So what is quantum mechanics?

People throw the words "quantum mechanics" around without really understanding what it really means. What is quantum mechanics? Why is it necessary?  These are the questions I will be answering.



Let's start with the first one. What is quantum mechanics? In a few words, it is the study of the dynamical (how things move) and static (what things do when they are still) properties of small objects. Technically, everything obeys the laws of quantum mechanics, from the largest galaxies, to the smallest of sub atomic particles. So you may ask, "What do I get if I use quantum mechanics to figure out how something much bigger than a molecule behaves?" The answer is very simple, the behavior is pretty much exactly how classical physics predicts. What is classical physics exactly? Classical physics assumes that if you know everything about a group of objects (physicists call this a system), you can predict what everything does. For example. I'm holding a ball.

Gravity does it's thing and the ball hits the ground. This is a simple example of what's called a classical system. What is in this system? Well, there's me, a ball, and the earth. The earth is the source of what's called a force, or what in physics is generally called an interaction. What's the lesson here? Classical physics has predictable results. Physicists called a classical system a deterministic system. Never did you question whether or not the ball would hit the ground. Every time you've dropped a ball it has hit the ground. The time it takes can be accurately predicted using classical physics. Now let's consider the quantum version of this experiment.

Confused? You should be. Have you ever seen this happen in real life? Sure, you may have seen a gust of wind pick up the ball and carry it away from where you intended to drop it, but have you ever seen a ball be in multiple places at the same time? Neither have I. This is because the larger an object, the less apparent "quantum effects" are. So how big is big enough? Generally, anything much bigger than an atom starts to lose quantum weirdness. Now you're thinking, okay Dan, you've told me classical systems are deterministic, but you haven't told me why your lame cartoon is so confusing. Well I'm about to. As it turns out, quantum mechanical systems are not deterministic. Imaging you're holding an atom. You look at it and decide you have its exact location. Imagine you are standing on a flat surface and nothing is in the area that will affect the atom except for the earth's gravity. You know from experience that if you drop something, it's sure as hell going to fall and hit the ground. You drop the atom and wait for it to hit the ground. After waiting sufficiently long, you check to see if it's there. Is it? Well quantum mechanics dictates that there is no way to know whether or not it is. You see, quantum mechanics says that the best you can ever get is a guess at where the atom is, and you can only truly know where it is when you try to measure where it is. If you repeat this experiment many times, on average, you'll find the atom has landed directly beneath you, right where you expect it!  Let's sum up.

Classical physics (or, everyday life) - if you know everything about a system, you can predict what will happen to the system.

Quantum physics (or, how small things behave) - if you know everything about a system, the best you can do is get a guess at what will happen.

There is a very very short summary of what quantum mechanics is. Details will be illuminated (hopefully) in future posts.

Now to the second question. Why is it necessary? I won't go into detail on this one quite yet, but if it weren't for our knowledge of quantum mechanics, you wouldn't be reading this. Computer technology relies HEAVILY on quantum mechanics. The transistor is an integral component in just about every electronic device, but transistors simply would not work if it weren't for quantum mechanics. The applications of quantum mechanics are vast (Can you think of some more uses?), and the subject itself is a very rich and very interesting subject.  I myself love to talk about quantum mechanics, so I look forward to future posts! If you have comments, suggestions, or wish to point out an error in this post, I welcome you. Please keep comments civil.