Projection Theory

Yiheng Qiu, Thomas M. Henderson, Jinmo Zhao +1 more

Coupled cluster theory is the method of choice for weakly correlated systems. But in the strongly correlated regime, it faces a symmetry dilemma, where it either completely fails to describe the system or has to artificially break certain symmetries. On the other hand, projected Hartree-Fock theory captures the essential physics of many kinds of strong correlations via symmetry breaking and restoration. In this work, we combine and try to retain the merits of these two methods by applying symmetry projection to broken symmetry coupled cluster wave functions. The non-orthogonal nature of states resulting from the application of symmetry projection operators furnishes particle-hole excitations to all orders, thus creating an obstacle for the exact evaluation of overlaps. Here we provide a solution via a disentanglement framework theory that can be approximated rigorously and systematically. Results of projected coupled cluster theory are presented for molecules and the Hubbard model, showing that spin projection significantly improves unrestricted coupled cluster theory while restoring good quantum numbers. The energy of projected coupled cluster theory reduces to the unprojected one in the thermodynamic limit, albeit at a much slower rate than projected Hartree-Fock.

David Halenta

The Quantum Continuum Theory (QCT) presents a meta-theoretical framework in which quantum mechanics, spacetime geometry, and cognition emerge as projections from an invariant pre-ontic continuum (Ω). Unlike Copenhagen, Many-Worlds, Bohm, or QBism, QCT eliminates auxiliary mechanisms—collapse, branching, hidden variables—by treating projection as the fundamental generative principle. Five axioms formalize invariance, emergent spacetime as projection interference, and relational objectivity through projection overlap. QCT resolves the measurement problem structurally, reinterprets entanglement without nonlocality, addresses the preferred-basis problem, and aligns with timeless quantum cosmology (Wheeler–DeWitt equation). By deriving coherence, nonlocality, and spacetime from projection principles rather than postulating them independently, QCT provides explanatory depth across quantum foundations, emergent gravity, and quantum cognition, maintaining formal equivalence with standard quantum mechanics in projection-dense limits.

George H. Weinberg, Diane L. Rowe

Sheldon Goldstein

Stephen Bruce Sontz

In a recent paper it was shown that all the Hilbert space formulas for quantum probabilities can be realized as functions of geometric properties of the associated projective space, but those functions were expressed using the structures of the associated Hilbert space. In this paper a direct description of all these probabilities is given as formulas involving only the geometric properties of the projective space itself without referring to the associated Hilbert space theory. In large part this depends on a projection theorem for complex projective space which is analogous to the projection theorem for Hilbert spaces. The importance of this is that this exhibits quantum probability in terms of the geometry of a Riemannian metric in a non-linear Kähler manifold without any reference to a linear Hilbert space. As such this is a part of a larger program of the geometrization of physics. This opens the possibility of generalizations of quantum theory in other similar geometric settings. The theory presented includes projective spaces of both finite and infinite dimension. Some comments explain how quantum theory based on a von Neumann algebra is compatible with this approach.