What is nothing?

One is tempted to say that the question doesn’t make sense—that it is, somehow, incoherent to even pose it. Nothing, after all, is not anything! If it makes you uneasy, you are in good company: this question has been met with skepticism and bewilderment for at least two and a half millennia. The Ancient Greek philosopher Parmenides, for instance—a contemporary of Socrates—warned against trying to answer it, for (as he claimed to have been told by a goddess) that which is not and must not be cannot be apprehended or described. Instead, Parmenides urged, the truer way of inquiry seeks that which is and must be.

The physicists and philosophers who have followed Parmenides have tried to take his goddess’s advice. They have spent a great deal of time and effort seeking to understand that which is—namely, stuff, the matter of which the world is composed. And they have been enormously successful. We have, particularly in the last century, come to understand the microscopic building blocks of our universe: the hundred or so kinds of atoms that make up our bodies and all the ordinary objects we see and use every day; the protons, neutrons, and electrons that compose those atoms; and, ultimately, the quarks that form protons and neutrons.

But in trying to understand the material world, physicists have also come to realize that Parmenides’ goddess got things backward. Yes, we wish to understand that which is. But to do so, it has turned out, we have also needed to understand that which is not. The key to developing a successful physics of stuff has been to understand the physics of nothing.

Or more precisely: to develop the physics of stuff, we have first needed to understand what the world would be like if there were nothing—no matter—in it. This was a central insight of the seventeenth-century physicist Isaac Newton. What Newton realized was that to even write down the laws of motion and gravitation for which he is now famous, he needed to first get clear on the basic structure of empty space (and time). He needed to define concepts such as distance, duration, and motion by saying what sorts of relations could hold between the places where things could be (but were not). This led him to propose a theory of nothing that formed the foundation for physics over the next two and a half centuries.

Even more striking, though, than the need for a theory of nothing in physics is that Newton’s theory of nothing has turned out to be, completely and fundamentally, wrong. There is not, after all, just one way that nothing can be—nothing is not simple or trivial. Just as the twentieth century revealed deep new insight into the nature of physical objects and the laws that govern them, it also revealed fundamental misunderstandings in the Newtonian conception of (empty) space and time.

We learned, first, from Albert Einstein’s theory of relativity—a new theory of gravitation that replaced Newton’s—that space and time are not fixed once and for all: they are (together) a dynamic entity that responds to the presence of matter and energy, and which can influence how matter behaves. And then, about three decades later, we learned from quantum field theory—the theory of matter underlying modern particle physics—that nothing is not so much the absence of matter as a particular configuration of matter: more like an ocean with no waves than the absence of the ocean; more like a TV with static but no picture than a blank screen.

Today we are faced with a basic tension. Each of these theories—relativity theory and quantum field theory—presents us with a conception of nothing. But they are essentially different conceptions. Bringing them together presents a puzzle that physicists have been trying to solve for nearly a century, but which remains so elusive that perhaps, alas, Parmenides’ goddess was right.

The videos that follow explore the physics of nothing, as we have understood it in the twentieth and twenty-first centuries, from a handful of different perspectives. They look at how physics and philosophy interact—how physics needs, or at least essentially involves, philosophy at the level of trying to uncover and articulate the conception of reality that goes along with our physical theories. They look at the basic features of general relativity and quantum field theory, including the structure of emptiness in these theories. And they suggest ways in which quantum theory and relativity may eventually be brought together to create a new theory of nothing.

—James Owen Weatherall

James Owen Weatherall is Professor of Logic and Philosophy of Science at the University of California, Irvine. He is the author of Void: The Strange Physics of Nothing (2016) and The Physics of Wall Street: A Brief History of Predicting the Unpredictable (2013). His next book, The Misinformation Age: How False Beliefs Spread, with the philosopher Cailin O’Connor, will be published in January 2019. He lives in Irvine, California, with his wife, twin daughters, three chickens, and an indeterminate number of cats.

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