Along with Santa Machines, one of the twin holy grails of the nanotechnology movement. Utility fog is an immense swarm of nanobots uniformly dispersed throughout a physical volume. Each nanobot in the utility swarm is a small computational structure with a dense bristle of telescoping arms that can be used to attach to other bots in the swarm. Each bot is considerably smaller than a molecule of air or water so you can have your entire local environment filled with utility fog while it remains filled with whatever other substances you need. The fog itself can dynamically mold itself into whatever structures your current experience requires.

It is really hard to get a grip on the implications of this. In some ways it seems to radically alter many of our most basic assumptions about how the world works. Utility fog calls into question our intuitive notions of what it means to experience something and what it means for something to be real.

The most obvious and first-mentioned applications of UF are virtual furniture that appears and disappears from nothing as needed and restraint systems (think airbags from hell) for vehicles. While these would be early applications, they are not the ones that will change our notions of reality. For that we have to wait for UF to gain the ability to completely replace our experienced environment.

Imagine that you are standing in the middle of a room filled with utility fog. It is impossible to tell, visually, that the room is filled. Your first hint of it is when you comfortably lay back in the air which has just developed exactly the resistance, springiness, and other tactile qualities of an expensive down mattress. This is possible because the foglets have extended their arms and knitted themselves together into a structure that supports your body exactly the way a mattress would.

After relaxing for a few moments you decide you want to see the Grand Canyon. The foglets in front of your eyes obediently change their arrangement so that the pattern of light reaching your eyes is exactly what you would see if you were looking over the guardrail at the Grand Canyon visitor's facility. Since you want to be outside the foglets surrounding you engage in the appropriate motions to move the air in the room over you in a way that perfectly simulates the breeze that your eyes tell you is blowing. Sound is also handled by the fog, once again in a way that perfectly matches what you see and feel.

But we're not done yet. Let's say that instead of using some sort of archival footage of the Grand Canyon (like what you'd get from an encyclopedia) that your visuals are constructed from a realtime video feed of a location at the Grand Canyon. We'll assume that the temperature and wind movement sensations you feel are being matched to those collected by sensitive weather measurement devices at the target location. Finally, forget about laying back in bed. Have the utility fog in your room position your body in some appropriate fashion as it feeds the exact tactile sensations into your entire body that you would be experiencing if you were standing next to the Grand Canyon; every pebble pushing into your foot perfectly matched by a pressure from the fog; every brush of dust as the wind lazily kicks desert sand along the rim of the chasm faithfully replicated as a quiet caress from a hand smaller than an air molecule.

At this point you are experiencing everything exactly as if you were at the Grand Canyon, even though you may be thousands of miles away. You are seeing the Grand Canyon, you are feeling the Grand Canyon, you are hearing the Grand Canyon, you are even smelling and tasting the air exactly as it would smell and taste if you were at the Grand Canyon.

Are you actually at the Grand Canyon, or are you still in your room back home? Later on your memories are going to insist that you were at the Grand Canyon, so it probably doesn't matter that your body was someplace else at the time.

Another hard problem raised by UF is that it obliterates our intuitive notion of how to tell who is and is not a person and who a particular person is. It removes every touchpoint we have currently have in establishing the identity of others.

Currently we identify people by visual recognition, but in a UF world we won't be able to. I'll always be able to present myself visually as anything I want by reversing the trick that casts the image to my eyes. I can arrange portions of my fog to reflect exactly the light that would be reflected from the volume I fill if I looked like, say, the President; this would control the image that was received by everybody within visual range who wasn't using their own UF to rewrite their visual input.

Oh, sometimes thinking about this stuff hurts my brain.