Problems with Many Worlds Interpretation

[Fredrik]

for readers who just want to know what QM says about reality. My advice is to start with the attitude that the wavefunction is not real, that it is just a calculating device, like a probability distribution.

I often say this too.

I've never seen anyone argue this position effectively.

Have you seen anyone argue effectively for the opposite position? I would say that it isn't possible to "argue effectively" for either position, because the subject is neither scientific nor purely mathematical. To add the statement "this theory describes reality" or its negation to the list of axioms that defines a theory wouldn't change the theory's predictions. That means that we're not talking about science.

That being said, I still think that the things I said in posts 40-41 here, and the posts I linked to in there, are pretty good reasons to not think of QM as a description of reality. (It may not be an "effective" argument by your standards, but I think those standards may be unreasonably high, considering the unscientific nature of what we're talking about).

Do you have an argument for the opposite position?

Your version reeks of double-think -- you are accepting QM as a good theory of reality while at the same time rejecting its description of reality.

There's nothing contradictory about it if we define a theory as an assignment of probabilities to possible results of experiments.

 

[chaszz]

The aspect of MWI that has always bothered me most is conservation of energy. Where is all the energy coming from to fuel all these new universes that keep popping into existence? There is enough of a problem trying to conceive where the energy came from in our universe without compounding the problem practically to infinity. Am I missing something in MWI which does attempt to account for this?

 

[BruceW]

Some people interpret MWI differently. In the most well-accepted version of MWI, no new universes are literally created. MWI in its most basic simply says that non-unitary collpase of the wavefunction does not happen.

 

[Fyzix]

Some people interpret MWI differently. In the most well-accepted version of MWI, no new universes are literally created. MWI in its most basic simply says that non-unitary collpase of the wavefunction does not happen.

Yes and this is the version that Mitchell Porter has just debunked.
Have you read the thread or did you just decide to jump in and make assumptions?
This is one of the most detailed debates on MWI on this site to date, so unless you got some detailed rebutle for Mitchell Porter's posts, let's not mess the thread up by stating stuff without having read the thread

 

[BruceW]

Yes and this is the version that Mitchell Porter has just debunked.
Have you read the thread or did you just decide to jump in and make assumptions?
This is one of the most detailed debates on MWI on this site to date, so unless you got some detailed rebutle for Mitchell Porter's posts, let's not mess the thread up by stating stuff without having read the thread

You think that Mitchell Porter has just debunked MWI? Well I guess he should be expecting his nobel prize in the post any day now.
Haha, sorry to joke, but seriously, I don't find anything In Mitchell Porter's posts that debunk MWI. He made one good point though: in MWI the Born rule must be one of the postulates.
You might argue that the fact that MWI requires the Born rule postulate makes MWI less likely, but it doesn't mean MWI is wrong.

Edit: As I said earlier in the thread, its all about aesthetical preference. There have been no experiments to distinguish between the two interpretations.

 

[Fyzix]

You think that Mitchell Porter has just debunked MWI? Well I guess he should be expecting his nobel prize in the post any day now.

Are you aware of how many interpretations there are? Do you really think you would win a Nobel Prize by showing how one or many of them are bad? Seriously?
When I say debunk I obviously mean "shows why it doesn't work".
Fact is: if MWI can't get Born rule right, it has infact been falsified...
QM is all about probabilities, if you can't get them right, you have no interpretation that fits reality.

However this thread brings up a lot more interesting problems that MWI has and if you put them all together you could say that a certain interpretation is infact debunked, yes.
Which is why I don't want the thread to side track, I want to see proponents of MWI to respond to Mitchell Porter's posts in a detailed manner, so that the discussion can go on and we might see where MWI fails in other areas etc.

I don't find anything In Mitchell Porter's posts that debunk MWI. He made one good point though: in MWI the Born rule must be one of the postulates.
You might argue that the fact that MWI requires the Born rule postulate makes MWI less likely, but it doesn't mean MWI is wrong.

He has brought up more problems than just the famous Born Rule problem and I think he will bring up more if the debate goes further.
But I may ask you, do you seriously think you can just say "I postulate Born Rule" and *poof* MWI makes sense?

You have to have a mechanism, some sort of explanation for how you postulate the Born Rule.
Is there a God selecting the amount of branches to fit with Born Rule? Are there hidden variables? Are we even talking about MWI anymore ?
If you are going to postulate you need to give a detailed account of HOW.

Edit: As I said earlier in the thread, its all about aesthetical preference. There have been no experiments to distinguish between the two interpretations.

Same goes for consciousness collapse, transactional interpretation, ensemble interpretation, deBroglie Bohm interpretation, many minds interpretation, itacha interpretation and 50 other interpretations.
Just because there are no experiment to distinguish these interpretations, one can still philosophically and technically pick certain interpretations apart and thus debunk them.

Like for instance people can say about Bohm: it violates relativity, how do you claim to fix that without modifying your interpretation or postulating new physics?
Same goes for all the problems in MWI...

 

[Hurkyl]

I'm just saying it's incomplete, that's all.

It sounds like more than that, but let me ask you a question:

You would advise the QM mechanics student not to form an idea about wave-functions corresponding to entities in reality.

Would you also advise the fluid mechanics student not to form an idea about fluids corresponding to entities in reality?

If your answer is yes:

Then I assert that all of this talk about wave-functions not being real and such is a red herring -- it has absolutely nothing to do with the question of how to interpret quantum mechanics. Instead, it is just a reminder that interpretations are merely interpretations. (But then... why do you think we needed a reminder? )​

If your answer is no:

Then what alternative interpretation? If none, how can I interpret your position as anything but a rejection of QM?

If there was no question of interpretations, the quantum mechanics student would absorb QM in the same way as other subjects, and form an interpretation along the lines of

• Wave-functions are real entities
• Wave-functions evolve according to Schrödinger's equation ...
• ... except when they evolve according to collapse
• Measuring devices (with numerical output) loosely correspond to operators
• A measurement somewhere involves a collapse according to the operator describing the measuring device.

and so forth.

I can see why someone would come along and suggest MWI instead -- even though we can't be sure it works out until quantum thermodynamics is more fully developed -- since it unifies the two forms of evolution and has the potential to expand the scope of applicability of QM.

If you are rejecting interpreting wave-functions as reality, and you are not supplying an alternative, what am I supposed to think your motivation is, if it's not a rejecting of using QM to understand reality?​

I really wonder how history would have differed if people figured out decoherence before they figured out the no-go theorem that unitary evolution cannot produce a lossy transformation such as (actual) collapse.

Well, this is interesting. Suddenly we have a preferred basis after all.

I agree with Delta Kilo; you're not really making sense. It's almost like you're determined not to see how things can be made to work out.

Consider, for a moment, a nice simple system: a qubit in a statistical mixture of 60% spin up about the Z axis, and 40% spin down about the Z axis.

Surely, you see that this state can be written as a statistical mixture of Z+ and Z-.

Can you also see that this state cannot be written as a statistical mixture of X+ and X-?

Can you also see that this state can be written as a statistical mixture of Z+, Y+ and Y-? (Specifically, it is 20% Z+, 40% Y+, and 40% Y-)

(P.S. since when did "preferred" come to be applied to descriptions derived from physical information? Do GPS coordintes count as a preferred coordinate system now?)

The observables are what QM is about: the predictable properties of the basic physical objects.

Possibly you are referring to eigenstates.

No, I mean taking the algebraic structure QM describes that contains elements corresponding to those "observables", a description of how they are supposed to relate to physical states, turning the mathematical crank (e.g. this), and *poof* states are wavefunctions.

 

[unusualname]

The MWI interpretation problems occur with the "splitting universes" stuff

There is a much more obvious alternative, that each individual event in the universe is random and results in an evolution at each step ( see http://www.jbg.f2s.com/quantum2.txt )

Now, the Born Rule should be calculable here...

 

[BruceW]

Same goes for consciousness collapse, transactional interpretation, ensemble interpretation, deBroglie Bohm interpretation, many minds interpretation, itacha interpretation and 50 other interpretations.
Just because there are no experiment to distinguish these interpretations, one can still philosophically and technically pick certain interpretations apart and thus debunk them.

Yes, I was trying to make it clear that all the interpretations are scientifically correct. (Since they correctly fit the results of experiments).
I'm guessing by 'debunk', you mean: show that a particular interpretation is not as good (in being a physical theory) as some other interpretation. The problem here is that different people will disagree on what makes a good theory.
I think a good theory has: as few postulates as possible, has as great a generality as possible, and makes predictions on the outcomes of experiments.
Even if we agree on this definition of a good theory, people will still disagree on which interpretation fits this definition the best.
This is why I think a discussion on 'the best interpretation' is kind of premature until physicists find empirical evidence that can be used to distinguish between the various interpretations.

 

[Delta Kilo]

If people insist on equating MWI with this 'the universe splits in half at every moment' comic-strip, I am prepared to abandon the use of term MWI and just say that I subscribe to bare-bones "No-Collapse" interpretation. At least I won't have to argue anymore about branches and splits, which are really just secondary artefacts. I am also willing for the time being to give up arguing about what's real and what's not as it does not change anything at all in the grand scheme of things.

I hope that a simple "Collapse/No Collapse" dichotomy has at least some physical sense behind it.
a) If there is an objective collapse, then Schroedinger equation needs to be modified.
b) If there is no collapse then the emergence of observables and Born rule needs to be demonstrated.
In any case a decent model of measurement process is badly needed because at the moment it is a mess and it sits smack in the middle of otherwise beautiful theory. Anyone approaching QM for the first time is faced straight away with those awkward questions about measurement, and these questions do not go away as you learn more.

 

[dm4b]

Hi jtbell,

from: http://www.anthropic-principle.com/preprints/manyworlds.html

"Political scientist" L David Raub reports a poll of 72 of the "leading
cosmologists and other quantum field theorists" about the "Many-Worlds
Interpretation" and gives the following response breakdown [T].

1) "Yes, I think MWI is true" 58%
2) "No, I don't accept MWI" 18%
3) "Maybe it's true but I'm not yet convinced" 13%
4) "I have no opinion one way or the other" 11%

Amongst the "Yes, I think MWI is true" crowd listed are Stephen Hawking
and Nobel Laureates Murray Gell-Mann and Richard Feynman. Gell-Mann and
Hawking recorded reservations with the name "many-worlds", but not with
the theory's content. Nobel Laureate Steven Weinberg is also mentioned
as a many-worlder, although the suggestion is not when the poll was
conducted, presumably before 1988 (when Feynman died). The only "No,
I don't accept MWI" named is Penrose.

Yet another reason for me to like Roger Penrose ;-)

 

[mitchell porter]

It sounds like more than that, but let me ask you a question:

You would advise the QM mechanics student not to form an idea about wave-functions corresponding to entities in reality.

Would you also advise the fluid mechanics student not to form an idea about fluids corresponding to entities in reality?

...

If your answer is no:

Then what alternative interpretation? If none, how can I interpret your position as anything but a rejection of QM?

If there was no question of interpretations, the quantum mechanics student would absorb QM in the same way as other subjects, and form an interpretation along the lines of

• Wave-functions are real entities
• Wave-functions evolve according to Schrödinger's equation ...
• ... except when they evolve according to collapse
• Measuring devices (with numerical output) loosely correspond to operators
• A measurement somewhere involves a collapse according to the operator describing the measuring device.

and so forth.
​

They don't learn statistical mechanics like this. People don't come away from the study of statistical mechanics thinking that probability distributions correspond to the actual state of anything. A probabilistic description of a physical state provides incomplete but nonetheless useful information about what is actually there. When you acquire new information, you have to update the probability distribution, and it "jumps" or "collapses".

The idea of wavefunction collapse as a physical process, and the idea that maybe wavefunctions don't collapse, both arise from treating wavefunctions as real, in a way that we would never do for probability distributions.

I agree with Delta Kilo; you're not really making sense. It's almost like you're determined not to see how things can be made to work out.

Consider, for a moment, a nice simple system: a qubit in a statistical mixture of 60% spin up about the Z axis, and 40% spin down about the Z axis.

Surely, you see that this state can be written as a statistical mixture of Z+ and Z-.

Can you also see that this state cannot be written as a statistical mixture of X+ and X-?

Can you also see that this state can be written as a statistical mixture of Z+, Y+ and Y-? (Specifically, it is 20% Z+, 40% Y+, and 40% Y-)

I don't know what your point is. My point is that if you believe in wavefunctions but you don't believe in collapse, then you need to explain what parts of the wavefunction correspond to reality. Suppose that we have a qubit whose density matrix is as described. What is the physical reality?

1) Two "branches", one with Z+ spin, one with Z- spin, and nothing else.

2) Two "branches", one with X+ spin, one with X- spin, and nothing else.

3) Three "branches", one with Z+ spin, one with Y+ spin, one with Y- spin, and nothing else.

4) All of the above and more: all possible basis decompositions of the qubit density matrix correspond to actually existing parallel branches. These noncommuting sets of parallel branches somehow exist in intersection in the wavefunction of the universe.

5) None of the above. Only the density matrix, considered globally and holistically, is the true reality.

6) There is a preferred basis selected by the qubit's environment.

7) Something else.

I would appreciate a straight answer to this question. And it needs an answer, because in the observed world - the one that physics is meant to be explaining - we always see just one possibility realized. If you want a "no-collapse, wavefunction-is-real" interpretation of QM to make sense, your very first step must be to explain what "part" or "aspect" of the wavefunction we should be looking at, in order to find what we see around us. The next step is then to explain where the empirical probabilities come from. But we can't even get to that point if you won't take the first step.​

 

[Hurkyl]

When you acquire new information, you have to update the probability distribution, and it "jumps" or "collapses".

Or, you don't update anything at all, and start asking questions about conditional probabilities (should you actually desire to condition things).

both arise from treating wave-functions as real, in a way that we would never do for probability distributions.

Think about your comment for a minute. Why wouldn't you do that for probability distributions? Interpreting probability distributions as being the actual reality is mathematically and theoretically indistinguishable from interpreting them as being ignorance probabilities.

So what grounds could you possibly have to insist that you would never do things that way?


The only (good) answer I know simply doesn't apply when we start considering collapse based versus decoherence based interpretations of quantum mechanics...

I don't know what your point is. My point is that if you believe in wavefunctions but you don't believe in collapse, then you need to explain what parts of the wavefunction correspond to reality.

If ignorance probabilities about which definite state a system is good enough for corresponding to reality, and that view is theoretically indistinguishable from a system really being in a mixed state, then mixed states are good enough for corresponding to reality.

If you can't agree on that point, then nothing else in the discussion matters.

 

[Delta Kilo]

What is the physical reality?
1)...
2)...
...
7) Something else.
I would appreciate a straight answer to this question.

The answer to this question is of course Forty-Two
Define 'physical reality' and I'll give you a straight answer.

And it needs an answer, because in the observed world - the one that physics is meant to be explaining - we always see just one possibility realized.

Just out of curiosity, do you accept that there might be "physical reality" which is not part of our "observed world"? Like for example inside the event horizon of a black hole or outside of the bubble of our currently observable universe?

This is pure metaphysics anyway, so the answer one way or another does not really change anything.

 

[mitchell porter]

Interpreting probability distributions as being the actual reality is mathematically and theoretically indistinguishable from interpreting them as being ignorance probabilities.

OK, so you flip a coin, I don't see the outcome. I could say that it is actually heads or that it is actually tails, I just don't know which. Or, I could say that the actual reality is 50% heads and 50% tails - but not that there are two worlds, one where it's actually heads and another where it's actually tails; that would be the naive view of well-defined, self-contained parallel worlds that is apparently being rejected by the defenders of MWI here. Instead, I am just to believe that the probability distribution itself is the reality - whatever that could mean.

Doing this with wavefunctions is even worse because of the multiple incompatible choices of basis. In that case it is even harder to defend the practice of "interpreting [the calculational device] as being the actual reality".

If ignorance probabilities about which definite state a system is good enough for corresponding to reality, and that view is theoretically indistinguishable from a system really being in a mixed state, then mixed states are good enough for corresponding to reality.

If you can't agree on that point, then nothing else in the discussion matters.

When I interpret the probability distribution "50% heads, 50% tails" to mean that the coin is actually heads or actually tails, but I don't know which, that is an "ignorance interpretation" of the probability distribution. I am advocating that wavefunctions be interpreted in exactly the same way - as an incomplete statement about the probable values of observables like "the side of the coin that is facing up". Since QM is incomplete, one has reason to speculate about what a more complete description of reality might be, and so maybe you could then try regarding the wavefunctions as real (and then you run into the problems that I have been pointing out) - and I thought this is what you were doing.

So afer all this, are you telling me that you do interpret wavefunctions as incomplete descriptions of reality?

 

[mitchell porter]

The answer to this question is of course Forty-Two
Define 'physical reality' and I'll give you a straight answer.

That which actually exists.

Why on Earth would you need to resort to a dodge like this? Here we are talking about what quantum mechanics means. Apart from "epistemological interpretations" which say, correctly but somewhat unhelpfully, that QM is about making correct predictions of the behavior of physical entities, I thought "interpretations" were supposed to restore to physical ontology the conceptual clarity it had before QM. Before QM, a theory might be right or wrong, but it clearly took a stand on what it is that exists. Although I maintain that the sensible "first intepretation" of QM is to say that the observables define the ontology of the theory, and the wavefunctions are a predictive device for its behavior, it's true that QM's idiosyncrasies give it a less clear-cut status than the classical theories. There is some ambiguity about what is supposed to exist, according to the theory, or else we wouldn't be having these discussions.

I thought the purpose of an interpretation of QM was to resolve that ambiguity by making definite claims about what it is that exists. The claims might be right or wrong, just as a classical theory might be right or wrong, but we would at least have a theory with ontological clarity, which took a stand about what it is that exists. By refusing to answer a simple multiple-choice question - which even included "none of the above" as an option, leaving you license to write your own explanation of the "no collapse" ontology - and by asking for a definition of "physical reality" instead, it begins to appear rather questionable that you even have a theory, in any conventional sense of the word.

Just out of curiosity, do you accept that there might be "physical reality" which is not part of our "observed world"? Like for example inside the event horizon of a black hole or outside of the bubble of our currently observable universe?

I said several times in this thread that I do not have an a-priori problem with the idea of parallel worlds with copies of me experiencing things I don't know about here. I also don't have a problem with the existence of things that no-one is experiencing. My whole problem is with people who say they have a theory and then cannot explain, on even the most basic level, what the theory says.

Hurkyl has just started talking about ignorance interpretations, which makes me wonder if he doesn't intend for wavefunctions to be the actual states of things after all. And your own earlier remarks about choosing to put aside the issue of what's real in the theory lead in the same direction. I have been saying wavefunctions are like probability distributions, and as Hurkyl points out, you can have a "no collapse" interpretation of probability distributions: Instead of changing the probability distribution in response to new information, you keep the distribution but then condition on the information in order to use it. Of course I don't disagree with anything there, but at this point it seems we have wandered far from MWI and even far from "MWI without collapse", and are no longer talking about what a complete description of reality might be like.

If you do settle for an ignorance interpretation of wavefunctions, then the measurement problem can disappear as a separate problem. You can indeed just have a joint wavefunction for measurement device and measured object, evolving according to the physics of the measurement interaction in question, and you then apply the Born rule (or its generalizations) to any observable that you care about - and here by observable I don't just mean observables that get diagonalized by the measurement interaction, I mean any property anywhere in the history of the joint system. The algebra of observables tells you that they cannot all take definite values, but coarse-graining as in decoherent histories let's you specify a set of commuting observables, and you can apply the Born rule to them if you want to ask yourself about their probable values, their correlations, and so on.

Two points: First, while this approach removes the problem of measurement as being a distinct sort of physical process, it does nothing about the incompleteness of quantum mechanics. It doesn't even tell us which observables, in the history of the universe, actually take values - as I said, it can't be all of them because they don't all commute, and yet presumably it is some larger subset of them than the minimalist solipsistic option of "positions on ions around neural membranes in my brain". It's a framework which allows us to be totally consistent with empirical QM, and yet the answer to "what is real" is anything from neuro-solipsism to a maximal set of commuting observables covering most of space-time. This is another way to see that QM is ontologically incomplete.

Second point: In this ignorance interpretation of QM, you still need the Born rule! Otherwise you can't make any statements about probable behavior of observables. So you can have a "no-collapse" ignorance interpretation, but you can't do without the Born rule as an independent postulate.

 

[Hurkyl]

Ugh, this is a lot of words. Sorry.

Hurkyl has just started talking about ignorance interpretations, which makes me wonder if he doesn't intend for wavefunctions to be the actual states of things after all.

That's to describe the way you're thinking about things. An ignorance interpretation is "reality is in one of those states, but I don't know which and I'm assigning probabilities to capture my ignorance". "Probability distributions are real" is not an ignorance interpretation.

OK, so you flip a coin, I don't see the outcome. I could say that it is actually heads or that it is actually tails, I just don't know which. Or, I could say that the actual reality is 50% heads and 50% tails ... I am just to believe that the probability distribution itself is the reality - whatever that could mean.

Right, we start with something like this. And we run with classical mechanics for a while to get used to the shift in perspective.

While before we described the coin with an actual value "Heads" or "Tails", we now describe the coin with a random variable.

We consider the fact that when I observe the coin, I will see one of the two values in the set {heads, tails}. Ah, that's accounted for in the fact the sample space of the random variable is the set {heads, tails}.

The fact that actual observation sees only one outcome? The trivial fact that P(X=a | X=a) = 1. We just used to using conditional probabilities with random variables, when before we talked about absolute probabilities involving indeterminate variables.


Happy? The important thing at this point isn't that you think "this is a wonderful way to think about classical mechanics" -- it's that "huh; this is self-consistent and physically indistinguishable to the ordinary way of thinking, even if it seems a little weird."


Now, let's continue applying this to quantum states. Traditionally, we think of results of measurements being classical ignorance probabilities across definite outcomes. But now, we're thinking of probability distributions as reality -- but it's a bit more convenient than the classical case, because it's already built into the mathematical description of quantum state -- e.g. a quantum state could be written as a weighted positive linear combination of density matrices -- rather than us having to layer probability theory on top of the quantum state space. Also, quantum states are somewhat more general.

Now we run into the question you had before:

Suppose that we have a qubit whose density matrix is as described. What is the physical reality?

The answer is: the physical reality is the wave-function. Everything else follows from that. I mentioned a quantum state earlier that was a mixed state that could be written as

60% Z+ and 40% Z-​

Since we made ourselves happy with probability distributions as reality, it's not difficult to understand a quantum particle as really being in this state that is a probability distribution over pure states.

We understand that this is completely indistinguishable from a definite-outcomes interpretation where we view it as an ignorance probability over definite states.

Also, we can understand a quantum particle as really being in the state

20% Z+ and 40% Y+ and 40% Y-​

Now, you were having a problem with the fact these were different sums describing the same quantum state. What is the reality?? But the question was already answered -- reality is the wave-function. A little thought should convince you that the difference between the sums is entirely superficial -- the two decompositions cannot be distinguished by any physical experiment, and therefore we shouldn't be quick to insist that we must interpret them as different.

(and if it doesn't, try computing the expectation of some observable -- say, spin around the axis half-way between the Z+ and Y+ axes)


Are you with me this far? Again, I've not said anything to answer "why should we think this way" (except possibly for comments about things being physically indistinguishable), but instead have discussed "why can we think this way".

We've seen that something we might have thought as classical ignorance probabilities weighted 60% Z+ and 40% Z- give the same results as thinking of a qubit in a certain mixed quantum state. And that classical ignorance probabilities weighted 20% Z+ and 40% Y+ and 40% Y- gives the same results as thinking of the cubit in a certain mixed quantum state.

And we saw the interesting new physics that the two mixed states are the same -- and the interesting result that the two superficially different classical ignorance probabilities actually describe the same physics... something we might not have otherwise noticed if we weren't thinking about mixed states.


The next two major points are the relative state of a subsystem (I was probably going to talk about the entangled photon scenario as an example), and the fact that relative states provide a way around the old no-go theorem -- when a system undergoes unitary evolution, its subsystems can decohere.

Can you see where I'm going with these, or do I need to continue?

Anyways, once we have these points, we have the basic premise behind the family of decoherence-based interpretations of quantum mechanics -- the ignorance probabilities that QM was traditionally thought in terms of work out to be indistinguishable from mixed states. Mixed states can be produced by decoherence of relative states of subsystems. Decoherence of subsystems can occur through unitary evolution ala the Schrödinger equation. We have a framework where unitary evolution at least has the potential to produce the same mathematical descriptions of state that appear in a collapse-with-ignorance-probabilities point of view.



Aside: the "good" reason to use ignorance probabilities I mentioned earlier was Occam's razor -- in classical statistical mechanics, the decomposition of a probability distribution into a weighted combination of individual states is absolute and eternal, unlike the weighted combinations that appear in the framework I described above. For classical mechanics, we lose nothing (except possibly a wider point of view) by deciding to interpret everything as ignorance probabilities, so Occam can be applied. But in QM, we do lose something by insisting on thinking in terms of ignorance probabilities, so Occam doesn't apply anymore.

 

[Delta Kilo]

By refusing to answer a simple multiple-choice question - which even included "none of the above" as an option, leaving you license to write your own explanation of the "no collapse" ontology - and by asking for a definition of "physical reality" instead, it begins to appear rather questionable that you even have a theory, in any conventional sense of the word.

It's not about a theory, its about interpretation. It would become a theory if, for example, someone manages to get Born rule out of unitary evolution, much like normal law appears from random walk. Decoherence does at least part of the job, it shows how complex wavefunction is reduced to a set of choices in environmentally-selected preferred basis, but the last step of getting the actual probabilities, I don't think we are there yet, unfortunately.

Regarding the definition of reality and its connection with the observable world, there is yet another radical alternative: our observable world might not be real. Say, we observe temperature and pressure, would you say they are real or would you say the motion of individual molecules is real and temperature and pressure are just emerging statistical artefacts? What if we are simply unable to perceive the 'real' thing due to us being simply too big, too macroscopic?

As in, what is more real, an object or a picture of it on TV? What if we are unable to leave the room and the only thing we can observe is a picture on TV? Well, we can still develop a good consistent theory explaining (and predicting!) what we see, like colors, shadows, occlusions, etc. Would we then consider our 3D model as more or less real than 2D picture we see on TV?

These are all rhetorical questions, I do not pretend to have answers for them. And it does not matter one way or the other. What matters is to have a good working model.

 

[Fredrik]

Hurkyl,

if I flip a coin and you don't see the result, would you say that the reality of the coin is a probability distribution with roughly equal probabilities for heads and tails? Based on what you said, I don't see how your answer can be anything but "yes". So reality is subjective? It sounds like you have simply redefined the word "reality" to refer to the mathematical object a person uses as a representation of reality.

Consider a wavefunction that's non-zero over a large region. QM tells us how to use it to calculate probabilities of possible results of experiments. QM doesn't say if the graph of the function |\psi|^2 describes the particle's current shape, or if particles are actually point-like objects that move in really weird ways. How can QM be said to describe reality when it doesn't even tell us a particle's shape?

Since QM doesn't tell us the shape, we are free to make assumptions about it. I would say that it's these (non-mathematical, non-scientific and unnecessary) assumptions that define descriptions of possible realities. I still don't see a reason to say that the theory describes a possible reality, let alone the actual reality.

I also don't consider this a rejection of QM, as you apparently do. To reject it would be to say that its predictions aren't very accurate (this is certainly not true), or that it doesn't improve our understanding of reality at all. I would say that it does. It just doesn't improve it as much as we want it to.

 

[Fredrik]

It's not about a theory, its about interpretation.

That's exactly why it doesn't make sense to demand a definition of "reality". In the context of interpretations, "reality" must be considered a primitive, a term that's left undefined. How would you define it? No, I'm not asking what definition you would choose. I'm asking what sort of thing you would consider a definition. Do you mean that we should associate the term with something in a theory of physics? Wouldn't the only point of having a definition be that we might be able to derive such an association from it? I would say "yes", and that this means that a definition of this sort would be worse than useless.

 

[Hurkyl]

Hurkyl,

if I flip a coin and you don't see the result, would you say that the reality of the coin is a probability distribution with roughly equal probabilities for heads and tails? Based on what you said, I don't see how your answer can be anything but "yes".

Yes.

So reality is subjective?

No. This is something that's awkward to discuss because of a lack of good words -- but I'm hoping that since he's using words that can sound like what I mean, that he's thinking what I mean.

For this particular scenario to result as it did, the reality of the coin and environment was that it was already in a mixed state pre-flip. The mixture persisted (as they do in classical mechanics) through the flip, and we supposed that the mixture was typical enough that the final state was close enough to 50-50.

(snip)

I also don't consider this a rejection of QM, as you apparently do. To reject it would be to say that its predictions aren't very accurate (this is certainly not true), or that it doesn't improve our understanding of reality at all. I would say that it does. It just doesn't improve it as much as we want it to.

One of the important features of physical theories is that they give us the language and ideas in terms of which we talk about reality. I do consider rejecting this application of QM as rejecting QM itself.

How much sympathy would you have for an alchemist who teaches his students chemistry, but advises his students that they should not form an idea that materials are made out of molecules and atoms and such; that they are just calculating devices to work out material properties or transmutations or what-not?

 

[Fredrik]

No. This is something that's awkward to discuss because of a lack of good words -- but I'm hoping that since he's using words that can sound like what I mean, that he's thinking what I mean.

I don't understand. If I've seen the result and you haven't, we would use different probability distributions for the coin. So if the probability distribution is reality, then we have different realities. Maybe you were talking about a probability distribution for the state of the entire universe, or at least a big enough part of it to include both of us and the coin.

How much sympathy would you have for an alchemist who teaches his students chemistry, but advises his students that they should not form an idea that materials are made out of molecules and atoms and such; that they are just calculating devices to work out material properties or transmutations or what-not?

Not much, but that's because I know that there's a good theory about "what things are made of". In the case of QM, there's no theory (good or bad) about what's really going on. All we have are non-scientific guesses. It's also possible that we have reached the limits of what the scientific method can do.

 

[Dmitry67]

Just wanted to say Hi, I am back after almost 3 month. I was trying to struggle with the internet addiction, so I tried to be away from the internet this summer. Finally, the victory is on the Internet's side. Anyway, what did I miss, and what I should begin with? :) Just shoked by so many MWI/collapse threads...

 

[Hurkyl]

Maybe you were talking about a probability distribution for the state of the entire universe, or at least a big enough part of it to include both of us and the coin.

Yes. Actually, at that point in the discussion I was happy for it to be just big enough to include the coin.

In the case of QM, there's no theory (good or bad) about what's really going on.

What's wrong with, say, QM? Seriously? Aside from incompatibility with GR, What deficiency does it have other than:

• It doesn't have a unique interpretation, and
• It offends many peoples' sensibilities?

I consider neither of those points relevant.

I suppose I have a big advantage in this matter, that my background includes pure math, computer science, and abstract gaming, so I'm very, very used to the idea of building an understanding based on what something is, rather than upon any preconceived notions of how it should work.

 

[mitchell porter]

I suppose I have a big advantage in this matter, that my background includes pure math, computer science, and abstract gaming, so I'm very, very used to the idea of building an understanding based on what something is, rather than upon any preconceived notions of how it should work.

Hilarious... Hurkyl, if I decide that I will or will not kick you in the balls on the basis of a coin toss, and then I toss a coin and act accordingly, you do not end up in a state of "50% kicked in the balls, 50% not kicked in the balls". At the end, either I left you alone, or you are doubled over in pain.

It is very gauche and aggressive of me to resort to reductio ad kick-in-the-balls, after such a dainty intellectual argument, but the line you are taking is just incredibly absurd if you try to apply it to anything real. Reality consists of a series of definite situations. Maybe there are other realities defined by a different series of situations. But if your model of reality contains no such definiteness anywhere, it is false.

 

[Delta Kilo]

What's wrong with, say, QM? Seriously? Aside from incompatibility with GR, What deficiency does it have other than:

• It doesn't have a unique interpretation, and
• It offends many peoples' sensibilities?

I consider neither of those points relevant.

Whats wrong you say? You're joking, right?
How about having two different incompatible laws for the same thing and no clear explanation why and how and at which point exactly one transitions into another?
I'm talking about unitary evolutuion vs. collapse obviously.

 

[K^2]

That's why we have MWI. It explains you when the transition happens.

 

[Demystifier]

Whats wrong you say? You're joking, right?
How about having two different incompatible laws for the same thing and no clear explanation why and how and at which point exactly one transitions into another?
I'm talking about unitary evolutuion vs. collapse obviously.

I think Hurkyl meant that collapse does not belong to QM itself, but to interpretations of QM.

 

[Fredrik]

What's wrong with, say, QM? Seriously?

I think I have answered that. QM tells us how to associate a unique probability with each experimentally verifiable statement, and it does that extremely well, but it doesn't define a description of a possible reality, at least not one that resembles the reality we live in. Descriptions of possible realities are defined by non-scientific assumptions that people like to add on top of QM.

If you think that QM defines a description of a possible reality, then please explain what that description says. Can you do it without making additional assumptions? If not, I would say that you're talking about a description defined by those assumptions, not a description defined by QM.

 

[Fredrik]

Just wanted to say Hi, I am back after almost 3 month. I was trying to struggle with the internet addiction, so I tried to be away from the internet this summer. Finally, the victory is on the Internet's side. Anyway, what did I miss, and what I should begin with? :) Just shoked by so many MWI/collapse threads...

Welcome back. I don't think you missed anything important (if you left after we switched to a new LaTeX system). Yes, there have been a lot of threads about "what causes collapse" recently. I'm a bit fed up with that topic, so I didn't even open those threads.

 

[BruceW]

Hilarious... Hurkyl, if I decide that I will or will not kick you in the balls on the basis of a coin toss, and then I toss a coin and act accordingly, you do not end up in a state of "50% kicked in the balls, 50% not kicked in the balls". At the end, either I left you alone, or you are doubled over in pain.

This is what happens in MWI. The state would be something like |kicked> + |not kicked>

 

[BruceW]

Just wanted to say Hi, I am back after almost 3 month. I was trying to struggle with the internet addiction, so I tried to be away from the internet this summer. Finally, the victory is on the Internet's side. Anyway, what did I miss, and what I should begin with? :) Just shoked by so many MWI/collapse threads...

I said on page 2 that there has not been any experiments so far to distinguish between the possible interpretations of QM. And I didn't get any disagreements, so maybe that's one thing to take away from this discussion.

Although, I am not looking through all the recent journals. If anyone has seen some new expeiments which disprove any of the interpretations, I'd find that interesting.

 

[Ken G]

Let's start with any different predictions, we'll need that even before there can be experiments. Can anyone suggest a single predicted outcome that comes out any different in any of the interpretations?

 

[Fredrik]

Let's start with any different predictions, we'll need that even before there can be experiments. Can anyone suggest a single predicted outcome that comes out any different in any of the interpretations?

If even one such experiment exists, this "interpretation" would by definition (at least by my definitions) be a theory, not an interpretation.

 

[xts]

Can anyone suggest a single predicted outcome that comes out any different in any of the interpretations?

There is a famous Tegmark's "quantum suicide" experiment. But it may convince only you...
In MWI every suicide attempt must fail - you always live in a world, where Russian rulette gun misfired.
Unfortunately this experiment may convince only you - other people live in worlds in which they cry at your funerals.

But we shouldn't take this argument just as a joke. Actually that is a pretty strong argument putting together the collapse/measurement/many-world-forking and consciousness.

 

[K^2]

While Quantum Suicide only works in MWI, MWI does not guarantee that Quantum Suicide works. There is still a lot of debate on that one. So even if you were absolutely dead set on proving MWI, that might not be the way to go.

 

[xts]

While Quantum Suicide only works in MWI, MWI does not guarantee that Quantum Suicide works.

Why? If only my suicide attempt is not 100% effective (well, even as all patrons are loaded, the one still may misfire...), my consciousness must survive!

I'll be grateful for any references to the debate on quantum suicide!

 

[Fyzix]

Why? If only my suicide attempt is not 100% effective (well, even as all patrons are loaded, the one still may misfire...), my consciousness must survive!

I'll be grateful for any references to the debate on quantum suicide!

No... A consciousness will survive, will it be you? not guaranteed at all.

For simplicity's sake say your suicide attempt can have 10 outcomes, 9 of which you will infact die and 1 where you will live.
Nothing will make the 9 consciousnesses that cease to exist JUMP over into the single branch that their "twin" will survive in.Jacques Mallah has written a lot on this: http://arxiv.org/ftp/arxiv/papers/0902/0902.0187.pdf

 

[JordanL]

No... A consciousness will survive, will it be you? not guaranteed at all.

For simplicity's sake say your suicide attempt can have 10 outcomes, 9 of which you will infact die and 1 where you will live.
Nothing will make the 9 consciousnesses that cease to exist JUMP over into the single branch that their "twin" will survive in.


Jacques Mallah has written a lot on this: http://arxiv.org/ftp/arxiv/papers/0902/0902.0187.pdf

When a death occurs, the experience of everyone else would be of that person dying, while that person would continue living from their subjective experience. However their "jumping" between worlds would indicate that consciousness transcends reality and knowledge, or any particular version of it. In order for this to make sense, at least from what I understand, awareness has to be more fundamental to existence than any other concept, including logic or realism, as consciousness would represent something which can exist both independent of and within anything that can be described within our Universe.

In this way, awareness must be something upon which everything else is based, instead of the other way around, if jumping were to occur. If the Quantum Suicide experiment were true, we would have found a plausible cause for the existence of the universe: so that (at least) we could experience it.

The Quantum Suicide experiment has less to do with MWI and more to do with meaning and existence, in my opinion. And in that way is interesting to ponder, but also not helpful for understanding the differences between CI and MWI.

 

[BruceW]

This quantum suicide thought experiment doesn't actually provide a way of discerning between MWI and CI, right?

 

[JordanL]

This quantum suicide thought experiment doesn't actually provide a way of discerning between MWI and CI, right?

It provides a way to help explain conceptual differences, but does not discern them or define them.

 

[Hurkyl]

In MWI every suicide attempt must fail - you always live in a world, where Russian rulette gun misfired.

This is a level slip -- it's confusing the frog's eye and the bird's eye view. No matter how much you want to pay attention to

P(You live) > 0​

you can't get around the fact that

P(You live | You died) = 0.​

You point out there are "worlds" where people live to attend your funeral. In those same "worlds" you get to attend your funeral dead.

 

[Dmitry67]

Yes, it is a confusion of 2 meanings of "YOU" - YOU as a worldline (frog) and YOU as a tree (Bird). These 2 meanings are different in MWI. "Observer" has 2 meanings as well in MWI.

 

[Hurkyl]

It is very gauche and aggressive of me to resort to reductio ad kick-in-the-balls, after such a dainty intellectual argument, but the line you are taking is just incredibly absurd if you try to apply it to anything real. Reality consists of a series of definite situations.

Yes it is. What does your proposal accomplish other than to make you look like a fool? The variable "X or not X" has only one outcome -- "true". Checking for that result has no power to distinguish between any alternatives at all, let alone the ones under consideration.

 

[mitchell porter]

Hilarious... Hurkyl, if I decide that I will or will not kick you in the balls on the basis of a coin toss, and then I toss a coin and act accordingly, you do not end up in a state of "50% kicked in the balls, 50% not kicked in the balls". At the end, either I left you alone, or you are doubled over in pain.

It is very gauche and aggressive of me to resort to reductio ad kick-in-the-balls, after such a dainty intellectual argument, but the line you are taking is just incredibly absurd if you try to apply it to anything real. Reality consists of a series of definite situations. Maybe there are other realities defined by a different series of situations. But if your model of reality contains no such definiteness anywhere, it is false.

Yes it is. What does your proposal accomplish other than to make you look like a fool?

It is supposed to make you remember the existence of pain, which in turn is supposed to make you realize that reality has definite properties.

Maybe I'm misunderstanding you; but Fredrik asked you directly, if he flips a coin, what is the state of that coin. You could have said that it's actually heads, or actually tails, and you don't know which; you could even have said that it's actually heads in one world, and actually tails in another world; but instead you said that the actual state is a probability distribution.

Your subsequent brag (about how your conceptual facility with mathematics and computers has equipped you to accept things as they are, in all their counterintuitiveness) reinforces my initial suspicion that there's something seriously lacking in your concept of what physics is about. Physics, including quantum mechanics, is supposed to describe the real world of stars and planets and living things. It is not just an abstraction living in your imagination or on your hard drive. One of the features of the real world is that specific things happen.

I have, I hope, been fairly clear about how I think quantum mechanics should be interpreted. Observables are real, wavefunctions are not, they just provide probabilities for the behavior of observables; that is how QM is meant to be used. I have so far failed to discern any similarly clear position from you. You objected when I said that wavefunctions aren't real, but your subsequent remarks have been entirely formal in character, and do not indicate where you think QM connects to reality. Well, there was the nonsensical affirmation that a probability distribution over the possible outcomes of a coin toss is the actual state of the coin. This is where the kick in the balls comes in. It is supposed to make you understand that things actually happen.

 

[Hurkyl]

you could even have said that it's actually heads in one world, and actually tails in another world; but instead you said that the actual state is a probability distribution.

Yes. (Aside: you do notice that, in classical mechanics "probability distribution over N states" is essentially identical to "N worlds (with weights that sum to 1)" right?)

Well, there was the nonsensical affirmation that a probability distribution over the possible outcomes of a coin toss is the actual state of the coin.

I maintain there is no experiment that can possibly be done to distinguish between definite outcomes and indefinite outcomes.

If there is no experiment that can distinguish between possibilities, there is no physical content in the assertion that reality has definite outcomes rather than indefinite outcomes.

If there is no physical content to the assertion that reality has definite outcomes rather than indefinite outcomes, then the claim that you're talking about reality and I'm talking about nonsense is just you blowing smoke.


Nothing QM specific is relevant to anything you've said in this post. You are familiar with probability distributions on classical state spaces, right? I challenge you to find an experiment that can distinguish between reality is a single point of phase space versus reality is a probability distribution on phase space.

 

[mitchell porter]

Yes. (Aside: you do notice that, in classical mechanics "probability distribution over N states" is essentially identical to "N worlds (with weights that sum to 1)" right?)

If you mean "possible worlds" then I think I accept the equivalence.

I maintain there is no experiment that can possibly be done to distinguish between definite outcomes and indefinite outcomes.

If there is no experiment that can distinguish between possibilities, there is no physical content in the assertion that reality has definite outcomes rather than indefinite outcomes.

If there is no physical content to the assertion that reality has definite outcomes rather than indefinite outcomes, then the claim that you're talking about reality and I'm talking about nonsense is just you blowing smoke.

Every second of your waking life tells you that at least one "definite outcome" exists. This whole idea of an "indefinite outcome" is a contradiction. Either something is happening or it isn't happening.

Many worlds makes sense as a belief in "many definite outcomes". It then founders on the problem of reproducing the Born probabilities, but at least there's a definite hypothesis.

 

[Hurkyl]

If you mean "possible worlds" then I think I accept the equivalence.

I don't know what you're trying to connote with "possible". My best guess is that you're using it to mean "reality is in one of these worlds", which is very much not what I'm talking about.

Every second of your waking life tells you that at least one "definite outcome" exists. This whole idea of an "indefinite outcome" is a contradiction. Either something is happening or it isn't happening.

Show me an experiment that can tell the difference. Anything at all. The criterion you're suggesting is to observe:

P(X or not X) = 1​

or possibly

P(X and not X) = 0​

but neither criterion differentiates between X being a definite value that is either "true" or "false" and X being a random variable with sample space {"true", "false"}.

Let me repeat that for emphasis. In an indefinite outcome interpretation of classical mechanics, if X is any proposition tested by experiment, then you are guaranteed to observe either X or not X. "X or not X" is definitely true, whether X is definite or not.

 

[Demystifier]

There is a famous Tegmark's "quantum suicide" experiment. But it may convince only you...

First, it will not "convince" only me, but also all my neighbors living in the branch in which I survive each time.

Second, I doubt that it will really convince anybody; it could also be that I was just lucky to survive each time, or that there is some secret mechanism providing my survival in a deterministic manner (for example, perhaps someone found a way to control the initial conditions of Bohmian hidden variables).

 

[mitchell porter]

I don't know what you're trying to connote with "possible".

I don't want to presuppose the actuality of all the worlds appearing in the probability distribution.

Every second of your waking life tells you that at least one "definite outcome" exists. This whole idea of an "indefinite outcome" is a contradiction. Either something is happening or it isn't happening.

Show me an experiment that can tell the difference. Anything at all.

The criterion you're suggesting is to observe:

P(X or not X) = 1​

or possibly

P(X and not X) = 0​

but neither criterion differentiates between X being a definite value that is either "true" or "false" and X being a random variable with sample space {"true", "false"}.

Let me repeat that for emphasis. In an indefinite outcome interpretation of classical mechanics, if X is any proposition tested by experiment, then you are guaranteed to observe either X or not X. "X or not X" is definitely true, whether X is definite or not.

The radical insanity of your position makes it difficult to rebut. Or rather, it is easy to rebut, but it is difficult to make you acknowledge the rebuttal. I can't say exactly what your problem is, but it has something to do with misuse of formal concepts like "random variable".

First you ask for an experiment that can tell the difference between a definite outcome and an indefinite outcome. I repeat that an indefinite outcome is an incoherent concept, a logical contradiction. The experiment has an outcome, or it doesn't. If it has an outcome, it is necessarily a definite outcome. There is no such thing as an experiment with an "indefinite outcome" in the sense you are discussing.

Your distinction between

X being a definite value that is either "true" or "false" and X being a random variable with sample space {"true", "false"}

appears to be a confusion of levels. I'll just quote Wikipedia:

In probability and statistics, a random variable or stochastic variable is, roughly speaking, a variable whose value results from a measurement on some type of random process. Formally, it is a function from a probability space, typically to the real numbers, which is measurable.

It's as if you're saying "Your experiment can't distinguish between the possibility that X is a value of a function, and the possibility that X is a function."

I am not sure about the following attempt at cognitive debugging, I can only guess at the details of the thought process which causes you to say what you do; but your remark may be arising from an inconsistent use of the concept "random variable". Either you are talking about it as a mathematical concept, or you are using it to refer to a physical quantity. If you are talking about an experiment, then your random variable is a physical quantity, in which case it necessarily satisfies your first option - 'a definite value that is either "true" or "false"'. The "function from a probability space", etc, specifies the mathematical formalism we use to describe the physical variable, and it says something about the properties of the possible values of the physical variable (their probabilities).