## Dec 13 Crab Gates

Apart from a brief meeting with them when I did a GCSE in systems and control technology, the first time I played around with logic gates was in Minecraft when I was at university. I used them to make a large calculator in our survival server out of redstone. I've also tried making them out of lego, although never to the same level of success as this guy.

It is actually a fun problem to try to design how you would make all the different combinations of gates out of mechanical objects such as levers and elastic bands. But what I want to show you today is a cute experiment which used live crabs to make OR and AND gates.

Groups of around 40 live crabs were put into compartments with a door which could be slid out. As the door was removed a piece of red card was pulled up into position behind the crabs which would scare it into scuttling forward as a group. In this situation the crabs are acting a bit like an electron in a wire, with their bodies carrying the information. Here’s a layout for a potential OR gate:

If swarms of crabs start at either x or y then there will be an output at the top.

However by slightly redesigning the corridors we can make an AND gate instead:

So if only one group of crabs moves then they will just carry on along the diagonal, but if both groups are activated at the same time then they will collide and end up waddling together along the middle output corridor.

The paper I read mentioned that the group had also implemented NOT gates which means that any computing you could conceive of is possible from crab gates alone. I couldn't find how they implemented the NOT gate, but I can think of a few ways and you might like to think about it too.

The particular crabs the used were a subspecies of Soldier Crabs who are known for their bunching behaviour and thus have the right property for the colliding needed for the AND gate. In the paper the team reported a 100% success rate with all three types of gates, which I find surprising quite frankly.

An AND gate