Is it Aether? A very 'Mythical' concept, used to denote a scientific idea which have since been disproven.
I've provided the link between man and machine.
The concept of aether, or similar ideas like 'quintessence' or 'elan vital' have been used in the past as the animating force causing life to differ from non-life.
In rivers and trees, respectively, I can be seen.
It was once believe that light / electromagnetic force must be carried through a substance called the 'luminiferous aether'. Thus aether would allow us to 'see' the rivers and trees.
I've been key in measuring the speed of light.
The idea of aether was supposed to determine the speed of light, based on how fast light could propagate through it. But the idea of aether was disproved in the Michelson Morley experiment.
The Pacific Ocean is where I developed flight.
Aether is a greek god, the god of the upper air. (Sadly, the pacific ocean doesnt fit here, the mediterranean would).
To measure current flux, I gave my label.
Cant figure out anything for this one.
I once was a particle, but my matter became unstable.
In ancient greek philosophy, aether was proposed as a fifth element, along with fire, earth, air, and water. But we have since learned better.
I think some of them dont fit well enough, sadly.