Professor Werner Heisenberg is speeding down the highway, when a cop pulls him over. The cop walks up to his car and asks, "Excuse me sir, do you know how fast you were going?" And Heisenberg responds, "No...but I know exactly where I am!"
If you understood this joke, read no further. However, if you're still a little confused, get ready to dive into some uncensored quantum mechanics. (Just kidding, I've removed the nastiest bits for you!)
This joke is based on the Heisenberg uncertainty principle--one of the fundamental tenets of quantum mechanics. The uncertainty principle can be summed up as: "The more precisely the position of a particle is determined, the less precisely the momentum is known in this instant, and vice versa." But to understand this statement, and hence the joke, it is first necessary to take on some quantum mechanics.
Quantum mechanics describes the behaviors of small particles, like electrons whizzing around an atomic nucleus. Compared to classical mechanics, which explains the behaviors of big objects like baseballs, airplanes, skiers--basically anything you encounter in your daily life--quantum mechanics seems very strange. To understand quantum mechanics, you need to let go of everything you've ever observed on the macro-scale and accept that there is a tiny world governed by its own unique laws.
The first principle of quantum mechanics you need to know in order to understand the joke is this: Subatomic particles are inherently fuzzy. An electron is not a fireball zooming around a chunky nucleus, but a negatively charged blanket that sometimes acts like a sheet crumbled into a ball and sometimes like a quilt spread across a bed. And because of this inherent fuzziness, the properties that describe the behaviors of these particles--such as position and momentum--are inherently fuzzy as well.
But the uncertainty principle is more specific than "particles are fuzzy." It holds that it is possible to determine either the position or the momentum of a particle, but it is impossible to determine both simultaneously. In other words, the more precisely we measure the momentum of a particle, the less precisely we can measure its position, and visa versa.
Are you laughing yet? Or are you beginning to wonder why the act of measurement seems to have such elevated importance? The way Heisenberg saw it, measuring any observable property of a particle actually affects that property. In this view, an electron doesn't even have a finite position and momentum until a scientist attempts to measure it, at which point the electron is forced to choose a state--like an atomic game of musical chairs where each player exists in every chair until the music stops. Not every physicist likes this interpretation of quantum mechanics--it always galled Einstein--but it's the one I'm sticking to for simplicity. After all, we're just trying to understand a joke here!
Still no guffaws? Okay, then it's time for an equation. As perplexing as the uncertainty principle is, it is represented by a very simple one: The product of the uncertainty in position and the uncertainty in momentum will always be greater than or equal to a constant. In other words, these two uncertainties are inversely related--if one increases, the other falls by a proportional amount.
However, the uncertainty principle can only be directly observed on the atomic scale. So sorry, Dr. Heisenberg, but it looks like you're getting a ticket after all.