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Research:

John Friedman has worked on a variety of problems in gravitational physics, involving neutron stars, black holes and gravitational collapse, the topology of spacetime, and topological questions in quantum gravity.

John Friedman has worked on a broad range of problems in relativistic astrophysics and gravitational physics.

Research in relativistic astrophysics

Friedman’s early work on relativistic stability with S. Chandrasekhar and Bernard Schutz culminated in the surprising result that, if one ignores viscosity, all rotating stars are unstable to nonaxisymmetric perturbations that radiate away their angular momentum in gravitational waves. In particular, this “CFS” instability of what are called Rossby modes or r-modes may set the upper limit on the spin of old neutron stars spun-up by accretion from a companion star. With a number of collaborators, including Sharon Morsink, former graduate student Keith Lockitch, Nils Andersson, Friedman has looked at a gravitational-wave driven instability of neutron stars that may limit the spin of newborn stars and also of old neutron stars spun-up by accretion from a giant companion.

By constructing the first accurate models of of rotating relativistic stars based on a range of candidate neutron-star equations of state, Friedman, James Ipser, and Leonard Parker — and, later, Nikolaos Stergioulas and Friedman — related observable neutron-star properties (mass, radius, surface redshift, moment of inertia) to the equation of state of matter at supernuclear density. Stergioulas’ rotating neutron star code, and a modified version by Morsink, is now the widely used public domain code> RNS .

With a number of collaborators, including Prof. Sharon Morsink (a former postdoc at UWM’s Center for Gravitation and Cosmology), recent PhD student Keith Lockitch,, and Prof. Nils Andersson, Friedman has looked at a gravitational-wave driven instability of neutron stars that may limit the spin of newborn stars and also of old neutron stars spun-up by accretion from a giant companion. A variant of the “CFS” instability (initially studied by S. Chandrasekar, Friedman, and Bernard Schutz), this unstable mode in low-mass x-ray binaries may radiate enough energy in gravitational waves to be seen by advanced LIGO.

Related work with Ipser and subsequently with (and primarily by) Stergioulas and Scott Koranda established upper limits set by causality on the mass and spin of rapidly rotating neutron stars (on on any cold, gravitationally bound star). Work with Ipser and Rafael Sorkin noted that a theorem of Sorkin immediately implies that relativistic stars with a given value of angular momentum are unstable to collapse by the time their mass reaches its maximum value for that angular momentum.

With Stergioulas, Friedman recently completed a monograph, Rapidly Rotating Relativistic Stars, published by Cambridge University Press. The text pulls together over 40 years of research to provide graduate students and researchers in astrophysics, gravitational physics, and astronomy with a largely self-contained treatment of the structure, stability and oscillations of rotating neutron stars.

With the first direct observations of gravitational waves expected in the next few years, we are at the dawn of gravitational-wave astronomy. Gravitational wave observatories in the US, Europe, Japan and India will soon begin to hear gravitational waves from the inspiral and coalescence of compact binary systems — systems with two neutron stars, one black hole and one neutron star, and two black holes. Friedman’s recent work is tied to a primary goal of this worldwide effort: to find the masses and radii of inspiraling neutron stars and, from them, to infer the behavior of cold matter above nuclear density – that is, to constrain the equation of state of neutron-star matter. Work with a number of collaborators, including (former) graduate students Ben Lackey, Jocelyn Read, and Charalampos Markakis, Masaru Shibata and (former) postdocs Koutarou Kyutoku, Keisuke Taniguchi, and Koji Uryu, all initially from Shibata’s group, estimates the accuracy with which gravitational waves from binary inspiral can measure the radii of neutron stars and constrain the equation of state.

With Tobias Keidl, , Alan Wiseman , Dong-Hoon Kim, Abhay Shah, Bernard Whiting and Thomas Linz, Friedman looked at a set of problems related to the gravitational waves from stellar size black holes spiraling in to the supermassive black holes that lie in the cores of nearly all large galaxies. These are a principal targets of a future space-based observatory. Results of the research include: Development of analytic and numerical tools to find the self-force on a particle in geodesic motion, using a formalism appropriate for rotating black holes; calculation of the force in this formalism for circular orbits; extract the analytical coefficients in the post-Newtonian expansion by calculating an essentially gauge-invariant redshift factor to extremely high accuracy; and finding renormalization coefficients for the self-force of a charged mass in an electrovac spacetime.

Biographical Sketch:

John Friedman received his BA from Harvard College in 1967 and his PhD from the University of Chicago in 1973, supervised by Nobel laureate S. Chandrasekhar. From 1974 to 1976 he was a Fermi Fellow at the University of Chicago. Since 1976 he has been at the University of Wisconsin-Milwaukee, where Friedman served for three years as Chair of the Department of Physics and where he currently holds the rank of University Distinguished Professor Emeritus.

Friedman is a Fellow of the American Physical Society and a former chair of its gravitational physics section. He has served on the editorial boards of several prestigious journals, including Classical and Quantum Gravity, Physical Review Letters and Physical Review D. Professor Friedman is currently a U.S. representative to the International Society of General Relativity and Gravitation.

Friedman’s BA is from Harvard in 1967, his PhD from University of Chicago in 1973. Following postdoctoral work at Yale and a Fermi Fellowship at Chicago, Friedman has been on the faculty at the University of Wisconsin-Milwaukee, where he was appointed as a University Distinguished Professor in 2007 and is now Professor Emeritus. A fellow of the American Physical Society, Friedman served for three years as Chair of the UWM Department of Physics and a term as Chair of the APS’s gravitational physics society, the Topical Group in Gravitation. He served two terms as Divisional Associate Editor of Physical Review Letters and a term on the Editorial Board of Classical and Quantum Gravity, and was a US representative to the International Society of General Relativity and Gravitation. He is grateful in retirement to be free of administrative responsibilities.