  Atom interferometers provide a powerful means of realizing quantum coherent
systems with increasingly macroscopic extent in space and time. These systems
provide an opportunity for a variety of novel tests of fundamental physics,
including ultralight dark matter searches and tests of modifications of
gravity, using long drop times, microgravity. However, as experiments operate
with longer periods of free fall and become sensitive to smaller background
effects, key questions start to emerge about the fundamental limits to future
atom interferometery experiments. We study the effects on atomic coherence from
hard-to-screen backgrounds due to baths of ambient particles with long-range
forces, such as gravitating baths and charged cosmic rays. Our approach -
working in the Heisenberg picture for the atomic motion - makes proper
inclusion of the experimental apparatus feasible and clearly shows how to
handle long-range forces and preferred frame ambiguities. We find that these
potential backgrounds are likely negligible for the next generation of
interferometers, as aggressive estimates for the gravitational decoherence from
a background bath of dark matter particles gives a decoherence timescale on the
order of years.
