There's always something to interact with. If you had a conscious being made out of star systems, it wouldn't meaningfully interact with anything within our lifetime, but over billions of billions of years, it would presumably shift entire galaxies. The rockputer just needs inputs that operate at its own scale, like the shape of the coastlines it's expanding into, information about geological processes, another rockputer competing for territory, and so on. Alternatively you could simulate a whole universe using these rocks, and feed simulated inputs to the being.
Of course, part of the difficulty of imagining a conscious rockputer is that it's also pretty hard to imagine its inputs :)
This is trivially true as you state it; that's why I added the qualifier "in appropriate ways". Not all interactions will produce consciousness. One obvious difference between us and your hypothetical "rockputer" is that the "rockputer" can't change its behavior based on its inputs in a way that improves its chances of survival; rocks simply aren't built that way. Neither are star systems or galaxies. But we are.
> the "rockputer" can't change its behavior based on its inputs in a way that improves its chances of survival
Yes it can. Some natural events, for example a flood or an earthquake, can destroy parts of the rockputer. It is therefore important for it to store the various parts of itself strategically. It shouldn't put its vital parts near the coast, or a tsunami may kill it. It should store its own consciousness in a robust way, so that it can recover from an earthquake. It's probably too slow to actually see either of them coming, but it can certainly prepare itself.
Or imagine you build two rockputers, one with black stones, another with white stones, and you have rules to remove stones when both rockputers try to expand into the same territory, a bit like in the game of Go. Then one can kill the other.
Star systems interact with each other through gravity, so you could conceive of them as some kind of gargantuan atoms, capable of making complex structures, including conscious ones. Granted, there doesn't seem to be an equivalent of the other forces at that scale, so probably it wouldn't work, but you see what I mean.
The rockputer comprises both the rocks and the mechanisms for moving the rocks in response to input. If the rock moving mechanism is structured properly, then the rock movement patterns could adapt to changes in the inputs to the overall rockputer system.
The level of complexity of the "passive" components of the system (i.e. the rocks) is irrelevant to whether or not the system can effect conscious-seeming behaviour when acting dynamically. Analogously, the underlying components of people, i.e. atoms, are clearly quite dumb on their own. When those atoms are allowed to evolve collectively over time, according to dynamics dictated by basic physical laws, conscious-seeming behaviour magically appears.
You can't deny the possibility of a conscious rockputer just by considering properties of the rocks.
> The rockputer comprises both the rocks and the mechanisms for moving the rocks in response to input.
Yes, and in that case, all the actual computation is being done by the mechanism, not the rocks. As you say, the rocks are just "passive" components.
I agree that, in principle, such a system could compute; but it still has the problem of computing on a time scale that supports appropriate interactions with its environment. Moving rocks around in accordance with some set of computational rules based on inputs is very slow--quite possibly too slow to respond appropriately to inputs. For example, if incoming light rays are carrying information about a tidal wave that is about to swamp the rockputer and destroy its structure, could the rockputer compute an appropriate response (such as moving itself to higher ground) quickly enough to save itself?
If it's too slow to interact with the rest of the world in appropriate ways, it wouldn't be a consciousness at all.