The bouncing droplets imply having to drop the Bohmian approach?

If you are interested in the area of QM foundations, then may be you should drop everything at once, and go, check out the latest pop-sci news report: “Famous experiment dooms alternative to quantum weirdness” by Natalie Wolchover in the Quanta Magazine [^].

Remember the bouncing droplets experiments performed by Yves Couder and pals? In 2006, they had reported that they could get the famous interference pattern even if the bouncing droplets passed through the double slit arrangement only one at a time. … As the Quanta article now reports, it turns out that when other groups in the USA and France tried to reproduce this result (the single-particle double-slit interference), they could not.

“Repeat runs of the experiment, called the “double-slit experiment,” have contradicted Couder’s initial results and revealed the double-slit experiment to be the breaking point of both the bouncing-droplet analogy and de Broglie’s pilot-wave vision of quantum mechanics.”

Well, just an experimental failure or two in reproducing the interference, by itself, wouldn’t make for a “breaking point,”i.e., if the basic idea itself were to be sound. So the question now becomes whether the basic idea itself is sound enough or not.

Turns out that a new argument has been put forth, in the form of a thought experiment, which reportedly shows why and how the very basic idea itself must be regarded as faulty. This thought experiment has been proposed by a Danish professor of fluid dynamics, Prof. Tomas Bohr. (Yes, there is a relation: Prof. Tomas Bohr is a son of the Nobel laureate Aage Bohr, i.e., a grandson of the Nobel laureate Niels Bohr [^].)

Though related to QM foundations, this thought experiment is not very “philosophical” in nature; on the contrary, it is very, very “physics-like.” And the idea behind it also is “simple.” … It’s one of those ideas which make you exclaim “why didn’t I think of it before?”—at least the first time you run into it. Here is an excerpt (which actually is the caption for an immediately understandable diagram):

“Tomas Bohr’s variation on the famous double-slit experiment considers what would happen if a particle must go to one side or the other of a central dividing wall before passing through one of the slits. Quantum mechanics predicts that the wall will have no effect on the resulting double-slit interference pattern. Pilot-wave theory, however, predicts that the wall will prevent interference from happening.”

… Ummm… Not quite.

From whatever little I know about the pilot-wave theory, I think that the wall wouldn’t prevent the interference from occurring, even if you use this theory. … It all seems to depend on how you interpret (and/or extend) the pilot-wave theory. But if applied right (which means: in its own spirit), then I guess that the theory is just going to reproduce whatever it is that the mainstream QM predicts. Given this conclusion I have drawn about this approach, I did think that the above-quoted portion was a bit misleading.

The main text of the article then proceeds to more accurately point out the actual problem (i.e., the way Prof. Tomas Bohr apparently sees it):

“… the dividing-wall thought experiment highlights, in starkly simple form, the inherent problem with de Broglie’s idea. In a quantum reality driven by local interactions between a particle and a pilot wave, you lose the necessary symmetry to produce double-slit interference and other nonlocal quantum phenomena. An ethereal, nonlocal wave function is needed that can travel unimpeded on both sides of any wall. [snip] But with pilot waves, “since one of these sides in the experiment carries a particle and one doesn’t, you’ll never get that right. You’re breaking this very important symmetry in quantum mechanics.””

But isn’t the pilot wave precisely ethereal and nonlocal in nature, undergoing instantaneous changes to itself at all points of space? Doesn’t the pilot theory posit that this wave doesn’t consist of anything material that does the waving but is just a wave, all by itself?


…So, if you think it through, people seem to be mixing up two separate issues here:

  1. One issue is whether it will at all be possible for any real physical experiment done up with the bouncing droplets to be able to reproduce the predictions of QM or not.
  2. An entirely different issue is whether, in Bohr’s dividing-wall thought-experiment, the de Broglie-Bohm approach actually predicts something that is at a variance from what QM predicts or not.

These two indeed are separate issues, and I think that the critics are right on the first count, but not necessarily on the second.

Just to clarify: The interference pattern as predicted by the mainstream QM itself would undergo a change, a minor but a very definite change, once you introduce the middle dividing wall; it would be different from the pattern obtained for the “plain-vanilla” version of the interference chamber. And if what I understand about the Bohmian mechanics is correct, then it too would proceed to  produce exactly the same patterns in both these cases.


With that said, I would still like to remind you that my own understanding of the pilot-wave theory is only minimal, mostly at the level of browsing of the Wiki and a few home pages, and going through a few pop-sci level explanations by a few Bohmians. I have never actually sat down to actually go through even one paper on it fully (let alone systematically study an entire book or a whole series of articles on this topic).

For this reason, I would rather leave it to the “real” Bohmians to respond to this fresh argument by Prof. Tomas Bohr.

But yes, a new argument—or at least, an old argument but in a remarkably new settings—it sure seems to be.


How would the Bohmians respond?

If you ask me, from whatever I have gathered about the Bohmians and their approach, I think that they are simply going to be nonchalant about this new objection, too. I don’t think that you could possibly hope to pin them down with this argument either. They are simply going to bounce back, just like those drops. And the reason for that, in turn, is what I mentioned already here in this post: their pilot-wave is both ethereal and nonlocal in the first place.


So, yes, even if Wolchover’s report does seem to be misguided a bit, I still liked it, mainly because it was informative on both the sides: experimental as well as theoretical (viz., as related to the new thought-experiment).

In conclusion, even if the famous experiment does not doom this (Bohmian) alternative to the quantum weirdness, the basic reason for its unsinkability is this:

The Bohmian mechanics is just as weird as the mainstream QM is—even if the Bohmians habitually and routinely tell you otherwise.

When a Bohmian tells you that his theory is “sensible”/“realistic”/etc/, what he is talking about is: the nature of his original ambition—but not the actual nature of his actual theory.


To write anything further about QM is to begin dropping hints to my new approach. So let me stop right here.

[But yes, I am fully ready willing from my side to disclose all details about it at any time to a suitable audience. … Let physics professors in India respond to my requests to let me conduct an informal (but officially acknowledged) seminar on my new approach, and see if I get ready to deliver it right within a week’s time, or not!

[Keep waiting!]]


Regarding other things, as you know, the machine I am using right now is (very) slow. Even then, I have managed to run a couple of 10-line Python scripts, using VSCode.

I have immediately taken to liking this IDE “code-editor.” (Never had tried it before.) I like it a lot. … Just how much?

I think I can safely say that VSCode is the best thing to have happened to the programming world since VC++ 6 about two decades ago.

Yes, I have already stopped using PyCharm (which, IMHO, is now the second-best alternative, not the best).


No songs section this time, because I have already run a neat and beautiful song just yesterday. (Check out my previous post.) … OK, if some song strikes me in a day or two, I will return here to add it. Else, wait until the next time around. … Until then, take care and bye for now…


[Originally published on 16 October 2018 22:09 hrs IST. Minor editing (including to the title line) done by 17 October 2018 08:09 hrs IST.]

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Off the blog. [“Matter” cannot act “where” it is not.]

I am going to go off the blogging activity in general, and this blog in most particular, for some time. [And, this time round, I will keep my promise.]


The reason is, I’ve just received the shipment of a book which I had ordered about a month ago. Though only about 300 pages in length, it’s going to take me weeks to complete. And, the book is gripping enough, and the issue important enough, that I am not going to let a mere blog or two—or the entire Internet—come in the way.


I had read it once, almost cover-to-cover, some 25 years ago, while I was a student in UAB.

Reading a book cover-to-cover—I mean: in-sequence, and by that I mean: starting from the front-cover and going through the pages in the same sequence as the one in which the book has been written, all the way to the back-cover—was quite odd a thing to have happened with me, at that time. It was quite unlike my usual habits whereby I am more or less always randomly jumping around in a book, even while reading one for the very first time.

But this book was different; it was extraordinarily engaging.

In fact, as I vividly remember, I had just idly picked up this book off a shelf from the Hill library of UAB, for a casual examination, had browsed it a bit, and then had began sampling some passage from nowhere in the middle of the book while standing in an library aisle. Then, some little time later, I was engrossed in reading it—with a folded elbow resting on the shelf, head turned down and resting against a shelf rack (due to a general weakness due to a physical hunger which I was ignoring [and I would have have to go home and cook something for myself; there was none to do that for me; and so, it was easy enough to ignore the hunger]). I don’t honestly remember how the pages turned. But I do remember that I must have already finished some 15-20 pages (all “in-the-order”!) before I even realized that I had been reading this book while still awkwardly resting against that shelf-rack. …

… I checked out the book, and once home [student dormitory], began reading it starting from the very first page. … I took time, days, perhaps weeks. But whatever the length of time that I did take, with this book, I didn’t have to jump around the pages.


The issue that the book dealt with was:

[Instantaneous] Action at a Distance.

The book in question was:

Hesse, Mary B. (1961) “Forces and Fields: The concept of Action at a Distance in the history of physics,” Philosophical Library, Edinburgh and New York.


It was the very first book I had found, I even today distinctly remember, in which someone—someone, anyone, other than me—had cared to think about the issues like the IAD, the concepts like fields and point particles—and had tried to trace their physical roots, to understand the physical origins behind these (and such) mathematical concepts. (And, had chosen to say “concepts” while meaning ones, rather than trying to hide behind poor substitute words like “ideas”, “experiences”, “issues”, “models”, etc.)

Twenty-five years later, I still remain hooked on to the topic. Despite having published a paper on IAD and diffusion [and yes, what the hell, I will say it: despite claiming a first in 200+ years in reference to this topic], I even today do find new things to think about, about this “kutty” [Original: IITM lingo; English translation: “small”] topic. And so, I keep returning to it and thinking about it. I still am able to gain new insights once in an odd while. … Indeed, my recent ‘net search on IAD (the one which led to Hesse and my buying the book) precisely was to see if someone had reported the conceptual [and of course, mathematical] observation which I have recently made, or not. [If too curious about it, the answer: looks like, none has.]


But now coming to Hesse’s writing style, let me quote a passage from one of her research papers. I ran into this paper only recently, last month (in July 2017), and it was while going through it that I happened [once again] to remember her book. Since I did have some money in hand, I did immediately decide to order my copy of this book.

Anyway, the paper I have in mind is this:

Hesse, Mary B. (1955) “Action at a Distance in Classical Physics,” Isis, Vol. 46, No. 4 (Dec., 1955), pp. 337–353, University of Chicago Press/The History of Science Society.

The paper (it has no abstract) begins thus:

The scholastic axiom that “matter cannot act where it is not” is one of the very general metaphysical principles found in science before the seventeenth century which retain their relevance for scientific theory even when the metaphysics itself has been discarded. Other such principles have been fruitful in the development of physics: for example, the “conservation of motion” stated by Descartes and Leibniz, which was generalized and given precision in the nineteenth century as the doctrine of the conservation of energy; …

Here is another passage, once again, from the same paper:

Now Faraday uses a terminology in speaking about the lines of force which is derived from the idea of a bundle of elastic strings stretched under tension from point to point of the field. Thus he speaks of “tension” and “the number of lines” cut by a body moving in the field. Remembering his discussion about contiguous particles of a dielectric medium, one must think of the strings as stretching from one particle of the medium to the next in a straight line, the distance between particles being so small that the line appears as a smooth curve. How seriously does he take this model? Certainly the bundle of elastic strings is nothing like those one can buy at the store. The “number of lines” does not refer to a definite number of discrete material entities, but to the amount of force exerted over a given area in the field. It would not make sense to assign points through which a line passes and points which are free from a line. The field of force is continuous.

See the flow of the writing? the authentic respect for the intellectual history, and yet, the overriding concern for having to reach a conclusion, a meaning? the appreciation for the subtle drama? the clarity of thought, of expression?

Well, these passages were from the paper, but the book itself, too, is similarly written.


Obviously, while I remain engaged in [re-]reading the book [after a gap of 25 years], don’t expect me to blog.

After all, even I cannot act “where” I am not.


A Song I Like:

[I thought a bit between this song and another song, one by R.D. Burman, Gulzar and Lata. In the end, it was this song which won out. As usual, in making my decision, the reference was exclusively made to the respective audio tracks. In fact, in the making of this decision, I happened to have also ignored even the excellent guitar pieces in this song, and the orchestration in general in both. The words and the tune were too well “fused” together in this song; that’s why. I do promise you to run the RD song once I return. In the meanwhile, I don’t at all mind keeping you guessing. Happy guessing!]

(Hindi) “bheegi bheegi…” [“bheege bheege lamhon kee bheegee bheegee yaadein…”]
Music and Lyrics: Kaushal S. Inamdar
Singer: Hamsika Iyer

[Minor additions/editing may follow tomorrow or so.]