  The Laser Interferometer Space Antenna (LISA) has the potential to reveal
wonders about the fundamental theory of nature at play in the extreme gravity
regime, where the gravitational interaction is both strong and dynamical. In
this white paper, the Fundamental Physics Working Group of the LISA Consortium
summarizes the current topics in fundamental physics where LISA observations of
GWs can be expected to provide key input. We provide the briefest of reviews to
then delineate avenues for future research directions and to discuss
connections between this working group, other working groups and the consortium
work package teams. These connections must be developed for LISA to live up to
its science potential in these areas.
