I QFT made Bohmian mechanics a non-starter: missed opportunities?

[Demystifier]

Isn't the point of measurement to figure out what nature is doing all the time? What makes a measurement special such that nature would behave differently? The standard CI inspired approach is deeply dissatisfying. Is there another scientific field that has this approach to measurement, other than psychology or sociology?

Those are indeed excellent questions! Most scientists would like to know what nature is doing all the time. However, the measurement by itself, obviously, cannot say what is nature doing in the absence of measurement. This is why science also has theories. The theories are guided by experiments, but they are really extrapolations from experiments. It is the scientific theories (or at least some of them) that tell us something about what is nature doing all the time.

 

[vanhees71]

This doesn't make sense. Science is about the objectively observable facts about Nature not some fictitious behavior of Nature, when it's "not observed". You can claim anything about this. To verify it, you have to observe it, and then it's not "not observed" any longer. Of course, philosophers love such oddities ;-)).

 

[gentzen]

Isn't the point of measurement to figure out what nature is doing all the time? What makes a measurement special such that nature would behave differently?

Neither measurements nor nature is special. What is special is that you have a theory whose predictions consists mostly of statistics. Like I explained to vanhees71 earlier in this thread:

I guess your basic problem is that you don't understand why you have to first fix a context before applying statistics. Many scientists in the "softer sciences" ran into the practical consequences of this misunderstanding. The currently favored solution is to preregister (i.e. fix a context for) studies that would risk to get into trouble with this.

In the end, you need stuff which can be reliably described and reproduced. At least, if the results of your experiment consists mostly of statistics.

 

[vanhees71]

Of course the context of the statistics is given by the measurement device used to measure some observable(s). That's what I always try to make clear: Nature doesn't consist of a rigged Hilbert space and self-adjoint operators!

 

[martinbn]

No, standard QM does not describe nature in the absence of measurement. Neither in a probabilistic sense (because the Born rule in arbitrary basis is only valid when an observable is measured, it cannot be universally valid due to the contextuality theorems), nor in a deterministic sense (Schrodinger equation is deterministic, but standard QM insists that nature is not deterministic).

Indeed, adherents of standard QM often emphasize that a physical theory should not describe nature in the absence of measurement, because any such description would necessarily be metaphysical. This fact (that standard QM does not describe nature in the absence of measurement) they see as a strength of standard QM, not as its weakness.

Come on! Standard QM says that each QM system is described mathematically by a wave function, which in the absence of measurement evolves according to the Schrodinger equation.

 

[martinbn]

Can you be more precise? What exactly is missing in these models to be more than "wishful thinking"?

I can, but you would have to give a specific paper.

 

[martinbn]

In physics symmetry refers to the laws (equations of motion), not to particular solutions.

But BM includes equations for the trajectories, right?

 

[gentzen]

But BM includes equations for the trajectories, right?

Violation of symmetry typically leads to violation of associated conservation laws. I strongly doubt that you will find violation of angular momentum convervation in BM. But for the symmetries that BM actually violates, you also find violations of some exact conservation laws, so that you are left with conservation only on average.

 

[Demystifier]

Come on! Standard QM says that each QM system is described mathematically by a wave function, which in the absence of measurement evolves according to the Schrodinger equation.

And what happens with the wave function at the time of measurement? Collapse? But collapse is non-local, and yet standard QM insists that Nature is local. Do you see a contradiction?

 

[Demystifier]

But BM includes equations for the trajectories, right?

Yes, and these equations have symmetries that I mentioned.

 

[Quantum Waver]

This doesn't make sense. Science is about the objectively observable facts about Nature not some fictitious behavior of Nature, when it's "not observed". You can claim anything about this. To verify it, you have to observe it, and then it's not "not observed" any longer. Of course, philosophers love such oddities ;-)).

Why would the observable facts of nature change just because we stopped observing? Shouldn't they remain the same, given all the observed regularities not just on Earth but throughout the cosmos? There's all sorts of things happening all the time that go unobserved, but we have a good scientific understanding for how stars are fusing hydrogen into helium far away or ants are building ant hills deep in the woods. It all hangs together. You slam the breaks and feel the inertia of all the mass in the universe acting on you.

 

[gentzen]

I can, but you would have to give a specific paper.

I can lookup the specific paper (or papers) which caused me to start this thread:

Even classically, we talk about the electromagnetic field instead of saying that a particle here interacts with a particle there. So the question should rather be whether we favor photons over configurations of an electromagnetic field to just keep things local.

I read in "Do We Really Understand Quantum Mechanics?" by Franck Laloë that a version of Bohmian mechanics using field configuration trajectories for the electromagnetic field and particle trajectories for Fermions (with stochastic creation and annihilation events) works actually quite well. One of the main drawbacks of Bohmian mechanics (including this version) is its non-locality, so the answer to the adjusted question about keeping things local could actually be yes, in a certain sense.

Maybe you know those anyway, so I don't need to select some specific paper, or decide which was the paper with the initial proposal, and which were papers only ment for people like me that "sometimes need to update their knowledge" before they can continue a discussion.

 

[Quantum Waver]

Of course the context of the statistics is given by the measurement device used to measure some observable(s). That's what I always try to make clear: Nature doesn't consist of a rigged Hilbert space and self-adjoint operators!

Then what does nature consist of? What is the mathematics describing and how is it predictive?

 

[gentzen]

Why would the observable facts of nature change just because we stopped observing?

Are you sure that you are not arguing against a strawman here? Einstein accused his opponents of that mindset, but I have yet to see somebody who really takes that position.

 

[gentzen]

Then what does nature consist of?

What sort of answer do you expect? An answer like Descartes dualism, that you can "prove" that you yourself exists, and then hope that somehing similar can be proved about the "external world" too?
Maybe have a look at my answer
https://philosophy.stackexchange.co...rgo-sum-that-we-can-be-certain-of/14461#14461
and especially try to think about what I wanted to communicate with that answer.

 

[Quantum Waver]

Are you sure that you are not arguing against a strawman here? Einstein accused his opponents of that mindset, but I have yet to see somebody who really takes that position.

I may not be phrasing it generously. What I'm arguing against is that the wave equation is just a predictive tool which doesn't describe what's happening to the unobserved system, because science can't say what happens in between observations. I disagree, because science offers explanations for all sorts of things happening around us which may go unobserved, like the chemistry in our bodies or solar radiation.

There are anti-realists who argue that unobservables are theoretical fictions used to make models work, and models are predictive of experimental results, not descriptive of nature.

 

[gentzen]

because science can't say what happens in between observations. I disagree, because science offers explanations for all sorts of things happening around us which may go unobserved.

I agree that QM also makes prediction which are not purely statistical. But those are not predictions about a naive "external world" either. So we test QM mostly with experiments that can only be interpreted statistically, but we deduce from this that also the non-statistical predictions of QM are reliable.

 

[Quantum Waver]

What sort of answer do you expect?

A physics answer like MWI or BM. One that's realist about the wave equation. I'm not really following your ontological commitment answer. I would say science is a guide to ontological commitments, within the limits of the scientific method.

 

[martinbn]

And what happens with the wave function at the time of measurement? Collapse? But collapse is non-local, and yet standard QM insists that Nature is local. Do you see a contradiction?

Why are you changing the subject! Is standard textbook QM a mathematical description or not?

 

[gentzen]

A physics answer like MWI or BM. One that's realist about the wave equation. I'm not really following your ontological commitment answer. I would say science is a guide to ontological commitments, within the limits of the scientific method.

I disagree with the naive conclusion of 'cogito ergo sum' as a proposition proved beyond doubt in some absolute sense, or otherwise of exceptional status. I suggested that it may just be a normal case of ontological commitment. (I later gave another more charitable answer that does grant some special status to it.)

With respect to Descartes, his dualism was a huge part of the revolution in mathematics and the sciences more general. But in the end, his dualism is probably not correct, and at some point it started to limit further scientific progress.

 

[gentzen]

Why are you changing the subject! Is standard textbook QM a mathematical description or not?

Some textbooks like Griffiths teach a literal collapse interpretation which is indeed non-local. It is true that he explains his (pedagocial) reasons for doing so in the book itself, but it still means that the simplest "calculation first" version of (nonrelativistic) QM is non-local.

You have to work much harder to avoid that straightforward non-locality, and risk that only very few people will understand what you are trying to communicate.

 

[martinbn]

Some textbooks like Griffiths teach a literal collapse interpretation which is indeed non-local. It is true that he explains his (pedagocial) reasons for doing so in the book itself, but it still means that the simplest "calculation first" version of (nonrelativistic) QM is non-local.

You have to work much harder to avoid that straightforward non-locality, and risk that only very few people will understand what you are trying to communicate.

But we are not diacussing locality! We are disagreeing on wether QM is a mathimatical discription of nature.

 

[gentzen]

But we are not diacussing locality! We are disagreeing on wether QM is a mathimatical discription of nature.

I agree that nonrelativistic QM is a mathematical description of many important features of nature, which goes far beyond classical physics. I don't want to deny QFT a similar status, but I am not expert enough in that subject to judge for myself.

 

[Structure seeker]

This doesn't make sense. Science is about the objectively observable facts about Nature not some fictitious behavior of Nature, when it's "not observed". You can claim anything about this. To verify it, you have to observe it, and then it's not "not observed" any longer.

Nothing is objectively observable by humans, it is only independently of researcher, time and place observable. And claims aren't proven by verification, they are made reasonable by passing multiple falsification possibilities again and again.

 

[vanhees71]

And what happens with the wave function at the time of measurement? Collapse? But collapse is non-local, and yet standard QM insists that Nature is local. Do you see a contradiction?

The wave function evolves according to all interactions of the system described by it, including its interaction with the measurement device. At this moment the time evolution is of course not described any longer by the wave function but by the corresponding reduced density matrix since now the system is an open system, and the time evolution is rather described by master equations than unitary time evolution. For me the best description is via the Kadanoff-Baym equations, but there are many more equations, using other and different approximation schemes like the Lindblad equations (with a lot of trouble like the lack of the correct thermalization in the long-time limit) describing a Markov approximation etc. etc.

 

[vanhees71]

I agree that nonrelativistic QM is a mathematical description of many important features of nature, which goes far beyond classical physics. I don't want to deny QFT a similar status, but I am not expert enough in that subject to judge for myself.

Of course both non-relativistic QM and relativistic QFT are mathematical descriptions of most important features of nature.

 

[Demystifier]

Why are you changing the subject! Is standard textbook QM a mathematical description or not?

Fair enough! I said that Bohmians assume that mathematical description of nature in the absence of measurement is possible. You then said that standard QM also makes such a description, in the form of wave function that evolves deterministically in the absence of measurement, tacitly implying that it collapses upon measurement, and noted that Bohmians are not satisfied with that description. You are absolutely right, that's a description and Bohmians are not satisfied with it. The reason why they are not satisfied is because this description is mathematically incomplete, in the sense that it does not contain a mathematical definition of measurement. So to make the long story short, it is a mathematical description, but it is an incomplete mathematical description. Bohmians want a complete one.

 

[vanhees71]

How is a mathematical description of nature possible "in absence of measurement"? What we can mathematically describe are quantified phenomena, i.e., phenomena which can be described by numerical data, and the mapping of properties of natural phenomena to numbers involves an operational definition of observables, i.e., some measurement procedure. Even on the very elementary level of classical mechanics that's the case. In some sense you can say modern physics starts with defining lengths and times by measurement procedures to describe the motion of a body in space as a function of time. The construction of such measurement devices ("clocks and compasses/rulers") of course also needs some hypothetical theory about space and time, which in Newtonian mechanics is absolute time and Euclidean space.

 

[martinbn]

Fair enough! I said that Bohmians assume that mathematical description of nature in the absence of measurement is possible. You then said that standard QM also makes such a description, in the form of wave function that evolves deterministically in the absence of measurement, tacitly implying that it collapses upon measurement, and noted that Bohmians are not satisfied with that description. You are absolutely right, that's a description and Bohmians are not satisfied with it. The reason why they are not satisfied is because this description is mathematically incomplete, in the sense that it does not contain a mathematical definition of measurement. So to make the long story short, it is a mathematical description, but it is an incomplete mathematical description. Bohmians want a complete one.

This is more belief than what you addmitted initially. Also, no matter what mathematical description you have there will be undefined terms like points and lines in Euclidean geometry. But no one is unhappy about geometry.

 

[Demystifier]

Also, no matter what mathematical description you have there will be undefined terms like points and lines in Euclidean geometry. But no one is unhappy about geometry.

Sure, there is always something undefined, no theory is absolutely complete. But some theories are "more complete" than some other theories. Theorists search for new theories that should be more complete than the old ones. A set theorist may be unhappy with geometry in its usual form and define points and lines in terms of sets, but even in set theory there are objects which are not defined. Likewise, someone working in quantum foundations may be unhappy with the usual formulation of QM and define some standard QM concepts in terms of new concepts that seem more fundamental. All that is completely normal for theorists.

 

[gentzen]

You are absolutely right, that's a description and Bohmians are not satisfied with it. The reason why they are not satisfied is because this description is mathematically incomplete, in the sense that it does not contain a mathematical definition of measurement.

So it turns out that I am definitively not a Bohmian. But I find it instructive nevertheless to try to understand the mathematical and physical properties of BM.

 

[Structure seeker]

What we can mathematically describe

Presupposing what we can and cannot mathematically describe is exactly the problem. One thing the CI doesn't describe is what happens at wavefunction collapse. Then it is like a gravity model that describes everything except black holes. Bohmians want to understand wavefunction collapse as well.

It's just that IMO the idea that a particle is hiding somewhere in the wave before measurement reduces the geniously devised quantum field waves to background influence rather than these waves being the actual form of the particle before measurement. I'd like these geniously devised fields to be accessible rather than an ethereal background

 

[Demystifier]

The wave function evolves according to all interactions of the system described by it, including its interaction with the measurement device. At this moment the time evolution is of course not described any longer by the wave function but by the corresponding reduced density matrix since now the system is an open system, and the time evolution is rather described by master equations than unitary time evolution.

But the open system is a part of the larger closed system, and the full closed system should be described by the wave function.

 

[Demystifier]

So it turns out that I am definitively not a Bohmian. But I find it instructive nevertheless to try to understand the mathematical and physical properties of BM.

Fair enough. What's your favored interpretation of QM?

 

[Demystifier]

Science is about the objectively observable facts about Nature

No, that's just your wishful thinking, to use the favored phrase of @martinbn. In reality, science is much more than that. It's an empirical fact that scientific journals publish a lot of research which are not only about observable facts. After all, the Bohm's paper is published in one such journal (Physical Review). By insisting that science is only about observable facts you deny observable facts.

 

[martinbn]

No, that's just your wishful thinking, to use the favored phrase of @martinbn. In reality, science is much more than that. It's an empirical fact that scientific journals publish a lot of research which are not only about observable facts. After all, the Bohm's paper is published in one such journal (Physical Review). By insisting that science is only about observable facts you deny observable facts.

What is your definition of science? What is published in journals?

 

[Demystifier]

What is your definition of science? What is published in journals?

Science is any research the results of which is publishable in scientific journals. To avoid circularity, scientific journal is any journal that the society proclaims to be so.

 

[martinbn]

Science is any research the results of which is publishable in scientific journals. To avoid circularity, scientific journal is any journal that the society proclaims to be so.

That is too restrictive, what about the times before journals. It is also too generous, there is a lot of cr@p that gets published.

 

[Demystifier]

That is too restrictive, what about the times before journals.

My definition refers to present times.

It is also too generous, there is a lot of cr@p that gets published.

Crap science is also science.

But I admit that my definition of science is not perfect. Do you have a better one?

 

[Demystifier]

How is a mathematical description of nature possible "in absence of measurement"? What we can mathematically describe are quantified phenomena, i.e., phenomena which can be described by numerical data, and the mapping of properties of natural phenomena to numbers involves an operational definition of observables, i.e., some measurement procedure.

The gauge potential is mathematical, it is even expressed in physical units, and yet the gauge potential is not measurable.

 

[gentzen]

Fair enough. What's your favored interpretation of QM?

I guess in the end I am still an instrumentalist, just like I was in Jan '17 when I reacted with huge anger against Steven Weinberg's "The Trouble With Quantum Mechanics" and his unfair characterization of instrumentalism.

I don't remember exactly when I learned about the Hanbury Brown effect (from Feynman's QED book), at least I know that in Oct '19 I tried to recall the name of that effect after "I talked with a colleague about interpretation of mathematics and physics, and I claimed that mathematics must secretly always be interpreted instrumentalistically. He said that this would be the same for physics, so I tried to remember the name of that effect"

which shows that my preferred instrumentalistic interpretation of QM is insufficient, and that there is some reality of the wavefunction with observable effects (a systematic pattern instead of mere noise) deep below the predictions of my instrumentalistic interpretation.

Maybe I could fix this specific defect, maybe not. But in the end it is also part of the instrumentalistic philosophy that I don't care too much that my intuitions and "interpretation miniatures" are not fully consistent, and that it may be impossible to make them fully consistent, even if I tried.

After the death of Steven Weinberg, I learned more about his motivation(s) for his attack against instrumentalism, and that he even had the audacity to compute "relaxation times" for specific scenarios in his QM textbook, and suggest that them being much bigger than the age of the universe had consequences like ... well, I don't want to dive into that sort of discussion here.
Let me just say that he cared deeper about that stuff than I do.

 

[Demystifier]

I guess in the end I am still an instrumentalist

So I guess you have seen my "Bohmian mechanics for instrumentalists".

"interpretation miniatures"

I guess you have seen that one too.

 

[vanhees71]

The gauge potential is mathematical, it is even expressed in physical units, and yet the gauge potential is not measurable.

So?

 

[Demystifier]

So?

So it answers your question "How is a mathematical description of nature possible in absence of measurement?".

 

[vanhees71]

No, it doesn't. The physical meaning of the electromagnetic field is given operationally, i.e., by giving a relation between what's described by the mathematics to quantitatively observable phenomena. The electromagnetic potentials themselves are not directly observable. They are not even uniquely determined by a given physical phenomenon but only "modulo arbitrary gauge transformations". They can be used, of course, to derive observables like the electromagnetic field.

 

[Morbert]

@vanhees71 everyone presumably agrees that physical theories have to ultimately make contact with measurement. But this does not mean measurement must have a foundational significance, with no ontic conceptualization preceding it.

 

[vanhees71]

What ontic conceptualization do you have in mind?

 

[Morbert]

What ontic conceptualization do you have in mind?

E.g. Instead of interpreting a quantum theory as a theory of measurement outcomes. We might first interpret it as a theory of possible events and their likelihoods, and then identify measurement outcomes with correlations between certain events.

 

[vanhees71]

Sure, you can use different words for observables, and call them events. What does this change?

 

[Morbert]

Not all events would be measurement events though. We would give significance to the an event like a microscopic system's angular momentum obtaining some value, even if no measurement is made.

As for what changes, my opinion is that probably not much changes. Instrumentalism is a defensible position for a quantum physicist even if it is not for a paleontologist. As a foundational project, it is probably the most complete.

At the same time it's also my opinion that it doesn't fully capture the modes of thinking quantum chemists and applied quantum physicists use in their day-to-day. Consistent histories seems to better capture convention here, even if the number of applied physicists who identify as CH proponents is approximately 0.