Quantum means "an amount," and mechanics means "the motion of that amount." Newtonian physics resembles cause and effect like a machine and works with things in human size, like balls on the ground, or larger things like planets and stars: push a ball on the ground with "X" amount of force and it will go "Y" far. When we get to subatomic particles, Newtonian physics doesn't work because an "X" amount of force may send 30% of the energy in the "A" direction and 70% of the energy in direction "B." We are forced to create a new mathematics that deals with probabilities instead of Newtonian machine-like mathematics with absolute outcomes, so we created Quantum Mechanics only for subatomic things.
Einstein's theory of general relativity worked on our scale and the astrophysics scale. It did not work on a subatomic scale because of a simple experiment that proves light acts like a wave and a particle. Nothing in Newtonian physics does that: energy moves through matter as a wave (radio waves, heat, microwaves) or is self contained in a particle. Einstein became the biggest opponent of Quantum Mechanics because he wanted to answer the holes in his own theory. If something fails at being proven wrong, it's right.
I love this :) After his Phd in physics, Antony Garrett Lisi moves to Maui and teaches surfing. On the side he pursues physics problems, coming up with his "An Exceptionally Simple Theory of Everything." His theory includes Einsteins theory of general relativity, explains gravity's weakness with 8 dimensions, includes Quantum Mechanics and predicts undiscovered elements in the periodic chart. Doesn't work with string theory though. So, gravity is weak because it only exists in our dimension a small amount. Being tested (proven) on the super collider at CERN right now. This is his diagram of how it all fits together. This guy could be the next Einstein. Then I'll be looking for my "Exeptionally Simple Explaination" of his theory.
What Comes Out Of This?
Einstein's theory of general relativity brought us the atom bomb. "An Exceptionally Simple Theory of Everthing" may bring us power over gravity, abilities to create any type of material ourselves, multidimensional travel, teleportation... that sort of thing. Probably some really great weapons...
Rant About How Long It Took To Learn This
You be the judge of whether Wikipedia does a better job of explaining. Better yet, do you're own web search and see if you aren't bogged down by multiple unrelated facts and still no answer.
So, I read some books. Hawking tells me if I travel to the edge of the universe I'll end up on the other side and if I get near a black hole I'll be stretched into an infinitely long string. New media icon Michio Kaku, who's area of expertise is String Theory, is telling me it's all waves and strings which will probably be disproved soon anyway. I was able to boil down four sentences after 375 pages of Gary Zukav's, "The Dancing Wu Li Masters," skipping the parts about photons being sentient.
Watch the science channel and you'll be bombarded with physics shows where the host appears with multiple copies of himself at a bar and then a humpback whale appears spontaneously in Times Square*. Holding his hands together in a prayer-like manner he somberly states to the camera, "That's the new physics." Wha??! I hold this wizardish, magical David Copperfield presentation of science personally responsible for things like the movie, "What The Bleep Do We Know." Imagine something that you want to happen, see if it happens. That's the "Bleep" experiment. It's called wishful thinking.
*At the big bang with all that energy released, anything could happen. Billions of years later, after energy organized itself into atoms, elements, stars, planets, and galaxies, the cascading organization of "stuff" in the universe means things like Humpback Whales falling out of the sky just won't happen.