Terms
Glossary
The words the debate about time is conducted in, defined briefly and linked to the pages where they matter.
- A-series
- McTaggart's name for the ordering of events as past, present and future. Unlike earlier and later, these positions change: an event is future, then present, then past.
- A-theory
- Any view on which the A-series is objective: there is a real, privileged present, and becoming past is a real change. Prior and Smolin hold versions of it.
- Akinetopsia
- Motion blindness: a rare loss, after brain damage, of the ability to see movement. Objects are seen, but only as a series of still positions.
- Arrow of time
- The asymmetry between past and future: we remember the past, not the future; causes precede effects; entropy rises. Most physicists trace it to the second law of thermodynamics.
- B-series
- The ordering of events by earlier than and later than. These relations never change: if one event is earlier than another, it always is.
- B-theory (tenseless theory)
- The view that the only temporal facts are B-series facts. There is no objective present; "now", like "here", picks out a moment from a point of view.
- Block universe (eternalism)
- The picture of all of spacetime, past, present and future, as equally real and laid out once and for all. Nothing in it moves or comes into being.
- Causation
- The relation of cause and effect. On Deutsch's account, X causes Y when both happen and, in most variants of our universe without X, Y does not happen either.
- Cinematographic illusion
- Bergson's charge against the intellect: it represents change as a series of still frames, like a film, and so misses the movement itself.
- Closed timelike curve
- A path through spacetime that loops back to its own past. Gödel found universes containing them among the solutions of Einstein's equations.
- Cosmological natural selection
- Smolin's proposal that black holes give rise to new universes with slightly altered constants, so that universes good at making black holes come to predominate.
- Counterfactual conditional
- A statement about what would have happened if things had been otherwise: "if Faraday had died young, technology would have been delayed". Deutsch argues that only the multiverse gives such statements a definite meaning.
- Determinism
- The property of laws by which the state of the world at one time fixes its state at all others. Classical and quantum laws are deterministic in both directions of time, though quantum determinism holds only for the multiverse as a whole.
- Duration (durée)
- Bergson's term for lived time: continuous, indivisible, with each moment carrying the ones before it, as in a melody.
- Entropy
- A measure of how many microscopic arrangements look the same at a coarser level of description. The second law of thermodynamics says that, in an isolated system, it does not decrease.
- Flow of time (passage)
- The supposed movement of the present from past to future, or of our consciousness through time. Deutsch argues that the idea is not merely false but incoherent; Maudlin defends a version of it.
- Many-worlds interpretation
- Hugh Everett's reading of quantum mechanics, published in 1957: the quantum state never collapses, and every outcome of a measurement occurs, in a different branch. Deutsch's multiverse is a version of it.
- Multiverse
- Deutsch's name for the whole of physical reality: a vast structure of snapshots, of which what we call other times and other universes are both parts.
- Now (Barbour)
- Barbour's term for a complete, static configuration of the universe. He holds that Nows are all that exist.
- Past Hypothesis
- The assumption that the universe began in a state of extraordinarily low entropy. The name is David Albert's; on many accounts the arrow of time depends on it.
- Platonia
- Barbour's name for the timeless space of all possible relative configurations of the universe.
- Presentism
- The view that only the present exists. The past no longer exists and the future does not exist yet. Prior defended it; Deutsch calls it a kind of solipsism.
- Relational quantum mechanics
- Rovelli's interpretation of quantum mechanics, on which the properties of a system exist only relative to other systems it interacts with.
- Relativity of simultaneity
- The result of Einstein's special relativity (1905) that observers in relative motion disagree about which distant events happen at the same time. Neither is wrong.
- Retrocausality
- Influence running from later events to earlier ones. Price has argued that it could explain quantum correlations without action at a distance.
- Snapshot
- In Deutsch's chapter, a universe at a particular time: everything that exists, throughout space, at one moment.
- Spacetime
- Space and time taken together as a single four-dimensional whole. Relativity made this view indispensable.
- Specious present
- The short stretch of time experienced as present at once, as when we hear a whole phrase of music. William James took up the phrase in 1890.
- Tense logic
- Prior's formal logic of tensed statements, with operators such as P ("it has been the case that") and F ("it will be the case that").
- Thermal time hypothesis
- The proposal by Connes and Rovelli that the flow of time is defined by a system's statistical state, and so emerges when we know a system only partially.
- Time capsule
- Barbour's term for a Now whose contents look like records of other Nows, such as fossils or memories. He argues that this is all our sense of the past amounts to.
- View from nowhen
- Price's name for a standpoint that favours neither direction of time, from which the laws of physics look time-symmetric.