I built a mechanical computer powered by marbles. What are its theoretical limitations?

Question

Over the last couple years, I built a mechanical computer powered by marbles and made a game out of it. It's similar to the old Digi-Comp II, except for two key differences:

1. Parts are repositionable on the board.
2. You can connect multiple 'bits' together using gears. When one of these bits is flipped, it flips the other bits connected to it.

The above link describes how it works. My question is, what are its theoretical limitations? My theoretical computing background is weak, so please ELI5.

edit: I'm not interested in the obvious limitations: speed (not going to win any races there...), board size, or # of marbles. I'm more interested in its theoretical limitations. Maybe it would help to split it into two questions:

1. How can it be proven (or disproven) to be Turing-complete?
2. If more than 3 gear bits are connected together, the friction becomes too great for a marble to turn all of them at once. Does that create additional limitations?

Thanks - I'm really excited to read your answers! I've been thinking about this for a long time.

Answer 1

What you have right now is a concrete computer. We cannot compare it to a computational model until it is properly formalized.

My intuition is that the board could be modeled as a dataflow architecture. Computational models conceived according to this paradigm can be Turing-complete, but (as was said in the comments) no concrete computer will ever be Turing-equivalent, and I don't think you should worry about this. All real computers are just (imperfect) working metaphors of formal computational models.

Should you come around to the idea of more closely imitating a Turing-equivalent dataflow machine, there are a few issues that could be addressed in order to "strengthen the metaphor", so to speak. Introducing cycles and composition of machines would be the two most important things, in my opinion, but I think your machine is already pretty amazing. It serves its purpose very well, and these "improvements" could sacrifice its usability.