Let’s say that in my fictitious, scientifically advanced world, humans create some “miniature” (and I say miniature because I’m not sure what prefix use: Nano? Micro? Or even smaller?) machines to use for medical purposes. Eventually this leads to the beginning of a “grey goo” scenario, that is ultimately averted, some event wiping out most of the machines code and rewriting it in a less harmful way.
These machines now keep replicating but not so aggressively, they “integrate” themselves with the humans and the terrain, rather than just repurposing everything. They are invisible to the naked eye unless they glob together, and permeate everything: everyone’s body is full of them, the terrain, the trees, the animals…
These machines have no longer much purpose on their own, they just are around, but they form a network of information, communicating with each other, and they can be programmed. The machines are good enough to intercept our senses signals before they arrive into our brain, and are able to edit this information if they are correctly programmed.
Hence we would have a society completely immersed in Augmented Reality. Reading our minds like if they were open books is impossible for these machines, but other than that they can do almost anything information related, and physically too.
Information related: they would work like our internet nowadays, but sans the need of a device. One could “ask the network” to show him a map of his surroundings and would be able to see it via de AR. Ask again the shortest route to certain street and, as long as someone programmed the street names… you have some kind of GPS. Record what you just have seen and send it as a video file to someone. Have long distance calls or even transmit sensations. Have “pop ups” appear when you look at products at a store with their price, or automatically translate what someone is saying in another language and make you hear the translation instead of the original. The sky is the limit, or more accurately: the minds of the programmers in that world is the limit.
Physically, since this machines are everywhere and are physical entities even if they are small, if correctly programmed they can interact with anything physically. Have them eliminate the cholesterol in your arteries, or fix the cavities in your teeth, eliminating the corrupted material and creating new “tooth material” to refill them. Ask them to clump together and form a club… or a gun. Have them scan an item and create an exact copy from scratch.
Now you know what these “machines” of mine can or should be able to do.
What size are they? I just know they have to be invisible to the naked eye unless the get together to make anything bigger, but I don’t know at what scale would this be true and one could still build a theoretically functioning machine. If the size is sub-atomic (which I doubt) that would be a little bit of a headache. The only thing I know about quantum mechanics is that my machines might or might not be there until resolved by observing them.
What material are they made of? In my mind, originally, they were made of metal, and this world had very few easily accessible sources of metal left due to this. But giving it a little more thought, they might be built with semiconductors… or some other material that makes more sense. If they are to be sub-atomic… Can we talk of materials at that size?
Note that I used the “science-based” tag, not hard science. I’m perfectly fine with “theoretical” solutions if there aren’t any real ones, like: they should be a material that does not exist but need to have this or that characteristics. I could very well handwave everything and say they are just small, and made of unobtanium, but having a sort of "real" frame of reference would do wonders to help me work with these setting and avoid making it look just like "space magic".