Reply To: God knows where all the particles are!

Dustin Lazarovici

Dear Reinhard,

thank you very much for your very nicely written responses. I’m still not sure that your hostility towards BM isn’t fueled by some factual misunderstandings (the things that Travis keeps pointing out), but I think I’ve got a better sense for where your objections come from.

Please allow me to add a few remarks. I don’t try to change your mind, I just try locate the core of the disagreement a bit better.

1) BM does not presuppose realism (whatever you mean by that, exactly). Accepting BM, you can take any philosophical attitutude you look like towards the particles and their trajectory. You can, for instance, take the anti-realist stance that they’re mere theoretical entities but still endorse BM as the most conceptually clear way of doing QM.

Of course, you CAN take BM seriously as a “realist” description of the physical world. At least it makes sense to ask: what would the world look like if BM were true (and how well does this agree with our actual physical world). It doesn’t make sense to ask: what would the world look like if textbook QM were true, because textbook QM does not provide a clear, coherent picture of the microscopic regime.

2) Micro-to macro transition is indeed a very subtle business. It’s probably not a coincidence that so many Bohmian have their background in this particular field of mathematical physics. I understand, on some level, why you would like to avoid this business, that you describe as “fuzzy”.

I just don’t think that it’s a good idea to embrace instead a set of “rules” that are fuzzy and vague to begin with and that don’t even try to achieve constiency between the microscopic and the macroscopic level. And I refuse to believe that your problems go so far that you literally don’t understand atomism, i.e. how a prediction about the distribution of particles in 3-dimensional space amounts to a prediction about the actual, observable world.

3) I don’t think anyone could convince you that BM solves any problems, because you’re obviously an expert in quantum phyiscs and you probably feel that, on the level on which you care, there are no problems with the theory. So let’s assume for the sake of argument that BM doesn’t solve any practical problems.

Still, as a quantum theory, it comes with a lot of virtues: mathematical precision (everything is defined by equations), observer-independence, consistency between the micro- and the macro level, just to name a few.

Until not so long ago, these features were considered as highly desirable by almost all scientists. I think they are still considered as highly desirable by most. However, at the advent of QM, many physicists gave them up, because they – mistakingly! – thought they had to.

Still, you are free to believe that these features are not only not desirable but actually very bad. That’s fine, I guess, it’s ok to have different preferences about what a physical theory should do. Just don’t pretend then that Bohmians have some naive philosophical prejudices, while you don’t. At the very least, we are all just as prejudiced, only in opposite directions.

Best, Dustin

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