Monday, January 16, 2017

Book Review: "Making Sense of Quantum Mechanics"

After much delay I had found the time to finish reading Jean Bricmont's "Making Sense of Quantum Mechanics" book.

The book is the best presentation of Bohmian interpretation I have ever read. It masterly combines the philosophical ideas with a bit of math, famous quotes, and some historical perspective. 

After preliminary topics in chapter one, chapter two discusses the first quantum "mystery": superposition, while chapter four discuses the second "mystery": nonlocality.  It was chapter three, a philosophical "intermetzzo" which took me a very loooong time to read and prevented me to write this review much sooner: one one end I could not write this post without reading it, and on the other end I was loosing interest very quickly in it after a couple of pages of historical review. Then Bricmont proceeds into presenting Bohniam mechanics - the heart of the book. 

Let's dig a bit deeper into it. Chapter two is a very well written introduction into why quantum mechanics is counter-intuitive. This is presented in the style of modern quantum foundation undergrad classes. Chapter four main idea is this: to many physicists Bell's result proved the impossibility of non-contextual hidden variables (or local realism) while Bell should be understood in conjunction with EPR: EPR+Bell = nonlocality. But what does nonlocality mean? Is it just higher than expected correlations? Here Bricmont makes a very bold and provocative claim: 

"the conclusion of his [Bell's] argument, combined with the EPR argument is rather that there are nonlocal physical effects (and not just correlations between events) in Nature."

To support this chapter 4.2 discusses "Einstein's boxes" [I had a series of posts discussing why in my opinion they do not represent an argument for nonlocality. What EPR+Bell shows is that the composition of two physical systems into a larger physical system does not respect the rules of classical physics - parabolic composability but new rules - elliptic composability. Nature is not "nonlocal" but "non-parabolic composable"]. 

Onto the main topic, the presentation of Bohmian mechanics is standard and what it is surprising is the degree on which the underdetermination issue is addressed: there are an infinite number of alternative theories (like Nelson's stochastic theory) which are in the same realistic vein and which make the same predictions as Bohmian mechanics. Chapter three discussion is invoked here but I feel the argument is very weak (not even a handwaving).

Then the book talks about alternative approaches to Bohmian mechanics courageously taking (some well deserved some not) shots at alternative interpretations (like GRW, MWI, CH, Qbism), and wraps up with historical topics and sociological arguments.

Now onto what the book covers poorly: surreal trajectories, and quantum field theory in Bohmian mechanics. Surreal trajectories are mentioned in passing in a quote while they are the major objection to the interpretation. As I said before, the very name "surreal trajectory" was a masterful catchy clever title for a paper but it backfired in the long term because it was attaching a stigma to Bohmian mechanics which in turned allowed Bohmian supporters to summarily and unfairly dismiss the argument. I will revisit the argument in next post. The key point of surreal trajectories paper is that the particle is detected where Bohmian mechanics predicts it must not go, and since the only thing "real" in Bohmian mechanics is the position of the particle, it represents a fatal blow to the Bohmian ontology. Currently, to my knowledge, there is no consensus inside the Bohmian community on the proper answer the surreal trajectory paper: some deny it is a problem at all while others acknowledge the problem and propose (faulty) ideas on how to deal with it. This is similar with the situation inside the MWI camp where the big pink elephant in the room there is the notion of probabilities: some in MWI disagree it is an issue while others attempt to solve it (but fail). 

Quantum field theory in Bohmian mechanics is another sore point which is not properly discussed. My take on the topic is that a Bohmian quantum field theory is impossible to be constructed, and I want to be proven wrong by a consistent proposal: show me the money, show me the archive paper where the problem is comprehensibly solved.

Bad points aside, overall I liked the book, I find it stimulating, and I enjoyed very much reading it (except chapter 3 which invariably succeeded putting me to sleep). The book is a must read for any person seriously interested in the foundations of quantum mechanics.


  1. Your views make absolutely no sense. If you realize that one can't write down a Bohmian quantum field theory, you surely understand that it's lethal for the theory - because the QFT phenomena self-evidently exist. So why are you reading and even praising Bohmian books then?

    Crackpot Bricmont surely doesn't say that QFT is not doable. On the contrary, on page 170, for example, he incorrectly states that "that is certainly doable".

    1. -why are you reading [...] Bohmian books ...?
      -on page 170, for example...

      So why *you* reading them? You are a hypocrite.

    2. I was not systematically reading it. I just know what is in it, just like I know what is in the brains of pretty much all well-known enough people confused about QM. It took me 30 seconds to find the page where Bricmont ludicrously says that Bohmian QFT may be done.

  2. I have heard of this book, but have yet to read it. Bohm's QM is not that high on my agendas. Bohm's QM is funny, for it follows pretty much right from the Schrodinger equation, but it can't be in total right.

    Here is a calculation I did, but never pursued any further or published. Take Bohm's QM result for the modified Hamilton-Jacobi equation and the continuity equation for the so called pilot wave. Now perform a canonical or symplectic transformation on the system. You get another H-J equation and continuity equation. This means that a classical canonical transformation shifts the problem from what Bohm et al called the active channel to another. Yet this must be a manifestation of the analysis and not something physical. So if one does a sum over all possible canonical transformations the result is a form of path integral. So Bohm-QM has a little bit of something going for it.

    If I had to say what the problem is it is that if you really had an active channel where some objective particle and its observables this runs afoul of the Born rule. If the Born rule is considered a hard aspect of QM (which I think most of us do), then it makes an odd situation for observables to be otherwise defined. This does suggest something with measuement, which Bohm originally this interpretation for. Using the language of "collapse" there is then one Feynman path which manifests itself to the exclusion of others. This then illustrates something strange about the incompatibility with the nature of measurement and quantum mechanics.

    1. To recover Born rule in Bohmian mechanics you need "quantum equilibrium". This is an additional assumption which needs to be postulated.

    2. I guess I have not heard of quantum equilibrium. It is interesting that Bohm's QM requires various auxiliary assumptions, such as guidance conditions or equations. I have questioned whether these things are telling us something about the inconsistency of quantum measurement with QM itself. I say this as someone who rejects all quantum interpretations as not only not being fundamental but also not really proper theories to begin with.