1. The methods and fruits of science

This chapter briefly discusses central key topics in the philosophy of science that the remainder of the book draws upon. It begins by considering the scientific method. ‘Induction’—the idea that we construct scientific theories just by generalizing from observations—is a very poor match to real science. ‘Falsification’—Popper’s idea that we create a theory, test against observation, and discard it if it fails the test—is much more realistic, but still too simple: data only falsifies data given auxiliary assumptions that can themselves be doubted. The issues are illustrated through an example from modern astrophysics: dark matter. The chapter then explores how we can resolve issues of underdetermination, where two theories give the same predictions. Finally, it introduces ‘scientific realism’, the view that our best theories tell us things about the world that go beyond what is directly observable.

2. Motion and inertia

This chapter explores the question of what it means for something to move, and why physics cannot be done without an answer to that question. It does so mostly in the context of Newtonian physics, leaving considerations of the theory of relativity to the next chapter. We cannot simply define motion of one body as relative to another body if we want to do physics—we have to introduce the idea of a ‘rest frame’ that defines which bodies are at rest (Newton called this rest frame ‘absolute space’). But physics also satisfies the relativity principle—it is impossible to distinguish the rest frame from another frame moving at constant speed in that frame. So what physics really requires is not a preferred rest frame, but a family of inertial frames, all moving at uniform speeds relative to one another. The notion of ‘spacetime’ has been introduced as a way of understanding this family of inertial frames, but philosophers of physics disagree as to whether spacetime explains the nature of motion in physics, or merely codifies it. The chapter concludes by explaining how gravity can be thought of as a change in the structure of the inertial frames: though it was Einstein who first saw this clearly, it has nothing to do with relativity and makes sense even in Newtonian physics.

6. Interpreting the quantum

This chapter surveys various proposals to interpret—that is, make sense of—quantum mechanics. We could attempt to think of quantum mechanics in purely instrumentalist terms, as an algorithm to predict observed experimental results. But this fits badly with scientific practice and is probably not viable. We could attempt to modify quantum mechanics itself to resolve the paradoxes, and there are some simple models that attempt to do that: some are ‘hidden-variable’ theories that add extra properties to the theory, some are ‘dynamical-collapse’ theories that modify the theory’s equations. But none of these models succeed in reproducing quantum theory’s predictions outside a relatively narrow range of applications. Or we could try to take the apparent indefiniteness of quantum mechanics literally, and interpret it as a theory of many parallel worlds. The correct interpretation of quantum mechanics remains controversial, but the search for understanding and interpretation of the theory has led to very substantial scientific results and is likely to lead to more.

5. Mysteries of the quantum

This chapter introduces the central mysteries of quantum mechanics. Quantum mechanics is an enormously successful theory that lies at the heart of modern physics, but there is no agreement on how to understand it. Simple experiments with light demonstrate why: in understanding those experiments, we have to shift inconsistently back and forth between thinking of the theory as assigning indefinite, delocalized, but known properties to a system, and assigning definite, localized, but unknown properties (this is called the ‘problem of measurement’). Furthermore, when we break a system into subsystems, the state of the system is not determined by the states of the subsystem (this is called ‘entanglement’), and simple arguments seem to tell us that the physical properties of entangled subsystems can influence one another non-locally—faster than light. These three mysteries—measurement, entanglement, non-locality—need to be addressed by any attempt to make sense of quantum theory.

Introduction

This introductory chapter provides an overview of philosophy of physics, which is an interdisciplinary field sitting between physics proper, mainstream philosophy, and the general philosophy of science, and communicating ideas and insights between them. Philosophy of physics is mostly concerned not with physics as a whole but with particular areas within it. Given a field in physics, one can consider the conceptual—that is, philosophical—questions that arise in that field, and the problems in each sub-field are distinctive. The chapter briefly discusses many of these, including some in cutting-edge areas of physics like quantum cosmology, black holes, and string theory. But it notes that the bulk of work in philosophy of physics is concerned with three areas where the physics is reasonably well established: the philosophy of spacetime; the philosophy of statistical mechanics; and the philosophy of quantum mechanics.

4. Reduction and irreversibility

This chapter considers philosophical issues that arise in statistical mechanics. Physics is not one theory but many, describing different bits of the world on different scales, and statistical mechanics is the field of physics that attempts to understand the relation between those descriptions. There are two alternative ways to think about statistical mechanics: either as a tool of inference, used to study complex systems in the face of our partial knowledge; or as a search for objectively true higher-level descriptions of those systems. On either conception, statistical mechanics needs to account for the emergence of irreversible, time-directed behaviour even though small-scale physics is reversible; it also needs to explain the role of probabilities, which are very widely used in statistical mechanics but have no uncontroversial interpretation.

3. Relativity and its philosophy

This chapter discusses how relativity theory affects our ideas about space, time, and motion. The special theory of relativity does not introduce the idea that motion is relative: it combines that idea, already present in Newtonian physics, with the idea that the speed of light does not depend on the motion of the source. This combination has surprising consequences: that moving clocks run slow; that moving rods shrink. This is apparently in flat contradiction with the relativity principle. The resolution of this paradox looks very different depending on one’s view of what spacetime is: is it simply a codification of physics, or can it do explanatory work in its own right. Thus the paradox lets us get clearer on what is at stake in these questions about the nature of spacetime. Relativity also imperils the idea that simultaneity—the relationship between two events when they occur at the same time—is relative and/or conventional. The epilogue of the chapter briefly discusses the general theory of relativity.