Yale University Press’s Foundational Questions in Science series takes the following as its credo: “At its deepest level, science becomes nearly indistinguishable from philosophy.” Those words, I think, are crucial to understanding Void: The Strange Physics of Nothing, a small but lively volume in the series. Arguably, the deepest level of contemporary science is the physical theory of “nothing,” a word actually applied to various, not necessarily distinct non-nothings. For example, in quantum field theory, “nothing” refers to the vacuum, or ground state, with respect to which other physical states are defined. In general relativity, it describes matter-free spacetimes, whereas in quantum gravity, it is a hypothetical state with no classical spacetime.
Of course, oracular pronouncements and disputes over “nothing” are not new to philosophers or scientists. Medieval philosophers argued that nihil fit ex nihilo (Latin for “out of nothing comes nothing”). Thomas Aquinas added that creatio non est mutatio (“creation is not change”), thus distinguishing between creation, the domain of theology, and change, the domain of science. Isaac Newton, Gottfried Leibniz, and their followers argued over empty space: Is it a primitive something, endowed with its own structure, or a derivative entity, best understood in terms of a separation between bodies?
James Clerk Maxwell filled the void with an ethereal medium deemed necessary for the propagation of electromagnetic energy across space. Albert Einstein abolished Maxwell’s ether but reinstated one of his own, the metric field. Martin Heidegger opined in 1929 that science cannot dismiss the nothing “with a lordly wave of the hand,” unaware that a year before, the lordly hand of Paul Dirac had filled the void with an infinite sea of negative-energy electrons. Much of the ado about nothing turns out to have been merely verbal, arising from an understandable but faulty assumption that space empty of stuff—that is, the perceptible absence of matter in the familiar sense—is nothing. Contemporary physics tells us that nothing could be further from the truth.
James Owen Weatherall, author of a popular account of how mathematical models are used to predict financial markets (The Physics of Wall Street: A Brief History of Predicting the Unpredictable, Houghton Mifflin Harcourt, 2013), takes up the topic of nothing in three chapters. Trained as both a physicist and a philosopher, Weatherall is an engaging writer, and he provides a readable tour of how the physics of nothing has developed. Chapter 1 discusses the early 18th-century conflict between Newton and Leibniz over absolute space. Leibniz maintained that space and time were ideal relations between material bodies. Newton, however, argued that space and time were absolute entities that exist even in the absence of bodies or material processes. In keeping with the practice of many contemporary philosophers of physics, Weatherall argues that the controversy was motivated by Newton’s implicit recognition that mechanics requires a space and time to define the notion of inertial structure. Such a rational reconstruction, however, downplays the highly nontrivial theological reasons underlying Newton’s insistence on absolute space and time.
Chapter 2 discusses empty space in relativity theory, from the geometrical structure of Minkowski spacetime to empty-space solutions of the field equations of general relativity, Schwarzschild black holes, Einstein’s discovery of wave-like solutions, and his infamous 1936 paper with Nathan Rosen denying the existence of gravitational waves. The chapter fittingly concludes with the Laser Interferometer Gravitational-Wave Observatory’s detection of a gravitational wave in September 2015.
The third chapter is devoted to quantum field theory and the notion of the vacuum state, a seething cauldron of virtual particle–antiparticle pairs popping into existence and then disappearing before energy conservation is violated. Mostly a history of early quantum electrodynamics, it is organized around two pairs of theoreticians: Dirac and Pascual Jordan, and Richard Feynman and Julian Schwinger. The chapter is the highlight of the book, for Weatherall tells his story convincingly and brings back into prominence the vital contributions of physicists such as Jordan and Schwinger, who are largely neglected in lay scientific literature. The epilog discusses the so-called string landscape or multiverse, the plethora of vacuum states allowed by string theory—according to Weatherall, “approximately 10500 different ways for there to be nothing.”
Two observations: First, for much of the book, “nothing” is characterized as the absence of “stuff.” But a principal conclusion of the third chapter is that the vacuum state, according to quantum field theory, “is very much a kind of stuff.” Despite the absence of particles, it is dynamic, hardly an attribute of nothing. Thus the vacuum state ensures, as Weatherall puts it, “a chance of finding stuff even when there’s nothing there.” The proper conclusion to draw may be that the dialectical trope of “stuff” versus “nothing” has simply become archaic.
Second, can one really say “if anything in quantum field theory is well understood, mathematically speaking, it is the vacuum”? A note refers interested readers to the characterization of special-relativistic vacuum states in algebraic quantum field theory. I think it is fair to say that the vacuum of renormalizable quantum field theories is mathematically understood in the absence of gravity. Without gravity, vacuum energy is not an observable; it cancels out in any measurable quantity and so can be defined to be zero. In the presence of gravity, however, no unique vacuum state exists in general, and it is not clear what the correct renormalization scheme is.
Thomas Ryckman is the author of Einstein (Routledge, 2017) and The Reign of Relativity: Philosophy in Physics 1915–1925 (Oxford University Press, 2005). He is a professor of philosophy at Stanford University in California.
