  The Magnetorotational Instability (MRI) has long been considered a plausibly
ubiquitous mechanism to destabilize otherwise stable Keplerian flows to support
radially outward transport of angular momentum. Such an efficient transport
process would allow fast accretion in astrophysical objects such as stars and
black holes to release copious kinetic energy that powers many of the most
luminous sources in the universe. But the standard MRI under a purely vertical
magnetic field has heretofore never been directly measured despite numerous
efforts over more than a decade. Here we report an unambiguous laboratory
demonstration of the spring-mass analogue to the standard MRI by comparing
motion of a spring-tethered ball within different rotating flows. The
experiment corroborates the theory: efficient outward angular momentum
transport manifests only for cases with a weak spring in quasi-Keperian flow.
Our experimental method accomplishes this in a new way, thereby connecting
solid and fluid mechanics to plasma astrophysics.
