Title:  Thinking Effectively About Gravity

Abstract:  

We live at a time of contradictory messages about how successfully we understand gravity. General Relativity seems to work well in the Earth’s immediate neighbourhood, but arguments abound that it needs modification at very small and/or very large distances. This talk 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 mixed: Some features of gravity are easier to modify than others. Also evidence from cosmology seems to prefer features (like light scalars and small vacuum energies) that are not generic to the long-wavelength limit of fundamental theories, and this is a crucial clue that would be silly to ignore. On the other hand, this clue seems difficult to use (so far) because no theories are known that everyone agrees describe all observations even in principle.