In my talk, I will provide an overview of recent advances in understanding and controlling wave propagation, as enabled by the concept of the Wigner–Smith operator. Originally proposed by Eugene Wigner and Felix Smith in the context of scattering time delay, this operator has meanwhile been generalised [1,2] to serve as a key tool in a variety of applications, including wave focusing [2,3], maximising optical forces and torques [2,4], optical trapping [5], efficiently cooling levitated particles [6], transmission through multi-mode fibers [7], estimating system parameters [8], and optical inverse-design [9]. Furthermore, I will demonstrate that the generalised Wigner–Smith operator is strongly connected to the forces of the quantum vacuum [10], and its expectation values can be unambiguously decomposed into geometric and dynamical parts [11]. This generalises Berry's seminal concept of the geometric phase to a broad class of scattering problems. References [1] Ambichl et al., Phys. Rev. Lett. 119, 033903 (2017). [2] Horodynski et al., Nature Photonics 14, 149 (2020). [3] Goıcoechea et al., Phys. Rev. Lett. 134, 183802 (2025). [4] Orazbayev et al., Nature Physics 14, 149 (2024). [5] Butaite et al., Sci. Adv. 10, eadi7792 (2024). [6] Hüpfl et al., Phys. Rev. Lett. 130, 083203 (2023). [7] Matthès et al., Phys. Rev. X 11, 021060 (2021). [8] Bouchet et al., Nature Physics 17, 564 (2021). [9] Horodynski et al., Nature 607, 281 (2022). [10] Rachbauer et al., J. Opt. Soc. Am. B 41, 2122 (2024). [11] Bliokh et al., arXiv:2506.07144.