# I Ontology of spin measurement

#### entropy1

Summary
If a measurement outcome depends on the measurement setup, is de measured not real or the measurement?
If the factual outcome of an electron-spin measurement depends on the orientation of the SG magnet, for instance up or down in one orientation and left or right in the other, does that mean that, since the outcome is dependent of the measurement setup, the spin is actually not real before measurement, or could it also mean that the measurement outcome is not real?

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#### A. Neumaier

This is interpretation dependent.

In the Copenhagen interpretation, the spin is not real before the measurement.

In Bohmian mechanics, the spin is determined before the measurement (by the beam in which it is)

In the thermal interpretation, the spin is a real number and the measurement outcome discretizes it, hence is only approximate.

#### DarMM

Gold Member
Summary: If a measurement outcome depends on the measurement setup, is de measured not real or the measurement?

If the factual outcome of an electron-spin measurement depends on the orientation of the SG magnet, for instance up or down in one orientation and left or right in the other, does that mean that, since the outcome is dependent of the measurement setup, the spin is actually not real before measurement, or could it also mean that the measurement outcome is not real?
Related to what @A. Neumaier said above, in the Copenhagen interpretation Spin is a phenomena that occurs in a classical system as a result of interaction with a microscopic system. It has no meaning outside of that interaction.

#### Demystifier

2018 Award
Summary: If a measurement outcome depends on the measurement setup, is de measured not real or the measurement?

If the factual outcome of an electron-spin measurement depends on the orientation of the SG magnet, for instance up or down in one orientation and left or right in the other, does that mean that, since the outcome is dependent of the measurement setup, the spin is actually not real before measurement, or could it also mean that the measurement outcome is not real?
In Bohmian mechanics, spin does not exist before measurement. Only particle positions do.

#### entropy1

In Bohmian mechanics, spin does not exist before measurement. Only particle positions do.
So what I am wondering about is if the spin is defined as a certain outcome of a certain measurement, or that it is regarded as something ontological real, in which case the measurement doesn't necessarily has to represent the value of it.

I am aware that these measurements don't commute, but if one took the correlation between two measurements on the same spin to be real, but not de measurements themselves*, then we might have less of a problem for instance in considering that a two dimensional operator yields two real worlds in MWI, because the measurement outcomes are not real*.

#### Demystifier

2018 Award
So what I am wondering about is if the spin is defined as a certain outcome of a certain measurement
Yes, that's how it is in Bohmian mechanics.

#### entropy1

Yes, that's how it is in Bohmian mechanics.
So, if, in general, a pair of spin measurements on a pair of entangled electrons correlate, what does correlate them?

#### Demystifier

2018 Award
So, if, in general, a pair of spin measurements on a pair of entangled electrons correlate, what does correlate them?
The common wave function that serves as a pilot wave for the particles.

#### Demystifier

2018 Award
But Figure 12 and 13 of
seem to show that there is a definite spin vector at each point of the Bohmian trajectory during the unitary development, i.e., - prior to any measurement!?
This "spin vector" is a property of the wave function, so exists even in standard QM. If only the particle is interpreted as ontological while the wave function is interpreted as something analogous to the Hamilton-Jacobi S-function in classical mechanics, then this spin vector is not interpreted as ontological.

#### A. Neumaier

This "spin vector" is a property of the wave function, so exists even in standard QM.
No. The spin vector in QM is an operator; the wave function has 2 complex components at every position, hence there is no trajectory of spin vectors. Is the Bohmian spin vector trajectory perhaps $\psi(t)(x(t))$?
In Bohmian mechanics, spin does not exist before measurement. Only particle positions do.
How then is spin measured and how does it get a unique result?

#### Demystifier

2018 Award
No. The spin vector in QM is an operator; the wave function has 2 complex components at every position, hence there is no trajectory of spin vectors.
They parametrize the wave function by certain angles in Eq. (16) and define the "spin vector" in terms of one of those angles. I don't think that it is a very good definition of spin vector, and it is certainly not standard in BM, but that's what they do. Don't blame me and don't blame BM. Some authors do silly things in standard QM too, but that doesn't make standard QM silly.

#### Demystifier

2018 Award
How then is spin measured and how does it get a unique result?
I thought it was answered like 100 times. In the Stern-Gerlach apparatus one measures whether the particle ends in the upper or the lower particle detector. That's all. It's not like the thermal interpretation where one needs a separate beable for each observable.

#### A. Neumaier

This "spin vector" is a property of the wave function, so exists even in standard QM. If only the particle is interpreted as ontological while the wave function is interpreted as something analogous to the Hamilton-Jacobi S-function in classical mechanics, then this spin vector is not interpreted as ontological.
How can the wave function be not ontic when its dynamics determines the positions at future times?
Something nonexistent cannot affect the existent.
They parametrize the wave function by certain angles in Eq. (16) and define the "spin vector" in terms of one of those angles. I don't think that it is a very good definition of spin vector, and it is certainly not standard in BM, but that's what they do. Don't blame me and don't blame BM. Some authors do silly things in standard QM too, but that doesn't make standard QM silly.
In the Stern-Gerlach apparatus one measures whether the particle ends in the upper or the lower particle detector. That's all.
But why can this then interpreted as a measurement of spin? Simply declaring it to be so is not an answer. In the analysis of
Figure 2 suggests that rather than measuring spin it measures starting in the upper part of the SG arrangement, independent of spin! Or is this just another silly thing done in BM?

#### DarMM

Gold Member
How can the wave function be not ontic when its dynamics determines the positions at future times?
Something nonexistent cannot affect the existent.
I think he means non-ontic like the action in Classical Mechanics.

#### microsansfil

How then is spin measured and how does it get a unique result?
For example by analogy: We never measure absolute positions or speeds, but always distances between several objects taken as a reference frame or relative speeds.

And so it is through an interactive process that we give birth to the physical quantities position and velocity, which do not "exist" in the absolute before any measurement.

/Patrick

#### A. Neumaier

I think he means non-ontic like the action in Classical Mechanics.
But the action is measurable, by measuring separately kinetic and potential energy.

#### Khashishi

This is a bit like observers arguing over which shadows are real in Plato's cave. We only see what we measure, and all of our measurements project what is measured into some simpler space.

When we make a measurement, there is some contribution from all the possible paths up to the measurement, so there is some contribution from both the spin up electron and spin down electron. The reason we consider the electron to have a definitive state is because we can repeat the measurement and measure the spin again and get the same result. But really, we are observing the result of all possible paths that go through the first measurement and then the second measurement, and there is only significant probability of measurement if the first and second measurements agree. This isn't enough to really say if the electron has a definitive state of spin when the electron isn't being measured.

#### microsansfil

I think he means non-ontic like the action in Classical Mechanics.
The action has the dimension M·L2·T-1. I don't think that the wave function has a dimension.

if the wave function is just a calculation tool for calculating measurement predictions, can it then be considered as an "ontic/beable entity"? A Platonist may answer yes, hence the ambiguity of these metaphysical notions of existence, ontology.

/Patrick

#### DarMM

Gold Member
The action has the dimension M·L2·T-1. I don't think that the wave function has a dimension.
That's correct, but why does that matter for the wavefunction being non ontological?

if the wave function is just a calculation tool for calculating measurement predictions, can it then be considered as an "ontic/beable entity"? A Platonist may answer yes, hence the ambiguity of these metaphysical notions of existence, ontology.
Well in Bohmian Mechanics the wave function isn't considered just a tool for calculating predictions, thats the point.

#### DarMM

Gold Member
But the action is measurable, by measuring separately kinetic and potential energy.
My understanding of the philosophy of Classical Mechanics is that the action is considered part of the metaphysics but not part of the ontology.

Similarly in Bohmian Mechanics the wave function is a "law of physics" thus part of the metaphysics, but not a "thing" of the theory and thus non-ontic.

#### A. Neumaier

My understanding of the philosophy of Classical Mechanics is that the action is considered part of the metaphysics but not part of the ontology.

Similarly in Bohmian Mechanics the wave function is a "law of physics" thus part of the metaphysics, but not a "thing" of the theory and thus non-ontic.
Surely the Hamiltonian is ontic in classical physics, as it represents the energy. But the Lagrangian and hence the action is computable from the Hamiltonian by a Legendre transform.

With your interpretation, it would follow that things computable from ontic stuff are not always ontic. Which rule then would guarantee that the (measurable) temperature of a classical gas is ontic?

#### Demystifier

2018 Award
How can the wave function be not ontic when its dynamics determines the positions at future times?
Something nonexistent cannot affect the existent.
Would you say that the Hamiton-Jacobi function $S({\bf x},t)$ existent in classical mechanics? The wave function in BM is existent/nonexistent in the same sense in which the $S$-function is existent/nonexistent in classical mechanics.

Figure 2 suggests that rather than measuring spin it measures starting in the upper part of the SG arrangement, independent of spin! Or is this just another silly thing done in BM?
From the Bohmian perspective it's indeed silly to call it measurement of spin. But Bohmians use such a silly language because that language is borrowed from standard QM (which is silly too, because standard QM says that spin doesn't exist before you measure it, so what does it mean to measure something which doesn't exist before measurement?). In other words Bohmians speak to "ordinary" physicists by saying something like this: The procedure that you call measurement of spin is really a measurement of position and I will tell you what is really going on when you think you measure spin.

#### Demystifier

2018 Award
With your interpretation, it would follow that things computable from ontic stuff are not always ontic. Which rule then would guarantee that the (measurable) temperature of a classical gas is ontic?
When Bohmians talk about ontology, they usually mean fundamental ontology. Temperature, in that sense, is not a fundamental ontology. Even kinetic energy of a classical nonrelativistic particle, given by $E=mv^2/2$, is not a fundamental ontology in classical mechanics. The only fundamental ontology in classical mechanics is the trajectory ${\bf x}(t)$, while everything else can be expressed in terms of that.

In the thermal interpretation of QM, on the other hand, there is no fundamental ontology from which everything else can be expressed. All observables are on the same footing. I find it very weird, especially if I look at the classical limit.

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#### DarMM

Gold Member
With your interpretation, it would follow that things computable from ontic stuff are not always ontic. Which rule then would guarantee that the (measurable) temperature of a classical gas is ontic?
Temperature usually isn't considered ontic. With energy it may or may not be counted as part of the ontology. Action usually is not as a particle does not possess action at a given moment since you need to integrate over time.

The action would be part of the metaphysics though.

"Ontology of spin measurement"

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