Reversibility [of networks and universe]
By including both forward and backward versions of every transformation it is straightforward to set up reversible rules for network evolution. It is not clear, however, whether the basic rules for the universe are really reversible. It could well be that the apparent reversibility we see arises because the universe is effectively on an attractor, as discussed on page 1018. Note that if pieces of the universe can break off, but cannot reconnect, then there will inevitably be an irreversible loss of information.