9 - Other Tests of Post-Newtonian Gravity

Summary

There remains a number of tests of post-Newtonian gravitational effects that do not fit into either of the two categories, classical tests or tests of SEP. These include the gyroscope experiment (Section 9.1), laboratory experiments (Section 9.2), and tests of post-Newtonian conservation laws (Section 9.3). Some of these experiments provide limits on PPN parameters, in particular the conservation-law parameters ζ₁, ζ>₂, ζ₃, ζ₄, that were not constrained (or that were constrained only indirectly) by the classical tests and by tests of SEP. Such experiments provide new information about the nature of post-Newtonian gravity. Others, however, such as the gyroscope experiment and some laboratory experiments, all yet to be performed, determine values for PPN parameters already constrained by the experiments discussed in Chapters 7 and 8. In some cases, the prior constraints on the parameters are tighter than the best limit these experiments could hope to achieve. Nevertheless, it is important to carry out such experiments, for the following reasons:

(i) They provide independent, though potentially weaker, checks of the values of the PPN parameters, and thereby independent tests of gravitation theory. They are independent in the sense that the physical mechanism responsible for the effect being measured may be completely different than the mechanism that led to the prior limit on the PPN parameters. An example is the gyroscope test of the Lense–Thirring effect, the dragging of inertial frames produced purely by the rotation of the Earth. It is not a preferred-frame effect, yet it depends upon the parameter α₁.