  In the understanding of the fundamental interactions, the origin of an arrow
of time is viewed as problematic. However, quantum field theory has an arrow of
causality, which tells us which time direction is the past lightcone and which
is the future. This direction is tied to the conventions used in the
quantization procedures. The different possible causal directions have related
physics - in this sense they are covariant under time-reversal. However, only
one causal direction emerges for a given set of conventions. This causal arrow
tells us the direction that scattering reactions proceed. The time direction of
scattering in turn tells us the time direction for which entropy increases -
the so-called arrow of thermodynamics. This connection is overlooked in most
discussions of the arrow of time.
