An Up (or Down) Quark.
You see me all day:
The little buggers are everywhere, in all matter. 
Quarks are a tiny, like the edge of an absurdly sharp blade. Some people would consider Quarks to be smart, as they are explained and though about by Physicists (ie: sharp people).
I show you your way
But you show me what to show you:
According to certain interpretations of Quantum Mechanics, the state of a quantum particle (ie: a Quark) is decided by whether it is observed or not. If it is unobserved, then it is in a state of Superposition, once it is observed its State Collapses.
I exist, but not in your dimension:
Quarks are subatomic matter, they are tiny. Relatively speaking, humans are huge. Dimension here refers to size.
I'm made up of down:
Hence Up or Down Quark (probably Up).
My older language is by rotation:
Quarks are imagined to 'spin' but this notion is misguided.
But now through light:
I can't explain this. It might have something to do with Quarks color charge.
(though in the end it comes down to the same shocking tale):
Quantum Mechanics is sometimes considered bizarre and shocking.
I usually look a certain way
But I change at certain times
Again with Quantum State.
Into myself, my thin sibling, into time,
and into ambiguity
or something with which you grasp
Changing states upon observation. 'Grasp' as in strong interaction.