Effective Theories and Modifications of Gravity

Abstract

We live at a time of contradictory messages about how successfully we understand gravity. General Relativity seems to work very well in the Earth's immediate neighborhood, but arguments abound that it needs modification at very small and/or very large distances. This essay tries to put this discussion into the broader context of similar situations in other areas of physics, and summarizes some of the lessons which our good understanding of gravity in the solar system has for proponents for its modification over very long and very short distances. The main message is that effective theories, in the technical sense of `effective', provide the natural language for testing proposals, and so are also effective in the colloquial sense.