Time
From the measure of motion to the relative flow of clocks — the shifting nature of what we call now.
Each star is a thinker or work; solid lines draw the constellation of a school, dashed threads the passage of ideas between eras.
Select any point on the timeline to read about it.
All entries by era
Time 400 BCE – 2030 CE
From the measure of motion to the relative flow of clocks — the shifting nature of what we call now.
- 350 BCE
Aristotle, Physics. Aristotle defines time as 'the number of motion with respect to before and after' — without change there is no time to measure. He binds time firmly to the physical world and to the counting mind, framing debates that would run for two thousand years.
- 400 CE
Augustine, Confessions XI. 'What then is time? If no one asks me, I know; if I wish to explain it, I do not know.' Augustine locates past and future in memory and expectation, making time as much a puzzle of the soul as of the cosmos and inaugurating its philosophy of consciousness.
- 1656 CE
Christiaan Huygens, pendulum clock. Huygens builds the first pendulum clock, cutting timekeeping error from minutes to seconds a day. Accurate, regular clocks make time a measurable quantity for science and commerce alike, and turn the abstract flow of duration into readable numbers.
- 1687 CE
Isaac Newton, Principia. Newton posits an absolute time that 'flows equably without relation to anything external', the same everywhere and independent of events. This universal clock underpins classical physics and shapes common sense about time for the next two centuries.
- 1865 CE
Rudolf Clausius, entropy. Clausius formulates the second law of thermodynamics and names entropy, showing that in isolated systems disorder tends to increase. For the first time physics distinguishes past from future: the arrow of time points the way entropy grows.
- 1884 CE
International Meridian Conference. Delegates fix Greenwich as the prime meridian and divide the world into standard time zones. Driven by railways and telegraphs, the reform replaces a patchwork of local solar times with a single global grid, making time a coordinated worldwide convention.
- 1905 CE
Albert Einstein, special relativity. Einstein shows that time is not absolute: moving clocks run slow and simultaneity depends on the observer. Newton's universal now dissolves, replaced by a spacetime in which each observer carries their own measure of duration.
- 1908 CE
Hermann Minkowski, spacetime. 'Henceforth space by itself, and time by itself, are doomed to fade away.' Minkowski recasts relativity geometrically, fusing the three dimensions of space with time into a single four-dimensional continuum — the stage on which all of modern physics unfolds.
- 1915 CE
Einstein, general relativity. General relativity reveals that gravity itself bends time: clocks tick slower deep in a gravitational field than high above it. Time becomes flexible, woven into the curvature of spacetime — an effect real enough that GPS satellites must correct for it daily.
- 1949 CE
Harold Lyons, first atomic clock. The first atomic clock ties timekeeping to the unchanging vibration of atoms rather than the motion of the heavens. By 1967 the second is redefined in terms of cesium's frequency, making time the most precisely measured quantity in all of science.
- 2010 CE
Optical lattice clocks. Optical clocks grow so precise they would lose less than a second over the age of the universe, and can detect the tiny slowing of time caused by raising a clock a few centimetres. Relativity's effects, once cosmic, are now measured on a laboratory bench.
The milestones
c. 350 BCE
Aristotle, Physics
Time as the measure of change
Aristotle defines time as 'the number of motion with respect to before and after' — without change there is no time to measure. He binds time firmly to the physical world and to the counting mind, framing debates that would run for two thousand years.
c. 400 CE
Augustine, Confessions XI
Time in the mind
'What then is time? If no one asks me, I know; if I wish to explain it, I do not know.' Augustine locates past and future in memory and expectation, making time as much a puzzle of the soul as of the cosmos and inaugurating its philosophy of consciousness.
1656
Christiaan Huygens, pendulum clock
Time made precise
Huygens builds the first pendulum clock, cutting timekeeping error from minutes to seconds a day. Accurate, regular clocks make time a measurable quantity for science and commerce alike, and turn the abstract flow of duration into readable numbers.
1687
Isaac Newton, Principia
Absolute, universal time
Newton posits an absolute time that 'flows equably without relation to anything external', the same everywhere and independent of events. This universal clock underpins classical physics and shapes common sense about time for the next two centuries.
1865
Rudolf Clausius, entropy
The arrow of time
Clausius formulates the second law of thermodynamics and names entropy, showing that in isolated systems disorder tends to increase. For the first time physics distinguishes past from future: the arrow of time points the way entropy grows.
1884
International Meridian Conference
Standard time zones
Delegates fix Greenwich as the prime meridian and divide the world into standard time zones. Driven by railways and telegraphs, the reform replaces a patchwork of local solar times with a single global grid, making time a coordinated worldwide convention.
1905
Albert Einstein, special relativity
Time is relative
Einstein shows that time is not absolute: moving clocks run slow and simultaneity depends on the observer. Newton's universal now dissolves, replaced by a spacetime in which each observer carries their own measure of duration.
1908
Hermann Minkowski, spacetime
Space and time as one
'Henceforth space by itself, and time by itself, are doomed to fade away.' Minkowski recasts relativity geometrically, fusing the three dimensions of space with time into a single four-dimensional continuum — the stage on which all of modern physics unfolds.
1915
Einstein, general relativity
Gravity slows time
General relativity reveals that gravity itself bends time: clocks tick slower deep in a gravitational field than high above it. Time becomes flexible, woven into the curvature of spacetime — an effect real enough that GPS satellites must correct for it daily.
1949
Harold Lyons, first atomic clock
Time kept by atoms
The first atomic clock ties timekeeping to the unchanging vibration of atoms rather than the motion of the heavens. By 1967 the second is redefined in terms of cesium's frequency, making time the most precisely measured quantity in all of science.
2010 →
Optical lattice clocks
Measuring time's warp on a tabletop
Optical clocks grow so precise they would lose less than a second over the age of the universe, and can detect the tiny slowing of time caused by raising a clock a few centimetres. Relativity's effects, once cosmic, are now measured on a laboratory bench.