What exactly is quantum mechanics anyway?

Frequently when I tell people I was a physics major, or that I teach physics, or that I had to take quantum mechanics in college, they contort their faces into a look of disgust and say something to the effect of "What exactly is quantum mechanics anyway?" My response is "Quantum mechanics is the physics of itty bitty things. When you get down to objects that are small enough (like down to the size of an atom or so), they don't behave the way we are used to things behaving."

That's not a bad introduction, and most people are satisfied with that. A few will push me further and ask for details. "Well," I say, "imagine you have a box with a baseball in it. If the box is sealed, there's no way that baseball is coming out, no matter what you do to the box. Now imagine a box with a single electron inside. If the walls are thin enough, there is a probability that the electron will appear on the outside of the box without damage to either the box or the electron. Now the electron doesn't pass through the wall of the box, it just appears on the other side of it." This is one of the bits of quantum theory that is pretty surprising to the average person (and rightly so), and it's known as quantum mechanical tunneling.

There are other bits of the theory that are quite odd, and here are a few listed:

• In Newtonian physics (the stuff usually taught in high school that does a pretty good job of explaining our daily experiences), knowing the position and momentum (speed) of an object is essential to describing its motion at some time in the future. In quantum theory, it's impossible to know the position of a particle at all. The best you can do is come up with a probability that a particle will be somewhere at a given time, so it's really more accurate to say the particle is everywhere, but it's not really anywhere either.
  
• The Heisenberg Uncertainty Principle says that it's impossible to know both the momentum and position of a particle. The more precisely you know its momentum, the less precisely you can know its position and vice versa.
  
• Making a measurement or observation of a system will cause it to settle into a certain state, where it will remain. In other words, once you make an observation, you throw the particle's probability distribution straight out the window.
• Light travels in the form of particles (or quanta) known as photons. Newton said that light traveled as a wave, and indeed there have been many tests to demonstrate that it does travel as a wave. However there are an equal number of tests showing that it is also a particle. Whatever your experiment is set up to measure is how you will observe the light.
  

You might look at this brief list and say "I don't buy it, it just can't be true." Hate to break it to you, but quantum mechanics is the most accurate scientific theory yet, and careful and repeatable experiments have proven beyond a doubt everything I've listed here, and much more.

So why do we need quantum theory at all? The answer is that Newtonian physics isn't always true. Newton made some great contributions in science, so many that most of what is taught today in your average high school physics class was developed by Newton. But when you get outside of the realm of our everyday experiences, for example near a star where gravity behaves much differently than we're used to here on Earth, Newtonian physics breaks down and no longer provides reasonable predictions. This gravitational snafu was addressed by Einstein in his theory of General Relativity, which has replaced Newton's gravity as the foundations of astrophysics for the last century. But when you go in the other direction, down to the scale of very small things, the theories of both Newton and Einstein break down completely and provide nonsensical predictions. In other words there are certain realms, i.e., when dealing with very small things, that both Newton and Einstein were wrong. Enter quantum mechanical pioneers like Fermi, Planck, Schroedinger, Heisenberg, Bohr, Pauli, and others, who began to think about a few cases where experimental results didn't agree with Newtonian predictions, and after a few decades quantum mechanics was born.

Quantum theory is unlike general relativity and classical Newtonian mechanics in one important regard: it doesn't appear to break down outside of its comfort zone. In other words, where both classical physics and general relativity break down when you look at very small things, quantum is still just as accurate when you look at larger scale situations, and in those special cases it essentially reduces to the same results as classical mechanics.

The reason for this post was the video below that I found this evening. It's a pretty good description of quantum mechanics. If it seems more philosophical than your usual science video it's because the nature of quantum mechanics makes it unavoidable.