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arXiv:2506.07632

Notes on Real Quantum Mechanics in a Kahler Space

I. Ya. Aref’eva, Igor Vasil'evich Volovich

Published Jun 9, 2025• 0 citations
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Abstract

The necessity of complex numbers in quantum mechanics has long been debated. This paper develops a real Kahler space formulation of quantum mechanics [19], asserting equivalence to the standard complex Hilbert space framework. By mapping the complex Hilbert space ${\mathbb C}^n$ to a real Kahler space $K ^{2n}$, i.e. ${\mathbb R}^{2n}$ equipped with a metric, a symplectic structure and an automorphism, we establish a correspondence between Hermitian operators in ${\mathbb C}^n$ and real operators in $K^{2n}$. While the isomorphism appears straightforward some subtleties emerge: (i) the overcounting of composite system states under tensor products in ${\mathbb R}^{2n}$, and (ii) the double degeneracy of operator spectra in the real formulation. Through a systematic investigation of these challenges, we clarify the structural relationship between real and complex formulations, resolve ambiguities in composite system representations, and analyze spectral consequences. Our results reaffirm the equivalence of the two frameworks while highlighting nuanced distinctions with implications for foundational debates on locality, phase invariance, and the role of complex numbers in quantum theory.

DOI
10.48550/arxiv.2506.07632

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•Apr 13, 2026•0 citations
The necessity of complex numbers in quantum mechanics has long been debated. This paper develops a real Kahler space formulation of quantum mechanics [19], asserting equivalence to the standard complex Hilbert space framework. By mapping the complex Hilbert space Cn{\mathbb C}^nCn to a real Kahler space K2nK ^{2n}K2n, i.e. R2n{\mathbb R}^{2n}R2n equipped with a metric, a symplectic structure and an automorphism, we establish a correspondence between Hermitian operators in Cn{\mathbb C}^nCn and real operators in K2nK^{2n}K2n. While the isomorphism appears straightforward some subtleties emerge: (i) the overcounting of composite system states under tensor products in R2n{\mathbb R}^{2n}R2n, and (ii) the double degeneracy of operator spectra in the real formulation. Through a systematic investigation of these challenges, we clarify the structural relationship between real and complex formulations, resolve ambiguities in composite system representations, and analyze spectral consequences. Our results reaffirm the equivalence of the two frameworks while highlighting nuanced distinctions with implications for foundational debates on locality, phase invariance, and the role of complex numbers in quantum theory.
In this paper, we demonstrate the equivalence between the complex Hilbert space and real Kahler space formulations of quantum mechanics. Complex numbers play an important role in the traditional formulation of quantum mechanics in complex Hilbert spaces. However, the necessity of complex numbers--as opposed to their mere convenience--remains a subject of debate. Several alternative formulations of quantum mechanics using real numbers have been proposed. In this paper, we demonstrate that standard quantum mechanics, formulated in a complex Hilbert space, admits an equivalent reformulation in a real Kahler space. By establishing a natural isomorphism between the operator theories of the complex Hilbert space and the real Kahler space, we prove the equivalence of the two formulations including composite system. This Kahler-space framework preserves all essential features of quantum mechanics while offering a key advantage: it inherently incorporates a Hamiltonian symplectic structure analogous to classical mechanics. This structural alignment provides a unified geometric perspective for both classical and quantum dynamics. Additionally, we show that the ergodicity of finite-dimensional quantum systems becomes manifest in this framework, resolving interpretational ambiguities present in conventional complex formulations.
Standard quantum mechanics employs complex Hilbert spaces, but whether complex numbers are fundamental or merely convenient has long been debated. For decades, real-valued equivalents were considered mathematically possible but cumbersome. However, a highly cited 2021 result claimed that any quantum theory based on real numbers is experimentally falsifiable via network Bell experiments. Yet, it remains an open question whether this falsification applies to all real-valued theories. Here we show that this conclusion rests on an incomplete real formulation, and we present a rigorous real-valued framework that perfectly reproduces all predictions of standard quantum mechanics. We demonstrate that the standard real tensor product (⊗R\otimes_{\mathbb{R}}⊗R​) used in previous no-go theorems is algebraically incompatible with the rich structure of conventional quantum mechanics. We present a real framework based on Kähler space and prove that it is exactly isomorphic to established quantum mechanics via an explicit bijection γγγ. The isomorphism extends to composite systems through a symplectic composition rule $\otimes^{\ks}$ that replaces the Kronecker product. Consequently, our formulation achieves the maximal CHSH3\mathrm{CHSH}_{3}CHSH3​ violation of 626\sqrt{2}62​ using purely real variables, demonstrating that the no-go theorem is specific to a particular real representation of states and operators and to the composition rule ⊗R\otimes_\mathbb{R}⊗R​ built upon it, neither of which extends to the present Kähler framework. These results demonstrate that complex numbers are not fundamentally required by nature; rather, they encode a deeper real geometric structure that governs quantum interference and entanglement, settling this long debate.
Abstract We investigate the isomorphism between the formulations of quantum mechanics in a finite-dimensional complex Hilbert space and in a real Kähler space. This investigation establishes a direct correspondence between Hermitian operators in the complex Hilbert space and real operators in the Kähler space. In this real formulation, we note the emergent property of spectral degeneracy. Moreover, we show that the structure of the Kähler space provides a natural description of composite systems in real quantum mechanics.
Barrios Hita et al. [Phys. Rev. Lett. 136\bf{136}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 JJJ with J2=−1J^2 = -\mathbb{1}J2=−1, and all physical effects, instruments, and dynamics must preserve the corresponding SO(2)SO(2)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\mathbb{R}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.
This work reformulates quantum mechanics on the real plane (R2)(\mathbb{R}^2)(R2), establishing an algebraic isomorphism to the complex field (C)(\mathbb{C})(C) and demonstrating that the complex structure inherent to the Schrödinger equation can be fully captured by coupled real-valued dynamics. By constructing a field (R2,+,⋅)(\mathbb{R}^2, +, \cdot)(R2,+,⋅) isomorphic to C\mathbb{C}C, the real counterpart of the Schrödinger equation is derived, yielding a system of coupled real partial differential equations. This system reveals a symplectic structure where the symplectic matrix J\mathcal{J}J geometrically represents the imaginary unit iii, acting as a phase-space rotation operator. Time-independent solutions decouple into identical real Helmholtz equations, preserving spectral equivalence. The reformulation provides geometric insights into foundational aspects: the geometric representation of iii, the emergence of probability conservation from real-component interactions, and the encoding of quantum phase relationships through the multiplicative structure of the R2\mathbb{R}^2R2 field. This perspective reinforces connections to classical Hamiltonian mechanics and geometric quantization, suggesting that complex numbers are structural, emerging from field operations on the real plane, rather than fundamental. Furthermore, quantum computation elements like qubits and gates are shown to have equivalent representations as real symplectic and special orthogonal matrices, maintaining unitary equivalence. We introduce a framework for quantum computation within a real vector space and provide a generalized protocol for generating entangled Bell states. These results offer a new perspective on the necessity of complex numbers in quantum theory, providing a framework with potential computational advantages for hybrid quantum-classical systems and enhanced geometric interpretability.
This paper proves that quantum mechanics emerges as a mathematical necessity when a deterministic classical system possesses compact Kähler dimensions inaccessible to observation. The proof rests on three independent results from established mathematics: Spectral discreteness: The spectral theorem for elliptic operators on compact manifolds guarantees a discrete eigenspectrum, producing superposition and complex phase structure. The uncertainty principle: The Gromov non-squeezing theorem establishes an irreducible lower bound on observable uncertainties, with Planck's constant identified as the symplectic capacity of the compact space. The Born rule: The unique Kähler-compatible L² inner product determines the probability rule P = |ψ|² with no alternatives. Together, these results demonstrate that the complete axiomatic structure of quantum mechanics — complex Hilbert space, superposition, interference, the uncertainty principle, the Born rule, and entanglement — is a necessary consequence of dimensional projection rather than a set of independent postulates. The paper further shows that: The complex numbers in quantum mechanics originate from the complex structure (J) of the compact manifold, providing a geometric explanation for recent experiments ruling out real-valued quantum mechanics (Renou et al., Nature 2021; Chen et al., PRL 2022). Unitary time evolution arises because the complex structure converts what would otherwise be dissipative dynamics into probability-preserving oscillation. The number field of quantum mechanics (real, complex, or quaternionic) is determined by the topology of the compact dimensions (Riemannian, Kähler, or hyperkähler respectively), making laboratory quantum experiments probes of extra-dimensional geometry. CPT symmetry corresponds to orientation-invariance of the compact manifold, connecting discrete symmetry tests to the topology of the extra dimensions. The theorem requires no physical assumptions beyond the existence of inaccessible compact Kähler dimensions. It applies to any number of dimensions, any compact Kähler geometry, and any deterministic dynamics. The paper concludes that quantum mechanics is not a law of nature but a law of perspective — the second such law in physics, following relativity. Where relativity revealed that the separation of spacetime into space and time is observer-dependent, this work shows that the separation of dynamics into deterministic and probabilistic is equally observer-dependent. Originally submitted to Foundations of Physics on March 16, 2026