Quantum Physics seminars

Consistency requirements on scattering amplitudes can constrain quantum gravity without a perturbative expansion. This talk applies analyticity, crossing and unitarity to maximally supersymmetric gravitational theories in ten and eleven dimensions. It extends bounds from the leading correction to higher-order effective couplings, comparing the allowed regions with string-theory and M-theory predictions. A nearby kink in eleven dimensions suggests a possible route toward characterizing the M-theory scattering matrix through consistency alone.

Can quantum uncertainty resolve cosmological singularities within the Wheeler–DeWitt description itself? Introducing an observer's proper time turns the problem into evolution resembling scattering in a central potential. The talk describes a smooth quantum bounce for a radiation-filled universe, with large scale-factor uncertainty, and explains why anisotropy introduces a critical attractive potential. That harder regime motivates extending the classical BKL account of singularities into quantum theory.

Representing quantum states through measurement probabilities can preserve subsystem structure or faithfully track nearby states, but not necessarily both. This creates robustness problems for axiomatic reconstructions based on generalized probabilistic theories. The talk introduces an operational alternative that treats a state as an observer's knowledge, aiming to compare possible physical theories while avoiding instability under small changes of the theory.

Operational definitions identify subsystems through the transformations an agent can perform. This talk classifies complementary, bicommutant-closed subsystems using unitary channels, whose disregard for global phase permits possibilities beyond conventional operator subalgebras. The classification involves graded subalgebras. Some resulting subsystems are nontrivial yet support no measurements, raising a foundational choice between excluding them and accepting agents that act without being able to observe.

This talk reexamines black-hole information through the completeness of observables at infinity in gravitational theories. That completeness challenges radial factorization of the bulk state space and assumptions underlying familiar information-loss arguments. Within this account, information is already represented asymptotically. Restricting an observer's available measurements can nevertheless produce Page curves, interpreted as redistribution among accessible observables rather than information newly escaping the hole.

Michael Borinsky introduces tropical quantum field theory as a simpler deformation limit that preserves useful information about an interacting theory. Its numerical algorithm computes massive-scalar perturbation theory with runtime polynomial in loop order and external-leg count, avoiding the factorial growth of a Feynman-diagram expansion and making very high loop orders feasible. Geometrically, amplitudes become volumes of graph-moduli spaces, whose recursions resemble those for Weil–Petersson volumes. The talk connects the limiting construction, geometry and algorithm.