and state 2 is when it's entangled with a macroscopic apparatus.)

My issue is that he uses words like "forever" and "impossible." These convey a sense of finality, but the decision of where to draw the boundary is subjective. The worlds can in principle (and under certain cosmological models, must) recohere.

See, for example: https://arxiv.org/abs/1105.3796

"Decoherence - the modern version of wave-function collapse - is subjective in that it depends on the choice of a set of unmonitored degrees of freedom, the "environment"."

See in particular Section 3.2 (Failure to irreversibly decohere: A limitation of finite systems)

(Edit: I should mention that I am not a physicist, by a long shot. Just a curious amateur.)

[kgwgk]

> the copenhagen interpretation, postulates a magical change in the wave function of the universe.

And the MWI postulates a magical branching into different "worlds", doesn't it?

[gpsx]

No. I may not have conveyed the idea very well in the comment. That is normal quantum mechanics. People just use the term "world" when it applies to these observers and not when it applies to something like an electron.

[kgwgk]

If this is normal quantum mechanics and the "worlds" are not real, separated physical objects -- and they are just mathematical constructions -- what problem does the MWI solve precisely?

If the universe is an isolated quantum system evolving unitarily, how does the MWI help to understand the laws of physics that we observe?

[nessus42]

> If the universe is an isolated quantum system evolving unitarily, how does the MWI help to understand the laws of physics that we observe?

It definitively answers the question of when wave collapse occurs and what causes it.

The answer given by the Everett Interpretation is simply that there is no wave collapse, and what we observe is the result of our brains being in a superposition of multiple states.

[kgwgk]

Consider this simple application of standard QM:

1) we have a one-particle system that has been prepared into a pure state by measuring the spin along the x-axis

2) we are going to measure the spin along the z-axis

3) the quantum state before is a superposition of the |up> and |down> states (in the basis corresponding to the Sz operator)

4) the theory predicts that we are going to find either |up> or |down> with equal probability

5) immediately after the measurement the quantum state will be either |up> or |down>, depending on the outcome

What is the answer given by the Everett Interpretation? What is the description of the initial conditions? What is the prediction of theory? What is description of the end state?

I hope the answer is not just handwaving and mumbling about "superposition".

[nessus42]

The prediction of the Everett Interpretation is that our brains end up in a superposition of states in which one of those states perceives an up particle and one of those states percieves a down particle, but that the original particle is still, in actuality, in a superposition of up and down states. I.e., the superposition has not actually collapsed into either up or down. It's just that our brains are now in a superposition of states too.

I don't understand why this would be difficult to understand.