Dr. Clifford Will (UF)
University of Florida Astronomy Colloquium - Jan. 21st, 2015
The Cosmic Barber: Counting Gravitational Hair in the Solar System and Beyond
According to general relativity, every self-gravitating object has ``hair'', an array of multipole moments of various types that characterize the body's exterior geometry. In alternative theories of gravity, bodies could also be endowed with more exotic tresses, such as scalar hair. We review how solar system experiments, such as light deflection and time-delay measurements, have placed stringent limits on scalar hair. We describe how experiments such as GRACE have measured with high precision the vast head of Newtonian hair possessed by the Earth. We discuss measurements of the angular-momentum hair of the Earth by Gravity Probe B and the LAGEOS project. At the relativistic extreme, black holes are almost bald, possessing only two strands of general relativistic hair, and we describe a possible way of measuring those follicles by tracking stars orbiting very close to our galactic center black hole.