Is the Many-Worlds Interpretation truly deterministic?

[vanhees71]

But it is precisely what quantum statistics does for you! It describes the behavior of classical, macroscopic systems in terms of the "relevant" macroscopic observables and explains, why these quantities behave in almost all case as described by classical physics. Classicality is the result of a sufficiently coarse-grained description. There is no other way to observe nature in its microscopic details than to use "amplification" and "measurement devices" that are macroscopic to make these observables comprehensible finally to our senses.

 

[stevendaryl]

But it is precisely what quantum statistics does for you! It describes the behavior of classical, macroscopic systems in terms of the "relevant" macroscopic observables and explains, why these quantities behave in almost all case as described by classical physics.

I don't think so. If macroscopic phenomena such as measurements are described in terms of microscopic phenomena, then the axioms of quantum mechanics should be expressible solely in terms of the microscopic phenomena. So there would be no need for an axiom saying "A measurement results in an eigenvalue with a probability given by..."

 

[A. Neumaier]

If a measurement is a process describable by ordinary physics (and thus, by QM), then it should be possible to reformulate the Born rule so that it doesn't mention measurements at all.

So ultimately, it seems to me that there should be a formulation of QM that doesn't mention anything about macroscopic concepts such as measurements or expectation values or irreversibility. Those statements should, in my opinion, be derivable from statements about what goes on at a microscopic level. And pure QM doesn't have such a description.

Such an improved formulation was given here.

My thermal interpretation is fundamental in the sense you require. It is meaningful without mentioning measurement at all, and implies the Born rule in the cases where the latter is appropriate. It is fully compatible with statistical mechanics (the theory of macroscopic implications of quantum mechanics, including irreversibility), from which it was in fact abstracted. And statistical mechanics is also the discipline in terms of which real measurement processes with real detectors can be analyzed. Indeed, to design good detectors one uses statistical mechanics and not the Born rules!

The thermal interpretation doesn't contain anything essentially new - it just places the emphasis in a way that makes the obvious more obvious instead of (as traditional interpretations do) placing counterintuitive axioms such as Born's rule (which are valid only in special contexts) into the center of attention.

 

[stevendaryl]

Such an improved formulation was given here.

No offense, but I don't agree that your reformulation solves the problem. Basically, you're saying that if we have a large system of many, many particles, then macroscopic variables (such as field averages) can have well-defined values. But that seems to me to be a matter of choosing a setting where the peculiarities of quantum mechanics are swamped out. It's not an explanation. You can still have quantum mechanics of a small number of particles; for example, in the EPR experiment. QM makes definite predictions about observations for such systems, and those predictions don't require any kind of thermal limit.

 

[A. Neumaier]

No offense, but I don't agree that your reformulation solves the problem. Basically, you're saying that if we have a large system of many, many particles, then macroscopic variables (such as field averages) can have well-defined values. But that seems to me to be a matter of choosing a setting where the peculiarities of quantum mechanics are swamped out. It's not an explanation. You can still have quantum mechanics of a small number of particles; for example, in the EPR experiment. QM makes definite predictions about observations for such systems, and those predictions don't require any kind of thermal limit.

Note: Expectation values are just values with an objective meaning in each state, whether macroscopic or microscopic. Expectations of a macroscopic observable are measurable by a single macroscopic measurement. Unlike measurements, expectations are part of the standard theory (shut-up-and-calculate). If you see the need to ban expectation values from a fundamental description you would also need to ban observables, and the theory would become impossible to formulate.

QM makes indeed lots of predictions about expectations of observations for microscopic systems, and those predictions don't require any kind of thermal limit. These expectations are correctly described by the thermal interpretation, even for microscopic systems. That the motivation of the interpretation comes from statistical mechanics doesn't mean that the interpretation itself is limited to macroscopic systems.

 

[vanhees71]

I don't think so. If macroscopic phenomena such as measurements are described in terms of microscopic phenomena, then the axioms of quantum mechanics should be expressible solely in terms of the microscopic phenomena. So there would be no need for an axiom saying "A measurement results in an eigenvalue with a probability given by..."

I don't understand what you want. The natural sciences are about observations/measurements of nature. It's not about fairy tales concerning some "underlying truth", or however you want to name it. Quantum statistics is by the way nothing else than the application of the quantum-theoretical formalism to macroscopic objects, i.e., it delivers in a scientific sense what you want, namely the understanding of macroscopic behavior from the underlying fundamental/microscopic dynamics.

 

[stevendaryl]

I don't understand what you want.

A formulation of quantum mechanics in terms of microscopic properties, such that the macroscopic rule--"When you measure an observable, you get an eigenvalue of the corresponding operator with a probability given by ..."--is derivable, rather than postulated. I thought that's what I said.

 

[vanhees71]

The question is derivable from what, and if you find such a formulation, what's the advantage compared to the "traditional" formulation which starts from what's really measured in the lab?

 

[stevendaryl]

The question is derivable from what, and if you find such a formulation, what's the advantage compared to the "traditional" formulation which starts from what's really measured in the lab?

What does "advantage" mean, here? Science was created to understand phenomena--light, planetary motion, properties of gases and liquids, the behavior of substances when they interact---that exist independent of any lab. Labs are invented to help get more information about the world, but the world and its phenomena don't depend on labs for their existence. Nuclear processes in stars work the same way even when there are no nuclear physicists around.

I can certainly understand the point of view that the only thing that is important for scientists is to explain the observations of scientists, but I think that's a sterile, solipsistic view of science. Nobody becomes a scientist with that interpretation of science in mind---that the point of science is to explain what happens in science labs. They become scientists because they are curious about what goes on outside the lab.

 

[A. Neumaier]

Nuclear processes in stars work the same way even when there are no nuclear physicists around.

But nothing ever is measured then.

"When you measure an observable, you get an eigenvalue of the corresponding operator with a probability given by ..."--is derivable, rather than postulated.

See the derivation in Section 10.5 (p.239) of my online book (version v2).

 

[nikkkom]

The same comment I made previously applies here: these things are only relevant for the MWI if quantum superpositions play a meaningful role. Do they? I don't see how they do for die rolls. Cups of water spontaneously freezing, possibly, but I'm doubtful.

MWI says that there is no collapse, ever. When you open the box, cat doesn't become dead or alive. Instead, you are now entangled with the cat's state, and both states of "I see dead cat and feel depressed" and "I see live cat and feel happy" exist in superposition.

IOW, MWI says that every possibility which (like cat state) arises from quantum superposition, is realized. And presumably, fair dice rolls depend on past history of the dice cubes and the person throwing it, and there is more than enough variability in their past that every result of dice roll result is possible, and therefore, according to MWI, every of those possibilities is realized in some branches.

 

[Mentz114]

That seems false to me. Do you have a reference that makes that claim?

I think there is general agreement that decoherence is responsible for destroying interference patterns, and making it impossible for a subsystem to be in a superposition of states. But I don't think there is any consensus that environmental effects select one possibility for the result of a measurement over another.

What else can it be ? Are you proposing that something that something outside the universe universe is affecting this ?

I've heard of thinking outside the box but come on ! That is not physics.

 

[stevendaryl]

What else can it be ? Are you proposing that something that something outside the universe universe is affecting this ?

I'm not proposing anything. QM does not specify how one alternative is chosen out of a set of possibilities. You seem to be claiming that it does, and I think that's false.

 

[stevendaryl]

I'm not proposing anything. QM does not specify how one alternative is chosen out of a set of possibilities. You seem to be claiming that it does, and I think that's false.

I should say that it's not part of standard mechanics. To quote Bill Hobba from this thread:
https://www.physicsforums.com/threads/decoherence-and-standard-formalism.890882/#post-5604515

there are 3 parts to the measurement problem.

1. The problem of non observance of interference
2. How the preferred basis emerges.This is why, for example, classical objects nearly always have a definite position
3. How an improper mixed state becomes a proper one.

The first 2 is explained by decoherence, the third some interpretations simply assume, while others explain.

The meaning of "how an improper mixed state becomes a proper one" is the issue of how one possibility is selected out of a set of possibilities. It is not explained by decoherence or environmental effects. Or at least, there is no consensus that it is.

 

[vanhees71]

This argument goes in circles. There is the state of a quantum system, described by a statistical operator. The state is called pure if the statistical operator is a projection operator otherwise it's called a mixed state. It is impossible to distinguish between what you call proper vs. improper mixed state. We have clarified this several times now.

 

[Mentz114]

The meaning of "how an improper mixed state becomes a proper one" is the issue of how one possibility is selected out of a set of possibilities. It is not explained by decoherence or environmental effects. Or at least, there is no consensus that it is.

This is getting off-topic. We've had this discussion before and we disagree deeply on this issue.

 

[stevendaryl]

This argument goes in circles. There is the state of a quantum system, described by a statistical operator. The state is called pure if the statistical operator is a projection operator otherwise it's called a mixed state. It is impossible to distinguish between what you call proper vs. improper mixed state. We have clarified this several times now.

I don't agree with your "clarification", but if you're happy with it, fine. Not everybody is.

 

[stevendaryl]

This is getting off-topic. We've had this discussion before and we disagree deeply on this issue.

I was responding to your claim "In the real world there is dissipation, non-unitary evolution, absence of superposition and other noise that let's the outcome be decided by the current state of the universe".

The outcome being determined by the current state of the universe is the part that I'm claiming is false.

 

[Mentz114]

I was responding to your claim "In the real world there is dissipation, non-unitary evolution, absence of superposition and other noise that let's the outcome be decided by the current state of the universe".

The outcome being determined by the current state of the universe is the part that I'm claiming is false.

Fair enough. That is my central thesis. What else could possibly decide ?

 

[stevendaryl]

Fair enough. That is my central thesis. What else could possibly decide ?

I'm not saying that I know the answer, I'm saying that the answer does not come from standard QM by taking into account the environment.

 

[Mentz114]

I'm not saying that I know the answer, I'm saying that the answer does not come from standard QM by taking into account the environment.

That's a reasonable view. Any ire I have is directed at MWI.

However, I think classical statistical mechanics and standard QM together offers an explanation. I would heartily recommend a close reading of chapter 2, especially section 2.6 of "The Quantum Theory of Motion" by Peter Holland. I get a clear mental structure from this, and no mystery.

I'm sure this is also done in the 'Thermal interpretation' but Hollands discussion is easier ( for me anyway).

[edited the title of the book]

 

[stevendaryl]

I'm not saying that I know the answer, I'm saying that the answer does not come from standard QM by taking into account the environment.

I don't want to spend more time on this, except to say that it would seem to me that the assumption that facts about the rest of the universe determine the outcome of measurements would contradict Bell's inequality. Maybe there is a loophole, but there certainly is no consensus that there is such a loophole.

 

[Jilang]

I don't want to spend more time on this, except to say that it would seem to me that the assumption that facts about the rest of the universe determine the outcome of measurements would contradict Bell's inequality. Maybe there is a loophole, but there certainly is no consensus that there is such a loophole.

I am reminded to remind you that quite recently you showed how the Bells inequality is violated for probabilities but not the probability amplitudes. In QT Amplitudes add, probabilities don't. Hidden variables only become a problem when one considers the probabilities rather than the amplitudes. Now, what would that suggest? (I have no suggestions)

 

[stevendaryl]

I am reminded to remind you that quite recently you showed how the Bells inequality is violated for probabilities but not the probability amplitudes. In QT Amplitudes add, probabilities don't. Hidden variables only become a problem when one considers the probabilities rather than the amplitudes. Now, what would that suggest? (I have no suggestions)

Yes, that is an interesting observation, but I don't know what to make of it.

 

[Mentz114]

I don't want to spend more time on this, except to say that it would seem to me that the assumption that facts about the rest of the universe determine the outcome of measurements would contradict Bell's inequality. Maybe there is a loophole, but there certainly is no consensus that there is such a loophole.

(my emphasis)
Do you mean through non-locality ?
What makes photon detectors click is correlations between probability amplitudes that happen right at the detector. These correlations could have been present after preparation - or is that just another NLHV theory ?

[Stephen, do you have a ref to the post where you did the work referred to by jilanq?]

 

[Jilang]

It was here at #11
https://www.physicsforums.com/threa...nonlocal-hidden-variable.875064/#post-5496299

 

[nolxiii]

So basically, either MWI, which is clearly crazy, or alternatively the universe depends on quantum decision fairies?

This more or less sum it up?

 

[PeterDonis]

What, exactly, are you doubtful about? Are you still talking about the issue of whether very weird outcomes actually happen in MWI?

Yes. The particular claim in question in what you quoted was the claim that when someone rolls a die, there must be six MWI branches created, one for each possible result of the die roll. I don't see why that must be the case, because I don't see how any quantum superpositions play a role in determining how the die comes up. As far as I know, die rolling is a classical process; there is no quantum indeterminacy involved. It's not the same as a radioactive atom decay determining some result. And if there is no quantum indeterminacy involved, then there is no MWI branching. There is just a classical deterministic process, which only looks random to us because we have insufficient knowledge of the initial conditions to predict the outcome. My more general point is that the same might apply to lots of cases which are cavalierly assumed, in pop science writings on the MWI, to lead to MWI branching.

MWI says that every possibility which (like cat state) arises from quantum superposition, is realized.

Yes; the cat state in the Schrodinger's cat experiment is explicitly assumed to depend on a quantum superposition, the decay of a radioactive atom. But that is not the case for die rolls. See below.

And presumably, fair dice rolls depend on past history of the dice cubes and the person throwing it, and there is more than enough variability in their past that every result of dice roll result is possible, and therefore, according to MWI, every of those possibilities is realized in some branches.

This "presumably" is precisely the presumption I am questioning. Variability in the past is not enough; we are not talking about the fact that over a large number of dice rolls, each number will come up, on average, 1 in 6 times. We are talking about a presumption that, for a single die roll, the outcome has some significant dependence on a quantum superposition, like it does in the cat experiment. That has to be the case for the MWI to apply at all; you agree with that in what I quoted above.

But a single die roll, as far as I can tell, does not depend on any quantum superposition. It depends on a variety of initial conditions that are, at least at our current level of technology, uncontrollable, but that's not the same as depending on a quantum superposition. There are no radioactive atoms inside dice whose decay affects the outcome. There is just, as I said above in response to stevendaryl, a classical process whose outcome is only unpredictable to us because we have insufficient knowledge of the initial conditions. If quantum indeterminacy were involved, even perfect knowledge of initial conditions would be insufficient to predict the outcome: but I am not aware that anyone actually thinks that is the case for die rolls. They just fail to realize they are implying that when they make claims about MWI branching.

 

[nikkkom]

> And presumably, fair dice rolls depend on past history of the dice cubes and the person throwing it, and there is more than enough variability in their past that every result of dice roll result is possible, and therefore, according to MWI, every of those possibilities is realized in some branches.

This "presumably" is precisely the presumption I am questioning. Variability in the past is not enough; we are not talking about the fact that over a large number of dice rolls, each number will come up, on average, 1 in 6 times. We are talking about a presumption that, for a single die roll, the outcome has some significant dependence on a quantum superposition, like it does in the cat experiment. That has to be the case for the MWI to apply at all; you agree with that in what I quoted above.

But a single die roll, as far as I can tell, does not depend on any quantum superposition.

It depends on the large set of prior events, some of them are clearly quantum. How strong the muscles of the thrower are, how precisely nerve impulses are translated into muscle action. Nerve impulses are electric. Human body experiences some 5000 radioactive decays per second from K40 and C14, and also cosmic radiation constantly showers us. Those are quantum processes, and they _directly_ add electric noise to nerve signals. Brownian motion of air molecular is quantum too. Muscle action is a chemical reaction in cells (some proteins change configuration), this is also affected by quantum effects.

 

[PeterDonis]

It depends on the large set of prior events, some of them are clearly quantum.

Sure, there are quantum events going on; but the question is whether the result of the die roll--which of the six sides comes up--depends on any of the superpositions in those events.

Let me try to reframe my point another way. In the Schrodinger's cat experiment, there are all sorts of quantum events going on inside the cat, but the key observable--whether the cat is dead or alive--does not, by hypothesis, depend on any of them. It depends only on whether or not a particular radioactive atom, outside the cat's body, decays within a certain period of time. The atom's state is in a superposition of decayed/not decayed, and the way the experiment is set up, the dynamics links that pair of atom states, decayed/not decayed, with the particular pair of cat states dead/alive. That is what makes MWI proponents say that the cat must end up in a superposition of dead/alive.

Compare this with a die roll. Sure, there are all sorts of quantum fluctuations going on in the die, in the thrower's hands and arms and nerves, in the surrounding air, etc. But none of them are linked to the result of the die roll--which side comes up--in the way the radioactive atom's state is linked to the cat's state above, just as in the above example, the cat's dead/alive state is not linked to any of the umpteen quantum superpositions inside its body, in the air molecules in the box, etc. Similarly, none of those umpteen quantum superpositions in the die and the thrower and the environment are linked, by the dynamics, to the die being in a superposition of coming up one/two/three/four/five/six. Or at least I would like to see some MWI proponent give me some reason to believe that there is some such link, other than "presuming" that this is the case. As far as I know, no MWI proponent has ever done that; they all just wave their hands and presume. That's what I'm objecting to.

 

[nikkkom]

Sure, there are quantum events going on; but the question is whether the result of the die roll--which of the six sides comes up--depends on any of the superpositions in those events.

Let me try to reframe my point another way. In the Schrodinger's cat experiment, there are all sorts of quantum events going on inside the cat, but the key observable--whether the cat is dead or alive--does not, by hypothesis, depend on any of them. It depends only on whether or not a particular radioactive atom, outside the cat's body, decays within a certain period of time. The atom's state is in a superposition of decayed/not decayed, and the way the experiment is set up, the dynamics links that pair of atom states, decayed/not decayed, with the particular pair of cat states dead/alive. That is what makes MWI proponents say that the cat must end up in a superposition of dead/alive.

Compare this with a die roll. Sure, there are all sorts of quantum fluctuations going on in the die, in the thrower's hands and arms and nerves, in the surrounding air, etc. But none of them are linked to the result of the die roll

Let's assume that the above is true.

There is another argument. MWI claims that not only this branch, with exactly this dice and this thrower exists before the throw. There are countless other branches, some of them very similar (so the dice throw is happening there too) but have slightly different dice, and/or slightly different thrower (stronger or weaker, or more excited than in other branch, so he throws the dice differently). In some of those branches, the result will be 6.

 

[PeterDonis]

MWI claims that not only this branch, with exactly this dice and this thrower exists before the throw. There are countless other branches, some of them very similar (so the dice throw is happening there too) but have slightly different dice, and/or slightly different thrower (stronger or weaker, or more excited than in other branch, so he throws the dice differently). In some of those branches, the result will be 6.

That just pushes the question back a step: how did these previously existing branches with slightly different initial conditions for the die and the thrower come into existence? Of course if branching events are ubiquitous it's easy to see how, but again, that's precisely the presumption that I am questioning. Schrodinger's Cat-type situations are not ubiquitous; it takes very careful experimental setups to create them.

In fact, the alternate argument you are giving here implicitly admits that branching events are not ubiquitous, because it implicitly admits that the die roll itself is not a branching event. There are branches in which the die comes up each way, but that's because there were already branches in which the die was set up to come up that way. The die roll itself causes no branching at all. But if we allow that the die roll doesn't cause branching, what justification is there for assuming that there are all these branches around to begin with?

 

[nikkkom]

That just pushes the question back a step: how did these previously existing branches with slightly different initial conditions for the die and the thrower come into existence?

By multiplying since the Big Bang.

For one, chemical processes are quantum. Which molecules clump into which dust grains in the proto-Solar system depends on molecules' states. There are enormous numbers of "Schrödinger's cat" situations in the protoplanetary cloud. Copenhagen says only one of the possible outcomes is realized; MWI says _all_ of them exist.

While most of them are vastly different from the branch we are in, some are only slightly different.

I think I'll stop repeating myself if you are still not getting it. I can't explain it any better.

 

[PeterDonis]

Which molecules clump into which dust grains in the proto-Solar system depends on molecules' states. There are enormous numbers of "Schrödinger's cat" situations in the protoplanetary cloud.

Hmm. So basically, the idea is that the emergence of classical initial conditions depends on quantum indeterminacy. That seems to lead to branching being somewhere in between the extreme MWI position I have been arguing against, and the position I have been arguing for (or at least stating as an objection). For example, rolling a die or flipping a coin would not cause branching; but somewhere in the far past there would have been branches created with different classical initial conditions that lead to different results for the die rolls or coin flips (or even different sets of die rolls or coin flips being made).

 

[akvadrako]

For example, rolling a die or flipping a coin would not cause branching;

Have you seen this paper?

Origin of probabilities and their application to the multiverse (2012)
https://arxiv.org/abs/1212.0953

They do some shallow analysis of a coin flip and billiards, concluding that quantum events almost completely determine the outcome.