2021 Paul Dirac Medal and Prize

Professor Steven Balbus for fundamental contributions to the theory of accretion-disc turbulence and the dynamical stability of astrophysical fluids, breaking new ground by establishing the critical role played by weak magnetic fields.

Professor Steven Balbus IOP Paul Dirac Medal and Prize winner 2021

Professor Steven Balbus ranks amongst the most influential astrophysical fluid dynamicists of his generation. His interests and contributions are wide-ranging.

His major contributions include, but are not limited to, explaining the origin of turbulence in accretion discs, discovering several important instabilities in magnetised plasmas that remain the focus of current research, developing a novel and accurate theory for the Sun’s internal rotation, and pioneering the theory of time-dependent black-hole accretion discs.

His 1991 paper with John Hawley is one of the most heavily cited theoretical papers in the astrophysical literature (3,315 citations), solving a decades-old problem by elucidating the mechanism that generates turbulence and transport in accretion discs. The magnetorotational instability (MRI) is present in any differentially rotating gaseous fluid with even trace ionisation.

With Balbus leading the analysis and Hawley the massive simulations, the well-paired team established that the MRI led to fully developed turbulence. Balbus and Hawley were recognised with the 2013 Shaw Prize in Astronomy for this accomplishment. The MRI is the basis for simulations of turbulent astrophysical discs, including those carried out for comparison with the observations made by the Event Horizon Telescope of galaxy M87’s black hole.

Balbus went on to apply the MRI to both low-ionisation gases and stellar interiors; the instability has been invoked by researchers to explain a class of gamma ray bursts formed by a ‘failed’ supernova core implosion.

Balbus also demonstrated that the dramatic dynamical influence of weak magnetic fields extends beyond rotating systems. By tapping into free-energy gradients, such fields can also thermally destabilise astrophysical plasmas. Many such instabilities have now been discovered, and are recognised to be important astrophysically.

Most recently, Balbus has led the development of the theory of time-dependent ‘thin’ relativistic discs appropriate for Kerr black holes and, with his PhD student Andrew Mummery, applied the formalism successfully to detailed spectral observations of discs formed from tidally disrupted stars that have passed dangerously close to a black hole.

Prior to the Savilian chair in the University of Oxford, Balbus held positions at the University of Virginia and École Normale Supérieure in Paris. His contributions to teaching have been formally recognised by a teaching award from the University of Oxford. He is a member of the US National Academy of Sciences and a Fellow of the Royal Society.