A better question is, what alternatives are there? In such a complex system with properly divided modules for physics, AI, etc., how else can you orchestrate these systems?

Message passing does seem to be the "best" solution to this problem. I can't think of alternatives right now. But there are plenty of examples of message passing in practice. In fact, operating systems use message passing for several of their functions. From Wikipedia:

Messages are also commonly used in the same sense as a means of interprocess communication; the other common technique being streams or pipes, in which data are sent as a sequence of elementary data items instead (the higher-level version of a virtual circuit).

(Interprocess communication is the communication between processes (i.e. running instances of programs) within the operating system environment)

So if it's good enough for an operating system, it's probably good enough for a game, right? There are other benefits as well, but I'll let the Wikipedia article on message passing do the explaining.

I also asked a question on Stack Overflow, "Data structures for message passing within a program?", which you might want to read over.

A better question is, what alternatives are there? In such a complex system with properly divided modules for physics, AI, etc., how else can you orchestrate these systems?

Message passing does seem to be the "best" solution to this problem. I can't think of alternatives right now. But there are plenty of examples of message passing in practice. In fact, operating systems use message passing for several of their functions. From Wikipedia:

Messages are also commonly used in the same sense as a means of interprocess communication; the other common technique being streams or pipes, in which data are sent as a sequence of elementary data items instead (the higher-level version of a virtual circuit).

(Interprocess communication is the communication between processes (i.e. running instances of programs) within the operating system environment)

So if it's good enough for an operating system, it's probably good enough for a game, right? There are other benefits as well, but I'll let the Wikipedia article on message passing do the explaining.

I also asked a question on Stack Overflow, "Data structures for message passing within a program?", which you might want to read over.