Void

The Strange Physics of Nothing

James Owen Weatherall

• Publisher: Yale University Press
• Year: 2016
• ISBN:
  9780300224498
• Topic: Physical Sciences
• Subtopic: Quantum Theory

The New York Times bestselling author of The Physics of Wall Street " deftly explains all you wanted to know about nothingness—a.k.a. the quantum vacuum" (Priyamvada Natarajan, author of Mapping the Heavens ). James Owen Weatherall's bestselling book, The Physics of Wall Street, was named one of Physics Today 's five most intriguing books of 2013. In this work, he takes on a fundamental concept of modern physics: nothing. The physics of stuff— protons, neutrons, electrons, and even quarks and gluons—is at least somewhat familiar to most of us. But what about the physics of nothing? Isaac Newton thought of empty space as nothingness extended in all directions, a kind of theater in which physics could unfold. But both quantum theory and relativity tell us that Newton's picture can't be right. Nothing, it turns out, is an awful lot like something, with a structure and properties every bit as complex and mysterious as matter. In his signature lively prose, Weatherall explores the very nature of empty space—and solidifies his reputation as a science writer to watch. Included on the 2017 Best Book List by the American Association for the Advancement of Science (AAAS) "An engaging and interesting account."— The Economist "Readers get a dose of biography while following such figures as Einstein, Dirac, and Newton to see how top theories about the void have been discovered, developed, and debunked. Weatherall's clear language and skillful organization adroitly combines history and physics to show readers just how much 'nothing really matters.'"— Publishers Weekly

CHAPTER 1

The Plenum and the Void

LATE IN the summer of 1665, as an epidemic of bubonic plague swept London, Cambridge University closed.⁷ It would remain closed for two years. Scholars and students evacuated the city for the countryside to avoid contracting plague. A twenty-two-year-old Isaac Newton had just finished his bachelor of arts degree at Trinity College. Unable to continue his studies, he returned to his mother’s home, Woolsthorpe Manor, in a small hamlet in the east of England.

The last time Newton had lived at Woolsthorpe Manor, he was seventeen years old. His mother had pulled him out of the King’s School, the grammar school where Newton had been educated since he was twelve, to help her run the farm. A grammar school in the seventeenth century taught just that: grammar, or more specifically, Latin grammar. Newton had studied no mathematics, no science, and no philosophy—and now he was expected to learn farming. He was a slow study, and he hated the work.

Within a few months, the headmaster at the King’s School had convinced Newton’s mother that she had made a mistake, and young Isaac had better go back to his studies. At least, he was not going to make a living as a farmer. He returned to the King’s School, finished, and went on to Cambridge, supporting himself as a sizar—a kind of work-study position, where he served as valet for the wealthier, better-connected students. But he didn’t mind: this was his chance to expand his intellectual horizons beyond Latin and farming to the best science, mathematics, and philosophy of the day. When Newton returned to Woolsthorpe during the plague year, he was a different man.

But he was poised to make an even more dramatic transformation. Left to his own devices in the countryside, he went from a student with undergraduate training to the leading mathematician in the world—though it would take some years before he was recognized as such. It was in Woolsthorpe that he single-handedly developed what he called his method of fluxions—the collection of mathematical methods we now call calculus, without which no modern mathematical science would be possible.⁸ He also performed his first experiments with light and prisms, developing a theory of optics that greatly advanced that subject.

By themselves, these contributions would have made him one of the most important mathematicians and physicists in history. But it is for a third subject—one that would consume him for the next twenty years—that he is best remembered. While living in Woolsthorpe during the plague year, Newton began to think about motion and about gravitation.

Newton’s laws of motion and his law of universal gravitation—which says that all bodies in the universe attract one another, with a force that increases with their masses and decreases with the square of their distances—revolutionized physics. Three and a half centuries later, we still teach Newtonian physics to high school and college students. Mastering Newton’s work, and the work of his followers, is a prerequisite to any real understanding of physics—something that cannot be said for any prior physicist. Indeed, although we have subsequently developed theories that, in various ways, supersede Newtonian physics, Newton’s laws remain a central pillar in our understanding of the physical world. For just one piquant example, : It was Newton’s laws that took us to the moon, and that will take us to Mars.

At the heart of Newton’s physics was a radical rethinking of our basic notions of space and time. Before Newton, our understanding of space and time was mired in metaphysical obscurity. Newton redefined these concepts in a way that allowed him to precisely describe various quantities of motion of physical objects, such as velocity and acceleration, and to relate these quantities to measurements we could make in a laboratory or observatory.⁹

Of course, Newton’s goal in all of this was to develop the physics of stuff—ordinary objects like tables and chairs, as well as planets and stars. But before he could do that, he had to say what the world would be like if there were nothing. He needed to describe the geometrical structure of empty space.

Newton’s masterpiece, the culmination of two decades of work on motion and gravitation, was the Philosophiae Naturalis Principia Mathematica—the Mathematical Principles of Natural Philosophy—or the Principia for short. It was first published in 1687, when Newton was forty-four years old. Although there are many documents, mostly unpublished during Newton’s lifetime, that record the development of his thinking on motion—including a remarkable document known colloquially as De Grav, which many scholars take to show that Newton already had his mature understanding of space and time by the late 1660s—the Principia was the definitive statement of Newton’s physics.¹⁰

In the physics of the Principia, there are bodies that, at any given instant, occupy places in space.¹¹ Bodies, for Newton, are extended objects—things like tables, chairs, and planets. Space, to a first approximation, may be thought of as an infinite, otherwise empty container in which bodies may be located.¹² Places are possible locations of bodies—literally, regions of space that could be filled by a body. We may think of space as all of the places—that is, all of the locations that could be occupied by a body at an instant. Between bodies, there is nothing but empty space.

Newton’s picture of bodies in space traces back to the ancient Greeks, most notably Democritus and Epicurus, who developed variants of a theory they called atomism.¹³ Atomism holds that the basic constituents of matter are tiny, indivisible bits of matter known as “atoms.”

Our contemporary notion of atom, as it appears in twenty-first-century chemistry and physics, owes much to this ancient idea, but it differs in important ways. The biggest difference is that, for the Ancients, atoms were indivisible. They did not have parts. Atoms as we understand them now do have parts. They are composed of electrons, protons, and neutrons; protons and neutrons, meanwhile, are composed of still more basic entities known as quarks. And atoms are certainly divisible: dividing them is the basis of nuclear fission, as used in nuclear weapons and nuclear power.¹⁴ Another difference is that the Ancients believed atoms could differ in properties such as size, shape, and color—though there was apparently no attempt to systematically classify atoms. Twenty-first-century chemistry, meanwhile, recognizes a periodic table of different atoms, with some 118 varieties, but the differences in species of atoms is determined by how they are composed. It would be very strange to think of modern atoms as having colors or shapes in the way the Ancients imagined—though they do, arguably, differ in size.

Newton believed that bodies have smallest parts, which could not be further divided, so he believed in something like the Ancients’ atoms.¹⁵ But for our purposes, atoms are not the important part of the atomistic view. Rather, it is that for Ancient atomists, atoms move around in the void, infinite space occupied by nothing. All physical phenomena, they believed, could be explained by motions of atoms in the void: tiny bits of stuff moving around in regions without stuff.

This idea that physical stuff comes in discrete chunks (indivisible or not), and that between bits of matter there are regions of space where there is nothing, may seem intuitively plausible or even obvious. After all, we are used to thinking about distinct things having some space between them. If you imagine sitting at a café with a friend, it is very natural to think that there’s you, your friend, and in between, some distance of space where there’s nothing. Of course, that is not literally right, since there is air between you and your friend. But we are taught that air is composed of various molecules, such as carbon dioxide and diatomic oxygen. And these molecules have nothing in between them.

If this idea does seem intuitive, it is because we are brought up in a world shaped by three and a half centuries of Newtonian science. But for millennia before that, the idea was extremely controversial.¹⁶ Aristotle, for instance—by far the most influential physicist before Newton—argued that Democritus’ views were not only wrong but incoherent. Motion, Aristotle claimed, could only arise from pushing or pulling of one thing by another. Motions that are not obviously of this form—smoke rising, rocks falling—occur because there is a natural order between different kinds of matter, and when matter is arranged out of that order, neighboring bits push and pull one another to restore the natural order. So a rock suspended in air and released is pushed downward by the air to join the other rocks on the ground, while the rock pushes up the air around it.

The important thing about this picture is that it precludes motion in the void. If an atom were surrounded by nothing at all, there would be nothing to exert any sort of pushing or pulling influence on it. If all motion arises from pushing and pulling by neighboring matter, and there is no neighboring matter, then motion is impossible. The upshot was supposed to be that the very idea of empty space is confused.

Aristotle’s physics dominated the science of the Western world for nearly two thousand years.¹⁷ By the time Newton was born, however, Aristotle’s influence was already on the wane. Aristotelianism had been challenged in the late sixteenth and early seventeenth centuries, by members of the vanguard of the scientific revolution, such as Galileo Galilei in Italy and René Descartes in France. But the new views that emerged in the wake of Aristotelian physics—at least in Continental Europe—were no more sympathetic to the idea of bodies moving through empty space than Aristotle had been.

Descartes was one of the most influential natural philosophers working in the first half of the seventeenth century.¹⁸ He rejected Aristotelian physics and attempted to replace it with a complete, systematic physics of his own. Part of this project involved developing a complete metaphysics, with accounts of space and time, God, and matter of all sorts. He was an early advocate of the idea that an adequate philosophy of nature must be based on mathematics. This approach led him to develop a theory of space, time, and matter based on principles of geometry. For Descartes, the primary property that an object had was its extension, that is, its shape in space. Space, meanwhile, was nothing but extension—the shapes of the bodies occupying space.

This tight connection between stuff and the space filled by stuff—so tight, in fact, that there was no difference between them as far as Descartes was concerned—led to a very quick argument that empty space is impossible.¹⁹ Suppose there were a void—some region of space that was empty of all matter. Then this region would have to have some extension: it would have to have nonzero volume, or else it would not be properly described as a region in the first place. But since extension simply is body, there would have to be some sort of body present in the region—that is, it would not be empty after all, by definition of being extended!

Thus, Descartes reasoned, all of space must be filled with some sort of very fine, unobservable stuff. He called this material the plenum. Ordinary objects can pass through the plenum, but anywhere that is not occupied by stuff like tables, chairs, and planets is occupied by this other sort of stuff instead.

If you think this idea seems strange, or even question-begging—after all, Descartes defines space in such a way that it can never be empty—then you are in good company: Descartes’ views on space and matter were flatly rejected by the leading physicists of the second half of the seventeenth century.²⁰ But as we will see, versions of this idea have resurfaced over the subsequent centuries. Arguably, even today, our best current physical theories posit a plenum—and at the very least, developments in twentieth-century physics have significantly blurred the distinction between “space” and “stuff” occupying that space, albeit in different ways from what Descartes envisioned.

In a letter dated February 5, 1676, Newton penned one of his most-quoted remarks: “If I have seen further, it is by standing on the shoulders of Giants.”²¹ He cannot take credit for this metaphor, which already had a five-hundred-year history by the time he wrote his letter—usually with the added twist that it is a dwarf benefiting from the giants’ height. And though the quote is often cited in connection with scientific humility, the passage may not be quite as modest as it seems: the letter was addressed to Robert Hooke, who was short and humpbacked, and the “Giants” to whom Newton referred were Hooke and Descartes, for whom he had little admiration. Indeed, by the time of writing, Newton and Hooke were serious rivals, disagreeing bitterly over the nature of light. Newton had even threatened to resign from the Royal Society of London, the most important scientific organization in England at the time, over Hooke’s criticisms of his work. Years later, Hooke, who was famously cantankerous, would claim that Newton had stolen his own theory of universal gravitation. With this context, it is hard not to see Newton’s reference to giants as tongue in cheek.

Still, Hooke’s stature notwithstanding, it is a remarkable fact that among Newton’s contemporaries were a number of physicists, mathematicians, and natural philosophers whose abilities and contributions rivaled his own. One of the most influential was Gottfried Wilhelm Leibniz.²² Two years younger than Newton, he was born in 1646 in the city of Leipzig, in the east of what is now Germany but was then part of the Holy Roman Empire. He spent most of his career as court librarian in Hanover, the seat of the Duchy of Brunswick-Lüneberg.

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