Complex Hilbert space is usually assumed at the start of quantum mechanics. This paper formulates a reverse construction for the one-particle and operator-algebraic setting. Starting from a real Hilbert space carrying an orthogonal representation, the admissible imaginary units are identified with the skew-adjoint square roots of − Id in the Real commutant of the complexified representation. The novelty is not the isolated use of complexification, spectral orientation, or polar decomposition, but their order of dependence: first the symmetry representation defines an intrinsic operator-valued phase space; then an oriented internal time flow selects a distinguished point of that phase space; only afterward are the usual complex one-particle, observable, probability, and Weyl–CCR structures invoked under their standard hypotheses. In the terminology used here, this oriented internal flow is the reversible or inner-generative side of a dual-time distinction. It is not a second observer clock and not an additional Lorentzian coordinate: mathematically it is the strongly continuous orthogonal group U ( t ) = exp ( t A ) on the real Hilbert space. Its Stone-dual variable is energy/frequency λ ; canonical conjugation pairs λ with − λ ; positive energy orients this pair and selects the canonical phase on the dynamic sector through A = J | A | = | A | J . Thus inner-generative time is the source of phase orientation, while irreversible record time, projection, memory, and decoherence enter only as later reduced-dynamics layers. No spectral gap at zero is required, and zero modes remain visible as residual phase data rather than being hidden by the construction. The selected phase is invariant under time-orientation-preserving symmetries and changes sign under time reversal, giving the usual antiunitary action after complex reconstruction. The paper also makes explicit the monoidal limitation: two selected local phases do not make the raw real tensor product the standard composite system; the standard composite is recovered only after imposing the J -balanced, equivalently complex, tensor product rule. Thus the result is a reconstruction of compatible complex kinematics on the selected one-particle/operator-algebraic sector, not a derivation of all quantum foundations from purely real data.
Complex Hilbert space is usually assumed at the start of quantum mechanics. This paper formulates a reverse construction for the one-particle and operator-algebraic setting. Starting from a real Hilbert space carrying an orthogonal representation, the admissible imaginary units are identified with the skew-adjoint square roots of − Id in the Real commutant of the complexified representation. The novelty is not the isolated use of complexification, spectral orientation, or polar decomposition, but their order of dependence: first the symmetry representation defines an intrinsic operator-valued phase space; then an oriented internal time flow selects a distinguished point of that phase space; only afterward are the usual complex one-particle, observable, probability, and Weyl–CCR structures invoked under their standard hypotheses. In the terminology used here, this oriented internal flow is the reversible or inner-generative side of a dual-time distinction. It is not a second observer clock and not an additional Lorentzian coordinate: mathematically it is the strongly continuous orthogonal group U ( t ) = exp ( t A ) on the real Hilbert space. Its Stone-dual variable is energy/frequency λ ; canonical conjugation pairs λ with − λ ; positive energy orients this pair and selects the canonical phase on the dynamic sector through A = J | A | = | A | J . Thus inner-generative time is the source of phase orientation, while irreversible record time, projection, memory, and decoherence enter only as later reduced-dynamics layers. No spectral gap at zero is required, and zero modes remain visible as residual phase data rather than being hidden by the construction. The selected phase is invariant under time-orientation-preserving symmetries and changes sign under time reversal, giving the usual antiunitary action after complex reconstruction. The paper also makes explicit the monoidal limitation: two selected local phases do not make the raw real tensor product the standard composite system; the standard composite is recovered only after imposing the J -balanced, equivalently complex, tensor product rule. Thus the result is a reconstruction of compatible complex kinematics on the selected one-particle/operator-algebraic sector, not a derivation of all quantum foundations from purely real data.
As earlier conjectured by several authors and much later established by Solèr (relying on partial results by Piron, Maeda–Maeda and other authors), from the lattice theory point of view, Quantum Mechanics may be formulated in real, complex or quaternionic Hilbert spaces only. Stückelberg provided some physical, but not mathematically rigorous, reasons for ruling out the real Hilbert space formulation, assuming that any formulation should encompass a statement of Heisenberg principle. Focusing on this issue from another — in our opinion, deeper — viewpoint, we argue that there is a general fundamental reason why elementary quantum systems are not described in real Hilbert spaces. It is their basic symmetry group. In the first part of the paper, we consider an elementary relativistic system within Wigner’s approach defined as a locally-faithful irreducible strongly-continuous unitary representation of the Poincaré group in a real Hilbert space. We prove that, if the squared-mass operator is non-negative, the system admits a natural, Poincaré invariant and unique up to sign, complex structure which commutes with the whole algebra of observables generated by the representation itself. This complex structure leads to a physically equivalent reformulation of the theory in a complex Hilbert space. Within this complex formulation, differently from what happens in the real one, all selfadjoint operators represent observables in accordance with Solèr’s thesis, and the standard quantum version of Noether theorem may be formulated. In the second part of this work, we focus on the physical hypotheses adopted to define a quantum elementary relativistic system relaxing them on the one hand, and making our model physically more general on the other hand. We use a physically more accurate notion of irreducibility regarding the algebra of observables only, we describe the symmetries in terms of automorphisms of the restricted lattice of elementary propositions of the quantum system and we adopt a notion of continuity referred to the states viewed as probability measures on the elementary propositions. Also in this case, the final result proves that there exists a unique (up to sign) Poincaré invariant complex structure making the theory complex and completely fitting into Solèr’s picture. This complex structure reveals a nice interplay of Poincaré symmetry and the classification of the commutant of irreducible real von Neumann algebras.
As earlier conjectured by several authors and much later established by Solèr, from the lattice-theory point of view, Quantum Mechanics may be formulated in real, complex or quaternionic Hilbert spaces only. On the other hand, no quantum systems seem to exist that are naturally described in a real or quaternionic Hilbert space. In a previous paper [23], we showed that any quantum system which is elementary from the viewpoint of the Poincaré symmetry group and it is initially described in a real Hilbert space, it can also be described within the standard complex Hilbert space framework. This complex description is unique and more precise than the real one as, for instance, in the complex description, all self-adjoint operators represent observables defined by the symmetry group. The complex picture fulfils the thesis of Solér’s theorem and permits the standard formulation of the quantum Noether’s theorem. The present work is devoted to investigate the remaining case, namely, the possibility of a description of a relativistic elementary quantum system in a quaternionic Hilbert space. Everything is done exploiting recent results of the quaternionic spectral theory that were independently developed. In the initial part of this work, we extend some results of group representation theory and von Neumann algebra theory from the real and complex cases to the quaternionic Hilbert space case. We prove the double commutant theorem also for quaternionic von Neumann algebras (whose proof requires a different procedure with respect to the real and complex cases) and we extend to the quaternionic case a result established in the previous paper concerning the classification of irreducible von Neumann algebras into three categories. In the second part of the paper, we consider an elementary relativistic system within Wigner’s approach defined as a locally-faithful irreducible strongly-continuous unitary representation of the Poincaré group in a quaternionic Hilbert space. We prove that, if the squared-mass operator is non-negative, the system admits a natural, Poincaré invariant and unique up to sign, complex structure which commutes with the whole algebra of observables generated by the representation itself. This complex structure leads to a physically equivalent reformulation of the theory in a complex Hilbert space. Within this complex formulation, differently from what happens in the quaternionic one, all self-adjoint operators represent observables in agreement with Solèr’s thesis, the standard quantum version of Noether theorem may be formulated and the notion of composite system may be given in terms of tensor product of elementary systems. In the third part of the paper, we focus on the physical hypotheses adopted to define a quantum elementary relativistic system relaxing them on the one hand, and making our model physically more general on the other hand. We use a physically more accurate notion of irreducibility regarding the algebra of observables only, we describe the symmetries in terms of automorphisms of the restricted lattice of elementary propositions of the quantum system and we adopt a notion of continuity referred to the states viewed as probability measures on the elementary propositions. Also in this case, the final result proves that there exists a unique (up to sign) Poincaré invariant complex structure making the theory complex and completely fitting into Solèr’s picture. The overall conclusion is that relativistic elementary systems are naturally and better described in complex Hilbert spaces even if starting from a real or quaternionic Hilbert space formulation and this complex description is uniquely fixed by physics.
Barrios Hita et al. [Phys. Rev. Lett. 136, 240202 (2026)] argued that quantum mechanics can be formulated over the real numbers by replacing the tensor-product postulate with a quotient-space construction, and concluded that complex numbers are therefore a matter of convenience. We show that the operational content of this construction is not that of a generic real Hilbert-space theory. Empirical equivalence requires a distinguished real linear operator J with J2=−1, and all physical effects, instruments, and dynamics must preserve the corresponding SO(2) gauge. Moreover, the composite-system rule is a balanced tensor product over this hidden complex structure, not the ordinary tensor product over R. In multipartite network scenarios, this changes the meaning of source independence: canonical real representatives are not source-factorizable in the usual tensor-product sense. Thus, the construction is best understood as standard complex quantum mechanics written in real notation, not as an independent real-amplitude theory. This clarifies what is, and is not, excluded by experiments testing the necessity of complex numbers.
We reconstruct complex quantum kinematics from a finite-dimensional real amplitude space equipped with projective measurement contexts and a positive mass geometry. Contextual completeness makes the generated algebra irreducible, so its neutral commutant has real, complex, or quaternionic scalar type. A positive-dimensional abelian group of neutral continuous phases selects exactly the complex branch. Normalizing its central generator yields a common operator J2 = −I, unique up to J ↦ −J, while the double commutant recovers the full algebra of complex-linear operators. The identity component of the orthogonal normalizer acts unitarily, and normalizer elements reversing the sign of J act antiunitarily. A commutator Laplacian identifies the neutral commutant and provides spectral-gap estimates for approximate neutrality and stable phase reconstruction. A three-context qubit example gives an explicit finite realization.
Paper XXII of the Six-Throat Directed Geometry program derives the mathematical structure of one-particle quantum mechanics from the directed spinor dynamics established on the regular closed six-throat spacetime. Starting from the conserved curved-spacetime Dirac current, the construction produces a positive and slice-independent inner product and its Hilbert-space completion. The corresponding geometric propagator is unitary and satisfies composition, inverse, and adjoint closure. Real physical observables are represented by self-adjoint operators with real spectra and orthogonal spectral resolutions. The compact global geometry yields an exact periodic commutator algebra, while the standard canonical position-momentum relation is recovered on regular local charts. The Robertson and Schrödinger-Robertson uncertainty relations then follow from the same Hilbert-space structure. The tetrad Clifford algebra generates the intrinsic spin-one-half representation, including the expected spin magnitude, the spinor sign reversal under a full two-pi rotation, and the rotational multiplets obtained by coupling orbital and intrinsic angular momentum. Detector-surface current is promoted from the direct Born-flux construction of Paper XXI to a general positive measurement structure described by a non-projective POVM and the trace probability rule. Density operators, purity, von Neumann entropy, conditional access states, and Kraus dephasing channels arise within the same measurement framework. The positive-energy Dirac branch reduces at low energy to Pauli dynamics and, when the spin-covariant terms commute appropriately, to the Schrödinger Hamiltonian. The same unitary framework yields the Liouville-von Neumann equation, the Heisenberg operator equation, and the Ehrenfest relations in the local directed potential. A forty-three-condition numerical closure audit satisfies all 43 declared conditions across the derived one-particle structure. Representative numerical results include Hilbert-product and unitary residuals at approximately machine precision, exact spin-one-half algebra to numerical precision, POVM completeness at machine precision, low-energy wave-packet infidelity of 3.16 × 10⁻¹⁰ for the stated audit configuration, and Schrödinger-Heisenberg expectation agreement at 5.66 × 10⁻¹⁷. The result is a single geometric chain from conserved spacetime flux to one-particle state space, unitary dynamics, observables, commutators, uncertainty, spin, measurement operators, statistical states, and the low-energy quantum equations. Composite systems, entanglement, exchange symmetry, quantum statistics, and variable-particle-number field theory remain subsequent structural layers and are not claimed as part of the present one-particle closure.
We formulate a second-order organization of quantum dynamics in which physical energy is identified with the magnitude of the temporal oscillation frequency, whereas the algebraic sign of the frequency specifies temporal phase orientation rather than the sign of energy. For any time-independent self-adjoint first-order evolution generator G^, the associated second-order parent wave equation naturally defines a nonnegative physical-energy operator through the spectral modulus E^pe=∣G^∣=G^2. Through the polar decomposition G^=S^E^pe, this construction decouples the nonnegative physical-energy magnitude from the signed spectrum of the first-order generator. Applied to the free Dirac equation, the Dirac Hamiltonian H^D retains its conventional unbounded spectrum (−∞,−mc2]∪[mc2,∞), whereas the physical-energy spectrum is bounded below by the rest energy, σ(E^pe)=[mc2,∞). For generators with a spectral gap around zero, the second-order dynamics admits a positive-definite conserved wave norm and an equivalent two-component unitary evolution. We further prove a positive-frequency spectral completeness theorem in L2(R) and show that the temporal Hilbert transform Ht provides a natural real-linear operator realization of the modulus mapping via ℏHt∂t↔ℏ∣Ω∣. Direct numerical diagonalization and high-precision time-domain integration of the one-dimensional Dirac equation confirm both the spectral modulus mapping and the exact conservation of quadratic wave energy across opposite temporal orientations. While this kinematic framework places quantum energy on the same nonnegative footing as classical field and signal energies, promoting the two temporal orientations to full particle and antiparticle sectors requires a second-quantized field theory incorporating charge conjugation, spin-statistics, locality, and interactions.