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Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:03:56 UTC Robert McNees

Physicist David Bohm, who developed a non-local formulation of quantum mechanics that he hoped would evade some of the conceptually thorny aspects of the Copenhagen Interpretation, and would later inspire the work of John Bell, was born #OTD in 1917.
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1. David Bohm, in his early 20s, wearing a jacket and tie. He is looking down and to the side in this black-and-white photo.
  https://files.mastodon.social/media_attachments/files/111/613/354/308/664/237/original/8aeaf50e4d46f145.jpeg
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:07:04 UTC Robert McNees
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  Bohm’s quantum mechanics textbook was published in 1951. It was very successful, and is still available from Dover as a reprint. It's a great book.
  I almost said “inexpensive reprint," but then I checked the Dover website. It's $40 now! A few years ago it was $25, and when I bought it, the book was $14. Dover-flation!
  https://store.doverpublications.com/0486659690.html
  Anyway, here's my copy:
  In conversation about a year ago permalink
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  1. The Dover edition of "Quantum Theory" by David Bohm. The cover has a light blue background and shows a diagram of an oscillating wave function with a Gaussian envelope. The area between the wave function and the horizontal axis is shaded in dark green, while the area between the Gaussian envelope and the axis is light green.
    https://files.mastodon.social/media_attachments/files/111/613/366/196/580/307/original/0d3035913b7e5d2c.jpeg
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:10:04 UTC Robert McNees
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  That same year, a crotchety old faculty member named Einstein encouraged the young Bohm to study von Neumann’s work scrutinizing “hidden variable theories.”
  These are theories that assert additional deterministic variables whose state and dynamics, were they known to us, would resolve the indeterminacy of quantum mechanics.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:11:18 UTC Robert McNees
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  Bohm devised his own formulation of QM that he felt evaded von Neumann's objections, and might avoid some troubling ideas associated with the Copenhagen Interpretation.
  His approach was similar to the “pilot wave” idea first presented by de Broglie at the 1927 Solvay Conference.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:13:51 UTC Robert McNees
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  This formulation, often referred to as De Brogle-Bohm Theory, was published in 1952. It appeared in a pair of papers, both in the January 15th issue of Physical Review.
  (1) "A Suggested Interpretation of the Quantum Theory in Terms of Hidden Variables. I"
  David Bohm, Phys. Rev. 85, 166
  https://journals.aps.org/pr/abstract/10.1103/PhysRev.85.166
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  1. The abstract reads: "The usual interpretation of the quantum theory is self-consistent, but it involves an assumption that cannot be tested experimentally, viz. , that the most complete possible specification of an individual system is in terms of a wave function that determines only probable results of actual measurement processes. The only way of investigating the truth of this assumption is by trying to find some other interpretation of the quantum theory in terms of at present "hidden" variables, which in principle determine the precise behavior of an individual system, but which are in practice averaged over in measurements of the types that can now be carried out. In this paper and in a subsequent paper, an interpretation of the quantum theory in terms of just such "hidden" variables is suggested. It is shown that as long as the mathematical theory retains its present general form, this suggested interpretation leads to precisely the same results for all physical processes..."
    https://files.mastodon.social/media_attachments/files/111/613/392/983/130/748/original/cb4b8e9b451868a2.png
  2. A Suggested Interpretation of the Quantum Theory in Terms of "Hidden" Variables. I
    
    The usual interpretation of the quantum theory is self-consistent, but it involves an assumption that cannot be tested experimentally, viz., that the most complete possible specification of an individual system is in terms of a wave function that determines only probable results of actual measurement processes. The only way of investigating the truth of this assumption is by trying to find some other interpretation of the quantum theory in terms of at present "hidden" variables, which in principle determine the precise behavior of an individual system, but which are in practice averaged over in measurements of the types that can now be carried out. In this paper and in a subsequent paper, an interpretation of the quantum theory in terms of just such "hidden" variables is suggested. It is shown that as long as the mathematical theory retains its present general form, this suggested interpretation leads to precisely the same results for all physical processes as does the usual interpretation. Nevertheless, the suggested interpretation provides a broader conceptual framework than the usual interpretation, because it makes possible a precise and continuous description of all processes, even at the quantum level. This broader conceptual framework allows more general mathematical formulations of the theory than those allowed by the usual interpretation. Now, the usual mathematical formulation seems to lead to insoluble difficulties when it is extrapolated into the domain of distances of the order of ${10}^{\ensuremath{-}13}$ cm or less. It is therefore entirely possible that the interpretation suggested here may be needed for the resolution of these difficulties. In any case, the mere possibility of such an interpretation proves that it is not necessary for us to give up a precise, rational, and objective description of individual systems at a quantum level of accuracy.
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:15:05 UTC Robert McNees
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  (2) ”A Suggested Interpretation of the Quantum Theory in Terms of Hidden Variables. II"
  David Bohm, Phys. Rev. 85, 180
  https://journals.aps.org/pr/abstract/10.1103/PhysRev.85.180
  In conversation about a year ago permalink
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  1. The beginning of the abstract reads: "In this paper, we shall show how the theory of measurements is to be understood from the point of view of a physical interpretation of the quantum theory in terms of "hidden" variables, developed in a previous paper. We find that in principle, these "hidden" variables determine the precise results of each individual measurement process. In practice, however, in measurements that we now know how to carry out, the observing apparatus disturbs the observed system in an unpredictable and uncontrollable way, so that the uncertainty principle is obtained as a practical limitation on the possible precision of measurements. This limitation is not, however, inherent in the conceptual structure of our interpretation..."
    https://files.mastodon.social/media_attachments/files/111/613/397/845/761/637/original/fab64f7ec2c7978b.png
  2. A Suggested Interpretation of the Quantum Theory in Terms of "Hidden" Variables. II
    
    In this paper, we shall show how the theory of measurements is to be understood from the point of view of a physical interpretation of the quantum theory in terms of "hidden" variables, developed in a previous paper. We find that in principle, these "hidden" variables determine the precise results of each individual measurement process. In practice, however, in measurements that we now know how to carry out, the observing apparatus disturbs the observed system in an unpredictable and uncontrollable way, so that the uncertainty principle is obtained as a practical limitation on the possible precision of measurements. This limitation is not, however, inherent in the conceptual structure of our interpretation. We shall see, for example, that simultaneous measurements of position and momentum having unlimited precision would in principle be possible if, as suggested in the previous paper, the mathematical formulation of the quantum theory needs to be modified at very short distances in certain ways that are consistent with our interpretation but not with the usual interpretation.We give a simple explanation of the origin of quantum-mechanical correlations of distant objects in the hypothetical experiment of Einstein, Podolsky, and Rosen, which was suggested by these authors as a criticism of the usual interpretation.Finally, we show that von Neumann's proof that quantum theory is not consistent with hidden variables does not apply to our interpretation, because the hidden variables contemplated here depend both on the state of the measuring apparatus and the observed system and therefore go beyond certain of von Neumann's assumptions.In two appendixes, we treat the problem of the electromagnetic field in our interpretation and answer certain additional objections which have arisen in the attempt to give a precise description for an individual system at the quantum level.
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:16:02 UTC Robert McNees
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  Unlike the Copenhagen Interpretation, Bohm’s formulation is completely deterministic and asserts that systems have a real and definite configuration that exists even when we haven't performed a measurement to observe it.
  The wave function is still there, shepherding particles along. Each particle would travel through one or the other opening in a double-slit experiment, while the wave function would pass through both and interfere, driving particles towards some spots and away from others.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:17:00 UTC Robert McNees
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  When one first learns QM, a very natural question is “particle or wave?” It’s natural because our macroscopic intuition, unattuned to the microscopic world, thinks of this as a one-or-the-other proposition.
  Bohm’s approach provides a simple answer: there’s a particle AND a wave.
  
  But Bohm’s theory is also non-local, exhibiting immediate action-at-a-distance. In that sense it was no less spooky to many physicists than the interpretation he sought to replace. So it didn’t win many adherents.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:18:12 UTC Robert McNees
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  Several years later, John Stewart Bell revisited the question of hidden variable theories. He was not satisfied with attempts to explain measurement in the Copenhagen Interpretation, and was encouraged by how the issue was addressed in Bohm’s theory.
  He proved a result now referred to as Bell’s Theorem, which essentially rules out the possibility of a local hidden variable theory reproducing the predictions of quantum mechanics. Bohm’s theory evades Bell's argument because it is non-local.
  
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- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:19:03 UTC Robert McNees
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  Bohm belonged to a number of organizations supporting communism when he was a graduate student. As a result, the military wouldn't grant him security clearance to work on the Manhattan Project.
  This was probably fine with Bohm, who opposed US involvement in WWII. But it put him on an unusual trajectory for completing his doctorate. Bohm had performed scattering calculations that were needed by folks working on the Manhattan Project. His work was immediately classified.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:19:32 UTC Robert McNees
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  Since his work was now classified, and he did not have security clearance, Bohm was not allowed to write his own thesis or present a defense!
  Oppenheimer (his advisor) had to go to the University on Bohm's behalf, and make special arrangements to grant his PhD.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:20:02 UTC Robert McNees
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  Bohm became a professor at Princeton after the war, but eventually the House Un-American Activities Committee came for him. Princeton caved and suspended him; he was eventually arrested for refusing to testify.
  Even though Bohm was acquitted, Princeton no longer wanted him on the faculty. Einstein unsuccessfully lobbied to keep him at the Institute for Advanced Study as a personal research assistant, and other jobs fell through. Bohm eventually secured a position in São Paolo.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:20:33 UTC Robert McNees
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  Bohm was still formally at Princeton when he submitted the two papers on his formulation of quantum mechanics, but he had relocated to Brazil by the time they were published. He would remain there for a few more years before moving to Israel, and eventually to Bristol in the UK.
  
  He would spend four years in Bristol before being appointed professor at Birkbeck College, University of London in 1961. Bohm remained there until he retired in the late 80s.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:21:42 UTC Robert McNees
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  While at Bristol, Bohm and Yakir Aharonov worked out what is usually called the "Aharonov-Bohm Effect." (They were not aware that Ehrenberg and Siday had suggested the same phenomenon a few years earlier.)
  The Aharonov-Bohm effect demonstrates of the reality of the vector potential in electrodynamics.
  Rather than working with the electric and magnetic fields that exert forces on charged particles, one can formulate electrodynamics in terms of what we call "potentials."
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:23:07 UTC Robert McNees
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  The electric and magnetic fields can be recovered from the way these potentials change from point-to-point in space, and over time.
  One of the potentials has a handy interpretation in terms of the work done on a charged particle as it is pushed around by an electric field.
  But the other potential, which we call the “vector potential,” has no such interpretation. It initially seems like a mathematical convenience that one introduces to simplify magnetic field calculations.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:23:42 UTC Robert McNees
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  Aharonov and Bohm pointed out that this vector potential has real physical consequences; it isn’t just a mathematical convenience.
  Imagine coiling a wire very tightly around a long cylinder. This is called a (cylindrical) “solenoid.” If you send a current through the wire you get an approximately constant magnetic field inside the cylinder, but essentially no magnetic field outside the cylinder.
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:24:43 UTC Robert McNees
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  Even though the magnetic field is zero outside the solenoid, the vector potential is not. And when a charged particle like an electron passes through this region, it "feels" the vector potential and acquires a shift in its quantum mechanical phase.
  The shift in the electron’s phase affects the way it interferes with itself in a double-slit experiment. You run the experiment with and without the solenoid turned on, and you measure a change in the interference pattern.
  https://pubs.aip.org/physicstoday/article/62/9/38/895362/The-Aharonov-Bohm-effects-Variations-on-a-subtle
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:26:18 UTC Robert McNees
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  So the vector potential is more than just a mathematical convenience. It's a lovely little result.
  If that Physics Today article is paywalled, you can access a copy that one of the authors hosts on his university's Digital Commons:
  https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1118&context=physicsfacpub
  
  In conversation about a year ago permalink
- N0tSure (n0tsure@mastodon.social)'s status on Wednesday, 20-Dec-2023 15:58:57 UTC N0tSure
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  @mcnees His book, "Wholeness and the Implicate Order" was important to me.
  
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- AstroHyde (astrohyde@mastodon.social)'s status on Wednesday, 20-Dec-2023 16:12:08 UTC AstroHyde
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  @mcnees omg, I LOVE this book, one of my all time favorites! My copy got lost years ago moving around the world and I always figured I’d be able to pick it up later, it seemed to expensive when I checked online... sounds like maybe everyone is having trouble getting copies, could explain the price hike😆
  
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- Dave Bacon (dabacon@ftl.chat)'s status on Wednesday, 20-Dec-2023 16:13:10 UTC Dave Bacon
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  @mcnees Another contribution is that Bohm, together with Pines, developed to the random phase approximation as it is used in condensed matter https://en.m.wikipedia.org/wiki/Random_phase_approximation
  In conversation about a year ago permalink
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  1. Random phase approximation
    
    The random phase approximation (RPA) is an approximation method in condensed matter physics and in nuclear physics. It was first introduced by David Bohm and David Pines as an important result in a series of seminal papers of 1952 and 1953. For decades physicists had been trying to incorporate the effect of microscopic quantum mechanical interactions between electrons in the theory of matter. Bohm and Pines' RPA accounts for the weak screened Coulomb interaction and is commonly used for describing the dynamic linear electronic response of electron systems. It was further developed to the relativistic form (RRPA) by solving the Dirac equation.In the RPA, electrons are assumed to respond only to the total electric potential V(r) which is the sum of the external perturbing potential Vext(r) and a screening potential Vsc(r). The external perturbing potential is assumed to oscillate at a single frequency ω, so that the model yields via a self-consistent field (SCF) method a dynamic dielectric function denoted by εRPA(k, ω). The contribution to the dielectric function...
- Andrew D🥛 (aducharme@mastodon.social)'s status on Wednesday, 20-Dec-2023 17:32:02 UTC Andrew D🥛
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  @mcnees Two members of my group went to undergrad at Chapman University in CA where Aharonov is still somehow a pretty active faculty member
  
  In conversation about a year ago permalink
- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 17:53:20 UTC Robert McNees
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  - Andrew D🥛
  @aducharme I had no idea!
  
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- Robert McNees (mcnees@mastodon.social)'s status on Wednesday, 20-Dec-2023 21:33:12 UTC Robert McNees
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  - Larry Smith For Harris
  @stargazersmith Sure!
  
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- Larry Smith For Harris (stargazersmith@social.linux.pizza)'s status on Wednesday, 20-Dec-2023 21:33:13 UTC Larry Smith For Harris
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  @mcnees
  Could a person with a mere BS in physics understand it?
  
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