Warrior-poet and former Secretary of Defense Donald Rumsfeld once deployed his particular brand of obtuse poetry to define 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. And let us not forget the “known unknowns”—information we know is out there but have not yet discovered.

Scientists wrestle with the last kind and in the process uncover the questions people never thought to ask.

Once the right question emerges, scientists 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. With enough time, sweat, and calculations, all unknowns shall become a thing of the past.

In 1927 Werner Heisenberg dashed that hope. He was already a noted physicist working with Niels Bohr in Copenhagen, Denmark, to develop the new field of quantum mechanics—the study of the interactions of subatomic particles such as protons, neutrons, and electrons. But that year he made his namesake contribution to science, the Heisenberg Uncertainty Principle, in which he proposed a different kind of scientific knowledge—the known unknowable.

The Uncertainty Principle stands on the idea of the “observer effect”: the act of observing something changes the thing itself. Heisenberg initially provided the example of a microscope—it uses photons of light to locate an electron, but the electron’s position and velocity are altered when it gets hit with a photon. At first glance, it seems that with better tools scientists could get around this problem. Not so, said Heisenberg; 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. In a universe governed by quantum mechanics, it is impossible to know precisely both the location and the momentum of a particle. To measure the location, one must interfere with its momentum, or vice versa. The same thing occurs with measuring both the time and energy of an event on the quantum scale—you can’t tell both at once.

Grappling with this indeterminacy, physicists settle for a blurrier vision of the world, perhaps that there is a 70 percent probability of pinpointing an electron within a particular locale at a particular time. They could shrink that area and derive a more accurate picture of the electron’s position—what mathematicians call narrowing the “probability distribution.” But then in walks Heisenberg’s 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 that reality existed outside of human interpretation. A tree falling the forest makes a sound whether you’re there or not, and it doesn’t need you around in order to have position, momentum, or any other physical property. Albert Einstein’s theories of relativity allow different viewers to perceive reality differently, but the universe they see is the same one.

Classical physics also depended on causality—one thing causes another, every action has a reaction. Quantum mechanics makes no such promises. Consider baseball for a moment. A batter with a .300 average 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. Seventy of a hundred atoms will decay, but which seventy?

We don’t know.

For the old guard, this looked like malarkey rooted in metaphysics. Einstein was among them. To his death he could not reconcile Heisenberg with his own conception of physics’ elegance. 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 won out. That’s because uncertainty isn’t simply some marvelous trick 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 lost grace Einstein mourned.

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.”