  In a model where a multiverse wavefunction explores a multitude of vacua with
different symmetries and parameters, properties of universes closely related to
ours can be understood by examining the consequences of small departures of
physical parameters from their observed values. The masses of the light
fermions that make up the stable matter of which we are made-- the up and down
quarks, and the electron-- have values in a narrow window that both allows a
variety of nuclei other than protons to exist, and at the same time allows
atoms with stable shells of electrons that are not devoured by their nuclei.
These fundamental parameters of the Standard Model are good candidates for
quantities whose values are determined through selection effects within a
multiverse, since a living world of molecules needs stable nuclei other than
just protons and neutrons. If the fermion masses are fixed by brane
condensation or compactification of extra dimensions, there may be observable
fossils of the branching event, such as a gravitational wave background.
