  Friction in liquids arises from conservative forces between molecules and
atoms. Although the hydrodynamics at the nanoscale is subject of intense
research and despite the enormous interest in the non-Markovian dynamics of
single molecules and solutes, the onset of friction from the atomistic scale so
far could not be demonstrated. Here, we fill this gap based on
frequency-resolved friction data from high-precision simulations of three
prototypical liquids, including water. Combining with rigorous theoretical
arguments, we show that friction in liquids emerges abruptly at a
characteristic frequency, beyond which viscous liquids appear as
non-dissipative, elastic solids; as a consequence, its origin is non-local in
time. Concomitantly, the molecules experience Brownian forces that display
persistent correlations and long-lasting memory. A critical test of the
generalised Stokes-Einstein relation, mapping the friction of single molecules
to the viscoelastic response of the macroscopic sample, disproves the relation
for Newtonian fluids, but substantiates it exemplarily for water and a
moderately supercooled liquid. The employed approach is suitable to yield novel
insights into vitrification mechanisms and the intriguing mechanical properties
of soft materials.
