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Comment by jws | Hacker News Reader

They claim to have built it already, and given that the AND gate is actually a tripartite output of a∧¬b, a∧b, ¬a∧b the NOT seems to be trivial.

Note: this is not level based logic, but billiard ball logic. If I understood that better I'd write a concise, illuminating explanation of why ¬a∧¬b is missing in the AND gate's outputs.

http://en.wikipedia.org/wiki/Billiard_ball_computer

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Uh, I think you're overthinking this. What is the output of a NOT crab-gate when there are no crabs in the input?

In a billiard-ball model one would construct a NOT/inverter with a crab source that normally flows straight to the output but can be misdirected into a drain by crabs coming in from the input. Of course you need a crab source for this, but this is little different from electronic transistor-based computers, where the inverse of Vss is Vdd and the Vdd comes from the power supply.

Right, yes, but the (semi-serious) point is that we don't have a crab grid in the same way we have a power grid, so actually implementing this (which is what the article is about, rather than just theory) might be hard. For the crab computer to "run" arbitrary computations it would need an arbitrarily large source of crabs.

A functional crab computer would probably have to be built from delay insensitive logic[1]. The outputs of the gates would be of the form X and !X, where the signal (crab cluster) would appear at the appropriate output.

With regards to crab supply, I think the sufficient study of reversible computing, and in particular their charge conservation properties, would address that.

You'll still have to have a mechanism to spray bits of dead fish around to feed the thing.

[1] http://en.wikipedia.org/wiki/Delay_insensitive_circuit

[2] http://en.wikipedia.org/wiki/Reversible_computing

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