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.
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 complex Hilbert space of standard quantum mechanics may be treated as a real Hilbert space. The pure states of the complex theory become mixed states in the real formulation. It is then possible to generalize standard quantum mechanics, keeping the same set of physical states, but admitting more general observables. The standard time reversal operator involves complex conjugation, in this sense it goes beyond the complex theory and may serve as an example to motivate the generalization. Another example is unconventional canonical quantization such that the harmonic oscillator of angular frequency ω has any given finite or infinite set of discrete energy eigenvalues, limited below by ℏω/2.
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.
In principle, the lattice of elementary propositions of a generic quantum system admits a representation in real, complex or quaternionic Hilbert spaces as established by Solèr’s theorem (1995) closing a long standing problem that can be traced back to von Neumann’s mathematical formulation of quantum mechanics. However up to now there are no examples of quantum systems described in Hilbert spaces whose scalar field is different from the set of complex numbers. We show that elementary relativistic systems cannot be described by irreducible strongly-continuous unitary representations of SL(2, C) on real or quaternionic Hilbert spaces as a consequence of some peculiarity of the generators related with the theory of polar decomposition of operators. Indeed such a ”naive” attempt leads necessarily to an equivalent formulation on a complex Hilbert space. Although this conclusion seems to give a definitive answer to the real/quaternionic-quantum-mechanics issue, it lacks consistency since it does not derive from more general physical hypotheses as the complex one does. Trying a more solid approach, in both situations we end up with three possibilities: an equivalent description in terms of a Wigner unitary representation in a real, complex or quaternionic Hilbert space. At this point the ”naive” result turns out to be a definitely important technical lemma, for it forbids the two extreme possibilities. \nIn conclusion, the real/quaternionic theory is actually complex. This improved approach is based upon the concept of von Neumann algebra of observables. Unfortunately, while \nthere exists a thorough literature about these algebras on real and complex Hilbert spaces, an analysis on the notion of von Neumann algebra over a quaternionic Hilbert space is \ncompletely absent to our knowledge. There are several issues in trying to define such a mathematical object, first of all the inability to construct linear combination of operators \nwith quaternionic coeffients. Restricting ourselves to unital real *-algebras of operators we are able to prove the von Neumann Double Commutant Theorem also on quaternionc \nHilbert spaces. Clearly, this property turns out to be crucial.
Abstract The functional integral representation of the density matrix at thermal equilibrium in non-relativistic quantum mechanics (QM) with many degrees of freedom, in the grand canonical formulation is introduced. In QM, Hamiltonians H(p,q) can be also expressed in terms of creation and annihilation operators, a method adapted to the study of perturbed harmonic oscillators. In the holomorphic formalism, quantum operators act by multiplication and differentiation on a vector space of analytic functions. Alternatively, they can also be represented by kernels, functions of complex variables that correspond in the classical limit to a complex parametrization of phase space. The formalism is adapted to the description of many-body boson systems. To this formalism corresponds a path integral representation of the density matrix at thermal equilibrium, where paths belong to complex spaces, instead of the more usual position–momentum phase space. A parallel formalism can be set up to describe systems with many fermion degrees of freedom, with Grassmann variables replacing complex variables. Both formalisms can be generalized to quantum gases of Bose and Fermi particles in the grand canonical formulation. Field integral representations of the corresponding quantum partition functions are derived.
In standard presentations of mathematics and quantum physics the imaginary unit i is introduced axiomatically as a formal symbol with i 2 = −1, and the complex field C is then postulated as the natural home for wave functions and amplitudes. The physical reasons why such a structure is singled out are usually left implicit. In this paper we give a systematic derivation and reinterpretation of i from three complementary viewpoints, organised within the PAC–µ 8 framework. Part I adopts an axiomatic + vibrational (AV) viewpoint: starting from a real Hilbert space describing vibrational modes, together with three simple assumptions about time– translation, isotropy and phase completeness, we show that there is a distinguished linear operator J with J 2 = −1 acting as a quarter–period phase shift. Complex numbers arise by representing J as multiplication by i, so that “i” is nothing but the abstract 90◦ rotation operator on the vibrational plane. Part II adopts an axiomatic + quantum (AQ) viewpoint. We reformulate standard quantum postulates in terms of a real Hilbert space equipped with a complex structure J and a positive spectral generator K, with Ω = K1/2 and H = ℏΩ. Recovering the usual unitary time evolution Ut = exp(−iHt/ℏ) then requires no additional postulate about i: the symbol i is simply the matrix representation of J in a preferred basis. Part III connects i to the Phase–Arithmetic Spectral (PAS) axis of PAC–µ 8 . We discuss how phase geometry, irrational numbers such as √ 2 and π, Euler’s formula e iθ, Dirichlet characters and arithmetic L–functions can all be understood as expressions of the same underlying complex structure. In this sense i is the pivot that relates geometric rotations, vibrational phases and arithmetic spectra. Part IV illustrates the structural role of i on the physical side through four canonical examples: the harmonic oscillator, spin–1/2, two–path interference, and one–qubit gates in quantum information. In each case we show how i implements a 90◦ phase rotation in an appropriate two–dimensional vibrational plane. Part V distils these insights into concrete recommendations for rewriting standard text book and axiomatic treatments of complex numbers and quantum mechanics. We propose replacing the bare postulate “work over C” by a vibrationally motivated assumption of a real Hilbert space equipped with a complex structure J, and we outline how this shift clarifies the role of i in both physics and arithmetic. The overarching message is that i need not be treated as an arbitrary symbol: in the PAC–µ 8 framework it is a derived and geometrically natural object—the abstract quarter– period operator that organises vibrations, quantum phases and arithmetic characters into a single spectral story.