In his 1996 paper on http://arxiv.org/abs/quant-ph/9609002v2" [Broken], Carlo Rovelli proposed that “a complete description of the world is exhausted by the relevant information that systems have about each other.” I’m interested in exploring here what such a description of the world looks like. I’m distinguishing here between the reality of physical things in themselves – particles, fields, spacetime, etc. – and the information about them that gets physically communicated to other things. Rovelli claimed that QM does not describe the nature of reality at all, only the nature of things as “observed” by other things. And he suggested that at a basic level, it’s not even meaningful to talk about the reality of things “in themselves” – since whether or not such a reality actually exists, all physics can ever find out about concerns the web of interaction that communicates empirical information. Of course this doesn’t mean we should stop talking about reality. Clearly it’s vastly more efficient to say what something really is than to say how it appears from many different viewpoints, in many different contexts. And we know that for most theoretical as well as practical purposes, this kind of objective, “object-oriented” description works extremely well. But we also know that QM, our best fundamental theory, does in fact represent “the state of a system” by a mathematical function that encapsulates all the possible ways it could be observed in all available contexts. Here I don’t want to argue about how to interpret QM. But however we might interpret it, I think Rovelli is certainly right: what physics actually deals with, in practice, is some kind of system of physical interactions that communicate information. That is, what we call reality is a highly consistent “picture” of the world that gets communicated through this system of physical appearances. This “picture” is the only reality we can know, empirically. Whether it corresponds to an absolute reality existing “in itself” is not a question for science. On the other hand, we do know that this communications system exists, if we know anything at all. So my question is, how can we best describe it? What do our best theories tell us, not about physical reality, but about how information gets communicated? I’ll offer for discussion, below, a few different ways to approach this question. But for the sake of argument, let’s accept a couple of Rovelli’s assumptions – that any physical system counts as an “observer” insofar as it exchanges information with other systems, and that any interaction is a “measurement” if it occurs in a context in which information can be extracted from it. That means, we’re not trying to add anything to physics that has to do with what’s special, say, about human consciousness. This is not about how we experience the world – I want to look at the interactive structure of the world itself, that can be “experienced” by a person or a measuring instrument or an atom.