Nonpareil
The Terrible Trivium
I don't see a fundamental difference - in a binary digital computer, patterns of bits interact; in a moving rock CA, patterns of rocks interact. The means are obviously different, but the principle is the same.
This is where we have our disconnect.
The patterns of rock are not interacting. The rocks are not moving save when Cueball moves them. They are the input and output given to and emerging from the actual computer, which is Cueball.
Cueball could hand the job of moving rocks to a hypothetical entity which only knew the rules for figuring out where the next rock goes, yes, and the ultimate result would still be that the computations for simulating the universe were still done - but, again, the computations would still be taking place inside this entity's head.
The rocks are functionally read-write memory. They can be referenced to perform computations, and altered to store data about the simulation, and an interpreter could look at them and figure out the state of the simulation whether or not the entity was consciously aware of its role, but there's still no actual computation going on in the rocks themselves.
Access to infinite memory is a nice thing for a computer to have, but the memory is not the computer. The input and output are not the same as the thing that does the computation.
Replace Cueball with a pen-wielding robot, the infinite plains with paper, and the rocks with ink dots, if it helps to visualize it that way. The pattern of dots is what the computer accepts as input and modifies as output, but it isn't part of the computer itself.
EDIT: In the same way, building a Turing-complete computer within the Game of Life has not actually turned the cells themselves into a computer. What you have done is create a visual programming language for a computer which already existed - the same one that was responsible for running the basic game.
Again, I may be wrong about this, but that's the way it appears to me at the moment. The frustrating thing, however, is that we're getting to the limits of what I know for certain on the subject of computer science, and I'm unsure of whether or not a computer constructed within the game of life would be considered a separate entity or just an extension of the computer already running it, or how that distinction would be made if it is. I'd be grateful if some lurking masked computer scientist swooped in to save us from this conundrum, but I recognize that this is unlikely.
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