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Complex Numbers in Quantum Theory

Glenn H. Maynard

Published Aug 1, 2015• 0 citations
Publisher

Abstract

In 1927, Nobel prize winning physicist, E. Schrodinger, in correspondence with Ehrenfest, wrote the following about the new theory: “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. Psi is surely fundamentally a real function.” This seemingly simple issue remains unexplained almost ninety years later. In this dissertation I elucidate the physical and theoretical origins of the complex requirement. I identify a freedom/constraint situation encountered by vectors when, employed in accordance with adopted quantum representational methodology, and representing angular momentum states in particular. Complex vectors, quite simply, provide more available adjustable variables than do real vectors. The additional variables relax the constraint situation allowing the theory’s representational program to carry through. This complex number issue, which lies at the deepest foundations of the theory, has implications for important issues located higher in the theory. For example, any unification of the classical and quantum accounts of the settled order of nature, will rest squarely on our ability to account for the introduction of the imaginary unit.

DOI
10.12794/metadc804988

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In 1927, Nobel prize winning physicist, E. Schrodinger, in correspondence with Ehrenfest, wrote the following about the new theory: “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. Psi is surely fundamentally a real function.” This seemingly simple issue remains unexplained almost ninety years later. In this dissertation I elucidate the physical and theoretical origins of the complex requirement. I identify a freedom/constraint situation encountered by vectors when, employed in accordance with adopted quantum representational methodology, and representing angular momentum states in particular. Complex vectors, quite simply, provide more available adjustable variables than do real vectors. The additional variables relax the constraint situation allowing the theory’s representational program to carry through. This complex number issue, which lies at the deepest foundations of the theory, has implications for important issues located higher in the theory. For example, any unification of the classical and quantum accounts of the settled order of nature, will rest squarely on our ability to account for the introduction of the imaginary unit.
Quantum mechanics seems to use complex numbers in a fundamentally different way when compared to classical physics. Selected episodes from early stages of the theory are presented to identify different approaches to the use of complex numbers. If for Heisenberg it seemed quite natural to use complex exponentials, Schrödinger clearly struggled with the complex nature of his wave function and tried to assign physical meaning only to its real component. Dirac was probably the first to stress that complex quantities in quantum mechanics cannot be split up into pure real and imaginary parts. Possible educational implications are outlined.
In this contribution I outline some of Schrödinger’s initial resistance to the complex nature of his wave function. It is shown how he first attached physical meaning only to its real component and even tried to avoid the explicit appearance of the imaginary unit in his fundamental (timedependent) equation. This attitude is quite understandable, since he was committed to the classical framework of wave theory, not only ontologically, but also formally. Around two years after the publication of his famous papers on wave mechanics, Schrödinger seems to have accepted that the wave function must be complex and that the physical interpretation is to be related to its absolute square. This is justified by him both due to equivalence considerations with matrix mechanics and in analogy with Maxwell’s equations. This episode can be of special pedagogical relevance as students often struggle with the complex nature of the wave function on their first encounter with it.
Abstract Real polarizations play an important part in linking classical and quantum mechanics. We shall see that picking a polarization in a classical phase space amounts to choosing a representation in the underlying quantum theory-’representation’ in the sense of a position or momentum space representation. We shall also come across complex representations, which in many ways are easier to handle. An obvious example is the Fock space treatment of the harmonic oscillator, in which the quantum Hilbert space is realized as a space of holomorphic functions Complex representations arise from complex polarizations, which are natural generalizations of real polarizations: one simply relaxes the condition that the Hamiltonian vector fields spanning P should be real.
The indispensable nature of complex numbers in quantum mechanics has beenexperimentally confirmed, yet its deep origin — whether complex numbers are anad hoc introduction or a logical consequence of some more fundamental constraint—remains insufficiently clarified. From the perspective of symmetry analysis, thispaper argues that the complex structure is not an accidental choice of quantumtheory, but an inevitable outcome under the joint constraints of spacetime transformation symmetry and algebraic completeness. Through a rigorous analysis ofthe covariance of the Schrödinger equation under Galilean transformations, thispaper reveals: if the transformation rule of the wave function is required to preserve the probability interpretation and satisfy spacetime symmetry, then the wavefunction must take values in the complex number field. This conclusion can beextended to relativistic cases, forming an intrinsic echo with the role of mass asa central charge in Bargmann’s theorem. On this basis, we establish a quantitative criterion for the strength of symmetry constraints, proving that the transitionfrom a real-valued formulation of quantum theory to a complex-valued formulationcorresponds to a central extension of the symmetry algebra, which is the uniqueway for a physical theory to realize symmetry in the sense of projective representations. This framework is compatible with recent experimental advances regardingimaginarity as a quantum resource and multipath interference tests of hypercomplex theories, and proposes a symplectic geometry-based scheme for measuring thestrength of symmetry constraints. This paper advances the foundational issue ofcomplex numbers in quantum theory from an ”ontological debate” to a new levelof ”symmetry constraint analysis.”
In this paper, some of Schrödinger's initial struggles with the complex nature of his wave function are outlined. It is shown that he first attached physical meaning only to its real component and even tried to avoid the explicit appearance of the imaginary unit i in his fundamental (time-dependent) equation. This attitude is quite understandable, since he was committed to the classical framework of wave theory, not only ontologically, but also formally. Around one year after the publication of his famous papers on wave mechanics, Schrödinger seems to have accepted that the wave function must be complex and that the physical interpretation is to be related to its absolute square. This is justified by him both due to equivalence considerations with matrix mechanics and in analogy with Maxwell's equations. It is argued that an awareness of the struggles Schrödinger went through to accept a complex wave function can be a relief to students who themselves wonder why ψ needs to be complex when they encounter the topic for the first time.
> In standard quantum mechanics, the imaginary part of the wavefunction is treated as a mathematical convenience. This work shows that it corresponds to a real internal coordinate in a strictly real two-component representation Ψ = (R, I). The quantum phase is identified as a rotation angle in this internal plane, and Schrödinger dynamics becomes a continuous rotation between R and I. Crucially, the I-component — though not directly measurable in a single projective measurement — can be fully reconstructed via controlled phase shifts and interferometry against a known reference. This explains why real-only models fail: they lack the second degree of freedom required to produce the sinθ term in interference patterns (P(θ) = A + B cosθ + C sinθ, with C ≠ 0). The model is empirically equivalent to complex quantum mechanics but provides a clear physical interpretation of the “imaginary” part as an internal information coordinate. > This document is self-contained, uses no complex notation, and is fully compatible with established quantum theory. It includes a testable experimental signature and connects to quantum tomography, interferometry, and foundational questions about the role of complex numbers in physics. This work is conceptually related to recent discussions on the necessity of complex numbers in quantum theory (Renou et al. 2021; Chen et al. 2022; Weilenmann et al. 2025), but takes a different approach by giving an explicitly real two-component interpretation of the imaginary part.”