Now, it turns out that the two fundamental theories
that are the quantum theory and general relativity are
difficult to reconcile with one another. When one tries
to merge them into a single theory, whether string theory,
quantum gravity, etc. their unification turns into
a confrontation: it leads to violations of some of the
basic principles. Typical manifestations are: an outrageous
value of the cosmological constant, the appearance
of Lorentz violations, the nonconstancy of fundamental
constants. Indeed, because they are intimately
connected, it is all aspects of the Einstein’s equivalence
principle that are subject to violations.