You're barking up the wrong tree. Even a Turing machine doesn't need to be computing "constantly"; it's perfectly legitimate to execute one transition every ten minutes and then just sit there.
That is fine, because we can just say that if a system is between two transitions, it is also computing. So as long as our TM executes more than a single transition, the definition is consistent.
On the other hand, if westprog wants to say that a rock computes (not in the TM sense, "switches" would be a better word) because it *might* satisfy the definition, some time or another, then he can do so.
But that is just semantic games, and if he wants to play that game all we have to do is come up with yet another word for "active" computation or somthing, and so on and so forth.
It's a more general problem. For example, if you've defined the rock as having two states (molten and solid), those two states are not, by themselves, sufficient to act as a Turing machine.
You would need some way of defining input strings, output strings, and a suitable storage mechanism (tape or tape-equivalent). And in particular, you'd need to show how a particular set of transitions from molten to solid and back acted in a way to operate on the storage mechanism in a manner equivalent to TM storage.
Simply saying "a rock has states, a TM has states, therefore they are equivalent" is not valid. A TM is actually a rather precisely-defined set of (mathematical) properties, properties that a rock appears not to have.
Yeah I know, and I am not trying to show that.
I was trying to get at the fundamental physical difference between systems that we consider necessary for computing (informally,
switches) and all other systems.
You could, if you were very clever, build a TM equivalent computer from a bunch of switches consisting of rocks + other stuff.
If I can show that switching (I have been calling that "computation," when I mention "computation" I am not speaking of the formal TM equivalent sense) has a physical definition then it is trivial to show that all the rest of computation theory can be physical as well -- all one needs to do is instantiate the abstract. But westprog wants that first step, so I have been trying to provide it for him.