Glossary

Generally speaking, this glossary limits itself to defining terms that recur in the text or that are of particular significance for a basic understanding of quantum theory. Other terms that occur only once or are of less fundamental importance are defined in the text itself, and these can be accessed through the index.

angular momentum: a dynamical quantity that is the measure of rotatory motion Balmer formula: a simple formula for the frequencies of prominent lines in the hydrogen spectrum Bell inequalities: conditions that would have to be satisfied in a theory that was strictly local in its character, with no non-local correlations bosons: particles whose many-particle wavefunctions are symmetric Bohmian theory: a deterministic interpretation of quantum theory proposed by David Bohm chaos theory: the physics of systems whose extreme sensitivity to details of circumstance makes their future behaviour intrinsically unpredictable classical physics: deterministic and picturable physical theory of the kind that Isaac Newton discovered collapse of the wavepacket: the discontinuous change in the wavefunction occasioned by an act of measurement

complementarity: the fact, much emphasized by Niels Bohr, that there are distinct and mutually exclusive ways in which a quantum system can be considered Copenhagen interpretation: a family of interpretations of quantum theory deriving from Niels Bohr and emphasizing indeterminacy and the role of classical measuring apparatus in measurement decoherence: an environmental effect on quantum systems that is capable of rapidly inducing almost classical behaviour degrees of freedom: the different independent ways in which a dynamical system can change in the course of its motion epistemology: philosophical discussion of the significance of what we can know EPR effect: the counterintuitive consequence that two quantum entities that have interacted with each other retain a power of mutual influence however far apart they may separate from each other exclusion principle: the condition that no two fermions (such as two electrons) can be in the same state fermions: particles whose many-particle wavefunctions are antisymmetric hidden variables: unobservable quantities that help to fix what actually happens in a deterministic interpretation of quantum theory interference phenomena: effects arising from the combination of waves, which may result in reinforcement (waves in step) or cancellation (waves out of step) many-worlds interpretation: an interpretation of quantum theory in which all possible outcomes of measurement are actually realized in different parallel worlds measurement problem: the contentious issue in the interpretation of quantum theory relating to how one is to understand the obtaining of a definite result on each occasion of measurement non-commuting: the property that the order of multiplication matters, so that AB is not the same as BA observables: quantities that can be measured experimentally ontology: philosophical discussion of the nature of being Planck’s constant: the fundamental new physical constant that sets the scale of quantum theory

positivism: the philosophical position that science is concerned simply with correlating directly observed phenomena pragmatism: the philosophical position that science is really about the technical capability for getting things done quantum chaology: the not-fully-understood subject of the quantum mechanics of chaotic systems quantum field theory: the application of quantum theory to fields such as the electromagnetic field or the field that is associated with electrons quarks and gluons: current candidates for the basic constituents of nuclear matter radiation: energy carried by the electromagnetic field realism: the philosophical position that science is telling us what the physical world is actually like Schrödinger equation: the fundamental equation of quantum theory that determines how the wavefunction varies with time spin: the intrinsic angular momentum possessed by elementary particles statistical physics: treatment of the bulk behaviour of complex systems on the basis of their most probable states statistics: the behaviour of systems composed of identical particles superposition: the fundamental principle of quantum theory that permits the adding together of states that in classical physics would be immiscible uncertainty principle: the fact that in quantum theory observables can be grouped in pairs (such as position and momentum, time and energy) such that both members of the pair cannot simultaneously be measured with precise accuracy. The scale of the limit of simultaneous accuracy is set by Planck’s constant wavefunction: the most useful mathematical representation of a state in quantum theory. It is a solution of the Schrödinger equation wave/particle duality: the quantum property that entities can sometimes behave in a particlelike way and sometimes in a wavelike way