12/12 26: The World According to Quantum Mechanics

Ready to have your mind blown?

So I've recently witnessed a lot of talk about quantum theory, particle physics, and all that good stuff, and I've been a part of a few of those discussions. After a bit of pondering, I decided that such concepts wouldn't be a bad idea for a new Tidbit. Of course, unlike most of my Tidbits, this is pretty heavy stuff, so if you aren't primed to think outside the box at the moment you might want to defer your reading of this post until a later, quieter time. I'm not going to deal with a lot of relativistic theory here... though I might in a future edition. So at least we won't be getting into stuff that confusing.

But if you're ready, let's dive in. In this Tidbit, I will attempt to introduce you to the basic concepts of quantum mechanics, explain what they mean, and extrapolate basic conclusions about our existence from what we know. I'm no expert in this field by any means, but I do a lot of reading on the subject, and I'm hoping that my explanations here will spark a serious interest in people who are prone to debating such confusing matters. Keep in mind that my journey through the subject here is going to be highly selective in order to make the material stomachable. But also keep in mind that everything we're discussing here actually is true. Very little of this is speculation or a product of uncertainty. We're talking about the very world around you in this article--everything you see as you look around from your vantage point sitting in your chair.

So let's start with the basics: what is quantum mechanics? Well, in short, it's the study of things of the very small--atoms, electrons, photons, and the like--in an attempt to make some sense of their erratic behaviors. Until around a century ago, it wasn't clear how anything worked apart from things closer to our scale of existence. In fact, what we see as a smooth and predictable universe, governed clearly by the laws of Newtonian physics, is in fact nothing like that at all. These Newtonian laws, such as calculations of gravity's effects on a ball tossed into the air, apply only in our large-scale world. Once you break things down into the world of particles, it's much more difficult to predict anything (in fact, it's actually impossible, as we'll see later on). The movements and actions of particles seems much more jittery than the smooth-looking stuff we perceive around us.

Anyway, a century ago, some German guy named Max Planck made some discoveries regarding the behavior of oscillating atoms, as well as a number known as Planck's constant, which relates the energy of an electromagnetic wave to its frequency. A little later on, Werner Heisenberg declared a fundamental--and rather startling--principle of quantum theory that shook the scientific world: he found that it is impossible to measure both a particle's velocity and position simultaneously with sufficient accuracy. In fact, the more you know about a particle's speed, the less you know about its location. That means it's just plain impossible to predict anything with accuracy relating to particle physics. Read that again just in case you didn't get the importance of it!

This discovery became known as Heisenberg's Uncertainty Principle, and it just so happens to be parodied in my signature. :) This stuff is true of ALL particles... it's just the way things are. If you're curious as to WHY... well, there really isn't a very good explanation... it's just a highly complex, so-called "entangled" quantum system (see later for more on entanglement).

So, you might ask, then why is it possible to measure the path of a ball through the air? Well, as I said before, in our large-scale world, we perceive only the "quantum average" of such events. In other words, we only see the grand scale result, which is always pretty much what we expect it to be here in the "big" world. Think of it like the pixels on a television screen; up close, they appear clearly separate and multicolored. But together, they blend to form a smooth and familiar picture that is interpreted effortlessly by our brains.

Are we having fun yet? Whew--not exactly the type of stuff you would want to talk about at parties.

The next big foundational principle--and this one will blow your mind--is the two slit experiment. Here's how it works. Imagine a wall with two small vertical slits cut into it at equal heights. Now, stand back from that wall and place a machine gun around twenty feet back, aimed at the center of the two slits. Fire it for a few seconds, and then walk behind the two-slit wall and check out the pattern of bullet holes on the wall behind it. Obviously, they're also arranged in a distinct pattern of two vertical lines.

Now, replace the machine gun with an electron gun. Fire off a few hundred electrons, and then check the back wall again. Same pattern, right? Heh, wrong. In fact, not even close. No two lines at all--instead, there's a muddled swelling and fading of vertical waves across the entire wall. How in the world did that happen?

Well, maybe the particles are interfering with one another; you know, bumping into each other as they fly toward and through the slits. That's got to be why. Let's test our theory to make sure. Now, we're going to fire one electron every few seconds, so that way we give them plenty of time to hit the wall on their own before firing off another one that might screw up its path... After a few hours, though, we end up with the exact same pattern. Is blood shooting out of your eyes yet?

In case you didn't catch all of that (I know it's pretty hard to get in writing), here's a visual demonstration of pretty much the same analogies I used.

The reason this occurs is thanks to a property known as wave-particle duality. Basically, that means that electrons and other very small things have to be thought of as both particles (bullets) and waves (like water). This is exemplified by the stuff you've heard of as "electromagnetic waves," which, by the way, is the same thing as light (it's just a stream of photons).

So, now, let's see which slit each electron travels through. We're going to place a detector (sort of like a camera we could say) by each slit to reveal which path each electron takes. We fire up our electron gun, start up the detectors, and then let it run for a few hours to collect some data for our graphs while we head to Wendy's for a #6 (Biggie-sized with a Fruit Punch to drink).

When we get back, we drop our food on the floor at the sight of not an interference pattern, but instead a uniform distribution along the wall that looks nothing like before. What in the world happened? How is that even possible? It's as though the electrons knew that we were watching them, so they magically decided to stop acting like waves and instead reverted to their usual particle selves.

In conclusion, the ONLY way to think of this problem is to imagine the electrons travelling through BOTH slits at the same time. Every electron goes through BOTH slits--they don't choose one or the other like you would think. This whole concept leads into a massively confusing approach involving probability waves, wavefunctions and their spontaneous collapse, among other completely unnecessary concepts that I'm not even going to go into here.

SO. That's the basics of quantum theory. From these experiments was born the study of such a counterintuitive subject that even Einstein himself hated it. His famous remark, "God does not play dice," was made in response to some of the quantum goings-on at the time. However, now we know that both quantum theory AND Einstein's theories are correct. The problem is, we have no idea how to connect them in harmony. That leads to a constant searching by scientists for a state of theoretical consonance between the two theories referred to as a "unified theory." Such a theory would be the ultimate "theory of everything"--and that is essentially now the primary goal of modern theoretical physics.

As an example of how these theories (and, in fact, reality) are at odds with Einstein's theory of general relativity, let's talk about quantum entanglement. If you ever really wanted to prove to yourself how very little we actually understand about how the world around us works, this is a good place to start. It is essentially a stated law (authored by Einstein) that nothing--and that means, nothing--can move faster than the speed of light. It's a sort of absolute speed limit for all matter and forces in our universe. However, oddly enough, scientists have recently found some disturbing (or interesting, depending on how you look at it) results from their experiments involving photons and prisms. If you split a photon with a special prism to result in two different photons, traveling in opposite directions from one another, they are what is called "entangled." Let's say you then let these photons continue on their opposite paths for millions of years.

Now, they're on opposite ends of the galaxy (or maybe they're even in different galaxies). If you are now to influence one photon's spin (a common property of all particles that determines their nature), the other particle's spin will instantaneously shift as well in the opposite way. This is horribly confounding to particle physicists, seeing as the only way this seems possible is if the communication between them moved faster than the speed of light. Some theories have been proposed that perhaps it is still limited by the speed of light, but instead the message travels through time as well (spacetime is indeed a strange thing... time doesn't actually progress the way we think it does. I'll get into this in a future Tidbit to twist your mind further). But currently, no one really has any idea, and this is something that science is still working on an explanation for.

But uncertainty is what it's all about anyway. Quantum mechanics reveals, according to what is called the Copenhagen interpretation, that it isn't possible to calculate things in our world to predict what's going to happen next. We don't live in a deterministic world, in other words... we live in a probabilistic world. Everything is based on a set of possible outcomes, not a value of solid certainty.

And one truly uncertain thing is whether or not I'll post a second edition to all of this stuff, dealing next time with advanced relativity concepts and how they intermingle with the confusion of our quantum reality. But either way, stay tuned. :)

Thanks to Stephen Hawking's A Brief History of Time and Brian Greene's The Fabric of the Cosmos for this Tidbit!