Here's an almost done version of something I had to write for class, explaining what the hell the Uncertainty Principle is:
Perhaps warrior-poet and former Secretary of Defense Donald Rumsfeld most deftly defined the many kinds of knowledge. There are “known knowns,” the things we think we’ve got a handle on. There are “unknown unknowns,” ideas or phenomena we’ve never even thought to investigate. Let us not forget “known unknowns” – information we know is out there but have not discovered.
Scientists thrive on the last kind. Experiments attempt to solve known problems. In the process, they uncover the questions people never thought to ask. Once scientists ask the right question, they restart the process.
That’s the textbook version. Behind it lies an ethos that, taken to its logical conclusion, presumes science can uncover all the rules that govern the universe. Unknowns shall become a thing of the past. Eventually.
In 1927 Werner Heisenberg cut into that foundation. He was already a noted physicist working with Niels Bohr in Copenhagen, Denmark, to open up the new field of quantum mechanics – the study of the properties of subatomic particles like protons, neutrons, electrons and their constituent parts. But that year he made his namesake contribution to science, the Heisenberg Uncertainty Principle. And he proposed a different kind of scientific knowledge – the known unknowable.
A simple version goes like this: It is impossible to know both the location and the momentum of a particle. In order to measure the location, one must slow down the particle, causing it to lose momentum. That’s not the only way to phrase the Uncertainty Principle, however. The same thing occurs with measuring the time of an event at the quantum scale and the energy involved – you can’t tell both at once.
So in a universe of quantum mechanics governed by Heisenberg’s principle, scientists must work with probabilities. Unable to know both momentum and position, they settle for a blurrier vision, perhaps that there is a 70 percent probability the electron is in a particular field at a particular time. They could shrink that area and derive a more accurate picture of the electron’s position, what mathematicians would call narrowing the “probability distribution.” But there’s a trade-off: the more you know about the position, the less you can know about the momentum.
This presented a bombshell of a philosophical break with the past. Classical physics presumed an absolute reality outside of subjective interpretation. Albert Einstein’s theories of relativity allow different viewers to perceive reality differently, but they perceive the same universe.
Classical physics also depended on causality – one thing causes another, every action has a reaction. Quantum mechanics makes no such promises. Consider baseball – a .300 batting average means a batter gets a hit in three out of every ten official at-bats. But that doesn’t mean he will or won’t get a hit in his next at-bat. That’s how Heisenberg’s principle works with physics. A radioactive atom may have a 70 percent chance of decaying in the next hour, but that’s no guarantee that it will. 70 out 100 atoms will decay, but which 70? Who knows.
The Uncertainty Principle even included this bit of post-modernism: the act of observing something changes the thing itself. Heisenberg’s initial explanation provided the example of a microscope – it uses photons of light to view an election, but hitting the electron with a photon of light changes the electron. At first glance, it seems that with better tools, scientists could get around this problem. Not so, Heisenberg says – uncertainty is a quality of the universe itself. To pull off this bit of mental gymnastics, he and Bohr abandoned the old arguments of realism. There are no states of absolute position or momentum in quantum mechanics – their existence depends on how a person chooses to measure them.
For many this looked like malarkey rooted in metaphysics. Einstein was among them. To his death he could not reconcile Heisenberg with his own conception of elegant cosmology. As former Cambridge University physicist David Lindley writes in "Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science," Einstein felt the Uncertainly Principle “was a sign of human inability to comprehend the physical world, not an indication of something strange and inaccessible about the world itself.”
In the modern era, however, Bohr and Heisenberg have won out. That’s because uncertainty isn’t simply some marvelous bit of modern philosophy – it works. The probabilities inherent in the Uncertainty Principle can tell researchers the likelihood a particular radioactive atom will decay, or a particular electron will jump to a higher energy level in an atom. But it can’t tell you which one, or when, and that’s the essential trade-off. Uncertainty balances the equations. It doesn’t answer the big questions. As Lindley wrote, “it’s not hard for scientists to use quantum mechanics without indulging in philosophical worries about the nature of the universe.”