Designing a planet's road network

Can you imagine designing the world's road network from scratch? Plus all the pavements, footpaths, bridges and shortcuts? Can you imagine designing a computer with the complexity of a planet?

In Douglas Adams' classic The Hitchhiker's Guide to the Galaxy, there's a whole planet devoted to designing other planets, and the Earth was one of their creations. In the story, Earth isn't just a planet: it's also the most powerful and most complicated computer ever made, and its job was to help explain the answer to the meaning of life. Aliens had to design every last bit of it - one character, Slartibartfast, had the particularly complex job of designing the world's coastlines. His favourite thing to make was fjords, because he liked the decorative look they gave to a country. (He even won an award for designing Norway.)

That's just a story though, right? Could anyone ever design a computer of planetary complexity from scratch? As it happens that is exactly the task facing modern computer chip designers.

It is often said that modern chips are the most complex things humans have ever created, and if you imagine starting to design a whole planet's road network, you will start to get the idea of what that means. The task is rather similar.

Essentially a computer chip is made up of millions of transistors: tiny elements that control how electrons flow round a circuit. A microscopic view of a chip like the one above looks very much like a road network with tracks connecting the transistors, which are a bit like junctions. Teams of chip designers have to design where the transistors go and how they are connected. The electrons flowing are a little like cars moving around the road network.

There's an extra complication on a chip though. Designers of a road network only have to make sure people can get from A to B. In a computer, the changing voltages caused by the electrons as they move around is how data both gets from one part of the chip to another. Data also get switched around and transformed as calculations are performed at different points in the circuit. That means chip designers have to think about more than just connecting known places together. They have to make sure that as the electrons flow around, the data they represent still makes sense and computes the right answers. That's how the whole thing is capable of doing something useful - like play music, give travel directions or control a computer game. It's like designing a planetary road network, except all the traffic has to mean something in the end! Just like the fictional version of the Earth, only in fact.

It's actually even harder for chip designers. Nowadays the connections they have to design are smaller than the wavelength of light. All that complexity has to fit, not on something as big as a planet, but crammed on a slab of silicon the size of your fingernail! Pretty impressive, but Earth's intricate fjords are still more beautiful (especially the ones in Norway).

This article is inspired by the Kilburn Lecture given by Professor Steve Furber of the University of Manchester on 20th June 2008.