Superselection sectors in asymptotic quantization of gravity

Using the continuity of the scalar psi(2) (the mass aspect) at null infinity through 1 degrees we show that the space of radiative solutions of general relativity can be thought of a fibered space where the value of psi(2) at 1 degrees plays the role of the base space. We also show that the restriction of the available symplectic form to each "fiber" is degenerate along directions that represent supertranslations of the EMS group. By factoring out these gauge directions we obtain the reduced phase space for the radiation data. This reduced phase space posses a global structure, i.e., it does not distinguishes between future or past null infinity. Thus, it can be used as the space of quantum gravitons. Moreover, a (pre)Hilbert space can be constructed on each "fiber" if an appropriate definition of scalar product is provided. Since there is no natural correspondence between the reduced phase spaces of different leaves they define superselection sectors on the space of asymptotic quantum states. We briefly review the dynamics of null surface quantization since these quantum objects are natural operators on the superselection sectors and also discuss the physical relevance of these sectors.