Tiny, Tiny, Tiny
Quantum Mechanics is about forces on a truly tiny scale, as in subatomic particles. It includes three of the Four Fundamental Forces: electromagnetism, strong nuclear force, and weak nuclear force.
At the atomic level, everything is random that whatever happens is based on chance. The fabric of space becomes chaotic through the lens of Quantum Mechanics. So to understand what's going on at that level, scientists use probability to predict an outcome.
In fact, according probability in Quantum Mechanics, it is actually possible for me to pass through a wall... if I tried for an eternity. (See NOVA's "The Elegant Universe")
But there's a problem... the math for Quantum Mechanics doesn't line up with the math for General Relativity!
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