First of all word nanobot is combined from two words: nano and bot = nanobot. Word nano comes from the size of the bots which is some nanometers depending on use. And bot refers to robot/some kind of mechanical structure that is able to do the given job by themselves. So nanobots are tiny robots.
Because we had to use the word nanobots we shall continue using that word even though it’s kind of science fiction like and nanorobots would be better expression.
Nanobots could have many functions (no actual mechanical nanobots have yet been constructed or created) like medical applications and actually the theoretical use of nanobots are endless because of their size. They could build anything that we know or even more. They are like small humans.
So what about the grey goo situation that is mentioned in the title. Well its very famous doomsday scenario given when talking about nanobots. The theoretical situation is that a swarm of nanobots gets out of control somehow. Maybe they would build a mind of their own or they get infected by a computer virus and they begin deconstructing all matter which results in everything on the planet turn into shapeless grey goo. This is very unlikely to happen but raises concerns about nanotechnology’s safety issues.
Scary isn’t it?
Juhana ja Teemu