  This short review is intended as a colloquium-level summary, for the Snowmass
2021 process, on recent theoretical results on infrared observables in quantum
gravity. We rely on simple physical arguments, most notably a random walk
intuition, to show how effects of quantum gravity in the ultraviolet (at the
Planck length $\ell_p \approx 10^{-35} \mbox{ m}$) may integrate into the
infrared when the large measurement length scale $L$ enters into the
observable. A quantum uncertainty at lightsheet horizons would give rise to an
accumulated effect of size $\delta L^2 \simeq \ell_p L/4 \pi$. We discuss how
the random walk intuition falls out from more formal calculations, such as from
AdS/CFT, from the dimensional reduction of the Einstein-Hilbert action to
dilaton gravity, from multiple gravitational shockwaves generated by vacuum
energy fluctuations, as well as from an effective description of gravity as a
fluid. We overview experimental prospects for measuring this effect with a
simple Michelson interferometer utilizing many of the tools developed for
gravitational wave observatories.
