Recordings

Understanding Classical Stability

Speaker: Jordi Gaset Rifà (CUNEF Universidad, Spain)  |  Date: October 20th, 2026
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Abstract

The main classical obstruction to the physical viability of higher-order field theories is their potential instability. In this talk, we will elucidate what stability of classical systems means and how to analytically (dis)prove it. We will start by defining and classifying the different notions of stability and their relation to other problems in physics and mathematics. Then, we will explain the existing analytical methods to prove stability, including the most general one based on confining functions. We will end with a discussion on the limits of current analytical methods, and the relevance of integrability in this context. For relevant publications see 2504.16169, 2408.16832 The slides used in the talk can be found here.

Recording

Stable Quantum Ghost

Speaker: Alexander Vikman (FZU - Institute of Physics of the Czech Academy of Sciences, Czech Republic)  |  Date: October 13th, 2026
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Abstract

I will discuss results of our recent work e-Print 2604.21823 [hep-th]. There we quantized a system consisting of a harmonic oscillator polynomially coupled to a ghost with negative kinetic energy. Such systems often arise from reformulating the Ostrogradski Hamiltonian of higher-derivative theories. In our previous work JCAP 11 (2023) 031, e-Print: 2305.09631 [gr-qc], we demonstrated that the interactions in this system are such that the motion is globally stable for all initial data. We proved that, owing to an integral of motion with a positive discrete spectrum, the following hold: i) the Hamiltonian has a pure point spectrum unbounded in both directions, ii) the evolution is manifestly unitary, iii) the vacuum is well defined, and iv) expectation values for the squares of canonical variables are all bounded. Numerical solutions of the Schrödinger equation confirmed these results. We argued that the discrete spectrum of the integral of motion enforces stability for extended interactions.

Recording

Higher Derivative Theories: the Good, the Bad and the Ugly

Speaker: Richard P Woodard (University of Florida, USA)  |  Date: October 6th, 2026
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Abstract

Aside from philosophical interest, Lagrangians that contain higher time derivatives hold great interest for quantum gravity owing to the theorem of the late Kelly Stelle that adding fundamental Ricci-squared and Weyl-squared terms to the Hilbert action would result in a perturbatively renormalizable theory. The reason we still have a ``problem of quantum gravity'' is that the Weyl-squared term is not allowed. I explain why not, and hopefully debunk some of the confused and confusing literature on this subject. This talk is based on astro-ph/0601672, arXiv:2306.09596, arXiv:1506.022 and arXiv:2602.16190.. The slides can be found here

Recording