  The gravitational-wave event GW170817 and the near-simultaneous corresponding
gamma-ray burst (GRB 170817 A) falsify modified gravity theories in which the
gravitational geometry differs non-conformally from physical geometry. Thus,
the observations of this event definitively rule out theories, such as TeVeS, a
suggested relativistic extension of Milgrom's modified Newtonian dynamics
(MOND), that predict a significantly different Shapiro delay for
electromagnetic and gravitational radiation. While not falsifying MOND per se,
GW170817 severely constrains relativistic extensions of MOND to theories that
do not rely on additional matter-coupling fields but rather upon modified field
equations for one universal gravitational and physical metric. Here I mention a
simple preferred-frame theory as an example.
