Can someone explain to me quantum mechanics in simple words/phrases?

Answer 1

"Quantum mechanics is a fundamental branch of physics with wide applications in experimental physics and theoretical physics that replaces classical mechanics and classical electromagnetism at the atomic and subatomic levels. Quantum mechanics is a more fundamental theory than Newtonian mechanics and classical electromagnetism, in the sense that it provides accurate and precise descriptions for many phenomena that these "classical" theories simply cannot explain on the atomic and subatomic level."

Answer 2

Quantum mechanics explains the behavior of very small particles. On the macroscopic (large) scale, chunks of matter seem to be located at a single position. If you have a rock sitting on a desk and you measure its location, assuming nothing hits the rock, you can measure it again later and you'll get the same result. Very small particles behave fundamentally different. They do not HAVE a definite position until you measure them. Likewise, the more accurately you measure position, the less you know about velocity, because the act of measuring position involves bouncing other particles off it, which drastically changes its velocity. Likewise, the more accurately you measure velocity, the less you know about the particle's position. This pair of variables, position and velocity (well, really momentum) has a special relationship in quantum mechanics. The uncertainty in position multiplied by the uncertainty in momentum is always larger than a certain value, or:
dx*dp >= hbar/2

This leads to the old joke about the cop stopping the physicist on the highway. The cop says, "Do you know how fast you were going?" The physicist replies, "No, but I know exactly where I am."

Answer 3

Think of a bumpy pool table. A basket ball rolls smoothely, a pool ball wiggles a bit, a marble goes every which way. You need quantum formulas to predict the path of the marble.