# Insights Against "interpretation" - Comments

#### stevendaryl

Staff Emeritus
Yes, this does answer my concern better. But I still have a question left. My point was that in Newtonian gravity I can by only observing one particle deduce something about the behavior of the other, without the need of me knowing the initial state of the universe, just local observations of one particle. That is not possible in quantum mechanics. Is it possible in Bohmian mechanics? If not, then the claim that the nonlicality of Bohmian mechanics is the same as that in Newton's gravity is simply incorrect. This was the only point that I wanted to make. If on the other hand it is possible, then it seems that Bohmian mechanics is substantially different from QM, not just an interpretation. So different that it is already ruled out.
I'm not sure I understand what distinction you are making. Bohmian mechanics is deterministic, just as Newtonian mechanics is. Each particle affects every other particle, just as with Newtonian mechanics.

That is, in principle, different from standard QM, which is nondeterministic, but in practice they make the same predictions. If you don't know the exact positions of every particle, but only a probability distribution, then Bohmian mechanics only allows you to make probabilistic predictions.

#### A. Neumaier

in Newtonian gravity I can by only observing one particle deduce something about the behavior of the other, without the need of me knowing the initial state of the universe
You can say nothing exact without knowing the state of the universe at the particular moment, since the dynamics of each particle depends on all others. This is as in Bohmian mechanics.

#### Demystifier

2018 Award
Why? Science is done by anthropomorphic entities such as Bob and Alice who do have wants. Seems counterproductive to insist otherwise.
That's true for a large part of science (applied science, mainly), but not for all science. (For instance, in cosmology one does not ask what happens if Alice creates a new galaxy or Bob creates a new quantum fluctuation as a seed for a new inflationary Universe.) The point of Bohmian mechanics is to interpret quantum mechanics in a manner that does not involve observers. So if one wants to understand Bohmian mechanics, then referring to Alice and Bob is, well, counterproductive.

#### Demystifier

2018 Award
To be convincing with his argument, Demystifier has to explain is why the Bohmian universe behaves such that seeming choices can be made by Alice!
Well, that's a problem for any physical theory that claims to be fundamentally deterministic or fundamentally probabilistic. If so, then where does the human free will come from? The most physical answer is that free will is an illusion. Things just happen due to the laws of physics, but our brain then somehow interprets some of those as being "chosen by oneself". How exactly that happens in the brain is something that physics alone cannot answer.

#### Demystifier

2018 Award
My point was that in Newtonian gravity I can by only observing one particle deduce something about the behavior of the other, without the need of me knowing the initial state of the universe, just local observations of one particle. That is not possible in quantum mechanics. Is it possible in Bohmian mechanics? If not, then the claim that the nonlicality of Bohmian mechanics is the same as that in Newton's gravity is simply incorrect. This was the only point that I wanted to make.
It is not possible in Bohmian mechanics, but to understand where does the difference come from you have to ask the following question: How exactly one observes a particle in Newtonian gravity? The point is that one observes it by a local non-gravitational interaction. Typically, one observes the stars and planets by watching them, which involves interaction with light. For instance when you watch the Moon, the light interacts with the Moon (by reflecting from it) only when the light touches the Moon. It is this locality of interaction that allows one to determine the position of the Moon. In Bohmian mechanics, on the other hand, particles do not interact via any local interactions at all (see my "Bohmian mechanics for instrumentalists"). In this sense, there is no analogue of "light" in Bohmian mechanics. Bohmian particles are analogous to dark matter in astrophysics, which, as you might know, cannot be directly observed precisely because it does not interact with light.

Or to make the long story short, nonlocality in Newtonian gravity is very much like nonlocality in Bohmian mechanics, but the difference is that, in Newtonian gravity, there is something additional which is local and non-gravitational.

#### martinbn

It is not possible in Bohmian mechanics, but to understand where does the difference come from you have to ask the following question: How exactly one observes a particle in Newtonian gravity? The point is that one observes it by a local non-gravitational interaction. Typically, one observes the stars and planets by watching them, which involves interaction with light. For instance when you watch the Moon, the light interacts with the Moon (by reflecting from it) only when the light touches the Moon. It is this locality of interaction that allows one to determine the position of the Moon. In Bohmian mechanics, on the other hand, particles do not interact via any local interactions at all (see my "Bohmian mechanics for instrumentalists"). In this sense, there is no analogue of "light" in Bohmian mechanics. Bohmian particles are analogous to dark matter in astrophysics, which, as you might know, cannot be directly observed precisely because it does not interact with light.

Or to make the long story short, nonlocality in Newtonian gravity is very much like nonlocality in Bohmian mechanics, but the difference is that, in Newtonian gravity, there is something additional which is local and non-gravitational.
I am not so sure about this. I can sit at the seashore and watch the tides. If the moon disappears I will realize that by the tides. I don't need to look at the moon. Newtonian gravity allows instantaneous signal transmission, QM doesn't. Does BM? I guess not, so it is not the same as Newtonian gravity.

#### stevendaryl

Staff Emeritus
Newtonian gravity allows instantaneous signal transmission, QM doesn't. Does BM? I guess not, so it is not the same as Newtonian gravity.
I think BM does allow instantaneous effects in the same sense that Newtonian gravity does. But it can't be used for communication unless you know the exact locations of every particle in the universe. Which you don't.

Let me illustrate with a very simplified version of EPR. Suppose that Alice is trying to transmit a single bit (0 or 1) to Bob. Alice has a device with two buttons labeled 0 and 1. Bob has a corresponding device with two LEDs, one labeled 0 and one labeled 1. Let's suppose that there is a bit-valued hidden variable $\lambda$ that is either 0 or 1, and that $\lambda$ depends in some sensitive way on the exact positions of every particle in the universe.

Let $A$ be Alice's choice, 0 or 1. Let $B$ be Bob's result, 0 or 1. Then the laws of the universe work so that:

$B = A + \lambda(1 - 2A)$

So if $\lambda = 0$, then $B = A$. If $\lambda = 1$, then $B= 1-A$.

I think it's clear that Alice can't dependably send a 0 to Bob unless she knows the value of $\lambda$. However, her choice does affect Bob's result, in the sense that if she makes the opposite choice, he will get the opposite value.

#### Demystifier

2018 Award
I am not so sure about this. I can sit at the seashore and watch the tides. If the moon disappears I will realize that by the tides. I don't need to look at the moon. Newtonian gravity allows instantaneous signal transmission, QM doesn't. Does BM? I guess not, so it is not the same as Newtonian gravity.
How would you make the Moon disappear? Which force would you apply for that? Gravitational force or some other force? If it is some other force, then, as I already pointed out, the difference emerges from the fact that in Newtonian gravity there are some additional forces.

#### cube137

Demystifier. Why does Bohmian Mechanics belong to "b) No" in the "Find Your Own Quantum Interpretation" chart for the question "Is the world completely described by the state in the Hilbert space? The "a) Yes" is for Many Worlds for example. You may argue the beabble in BM is only for the particles like tables or chairs. But what would be wrong by stating the wave function in BM is also real like in Many worlds? What theoretical conflict can occur?

#### stevendaryl

Staff Emeritus
Demystifier. Why does Bohmian Mechanics belong to "b) No" in the "Find Your Own Quantum Interpretation" chart for the question "Is the world completely described by the state in the Hilbert space?
The answer is "no" because the world is described by the state in Hilbert space AND the instantaneous positions of every particle. The state alone is not a complete description.

#### cube137

The answer is "no" because the world is described by the state in Hilbert space AND the instantaneous positions of every particle. The state alone is not a complete description.
Why, in many worlds, there are no instantaneous positions of every particle since the world is conpletely described by the state in Hilbert space?

#### stevendaryl

Staff Emeritus
Why, in many worlds, there are no instantaneous positions of every particle since the world is conpletely described by the state in Hilbert space?
I'm not sure how to answer a "why" question like that. It just doesn't. In the simplest case of one particle, you have a wave function $\psi(x,t)$. The particle has a probability of being here or there, but the wave function doesn't tell you where the particle is.

#### cube137

I'm not sure how to answer a "why" question like that. It just doesn't. In the simplest case of one particle, you have a wave function $\psi(x,t)$. The particle has a probability of being here or there, but the wave function doesn't tell you where the particle is.
I know. But why did you state that "the world is described by the state in Hilbert space AND the instantaneous positions of every particle"? Is this only for Bohmian Mechanics?

#### stevendaryl

Staff Emeritus
I know. But why did you state that "the world is described by the state in Hilbert space AND the instantaneous positions of every particle"? Is this only for Bohmian Mechanics?
Yes, in Bohmian mechanics, the world is described by the wave function and the positions of every particle. In Many-Worlds, the world is described by just the wave function.

#### cube137

Yes, in Bohmian mechanics, the world is described by the wave function and the positions of every particle. In Many-Worlds, the world is described by just the wave function.
Ok. That's clear enough.

About many worlds. I'd like to know something. In a cat. How often are there quantum choices in the atoms and molecules of the cats enough to create different mixed states or worlds? And if the quantum choices affect the organs and endocrine system. So the cats can be in different moods in different worlds?

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