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

[PeterDonis]

you get chairs and tables which have never been part of QM

Sure they are: chairs and tables are just bound states containing $10^{25}$ or so atoms.

getting chairs from probabilities

Nobody claims that QM can do this, and it's the wrong way to get chairs and tables from QM anyway.

 

[Vanadium 50]

Why don't we see events with probability 10^-1000? Because the probability is 10^-1000! Interpretations don't even enter into it.

 

[Quantum Waver]

But isn't this as fundamental an observational fact as it is in classical physics? If we do a double-slit experiment with electrons, we just prepare a beam of electrons (or many single electrons) hitting the double slit with a pretty well defined momentum and then register, on which point of a screen far enough away from the double slit. It's a basic observational fact that for any electron we get "one pixel" registering one electron (where "a pixel" is just a macroscopically small region on a pixel detector, photoplate, etc.). According to QT where each individual electron is registered is random, and the probability distribution function is given by Born's rule from the calculated wave function at the place where we register the electrons. These are the simple observational facts we describe probabilistically with QT.

Of course, you can always ask, whether the necessity for a probabilistic description is due to some ignorance about the state of the electron in this situation, i.e., that there are maybe "hidden variables", whose determination would also determinate precisely the spot, at which we'll register a specific single electron. What's ruled out by many Bell-test experiments is that such a hidden-local-variable model can be made deterministic (realistic = all observables always take determined values) and local (space-like separated events cannot be causally connected).

Whether there may be some future deterministic theory describing everything in Nature, nobody can know, but the given empirical evidence is completely described by Q(F)T, except that there is no satisfactory QT which takes into account the gravitational interaction (or, if one takes the geometrodynamical paradigm of GR literally a QT of spacetime itself). IMHO that's the big open problem of contemporary physics and not some interpretational issues of QT, which are all solved theoretically (with relativsitic local QFT describing everything except the gravitational interaction) and consolidated empirically (with the long overdue Nobel prize of last year).

At the end of the DeWitt article on page 35, he suggests that the initial coherence of the universal wave function in the Big Bang may have empirical implications for cosmology. Sean Carrol has stated that MWI is popular among cosmologists. Carroll has mentioned working on showing how spacetime and gravity could emerge from Hilbert space (maybe in this paper: https://arxiv.org/pdf/2103.09780.pdf). So it's possible an interpretation of QM will have results for future physics. If you listen to people working in the foundation of QM, they do see it as making progress in physics, not simply philosophizing.

Aside from the above, the motivation is an attempt to understand what nature's doing when not being measured. The wave function works as a predictive model, but how is that so? If it's not describing anything real, then what makes it predictive? The beam of electrons are doing something in between being emitted and being registered at the wall. Unless you think they only exists as measurements, which raises the question of what does exist in the interim. A probability wave? What is that exactly? A potential value? What triggers the stochastic collapse from potential to a real, determinate value? Why does measurement make the difference? How is the measuring device not also a potential? DeWitt mentions on page 31 how the final state vector in his fifth equation does not have a unique value, because the apparatus has gone into superposition.

 

[PeterDonis]

What law of physics postulates that quanta must form bound states

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.

and act classically as chairs?

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.

There is no adequate way to get chairs and tables from QM.

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.)

 

[Quantum Waver]

Sure they are: chairs and tables are just bound states containing $10^{25}$ or so atoms.Nobody claims that QM can do this, and it's the wrong way to get chairs and tables from QM anyway.

Under MWI, the bound states of those atoms behave quantum mechanically, so it should be possible to calculate the likelihood that all the atoms would tunnel at the same time through the wall the table is next to. In the DeWitt article you linked to, he calls these "Maverick worlds" on page 34. However, he does admit that if Hilbert space is small enough, they may be excluded by the universal wave equation. Also that if there are Maverick worlds where the very low probability events happen regularly, then life might not be able to evolve and it would be devoid of observers.

 

[Quantum Waver]

Why don't we see events with probability 10^-1000? Because the probability is 10^-1000! Interpretations don't even enter into it.

True, it's overwhelmingly likely that we're normal observers and don't get to see those extremely rare events. But even without MWI, if the universe is infinite with a normal distribution of matter throughout, then it follows there will be regions with observers like us who do witness extremely low probability events. Given the measured topology is near flat, that's a possibility in cosmology.

 

[PeterDonis]

Under MWI, the bound states of those atoms behave quantum mechanically, so it should be possible to calculate the likelihood that all the atoms would tunnel at once through the wall the table is next to.

Such a calculation assumes that the potential and Hamiltonian for the case "macroscopic object next to wall" are sufficiently similar to the potential and Hamiltonian for the astronomically simpler case of "single quantum particle next to microscopic potential barrier" to make the known solution of the former an acceptable proxy for the unknown (since we have no way of actually writing down the applicable potential and Hamiltonian) solution of the latter. But this assumption seems to me to be handwaving to support an already determined conclusion, rather than anything that can reasonably be extracted from QM.

 

[Quantum Waver]

Such a calculation assumes that the potential and Hamiltonian for the case "macroscopic object next to wall" are sufficiently similar to the potential and Hamiltonian for the astronomically simpler case of "single quantum particle next to microscopic potential barrier" to make the known solution of the former an acceptable proxy for the unknown (since we have no way of actually writing down the applicable potential and Hamiltonian) solution of the latter. But this assumption seems to me to be handwaving to support an already determined conclusion, rather than anything that can reasonably be extracted from QM.

But under MWI, all the particles are in a superposition for all possible states, which includes the particles being on the other side the wall. The table and wall aren't fundamental to this. They're just emergent configurations of particles in decohered branches, but quantum mechanically speaking, they would be in a superposition. Unless there's something about the bound states prohibiting this.

 

[PeroK]

True, it's overwhelmingly likely that we're normal observers and don't get to see those extremely rare events. But even without MWI, if the universe is infinite with a normal distribution of matter throughout, then it follows there will be regions with observers like us who do witness extremely low probability events. Given the measured topology is near flat, that's a possibility in cosmology

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.

 

[PeterDonis]

under MWI, all the particles are in a superposition for all possible states

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.

 

[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.