John Friedman has worked on a broad range of problems in
relativistic astrophysics and gravitational physics. A fellow of the
American Physical Society, Friedman served for three years as Chair
of the UWM Department of Physics and recently completed 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 he is currently a US representative to the
International Society of General Relativity and Gravitation.
of UWM's Department of Physics. He is grateful to
be largely free this year of major administrative responsibilities.
Friedman's most recent work
(see Phys. Rev. D70,
044044 (2004)., and
Phys.Rev. D65 (2002) 064035)
in collaboration with recent postdoc Koji Uryu, Prof. Masaru Shibata,
recent PhD student Antonios Tsokaros,
is related to a key problem in gravitational-wave astronomy: accurate
numerical modeling of binary systems comprising two neutron stars or
two black holes (or one black hole and one neutron star). A related
collaboration that also includes present and former members of Richard
Price's gravity group and current UWM postdoc Shin Yoshida, hopes to
find models that are stationary in the frame of the orbiting stars
and that satisfy the full set of Einstein equations, with equal
amounts of ingoing and outgoing radiation.
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.
Earlier collaborations [with Prof. James Ipser, Prof. Leonard Parker,
Prof. Nikolaos Stergioulas (then a PhD student at UWM), senior scientist
Scott Koranda] constructed models of rapidly rotating relativistic
stars. Among the results of this work are
upper limits set by causality
on the spin and mass of rotating neutron stars (on
on any cold, gravitationally bound star). Stergioulas'
rotating neutron star code, and a modified version by Sharon Morsink,
is now a widely used public domain code>,
rns.
A decade ago, Friedman and a group of students and post docs
pursued a substantial program to study implications of a
microscopic world
in which the causal structure and topology of space time may fluctuate at the smallest scales.
The ordinariness of macroscopic topology is explained in part by the
Topological Censorship Theorem,
proved by Friedman, Kristin
Schleich, and former student Donald Witt.
The ordinariness of macroscopic causal structure -- the absence of time
machines -- is
the content of what Stephen Hawking calls the Chronology Protection
Conjecture. Interest in the question was stimulated by work begun by
Kip Thorne and former UWM postdoc Michael Morris, and summarized in a
1990 PRD paper
by a group of workers, including Friedman. For a summary that includes Friedman's
more recent collaborations on classical and quantum fields on spacetimes that have closed
timelike curves or are time nonorientable, see
Cargese review.
Finally, Friedman is proud of early
work
with Rafael Sorkin, showing
in the context of canonical quantum gravity, that toplogical geons
(technically, quantum states of asymptotically flat spacetimes with
noneuclidean topology) can have half-integral spin. Related work
with Donald Witt
(
Topology, 25, 35-44, 1986,
) showed that some of the topologies
that admit half-integral spin were the first examples of 3-manifolds
for which a diffeo could be homotopic but not isotopic to the
identity.
