Pulsar Timing — Strong Gravity Clock Experiments

Abstract

25 years ago, in summer 1974, Joseph Taylor and Russell Hulse discovered the first binary pulsar, a pulsar in orbit with a compact companion which itself is most likely a neutron star. This pulsar, denoted PSR B1913+16, turned out to be the most exciting laboratory for testing relativistic gravity theories. Before the discovery of PSR B1913+16 all gravity experiments were confined to our solar system with its very weak gravitational fields. Hence, it has been possible to test gravity theories only in the first post-Newtonian approximation. Binary pulsars take us beyond the weak-field context because of their high orbital velocity and/or the strong self-gravitational fields of neutron stars.

To date, more than 70 binary pulsars have been discovered, most of them in orbit with a neutron star or a white dwarf. Many binary pulsars belong to a group of so-called millisecond pulsars which have very short rotational periods (< 20 ms) and slowdown rates of typically 10^p-20, proving to be extremely accurate clocks. This gave rise to a variety of new gravity experiments, like tests for the strong equivalence principle. After a brief introduction to pulsars, the technical and theoretical aspects of binary- pulsar gravity experiments are reviewed. The latest results are presented and an outlook is given to future improvements of these experiments.