Concepts of Time

Ancient Greece

Different philosophers and scientists have had diverse notions throughout history related to the concept of time. One of the earliest viewpoints—namely, the Greek philosopher Aristotle (384–322 BC)—defined time as “a number of movements in respect of the before and after.” He saw time as a measure of change, an inevitable cycle of motion or transformation. Aristotle believed time was infinite and continuous and the universe eternal. Perhaps most notably, he was one of the first people to ask if time really exists, since it is made up of two kinds of non-existence: the past and the future. This is one view of many here, causing both very large debates in ways between Sir Isaac Newton and Gottfried Leibniz.

Newton & Leibniz

In Philosophiæ Naturalis Principia Mathematica, Newton argued that time was absolute, moving uniformly and independently from external factors. So he named this “duration,” insisting that though absolute time does exist mathematically, it is undetectable. Relative time—which humans experience—is measured by the movements of celestial bodies like the sun and moon. This idea is called Newtonian time.

Leibniz, however, rejected absolute time. Time, he argued, exists only in connection with objects and events, a means of organizing sequences of events rather than an entity unto itself. This idea, called relational time, means that time is not measureable itself but that humans created it as a means of organizing experiences.

The bucket argument (also called Newton’s bucket) was one of the central points of debate between Newton’s advocate Samuel Clarke and Leibniz. For example, Newton gave a commonly cited example of spinning a bucket of water: when the bucket is still, the water is level, but as the bucket spins the water will have a concave surface. The water keeps swirling and bent in shape even after the bucket comes to a halt. Because this curvature was not caused by the bucket itself, Newton concluded that the water must be rotating with respect to an absolute space. This idea prevailed in physics for almost two centuries, despite Leibniz’s protests.

Einstein

Much later, Einstein completely reshaped our conception of time with the theory of relativity and he expanded Leibniz’s thoughts. Unlike Newton, who considered time equally uniform for every observer, Einstein now proposed that space and time were connected — a dimension called spacetime. He claimed that the speed of light (c) is invariable in respect to all observer, whether in motion or not. This realization led to the understanding that time and space are relative; an observer would experience time much slower than a stationary observer would if they were moving at higher velocities.

A well-known example is a spaceship flying near the speed of light. An observer aboard another spaceship traveling at a different velocity would notice time on the near-light-speed spacecraft seeming to slow down. So if a spaceship were able to move at such a speed that it could even reach the speed of light, time would theoretically freeze, as it were.

In the simplest terms, objects moving faster through space experience slower time, and slower-moving objects experience faster time. This has to happen in order to keep the speed of light the same.

Einstein’s general theory of relativity also solved Newton’s bucket argument. He also introduced the idea of geodesics—the natural paths that objects take in curved spacetime. Dan McKenzie in the 1970’s who realized that according to general relativity, a geodesic is motion with no force and involves free fall, while an object resisting that motion (for example, if the bucket were at rest on the surface of the Earth) is under force. In short, the water’s concavity is not because it is rotating with respect to a fixed space, but because it is rotating with respect to a geodesic.

Over the years, developing theories have demolished older ideas, showing even commonly held scientific wisdom is subject to change. No matter how advanced quantum physics becomes, time is a slippery thing. Who knows—Einstein’s speed-of-light constant might be proved wrong one day, and time travel might be the norm!

Measuring Time

The Calendar and the Clock

Nowadays we use calendars and clocks, both of which were originally based on sexagesimal numeral system (base 60 hypothesis). The Sumerians developed a place-value numbering system in the 3rd millennium BC, which the Babylonians adopted, and that is still in use today because it is mathematically efficient. An easy divisor into minutes and secondsThe number 60 has many factors.

Evolution of Time Units

The Egyptians were among the first to subdivide the day into smaller sections, using sundials to measure daylight hours. They divided daylight into 12 pieces, then split nights between stars into 12 parts as well, perhaps inspiring the modern 24-hour day. But their system was seasonal, with extended summer days and short winter days.

Around 147–127 BC, the Greek astronomer Hipparchus suggested dividing the day into 24 equal hours according to the equinox. Later, he created a system of longitude that divided circles into 360 degrees, each degree divided into 60 minutes and each minute into 60 seconds—a method still employed today.

Although several civilizations developed their own calendars, the most popular today is the Gregorian calendar, ordered by Pope Gregory XIII in 1582. It updated the earlier Julian calendar, which had inaccuracies that caused the equinoxes to shift over time.

Early Timekeeping Devices

Religious practices and work caused different cultures to come up with different time-keeping methods. Some early devices include:

Oil lamps & candle clocks served for marking time intervals, not measuring the exact time.

Water clocks (clepsydras)—They were among the most accurate ancient clocks, using a controlled outflow of water to measure time.

Hourglasses (sandglasses): For timing events, later calibrated with clocks.

Pendulum clocks: In 1656, Christiaan Huygens invented the first practical pendulum clock, achieving a daily error of less than 10 seconds.

Atomic clocks: These are the most accurate timekeeping devices in use today. Atomic clocks are based on the resonance of cesium. The second, the SI unit of time, is defined by vibrations of the cesium atom.

The measurement of time has progressed from sundials to atomic clocks, yet the fundamental fabric of time itself is among the greatest mysteries of science.