# Questions about the de-Broglie Bohm interpretation

1. Sep 1, 2009

### QMister

These questions were originally asked to one of PF.com's member, but he wanted me to post it publically so more people could benefit from the answers:

#1
In dBB (deBroglieBohm), what does the pilot wave consist of if not matter?
How can this wavefunctional "non matter" control all the matter in the universe?
I've read some different explanations of this like "it exists in some high dimensional abstract space".
This SCREAMS crackpot to me.
If you are going to postulate the existence of some unobserved dimension and space, you need extraordinary evidence or all you are doing is spitting on Occam's grave.

#2
In the double slit experiment, deBroglie's explanation makes sense to me, until you bring in the fact about placing a detector at one of the slits which in return removes the interference pattern, how does dBB explain this ?
Why do a detector **** up the wave pattern if there truly is a wave there?
In a little (layman) detail if you wouldn't mind.

#3
Also why doesn't the pilot waves continue to affect our world? (by the way, is there only ONE universal pilot wave, or many for each particle?)
If I emit a photon, the photon is a quanta/particle surfing on this wave, but after the photon is absorbed, the wave continues to propagate through space.
However, it's without any detectable influence on any other matter anymore... Why is this? Isn't this indirect disproof of it's existence?

#4
Nonlocality, infinite speed, or finite but faster than light speed?
Also where exactly (in layman terms) are the nonlocality introduced to Bohms interpretation.
What does it explain in dBB?
Why is it needed?

#5
According to all I've read on dBB it's 100% deterministic, so uncertaintly principle doesn't really state that particles doesn't have definite location and momentum, because they do have that, but rather that we as humans are limited so we can't figure out both at the same time right?
Also there is no inherit randomness in nature at all, it's all 100% deterministic and objective(according to dBB) ?

#6
Why Bohm instead of Copenhagen, MWI, TI etc.?
Is there any experiments that lead you (the supporter of Bohm) to believe dBB is more likely/has indirect evience favouring it over other interpretations?

Last edited: Sep 1, 2009
2. Sep 1, 2009

### Demystifier

Here I give you short answers, which later can be expanded if needed:

1. The word "matter" is somewhat vague in physics, but you can say that (according to the Bohmian interpretation) the wave function consists of "matter" if it makes it intuitively clearer to you.
The wave function lives in the multi-dimensional configuration space in ANY interpretation, so it is not crackpot. Such a formulation of quantum mechanics leads to experimentally verified predictions, such as entanglement and nonlocal correlations, which cannot be explained without such a multi-dimensional space.

2. and 3. To understand this, the Bohmian interpretation is not essential. Instead, the essential stuff that you need to understand is decoherence, which is an experimentally verified phenomenon. Try to google something about it before asking more questions.

4. Nonlocality is closely related to the fact that the wave function lives in the multi-dimensional space. We need nonlocality because the Bell theorem asserts that any realistic theory compatible with quantum mechanics must necessarily be nonlocal. Nonlocality (more precisely, nonlocal correlations) are an experimentally verified fact.

5. Yes and yes.

6. There is no experiment that prefers any interpretation over any other. The reasons for Bohm are theoretical. For example, in MWI it is difficult to explain the usual probabilistic rules of quantum mechanics. Copenhagen does not offer an answer to the question whether anything exists at all without measurements. Concerning the transactional interpretation, I do not understand it, but I would be very grateful if someone could explain it to me.

3. Sep 1, 2009

4. Sep 1, 2009

5. Sep 1, 2009

### DrChinese

Demystifier, a quick question if you have time.

The guidance equation is a function of instantaneous positions of all other particles, which themselves have positions which are unknown/unknowable. OK, that seems reasonable to me (as being a source of apparent indeterminism).

Now, the issue with that we are saying the "instantaneous position" of other particles, which gives rise to non-local influences. Why couldn't the equation be a function of their positions, but NOT their instantaneous positions? Perhaps the influence is time-delayed so that c is respected. Wouldn't that allow sufficient room for the equations to have a similar effect (i.e. apparent indeterminism but actually is deterministic)? Or perhaps it is not a past configuration that controls, but a future one. The question is whether the guidance equation demands instantaneous positions only for the influence, or could there be other solutions as well?

(Hopefully my question makes sense.)

6. Sep 1, 2009

### Demystifier

A good question!
It is perhaps possible to do it in principle, but equations would look much more complicated then. More about it later ...

7. Sep 1, 2009

### QMister

Well I don't find the answers on google that you asked me to find...
Can you just explain them simply?
Why does the detector mess up the wave pattern?
And why doesn't the wave continue to affect our world?

The papers you provide was good, but some of the stuff was too complicated to understand for a layman.

A while back there was a thread on dBB and someone (think it was you Demystifyer) said that ****SOMETHING I DONT REMEMBER**** was indirect evidence of the pilot wave which was the reason you believe in dBB.

Also you always hear that QM is all about probabilty, but dBB just says this is BS, the universe is as Einstein said: 100% deterministic, causal, nothing to do with probability?

8. Sep 2, 2009

### Demystifier

Basically, you want an explanation without equations, right? OK, here is an intuitive picture. The wave function can be thought of as a big tree (it is a tree in the multi-dimensional configuration space, but for the sake of intuitive visualization you can ignore it), with many branches, branches of branches, etc. The particle is a little ant that walks up along the tree. He never returns down. (This is because the up direction corresponds to the direction of time.) Whenever he enters some branche, he looses possibility to enter another one (it's much easier to draw than to explain by words, but I hope you understand what I mean).
Thus, these other branches become irrelevant for him because they do not influence his future walk. For all practical purposes he can say that other branches no longer exist, that they are destroyed. Yet, the other branches are still there.

Essentially, Bohmian mechanics is walking of the ant guided by the tree. The many world interpretation is the tree without the ant.

This is like playing roulette. It is all about probability. Yet, it is completely deterministic.

9. Sep 2, 2009

### Demystifier

OK, now a more serious answer. For some configurations (i.e., wave functions, experimental arrangements, etc.), it is possible to do that, but not for all. For example, for the configuration associated with the Aspect experiment, it isn't possible (except with superdeterminism).

10. Sep 2, 2009

### Demystifier

I think it was about delayed choice quantum eraser experiments, which show (or at least suggest) that wave functions do not really collapse. Instead, they have a tree structure as explained above, with a possibility that sometimes different branches can join again and branch in a different way.

11. Sep 2, 2009

### Dmitry67

Bravo!!!
I really liked it!

Anything about CI? Something about the wood cutter cutting all branches except the only one?

Last edited: Sep 2, 2009
12. Sep 2, 2009

### Demystifier

This is the von Neumann variant of CI, but there are also other variants:

By the way, I would prefer MWI over Bohm if someone could give me a SIMPLE explanation of the Born rule within MWI.

Last edited: Sep 2, 2009
13. Sep 2, 2009

### QMister

Very great analogies about tree and ant, however if you mean its still probability who rules, what decides which branch the ant climbes? that seems like indeterminism to me.

Why would you prefer MWI over Bohm when its basically proven it CAN'T make sense of anything we observe?

14. Sep 2, 2009

### Dmitry67

In Bohm there are hidden variables which deterministically decide which branch to chose. As they are hidden it looks random for any observer.

About MWI - what is proven? It is perfectly compatible with all we observe. In fact, based on the recent polls, it becomes interpretation #1

15. Sep 2, 2009

### DrChinese

That made me smile...

I certainly prefer knowing you are investigating BI...

16. Sep 2, 2009

### zenith8

As a Bohmian who occasionally responds to Bohmian posts, I apologize for not responding to this one. But one sometimes loses the will to live, especially if (a) you ask far too many questions, the answers to which are mostly covered in standard presentations, such as http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html" [Broken], and (b) you start off by calling us crackpots.
To kick off with your first question, I just wrote something about this in post #19 of https://www.physicsforums.com/showthread.php?t=334130", the relevant bit of which I take the liberty of reposting (out of context) below:

The use of a wave function defined on a multi-dimensional configuration space does not imply that this space exists in the same sense that the physical three-dimensional space may be said to exist. (Remember even in classical mechanics we tend to use a configuration space description).

In classical mechanics the config space representation is just a convenient summary of the positions of all the particles; in QM the situation is different because the physics is different - there is the possibility of entanglement due to non-local interactions. So a simply-connected 3d space alone cannot describe the holistic quantum connectiveness and nonlocality features of multi-particle quantum systems. Instead this is done formally by employment of the n-dimensional config space.

The problem with such a space actually existing are considerable, and include:

(1) needing at least 3 separate dimensions for every particle in the universe

(2) the total number of dimensions in the universe varying from moment to moment along with the creation and annihilation of particles.

(3) the extra dimensions always being completely unnoticeable at macroscopic scales, and

(4) a complete lack of any experimental evidence for the existence of multi-dimensional physical spaces.

An additional point is that we do not currently know the 'means' by which quantum non-local connections are actualized. This is not because of the non-relativistic context since non-locality is also present in relativistic versions of QM.

Given the strong reasons against taking multi-dimensional space as real, the vast amount of evidence in favour of physically real wave fields, and the absence of information about the 'means' of non-local connections, it is a coherent position to take the wave function to be a mathematical representation of a real field in physical space.

The notion of an n-particle system is described in Bohm theory by its trajectory which is traced out in 3n-dimensional config space. Even though this description is given using a multi-dimensional space, the motion of individual particles can be calculated since there is a natural mapping from the system's trajectory in 3n-dimensional space to trajectories in 3d space.

Maybe when we have discovered (or developed a model of) the 'means' by which quantum non-local connections are actualized then we will be able to describe the wave field in physical 3d space. That'll be the day.

Last edited by a moderator: May 4, 2017
17. Sep 3, 2009

### Demystifier

The branch is decided by the initial position of the ant. If it is not known to you, then you can only predict probabilities that the ant will end up in this or that branch.

It isn't proven.

18. Sep 3, 2009

### Demystifier

You shouldn't be surprised that a Bohmian has a positive opinion on MWI. These two interpretations have very much in common. Basically, BI asserts that MWI is correct but not complete.

In fact, I have realized that MWI has many positive features after reading the explanation of MWI by Bohm and Hiley (in their book "The Undivided Universe").

See also this paper, written by authors quite famous for their research on BI:
http://xxx.lanl.gov/abs/0903.2211

I believe that the many-world interpretation would make much more sense to many people if someone would ONLY change its name into many-branch interpretation. Indeed, according to MWI, there is ONLY ONE WORLD, but many branches. And the existence of these branches can be DERIVED from the Schrodinger equation. And experimental verification of partial decoherence is an experimental verification that these branches are in a certain sense "real". The only true question is not whether these branches exist (they apparently do), but whether their existence is enough to explain everything we observe. Adherents of strong MWI believe that it is enough, while Bohmians (like me) disagree.

19. Sep 3, 2009

### QMister

So except for the probability problem in MWI you are proponent of it?
You'd abounden BM, for MWI, if the probability problem was solved?
Also if it's true that the particle surf on the wave, why doesn't all particles end up in the same spot, what changes the initial position of the particle on the wave?

20. Sep 3, 2009

### zenith8

Over an ensemble of experiments the initial positions of the particles are distributed as the absolute square of the wave field. If at some time in the past the particles were not so distributed, then it is easy to show (theoretically or through numerical simulation) that they will become so distributed as a result of the Schroedinger evolution of the guiding wave.

Or, put another way, when we do a 'state preparation procedure' it is normally assumed that we can produce identical starting conditions for every run of the experiment. However, this is only true for the state of the wave field. The particles will have some random starting position over the $$\Psi^2$$ distribution.

Last edited: Sep 3, 2009