I Multiverse theory -- Why don't strange things happen here sometimes?

[PeterDonis]

this is a common hand-waving claim made in discussions of the MWI

Btw, I've often wondered why MWI proponents make such claims, since it seems like they're sawing off their own branch. It would seem to make much more sense under the MWI to explain the fact that we don't observe things like macroscopic objects tunneling through walls by invoking the fact that such outlandish things never had a nonzero amplitude in the universal wave function to begin with.

 

[Quantum Waver]

No, they're not, although this is a common hand-waving claim made in discussions of the MWI. The universal wave function only includes terms that arise by unitary evolution from the universal wave function at the Big Bang and the applicable Hamiltonian. There is no guarantee that "all possible states" as we would naively interpret that term are included.

Yeah, but the argument would be that the position of the particles in both the wall and table had become entangled at some point since the Big Bang because of some interaction like radioactive decay. They're not isolated systems.

Btw, I've often wondered why MWI proponents make such claims, since it seems like they're sawing off their own branch. It would seem to make much more sense under the MWI to explain the fact that we don't observe things like macroscopic objects tunneling through walls by invoking the fact that such outlandish things never had a nonzero amplitude in the universal wave function to begin with.

Because MWI isn't classical and QM allows for such possibilities. It's only outlandish to us normal branchers.

 

[PeterDonis]

the argument would be that the position of the particles in both the wall and table had become entangled

According to the MWI pretty much everything in the observable universe would have some entanglement with pretty much everything else. But that's not enough by itself to show that the table tunneling through the wall has a nonzero amplitude.

However, to get back to the original point of the thread, it's not even necessary to postulate events like "table tunneling through wall" to see that the MWI implies that there are "outlandish" branches in the wave function. A simple series of a sufficient number of Stern-Gerlach experiments is enough. So the specific question about tables tunneling through walls is really moot as far as this thread is concerned.

 

[Quantum Waver]

There was a thread somewhat recently about Bayesian methods being used to calculate the "probability" that the universe is infinite.

However, if you stick to purely frequentist probability methods, there is no way to calculate such a probability.

It's not that the Bayesians are wrong, but that statements like "it's 90% likely the universe is infinite" may never have a fully objective meaning.

That's why, IMO, statements about hypothetical planets where the laws of physics continue indefinitely to appear not to hold are potentially meaningless. The existence of such worlds is always predicated on some aspect of the theory that can never be observed.

This is very different from mathematical infinities, such as an infinite sequence, which can be proven to exist abstractly based on definite axioms.

I don't know whether statistics is necessary. It's really a question of whether the universe has zero curvature, in which case space extends infinitely. Then it becomes a question of whether matter/energy are uniformly distributed, and the observable universe is typical of that. It's known the actual universe is larger than the observable one, otherwise what's observable wouldn't increase over time as light has a chance to reach us from more distant regions.

The point is that a large enough (wouldn't have to be infinite) universe will have every physically possible event happen, including the extremely low probability ones. So it's not just an oddity of MWI.

 

[PeterDonis]

The point is that a large enough (wouldn't have to be infinite) universe will have every physically possible event happen

As long as "physically possible" means "consistent with the initial conditions". Again, we don't know that the initial conditions included everything that is possible according to the laws of physics.

 

[Nugatory]

Bound quantum states that collectively act classically... Okayish for a description but hardly any reason why they have to. What law of physics postulates that quanta must form bound states and act classically as chairs?

None.
All we have is that if the initial configuration is $10^{25}$ particles forming a chair, forward quantum mechanical evolution of that multiparticle system will, with probability very close to certainty, lead to continued chair-like behavior (as opposed to something bizarre such as the chair tunneling through the floor).

But that is also the case with classical analysis of macroscopic systems. It’s tempting to think that the classical treatment starts with our $10^{25}$ particles in a known configuration; we turn that configuration over to Laplace’s demon; the demon calculates the forward evolution into some other configuration which is also a well-behaved chair; low-probability weird behavior is precluded because the entire process is deterministic.
But of course we do nothing of the sort - we describe the chair using the bulk properties of the materials it’s made from, and these are inherently statistical in nature. Out of all the inconceivably huge number of possible microstates that correspond to the macrostate “chair” there will be some in which Laplace’s demon finds that eventually all the molecules in the seat are thermally moving in the same direction at the same time, and we would observe decidedly unchairlike weird behavior. As with quantum mechanical weirdness, the probability of actually observing such things in a chair-sized system is vanishingly small so we don’t consider it.

 

[PeroK]

I don't know whether statistics is necessary

Your whole argument, from start to finish, is statistical.

. It's really a question of whether the universe has zero curvature, in which case space extends infinitely.

That is a question that may not have an objective answer. A mathematical model of the universe may be infinite, but that does not imply certainty of everything predicted by the model.

Then it becomes a question of whether matter/energy are uniformly distributed, and the observable universe is typical of that. It's known the actual universe is larger than the observable one, otherwise what's observable wouldn't increase over time as light has a chance to reach us from more distant regions.

The point is that a large enough (wouldn't have to be infinite) universe will have every physically possible event happen, including the extremely low probability ones. So it's not just an oddity of MWI.

If the universe is finite, then there can only be finitely many planets, for example. You cannot therefore have "all possible" planets.

 

[PeroK]

None.
All we have is that if the initial configuration is $10^{25}$ particles forming a chair, forward quantum mechanical evolution of that multiparticle system will, with probability very close to certainty, lead to continued chair-like behavior (as opposed to something bizarre such as the chair tunneling through the floor).

But that is also the case with classical analysis of macroscopic systems. It’s tempting to think that the classical treatment starts with our $10^{25}$ particles in a known configuration; we turn that configuration over to Laplace’s demon; the demon calculates the forward evolution into some other configuration which is also a well-behaved chair; low-probability weird behavior is precluded because the entire process is deterministic.
But of course we do nothing of the sort - we describe the chair using the bulk properties of the materials it’s made from, and these are inherently statistical in nature. Out of all the inconceivably huge number of possible microstates that correspond to the macrostate “chair” there will be some in which Laplace’s demon finds that eventually all the molecules in the seat are thermally moving in the same direction at the same time, and we would observe decidedly unchairlike weird behavior. As with quantum mechanical weirdness, the probability of actually observing such things in a chair-sized system is vanishingly small so we don’t consider it.

I would present the case like this. You do an experiment where you build a wall and put a chair on one side of it and wait.

Eventually, the chair and wall biodegrade and all we get from the experiment is the expected macroscopic behaviour.

Then we repeat the experiment. And this outcome repeats until the universe gets to the point where the experiment is no longer possible. And still, statistically, there is no realistic possibility of seeing anything remotely like quantum tunneling of the chair.

So, in what sense, will the chair eventually tunnel through a wall?

 

[Quantum Waver]

If the universe is finite, then there can only be finitely many planets, for example. You cannot therefore have "all possible" planets.

There's a finite number of possible physical states. All possible planets is therefore finite. A large enough region of space would contain every kind of planet. Max Tegmark calculates how far
you identical copy would be provided:

Level I: A generic prediction of cosmological inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions - including an identical copy of you about 10^1029 meters away. https://space.mit.edu/home/tegmark/crazy.html

So, in what sense, will the chair eventually tunnel through a wall?

Eventually a Boltzmann brain could fluctuate into existence and hallucinate a chair tunneling through a wall. If the eternal future of the universe is de Sitter space, than any possible fluctuation will happen.

So, in what sense, will the chair eventually tunnel through a wall?

If there's no sense in which a chair could tunnel through a wall, why is there a non-zero probability? What does that mean? In MWI, it means it happens in some branches, assuming chair-tunneling is part of the universal wave function's evolution. If it's not, then that should mean the probability is actually zero and we're calculating it wrong.

 

[PeroK]

There's a finite number of possible physical states. All possible planets is therefore finite. A large enough region of space would contain every kind of planet.

Okay, you may be able to put an upper bound on the number of distinct planets. You can postulate that all those planets exist, but experimental corroboration would be practically impossible.

Eventually a Boltzmann brain could fluctuate into existence and hallucinate a chair tunneling through a wall.

Not necessarily, for the same reason as above. You can postulate such things, but have no experimental corroboration.

If there's no sense in which a chair could tunnel through a wall, why is there a non-zero probability? What does that mean?

Precisely my point. What is the physical significance of a probability of an event that is so low that nothing even remotely like that event will ever be observed?

In MWI, it means it happens in some branches, assuming chair-tunneling is part of the universal wave function's evolution. If it's not, then that should mean the probability is actually zero and we're calculating it wrong.

One problem with MWI is that it postulates a wave function with an infinite amount of information. That raises issues with blithely applying probabilities to events.

But, the fundamental problem with saying that things definitely exist is that if your theory is modified, where did your certainty go?

 

[GarberMoisha]

You must be joking. The existence of bound states, for systems where they exist, follows from the Hamiltonian and Schrodinger's Equation. I think you need to learn some basic QM.The laws of physics don't "postulate" chairs. Chairs are just one of a zillion possible bound states that are allowed by the laws of physics. Expecting the laws of physics to specifically tell you anything about chairs is foolish.If you're going to be strict about "adequate" and require an explicit derivation, then classical physics is no more "adequate" than QM, since it doesn't say anything specific about chairs either. (In fact, strictly speaking, it's less adequate, since classical physics can't even explain atoms.)

'Allowed' doesn't mean chairs. You are too lax with the conclusions and assumptions.
'Allowed' as you use the term means observed which is a tautology. You try to explain chairs by postulating that quantum states allow chairs.

The fact is neither classical physics nor QT has an adequate explanatjon for the two states of matter(classical and quantum mechanical). If you were small enough, you'd never withness the Newtonian scales and laws and if you were big enough, it would take 200 000 years of evolution to notice the quantum state of matter.

Which means that both states of matter are aspects of something else that is matter in reality and neither of them is the single explanation of what matter is.

If you insist that chairs are quantum mechanical in nature, you don't have an adequate explanation for the existence of chairs in the first place. If you insist that they are classical, you'd fail to account for the stability of matter that comptises them. Which leads to the conclusion that both concepts from QT and CM are inadequate, seprately or taken together.

 

[Drakkith]

You try to explain chairs by postulating that quantum states allow chairs.

How is this any different than saying quantum states allow a hydrogen atom?

 

[Nugatory]

You try to explain chairs by postulating that quantum states allow chairs.

No, we calculate that quantum states predict stable atoms and chemical bonding between them. From there we get the bulk properties of matter and these explain chairs.

 

[PeterDonis]

You are too lax with the conclusions and assumptions.

You are too confident and dogmatic with your assertions.

neither classical physics nor QT has an adequate explanatjon for the two states of matter(classical and quantum mechanical)

"Classical" and "quantum mechanical" aren't states of matter. They are theoretical frameworks.

States of matter are things like "solid", "liquid", "gas", "plasma", "white dwarf matter", "neutronium", "quark-gluon plasma", etc. Classical physics cannot explain the existence of such states. QM can. @Nugatory explained how.

both states of matter are aspects of something else that is matter in reality and neither of them is the single explanation of what matter is

This looks like personal speculation, which is off limits here.

 

[GarberMoisha]

I am not speculating. I am pointing out that there is a conceptual issue with both frameworks. QM does not talk about things existing in and of themselves with definite properties but of probabilities, hence the existence of chairs is problematic if this is the chosen framework that is to be taken as fundamental and giving rise to everything else.
Classical physics is incompatible with the quantum side of physical matter and is inadequate as a thorough framework.
When you choose one aspect to be fundamental you get MWI, Schroedingers cat, Wigner's friend, etc unobserved quantum paradoxes

 

[PeterDonis]

QM does not talk about things existing in and of themselves with definite properties but of probabilities

QM talks about probabilities of measurement results. This talk already assumes that there exist things being measured and things doing the measurements. For example, QM might talk about the probabilities of measurement results on a chair. But such talk makes no sense unless the chair exists, as well as the device we are using to measure it.

Different interpretations of QM make different claims about the "reality" that underlies the probabilities of measurement results, but none of them, as far as I know, claims that in that "reality" the things being measured and the measuring devices don't exist.

 

[hutchphd]

For example, QM might talk about the probabilities of measurement results on a chair. But such talk makes no sense unless the chair exists, as well as the device we are using to measure it.

I don't know that it makes no sense without the pre-existance of the chair. I see in the argument by the OP the usual creationist fallacy only this time applied to a chair.
The possibility of the chair tunneling is small
The possibility of the chair even existing is small and even smaller if their is no chairmaker.
The possibility of the existance of the chairmaker is also very small.
The possibility of the chair existing given the existance of the chairmaker is not so small
This is a longwinded way of saying that I think any particular world is wildly improbable (independent of Quantum Classical Everett or Bohm) and yet here we are.

 

[PeterDonis]

I don't know that it makes no sense without the pre-existance of the chair.

If the chair doesn't exist, how can "these are the probabilities for different possible results of this measurement on the chair" make sense?

 

[GarberMoisha]

QM talks about probabilities of measurement results. This talk already assumes that there exist things being measured and things doing the measurements. For example, QM might talk about the probabilities of measurement results on a chair. But such talk makes no sense unless the chair exists, as well as the device we are using to measure it.

Different interpretations of QM make different claims about the "reality" that underlies the probabilities of measurement results, but none of them, as far as I know, claims that in that "reality" the things being measured and the measuring devices don't exist.

If you believe the Universe is quantum, you should not assume the existence of classical stuff like measurement apparati.
Obviously, the Universe in not just quantum and you assuming the reality of the measurement apparatus proves this point. It is quantum and classical. You don't even directly observe the quantum side - you infer this from experiments. This Newtonian-quantum nature of matter is certainly inadequate(note to the other moderators - nowhere did I propose what matter really is; if I had to pick one interpretation, it would be closer to QBism - there matter and the experience of matter are not two separate concepts ). https://arxiv.org/abs/1705.03483

The agent experiences 2 different aspects of matter(quantum/classical) depending on the way of the enquiry.

 

[hutchphd]

Oh god I knew better than to touch the tar-baby . My point was that the very existance of the chair is far less likely than it tunneling spontaneously through wall. Using the conditional probabilities assuming preexistance of a carpenter then probably not.

 

[PeterDonis]

If you believe the Universe is quantum, you should not assume the existence of classical stuff like measurement apparati

If you believe the universe is quantum, then there is no such thing as "classical stuff"; measurement apparati are just as quantum as everything else. Which is still no argument for them not existing.

Obviously, the Universe in not just quantum

No, this is not at all obvious. What is obvious is that classical physics works, in the sense of making reasonably accurate predictions, for a wide variety of phenomena, including the behavior of things like chairs, given that such things exist (although, as already noted, classical physics cannot explain how such things exist, since it can't explain why atoms exist). But that in itself does not mean "the universe is quantum" is impossible. It means that if the universe is quantum, then classical physics must be derivable as an approximation to QM under appropriate conditions. Which is precisely what many results in QM, such as the Ehrenfest theorem, are intended to show.

if I had to pick one interpretation, it would be closer to QBism

If this is your preferred interpretation, that's fine--but then you should be using it. It doesn't appear to me that you are, since many of the things you are saying don't look anything like QBism to me, including the things I have responded to earlier in this post, and in my previous posts.

 

[PeterDonis]

My point was that the very existance of the chair is far less likely than it tunneling spontaneously through wall.

What are you basing this claim on? On its face it's obviously false, since the existence of chairs is a fact of everyday observation but nobody has ever observed a chair tunneling through a wall, so if QM's predicted probabilities were as you claim here, QM would be an egregiously wrong theory. Which it isn't.

 

[Nugatory]

When you choose one aspect to be fundamental you get MWI, Schroedingers cat, Wigner's friend, etc unobserved quantum paradoxes

Perhaps better to say that if we choose one aspect to be fundamental and then misrepresent it we will get paradoxes. The modern understanding of quantum mechanics leads to neither Wigner's Friend nor Schrodinger's Cat problems and I am totally unclear as to why you think MWI is somehow paradoxical.

 

[hutchphd]

What are you basing this claim on?

If there were no carpenters in the universe, what would be the probability of a "spontaneous" chair appearing at a place certain?
I should never try to argue philosophy. As a courtesy I will try to quantify if you really insist, but I think the outcome will not be very enlightening.

 

[PeterDonis]

If there were no carpenters in the universe, what would be the probability of a "spontaneous" chair appearing at a place certain?

Still a lot larger than the probability that such a chair would tunnel through a wall in such a universe, since to get the latter probability you have to multiply the tunneling probability we have been talking about, which is conditional on the chair being there next to the wall in the first place, by the probability that the chair will be there.

 

[Drakkith]

Obviously, the Universe in not just quantum and you assuming the reality of the measurement apparatus proves this point. It is quantum and classical. You don't even directly observe the quantum side - you infer this from experiments.

I don't buy this argument. Quantum physics tells us how things work at the very smallest scales and, conveniently, most of the complicated bits can be left out as we move to larger scales so we can do things like build buildings and cars and such with much, much easier math. When your allowed tolerances are millions of atoms across you don't care very much about the uncertainty of the position of a single electron or the like. This doesn't make the universe non-quantum, it just lets us use a simpler set of rules for many things at our scale.

 

[Drakkith]

If there were no carpenters in the universe, what would be the probability of a "spontaneous" chair appearing at a place certain?

The chair and the wall don't matter. We might as well talk about a rock tunneling through the rock next to it.

 

[GarberMoisha]

I don't buy this argument. Quantum physics tells us how things work at the very smallest scales and, conveniently, most of the complicated bits can be left out as we move to larger scales so we can do things like build buildings and cars and such with much, much easier math.

This is what I have been arguing against. Rather than assuming that quantum physics is telling you how things work at the smallest scales, you should infer that it is actually preventing you from knowing exactly how things work there. Then, once you get rid of that common misunderstanding, my argument about the nature of matter will appear more naturally.

When your allowed tolerances are millions of atoms across you don't care very much about the uncertainty of the position of a single electron or the like. This doesn't make the universe non-quantum, it just lets us use a simpler set of rules for many things at our scale.

You will always be stuck with the act of measurement. This is not just unsatisfactory but hints that the commonly held belief that matter is both Newtonian and quantum depending on how you look, is wrong. These two concept don't work together in a coherent way.

 

[Drakkith]

Rather than assuming that quantum physics is telling you how things work at the smallest scales, you should infer that it is actually preventing you from knowing exactly how things work there.

Why would I do this?

You will always be stuck with the act of measurement. This is not just unsatisfactory but hints that the commonly held belief that matter is both Newtonian and quantum depending on how you look, is wrong. These two concept don't work together in a coherent way.

No one is saying matter is Newtonian. We're saying that Newtonian physics is 'close enough' for work at our everyday scale.

 

[GarberMoisha]

Quantum mechanics in its standard form without additional assumptions does not naturally lead to infinite universes and MWI but to ignorance. This is enoded in the HUP in a very robust way and is easily visible in all iterations of the double split experiment.

 

[Drakkith]

Quantum mechanics in its standard form without additional assumptions does not naturally lead to infinite universes and MWI but to ignorance.

I assume you're referring to uncertainty when you mention 'ignorance'?

 

[GarberMoisha]

I assume you're referring to uncertainty when you mention 'ignorance'?

It is the same. Ignorance via 'uncertainty'. The fundamental term is ignorance which arises via uncertainty.

 

[Drakkith]

It is the same. Ignorance via 'uncertainty'. The fundamental term is ignorance which arises via uncertainty.

This sounds purely like a personal opinion and not something based in mainstream science. I remind you that PF rules do not allow for discussion of personal opinions in most circumstances since we aim to teach mainstream science as it is understood and practiced by the professional scientific community. You can have whatever opinion you like, but I ask that you stay within PF rules. Further posts of this nature will likely result in a thread ban or an infraction.

 

[kered rettop]

Seems an odd question. "Strange" presumably means "unlikely enough that it would cause extreme surprise if it actually did happen", and, according to both MWI and all true multiverse theories, unlikely things don't happen very often. So where's the problem? Sure, this very world you are experiencing right now will, according to MWI, have split into an incalculably vast number of worlds before you've finished reading this sentence. And a huge number of those worlds will be ones where pigs have been observed flying to a warmer clime for the winter. But the huge number is miniscule compared with the number of worlds in which this hasn't happened. So the chances of you being in a world where it has happened are vanishingly small.

 

[PeterDonis]

a huge number of those worlds will be ones where pigs have been observed flying to a warmer clime for the winter

This will only be true if there is a nonzero amplitude for the world you were in when you started reading, to transition to the world you describe by the end of your reading. Which is preposterous.

The MWI doesn't say that anything you can concoct in your imagination will happen. It only says that anything that has a nonzero amplitude in the wave function will happen.

 

[kered rettop]

This will only be true if there is a nonzero amplitude for the world you were in when you started reading, to transition to the world you describe by the end of your reading.

Correct, and I am saying that if a flying-pig world is possible in the sense of not being self-contradictory then there will be flying-pig states which are not precisely orthogonal to "our" state. Even MWI/decoherence doesn't claim that for its worlds.

Which is preposterous.

Not to put too fine a point on it, that is, at best, appealing to emotion, and at worst, begging the question.

The MWI doesn't say that anything you can concoct in your imagination will happen. It only says that anything that has a nonzero amplitude in the wave function will happen.

Which is precisely why I did no such thing. Of course if you have a proof that the amplitude is necessarily precisely zero for all values of flying pig, I would love to know about it.

 

[PeterDonis]

if a flying-pig world is possible in the sense of not being self-contradictory then there will be flying-pig states which are not precisely orthogonal to "our" state.

This is something you should be making an argument for, not just handwaving.

Of course if you have a proof that the amplitude is necessarily precisely zero for all values of flying pig, I would love to know about it.

The burden of proof is on you, not on me. Show me a wave function for our current state that has a nonzero amplitude to transition into a flying pig state.

 

[Frabjous]

Show me a wave function for our current state that has a nonzero amplitude to transition into a flying pig state.

I can almost get there. I can get to a frying pig state.

 

[kered rettop]

This is something you should be making an argument for, not just handwaving.

The burden of proof is on you, not on me. Show me a wave function for our current state that has a nonzero amplitude to transition into a flying pig state.

That's a cop-out. You're the mentor round here. You have a perfect opportunity to show me where my reasoning is wrong. Or not even wrong if it comes to that. I'd learn something and so would the OP.

 

[PeterDonis]

That's a cop-out.

To it it's a cop-out to reverse the burden of proof when you are challenged to support a positive claim.

You're the mentor round here.

That doesn't mean I'm obligated to give you a response just because you think you're entitled to it.

You have a perfect opportunity to show me where my reasoning is wrong.

I've already told you: I don't think the wave function has a nonzero amplitude for what you're claiming. It's up to you to show that it does, since you are the one that made the claim about flying pigs. You have already agreed with me that a nonzero amplitude in the wave function for such a transition is necessary to support your claim. So it's up to you to show that in fact such a nonzero amplitude exists. You can't just assume that there is a nonzero amplitude for anything you like.

 

[kered rettop]

To it it's a cop-out to reverse the burden of proof when you are challenged to support a positive claim.That doesn't mean I'm obligated to give you a response just because you think you're entitled to it.I've already told you: I don't think the wave function has a nonzero amplitude for what you're claiming. It's up to you to show that it does, since you are the one that made the claim about flying pigs. You have already agreed with me that a nonzero amplitude in the wave function for such a transition is necessary to support your claim. So it's up to you to show that in fact such a nonzero amplitude exists. You can't just assume that there is a nonzero amplitude for anything you like.

Well, that's exactly what I do assume - on the basis that people who appear to know what they're talking about make it. This whole thread is predicated on a) such worlds being physically possible and b) that transitioning is possible.
For instance, the third paragraph of Sascha Vongher's article
https://www.science20.com/alpha_meme/are_terrible_quantum_states_phenomenal-87197
simply asserts both. (If you are squeamish, I recommend you do not proceed any further in the article. Vongher clearly intends to shock.)

To it it's a cop-out to reverse the burden of proof when you are challenged to support a positive claim.
That doesn't mean I'm obligated to give you a response just because you think you're entitled to it.

I don't recollect saying that I was entitled to anything. But assuming the goal of our discussion is to reach an agreement, either party could move it on if they address what the other says. And I think you are capable.
I'm not - not in the terms you want. So the discussion will remain stalled unless you choose to enlighten me.

I've already told you: I don't think the wave function has a nonzero amplitude for what you're claiming. It's up to you to show that it does, since you are the one that made the claim about flying pigs. You have already agreed with me that a nonzero amplitude in the wave function for such a transition is necessary to support your claim. So it's up to you to show that in fact such a nonzero amplitude exists. You can't just assume that there is a nonzero amplitude for anything you like.

And you can't just assume that there is a zero amplitude for anything you don't like.

 

[PeterDonis]

that's exactly what I do assume - on the basis that people who appear to know what they're talking about make it

I've seen it asserted without argument in articles, yes. I've never seen it actually argued for. So I remain skeptical. But I don't think we're going to resolve that here.

assuming the goal of our discussion is to reach an agreement

In cases like this, there is no generally accepted mainstream answer, so we should not necessarily expect to reach agreement.

you can't just assume that there is a zero amplitude for anything you don't like.

It's not a matter of me not liking the flying pigs. It's a matter of Occam's Razor. In the absence of some kind of evidence or argument that a transition from our current world to a world of flying pigs is possible, it seems to me that the more parsimonious assumption is that it's not. Either that or we should just refuse to take a position at all until somebody comes up with some kind of actual argument.

 

[PeterDonis]

This whole thread is predicated on a) such worlds being physically possible and b) that transitioning is possible.

The thread is about whether such things are possible. It is not about just assuming it.

 

[kered rettop]

I've seen it asserted without argument in articles, yes. I've never seen it actually argued for. So I remain skeptical. But I don't think we're going to resolve that here.

In cases like this, there is no generally accepted mainstream answer, so we should not necessarily expect to reach agreement.

It's not a matter of me not liking the flying pigs. It's a matter of Occam's Razor. In the absence of some kind of evidence or argument that a transition from our current world to a world of flying pigs is possible, it seems to me that the more parsimonious assumption is that it's not. Either that or we should just refuse to take a position at all until somebody comes up with some kind of actual argument.

Well I'm willing to bet that the assertions are grounded in a decent argument but as you don't know of one and neither do I, we appear to have reached an impasse.

 

[PeterDonis]

I'm willing to bet that the assertions are grounded in a decent argument

If there is one, it should appear somewhere in one of the articles on the subject, or, better still, in a peer-reviewed paper.

 

[PeroK]

That's a cop-out. You're the mentor round here. You have a perfect opportunity to show me where my reasoning is wrong. Or not even wrong if it comes to that. I'd learn something and so would the OP.

Let's start with a model of the solar system where the Sun and planets are assumed to be perfect spheres. This model works for solar system dynamics but can't be taken literally. If all we knew about the planets was their orbits, then there would be no reason to assume anything else. It would, however, be scientifically invalid to postulate the planets are perfect spheres. This is perhaps a subtle point, but it's worth thinking about. Until you have examined a planet, it's invalid to publish a theory of planetary geology based on this assumption. Even though the "perfect sphere" model is the best you have available.

Next, let's consider the model of an infinite, globally flat universe. This model is supported by the cosmological data and is the best available. Again, however, it's invalid to assume this model is literally true and make assertions like there being an infinite number of duplicate planets Earth out there. Until you have evidence for these duplicates, it's an invalid application of the infinite universe model.

Finally, if we have a mutliverse theory or MWI of QM, then the same scientific rigour applies. Just because that model is the best fit for a certain set of observations does not allow us to take model literally, and extrapolate it without limit. These alternative worlds with habitually flying pigs cannot be assumed to exist until the model has been tested in this respect.

One problem with stating the existence of such worlds is that the theory only needs to be tweaked - in a way that does not significantly alter the model's predictions in its current domain of applicability - in order for all the unobserved but extrapolated phenomena to vanish.

In the above cases, the Earth's active surface does not affect its orbital dynamics. In the second case, the universe could turn out to be large but finite. In the third case, a tweak to QM could rule out the existence of habitually bizarre worlds.

In my opinion, it's pointless to discuss such extrapolations as though they must or may be valid.

 

[PeroK]

Well I'm willing to bet that the assertions are grounded in a decent argument but as you don't know of one and neither do I, we appear to have reached an impasse.

One thing you have to be aware of is that even the greatest physicists may have debatable ideas when they step beyond theoretical physics. Personally, I don't feel obliged to believe what may be wild speculation, no matter who is doing the speculating.

 

[gentzen]

And you can't just assume that there is a zero amplitude for anything you don't like.

If an amplitude of $10^{-1000}$ leads to totally different conclusions than an amplitude which is exactly zero, then the corresponding interpretation has robustness issues. In fact, the consistent histories approach runs into this kind of trouble. One way how Griffiths tried to overcome this was by hoping that some input data could be slightly perturbed, and that thereby all those annoying nearly-zeros could be turned into exact zeros. PeroK above has basically suggested the same thing.

However, one of the strength of MWI compared to consistent histories is that it is perfectly fine with such approximate zeros. At least for the popular "self-locating uncertainty" approach to probability in MWI, this follows from the observation that from a decision theoretic perspective the probabilities from MWI should (or even "must") be treated like normal probabilities. And this implies that a possible outcome with a probability amplitude of $10^{-1000}$ should have a negligible impact on your decisions.

(If somebody objects that this amplitude of $10^{-1000}$ still implies the existence of some outcome, one could counter this by analyzing the role played by that existence in some popular MWI approaches, and how MWI actually makes things better instead of worse by not insisting on the impossibility of those outcomes.)

 

[Morbert]

If an amplitude of $10^{-1000}$ leads to totally different conclusions than an amplitude which is exactly zero, then the corresponding interpretation has robustness issues. In fact, the consistent histories approach runs into this kind of trouble. One way how Griffiths tried to overcome this was by hoping that some input data could be slightly perturbed, and that thereby all those annoying nearly-zeros could be turned into exact zeros.

Do you have a reference for this discussion?

 

[gentzen]

(If somebody objects that this amplitude of $10^{-1000}$ still implies the existence of some outcome, one could counter this by analyzing the role played by that existence in some popular MWI approaches, and how MWI actually makes things better instead of worse by not insisting on the impossibility of those outcomes.)

For example, if the measure of existence concept is used, then there is no significant difference between a nearly vanishing measure of existence and a completely vanishing one. An analogy which came to my mind triggered by the word "measure" is that mathematics avoids problems by accepting the existence of sets of measure zero. It even accepts the existence of non-measurable sets... And in general, this strategy tends to reduce the number of problems, except when people start taking "mathematical existence" literally as for example in the Banach-Tarski paradox.