Remember the metallic substance that the T-1000 was made out of in the movie T2: Judgment Day? I want to create a substance like that in a story with some modifications...
So, the substance can move and take just about any shape, but it does not create more mass. For example, if there was a ton of it, it could take the shape of an elephant that weighed a ton, but not take the shape of a whale that weighs 20 tons. It cannot create something from nothing.
It can come apart into smaller components. For example: If there was a ton of it, it could take the shape of two separate elephants that each weigh half a ton.
It can only take the shape of something. It does not actually become that thing unless that thing is mechanical. So it could take the shape of a car but it would still need gasoline, a car battery, etc.
It could mimic a person, but not down the fine microscopic details.
It could withstand the vacuum of space.
It could become very hard or very malleable. When its malleable it relaxes and encompasses a bit more space.
It obeys commands from a computer that sends out signals to it. Without the signals it just falls into a malleable ball, but not a liquid puddle.
Also, and here’s the kicker, I want it to be able to float in the air. That doesn’t necessarily have to mean “anti-gravity” though. Maybe there a magnetic wave that it works within, or something like that(?)
Does anyone want to take a crack at proposing what might comprise such a thing? “Magic” not allowed.
Re-edit: Michio Kaku describes what I mean here-
Re-re-edit: And yet another possibility...