Theoretical Physics seminars

Quasinormal-mode frequencies characterize black-hole spacetimes and can be measured with gravitational waves, but small perturbations to the gravitational potential can substantially change the spectrum. This seminar studies that spectral instability in a time-domain waveform model. A frequency shift appears only after a delay set by the echo time between the perturbation and the primary potential. In the perturbative limit, the post-echo shift agrees with the usual frequency-domain result; outside that limit, the intermediate-time signal cannot be represented by a constant frequency shift.

This talk is about the proposal that (1) quantum states that violate the momentum and Hamiltonian constraints represent instances of time, and (2) time evolution corresponds to the gradual collapse of an initial state toward a diffeomorphism-invariant state, driven by stochastic fluctuations of the lapse and shift. During the wavefunction collapse, the scale factor increases monotonically and thus acts as a clock. Scalar, vector, and tensor gravitons arise as physical excitations. In the long-time limit, the tensor gravitons exhibit emergent unitary dynamics, while the extra modes are damped by the non-unitary dynamics that suppress constraint-violating excitations. I will also comment on possible physical implications of these extra graviton modes.

Salvatore Pace discusses the order of anomalies in global symmetries: the number of identical copies needed before their diagonal symmetry becomes anomaly-free. Familiar lattice examples often have finite order, whereas perturbative continuum anomalies have infinite order. The seminar examines Onsager symmetry as a lattice counterpart of chiral symmetry for a massless Dirac fermion in one spatial dimension. Its anomaly has order two, but imposing CPT symmetry on the lattice changes it to infinite order, bringing the lattice description closer to the continuum. The work was carried out with Elijah Lew-Smith and Shu-Heng Shao and is reported in arXiv:2606.12510. Attendance is at the IAS Bloomberg Lecture Hall; the organizer also offers Zoom access by request through the contact listed on the event page.