All branches of the wavefunction are equally real.

Why is this one real as opposed to the others I'm not experiencing where I instead went to sleep instead of replying to your post, now?

Why is this one real as opposed to the others I'm not experiencing where I instead went to sleep instead of replying to your post, now? — Question

Because in this one you're replying to the post. In another one, you're sleeping. What you're asking is why you're experiencing posting and not going to sleep. That's because you're not that "version" of you. That other you experienced it.

According to Everettian QM, the other branches are real. You are experiencing the other branches that you happen to be in. Your counterparts in other branches are just as real as you are.

It'll just be the (weak) anthropic principle.

Why can't I be in the possible world where I played the lottery and won?

Common!

what puzzles me about this interpretation of QM is how is this reality (the one here, at this moment which you are reading here on Earth) real as opposed to other possible worlds? — Question

You will notice that frequently in this discussion, the reality of the other worlds is put in quotes- 'other', or 'real'. A typical example is the Wikipedia entry on Many Worlds, the second sentence of which reads:

Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe").

Note the scare quotes around world and universe. So are they 'worlds' - or worlds? Are they real - or 'real'?

I don't think we will ever know.

I have often asked the question, both here and on Physics Forum, that if Many Worlds is the solution, then what is the problem? And that is a question I never get a straight answer to.

Try this paper:

http://xxx.lanl.gov/abs/quant-ph/0104033

I take issue with the first sentence:

The idea that quantum theory is a true description of physical reality

If it's not true, then what? That is the philosophical question.

I take issue with the first sentence:

The idea that quantum theory is a true description of physical reality


If it's not true, then what? That is the philosophical question. — Wayfarer

But that isn't the first sentence, is it?

sorry, it's the first phrase.

...."led Everett..."

I don't think you are even attempting to be honest.

1. Introduction
The idea that quantum theory is a true description of physical reality led Everett (1957) and many subsequent investigators (e.g. DeWitt and Graham 1973, Deutsch 1985, 1997) to explain quantum-mechanical phenomena in terms of the simultaneous existence of parallel universes or histories.

The question I am asking is, if 'parallel universes or histories is the solution, what is the problem?' Why is it necessary to postulate such an apparently bizarre notion in the first place? What problem is it trying to solve?

Note the scare quotes around world and universe. So are they 'worlds' - or worlds? Are they real - or 'real'?

I don't think we will ever know. — Wayfarer

If Many-Worlds is proven false, we will know for sure that the worlds are not real.

The question I am asking is, if 'parallel universes or histories is the solution, what is the problem?' Why is it necessary to postulate such an apparently bizarre notion in the first place? What problem is it trying to solve? — Wayfarer

I think the problem is that physicists are trying to understand the true nature of real possibility, with an inadequate understanding of the nature of time. In other words, their understanding of physical activity has come to the limits allowed, by their understanding of the non-physical time. Since time is what makes physical activity possible, and physicists do not have an adequate understanding of time, physicists do not have adequate premises for understanding the possibility of physical activity.

Speculations such as Many-Worlds are just examples of what comes about when a science pushes the limits of its premises. When premises are taken to the point where they produce absurd conclusions, what is demonstrated is a need to revisit the premises. In the case of quantum physics, what is glaringly obvious, is the inadequate understanding of time.

According to "many worlds," all possibilities are real/are actualized. You're in this particular actualized possibility. You're not in other ones.

This isn't to suggest that I support "many worlds," that I think it's not just nonsense applied as a way to understand a particular set of mathematical constructions, but nevertheless, that's the idea.

If I understand correctly, then according to the modal logic of "possible worlds", there is absolutely no way to distinguish one possible world from another as "the real world", any possible world is just as real as any other. If Many Worlds interpretation gives actual reality to possible worlds, then there is no basis for Question's questions which assume this world as "the real world". Such an assumption contradicts the primary premise that no one world is distinguishable from the others as the real world.

Charles Sanders Peirce's careful distinction of existence from reality might be helpful here. "Real" means that something possesses properties sufficient to identify it, regardless of whether anyone ever attributes them to it, while "exists" means that something reacts with other things. Hence all possible worlds are real, but only our actual world exists. An interesting corollary of this usage of terminology is that Peirce argued for the reality of God, but not the existence of God.

The question I am asking is, if 'parallel universes or histories is the solution, what is the problem?' Why is it necessary to postulate such an apparently bizarre notion in the first place? What problem is it trying to solve? — Wayfarer

You could ask what problem does entanglement solve? What problem does superposition solve? What problem does the Heisenberg Uncertainty Principle solve?

Or, when dealing with General Relativity, what problem do black holes solve? What problem do gravitational waves solve?

Parallel "universes" are a prediction of unitary i.e. Everettian quantum mechanics, which, as a by product, solve a number of other problems: e.g.

The Measurement Problem.
The nature of probability.
The arrow of time.
The meaning of counterfactuals.
The meaning of complexity.

A couple of other discoveries that were made within Everettian QM, but are arguably theory-neutral ( I disagree)

The possibility of quantum computers.
The existence of decoherence.

So, taking QM seriously as a theory of reality solves many problems, and renders it testable. Parallel "worlds" are a prediction.

So, taking QM seriously as a theory of reality solves many problems, and renders it testable. Parallel "worlds" are a prediction. — tom

In what sense is this particular prediction "testable"? What specific experiential consequences can we deductively explicate from it? How would we then go about inductively evaluating whether there really are parallel "worlds"?

Charles Sanders Peirce's careful distinction of existence from reality might be helpful here. "Real" means that something possesses properties sufficient to identify it, regardless of whether anyone ever attributes them to it, while "exists" means that something reacts with other things. Hence all possible worlds are real, but only our actual world exists. — aletheist

Um, perhaps you've not been paying attention, but we know the other worlds exist because they interact with each other and with our world.

Many Worlds is the only interpretation of QM that exists. The other theories are either not testable or proved wrong:

https://arxiv.org/abs/1508.02048

The only known theory that can reproduce the results of QM, at least up to quantum field theory, is Many Worlds.

Um, perhaps you've not been paying attention, but we know the other worlds exist because they interact with each other and with our world. — tom

I concede that I am not fully up to speed on QM, but it strikes me as a bit presumptuous to claim that we know the other worlds exist, and that they interact with our world. To be honest, I am not even sure what that would mean, or how one would demonstrate it.

In any case, the OP asked about distinguishing the world that we are actually experiencing from the "other" possible worlds. I was merely suggesting one terminological option for doing so.

but when the initial decision is made to entertain the notion of many worlds, then a whole series of consequences flow on from that. But I'm sceptical of the very first assumption. Actually, I'm not just sceptical - I'm dismissive of it. I think it is a fantasy. Everett himself says he had been drinking when the idea came to him. 'Hey, what if all the outcomes are real?'

Well, what if they're not? What if you refuse to entertain the notion? I think Neils Bohr refused to entertain it. Sure, it leaves all kinds of unanswered questions, but the lack of an answer migh be better than a clever fallacy.

And sure it's true that other interpretations are not testable - but that might just be an indication that we're dealing with a problem which is on the border of what is knowable. Again it might be better to acknowledge that than to build an edifice on fantasy.

I may be uneducated on the matter; but, why is this timeline where I'm asking these questions apparent to me and not any other? I'm not sure if I'm being clear enough.

Thank you.

I may be uneducated on the matter; but, why is this timeline where I'm asking these questions apparent to me and not any other? — Question

Perhaps you are in a different world? It seems like every participant in this thread is in one's own world. At least we are somewhat capable of interacting though.

But, the laws of physics are the same for all of us and we occupy the same space. So, hypothetically we are all experiencing the same history of the world (or what can be called the evolution of the wavefunction).

Hmm, here's a question for you then. If the history of the world is called the evolution of the wavefunction, then what is the future of the world?

but when the initial decision is made to entertain the notion of many worlds, then a whole series of consequences flow on from that. But I'm sceptical of the very first assumption. Actually, I'm not just sceptical - I'm dismissive of it. I think it is a fantasy. Everett himself says he had been drinking when the idea came to him. 'Hey, what if all the outcomes are real?' — Wayfarer

I think the issue is that if Everettian QM both explains our observations and solves a bunch of problems, as Tom points out, then on what grounds are you dismissing it?

In terms of consequences, we can continue to hold the same kinds of views around personal identity and decision-making as we always have. It's just that the world would be larger than previously thought.

on the grounds of the inherent implausibility there being parallel universes. Given the assumption there are, there are mathematical 'solutions' to various paradoxes and conundrums. But if their grounding assumption is unreal, then what kinds of 'solution' are they really?

Many Worlds is the only interpretation of QM that exists. — tom

Haha, it's disturbing to imagine what you'd be willing to do in the presence of Deutsch.

Anyway, the first big problem with the paper you linked to is that in section 2, what he describes as essentially the Popperian approach is not at all at odds with the Bayesian, instrumentalist or positivist approach to experimentation, yet Douche describes the latter as contradictory to the former.