Saturday, June 20, 2009

Switches and Gates

The humblest elements of our technology are usually the most important, and one of the humblest "bits" is the switch. The essence of the switch is a system with two or more stable states, separated by less stable regions. We often call those states "on" and "off" since the switch is usually used as some kind of gate.

The ordinary or "garden" variety of gate may be one of the oldest uses of the technology. If you have a wall, it usually has the function of keeping something out (or in), but sometimes you want to go out or in, or let someone else out or in, and this is much simpler if one has a gate or door. From an expansive enough view a lot of things can be considered to be gates (a flint and steel can be seen as a "gate" between the burning and non burning states) but I will try to confine the usage to more explicit gates. From the light switch to the transistor "bits" that are at the heart of our computers, switches and gates are ubiquitous in modern technmology, but are there other examples than gates from ancient times?

I can only think of a few - using a switch as a signal for example. One if by land, two if by sea. Do you know any others?

As usual, nature got there first. By a few billion years. Switches are everywhere in biology and central to life. The central elements of life, metabolism, reproduction, and movement all involve switches. Most metabolic processes need to be metered, and the same is true of DNA expression. Every cell needs to regulate the inflow and outflow of water, nutrients and other essential elements. Every neuron is itself a switch, and at the center of its function are molecular gates which control the inflow and outflow of sodium and potassium ions. Neurons are usuall assembled into complexes which form more complicated switches, controlling larger scale phenomena like what muscle to twitch. Muscles themselves are made up of intricate switches. Even the molecular machines that propel muscle movement, a series of little bars that connect to adjacent fibers and flip to propel muscle movement look a little like light switches. At the most fundamental level, they are switches, and are controlled by yet other larger and smaller switches.

The transistor found its most crucial use as a gate - and is itself a kind of gate, and those gates are the most essential elements of a computer. The bits that are stored in our computers are represented in several different forms, but all are switches. The information processing or "thinking" done by those computers is done by the transistors though. Assembled in various combinations they form latches for storing information and logic gates for evaluating decisons and doing arithmetic.

We could make our computers out of other kinds of gates. People have even figured out how to make fluid computers that implement all the logic in flows of streams of fluid. They are about a million times slower than the electronic kind, of course, and much bulkier.

Next time you turn on a light, use a computer, move, or think, thank a few trillion switches. They were all designed (or more commonly, evolved) to allow you to do just that.