  The central engines of disc-accreting stellar-mass black holes appear to be
scaled down versions of the supermassive black holes that power active galactic
nuclei. However, if the physics of accretion is universal, it should also be
possible to extend this scaling to other types of accreting systems,
irrespective of accretor mass, size, or type. We examine new observations,
obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and
young stellar objects. Every object in the sample displays the same linear
correlation between the brightness of the source and its amplitude of
variability (rms-flux relation) and obeys the same quantitative scaling
relation as stellar-mass black holes and active galactic nuclei. We also show
that the most important parameter in this scaling relation is the physical size
of the accreting object. This establishes the universality of accretion physics
from proto-stars still in the star-forming process to the supermassive black
holes at the centers of galaxies.
