7 - The Classical Tests

Summary

With the PPN formalism and its associated equations of motion in hand, we are now ready to confront the gravitation theories discussed in Chapter 5 with the results of solar system experiments. In this chapter, we focus on the three “classical” tests of relativistic gravity, consisting of (i) the deflection of light, (ii) the time delay of light, and (iii) the perihelion shift of Mercury.

This usage of the term “classical” tests is a break with tradition. Traditionally, the term “classical tests” has referred to the gravitational redshift experiment, the deflection of light, and the perihelion shift of Mercury. The reason is largely historical. These were among the first observable effects of general relativity to be computed by Einstein. However, in Chapter 2 we saw that the gravitational red-shift experiment is really not a test of general relativity, rather it is a test of the Einstein Equivalence Principle, upon which general relativity and every other metric theory of gravity are founded. Put differently, every metric theory of gravity automatically predicts the same red-shift. For this reason, we have dropped the red-shift experiment as a “classical” test (that is not to deny its importance, of course, as our discussion in Chapter 2 points out). However, we can immediately replace it with an experiment that is as important as the other two, the time delay of light.