This chapter gives a quantitative introduction to decoherence theory, including density matrix formalism in the context of quantum field theory, and a survey of the quantum trajectories method. Finally, the mathematical structure for a new proposal for spontaneous collapse, introduced nonmathematically in Chapter 6, is given.

This chapter discusses what we mean by particle detectors and “quantum jumps.” Modern results are presented that show that particle detection is not instantaneous, and that the photoelectric effect does not prove the existence of particles; it is a purely wavelike effect. The Born rule for random clicks of measurements in detectors is introduced and discussed, and quantum “uncertainty” is introduced.

In Chapter 2, we will review basic facts of quantum measurement that are usually discussed in basic texts on quantum mechanics. These include a motivating experiment – the Stern–Gerlach effect – and discussions of measurement results, statistics, the Born rule, and wavefunction collapse.

This chapter reviews the original transactional interpretation as based on the Wheeler–Feynman absorber theory of radiation and demonstrates how the Born Rule for the probabilities of outcomes is a direct physical consequence of the transactional formulation. The chapter also accounts for the nature of “null measurements” and applies the transactional picture to the famous two-slit experiment to gain a better understanding of what it means for the experiment to be “finished” and how transactions illuminate the appearance and disappearance of interference based on the type of measurement being performed.

This chapter provides a brief overview of the history of the development of quantum theory, with a critical focus on the antirealist tradition inaugurated by Niels Bohr. The distinction between “principle theories” and “constructive theories” is discussed, and it is noted that quantum mechanics is a “principle theory.” It is argued that quantum theory is amenable to a fully realist interpretation provided we let go of the demand that reality be classically picturable.