Under what circumstances does the wave function collapse?

[Ken G]

Regarding the Copenhagen interpretation, I've been thinking along similar lines as Ken G. Maybe the lesson of the measurement problem is not that something is wrong with QM (there are arguably no experimental hints of this) but that something is wrong with the naive realist interpretation of previous theories.

I agree with that, though it is only part of what I'm saying, because I'm also saying that what is wrong with the naive realist interpretation is only what is wrong with all interpretations-- they are framed as something they are not. Bohm did us a service by showing us that a type of naive realist interpretation still exists, but we turn that service into a disservice if we think that means nature really is doing that. Interpretations are nothing more than a bucket of syntactic postulates that generate the experimental outcomes by going outside what can experimentally be demonstrated as true. That replacement of semantic truth with syntactic structure is just what an interpretation is supposed to be, so we err if we interpret the syntactic structure as a new set of semantic truths. That would only leave us with a need for a new syntactic structure to be able to derive the semantic claims of that interpretation, it's not what interpretations are for unless we want an unending stream of unresolvable problems. What is interesting about any interpretation are what postulates it requires to get the necessary syntactic structure, and every one of those sets of viable postulates is what presents us with an understanding of nature, but we get a different package of understanding from every valid interpretation, so vive la difference.

 

[atyy]

But we do have a problem there, we have a completely deterministic theory built from fundamentally time reversible equations that is trying to describe a set of experiences where we seem to have choices and a special arrow to time. We just kind of stopped bothering with these problems when we discovered it was no longer the most fundamental theory available. We should expect the same eventual fate for the quantum measurement problem, if history will be our guide.

Indeed, that is the Bohmian or Bohmian-like solution.

That is one way to frame the measurement problem, but that's not how I look at it. What I think you are saying is that you have found a particular interpretation of quantum mechanics, a metatheory that adds axioms that allow the semantic truths of experimental outcomes in quantum type experiments to be derived from the syntactic structure of that particular metatheory. Yet that's what all the interpretations of quantum mechanics do, so why would that imply there would be a measurement problem without the Bohmian metatheory? In my mind, the measurement problem takes your solution to the next step and asks "is reality really doing what your interpretation says it is doing?" That's the "problem" that cannot be scientifically resolved (unless we get some new predictions to test), and which I am arguing is only a problem if we think that's a valid question to ask in the first place.

The Bohmian solution does predict deviations from quantum mechanics. It gives teeth to the possibility that quantum mechanics is not the final theory.

The Michelson-Morley experiment is an example of an experiment that comes out A if we resolve the cut one way and B if we resolve it a different way. That means the cut is scientifically explorable. The "cut" between what nature is actually doing, and our approaches to understanding nature, is not explorable as long as we frame physics to be something that does not have us in it.

But the point is do we really believe in the cut? Do we believe that special relativity is the theory in which the observer measures the speed of light to be the same in all reference frames, or do we prefer the formulation in which special relativity means the laws of physics have Poincare symmetry?

Or in general relativity, do we believe that test particles move on geodesics? There is of course no problem with that, if there are test particles without mass-energy. But there are no such test particles. So when we say test particles move on geodesics, we really mean that metric and matter are minimally coupled.

 

[stevendaryl]

It certainly is evidence that the mathematical construct has value to us, that it connects in some useful way to nature. Is there something else that a scientist means by "what's true in the world"? Do not scientists like to stay within what can be demonstrated, and shy away from what others could regard purely as an unevidenced belief system?

I think scientists are people who are curious about the world. There can and should be rigorous standards for what should be published as a scientific theory, but I'm very much opposed to a dogma that says that as a scientist, you're not allowed to think about certain things, not allowed to ask certain questions.

Of course, in some cases, "Your question has no answer" IS the answer.

I think that in many cases in science, asking the natural: "What's REALLY going on?" is very fruitful. Ptolemy's circles within circles had quite a bit of predictive power in describing the apparent motions of stars and planet. In contrast, Copernicus' alternative theory, which had planets moving in circles around the sun, was initially LESS accurate. I don't know what his motivation was, but I don't think it was agreement with experiment, it was an attempt to make sense of observations. More recently, Balmer had an ad hoc theory for computing the possible energy levels of hydrogen, and it was pretty accurate. But quantum mechanics really got started when people started to understand why the energy levels took on those values.

At some point, philosophers of science and scientists themselves started being dogmatic about what kind of questions you ask, and what theories you could pose and still be considered a scientist. I think that is completely wrong-headed. You should always ask about any question: What kind of evidence would shed light on that question? Or about a theory, we should always ask: What evidence would support or falsify that theory? (Yeah, I know Popper says that evidence only falsifies, never confirms, but psychologically, people think in terms of supporting a theory). But I think it's ridiculous to demand that people not think about anything beyond predicting outcomes of experiments.

 

[stevendaryl]

Empirically, investigations into the meaning of quantum mechanics, and attempts to understand how nature does what it does have been fruitless. So I think that in that particular case, it may be reasonable to say that maybe it's not worth spending a lot of time on it. But I think it's absolute bad advice to tell a scientist, in general, that he should never spend time thinking trying to understand the meaning of a scientific formalism.

 

[jaydnul]

I'm definitely lost but please continue because I suspect this will be a great reference to come back to once I have more of an understanding. Just to butt in really quickly, the only thing that I think I'm able to extract from this thread (at this time) is that it isn't quite clear yet how our classical world emerges from the quantum. Is that right?

 

[atyy]

I'm definitely lost but please continue because I suspect this will be a great reference to come back to once I have more of an understanding. Just to butt in really quickly, the only thing that I think I'm able to extract from this thread (at this time) is that it isn't quite clear yet how our classical world emerges from the quantum. Is that right?

Read the literature and decide for yourself.

I recommend the discussion in http://arxiv.org/abs/quant-ph/0209123. Weinberg's quantum mechanics book is also very good https://www.amazon.com/dp/1107028728/?tag=pfamazon01-20. I think most will agree with those two recommendations. The one contentious recommendation is Ballentine, which most people like, but I think is somewhere between misleading and wrong.

 

[Ken G]

The Bohmian solution does predict deviations from quantum mechanics. It gives teeth to the possibility that quantum mechanics is not the final theory.

Whenever an interpretation makes predictions outside of the theory it was originally designed to interpret, it has graduated from an interpretation to a scientific hypothesis. I have no objection to regarding Bohmian physics as a scientific hypothesis, but then we have nothing to discuss except how to do the experiment. If we can't have that discussion because the experiment is too difficult, then we are back to treating it as an interpretation, and my objections to how people treat interpretations comes into play.

But the point is do we really believe in the cut? Do we believe that special relativity is the theory in which the observer measures the speed of light to be the same in all reference frames, or do we prefer the formulation in which special relativity means the laws of physics have Poincare symmetry?

We are scientists, we don't believe things, we test them. We design syntactic structures that allow us to derive symbolically the semantic truths that we discover by experiment. We can marvel at the attributes of those syntactic structures, and claim an advanced understanding for having created them and tested them, but there is never any reason to believe them. Understanding does not come from belief, belief comes at the exact place where understanding has ended.

 

[atyy]

Whenever an interpretation makes predictions outside of the theory it was originally designed to interpret, it has graduated from an interpretation to a scientific hypothesis. I have no objection to regarding Bohmian physics as a scientific hypothesis, but then we have nothing to discuss except how to do the experiment. If we can't have that discussion because the experiment is too difficult, then we are back to treating it as an interpretation, and my objections to how people treat interpretations comes into play.

Sure, but that also means one must object to all research in quantum gravity - it is not science, just interpretation.

We are scientists, we don't believe things, we test them. We design syntactic structures that allow us to derive symbolically the semantic truths that we discover by experiment. We can marvel at the attributes of those syntactic structures, and claim an advanced understanding for having created them and tested them, but there is never any reason to believe them. Understanding does not come from belief, belief comes at the exact place where understanding has ended.

Sure one can say that. But then your belief in the existence of distant reality is just a belief. You cannot distinguish it experimentally from the nonexistence of distant reality. Yet you preferred one over the other.

 

[Ken G]

I think scientists are people who are curious about the world. There can and should be rigorous standards for what should be published as a scientific theory, but I'm very much opposed to a dogma that says that as a scientist, you're not allowed to think about certain things, not allowed to ask certain questions.

Is it not part of scientific investigation to be able to decide what is a scientific question, and what is not? No scientist is prohibited from asking questions, but the scientist should understand what a scientific question is in the first place. We actually get quite little formal guidance into this, it lives in a place firmly "under the rug" where we swept it, hoping that in practice it wouldn't matter all that much. Usually it doesn't-- sometimes it does.

Ptolemy's circles within circles had quite a bit of predictive power in describing the apparent motions of stars and planet. In contrast, Copernicus' alternative theory, which had planets moving in circles around the sun, was initially LESS accurate. I don't know what his motivation was, but I don't think it was agreement with experiment, it was an attempt to make sense of observations.

Yet Copernicus' alternative theory was only a stepping stone as well, when relativity revealed that all language about motion is actually just a choice of coordinates and not a statement of reality. Thus, nothing about Copernicus' success had anything to do with it being "what was really happening", all its lasting success was around "here's a better way to think about what is happening, and there might also be other good ways to change how we think about what is happening." It's always an end to understanding, not a start, to say "this is what is really happening." That is the sound of a door closing, a door opening sounds like "how do you know that is what is really happening?"

More recently, Balmer had an ad hoc theory for computing the possible energy levels of hydrogen, and it was pretty accurate. But quantum mechanics really got started when people started to understand why the energy levels took on those values.

And that's what gets us into all the trouble, as soon as we frame the energy levels as what nature is really doing, we immediately have to wonder if nature is really unitary, and if it is, how can we get individual outcomes when we do experiments? I hear the door slamming shut again, when a swinging open door sounds like "that's one way to think about what is setting those energy levels, what other ways might also work?"

But I think it's ridiculous to demand that people not think about anything beyond predicting outcomes of experiments.

Yet we do have to have a definition of science so that we know when we are doing it, do we not? You can say you think we need a different definition of science, but we still need to know what differentiates science from all the other people who believe they know the things you believe you know just as fervently as you believe you know them.

 

[stevendaryl]

We are scientists, we don't believe things, we test them. We design syntactic structures that allow us to derive symbolically the semantic truths that we discover by experiment. We can marvel at the attributes of those syntactic structures, and claim an advanced understanding for having created them and tested them, but there is never any reason to believe them. Understanding does not come from belief, belief comes at the exact place where understanding has ended.

I think that's BS. Scientists are just people. They are just ordinary people. They believe things, sometimes for good reasons, and sometimes for silly reason. There's nothing that scientists are required or forbidden to do.

You could say that science is a certain way of exploring nature through formulating and testing hypotheses. But to formulate a good theory or to come up with a good test for an existing theory requires thinking about the theory. It's way too rigid to say: "When you're doing science, you can only think about certain things." A scientist can think about whatever he wants to think about. You can't tell him what to think.

I think you have to be careful about creating thought police to inflict on scientists.

 

[stevendaryl]

Is it not part of scientific investigation to be able to decide what is a scientific question, and what is not?

I think it's mostly BS, and not worth spending time on.

 

[Ken G]

Sure, but that also means one must object to all research in quantum gravity - it is not science, just interpretation.

And that objection is indeed raised! But note, there is still hope of generating scientific hypotheses using quantum gravity investigations. There is nothing wrong with looking for signposts to the next theory, and interpretations can do that. But note that is still all about the syntax of the interpretation, one simply takes that syntactic structure and extends it. That's what Einstein did to get from SR to GR, he learned some lessons from his own interpretations of the semantic truths represented in experiments like Michelson-Morley, translated them into a syntactic structure, and extended the form of that structure into the arena of gravity. That is all completely within the realm of "what interpretations are supposed to do", but none of that ever had to look like an argument about which interpretation is a correct description of what nature is actually doing. Not only is the latter pure psychology rather than physics, it's worse-- it's a fundamental misunderstanding of what physics does, and what a physics question looks like.

So I say more power to anyone who presents an argument like "if we interpret QM through the eyes of interpretation X, it suggests we do experiment Y, because we might get surprised by that outcome." Like Bell did. So we get "any interpretation that looks like Z doesn't work for experiment Y", but someone is then free to modify interpretation Z into Z' such that it still works. That's what Bohm did. That's all good stuff, it's using interpretations the way they are meant to be used, no debate around which one is what reality is actually doing because that's not even a physics discussion any more. The physicist should not expect his/her interpretations to represent their own semantic truths about reality, they should regard them as syntactic structures that can be used to derive semantic truths that are verifiable by experiment.

What I'm saying is that principles of physics theories are not semantic truths. So conservation of energy is not a semantic truth, it is a synactic structure that let's us predict the semantic truths we test. The idea that all observers will get the same result for the speed of light is not a semantic truth, it is a syntactic structure that let's us derive the semantic truth that is the Michelson-Morley experiment. But we are free to do an experiment tomorrow that violates either of those principles, all it means is that we need a new syntactic structure to get our semantic truths. So has it always been, throughout the history of science.

Sure one can say that. But then your belief in the existence of distant reality is just a belief. You cannot distinguish it experimentally from the nonexistence of distant reality. Yet you preferred one over the other.

I actually don't prefer a belief in a distant reality, I prefer that mode of understanding. I don't actually believe anything is true in some sense outside my understanding of it, I'm quite skeptical that all our modes of understanding are highly restricted by our limitations, but all I mean by the "truth" concept is how I understand things.

 

[atyy]

Read the literature and decide for yourself.

I recommend the discussion in http://arxiv.org/abs/quant-ph/0209123. Weinberg's quantum mechanics book is also very good https://www.amazon.com/dp/1107028728/?tag=pfamazon01-20. I think most will agree with those two recommendations. The one contentious recommendation is Ballentine, which most people like, but I think is somewhere between misleading and wrong.

Oh yes, and before you read those - read Landau and Lifshitz https://www.amazon.com/dp/0750635398/?tag=pfamazon01-20 - that's a real physics book - it does not shy away from interpretation, it states a practical view and its limitations clearly, as a basis for physics.

After Weinberg that you can also read Haag https://www.amazon.com/dp/3540610499/?tag=pfamazon01-20 - another real physics book that shows that interpretation is central to the thinking of great physicists.

 

[stevendaryl]

Oh yes, and before you read those - read Landau and Lifshitz https://www.amazon.com/dp/0750635398/?tag=pfamazon01-20 - that's a real physics book - it does not shy away from interpretation, it states a practical view and its limitations clearly, as a basis for physics.

After Weinberg that you can also read Haag https://www.amazon.com/dp/3540610499/?tag=pfamazon01-20 - another real physics book that shows that interpretation is central to the thinking of great physicists.

As I said in another post, you could make a plausible argument that it's probably a waste of time to worry too much about interpretations, just because it has been 90-something years and very little progress has been made toward coming up with an interpretation that satisfies everyone. You might as well move on to some topic where there's more of a chance of getting somewhere. But I think it's way too strong to say that no scientist should ever think about interpretations.

 

[Ken G]

I think that's BS. Scientists are just people. They are just ordinary people. They believe things, sometimes for good reasons, and sometimes for silly reason. There's nothing that scientists are required or forbidden to do.

I agree that scientists are also people, but they have a "scientist hat" they put on when they wish to be regarded as a scientific authority, and a "person hat" they put on when they just want to share their beliefs with everyone else. This is quite important, because when these hats get mixed up, we lose the authority of science and just become two people spouting their own faith systems. Of course people like creationists will always want to turn the debate into something of that form, but the scientist must not fall for the trick-- the scientist is only a scientist when they are actually doing science, and they only have a right to speak in a science classroom, or science TV show, etc., when they are wearing the right hat.

You could say that science is a certain way of exploring nature through formulating and testing hypotheses. But to formulate a good theory or to come up with a good test for an existing theory requires thinking about the theory. It's way too rigid to say: "When you're doing science, you can only think about certain things."

I never said what you can think about, and I certainly never said you shouldn't think about interpretations! I said you should understand what the thought process is that is actually happening there when you do think about interpretations. You should understand that the thought process is "I gain understanding A of experimentally verified semantic truth B by contemplating syntactic structure C." That's understanding what an interpretation is, and what science is, and you get everything that every scientist ever got-- and you never needed to say "and I can only get these things if I believe C is what nature is really doing."

I think you have to be careful about creating thought police to inflict on scientists.

It is the scientists who need to understand their own thoughts, they need to know when they are not doing science. It's fine if it is the scientists who decide this-- your model is only a problem if it comes from outside science, from people who do not understand science. But when scientists themselves lose track of what doing science actually is, this is a problem, and the way "the measurement problem" is sometimes framed is actually this problem.

 

[atyy]

And that objection is indeed raised! But note, there is still hope of generating scientific hypotheses using quantum gravity investigations. There is nothing wrong with looking for signposts to the next theory, and interpretations can do that. But note that is still all about the syntax of the interpretation, one simply takes that syntactic structure and extends it. That's what Einstein did to get from SR to GR, he learned some lessons from his own interpretations of the semantic truths represented in experiments like Michelson-Morley, translated them into a syntactic structure, and extended the form of that structure into the arena of gravity. That is all completely within the realm of "what interpretations are supposed to do", but none of that ever had to look like an argument about which interpretation is a correct description of what nature is actually doing. Not only is the latter pure psychology rather than physics, it's worse-- it's a fundamental misunderstanding of what physics does, and what a physics question looks like.

So I say more power to anyone who presents an argument like "if we interpret QM through the eyes of interpretation X, it suggests we do experiment Y, because we might get surprised by that outcome." Like Bell did. So we get "any interpretation that looks like Z doesn't work for experiment Y", but someone is then free to modify interpretation Z into Z' such that it still works. That's what Bohm did. That's all good stuff, it's using interpretations the way they are meant to be used, no debate around which one is what reality is actually doing because that's not even a physics discussion any more. The physicist should not expect his/her interpretations to represent their own semantic truths about reality, they should regard them as syntactic structures that can be used to derive semantic truths that are verifiable by experiment.

What I'm saying is that principles of physics theories are not semantic truths. So conservation of energy is not a semantic truth, it is a synactic structure that let's us predict the semantic truths we test. The idea that all observers will get the same result for the speed of light is not a semantic truth, it is a syntactic structure that let's us derive the semantic truth that is the Michelson-Morley experiment. But we are free to do an experiment tomorrow that violates either of those principles, all it means is that we need a new syntactic structure to get our semantic truths. So has it always been, throughout the history of science.

When you say "But we are free to do an experiment tomorrow that violates either of those principles, all it means is that we need a new syntactic structure to get our semantic truths. So has it always been, throughout the history of science." is the existence of the "we" that you refer to a semantic truth or syntactic structure?

Also, I don't think you are consistent on the issue of quantum gravity. First you say that being testable in principle is not enough, then you say that there is still hope for generating testable hypotheses from quantum gravity. Presumably you mean the hope is not scientific, it's just a belief and those who investigate that hope are not scientists?

At any rate, here is some work using two realist "interpretations" of quantum mechanics to make predictions - I put "interpretation" in quotes because these both say that quantum mechanics is probably only an effective theory, and will accordingly fail somewhere, just as GR probably will fail. I say that GR will "probably" fail because it has not been ruled out that GR is Asymptotically Safe, just as we have not ruled out Many-Worlds as a coherent possibility.
http://arxiv.org/abs/0805.0163
http://arxiv.org/abs/1306.1579
http://arxiv.org/abs/1405.2868

 

[Sugdub]

under what circumstances does the wavefunction collapse?

I can't see any empirical evidence for the so-called "collapse" of the state vector. For me, the “collapse" is just an unavoidable “built-in” side effect of QM interpretations which assign the state vector as a property of the “preparation” (or of any “system” or “ensemble of systems” that it delivers), whilst the theoretician can set arbitrarily the boundary between the “preparation” and the “measurement apparatus”. Different values of the state vector are then assigned to different locations inside the device of a single experiment. Hence the belief whereby a dis-continuous physical transformation occurs inside the experimental device, which is then reflected into a discontinuous evolution of the state vector. But there is NO empirical support to such a belief, since it is not legitimate to assign two values of the state vector to the same experiment.

Let's start from the experimental evidence. Somehow the “state vector” deals with the reproducible statistical distribution of a flow of discrete events produced by an experiment. Different values of the state vector can only be obtained through performing different experiments entailing different device setup. Therefore the assignment of more than one value of the state vector to a single experiment goes far beyond the empirical evidence, it cannot be subjected to experimental verification, it is necessarily part of an “interpretation”, leading to metaphoric statements, beyond knowledge. Whether the “state vector” gets assigned as a “property” of a “preparation” (i.e. a subset of the experimental device) or of a discrete “system” (that gets exhibited somewhere in front of the “measurement apparatus”) or of an “ensemble” of such “identically prepared systems”... does not make any difference: different values of the state vector are then assigned to different locations inside the experimental device between the “source” and the “measurement apparatus”, hence the metaphoric view whereby a “hidden” physical process occurs there which effects the state of “something” in the world and gets reflected into the “collapse” of the state vector.

There is no empirical support to the existence of a disruptive physical process acting inside the experimental device during the experiment. Indeed physicists have the right to invent a “hidden” physical process and to explore the consequences of this hypothesis, but they can't forget that it is pure speculation. There is only one factual statement: a discontinuous change of the experimental setup (such as the addition of a new SG analyser) can produce a discontinuous change of the observed statistical distribution and gets reflected into a discontinuous change of the state vector. So the (dis)continuous change of the state vector takes place in a configuration space which traces the evolutions of the observed statistical distribution in response to various (dis)continuous changes of the experimental setup. It does not take place in the 3D physical space and neither in time during the experiment.
Please comment.

 

[Ken G]

When you say "But we are free to do an experiment tomorrow that violates either of those principles, all it means is that we need a new syntactic structure to get our semantic truths. So has it always been, throughout the history of science." is the existence of the "we" that you refer to a semantic truth or syntactic structure?

The "we" concept is a complex notion, not a simple semantic truth or syntactic structure, and not strictly a physics notion either-- nor do I invoke any scientific authority when I invoke that term, indeed I know no more about that term than a person who has never sat in a physics class in their life. There are many things we do not have physics for, that's one of the reasons that the physicist is not yet in the physics. We use semantic truths and syntactic structures when we do physics, but that doesn't mean we don't also invoke more complex notions when we need them. We couldn't even get out of bed in the morning if all we had to go on was physics!

Also, I don't think you are consistent on the issue of quantum gravity. First you say that being testable in principle is not enough, then you say that there is still hope for generating testable hypotheses from quantum gravity. Presumably you mean the hope is not scientific, it's just a belief and those who investigate that hope are not scientists?

A scientist can have hope, when did I say they cannot? Their hope is not their science, however-- they have a hope their approach will lead them to a scientific result that will not be just another footnote to history. But whether it will or not will be tested by science, not by hope or belief.

At any rate, here is some work using two realist "interpretations" of quantum mechanics to make predictions - I put "interpretation" in quotes because these both say that quantum mechanics is probably only an effective theory, and will accordingly fail somewhere, just as GR probably will fail. I say that GR will "probably" fail because it has not been ruled out that GR is Asymptotically Safe, just as we have not ruled out Many-Worlds as a coherent possibility.
http://arxiv.org/abs/0805.0163
http://arxiv.org/abs/1306.1579
http://arxiv.org/abs/1405.2868

Yes, I have no issue with using an interpretation to guide scientific investigations, and I am quite sympathetic to the idea that all physics theories are "effective" theories.

 

[atyy]

Yes, I have no issue with using an interpretation to guide scientific investigations, and I am quite sympathetic to the idea that all physics theories are "effective" theories.

What I don't get about your position is: if quantum mechanics is an effective theory, then there is a problem - quantum mechanics is incomplete.

The measurement problem asks - can we understand quantum mechanics now, in its present form with a Heisenberg cut as a complete theory, or the most complete theory possible? The Bohmian interpretation, and other realist interpretations are that we cannot understand quantum mechanics as a complete theory, but we can understand it as an effective theory. Are you saying that we can understand quantum mechanics as a complete theory, or the most complete possible, and there is possibly no effective theory underlying it? That is coherent, but I think you asserted both that there is no measurement problem (ie. quantum mechanics is complete) and that quantum mechanics is an effective theory. Why would one think quantum mechanics is an effective theory if quantum mechanics is conceivably complete?

 

[Ken G]

What I don't get about your position is: if quantum mechanics is an effective theory, then there is a problem - quantum mechanics is incomplete.

True, but that problem comes with the territory of science, so in some sense it's not a problem-- it's just science. Sure we always want better science, so we have a "problem", but our problem is not that our theories are effective theories, our problem is that we have more to learn. But isn't that why we do science in the first place? So there's nothing wrong when science has more to learn.

The measurement problem asks - can we understand quantum mechanics now, in its present form with a Heisenberg cut as a complete theory, or the most complete theory possible? The Bohmian interpretation, and other realist interpretations are that we cannot understand quantum mechanics as a complete theory, but we can understand it as an effective theory. Are you saying that we can understand it as a complete theory, or the most complete possible, and there is possibly no effective theory underlying it?

I'm saying we shouldn't even try to understand quantum mechanics, or any physics theory, as a "complete theory." That is not something we test, it's not an attribute of any theory. It's certainly not a semantic truth, because it is not testable, and it cannot be part of the syntactic structure of any theory, any more than there can be an axiom of arithmetic that says arithmetic is complete. Indeed, as I'm sure you know, Godel proved that putting an axiom like that into arithmetic is guaranteed to make it inconsistent. What I want to know is, from where comes this fascination to regard any physics theory as a complete theory? None has ever been, yet countless times the proponents of various theories expected them to be. I think at some point we step back and learn our lesson as to what physics really does, and what it does not do.

That is coherent, but I think you asserted both that there is no measurement problem and quantum mechanics is an effective theory. Why would one think quantum mechanics is an effective theory if there is no problem to be solved?

There is no measurement problem because quantum mechanics is an effective theory. The "measurement problem" is the question "which interpretation is what nature actually does, such that quantum mechanics plus that interpretation is a complete description of reality." That's not a scientific problem, because a proper framing of science is not about that, it's about creating effective theories like all the other theories of science.

Indeed, I would ask, since there is no measurement problem in any of the interpretations of quantum mechanics, because those interpretations are all designed to remove the measurement problem, then how can we say there's a measurement problem at all, unless what we are really asking is which one of them is what nature is actually doing? But answering that is not what interpretations do, interpretations merely provide a syntactic structure that let's you formalize elements of the theory that are otherwise ad hoc "rules of thumb" with the flavor of "do this because it works" when in the hands of practicing physicists. Those rules of thumb are what I mean by a semantic truth, something we find true by experience but we don't know why it's true, it cannot be formally proven true. So we construct a syntactic scaffolding around that thing we wish to regard as a truth, and that let's us prove it formally, which makes it a syntactic truth instead of a semantic one.

But if we are not happy with that state of affairs, and want to be able to interpret the syntactic truths of the scaffolding as their own semantic truths (things that are true facts of nature), then we need a scaffolding around those truths so that they can be syntactically derived. When we start building interpretations of our interpretations, and experimental results have been left far behind, then we have really lost sight of where science lays its anchors.

 

[atyy]

There is no measurement problem because quantum mechanics is an effective theory.

Well, those are the exact words I would use in my answer, but I don't know if we mean the same thing by the same words. So let me ask: would there be a measurement problem if quantum mechanics were not an effective theory?

 

[Ken G]

That's hard to answer, because it is basically asking if quantum mechanics wasn't quantum mechanics, and physics wasn't physics, and humans weren't humans, would there be a measurement problem? But I think you are asking if quantum mechanics was what nature is actually doing, and one of its interpretations was actually the complete truth, would there be a measurement problem? I would have to say "no" to that, because the "true interpretation" has done away with the apparent problem of the "Heisenberg cut." In that case, we would have eliminated the Copenhagen and ensemble interpretations, because those interpretation more or less assert that quantum mechanics is by its nature an effective theory (in different ways, but both say it's effective because it doesn't say how collapse happens after decoherence has set the stage). If Many-Worlds were "true", then there's no measurement problem because it is the nature of reality that each consciousness cannot know all of it (so quantum mechanics is not an effective theory, but scientists are ineffective witnesses). If Bohm were "true", then there's no measurement problem because the "classical world" we imagined pre-quantum is actually quite right, but there are additional "spooky" elements to it that we did not appreciate before. But I'm saying that the reason there is no measurement problem is because it is not actually a "problem" that quantum mechanics is an effective theory, as "effective theory" is redundant word use.

 

[atyy]

That's hard to answer, because it is basically asking if quantum mechanics wasn't quantum mechanics, and physics wasn't physics, and humans weren't humans, would there be a measurement problem? But I think you are asking if quantum mechanics was what nature is actually doing, and one of its interpretations was actually the complete truth, would there be a measurement problem? I would have to say "no" to that, because the "true interpretation" has done away with the apparent problem of the "Heisenberg cut." In that case, we would have eliminated the Copenhagen and ensemble interpretations, because those interpretation more or less assert that quantum mechanics is by its nature an effective theory (in different ways, but both say it's effective because it doesn't say how collapse happens after decoherence has set the stage). If Many-Worlds were "true", then there's no measurement problem because it is the nature of reality that each consciousness cannot know all of it (so quantum mechanics is not an effective theory, but scientists are ineffective witnesses). If Bohm were "true", then there's no measurement problem because the "classical world" we imagined pre-quantum is actually quite right, but there are additional "spooky" elements to it that we did not appreciate before. But I'm saying that the reason there is no measurement problem is because it is not actually a "problem" that quantum mechanics is an effective theory, as "effective theory" is redundant word use.

Sure, no disagreement there.

 

[Ken G]

OK, so if you agree that "effective theory" is redundant, then you must feel the value of Bohmian mechanics is not that it allows us to reframe quantum mechanics as a complete theory of nature, but rather simply that it provides a signpost for a direction to a new theory that might one day be confirmed to make different predictions than quantum mechanics? In that case, there's no problem, because it will be those future observations that decide the issue, and the only discussion we could have now is what are the right observations to do to test this hypothesis.

 

[atyy]

OK, so if you agree that "effective theory" is redundant, then you must feel the value of Bohmian mechanics is not that it allows us to reframe quantum mechanics as a complete theory of nature, but rather simply that it provides a signpost for a direction to a new theory that might one day be confirmed to make different predictions than quantum mechanics? In that case, there's no problem, because it will be those future observations that decide the issue, and the only discussion we could have now is what are the right observations to do to test this hypothesis.

Yes, but not just that - if someone says that quantum mechanics is a fundamentally new type of theory, and that the "true interpretation" must be the Copenhagen interpretation, because it is impossible to construct any theory of which quantum mechanics is an effective theory - then theories like Bohmian Mechanics show that that need not be true. In other words, I can actually back up my statement that "The measurement problem can be solved by considering quantum mechanics as an effective theory" by providing examples of theories of which quantum mechanics might be an effective theory.

For me, I would prefer to say the measurement problem exists and has been at least partially solved, rather than that it does not exist. The reason is that even if all theories are effective theories, they divide (as a matter of practice) into two types. The first type of effective theory does not reveal any obvious incompleteness in itself (even if you use Goedel's incompleteness theorem, a theory that is subject to it need not be obviously incomplete, because all Goedel sentences are of the form "for all ...", whereas an experiment usually tests a statement of the form "there exists ..."). So usually we only know how to falsify this first type of effective theory by experiment. The second type of effective theory reveals an obvious incompleteness in itself, and provides a theoretical opportunity - a signpost to new physics - as you say. Quantum general relativity (if it is not asymptotically safe) is one such theory. Newtonian gravity and Maxwell's equations were another such theory. And I think the measurement problem indicates that quantum mechanics also belongs in this second class of effective theories which signal their own incompleteness, even before any experiments have falsified them.

In both quantum general relativity and quantum mechanics, since we have not ruled out that quantum general relativity is asymptotically safe nor have we ruled out Many-Worlds as a possible interpretation, it remains possible that they are effective theories of the first type.

 

[Ken G]

Yes, but not just that - if someone says that quantum mechanics is a fundamentally new type of theory, and that the "true interpretation" must be the Copenhagen interpretation, because it is impossible to construct any theory of which quantum mechanics is an effective theory - then theories like Bohmian Mechanics show that that need not be true. In other words, I can actually back up my statement that "The measurement problem can be solved by considering quantum mechanics as an effective theory" by providing examples of theories of which quantum mechanics might be an effective theory.

I agree with that, I just don't think it matters if there is a theory that quantum mechanics is an effective theory of-- because physics theories are always effective theories. But that only means that your point has no traction with me, it certainly would have traction with anyone who thinks quantum mechanics is like a "dead end" sign. I just can't see that view, why would this one theory, out of the long list in our advancing understanding, be the dead end? I tend to think the differences between quantum mechanics and classical physics are highly overblown, as would a Bohmian, but I may think that for a different reason: I think classical physics had the same "problems that are not really problems unless we take the theory too literally" that quantum mechanics does, whereas a Bohmian tends to think neither one has a problem being taken literally.

For me, I would prefer to say the measurement problem exists and has been at least partially solved, rather than that it does not exist. The reason is that even if all theories are effective theories, they divide (as a matter of practice) into two types. The first type of effective theory does not reveal any obvious incompleteness in itself (even if you use Goedel's incompleteness theorem, a theory that is subject to it need not be obviously incomplete, because all Goedel sentences are of the form "for all ...", whereas an experiment usually tests a statement of the form "there exists ..."). So usually we only know how to falsify this first type of effective theory by experiment. The second type of effective theory reveals an obvious incompleteness in itself, and provides a theoretical opportunity - a signpost to new physics - as you say.

Yes, I agree with that basic dichotomy, though I think it's odd that many people see the larger problem as the second type! Instead, I see that second type of "bug" as a feature. A theory that gives you no clue when it breaks down just beguiles you into thinking it might not be an effective theory, but of course it is, and gives you no guidance for how to explore that. A theory that gives you a clear signpost as to where it is difficult to regard as anything but an effective theory is a good thing, because it does give guidance to the next theory. What's more, it is not necessary to frame the search for the next theory as a search for the complete theory, it is only necessary to build a syntactic scaffolding around the semantic truths that are essentially "rules of thumb" in the one theory (and all theories have these, they are the places you can ask "but why is..."), such that you can derive those semantic truths from the syntax of the interpretation. Then you extend that syntax to new semantic truths that have not been observed yet, and voila, there's your guidance to the new theory (general relativity is a perfect example of this approach). At no point does any of that have to look for a search for a theory that is not an effective theory, and I would call it an error to frame it as such.

Quantum general relativity (if it is not asymptotically safe) is one such theory. Newtonian gravity and Maxwell's equations were another such theory. And I think the measurement problem indicates that quantum mechanics also belongs in this second class of effective theories which signal their own incompleteness, even before any experiments have falsified them.

I agree, but I take that as my default stance about all theories, so I don't need the theory to tell me it is of the second type. All that matters is whether the theory is giving us guidance as to where to look for how to advance to the next theory, and it's hard to know that until we try. Winners write the history on this! If the Bohmian approach provides true guidance to the next theory, everyone will say it was obvious that quantum mechanics had this Heisenberg cut problem, and they eventually got the better theory by rejecting that cut. But if the Bohmian predictions don't work out, it will be forgotten that anyone ever suggested they would!

For my own part, I feel that even if you are right that the Bohmian approach can improve quantum mechanics, it would represent a relatively minor breakthrough in its technological and predictive power, and it would have the unfortunate psychological consequence of making people, once again, think that classical pictures are what reality is actually doing. I feel the next really big breakthrough in physics won't come until we put the physicist into the physics, because that is what will significantly advance how we frame physics itself, rather than being something of a throwback to how physics was framed in Newtonian times. So I'm rather hoping that the Bohmian approach doesn't work out, because then it will force us forward rather than backward, into realizing that our job is not only to build better physics theories, but also to build a more coherent and comprehensive description of what physics itself is trying to do. Of course, I have no idea what the observations will actually end up showing is the next set of semantic truths that we must grapple with, that's the real fun of physics.

In both quantum general relativity and quantum mechanics, since we have not ruled out that quantum general relativity is asymptotically safe nor have we ruled out Many-Worlds as a possible interpretation, it remains possible that they are effective theories of the first type.

Yes, and if that remains the "end of the story", it would be even worse than if Bohmian mechanics turned out to be right! That would be the worst case of all-- for the present state of affairs, where we have many worlds and multiverses and machinery that is flexible enough to explain anything you want without any chance of ruling it out, to continue indefinitely.