OBSERVERS IN QUANTUM GRAVITY II
Relativistic properties of the interacting kappa-Poincaré model
I will discuss the laws of transformation between observers in models where the Planck scale kinematics is governed by kappa-Poincaré symmetries. Such models are most simply realised in momentum space, but can also be endowed with a full phase space construction, provided by the relative locality framework. When looking at systems of particles interacting via causally connected vertices, relativistic invariance in momentum space is achieved if the boost parameter acting on each given particle receives a backreaction from the total momentum of all the particles that are causally connected, even those that do not directly enter the vertex. Within the relative locality framework the picture is Lorentz invariant if one uses an appropriate "total boost" to act on the particles' worldlines and on the interaction coordinates. The momentum space backreaction turns out to be just a manifestation of this "total boost" action. These findings provide the basis to consistently define distant relatively boosted observers in the relative locality framework, while raising questions concerning the amount of information that is needed to define such observers.