  Consider a system of N identical hard spherical particles moving in a
d-dimensional box and undergoing elastic, possibly multi-particle, collisions.
We develop a new algorithm that recovers the pre-collision state from the
post-collision state of the system, across a series of consecutive collisions,
with essentially no memory overhead. The challenge in achieving reversibility
for an n-particle collision (where, n << N) arises from the presence of nd-d-1
degrees of freedom during each collision, and from the complex geometrical
constraints placed on the colliding particles. To reverse the collisions in a
traditional simulation setting, all of the particular realizations of these
degrees of freedom during the forward simulation must be saved. This limitation
is addressed here by first performing a pseudo-randomization of angles,
ensuring determinism in the reverse path for any values of n and d. To address
the more difficult problem of geometrical and dynamic constraints, a new
approach is developed which correctly samples the constrained phase space. Upon
combining the pseudo-randomization with correct phase space sampling, perfect
reversibility of collisions is achieved, as illustrated for n <= 3, d=2, and
n=2, d=3 (and, in principle, generalizable to larger n). This result enables
for the first time reversible simulations of elastic collisions with
essentially zero memory accumulation. The reverse computation methodology
uncovers important issues of irreversibility in conventional models, and the
difficulties encountered in arriving at a reversible model for one of the most
basic physical system processes, namely, elastic collisions for hard spheres.
Insights and solution methodologies, with regard to accurate phase space
coverage with reversible random sampling proposed in this context, can help
serve as models and/or starting points for other reversible simulations.
