Improbability of the Many-Worlds Interpretation?

[PeterDonis]

One odd one is that technically there is some subset of the state space of the gunpowder where due to entropy decrease the gun doesn't fire for an incredibly rare entropic decrease. So if you get somebody to fire a machine gun straight at you there's a world where every bullet failed to fire.

You're assuming that there is quantum uncertainty involved in gunpowder firing. That's an example of the kind of assumption I am objecting to.

 

[DarMM]

Do physicists not really think about this stuff? How information rules everything?

"Everything" and "rules" meaning what exactly?

I also wouldn't connect a clarification request from me about what exactly you were referring to to be related in any way to what physicists think about or don't think about. I just wasn't sure precisely what you meant.

 

[DarMM]

You're assuming that there is quantum uncertainty involved in gunpowder firing. That's an example of the kind of assumption I am objecting to.

Doesn't tunneling permit exploration of the chemical state space even if it's very small? My (very rough) intuition is that quantum effects in chemistry are suppressed when you reach arsenate molecules.

 

[PeterDonis]

Doesn't tunneling permit exploration of the chemical state space even if it's very small?

The state space of what? A single gunpowder molecule? (Well, I know it isn't made up of molecules of a single chemical species, but you get the idea.) You can't fire a bullet with a single gunpowder molecule. You need something like an Avogadro's number of them. Yes, quite possibly some miniscule fraction of that Avodagro's number of molecules will be in a "can't fire" state due to quantum tunnelling, but so what? Enough will be in that state to fire the bullet. Or if the bullet doesn't fire it will be for classical reasons, like you left the ammunition unused too long and exceeded its shelf life, or the firing pin didn't work. It won't be because quantum uncertainty put the Avogadro's number of gunpowder molecules into a superposition of "can fire" and "can't fire", which is what would need to happen for quantum uncertainty to cause a splitting of worlds according to the MWI when the firing was attempted.

 

[DarMM]

Yes, quite possibly some miniscule fraction of that Avodagro's number of molecules will be in a "can't fire" state due to quantum tunnelling, but so what?

Because then isn't there a chance they'll all be in the "can't fire" state?

 

[PeterDonis]

Because then isn't there a chance they'll all be in the "can't fire" state?

No. Or at least, if you're going to allow for that possibility, then you are allowing for possibilities that invalidate our belief in past data, and that version of the MWI undermines itself, as I argued in post #24. The chance that enough of the molecules will happen to all have tunneled into the "can't fire" state, and none of them will have tunneled back into the "can fire" state, at the exact instant that you try to fire the bullet, is comparable to the chance that @DrChinese actually was president of the US 5 minutes ago, but then a quantum fluctuation happened that changed everything on Earth, including all of our memories, to the state we all perceive and remember now. And if you allow for possibilities like that, all bets are off and there's no point in doing science at all.

 

[DarMM]

What prevents it though if you permit any arbitrary tunnling outcome to occur? What principle would stop me from allowing it in the space of outcomes?

 

[PeterDonis]

What prevents it though if you permit any arbitrary tunnling outcome to occur? What principle would stop me from allowing it in the space of outcomes?

If you take the MWI entirely literally, nothing does. And I'm saying that doing that makes the MWI undermine itself, since it tells us we can't trust our memories and records of past data, because we could be in one of the worlds where a quantum fluctuation changed them. And what forces us to adopt QM in the first place is our memories and records of past data. If we can't trust those, what reason do we have to even believe QM, let alone the particular MWI interpretation of it?

 

[DarMM]

I'll need to chew on that, very interesting.

What do you make of the fact that in "single world" QM there is still a small chance that such an event could occur. Is it basically that such an event essentially has never occurred so we can ignore it, but that in MWI you know that there are definitely "science nullifying" worlds.

I suppose there would also be worlds where the Born rule was never validated due to unfortunate fluctuations causing it to be rejected.

 

[PeterDonis]

What do you make of the fact that in "single world" QM there is still a small chance that such an event could occur.

That depends on whether you think sufficiently small probabilities--something like 1 expected event per $10^{70}$ years, say, where our universe has only existed for $10^{10}$ years--are actually meaningful. I personally think probabilities that small are just a sign that the theory is incomplete, just as the presence of singularities in certain GR solutions is a sign that GR is incomplete.

 

[PeterDonis]

I suppose there would also be worlds where the Born rule was never validated due to unfortunate fluctuations causing it to be rejected.

According to the MWI, yes, there should be worlds (lots of them) in which the Born rule does not hold. I have seen papers claiming to show that the set of such worlds is a "set of measure zero", or something similar, but I'm personally not convinced by such arguments.

 

[DarMM]

That depends on whether you think sufficiently small probabilities--something like 1 expected event per $10^{70}$ years, say, where our universe has only existed for $10^{10}$ years--are actually meaningful. I personally think probabilities that small are just a sign that the theory is incomplete, just as the presence of singularities in certain GR solutions is a sign that GR is incomplete.

You might be interested that Roland Omnès has similar views to be seen on pages 82-84 and 235-236 of his "Understanding Quantum Mechanics". Ultimately these are related to earlier views due to Émile Borel in:
"Le jeur, la chance et les théories scientifiques modernes" (1941)

 

[Klystron]

Do physicists not really think about this stuff? How information rules everything?

Information theory expands the latter question to:
"How information describes everything?"
Otherwise, are we not limited to rule-based descriptions?

Broader question:
Do physicists (scientists) think everything can be known?

Self-referential form:
Does Information inform everything knowable?

 

[Hans de Vries]

And if the MWI is true, those boundary conditions are already going to include a huge number of terms in a vast superposition, based on what happened in the entire past light cone up to that point and all of the possible quantum branch points that occurred.

I should try not to get involved with quantum philosophical discussions, but okay, having done this sort of "hypothetical MWI" calculations in the past:

- There are ~$10^{80}$ elementary particles in the universe.
- Interactions / State transitions take place at the femto second level in stars.
- The Universe is circa $4*10^{32}$ femto seconds old.
- Each event may actually span much more than 2 branches, say $N$ branches.

So very, very roughly there may be $N^{10^{112}}$ different universes right now.
The total number of particles (and thus energy) of the multiverse increases by a factor of $N^{10^{95}}$ per second.

Each of the $10^{80}*N^{10^{112}}$ particles in the multiverse somehow knows exactly to which of the $N^{10^{112}}$ universes it belongs and only interacts with the particles labeled to be in the same universe. Each particle would need some universe label to distinguish between other universes. The number of bits you would need for one such label for one particle would be far more then the total information content of an entire universe.

The whole universe must split near instantaneously over the entire size of the universe according to some absolute reference frame to which time is synchronized, otherwise the order of splitting and the universe labeling would mess up completely. The average time between two splits is $10^{-95}$ seconds during which the split has to propagate circa $5*10^{10}$ light years.

So the minimum speed at which the splitting propagates must be something in the order of $10^{114}$ times faster as the speed of light.Yeah really, MWI makes the task of understanding the physics of nature so much easier...

 

[PeroK]

I should try not to get involved with quantum philosophical discussions, but okay...

Perhaps you should read this first:

http://www.preposterousuniverse.com...ion-of-quantum-mechanics-is-probably-correct/
It might save you from tilting at straw men.

 

[akvadrako]

No. Or at least, if you're going to allow for that possibility, then you are allowing for possibilities that invalidate our belief in past data, and that version of the MWI undermines itself, as I argued in post #24. The chance that enough of the molecules will happen to all have tunneled into the "can't fire" state, and none of them will have tunneled back into the "can fire" state, at the exact instant that you try to fire the bullet, is comparable to the chance that @DrChinese actually was president of the US 5 minutes ago, but then a quantum fluctuation happened that changed everything on Earth, including all of our memories, to the state we all perceive and remember now. And if you allow for possibilities like that, all bets are off and there's no point in doing science at all.

I don't think taking MWI literally requires the assumption that every massive tunneling event is realized. Incredibly unlikely things might not be stable enough to form recognizable branches, the interference effects might be too strong before they fully split, or they might not contain something that can be considered a future version of one's self, so must be ignored.

Looking at it backwards is also helpful. Maybe there is a very small contribution to my current state from almost-orthogonal past states where Dr. Chinese was president. But that past state is so incompatible with my current personal state (containing my memories) the contribution will be too small to notice.

I'm not suggesting these are solved problems, because I suspect one needs to really understand how people perceive the world and form their sense of self to fully connect objective unitary evolution to the subjective experience of life inside that universe.

 

[akvadrako]

"Everything" and "rules" meaning what exactly?

I also wouldn't connect a clarification request from me about what exactly you were referring to to be related in any way to what physicists think about or don't think about. I just wasn't sure precisely what you meant.

I see; sometimes I'm just surprised by what things I say turn out to be least clear :) I meant once you have a way to try every possible program, you have the power to do (almost) anything not ruled out by the laws of physics.

 

[Hans de Vries]

Perhaps you should read this first:
It might save you from tilting at straw men.

I know Sean Carrol's arguments but quote:

Everett, by contrast, says that the universe splits in two: in one the cat is awake, and in the other the cat is asleep. Once split, the universes go their own ways, never to interact with each other again.

I don't see how Sean Carrol's following arguments help in anyway:

1) All universes are in superposition and superposition is normal in QM
2) In any universe all particles are entangled so they can't interact with particles in other universes.
3) And because of decoherence particles in one universe do not interfere with particles in other universes.

- At the end you need the same amount of independent universes. Since the splitting started right at the big bang we can be assured that universes are totally different from each other.

- The entanglement claim does not hold. Only certain properties can be entangled and momentum is not one of them. You can reflect entangled photons on mirrors, guide them through glass fiber and so on. I don't see how entanglement justifies the claim that particles can not interact with each other anymore. It's just a subset of their properties that is entangled.

- Also the decoherence argument for non-interference runs counter to accepted physics. A particle in two worlds may obtain different momentum states in each world. In QED, and subsequently QFT, a particle in a superposition of 2 momentum states gives rise to interference currents which are the source of the bosons of the Standard model.

The rest of the article is rather denigrating and political. It goes about the skeptics emotional states: "angry", "denial" and finally "acceptance". The latter is of course to accept the Everettian MWI approach and to forget the "silly" objections.Finally: I do not choose sides with any of the quantum philosophical interpretations, simple because I'm highly skeptical to all of them

 

[PeroK]

I know Sean Carrol's arguments but quote:
I don't see how Sean Carrol's following arguments help in anyway:

1) All universes are in superposition and superposition is normal in QM
2) In any universe all particles are entangled so they can't interact with particles in other universes.
3) And because of decoherence particles in one universe do not interfere with particles in other universes.

I must confess, I find this post quite bizarre. I'm not an advocate of MWI, nor do I know the Carrol piece particularly well. But, my first reaction was "I didn't think he said any of that". So, I went back to his blog, and nothing you ascribe to him he actually says!

Those three points, as far as I can tell, are entirely your own invention!

My understanding of Carrol's argument (reading what he actually says) is:

Fundamentally, despite its name, in MWI there is only one universe. But, it contains ultimately a superposition of all (measurement) outcomes. Like orthodox QM does before you measure it. Unlike orthodox QM, the superposition is never resolved, but continues indefinitely.

In the infamous cat experiment, there is only ever one cat. It's the state (of the particles that make up the cat) that is in superposition; not that an extra cat has been brought into existence. When you open the box, in orthodox QM the state resolves itself in one or the other. In MWI, the two possibilities continue to exist, in some sense - but, through decoherence, there is no subsequent mixing of the two possibilities

It's the branches of the wave function that decohere, so that they do not in general interfere with each other. Not that new universes of trillions of particles are continuously created.

PS the crux of Carrol's argument is here (direct quotation, with my underlines):

"All of this exposition is building up to the following point: in order to describe a quantum state that includes two non-interacting “worlds” as in (2), we didn’t have to add anything at all to our description of the universe, unlike the classical case. All of the ingredients were already there!"

 

[Hans de Vries]

I must confess, I find this post quite bizarre. I'm not an advocate of MWI, nor do I know the Carrol piece particularly well. But, my first reaction was "I didn't think he said any of that". So, I went back to his blog, and nothing you ascribe to him he actually says!

Those three points, as far as I can tell, are entirely your own invention!

I would urge you to read carefully through the text as accurate and painstakingly as I have done (and anybody can do in the link at the bottom), before you come out in the aggressive way you did. Let's carefully go through the 3 statements (as I ascribed them to Carrol) one by one:

1) All universes are in superposition and superposition is normal in QM

First Carrol comes up with the theory that both particle and observer (apparatus or human) are in a superposition state with different versions of them self:

But there is clearly another possibility. If the particle can be in a superposition of two states, then so can the apparatus...What would it be like to live in a world with the kind of quantum state we have written in (2)? It might seem a bit unrealistic at first glance; after all, when we observe real-world quantum systems it always feels like we see one outcome or the other. We never think that we ourselves are in a superposition of having achieved different measurement outcomes.

If the human being is already in superposition after the split then both human beings need their own versions of the universe because:

Everett, by contrast, says that the universe splits in two: in one the cat is awake, and in the other the cat is asleep. Once split, the universes go their own ways, never to interact with each other again.

2) In any universe all particles are entangled so they can't interact with particles in other universes.

But there is clearly another possibility. If the particle can be in a superposition of two states, then so can the apparatus.... There are more things in the universe than our particle and the measuring apparatus; there is the rest of the Earth, and for that matter everything in outer space. That stuff — group it all together and call it the “environment” ... We expect the apparatus to quickly become entangled with the environment, if only because photons and air molecules in the environment will keep bumping into the apparatus. As a result, even though a state of this form is in a superposition, the two different pieces (one with the particle spin-up, one with the particle spin-down) will never be able to interfere with each other

Here we see that the apparatus (or human observer) is expected by Carrol to become entangled with the environment which includes "everything in outer space". The second odd thing here is that, while Carrol mentions entanglement, A particle spin-up never interferes with particle spin-down because the two states are orthogonal. This is a very well known property of the Dirac field! Entanglement has nothing to do with this !

3) And because of decoherence particles in one universe do not interfere with particles in other universes.

This is where the magic of decoherence comes in... As a result, even though a state of this form is in a superposition, the two different pieces (one with the particle spin-up, one with the particle spin-down) will never be able to interfere with each other. Interference (different parts of the wave function canceling each other out) demands a precise alignment of the quantum states, and once we lose information into the environment that (interference) becomes impossible. That’s decoherence

Here Carrol blames decoherence for the loss of interference between the two states in superposition.

My advice would be (to Carrol as well) : Study at least QED.http://www.preposterousuniverse.com...ion-of-quantum-mechanics-is-probably-correct/

 

[PeterDonis]

I don't think taking MWI literally requires the assumption that every massive tunneling event is realized

Perhaps not, but many MWI proponents seem to talk as if it does.

 

[PeterDonis]

Only certain properties can be entangled and momentum is not one of them.

Where are you getting this from? Momentum can certainly be entangled. To give just two examples: in a Stern-Gerlach apparatus, the momentum of a particle like an electron is entangled with its spin; and in a process where two photons are created by particle-antiparticle pair annihilation, the momenta of the two photons are entangled since they must sum to zero in the center of mass frame.

 

[Hans de Vries]

Where are you getting this from? Momentum can certainly be entangled. To give just two examples: in a Stern-Gerlach apparatus, the momentum of a particle like an electron is entangled with its spin;

and in a process where two photons are created by particle-antiparticle pair annihilation, the momenta of the two photons are entangled since they must sum to zero in the center of mass frame.

It's not as simple as just energy conservation. Do you have an example of a correlation experiment using momentum as an entangled quantum state?

 

[PeterDonis]

Do you have an example of a correlation experiment using momentum as an entangled quantum state?

The first example I gave is an experiment that has been done.

The second example I gave might not have been realized in precisely the form I gave it, but since it's just an example of momentum conservation (not energy conservation), and momentum conservation has been verified by countless experiments, I don't see what the issue is.

 

[Hans de Vries]

The first example I gave is an experiment that has been done.

The second example I gave might not have been realized in precisely the form I gave it, but since it's just an example of momentum conservation (not energy conservation), and momentum conservation has been verified by countless experiments, I don't see what the issue is.

Well yes, that is of course that's what I wanted to say: It's more than just momentum conservation.

One would expect at least some "action-at-a-distance" correlation effect in need for an explanation. For instance a higher probability that both green detectors go off or both red detectors go off because the entanglement relation maintains the momentum relation somehow at a distance, even after going through the mirrors. What is entanglement without "action-at-a-distance"? I'm sure if you do this experiment that you won't see this kind of correlation though. It is just some example.

Note that I made the remark to counter the claim suggesting there is no interaction possible when all particles in a superposition states are entangled. This clearly suggests an "action-on-a-distance" for momentum which I do not believe to be correct.

 

[PeroK]

I would urge you to read carefully through the text as accurate and painstakingly as I have done (and anybody can do in the link at the bottom), before you come out in the aggressive way you did. Let's carefully go through the 3 statements (as I ascribed them to Carrol) one by one:

1) All universes are in superposition and superposition is normal in QM

Carrol does not say this.

2) In any universe all particles are entangled so they can't interact with particles in other universes.

Carrol does not say this either. This is your interpretation of MWI.

3) And because of decoherence particles in one universe do not interfere with particles in other universes.

Carrol does not say this either.

I fail to see the connection between Carrol's text and your analysis of it. In fact, your analysis is precisely the misunderstanding of MWI that his blog was intended to address.

You are free to believe that all the advocates of MWI are either mad or ignorant. But, I don't buy that. I don't buy that you know so much more about QM that its professional advocates. Especially, as it seems quite clear to me you are tilting at a straw man of your own creation.

 

[PeterDonis]

One would expect at least some "action-at-a-distance" correlation effect in need for an explanation.

This is true of any experiment involving spacelike separated measurements on entangled particles, since any such experiment can produce correlations which violate the relevant Bell inequalities.

This clearly suggests an "action-on-a-distance" for momentum which I do not believe to be correct.

Then you evidently don't believe in conservation of momentum, since conservation of momentum plays the same role in an experiment involving momentum entanglement that conservation of angular momentum plays in an experiment involving spin entanglement. The "action at a distance" effect on correlations is the same in both cases.

 

[Hans de Vries]

Then you evidently don't believe in conservation of momentum, since conservation of momentum plays the same role in an experiment involving momentum entanglement that conservation of angular momentum plays in an experiment involving spin entanglement. The "action at a distance" effect on correlations is the same in both cases.

Conservation of momentum is maintained at all times, always. One of the most basic laws of nature. Who would ever make such a ridiculous claim?

- Momentum can also be absorbed by the silver mirrors.
- Angular momentum can be absorbed by Wollaston prisms.

So a loss of correlation at the detectors does not violate any conservation laws.

 

[PeterDonis]

For instance a higher probability that both green detectors go off or both red detectors go off because the entanglement relation maintains the momentum relation somehow at a distance, even after going through the mirrors

The photons can transfer momentum to the mirrors, so once they are involved the momentum of the two photons by themselves is not necessarily conserved.

 

[PeterDonis]

a loss of correlation at the detectors does not violate any conservation laws

Indeed, as I just posted myself in response to you. Which simply means this experiment is not a good one for testing momentum entanglement. But if you take out the mirrors, and just use pairs of detectors in opposite directions, and do coincidence counting, it is.

 

[Michael Price]

It's not clear to me that Michael Price is saying Dr. Chinese is president right now. For me that doesn't make sense, since he clearly isn't president based on the information contained in this lab. I'm not even sure you can compare times between different worlds as they are casually disconnected from the point of branching.

Dr Chinese is President of the US in some parallel timelines right now, in the sense that those timelines use our dating system and show a date of 2019.

 

[Michael Price]

But I also think you can't just consider classical evolution since inflation. When people perform quantum measurements now, it forms new branches. This is most clearly true when people use quantum outcomes to make macroscopic decisions. It's not clear if you are suggesting an alternative view, where quantum outcomes are overwhelmed by classical evolution and the branch we are living in is actually deterministic.

I also think quantum branching and decoherence is ubiquitous, though it's more controversial. This is based on the only analysis I've seen on the topic, which I linked in the other linked thread, looking at how the average coin flip is an amplification of quantum uncertainty.

Yes, and this branching occurs not just at quantum measurements, but at every entropic event.
PS, nice article.

 

[Michael Price]

.
The point I'm making is that all this blithe talk about "multiple worlds" fails to pay attention to specifically how such multiple worlds get created, if the MWI is true. They don't get created by magic. They don't get created just because we humans can imagine them. They get created by having genuine quantum mechanical uncertainty, "

Of course these multiple worlds (or timelines, as I prefer) don't get created by magic. I am at a loss to see why you thought I said or implied otherwise.

 

[Quanundrum]

Of course these multiple worlds (or timelines, as I prefer) don't get created by magic. I am at a loss to see why you thought I said or implied otherwise.

So what you contend is essentially that at least one of the branches that branch from THIS exact universe at THIS exact moment will reconfigure all particles in our lightcone to put you in the white house and every citizen of Earth will have their brain (again just particle reconfiguration) filled with memories where Michael Price (or whoever else) as president make sense. Right?

 

[Mentz114]

Dr Chinese is President of the US in some parallel timelines right now, in the sense that those timelines use our dating system and show a date of 2019.

How can possibly claim that from the laws of physics ? Are you a clairvoyant now ?

Being so certain in your proclamations is not becoming in a scientist.