The Gravity of Tinkering with Einstein

We live at a time of contradictory messages about how successfully we
understand gravity. General Relativity seems to work superbly well in the
Earth’s immediate neighbourhood, but arguments also abound that it needs
modification at very small and/or very large distances. This talk tries
to put this discussion into perspective by embedding it into a broader 
context: how similar situations have played out in other areas of physics.

I summarize some of the lessons that abundant testing of gravity in the 
solar system and beyond has for proponents for its modification. The main 
message is mixed: Some features of gravity are easier to modify than others. 
But evidence from cosmology in particular seems to suggest that something is 
indeed missing. Better yet some features that seem to be missing also seem
not to be generic to our understanding of how gravity works, and these could 
be crucial clues that would be silly to ignore.