Sub-atomic particles
Atoms
Molecules
Cells
Structures

?

Particles -> x/y/type/state
Atoms -> x/y, particles{}
Molecules -> x/y, atoms{}
Cells -> x/y, molecules{}
Structures -> x/y, cells{}
(velocity)

?

Each type of thing is handled separately, once something is consumed by a higher level, it disappears from the lower level section.
This way we can save processing power. It means however that things at a higher level can't combine at a lower level. :/

spacial hash? elements can interact with each other only in the same hash, hashes overlap?

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Particles spawn randomly within a radius specified by the farthest traveled particle.
Particles spawn with different types, each has a specific lifetime. (Use a particular generator for randomness of lifetimes.)
Velocities are random, seeded generator?