Operator size at finite temperature and Planckian bounds on quantum dynamics

It has long been believed that dissipative time scales $\tau$ obey a "Planckian" bound $\tau \gtrsim \frac{\hbar}{k_{\mathrm{B}}T}$ in strongly coupled quantum systems. Despite much circumstantial evidence, however, there is no known $\tau$ for which this bound is universal. Here we define operator size at finite temperature, and conjecture such a $\tau$: the time scale over which small operators become large. All known many-body theories are consistent with this conjecture. This proposed bound explains why previously conjectured Planckian bounds do not always apply to weakly coupled theories, and how Planckian time scales can be relevant to both transport and chaos.