I heard in a conference that gravity forbids to construct local gauge invariants like Tr{−14FaμνFμνa} and only allows non-local gauge invariant quantities like Wilson Loops: TrPexp[∮γAadxa]. Could someone explain me where does it come from?
This post imported from StackExchange Physics at 2014-05-01 12:18 (UCT), posted by SE-user toot