Moving up and down in the generic multiverse
Joel David Hamkins, Benedikt Löwe

Abstract:
We investigate the modal logic of the generic multiverse which is a
bimodal logic with operators corresponding to the relations "is a
forcing extension of" and "is a ground model of". The fragment of the
first relation is the modal logic of forcing and was studied by the
authors in earlier work. The fragment of the second relation is the
modal logic of grounds and will be studied here for the first time. In
addition, we discuss which combinations of modal logics are possible
for the two fragments.