  The question of where the inertial properties of matter come from has been
open for a long time. Isaac Newton considered inertia an intrinsic property of
matter. Ernst Mach held a different view whereby the inertia of a body comes
from its interaction with the rest of the universe. This idea is known today as
Mach's principle. We discuss Mach's principle based on transactional gravity,
the recently developed completion of the entropic gravity program by the
physics of quantum events induced by transactions. A consequence of the
analysis is a fundamental relation between the gravitational constant G and the
total mass in the causal universe, derived by means of entropic principles.
