Quantum-Spacetime Phenomenology.

I review the current status of phenomenological programs inspired by quantum-spacetime research. I stress in particular the significance of results establishing that certain data analyses provide sensitivity to effects introduced genuinely at the Planck scale. My main focus is on phenomenological programs that affect the directions taken by studies of quantum-spacetime theories.

Challenge to macroscopic probes of quantum spacetime based on noncommutative geometry.

Over the last decade, a growing number of quantum-gravity researchers has been looking for opportunities for the first ever experimental evidence of a Planck-length quantum property of spacetime. These studies are usually based on the analysis of some candidate indirect implications of spacetime quantization, such as a possible curvature of momentum space. Some recent proposals have raised hope that we might also gain direct experimental access to quantum properties of spacetime, by finding evidence of limitations to the measurability of the center-of-mass coordinates of some macroscopic bodies. However, I here observe that the arguments that originally led to speculating about spacetime quantization do not apply to the localization of the center of mass of a macroscopic body. And, I also analyze some popular formalizations of the notion of quantum spacetime, finding that when the quantization of spacetime is Planckian for the constituent particles, then for the center of mass of a composite macroscopic body the quantization of spacetime is much weaker than Planckian. These results suggest that the center-of-mass observables of macroscopic bodies should not provide good opportunities for uncovering quantum properties of spacetime. And, they also raise some conceptual challenges for theories of mechanics in quantum spacetime, in which, for example, free protons and free atoms should feel the effects of spacetime quantization differently.

Taming nonlocality in theories with Planck-scale deformed Lorentz symmetry.

We report a general analysis of worldlines for theories with deformed relativistic symmetries and momentum dependence of the speed of photons. Our formalization is faithful to Einstein's program, with spacetime points viewed as an abstraction of physical events. The emerging picture imposes the renunciation of the idealization of absolutely coincident events, but is free from some pathologies which had been previously conjectured.

Constraining the energy-momentum dispersion relation with Planck-scale sensitivity using cold atoms.

We use the results of ultraprecise cold-atom-recoil experiments to constrain the form of the energy-momentum dispersion relation, a structure that is expected to be modified in several quantum-gravity approaches. Our strategy of analysis applies to the nonrelativistic (small speeds) limit of the dispersion relation, and is therefore complementary to an analogous ongoing effort of investigation of the dispersion relation in the ultrarelativistic regime using observations in astrophysics. For the leading correction in the nonrelativistic limit the exceptional sensitivity of cold-atom-recoil experiments remarkably allows us to set a limit within a single order of magnitude of the desired Planck-scale level, thereby providing the first example of Planck-scale sensitivity in the study of the dispersion relation in controlled laboratory experiments.