Thinking Effectively About Gravity
Clifford Burgess
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 neighborhood, 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
and elsewhere has for proponents for its modification over very long
and very short distances. The main message is mixed: On one hand
short-distance quantum effects are notoriously difficult to control in
gravity and cosmology seems to like features (like light scalars and
small vacuum energies) that are not generic to the long-wavelength
limit of fundamental theories. These are crucial clues that would be
silly to ignore. On the other hand, General Relativity successfully
passes many stringent new observational tests and also seems to be
almost unique in its ability to reconcile quantum effects with gravity
on longer distances without being inconsistent. Neither of these seems
to offer much scope for modification. I try to organize what the
successes of GR might be telling us, and provide a score-card about it
says about the various alternatives that have been proposed.