Does a measurement setup determine the reality of spin measurement outcomes?

In summary, the concept of spin in the Copenhagen interpretation is not considered to be real before a measurement is performed. In Bohmian mechanics, spin is determined before the measurement by the wave function, which is considered to be ontologically real. However, in this interpretation, spin does not exist as a separate entity, only particle positions do. The measurement of spin in Bohmian mechanics is simply the measurement of whether the particle ends in the upper or lower detector in a Stern-Gerlach apparatus. In some interpretations, such as the thermal interpretation, spin is considered to be a real number that is only discretized by measurement. In the Copenhagen interpretation, spin is not considered to be real until measured, while in Bohmian mechanics it
  • #1
entropy1
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TL;DR Summary
If a measurement outcome depends on the measurement setup, is de measured not real or the measurement?
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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  • #2
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.
 
  • #3
entropy1 said:
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.
 
  • #4
entropy1 said:
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.
 
  • #5
Demystifier said:
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*.
 
  • #6
entropy1 said:
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.
 
  • #7
Demystifier said:
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?
 
  • #8
entropy1 said:
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.
 
  • #9
Demystifier said:
In Bohmian mechanics, spin does not exist before measurement. Only particle positions do.
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!?
 
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  • #10
A. Neumaier said:
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.
 
  • #11
Demystifier said:
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))##?
Demystifier said:
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?
 
  • #12
A. Neumaier said:
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.
 
  • #13
A. Neumaier said:
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.
 
  • #14
Demystifier said:
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.
Demystifier said:
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 said:
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?
 
  • #15
A. Neumaier said:
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.
 
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  • #16
A. Neumaier said:
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
 
  • #17
DarMM said:
I think he means non-ontic like the action in Classical Mechanics.
But the action is measurable, by measuring separately kinetic and potential energy.
 
  • #18
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.
 
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  • #19
DarMM said:
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
 
  • #20
microsansfil said:
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?

microsansfil said:
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, that's the point.
 
  • #21
A. Neumaier said:
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.
 
  • #22
DarMM said:
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?
 
  • #23
A. Neumaier said:
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.

A. Neumaier said:
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.
 
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  • #24
A. Neumaier said:
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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  • #25
A. Neumaier said:
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 possesses action at a given moment since you need to integrate over time.

The action would be part of the metaphysics though.
 
  • #26
Demystifier said:
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.
This is not correct. In the thermal interpretation, the fundamental observable quantities are the q-expectations (of which some have a special meaning), whereas all other properties that exist (= are objective = are real), such as temperature, are functions of these.
 
  • #27
DarMM said:
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 possesses action at a given moment since you need to integrate over time.

The action would be part of the metaphysics though.
This is a very artificial nomenclature. According to it,

The Lagrangian would be ontic (since it is defined at any given moment in terms of kinetic and potential energy), while temperature, pressure, and the duration of a solar eclipse, say, would only be metaphysics!??
 
  • #28
A. Neumaier said:
This is a very artificial nomenclature. According to it,

The Lagrangian would be ontic (since it is defined at any given moment in terms of kinetic and potential energy), while temperature, pressure, and the duration of a solar eclipse, say, would only be metaphysics!??
It's just the typical nomenclature in philosophy, "ontic" refers to the objects in the theory but not "laws" or similar things. I'm not sure exactly where the division is for each individual case but there is usually considered to be a difference between an object and a law. You seem to be treating it like non-ontic means "not real".
 
  • #29
DarMM said:
It's just the typical nomenclature in philosophy, "ontic" refers to the objects in the theory but not "laws" or similar things. I'm not sure exactly where the division is for each individual case but there is usually considered to be a difference between an object and a law. You seem to be treating it like non-ontic means "not real".
Yes. This seems to me the general usage. For example https://en.wikipedia.org/wiki/Ontic says:
Wikipedia said:
In philosophical ontology, ontic (from the Greek ὄν, genitive ὄντος: "of that which is") is physical, real, or factual existence. [...]
For Heidegger, "ontical" signifies concrete, specific realities, whereas "ontological" signifies deeper underlying structures of reality. [...]
Wesley Salmon's ontic conception of explanation, for instance, claims that explanations are ontic only if they are mind-independent things in the world. [...]
Harald Atmanspacher suggests that accurate claims about "ontic states describe all properties of a physical system exhaustively.
Maybe your usage is specific to recent quantum foundation discussions? Where can I find a critical discussion of the term?
 
  • #30
A. Neumaier said:
This is not correct. In the thermal interpretation, the fundamental observable quantities are the q-expectations (of which some have a special meaning), whereas all other properties that exist (= are objective = are real), such as temperature, are functions of these.
Yes, but q-expectations of what? Of all observables. In particular, there is an observable (essentially Hamiltonian times inverse number of particles) the q-expectation of which is the temperature.
 
  • #31
Demystifier said:
Yes, but q-expectations of what? Of all observables. In particular, there is an observable (essentially Hamiltonian times inverse number of particles) the q-expectation of which is the temperature.
No. Your observable ##H/N## is the mass-based energy density, which is quite independent from the temperature.

The fundamental observable quantities are the q-expectations of quantities. The latter are defined as the densely defined operators. The temperature is not such a quantity. It is instead a parameter in the grand canonical definition of the density operator.
 
  • #32
DarMM said:
in Bohmian Mechanics the wave function isn't considered just a tool for calculating predictions

DarMM said:
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

Don't these two statements contradict each other? I'm really confused by your terminology.
 
  • #33
PeterDonis said:
Don't these two statements contradict each other?
What is the contradiction? My understanding is that there can be elements of a theory that are not "things" but are also not just epistemic quantities. For example the Euler Langrange equations are not "things" of classical theory, i.e. objects posited to exist thus not part of the ontology. They are not however epistemic, they are objective relations between the objects.

Thus they are part of the metaphysics but not elements of the ontology.

As far as I am aware this is not "my terminology" but reasonably common terminology, this article might help:
https://plato.stanford.edu/entries/metaphysics/
 
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  • #34
DarMM said:
As far as I am aware this is not "my terminology" but reasonably common terminology, this article might help

I'll take a look. I am generally not a fan of using terminology in physics that is made up by philosophers instead of physics.
 
  • #35
PeterDonis said:
I'll take a look. I am generally not a fan of using terminology in physics that is made up by philosophers instead of physics.
Doesn't "ontology" and "epistemic" come from philosophy? Is the issue with using the word "metaphysics" in the sense of seperating "things" from "relations"?

If we don't use any philosophical terminology I guess we could say in Bohmian Mechanics the wave function is a relation obeyed by particles like the Euler Lagrange equations, an equation of motion. Not an actual propogating field like the EM field.
 
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<h2>1. What is a measurement setup in the context of spin measurement outcomes?</h2><p>A measurement setup refers to the experimental apparatus used to measure the spin of a particle, such as a Stern-Gerlach apparatus or a magnetic resonance imaging (MRI) machine.</p><h2>2. Can the measurement setup influence the outcome of a spin measurement?</h2><p>Yes, the measurement setup can influence the outcome of a spin measurement. Factors such as the strength and orientation of magnetic fields, as well as the sensitivity and precision of the measurement instrument, can affect the results.</p><h2>3. Does the measurement setup determine the reality of spin measurement outcomes?</h2><p>No, the measurement setup does not determine the reality of spin measurement outcomes. The outcome of a spin measurement is determined by the intrinsic properties of the particle being measured, and the measurement setup simply allows us to observe and quantify these properties.</p><h2>4. How do scientists account for the influence of the measurement setup on spin measurement outcomes?</h2><p>Scientists take great care in designing and calibrating their measurement setups to minimize any potential influence on spin measurement outcomes. They also conduct multiple measurements and analyze the data statistically to account for any experimental errors.</p><h2>5. Are there any limitations to the accuracy of spin measurements due to the measurement setup?</h2><p>Yes, there are limitations to the accuracy of spin measurements due to the measurement setup. These limitations can be minimized through advancements in technology and careful experimental design, but they cannot be completely eliminated due to the inherent uncertainty and complexity of quantum systems.</p>

1. What is a measurement setup in the context of spin measurement outcomes?

A measurement setup refers to the experimental apparatus used to measure the spin of a particle, such as a Stern-Gerlach apparatus or a magnetic resonance imaging (MRI) machine.

2. Can the measurement setup influence the outcome of a spin measurement?

Yes, the measurement setup can influence the outcome of a spin measurement. Factors such as the strength and orientation of magnetic fields, as well as the sensitivity and precision of the measurement instrument, can affect the results.

3. Does the measurement setup determine the reality of spin measurement outcomes?

No, the measurement setup does not determine the reality of spin measurement outcomes. The outcome of a spin measurement is determined by the intrinsic properties of the particle being measured, and the measurement setup simply allows us to observe and quantify these properties.

4. How do scientists account for the influence of the measurement setup on spin measurement outcomes?

Scientists take great care in designing and calibrating their measurement setups to minimize any potential influence on spin measurement outcomes. They also conduct multiple measurements and analyze the data statistically to account for any experimental errors.

5. Are there any limitations to the accuracy of spin measurements due to the measurement setup?

Yes, there are limitations to the accuracy of spin measurements due to the measurement setup. These limitations can be minimized through advancements in technology and careful experimental design, but they cannot be completely eliminated due to the inherent uncertainty and complexity of quantum systems.

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