The claim that “for an atom, no attributes even exist unless it is brought into interaction with a classical measuring device”
is central to the Copenhagen interpretation of quantum mechanics. But John Bell—one of the most influential
physicists of the 20th century—challenged this view deeply and directly.

1. Bell’s Critique of Measurement-Only Reality

Bell found the measurement-centered view of quantum mechanics unsatisfying and ambiguous:

“Either the wavefunction, as given by the Schrödinger equation, is not everything, or it is not right.”

— J.S. Bell, “Against ‘Measurement’”

He was frustrated by the idea that quantum properties “don’t exist” until measured, viewing it as a philosophically weak stance
and potentially bordering on solipsism.

2. Bell’s Theorem and Hidden Variables

Bell’s famous 1964 theorem asked whether it was possible to build a theory in which physical properties exist
independently of observation. He demonstrated that any local hidden variable theory must obey certain constraints—
Bell inequalities—which quantum mechanics violates.

Experimental results support quantum mechanics, but Bell’s aim wasn’t to disprove realism—it was to test the limits of
locality and show that Copenhagen-style interpretations aren’t inevitable.

3. Bell on the Classical–Quantum Divide

Bell criticized the unclear boundary between classical and quantum systems in the Copenhagen view:

“What exactly qualifies some physical systems to play the role of ‘measurer’? Was the wavefunction of the world waiting to jump
for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer
for some better qualified system … with a Ph.D.?”

— J.S. Bell, “Against ‘Measurement’” (1990)

He viewed this as a fundamental vagueness that undermined the coherence of the theory.

4. Beables vs Observables

Bell introduced the term “beables”—as opposed to “observables”—to refer to elements of a theory that
exist independently of measurement:

“The beables of the theory are those elements which might correspond to elements of reality, to things which exist.”

— J.S. Bell

He favored interpretations like Bohmian mechanics, which provide a realist account of particles and trajectories,
guided by the wavefunction.

Conclusion

Bell firmly rejected the idea that “attributes don’t exist unless measured.” He saw it not as a necessity, but as a flaw of
the Copenhagen interpretation—a sign that quantum mechanics, in its standard form, might be incomplete.

For Bell, the goal was to restore a kind of objective reality to physics—a world in which things exist whether or not
anyone is looking.

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Electron Spin in Bohmian Mechanics

Does Bohmian mechanics predict electron spin?

No, Bohmian mechanics does not independently predict the existence of spin.

Instead, it reproduces the predictions of standard quantum mechanics by incorporating spin into the wave function.

1. Original Bohm Model (1952)

• Treated spinless particles.
• Particles have definite positions, guided by the wavefunction.
• Wavefunction evolves via the Schrödinger equation.

2. Inclusion of Spin

• Wavefunction becomes a two-component spinor:
  ψ = [ψ+, ψ-]
• Evolves under the Pauli equation.
• Guiding equation still only involves position.

3. What About Spin Itself?

• No hidden variable for spin in standard Bohmian mechanics.
• Spin affects motion through the wavefunction’s internal structure.
• Spin measurement outcomes result from:
  • Definite particle position
  • Wavefunction structure under external fields

Experimental Predictions

Bohmian mechanics gives the same predictions as standard quantum theory for spin experiments (e.g., Stern–Gerlach), but explains them deterministically.

Summary

Bohmian mechanics assumes spin as part of the wavefunction structure, and explains spin measurement outcomes via deterministic particle motion — but it does not derive spin from deeper first principles.

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This is a deep and fascinating question — and it’s insightful to sense a connection. While Christoffel symbols and projection operators arise in different mathematical settings (geometry vs. quantum mechanics), they both deal with how structures change or decompose in curved or constrained spaces.

Christoffel Symbols (Geometry)

• Appear in differential geometry.
• Encode how coordinate bases twist and turn in a curved manifold.
• Used to define the covariant derivative.
• Not tensors themselves, but help define how vectors change across space.

Formally:

Projection Operators (Quantum Mechanics)

• Act in Hilbert spaces, often infinite-dimensional vector spaces.
• A projection operator satisfies:

     

• Projects a vector onto a subspace (e.g., eigenstate space of an observable).
• Encodes measurement in quantum mechanics.

Conceptual Bridge

Both structures deal with components of vectors under constraints:

Concept	Christoffel Symbols	Projection Operators
Setting	Curved space/manifold	Hilbert space
Purpose	Adjust derivative to follow curvature	Extract component in a subspace
Key Structure	Covariant derivative	Operator with
Acts On	Geometric vectors/tensors	Quantum states/vectors
Related To	Parallel transport, geodesics	Measurement, observables

Deeper Similarity: Connections and Decompositions

– Christoffel symbols decompose how a vector changes: intrinsic vs. extrinsic parts.
– Projection operators decompose quantum states into subspaces.

Both are types of connections:

• In geometry: a connection tells you how to compare vectors at different points in space.
• In quantum theory: projections (and Berry connections) tell you how quantum states evolve in parameter space.

Advanced View: Gauge Theory Link

In modern physics:

• Christoffel symbols behave like gauge connections for gravity.
• Projection operators appear in Berry phase phenomena and in quantum gauge structures.

In both cases, we talk about how quantities transform when moving around in space or state space.

Connections define how things change when you move around in space (or Hilbert space), and curvature arises when this change depends on the path.

TL;DR

They live in different worlds but share a core idea:

Both Christoffel symbols and projection operators are tools for tracking and constraining change under structure — whether it’s spatial curvature or Hilbert space decomposition.

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1. How Projection Operators Relate to Group Theory

Projection operators appear in quantum mechanics whenever we have symmetries described by a group \( G \). They help decompose Hilbert spaces into irreducible representations of these groups.

Key Properties:

• Idempotency: \( P^2 = P \) (applying twice is the same as once).
• Orthogonality (for distinct eigenvalues): \( P_i P_j = 0 \) if \( i \neq j \).
• Completeness: The sum of all projectors over a complete basis gives the identity:
  \[
  \sum_i P_i = I.
  \]

2. Projection Operators in Representation Theory

If a quantum system has a symmetry group \( G \), then its Hilbert space can be decomposed into irreducible representations (irreps). The projection operators onto these representations are:

where:

• \( d_\lambda \) is the dimension of the irrep labeled by \( \lambda \).
• \( \chi_\lambda(g) \) is the character of \( g \) in the representation.
• \( U(g) \) is the unitary representation matrix of \( g \).

Visual Representation:

        Group Symmetry in Quantum Mechanics
        --------------------------------------
        | Irrep 1 | Irrep 2 | Irrep 3 | ...
        --------------------------------------
        |   P₁    |   P₂    |   P₃    | ...
        --------------------------------------
            ⬇         ⬇         ⬇
        |ψ⟩ = c₁P₁ + c₂P₂ + c₃P₃

3. Projection Operators in Angular Momentum (SU(2) Symmetry)

In quantum mechanics, the rotation group SO(3) and its double cover SU(2) play a key role.

Decomposing Angular Momentum:

The total angular momentum operator \( J^2 \) commutes with all rotations, meaning its eigenspaces define invariant subspaces. The projection operator onto a definite angular momentum \( j \) is:

Spin-1 Representation of SU(2):

            Angular Momentum Subspaces
        ---------------------------------
        | J=1, m=1 | J=1, m=0 | J=1, m=-1 |
        ---------------------------------
        |    P₊    |    P₀    |    P₋    |
        ---------------------------------
            ⬇          ⬇         ⬇
        |ψ⟩ = aP₊ + bP₀ + cP₋

4. Projection Operators in Parity Symmetry (Z₂ Group)

In systems with parity symmetry, the parity operator \( P \) has eigenvalues \( \pm 1 \). The projection operators are:

Even/Odd Parity States:

        Classical Parity Transformation
        ---------------------------------
        | Even States  (P=+1)  | Odd States  (P=-1) |
        ---------------------------------
        |       P₊        |       P₋        |
        ---------------------------------
            ⬇                    ⬇
        |ψ⟩ = c₊ P₊ + c₋ P₋

5. Projection Operators in Quantum Measurement

In quantum measurement, projection operators describe observable eigenstates and their probabilities follow the Born rule:

Measurement and Decoherence:

        Quantum Measurement and Decoherence
        ---------------------------------------
        | State Before Measurement:  |ψ⟩        |
        ---------------------------------------
        | Projectors:   P₁, P₂, P₃,...            |
        ---------------------------------------
        | Probabilities: P₁⟨ψ|P₁|ψ⟩, P₂⟨ψ|P₂|ψ⟩,... |
        ---------------------------------------

6. Conclusion

• Projection operators decompose Hilbert spaces into irreducible representations.
• They appear in angular momentum (SU(2)), parity symmetry (Z₂), and representation theory.
• They ensure that quantum measurements follow the Born rule and describe state decoherence.

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Read this post on Projection Operators first.

1. What Is Gleason’s Theorem?

Gleason’s theorem states that in a Hilbert space of dimension , the only valid probability measure for quantum measurements must follow the Born rule:

where:

• is the probability of measuring outcome .
• is a **projection operator** representing a measurement.
• is the **density matrix** of the quantum state.

2. Single-Particle Spin Measurement

Consider a spin- particle (like an electron) measured along the -axis.

Spin Observable

The spin operator is:

The possible measured values (eigenvalues) are:

• (Spin up, )
• (Spin down, )

Projection Operators

Each measurement outcome corresponds to a projection operator:

Measurement Probabilities

If the quantum state is , the measurement probabilities are:

Single-Particle Measurement Diagram

       Spin Measurement Device (Stern-Gerlach)
                      |
      ↑ ( +ℏ/2 )      |       ↓ ( -ℏ/2 )
  --------------------->--------------------
        |ψ⟩ = α|+⟩ + β|−⟩

3. Two-Particle Entangled State

Consider two spin- particles in the Bell state:

Observable: Total Spin Along -Axis

The total spin operator is:

The possible measured values are:

• (both particles spin up)
• (both particles spin down)
• (one up, one down)

Measurement Probabilities

For the Bell state , Gleason’s theorem ensures that the measurement outcomes must obey:

Entanglement Measurement Diagram

        Particle A                        Particle B
       -----------                      -----------
       |  +⟩   -⟩ |                      |  +⟩   -⟩ |
       ------------------                 ------------------
                 |  Bell State: |Φ+⟩ = 1/√2 (|+⟩|+⟩ + |−⟩|−⟩)
                 |  
                 |  If A is measured as +ℏ/2, then B must be +ℏ/2.
                 |  If A is measured as -ℏ/2, then B must be -ℏ/2.

4. Conclusion

• Gleason’s theorem proves that quantum measurement probabilities must follow the Born rule.
• Any attempt to assign classical probabilities to measurement outcomes contradicts the additivity condition.
• This rules out non-contextual hidden-variable theories and reinforces the fundamental role of quantum uncertainty.

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Projection Operators and Measurement Outcomes

1. Single-Particle Spin Measurement

Consider a quantum system where a spin-\( 1/2 \) particle (e.g., an electron) is measured along the \( z \)-axis.

Observable: Spin along \( z \)-axis (\( S_z \))

The spin operator \( S_z \) is:

The possible measured values (eigenvalues) are:

• \( +\hbar/2 \) (Spin up, \( |+\rangle \))
• \( -\hbar/2 \) (Spin down, \( |-\rangle \))

Projection Operators:

The corresponding projection operators are:

Measurement Probabilities:

If the quantum state is \( |\psi\rangle = \alpha |+\rangle + \beta |-\rangle \), the probability of measuring \( +\hbar/2 \) (spin up) is:

Similarly, the probability of measuring \( -\hbar/2 \) (spin down) is:

Measurement Process Visualization:

       Spin Measurement Device (Stern-Gerlach)
                      |
      ↑ ( +ℏ/2 )      |       ↓ ( -ℏ/2 )
  --------------------->--------------------
        |ψ⟩ = α|+⟩ + β|−⟩
    

2. Two-Particle Entangled State

Now, consider a system of two entangled spin-\( 1/2 \) particles in the Bell state:

Observable: Total Spin along \( z \)-axis

The total spin operator is:

The possible measured values are:

• \( +\hbar \) (Both particles spin up)
• \( -\hbar \) (Both particles spin down)
• \( 0 \) (One particle spin up, one spin down)

Projection Operators:

For these measurement outcomes, the projection operators are:

Measurement Probabilities:

For the Bell state \( |\Phi^+\rangle \), we calculate:

Entanglement Measurement Visualization:

        Particle A                        Particle B
       -----------                      -----------
       |  +⟩   -⟩ |                      |  +⟩   -⟩ |
       |   |    |  |                      |   |    |  |
       ------------------                 ------------------
                 |  Bell State: |Φ+⟩ = 1/√2 (|+⟩|+⟩ + |−⟩|−⟩)
                 |  
                 |  If A is measured as +ℏ/2, then B must be +ℏ/2.
                 |  If A is measured as -ℏ/2, then B must be -ℏ/2.
    

3. Conclusion

• Single-particle case: Projection operators extract probabilities of spin measurements.
• Two-particle case: Projection operators help analyze entanglement, showing how quantum correlations affect measurement outcomes.

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Projection Operators and Gleason’s Theorem

1. Projection Operators in Quantum Mechanics

A projection operator \( P \) is a Hermitian operator satisfying:

These operators represent quantum measurement outcomes. If a system is in state \( |\psi\rangle \), the probability of measuring an outcome associated with projection \( P \) is:

2. Projection Operators in Observables

An observable \( A \) with discrete eigenvalues \( a_i \) can be expressed using projection operators \( P_i \):

The probability of measuring \( a_i \) is given by the Born rule:

3. Gleason’s Theorem

Gleason’s theorem states that in a Hilbert space of dimension \( d \geq 3 \), the only possible probability measure satisfying quantum additivity must be:

where \( E \) is a projection operator and \( \rho \) is a density matrix.

4. Implications of Gleason’s Theorem

• Justifies the Born Rule: Probability assignments must follow the standard quantum probability formula.
• Rules out Non-Contextual Hidden Variables: If measurement outcomes are predetermined, the additivity assumption is violated.
• Constrains Deterministic Quantum Theories: No assignment of definite values (0 or 1) to projection operators is consistent with quantum mechanics.

5. Connection to Bell’s Theorem

Gleason’s theorem disproves non-contextual hidden-variable theories but does not rule out contextual hidden-variable theories. Bell later extended this result with Bell’s inequalities, showing that hidden-variable theories must be nonlocal.

6. Conclusion

• Projection operators define quantum measurement outcomes.
• Gleason’s theorem proves that probability in quantum mechanics must follow the Born rule.
• Hidden-variable theories that assume predetermined values contradict quantum probability rules.

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Gleason’s Theorem and Hidden Variables

1. Projection Operators

A projection operator is a Hermitian operator satisfying:

These operators represent measurement outcomes in quantum mechanics. If a system is in state , the probability of measuring an outcome associated with projection is:

2. Additivity Assumption in Quantum Mechanics

Ballentine references an additivity assumption for commuting observables:

This assumption is valid for classical probability but has deeper implications in quantum mechanics.

3. Gleason’s Theorem (1957)

Gleason’s theorem states that in any Hilbert space of dimension ≥3, the only probability measure that satisfies the additivity assumption must follow the Born rule:

This implies that hidden-variable models must reproduce quantum probabilities exactly.

4. Dispersion-Free States and Their Impossibility

A dispersion-free state is a hypothetical state in which every observable has a definite pre-determined value (0 or 1 for projection operators). Gleason’s theorem shows that:

• If every projection operator had a unique 0 or 1 value, the additivity assumption would be violated.
• This means no hidden-variable theory can assign definite values to all quantum observables in a way that is consistent with quantum probability rules.

5. Bell’s Response and Contextuality

John Bell (1966) argued that:

• Hidden-variable theories could still exist if measurement outcomes depended on the whole experimental arrangement.
• This aligns with Bohr’s (1949) view that measurement depends on context.

6. Criticism of Gleason’s Assumptions

Ballentine criticizes Gleason’s theorem for assuming that every projection operator represents an observable, which is unrealistic. Some operators (e.g., ) may not correspond to real measurable quantities.

7. Conclusion

• Gleason’s theorem proves that quantum probability must follow the Born rule.
• Dispersion-free states (deterministic hidden-variable states) violate the additivity condition.
• Bell’s rebuttal suggests that contextual hidden-variable models might still be possible.

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The Hidden Variables section in Ballentine’s Statistical Interpretation of Quantum Mechanics examines the possibility of supplementing quantum mechanics with additional parameters (hidden variables) that determine the outcome of individual measurements, rather than relying on probabilistic quantum states.

Von Neumann’s Theorem

Von Neumann’s theorem aimed to show that no hidden-variable theory could reproduce all the statistical predictions of quantum mechanics. His proof relied on the assumption that expectation values should be additive:

This condition holds for quantum ensembles but assumes it should also apply to hidden-variable models.

Key Issues in Von Neumann’s Proof

• Noncommuting Observables: Quantum mechanics includes observables that do not commute, such as position q and momentum p. Von Neumann’s theorem does not properly account for these cases.
• Strong Assumption of Linearity: Expectation value linearity does not necessarily hold in hidden-variable models.
• Implication: If Von Neumann’s assumptions were correct, quantum mechanics would be the only possible theory, ruling out hidden variables.

Bell’s Rebuttal

John Bell revisited Von Neumann’s proof and identified its flaws. He pointed out that the assumption:

is not valid for hidden-variable theories since it assumes quantum mechanical averages apply to individual measurements.

Bell’s Key Arguments

• Hidden-Variable Theories Do Exist: Bohmian mechanics (Bohm, 1952) reproduces all quantum statistical predictions.
• Misinterpretation of Linearity: Expectation values of noncommuting variables need not sum linearly.
• Constructing a Working Hidden-Variable Model: Bell provided counterexamples demonstrating that hidden variables could exist.

Bell’s Theorem: A Stronger No-Go Result

While Bell criticized Von Neumann’s proof, he later formulated Bell’s theorem, which provided a stronger argument against local hidden-variable theories. His theorem is based on Bell inequalities,

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Key Points:

1. Marginal Distributions Must Agree with Quantum Theory

The joint probability distribution must reproduce the standard quantum probability distributions when integrated over one of the variables:

where ψ(q) and φ(p) are the wavefunctions in position and momentum space, respectively.

2. Fourier Transform Approach

The characteristic function of an observable A is given by:

By analogy, a joint characteristic function for position and momentum can be introduced, leading to a proposed joint probability distribution.

3. Wigner Function and Negativity Issue

One approach is to define a phase-space distribution such as the Wigner function:

However, the Wigner function can take negative values, which prevents it from being interpreted as a genuine probability distribution.

4. Impossibility of a Classical Joint Distribution

Analysis by Cohen and Margenau shows that it is impossible to construct a classical probability distribution P(q, p; ψ) that satisfies all quantum mechanical requirements, particularly those related to operator ordering and the uncertainty principle.

Conclusion

While various attempts have been made to construct joint probability distributions for position and momentum, they either fail to meet quantum consistency conditions or lead to negative probabilities. This demonstrates a fundamental departure of quantum mechanics from classical probability theory.

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