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How I Stopped Worrying and Learned to Love Orthodox Quantum Mechanics

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Many people here know that I am a “Bohmian”, i.e. an adherent of a very non-orthodox interpretation of quantum mechanics (QM). Indeed, in the past I have published a lot of papers on Bohmian mechanics in peer reviewed journals from 2004 to 2012. So how can I not worry and love orthodox QM? As a “Bohmian”, shouldn’t I be strictly against orthodox QM?

No. If, by orthodox QM, one means instrumental QM (which is well explained in the book “Quantum Theory: Concepts and Methods” by A. Peres), then orthodox QM is fully compatible with Bohmian QM. I am not saying that they are equivalent; indeed Bohmian QM offers answers to some questions on which instrumental QM has nothing to say. But I am saying that they are compatible, in the sense that no claim of instrumental QM contradicts any claim of Bohmian QM.

But still, if instrumental QM has nothing to say about certain questions, then why am I not worried? As a “Bohmian”, I certainly do not consider those questions irrelevant. So how can I not worry about it when it says nothing about questions that I find relevant?

The answer is that I stopped worrying and learned to love orthodox QM precisely because I know about Bohmian QM. But let me explain it from the beginning.

I always wanted to study the most fundamental aspects of physics. Consequently, as a student of physics I was much more fascinated by topics such as particle physics and general relativity than about topics such as condensed-matter physics. Therefore, my graduate study in physics and my PhD were in high-energy physics. Nevertheless, all the knowledge about quantum field theory (QFT) that I acquired as a high-energy physicists did not help me much to resolve one deep puzzle that really bothered me about QM. The thing that bothered me was how could Nature work like that? How could that possibly be? What could be a possible physical mechanism behind the abstract rules of QM? Should one conclude that there is no mechanism at all and that standard QM (including QFT) is the end of story?

But then I learned about Bohmian QM, and that was a true revelation. It finally told me a possible story how could that be. It didn’t definitely tell how it is (there is no direct evidence that Bohmian mechanics is how Nature actually works), but it did tell how it might be. It is comforting to know that behind the abstract and seemingly paradoxical formalism of QM may lie a simple intuitive mechanism as provided by Bohmian QM. Even if this mechanism is not exactly how Nature really works, the simple fact that such a mechanism is possible is sufficient to stop worrying and start to love instrumental QM as a useful tool that somehow emerges from a more fundamental mechanism, even if all the details of this mechanism are not (yet) known.

However, something important was still missing. Bohmian QM looks nice and simple for non-relativistic QM, but how about relativistic QFT? In principle, Bohmian ideas of that time worked also for relativistic QFT, but they did not look so nice and simple. My question was, can Bohmian ideas be modified such that it looks nice, simple and natural even for relativistic QFT? That question motivated my professional research on Bohmian QM and I published a lot of papers on that.

Nevertheless, I was not completely satisfied with my results. Even though I made several interesting modifications of Bohmian QM to incorporate relativistic QFT, neither of those modifications looked sufficiently simple and natural. Moreover, in arXiv:1309.0400, the last specialized paper on Bohmian mechanics I have written, a referee found a deep conceptual error that I was not able to fix. After that, I was no longer trying to modify Bohmian QM in that way.

Nevertheless, a partial satisfaction came from a slightly different angle. In an attempt to make sense of local non-reality interpretation of QM, I developed a theory of solipsistic hidden variables which is a sort of a hybrid between Bohmian and Copenhagen QM. In this theory an observer does play an important role, in the sense that Bohmian-like trajectories exist only for degrees of freedom of the observer and not for the observed objects. That theory helped me to learn that, in order to understand why do we observe what we observe, it is not necessary to know what exactly happens with observed objects. Instead, as solipsistic hidden variables demonstrate, in principle it can be understood even if the observed objects don’t exist! It was a big conceptual revelation for me that shaped my further thinking about the subject.

But it does not mean that I became solipsist. I don’t believe that observed objects don’t exist. The important message is not that observed objects might not exist. The important message is that the exact nature of their existence is not really so important to explain their observation. That idea helped me a lot to stop worrying and learn to love orthodox QM.

But that was not the end. As I said, in my younger days, my way of thinking was largely shaped by high-energy physics and not by condensed-matter physics. I thought that condensed-matter physics cannot teach me much about the most fundamental problems in physics. But it started to change in 2010, when, by accident, I saw in Feynman Lectures on Physics that Bohmian mechanics is related to superconductivity (see here) That suddenly made me interested in superconductivity. But superconductivity cannot be understood without understanding other more basic aspects of condensed-matter physics, so gradually I became interested in condensed-matter physics as a field. One very interesting thing about condensed-matter physics is that it uses QFT formalism which is almost identical to QFT formalism in high-energy physics, but the underlying philosophy of QFT is very different. Condensed-matter physics taught me to think about QFT in a different way than I was used to as a high-energy physicists.

One of the main conceptual differences between the two schools of thought on QFT is the interpretation of particle-like excitations resulting form canonical quantization of fields. In high-energy physics, such excitations are typically interpreted as elementary particles. In condensed-matter physics, they are usually interpreted as quasiparticles, such as phonons. Since I was also a Bohmian, that led me to a natural question: Does it make sense to introduce a Bohmian trajectory of a phonon? An obvious (but somewhat superficial) answer is that it doesn’t make sense because only true particles, and not quasiparticles, are supposed to have Bohmian trajectories. But what is a “true” particle? What exactly does it mean that a photon is a “true” particle and a phonon isn’t?

It was this last question that led me to my last fundamental insight about Bohmian mechanics. As I explained in Sec. 4.3 of arXiv:1703.08341 (accepted for publication in Int. J. Quantum Inf.), the analogy with condensed-matter quasiparticles such as phonons suggests a very natural resolution of the problem of Bohmian interpretation of relativistic QFT. According to this resolution, the so-called “elementary” particles such as photons and electrons described by relativistic QFT are not elementary at all. Instead they are merely quasiparticles, just as phonons. Consequently, those relativistic particles do not have Bohmian trajectories at all. What does have Bohmian trajectories are some more fundamental particles described by non-relativistic QM. Non-relativistic QM (together with Bohmian interpretation) is fundamental, while relativistic QFT is emergent. In this way, the problem of Bohmian interpretation of relativistic QFT is circumvented in a very elegant way.

There is only one “little” problem about that idea. There is no any experimental evidence that such more fundamental non-relativistic particles actually exist in Nature. Perhaps they will be discovered one day in the future, but at the moment it is only a theory. In fact it is not even a proper theory, because it cannot tell anything more specific about the exact nature of those hypothetical non-relativistic particles.

Nevertheless, there are at least two good things about that. First, unlike most other versions of Bohmian mechanics, this version makes a testable prediction. It predicts that, at very small distances not yet accessible to experimental technology, the Nature is made of non-relativistic particles. Second, at distances visible by current experimental technology, this version of Bohmian QM says that Bohmian trajectories are irrelevant. This means that, as far as relativistic QFT is concerned, I do not need to worry about Bohmian trajectories and can love orthodox QFT, without rejecting “common sense” in the form of non-relativistic Bohmian mechanics on some more fundamental scale. That’s how I finally I stopped worrying and learned to love orthodox QM.

 

 

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  1. D
    Demystifier says:
    name123

    Thanks for your help, and clearing up my misconception (of thinking that the guiding wave/field propagating at faster than light speed was the explanation) :)

    Thank you for asking sharp questions. It's a pleasure to explain things to people who know what confuses them.

  2. F
    fanieh says:
    name123

    Thanks for your help, and clearing up my misconception (of thinking that the guiding wave/field propagating at faster than light speed was the explanation) :)

    Why didn't you mention about the quantum potential, name123? I heard the quantum potential is non-local in that it can track a particle somewhere in Andromeda galaxy and a particle on earth especially if they are entangled.. so it's like the quantum potential can track all the particles in the universe at once. What do you think?

  3. N
    name123 says:
    fanieh

    Why didn't you mention about the quantum potential, name123? I heard the quantum potential is non-local in that it can track a particle somewhere in Andromeda galaxy and a particle on earth especially if they are entangled.. so it's like the quantum potential can track all the particles in the universe at once. What do you think?

    I am not sure what it is. Is it supposed to be a field or a particle or something else? My physics is pretty poor, so if you think it might be an answer maybe one of the advisers could help. As I understand it tachyon fields are not theorised to travel faster than the speed of light, only tachyon particles. Apart from spacetime, fields, and particles (in some theories) I am not aware of anything else being said to exist in a physical universe.

  4. F
    fanieh says:
    name123

    I am not sure what it is. Is it supposed to be a field or a particle or something else? My physics is pretty poor, so if you think it might be an answer maybe one of the advisers could help. As I understand it tachyon fields are not theorised to travel faster than the speed of light, only tachyon particles. Apart from spacetime, fields, and particles (in some theories) I am not aware of anything else being said to exist in a physical universe.

    How come tachyon fields are not theorized to travel faster than the speed of light, while tachyon particles can? May I know what is the explanation based on what you learnt?

  5. F
    fanieh says:
    name123

    I am not sure what it is. Is it supposed to be a field or a particle or something else? My physics is pretty poor, so if you think it might be an answer maybe one of the advisers could help. As I understand it tachyon fields are not theorised to travel faster than the speed of light, only tachyon particles. Apart from spacetime, fields, and particles (in some theories) I am not aware of anything else being said to exist in a physical universe.

    according to wiki: https://en.wikipedia.org/wiki/Quantum_potential

    "Bohm and Basil Hiley also called the quantum potential an information potential, given that it influences the form of processes and is itself shaped by the environment.[9] Bohm indicated "The ship or aeroplane (with its automatic Pilot) is a self-active system, i.e. it has its own energy. But the form of its activity is determined by the information content concerning its environment that is carried by the radar waves. This is independent of the intensity of the waves. We can similarly regard the quantum potential as containing active information. It is potentially active everywhere, but actually active only where and when there is a particle." (italics in original).[73]"

    But Demystifier and other researchers think de Broglie pilot wave approach without quantum potential is more elegant.. but isn't Bohm Quantum Potential also elegant in that this is directly connected to his idea of the Implicate Order? This is closer to AdS/CFT idea than the approach used by Valentini where the quantum vacuum is some kind of fluid of hydrodynamics? Is it not Demystifier? So does it depend on researchers if quantum potential is elegant or not.. or it's just not or never will be elegant?

  6. N
    name123 says:
    fanieh

    How come tachyon fields are not theorized to travel faster than the speed of light, while tachyon particles can? May I know what is the explanation based on what you learnt?

    Regarding tachyon fields I had read in wiki

    The term "tachyon" was coined by Gerald Feinberg in a 1967 paper[7] that studied quantum fields with imaginary mass. Feinberg believed such fields permitted faster than light propagation, but it was soon realized that Feinberg's model in fact did not allow for superluminal speeds.[6] Instead, the imaginary mass creates an instability in the configuration: any configuration in which one or more field excitations are tachyonic will spontaneously decay, and the resulting configuration contains no physical tachyons. This process is known as tachyon condensation. A famous example is the condensation of the Higgs boson in the Standard Model of particle physics.

    But with tachyon particles I assume they are not theorised to undergo tachyon condensation (something only fields do maybe), and their imaginary mass allows faster than light speed in the relativity equations. As I mentioned though, I was just assuming.

  7. N
    name123 says:
    fanieh

    according to wiki: https://en.wikipedia.org/wiki/Quantum_potential

    "Bohm and Basil Hiley also called the quantum potential an information potential, given that it influences the form of processes and is itself shaped by the environment.[9] Bohm indicated "The ship or aeroplane (with its automatic Pilot) is a self-active system, i.e. it has its own energy. But the form of its activity is determined by the information content concerning its environment that is carried by the radar waves. This is independent of the intensity of the waves. We can similarly regard the quantum potential as containing active information. It is potentially active everywhere, but actually active only where and when there is a particle." (italics in original).[73]"

    But Demystifier and other researchers think de Broglie pilot wave approach without quantum potential is more elegant.. but isn't Bohm Quantum Potential also elegant in that this is directly connected to his idea of the Implicate Order? This is closer to AdS/CFT idea than the approach used by Valentini where the quantum vacuum is some kind of fluid of hydrodynamics? Is it not Demystifier? So does it depend on researchers if quantum potential is elegant or not.. or it's just not or never will be elegant?

    My mathematical knowledge is poor and so is my understanding of physics described mathematically rather than conceptually. From what I read in that wiki link I thought the quantum potential was being described as a feature of the guiding wave. So I am not sure how it can be done away (it seems to be part of the equation). Perhaps reply to a Demystifier post directly, and he can explain his position concerning it.

  8. F
    fanieh says:
    name123

    My mathematical knowledge is poor and so is my understanding of physics described mathematically rather than conceptually. From what I read in that wiki link I thought the quantum potential was being described as a feature of the guiding wave. So I am not sure how it can be done away (it seems to be part of the equation). Perhaps reply to a Demystifier post directly, and he can explain his position concerning it.

    Are you saying the quantum potential is like the guiding wave where the guiding wave can't affect the quiding wave of different particles? But it seems the quantum potential can affect quantum potential of different particles.. wiki mentioned "David Bohm and Basil Hiley in 1975 presented how the concept of a quantum potential leads to the notion of an "unbroken wholeness of the entire universe", proposing that the fundamental new quality introduced by quantum physics is nonlocality"

    So i'm thinking the quantum potential is the cause of your "theoretical explanation of how such "spooky action at a distance" could happen in a physical universe".. but Demystifier seems to say no. So i'm now kinda confused. I'll think about it more. You research it too. Thanks.

  9. A. Neumaier
    A. Neumaier says:

    I just found this site:

    Problems with Bohmian mechanics
    <mod: approved link>

    While this web site is (like any web site) not peer reviewed, it contains numerous references to peer-reviewed work substantiating that the claims made there are not just those of a crank but have a significant support in the scientific community. In fact, much of the contents of the site may be viewed as a review of critiques of Bohmian mechanics. Some references to web sites supporting Bohmian mechanics are also given.

    Enjoy!

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