8 Closing Remarks

Clearly, the most significant development of these first few years of quantum-spacetime phenomenology has been our ability to uncover some experimental/observational contexts in which, through appropriate data analyses, we could gain access to effects introduced genuinely at the Planck scale. The compellingness of such instances of genuine Planck-scale sensitivity, which are most simply and clearly illustrated in Section 1.5, should be contrasted to the more frequent case of “dimensional-analysis Planck-scale sensitivities”, which typically involve a description of a plausible quantum-spacetime effect in terms of a dimensionless parameter, estimated arbitrarily as a ratio of the Planck length and some characteristic length scale of the problem.

Looking at the results summarized in this review, different readers, depending on how stringent their criteria for genuine Planck-scale sensitivity, will only recognize one or two examples. Not much, but much better than expected even just 15 years ago. And, as stressed, we do have, at this point, a rather encouraging list of contexts in which, while the availability of genuine Planck-scale sensitivity has still not been fully established, it appears that sensitivity to effects introduced genuinely at the Planck scale could be achieved in a not-so-distant future.

The fact that the development of this phenomenology is proving beneficial for the study of the idea of spacetime quantization is perhaps best testified by the fact that it is already managing to truly affect the directions taken by more formal work on spacetime quantization, especially in the areas of LQG and spacetime noncommutativity. Theorists in these areas follow the developments on the phenomenology side and do their best (the technical challenges they are facing are very severe) to derive results that can be exploited for the opportunities in phenomenology that are being established. In turn the phenomenology takes notice of the developments on the theory side, finding in them new input for enlarging the list of candidate quantum-spacetime effects that one could attempt to investigate experimentally.

The goal of testing/falsifying rigorous theories of spacetime quantization appears to still be beyond our present reach. But while most of the work in quantum-spacetime phenomenology so far has relied on simple-minded test theories describing candidate quantum-spacetime effects, I see first indications of a phase of further maturation of this phenomenology, in which we will actually test/falsify at least the most virulent rigorous formalizations of quantum spacetime. Planck-scale theories formulated in noncommutative versions of Minkowski spacetime are the example where we are presently closer to this goal.

The (however limited) information presently available to us appears to provide a clear invitation to continue to focus most of our efforts in the search for effects describable in terms of a (low-energy) expansion in powers of the Planck length, though other opportunities clearly should not be overlooked. Concerning the type of data on which quantum-spacetime phenomenology can rely, I have attempted to maintain throughout this review some visible separations between different proposals on the basis of whether they concern astrophysics, cosmology or controlled laboratory experiments. It is very clear that astrophysics has so far provided the most fruitful arena, but cosmology has the greatest potential reach (although for the most part this potential has not yet materialized). The role played so far in quantum-spacetime phenomenology by controlled laboratory experiments is rather marginal, but it would be important for the future development of quantum-spacetime phenomenology to find more opportunities for controlled laboratory experiments.