Time is the structure by which events are ordered, intervals are compared, and change is described.

This working definition contains three elements:

1. Order: one event is earlier than, later than, or simultaneous with another;
2. Duration: a process lasts for an interval, and intervals can be compared;
3. Change: an object or system has different states at different times.

The definition does not decide whether time is an independent part of reality, a system of relations among events, a dimension of spacetime, or an emergent feature of a deeper theory. Physics specifies how temporal quantities are measured and used in theories. Metaphysics asks what temporal structure implies about reality. Psychology and phenomenology examine how duration, succession, and passage are experienced.

Time, Clocks, and Measurement

A clock is not time itself. It is a physical system that uses a stable, repeatable process to compare durations. Pendulum clocks count swings, quartz clocks count oscillations, and atomic clocks use frequencies associated with atomic transitions.

The SI second is defined by fixing the numerical value of the unperturbed ground-state hyperfine transition frequency of cesium-133 at 9,192,631,770 hertz. This makes a unit of duration reproducible; it does not define the metaphysical nature of time. BIPM: The Second

Time, the unit called a second, a physical clock, and a coordinated time scale are therefore different concepts. Measurement establishes reliable relations among physical processes. It does not by itself determine whether time exists independently of those relations.

Time and Change

Change means that a system has different states at different times. Time supplies the order and interval by which those states are compared.

A relational theory may argue that temporal structure is constituted by relations among changes, but it must still explain order, duration, and the comparison of different processes. Time and change are closely connected without being identical concepts.

Metaphysical Accounts of Time

Substance and Relation

On a substantival view, time exists independently of particular events. Physical processes occur within a temporal framework that would remain even if nothing changed. Newton’s conception of absolute time is the standard historical example.

On a relational view, time consists in relations such as earlier than, later than, simultaneous with, and lasting longer than. Leibniz is closely associated with this position.

Substantivalism treats time as part of the world’s independent structure. Relationism treats temporal order as constituted by relations among things and events.

Order and Passage

The existence of temporal order does not by itself establish that time passes.

McTaggart distinguished the A-series, which classifies events as past, present, or future, from the B-series, which orders events as earlier than, later than, or simultaneous with one another.

In an A-theoretic account, an event may first be future, then present, and finally past. If these tensed properties are objective, becoming and passage may be features of reality.

In a B-theoretic account, the fundamental facts are tenseless relations among events. Temporal order can be objective without an additional moving present.

Past, Present, and Future

Presentism must explain what makes true statements about a nonexistent past true. Eternalism must explain why the present appears privileged in experience. Growing-block theories preserve objective becoming but must explain the boundary at which reality grows.

These are metaphysical interpretations, not direct experimental results. Physical theories constrain them, but measurements do not select one position without additional philosophical assumptions. Stanford Encyclopedia of Philosophy: Time

Persistence

Objects change while remaining identifiable as the same objects. Endurantist theories hold that an object is wholly present at different times. Perdurantist theories hold that an object extends through time and has different temporal parts, much as a spatially extended object has different spatial parts. The issue connects time with identity, change, and causation. Stanford Encyclopedia of Philosophy: Temporal Parts

Time in Classical Mechanics

Classical mechanics usually represents time with a universal parameter, t. Ideal observers share the same simultaneity relation and temporal intervals, while motion occurs against a common temporal background.

This framework remains highly accurate for ordinary speeds and weak gravitational fields. Relativity identifies the conditions under which Newtonian time is an effective approximation.

Time in Special Relativity

Special relativity removes absolute simultaneity for spatially separated events. Observers in different states of inertial motion can disagree about whether distant events are simultaneous.

This does not make time a matter of opinion. Measurements in different inertial frames are related by Lorentz transformations, and the causal structure of spacetime remains objective. Einstein Online: Relativity, Space and Time

The time accumulated by an ideal clock along its worldline is called proper time. In flat spacetime:

dτ = dt × √(1 - v²/c²)

Here dt is coordinate time in an inertial frame, dτ is the proper time accumulated by the moving clock, v is relative speed, and c is the speed of light.

Clocks that follow different paths can reunite with different elapsed times. Time dilation is a measurable difference between clock readings, not a visual illusion.

Relativity removes universal simultaneity, but not every invariant temporal order. If two events can be causally connected by a signal traveling at or below light speed, their causal order cannot be reversed by changing inertial frames.

Time in General Relativity

General relativity treats space and time as a dynamical spacetime geometry. Matter and energy affect that geometry, and the geometry influences the motion of matter and light.

The proper time accumulated by a clock depends on its path and gravitational conditions. Clocks at different gravitational potentials or moving at different speeds can accumulate different durations. Satellite navigation systems must correct for both gravitational and velocity-related effects. NIST: Putting Einstein to the Test

Coordinate time is a chosen coordinate used to describe a region of spacetime. Proper time is the reading of an ideal clock along a specific worldline. Coordinate descriptions can vary, while proper-time comparisons correspond to physical measurements.

The Thermodynamic Arrow of Time

Macroscopic processes have a clear direction. Heat spreads, gases disperse, broken objects do not ordinarily reassemble, and records encode earlier conditions.

Many microscopic dynamical laws are approximately invariant under time reversal, yet macroscopic behavior is strongly asymmetric. Statistical mechanics connects this asymmetry with entropy. Under suitable conditions, a closed system is overwhelmingly likely to evolve from a lower-entropy macrostate toward a higher-entropy one.

Entropy is not identical with time. It describes macroscopic states, while time provides the ordering in which change is described. Entropy increase helps explain irreversible direction.

The account also depends on a low-entropy past. Why the universe had such a special boundary condition remains an open foundational question. Stanford Encyclopedia of Philosophy: Thermodynamic Asymmetry in Time

Time in Quantum Theory

Standard quantum mechanics generally treats time as an external parameter with respect to which quantum states evolve. General relativity, however, makes spacetime itself dynamical.

Attempts to construct a quantum theory of gravity therefore face the problem of time: if spacetime is part of the quantum system, an external classical time parameter cannot simply be assumed for the universe as a whole.

Different approaches attempt to recover time from internal variables, correlations, or more fundamental quantum structures. There is currently no universally accepted complete theory. Stanford Encyclopedia of Philosophy: Quantum Gravity

Temporal Experience

The experienced present is not equivalent to a durationless mathematical instant.

Hearing a melody requires some retention of earlier notes and anticipation of what follows. Without this short temporal integration, a listener would experience disconnected sounds rather than a musical phrase. Philosophical and psychological accounts describe this temporally extended awareness through concepts such as the specious present. Stanford Encyclopedia of Philosophy: Temporal Consciousness

Experienced duration varies with attention, emotion, memory, and event density. Waiting can make an interval feel long. Absorbed activity can reduce awareness of passing time. A period with many distinctive memories can appear longer in retrospect.

These are changes in temporal experience, not changes in the proper time measured by an external clock.

Physics and Philosophy

Physics describes measurable relations among clocks, events, causal structures, and spacetime intervals. Philosophy asks what those relations imply about the nature of reality.

Relativity makes a single observer-independent cosmic present difficult to maintain, but it does not by itself prove eternalism. Thermodynamics explains much of the direction of macroscopic processes, but it does not establish that temporal passage is fundamental. Psychology explains temporal experience without reducing physical time to a mental construction.

Conclusion

Time has at least four connected aspects:

1. it orders events and allows intervals to be compared;
2. it functions as a measurable structure in physical theories;
3. it is related to the macroscopic direction of physical processes;
4. it structures conscious experience through memory, present awareness, and anticipation.

Classical mechanics approximates time as a universal background. Relativity connects elapsed time with paths through spacetime, relative motion, and gravity. Thermodynamics explains why macroscopic processes exhibit a preferred direction. Quantum gravity raises the possibility that time is not fundamental.

Metaphysics asks whether time exists independently, whether passage is real, whether the present is objectively privileged, and how past and future entities exist.

A claim about time should therefore identify its level. Event order, measured duration, proper time, coordinate time, thermodynamic direction, temporal experience, and temporal existence are related concepts, but conclusions about one do not automatically settle the others.