Approaches to Quantum Gravity

Quantum Gravity phenomenology 435
popular effects is decoherence. This may be also motivated using heuristic
arguments, based mainly on quantum field theory in curved spacetimes, which
suggest that black holes radiate thermally, with an associated “information loss
problem”.
22.2.5 Planck-scale departures from the equivalence principle
Various perspectives on the Quantum Gravity problem appear to suggest departures
from one or another (stronger or weaker) form of the equivalence principle.
For brevity let me just summarize here my preferred argument, which is based
on the observation that locality is a key ingredient of the present formulation of
the equivalence principle. In fact, the equivalence principle ensures that (for the
same initial conditions) two point particles would go on the same geodesic independently
of their mass. But it is well established that this is not applicable to
extended bodies, and presumably also not applicable to “delocalized point particles”
(point particles whose position is affected by uncontrolled uncertainties). If
spacetime structure is such to induce an irreducible limit on the localization of
particles, it would seem natural to expect some departures from the equivalence
principle.
22.2.6 Critical-dimension superstring theory
The most popular realization of string theory, with the adoption of supersymmetry
and the choice of working in a “critical” number of spacetime dimensions, has
given a very significant contribution to the conceptual aspects of the debate on
Quantum Gravity, perhaps most notably the fact that, indeed thanks to research on
string theory, we now know that Quantum Gravity might well be a perturbatively
renormalizable theory (whereas this was once thought to be impossible). But for
the prediction of physical effects string theory has not proven (yet?) to be rich. In
spite of all the noteworthy mathematical structure that is needed for the analysis
of string theory, from a wider perspective this is the approach that by construction
assumes that the solution to the Quantum Gravity problem should bring about a
rather limited amount of novelty. In particular, string theory is still introduced in
a classical Minkowski spacetime and it is still a genuinely quantum-mechanical
theory. None of the effects possibly due to spacetime quantization is therefore
necessarily expected and all the departures-from-quantum-mechanics effects, like
decoherence effects, are also not expected.
But on the other hand, as mentioned, string theory is turning out to be a
remarkably flexible formalism and, therefore, while one can structure things in
such a way that nothing interestingly new happens, one can also mould the