Hydrodynamic turbulence cannot transport angular momentum effectively in astrophysical disks

The power behind the stars The accretion disks that harness gravitational energy to power quasars or form stars and planets are among the most efficient energy sources known. A disk around a black hole, for instance, converts rest-mass energy to radiation at up to 40% efficiency. The nature of this conversion remains something of a mystery. Taylor–Couette experiments (involving fluid flow between rotating cylinders) are central to studies of nonlinear fluid dynamics and transition to turbulence, but mostly in flow regimes irrelevant to astrophysics. In a rare example of experimental astrophysics, a Taylor–Couette apparatus was used to model the forces involved in a rotating fluid in astrophysical conditions. The results rule out purely hydrodynamic turbulence, thereby supporting magnetorotational instabilities as the likely cause of turbulence.