  Together with the concept of reversibility, another relevant physical notion
is time-symmetry, which expresses that there is no way of distinguishing
between backward and forward time directions. This notion, found in physical
theories, has been neglected in the area of discrete dynamical systems. Here we
formalize it in the context of cellular automata and establish some basic facts
and relations. We also state some open problems that may encourage further
research on the topic.
