Reply To: God knows where all the particles are!

[Dustin Lazarovici]

Dear Reinhard (if I may call you Reinhard),

there seems to be a misunderstanding. Of Course, according to Bohmian mechanics, macroscopic objects are composed of microscopic (Bohmian) particles. And indeed, as Travis explains, the empirical content of the theory is in the position/configuration of macroscopic objects, including measurement devices, records and so on.

Now you seem to suggest that the fact that you cannot observe the trajectory of a Bohmian particles implies that you couldn’t observe the trajectory of a macroscopic object composed of Bohmian particles. But this is not correct.

“Absolute uncertainty” – which is a Theorem in BM – implies that you cannot know more about a system than its psi^2-distribution, psi being its effective wave-function. Applied to a single particle, this means that you cannot know/measure/observe its precise trajectory. Applied to a macroscopic object, composed of a great number of particles, this means that you cannot know its exact microscopic configuration, i.e. the position of every single constituent particle. However, a huge number of different microstates coarse grain to one and the same macrostate.

In other words: you don’t have to know/measure/observe the exact position of 10^24 microscopic particles composing a table to know/see the position of the table in your office.

Of course, if you were to spell out the Bohmian account in more Detail, it would involve decoherence and the fact that macroscopic wave-function can be sufficiently well localized on sufficiently large time-scales and it would involve the fact that typically, the microscopic configuration is roughly where the bump in the wave-function is. But these are all theorems in BM, i.e. they can be proven.

To summerize: that, according to BM, you cannot observe the trajectory of microscopic particles is true, but not really a problem. That, according to BM, you cannot observe the macro-trajectory of macroscopic objects would be a problem, but it’s not true.

@Miroljub No, I don’t agree that Bohmian explanations require a huge leap of faith and yield only modest benefit. I’m not sure why I should.