Which theory completes general relativity at the Planck scale?
In plain words
String theory, asymptotic safety, loop quantum gravity and others each propose what happens at the shortest distances. None has been confirmed, and most make no prediction testable at reachable energies.
Precise statement
For a named candidate completion of $3+1$ gravity coupled to the Standard Model (string theory, asymptotic safety, loop quantum gravity, causal dynamical triangulations, Horava gravity or another), derive a low-energy observable, at energies far below $E_P=\sqrt{\hbar c^5/G}$, approximately 1.2 x 10^19 GeV (2 x 10^16 erg), whose predicted deviation from general relativity plus the Standard Model is computed with stated uncertainty and is within reach of current or planned measurements. An answer is a falsifiable prediction with a stated magnitude that distinguishes the candidate from the alternatives.
What would settle it
An observation that confirms a prediction unique to one candidate completion.
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