May 16th, 2016
The simple answer is that quantum physics is newer, and therefore more advanced than what we call mechanical physics (or ‘regular’ physics). However, modern quantum mechanics, our present-day method of studying physics (nuclear, chemical, or astronomical) is so complex that its 1st quarter-century, from 1900-1925, is now referred to as ‘Old Quantum Theory’. In that first, primitive form, Niels Bohr and a bunch of other guys noticed that electrons orbit a nucleus at different levels—never in-between the levels. They called the ‘steps’ from one level to another ‘quanta’ (the plural of ‘quantum’, both from the Latin quantus (“how much”).
Actually, they used ‘quantum’ to refer to the miniscule amount of energy lost or gained when an electron moved from one orbit to another. They realized that quanta are limited—down at that level, energy doesn’t slide smoothly up and down a scale, but jumps from one quantum level to another. And this is just one of the ways in which very-small-scale (or nuclear) physics differ from what we call macroscopic physics (like throwing a baseball or flying a plane).
Another example is indeterminacy—usually referred to as Heisenberg’s Uncertainty Principle. What Heisenberg said was: you can’t see a thing without bouncing something off of it—usually a photon of light. But when things get very, very tiny you can’t bounce something off of it without moving it, or changing it somehow. So he concluded that you can’t look at something without changing the thing you’re looking at. It’s a great principle because it’s true of sub-atomic particles, but it’s also true of people—even of groups of people—if you watch them, they notice you’re watching them—and they change their behavior. But that’s not physics—it’s more like a coincidence.
The biggest obstacle to understanding quantum mechanics is that it’s based on the idea that there are more dimensions than we know of, or are aware of—the usual three dimensions of Space, and the fourth dimension of Time. They theorize that there are many more dimensions—maybe eleven or twelve, nobody really knows yet. The dimensions we know of seem so basic, so much a part of reality, that’s it’s nearly impossible to imagine what a fifth or sixth dimension would do, or where it would go. But mathematics can let theoretical physicists play around with the idea and try to get something out of it that humans can understand, at least partly. Still, you can see why there aren’t a lot of theoretical physicists—it’s kind of a headache.
Also, Multiple Dimensions pose the same problem as Dark Matter or Dark Energy—we only have so much empirical evidence to work with—the rest is all theories—and those theories, being about things we don’t see, or can’t comprehend, make it hard to come up with real-world experiments that could prove the theories.
To prove the existence of the Higgs boson (the ‘God’ particle) CERN had to build the Large Hadron Collider, which straddles the border between Switzerland and France—it is a circular structure 17 miles in circumference. It took ten years to build it. Peter Higgs came up with the theory in 1964—but he didn’t win the Nobel Prize until 2013. There were several other scientists involved, but I don’t want to complicate this more than I have to. The famous Stephen Hawking experienced the same sort of thing—he theorized the Big Bang in his graduate thesis, and described theoretical properties of Black Holes—and had to wait many years before people stopped laughing at him and started respecting him for being right—just like Higgs.
This is not the first time theory came long before experimental confirmation—when Einstein wanted to prove that gravity bent light, he devised an experiment that measured the apparent position of Mercury just before it passed behind the Sun. Because that light would have to pass by a big gravity-well like the Sun, the light gets bent and the apparent position of Mercury would differ from the known position of Mercury. The experiment had to be delayed because World War I U-boats made it impossible to go to the exact place on Earth where the observations had to be made—Einstein’s Special Theory of Relativity wasn’t published until after the war, when the experiment could finally be done. And that was before Quantum Physics even came into the picture.
So, if pressed, I would have to say that the main difference between Mechanical Physics and Quantum Physics is that Mechanical Physics is human-oriented—Newton based his Laws of Motion and Universal Gravitation on careful observation—he described what he saw, and pointed out the mathematical relationships of physical phenomena, for instance, that gravity decreased in proportion to the square of the distance between two objects.
Quantum Mechanics, on the other hand, is based on accepting that human limits are not the end of the story—that the universe is a strange place with more to it than we can see, or even imagine. It even opens up the possibility that a human brain may not ever be able to fully understand the universe—which makes Quantum Mechanics a glorious, even quixotic, quest for knowledge.