Comment by Aardwolf | Hacker News Reader

Comment by Aardwolf | Hacker News Reader

Here's something I'm wondering: Is it possible to create mechanical logic gates that do not involve any friction, elasticity, springs, gravity, as well as any active powered control, but only frictionless motions.

Example: a NOT gate is trivial to make without any such friction mechanics, just a lever (one side goes up, other goes down, so 'NOT') or two gears (one rotates one way, the other the other way).

But an AND or OR gate, needed to make the universal NAND or NOR gate to get any arbitrary logic, seem to always require some kind of elastic band, spring or gravity based mechanism to return the gate to initial position.

Friction / elastic bands, etc... cause wear and tear, which is why it'd be interesting to see a mechanism that's as frictionless as the NOT gate described above.

Things that are fine to use according to what I mean above would be: gears (including non-circular shaped ones), planetary gears / differential, levers, ... An example of something that is not fine is: an OR gate made by two sticks that can push an object forward, and an elastic band (or gravity or spring) makes the object move backwards again in the case both sticks are retracted backwards.

I mean theoretically, there's always friction in gears anyway but I don't mean avoiding that one.

There is a large amount of theoretical research on the subject of energy limits in computing. For example, Landauer's principle states any irreversible change in information requires some amount of dissipated heat, and therefore some energy input [1].

Reversible computing is an attempt to get around this limit by removing irreversible state changes [2].

[1] https://en.wikipedia.org/wiki/Landauer%27s_principle

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

Then it's about finding a mechanical toffoli or fredkin gate liie this

Springs do not have to wear out. If springs are made from a material such as steel that has a fatigue limit and the stress they experience is less than the fatigue limit, they may be deformed forever.

As for the answer, it appears to be yes. It is possible to make logic gates which only use rigid rotary links[0].

[0]https://arxiv.org/abs/1801.03534

Steve Mould did one with water: https://youtu.be/IxXaizglscw

This one uses gravity though! Not the same thing as friction causing wear and tear, but still a similar dependency on a force as the springs/elastic bands. Plus use of gravity means a lot of force is needed to operate many of the gates at the same time

ok but gravity scales quite nicely as force right?

And he did one with spintronics:

https://www.youtube.com/watch?v=QrkiJZKJfpY

Rod logic seems to fit the bill (https://anthony-zhang.me/blog/rod-logic/).

Check out https://en.wikipedia.org/wiki/Billiard-ball_computer It's theoretical but there are some great ideas in the general area of mechanical reversible computing.

Check out: https://m.youtube.com/watch?v=kQeDnFUNW-Q

And gate starts at 4:49. Or gate starts at 6:27. The trick is to make some gears not stationary, moving along as other gears push them.

OR is easy: a rod that is fixed to stay horizontal but can slide up and down. There are rods under it at either end for the two input bits. If either input goes up, the horizontal result rod goes up.

That requires a gravity based mechanism to return it back down though.

That requires a gravity based mechanism to return it back down though.

Or gate could be a ratchet in your gear example.

On a bike, it moves if you have momentum OR you pedal.

I guess the ratchet needs a spring though? Doh!

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