Why does nothing happen in MWI?

[Ilja]

Actually God's decisions are explained throughout the Bible - in Biblical terms. It probably isn't a good idea to jump on the fashionable bandwaggon of holding up Biblical belief as a model of irrationality. Some people might get offended.

Sorry, but I have described it here as more rational than MWI, so there is no base for christians to be offended, SCNR. And I can refer to Romans 11,33.

MWI also explains itself - the particular world you experience is one of many. MWI does not single out any particular world and then put "you" in it. You are in every world. All worlds are equally real, you seem to be expecting MWI to single out one world to be more real than the others. You are free to make up an interpretation that does this, and it will certainly require extra structure, but it will not be MWI.
What's that got to do with anything at all? The question isn't whether you exist but how MWI explains it.

Sorry, but this is an "explanation" which does not explain anything. It is antiscience pure. If this would be MWI, MWI should be rejected as anti-scientific. Those who argue that MWI needs more structure are, in fact, assuming that MWI is part of science, and, in particular, an interpretation of quantum theory. If one follows your variant of MWI, it should be rejected without further need of argumentation, simply as obviously anti-scientific.

It would be, in particular, in disagreement with claims that MWI gives the Born rule, because the Born rule makes differences between different worlds, assigning different probabilities to them.

 

[atyy]

The measurement problem *is* open. There is no consensus definition of a measurement. That's hardly MWI's fault.

But the point of MWI is to solve the measurement problem, so if MWI doesn't solve it, it fails.

MWI, however, removes the observation postulate. Instead, measurements are derived or described. But this means that the orthodox assumptions come to the surface. The tape measure is, rather appropriately, replaced by entanglement, it seems we can describe the physical process quite well in this quasi-operational way. The noting down of results is however, less clear. Is leaving a permanent record a sufficient or necessary condition? etc. Quantum erasure highlights the nee3d to be consistent in what you want MWI to explain.

Here you seem to be saying MWI does solve the measurement problem (remove the observation postulate). In MWI the problem is if everything happens, including seeing dead and alive cats, then MWI has to explain why we don't see such things.

One way it can do so is to say that consciousness is not possible if one does not see classical outcomes. But that is a theory of consciousness, and certainly is an additional postulate.

The other way to do it is to use the stability criterion, but then here if one is not making a statement about consciousness, then one is making a statement about what additional structure is real - ie. it exists even if there are no observers. But if one is talking about reality, the stability criterion seems to be something like Bohmian trajectotries.

 

[Derek Potter]

The axiom of choice guarantees that you can select one member of the collection. But the physical question is - which one? The axiom of choice does not answer that crucial physical question, so that's why it is irrelevant in this context.

OK, but why is the question crucial? There is nothing special about the cat factorization except that we happen to be thinking about it. Why does MWI have to explain my choice to think about cats?

Your analogy with classical phase space:

How to avoid the conclusion that nothing happens in a universe with two particles? Only by saying that physically we really do have two particles (in 3 dimensions) and not one particle (in 6 dimensions). But we cannot say that only from the mathematical structure of classical mechanics. There must be some additional structure involved, something which tells us that two particles in 3 dimensions is not the same as one particle in 6 dimensions. So pure classical mechanics cannot explain why something happens.
Likewise, pure quantum mechanics (the Schrodinger equation and nothing else) also cannot explain why something happens.

is helpful but needs fleshing out. Instead of saying "we really have two particles" we could equally well define, say a collision as a certain subspace. Since the time dependency has been removed to the coodinate system, this means that the subspace moves around and a collision happens when the subspace hits the particle. Events therefore are made possible if one defines a moving subspace in the otherwise nothing-happens frame. I think the same thing applies in QM - events only happen relative to a suitable frame but they do happen.

 

[Derek Potter]

Sorry, but this is an "explanation" which does not explain anything. It is antiscience pure. If this would be MWI, MWI should be rejected as anti-scientific. Those who argue that MWI needs more structure are, in fact, assuming that MWI is part of science, and, in particular, an interpretation of quantum theory. If one follows your variant of MWI, it should be rejected without further need of argumentation, simply as obviously anti-scientific.

That's ridiculous. I am not saying MWI requires extra structure. I am saying that a version of MWI which singles out one world would require extra structure. MWI does not single out one world. That was a requirement added by you.

It would be, in particular, in disagreement with claims that MWI gives the Born rule, because the Born rule makes differences between different worlds, assigning different probabilities to them.

Do you mean different probabilities within each world or different probabilities of each world in an ensemble - the weighting problem?

 

[Derek Potter]

But the point of MWI is to solve the measurement problem, so if MWI doesn't solve it, it fails.

Right, so MWI doesn't solve the clabbernuccle problem.
What clabbernuccle problem?
I don't know, I thought MWI was supposed to solve it. Not much use if it can't even say what it's supposed to solve.

Here you seem to be saying MWI does solve the measurement problem (remove the observation postulate). In MWI the problem is if everything happens, including seeing dead and alive cats, then MWI has to explain why we don't see such things.

Are you sure MWI has to explain anything that is specifically about us? A sensible definition of "see" would be "affected by" and "us" would be "any other system".

One way it can do so is to say that consciousness is not possible if one does not see classical outcomes. But that is a theory of consciousness, and certainly is an additional postulate.

Well it shouldn't be a theory of consciousness, it should be a theory of persistent observer states. That means we're talking about ...

The other way to do it is to use the stability criterion, but then here if one is not making a statement about consciousness, then one is making a statement about what additional structure is real - ie. it exists even if there are no observers.

MWI is generally ontic. Ontology is not mathematical nor physical structure.

But if one is talking about reality, the stability criterion seems to be something like Bohmian trajectotries.

What? Impossible to define consistently? :p

 

[Derek Potter]

OK, so this topic has bumped my post count up by 50 which suggests, correctly, that I have no life outside PF. In answer to Demystifier, No, I do not understand Schwindt's idea any better at all. I have had to clarify a few peripheral issues about the measurement problem but the thread has degenerated into wrangling about whether MWI is a viable interpretation - a long stretch from Schwindt's claim that Nothing happens in the Universe of the Everett Interpretation. Unfortunately, one must give people the benefit of the doubt and assume that what they have said is intended to be relevant and therefore must be responded to. The thread therefore branches more than a cat in MWI.

I would respectfully request that any further posts be kept on-topic and directly to the point. Thanks.

 

[Demystifier]

OK, but why is the question crucial? There is nothing special about the cat factorization except that we happen to be thinking about it. Why does MWI have to explain my choice to think about cats?

We think about cats because we observe cats. Why do we observe cats? The role of (theoretical) physics is to explain the observations,
so physics should explain why do we observe cats. If MWI cannot explain the observations, or more specifically if it cannot explain why do we observe cats, then MWI has not much to do with physics (not even with philosophy of physics). In that case, we should discuss MWI in some other forum.

Or let me put it in a different form. If MWI is the answer, then what is the question?

 

[Derek Potter]

We think about cats because we observe cats. Why do we observe cats? The role of (theoretical) physics is to explain the observations,
so physics should explain why do we observe cats. If MWI cannot explain the observations, or more specifically if it cannot explain why do we observe cats, then MWI has not much to do with physics (not even with philosophy of physics). In that case, we should discuss MWI in some other forum.

Who says MWI doesn't explain why we observe cats? That is precisely what it does do.

 

[Demystifier]

Who says MWI doesn't explain why we observe cats? That is precisely what it does do.

I think we are moving in circles:
Now I will ask - how does it explain it?
You will say - by decoherence.
I will say - but decoherence requires a split between "system" and "environment".
You will say - but the split is arbitrary made by our way of thinking, and MWI does not need to explain our thinking.
Then I will say - but we think in that way because this is what we observe.
Then you will say - MWI can explain why do we observe that.
I will ask - how?
You will say - by decoherence.
... and so on, and so on ...

Can you step out from that circle?

 

[Demystifier]

events only happen relative to a suitable frame but they do happen.

Are you really saying that all frames are real, and that "we" are just one of these frames?

If so, then in MWI "we" are just one of the branches defined by one of the frames. It does make some sense, but then the MWI reality is not merely the vector in the Hilbert space. Instead, the MWI reality is the collection of all possible decompositions of that vector in various bases. Different decompositions of the same vector are not considered equivalent. Is it what you are saying?

Very roughly, it is as if in classical physics 3 apples, 1+1+1 apples, 1+2 apples, and 2+1 apples were different things, and when you have 3 apples you really simultaneously have all these combinations as different objects.

If this is what you mean, then I can understand you. But then you should also be able to understand the paper we are discussing. In that paper the author assumes that different decompositions of the same vector are the same object. If that assumption is rejected, then his conclusions are no longer valid.

 

[Derek Potter]

Are you really saying that all frames are real, and that "we" are just one of these frames?

If so, then in MWI "we" are just one of the branches defined by one of the frames. It does make some sense, but then the MWI reality is not merely the vector in the Hilbert space. Instead, the MWI reality is the collection of all possible decompositions of that vector in various bases. Different decompositions of the same vector are not equivalent. Is it what you are saying?

Very roughly, it is as if in classical physics 3 apples, 1+1+1 apples, 1+2 apples, and 2+1 apples were different things, and when you have 3 apples you really simultaneously have all these combinations as different objects.

If this is what you mean, then I can understand you. But then you should also be able to understand the paper we are discussing. In that paper the author assumes that different decompositions of the same vector are the same object. If that assumption is rejected, then his conclusions are no longer valid.

That is exactly how I see it. If Schwindt makes that assumption then he must have a very strange idea of what comprises an object! The amplitude of |dog> in a Schrodinger's cat experiment may be extremely small, but it is a valid basis vector in the DOG decomposition. Schrodinger's cat is a dog!

 

[Ilja]

That's ridiculous. I am not saying MWI requires extra structure. I am saying that a version of MWI which singles out one world would require extra structure. MWI does not single out one world. That was a requirement added by you.

You have misunderstood. MWI requires additional structure to recover the laws of physics. This is the argument of the paper discussed here, that without additional structure MWI would be compatible with Nirvana, which is obviously very different physics, and it was also my argument in http://arxiv.org/abs/0901.3262 that without additional structure it would be compatible with very different physics. You have, AFAIU, not questioned these arguments themself.

So, if you reject additional structure, the argument is not about singling out a particular world, but about singling out a particular set of general laws of physics.

 

[Derek Potter]

You have misunderstood. MWI requires additional structure to recover the laws of physics. This is the argument of the paper discussed here, that without additional structure MWI would be compatible with Nirvana, which is obviously very different physics, and it was also my argument in http://arxiv.org/abs/0901.3262 that without additional structure it would be compatible with very different physics. You have, AFAIU, not questioned these arguments themself.

So, if you reject additional structure, the argument is not about singling out a particular world, but about singling out a particular set of general laws of physics.

Oh. Well something you said did seem to suggest that but I thought it was too far-fetched for you to really mean it. I don't think you're right in your interpretation of Schwindt. I think Schwindt assumes normal physics, the unitary evolution under a Hamiltonian law which *is* all relevant physics. He creates a Nirvana frame with normal physics driving the evolution of the state. I am not really interested in any wild claims that MWI can derive the magnitude of electric charge and the mass of the fundamental particles purely from tensor theorems.

 

[Ilja]

Oh. Well something you said did seem to suggest that but I thought it was too far-fetched for you to really mean it. I don't think you're right in your interpretation of Schwindt. I think Schwindt assumes normal physics, the unitary evolution under a Hamiltonian law which *is* all relevant physics. He creates a Nirvana frame with normal physics driving the evolution of the state. I am not really interested in any wild claims that MWI can derive the magnitude of electric charge and the mass of the fundamental particles purely from tensor theorems.

Of course it is not the job of MWI to deifne particle masses. But it is the job to to allow for sufficient structure to define fixed laws of physics. And in this sense, MWI is insufficient. Say, classical mechanics is fine with defineing that there is some abstract configuration space - with some examples for illustration - together with a minimum principle for some Lagrangian. This specifies what you have to define to specify the complete theory.

 

[Demystifier]

That is exactly how I see it.

OK, it's good to know that now we understand each other.

If Schwindt makes that assumption then he must have a very strange idea of what comprises an object!

I would put it differently. He takes that assumption not because he likes it, but because he thought it is assumed by those who pursue MWI. And then he (correctly) shows that such an assumption is not consistent with the existence of physical objects. So at the very least, he has proved a no-go theorem which shows how MWI should not be understood: In MWI, the reality is not the vector in the Hilbert space.

He does not say what is the reality in MWI, but now the compact answer seems to be: the set of all possible decompositions of the vector in the Hilbert space.

So, what to say about such an interpretation of QM? First, I think it is logically and mathematically consistent. Second, I think it is very inelegant. So I don't reject it, but I also don't like it.

 

[Demystifier]

So your solution is that everything happens (in contrast to nothings happens). Then we just pick what we like.

Yes. More precisely, each possible decomposition of the vector in the Hilbert space counts as a different object, and each of them is simultaneously real. But they are not all equal. Some of those objects have a decohered structure in it, and for some reason (which might have something to do with consciousness, according to Tegmark) the world we see is a part of one of those decohered objects.

So it should not be called "many world interpretation". It should be called "many many world interpretation", because we have two levels of many-worldness. At the first level, each decomposition of a given vector in the Hilbert space counts as a different many-world object. At the second level, given a decomposition, each term in the decomposition can be counted as a different world.

 

[atyy]

Yes. More precisely, each possible decomposition of the vector in the Hilbert space counts as a different object, and each of them is simultaneously real. But they are not all equal. Some of those objects have a decohered structure in it, and for some reason (which might have something to do with consciousness, according to Tegmark) the world we see is a part of one of those decohered objects.

So it should not be called "many world interpretation". It should be called "many many world interpretation", because we have two levels of many-worldness. At the first level, each decomposition of a given vector in the Hilbert space counts as a different many-world object. At the second level, given a decomposition, each term in the decomposition can be counted as a different world.

To paraphrase some MWI proponents: MWI is just BM with no worlds picked out: Bohmian Many World (BMW) And as you point out, there are many BMs, depending on choice of hidden variable, dynamics, degree of solipsism etc, so it is BM^MMMW.

 

[tzimie]

But the point of MWI is to solve the measurement problem, so if MWI doesn't solve it, it fails.

Unless the measurement problem itself is formulated incorrectly in the MWI framework.

 

[tzimie]

The advantage is that it merges the measurement problem with the hard problem of consciousness.

I really like this.
And thank you for mentioning the "hard problem of consciousness"

 

[maline]

Let me get this straight: is the argument saying that MWI predicts an unchanging universe and is falsified by observation, or just that the factorisations we use aren't "preferred" by the hamiltonian since the "Nirvana basis" should be more "preferred"?

 

[Demystifier]

Let me get this straight: is the argument saying that MWI predicts an unchanging universe and is falsified by observation, or just that the factorisations we use aren't "preferred" by the hamiltonian since the "Nirvana basis" should be more "preferred"?

It's a kind of both, but the latter is more straightforward because it requires fewer assumptions.

 

[maline]

Please clarify. What are the assumptions that would lead to predicting a static universe?

A side point: does the solution you are proposing & disliking mean that there are infinitely many copies of the hilbert space, each with a built-in factorisation? If so, I don't think that's what Derek intends.

 

[Derek Potter]

It should be called "many many world interpretation"

Which is what I often have called it. But I also call it "The Too Many Worlds Interpretation" for obvious reasons.

 

[Derek Potter]

Please clarify. What are the assumptions that would lead to predicting a static universe?

A side point: does the solution you are proposing & disliking mean that there are infinitely many copies of the hilbert space, each with a built-in factorisation? If so, I don't think that's what Derek intends.

Well it's always dangerous to try to guess what I mean - seems like it takes 50 posts to get a measure of understanding.
I do not see why it is necessary to postulate copies of the Hilbert space. In fact I don't know what that would mean as the space is not a physical object it's just somewhere to put a vector. The plurality is in the ways we can factorize the space. The factorized spaces coexist in the sense that 3+4 = 7 coexists with 6+1 = 7. You don't need separate copies of "7".

 

[Derek Potter]

OK, it's good to know that now we understand each other.I would put it differently. He takes that assumption not because he likes it, but because he thought it is assumed by those who pursue MWI. And then he (correctly) shows that such an assumption is not consistent with the existence of physical objects. So at the very least, he has proved a no-go theorem which shows how MWI should not be understood: In MWI, the reality is not the vector in the Hilbert space.

He does not say what is the reality in MWI, but now the compact answer seems to be: the set of all possible decompositions of the vector in the Hilbert space.

So, what to say about such an interpretation of QM? First, I think it is logically and mathematically consistent. Second, I think it is very inelegant. So I don't reject it, but I also don't like it.

Hmm! What do you think is inelegant about it?

 

[mfb]

But there is a basis in which it does not move around. The basis in which only the phase changes.

Perhaps it is easier to understand it in a classical analog:

Consider a 3-dimensional universe containing nothing but one classical pointlike particle moving around. Does anything happen in such a universe? You can consider the system in coordinates (the classical analog of basis) comoving with the particle. In these coordinates the particle does not move. So nothing happens in the universe with one particle.

How about the 3-dimensional universe with two particles? Now there is relative motion of two particles, so one may think that something really happens in the case of two particles. However, the mathematics of two particles in 3 dimensions is the same as mathematics of one particle in 6 dimensions. So mathematically, we may say that we still have one particle, only in 6 dimensions. So from that point of view nothing happens even with two particles, because they are mathematically equivalent to one particle.

How to avoid the conclusion that nothing happens in a universe with two particles? Only by saying that physically we really do have two particles (in 3 dimensions) and not one particle (in 6 dimensions). But we cannot say that only from the mathematical structure of classical mechanics. There must be some additional structure involved, something which tells us that two particles in 3 dimensions is not the same as one particle in 6 dimensions. So pure classical mechanics cannot explain why something happens.

Likewise, pure quantum mechanics (the Schrodinger equation and nothing else) also cannot explain why something happens.

The one particle in 6 dimensions would accelerate with the usual coordinate system, but you can find coordinates where the 6-dimensional particle is at rest. Those coordinates have to be quite special, and designed to make the particle resting. So I think you are hiding the "something happens" in the coordinates. That applies to classical mechanics and quantum mechanics with MWI in the same way.
I made an example for a pendulum in a previous thread.

Either nothing happens in classical mechanics (and I disagree with that view), or something happens in MWI as well.

 

[Derek Potter]

I really like this.
And thank you for mentioning the "hard problem of consciousness"

Well, atyy is making a bit of a stretch there. The best approach is that although the HP is unsolved and (say some) unsolvable, we can safely assume that one's mental state supervenes on the state of the brain and thus, in part, on the physical state of our sense organs. Add consciousness to taste.

 

[Derek Potter]

The one particle in 6 dimensions would accelerate with the usual coordinate system, but you can find coordinates where the 6-dimensional particle is at rest. Those coordinates have to be quite special, and designed to make the particle resting. So I think you are hiding the "something happens" in the coordinates. That applies to classical mechanics and quantum mechanics with MWI in the same way.
I made an example for a pendulum in a previous thread.
Either nothing happens in classical mechanics (and I disagree with that view), or something happens in MWI as well.

If I am a chicken laying an egg I can always pretend to be doing nothing at all, it's just the rest of universe doing crazy things around me. That would appear to confirm that nothing happens in classical mechanics. I think this is what Ilja is saying. However I am not sure what we should make of this. We don't want to insist on a preferred basis/frame unless it's forced upon us. Perhaps we should say that the idea of something happening needs to be refined - what does it mean to lay an egg if some pathological coordinate system sees me as a full orchestra playing Beethoven's Fifth? Schwindt talks about the system being nice or unpleasant according to the frame used but there has to be a better criterion than Schwindt's personal likes and dislikes.

 

[Ilja]

If I am a chicken laying an egg I can always pretend to be doing nothing at all, it's just the rest of universe doing crazy things around me. That would appear to confirm that nothing happens in classical mechanics. I think this is what Ilja is saying.

?
What I'm saying was completely different. Classical mechanics in general does not specify the particular configuration space and the particular Lagrangian, but it specifies that a complete classical theory specifies a configuration space, a Lagrangian, and then everything else, in particular the evolution equation, is fixed. Then, the evolution of our particular universe is defined by the initial values. And the theory also specifies what is the form of these initial values - an initial configuration q(t) and its first time derivative. And once all this is fixed, one cannot pretend that there is no evolution if there is evolution.

The only thing one can pretend to get rid of motion is that one has not correctly identified what is at rest an what not, thus, that one has made an error in the definition of \dot{q}(t) which should have been \dot{q}(t)-v. This is relevant only for positivists, who want to be able to identify everything based on observation, a stupid but popular philosophical idea. For everybody else this is quite irrelevant.

Then, there remains to be the final freedom - where we are in this particular solution. For this, in classical mechanics a particular event (where I am now) has to be specified.

In general, every physical theory should follow a similar scheme. The general scheme should define what has to be defined to define a particular physical theory. Given these additional data, which define the particular physical theory, the theory should define the set of additional information which is required to specify the particular universe (or multiverse, whatever this means). And, then, finally, it should be clear what has to be specified to identify my own position in this uni- or multiverse. If some general scheme does not clarify these points, it should be rejected as unphysical.

Quantum mechanics in its Copenhagen as well as its dBB variant does all this. The particular theory has to specify a configuration space Q. The state is specified by a wave function on the configuration space and a particular configuration. In the Copenhagen variant, there is some strange subdivision of the configuration space into a classical and a quantum part, where the wave function is defined only on the quantum part and the configuration only on the classical part. Then, there is a Hamiltonian operator which defines the evolution of the wave function, and the evolution of the configuration is defined by the guiding equation, in Copenhagen by a classical evolution equation on the classical part. And where we are ourself is also well-defined, in the same way as in classical mechanics, as a part of the configuration near a given event defined by its space coordinate at a given moment of time.

Thus, a complete quantum theory also specifies a configuration space, and defines what is a changing of the configuration in time. The positivistic problem of identifying absolute rest exists too, and is irrelevant for a realistic understanding too, as well for the question which is discussed here - if we see some motion in our world, it is impossible to uses this Galilean symmetry to get rid of the motion.

To accept MWI as a reasonable interpretation of quantum physics I require a similar scheme. But it is not given. The wave function, together with some abstract unitary evolution, is clearly not sufficient to specify the physics.

 

[Derek Potter]

?
What I'm saying was completely different.

I am still not sure whether I understand you. How exactly does classical mechanics specify that a classical theory must specify a particular configuration space? edit - Why shouldn't a theory consider different spaces and develop transforms to go from one to another? In QM, polarization states do not have a unique "configuration space", i.e. basis. Yet it's easy to say *this* state (suitably prepared) is a|A>+ b|B> in one basis and |c|C>+d|D> in another. Why is it non-physical to talk of the photon state as being different things in different configuration spaces? I don't see why you should reject MWI as unphysical because it accommodates talking about circular polarization and linear polarization at the same time.