[Dave]

To return to the "grey goo" - a term I haven't come across before (any references?) I like the idea of a nano-forge or Star Trek-style Replicator, in which any complex molecule could be assembled from a stock of its basic constituent atoms, and I think it has to come soon. But, there lies the problem - without a sufficient topping up of stock of the basic constituent atoms the machine would eventually run out of say, Silicon or Carbon, and come to an abrupt halt.

Now, I guess we are talking about more complex structures, the kind of things just mentioned from Michael Crichton's Prey or Wesley Crusher's nanobot school science project in Star Trek. They are self-replicating, so they have one of the criteria for being classed as a living thing, but they would still be limited by their environmental conditions. They would still consume all the raw materials around them, and then stop, probably to be poisoned on their own waste products, much as bacteria do. I think they would be more "grey scum" than "grey goo". But, to actively seek out new raw materials, wouldn't that also require some intelligence, not just an ability to work together? The Prey bots did achieve this, but the 'how' was not explained.

I think we are much too familiar with those apocalyptic sci-fi story lines - Michael Crichton's Prey, Star Trek Nanites , Ice-9 from Kurt Vonnegut's Cat's Cradle or the engineered bacteria in Neal Stephenson's Zodiac - the kind of story where the new invention inevitably runs out of control - it's a classic concept - it goes back to Frankenstein's creation.

In reality, I would think any leak would only cause a local problem. Something like the early non-biodegradable detergents that caused quite a problem with froth and foam in waterways in the late sixties.