Nature Of Time

• Type of physical science: Relativity
• Field of study: General relativity

Science has proceeded for centuries on the supposition that time is an absolute entity in the physical world. The theory of relativity demonstrated that time is intertwined with the space and materials in the universe. This relation of time with space and matter becomes significant in high-speed space travel and cosmology.

Overview

Time is a mysterious aspect of the physical world. The human concept of time is deeply rooted in everyday experience of the world and in common sense. It is such a fundamental concept that humans seldom inquire into its nature. To elucidate its nature, it is necessary to turn to other concepts, which are themselves closely related to time.

Two time-related entities that are used to characterize time are space and motion. Time is measured by motion in space—one measures the time of day by the motion of the Earth about its axis and the time of year by the rotation of the Earth around the Sun. Time is also measured by using clocks and watches based on the motion of wheels and pendulums. Motion itself, however, is measured by time and space: To describe the motion of a moving object, one must specify the space in which the object moves and the time it takes for that object to move through that space.

Finally, space can be measured by time and motion. Ancient Persians used the “parsang,” the distance on level ground that an average person could walk in one hour. Even today, some distances are described in terms of space and time. The “light-year,” a common measure of distance used in astronomy, is the distance that light travels in one year.

The use of this time-space-motion triangle for characterizing time has created much debate among scientists and philosophers for centuries. In classical physics (before Albert Einstein’s theory of relativity), time and space were treated as absolute and independent entities. Specifically, time was assumed to be a universal concept, and the time between two events was considered independent of the state of motion of the body of reference.

With this concept of time and space, all physical objects could be located in absolute space, and all events could be assigned positions objectively in a fixed three-dimensional space and on an ever-flowing stream of absolute time. Concerning the nature of time, Sir Isaac Newton states in his Philosophiae Naturalis Principia Mathematica (1687; principles of natural philosophy): “Absolute, true, and mathematical time, of itself, and from its own nature flows equably without regard to anything external.” Newton believed that the rate at which time moments succeed each other is independent of all events and processes in the physical world.

The idea of absolute and uniformly flowing time has been criticized by a number of philosophers and scientists. Gottfried Wilhelm Leibniz, a philosopher and mathematician who was a contemporary of Newton, rejected the idea of absolute time and argued that events are more fundamental than moments (this is known as the relational theory of time). He defined time as the order in which events occur. In other words, events do not occur in “time”; time is derived from the occurrence of events.

Despite opposing views such as that of Leibniz, the classical view of absolute time prevailed until 1905, when Einstein published his work on the special theory of relativity. This work was based on the notion that laws of nature must remain the same for all inertial observers.

As a result of this theory, the concepts of absolute time and absolute space were abolished.

Time (as well as space) was found to be an aspect of the relationship between the universe and the observer. Accordingly, time and space must be regarded as interchangeable; only jointly in the form of a four-dimensional entity called space-time can they be used to characterize the physical world. Hermann Minkowski (a mathematician and contemporary of Einstein) was the first person to formulate the properties of space-time mathematically. In Minkowski’s words: “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”

One consequence of the theory of relativity is that clocks run more slowly at high speeds; that is, there is a longer wait (according to a stationary clock) between ticks of a moving clock. This is called time dilation. A thought experiment, referred to as the twin paradox, has been given by Einstein to illustrate this concept. It involves two young twins; one stays on Earth, and the other goes for a long space journey at high speeds. When the traveling twin returns to Earth, still young, he finds his Earth twin quite old. Time in the spacecraft has passed at a slower rate than time on Earth, and as a result, the twin in the spacecraft has aged less.

For long distances and at speeds close to the speed of light, time dilation becomes significant.

The time dilation effect was verified by clocks flown on jet planes around the world in 1971.

In the general theory of relativity, Einstein included the effects of gravity. The general theory (or the theory of gravity) is based on the notion that laws of nature must remain the same for all observers regardless of their motion. Space-time, which in the special theory is the (objective) measure of time and space, in the general theory becomes dependent on matter. In the general theory of relativity, only the oneness of space, time, and matter has an independent existence.

According to general relativity, gravity has a slowing effect on time. This means that, in a stronger gravitational field, any cyclical motion, such as the ticking of a clock or the vibration of the nucleus of an atom, would take place more slowly.

The effect of gravity on time has been investigated and quantitatively verified since the 1950s. On Earth, because of small gravitational changes, such effects are too small to be measured by conventional timekeeping devices. In 1958, Rudolf Ludwig Mössbauer, a German scientist, discovered a phenomenon that opened the way to high-precision time measurement.

Mössbauer timekeeping is based on the concept that, upon excitation, nuclei of certain atoms vibrate and emit electromagnetic waves with frequencies on the order of a million million million oscillations per second. Atomic clocks that utilize the Mössbauer effect are accurate to a fraction of a second in millennia.

Direct verification of the effect of gravity on time was made in a series of experiments on US Navy aircraft in the Chesapeake Bay between September 1975 and January 1976. In these experiments, atomic clocks were carried on aircraft and flown at an altitude of more than 9,000 meters (29,528 feet). The clocks aboard the aircraft (where gravitation was weaker as a result of the high altitude) gained about three-billionths of a second every hour, which was within 1 percent of the prediction of the general theory of relativity. Later, in June 1976, an atomic clock was run at an altitude of approximately 9,600 kilometers (5,965 miles). At that altitude, the clock ran faster than similar clocks on the Earth’s surface by one-billionth of a second every second.

The slowing effect on time of gravitational fields has also been investigated indirectly by studying the spectra of light emitted by massive bodies. This is based on the concept that light, in trying to escape from a massive object, loses energy in much the same way as an object moving away from the Earth, and in doing so, its frequency decreases and its color becomes “redder.” This is referred to as the “gravitational redshift.” In a series of experiments at Harvard in 1959 and 1965, Robert V. Pound and his colleagues measured the effects of the Earth’s gravity on the frequency of light. In their tests, these researchers used the Mössbauer effect to measure the change in the frequency of light falling from the top to the bottom of a 22.5-meter (74-foot) tower.

Their experiments indirectly confirmed the effect of gravity on time to within 1 percent of the prediction of the general theory of relativity.

There have been other gravitational redshift tests, confirming the effect of gravity on time since the 1950s. Gravitational redshift tests for the Sun were performed by French astronomers in 1962. Another study was a redshift analysis of the spectra of light from more than one dozen white dwarfs, conducted jointly by American and British astronomers in the 1970s. The results of these two sets of tests indicated the slowing of time by, respectively, about one minute per year on the surface of the Sun and more than one hour per year on white dwarfs.

Among massive objects in the universe, black holes have the most amazing effect on time. Because of the tremendous gravity of black holes, the slowing of clocks in their vicinity would be extreme. In fact, to a distant observer, time near a black hole appears to slow dramatically and approach a stop at the event horizon; a hypothetical observer outside the event horizon could not detect any passage of time inside the black hole.

Applications

The relativity and subjectivity of time become pronounced in situations involving vast distances, speeds near the speed of light, and the presence of strong gravitational fields.

The relation of time and space has some very interesting implications concerning high-speed and long-distance space travel. The slowing of time and the foreshortening of distances at high speeds make possible long journeys to distant reaches of the universe.

According to the relativity theory, the size of the universe and the travel time are related to the speed of the traveler. The faster one travels, the more distances along the travel direction shrink, and the more slowly clocks on the spacecraft run. At high speeds, this combination would result in tremendous reductions in travel time between the point of departure and the destination. To illustrate, consider that it takes light from the Sun about eight minutes to reach the Earth. Despite the fact that no speed can exceed the speed of light (as special relativity reveals), the travel time measured by a moving observer could be shorter due to relativistic effects, although no object can exceed the speed of light. At a speed one-tenth of the speed of light, the trip would take about eighty minutes. To make the trip in eight minutes, the speed would need to be only about 70 percent of the speed of light. At 99 percent of the speed of light, the trip would take less time for the traveler due to time dilation. In principle, objects with mass cannot travel at the speed of light; however, as speeds approach the speed of light, time experienced by the traveler decreases relative to a stationary observer. Thus, while the trip may take less time for the traveler, it does not occur in zero time. Indeed, traveling at speeds close to the speed of light, one could cross great distances in the universe in less time for the traveler, and at such a speed, the words “time” and “space” depend on the observer’s frame of reference. In practice, however, such travels will likely remain in the realm of science fiction, because as the speed of the spacecraft increases, so does its effective mass and the energy required to accelerate it to such high speeds. It should also be noted that it is only by the Earth’s clocks that life in the spacecraft is different. Based on the time in the spacecraft, the traveler lives a normal life and ages at the normal rate.

One practical application of the effect of gravity on time is in contemporary navigation systems. Navigation systems operate on the basis of data on time and distances, which are communicated to them by satellites in low-gravity space outside the Earth’s atmosphere. Ignoring the predictions of general relativity concerning the effect of gravitation on time could amount to an error of several kilometers in calculated distances.

The general theory of relativity also has implications regarding the beginning and the end of time. This characteristic of time is usually studied in cosmology, to which general relativity has made its greatest contributions so far. A major discovery in cosmology in the twentieth century was Edwin Powell Hubble’s observation that distant galaxies are moving away from the Earth. This observation brought about the notion of the “expanding universe” and the “big bang” theory. According to the Big Bang theory, based on the present expanding rate of the universe, at some time, fifteen to twenty billion years ago, all the matter in the universe was concentrated in one incredibly dense “point.” The universe then began as a result of a big explosion—the Big Bang. There have been other theories of the origin and the behavior of the universe, but more and more observational evidence supports the Big Bang theory of the expanding universe. In 1965, Arno A. Penzias and Robert W. Wilson of Bell Laboratories made a discovery that strongly supports the Big Bang theory. The discovery was the detection of cosmic microwave radiation that fills the entire universe and is believed to be the remnant of an initial explosion.

The general theory of relativity supports the Big Bang expanding model of the universe.

The theory was first used by Alexander Friedmann (a Russian meteorologist and mathematician) before Hubble’s observation to model the Big Bang. In Friedmann’s models, space-time and matter came into existence at the time of the Big Bang. Therefore, in the context of the general theory of relativity, the Big Bang was the beginning of time for our universe, and the question “What happened before the Big Bang?” becomes meaningless. According to the general theory, time and space are possible only when matter is present, and therefore, within the physical world, it is matter and energy that ultimately determine the nature of time.

It is possible that the presently expanding universe will stop expanding and begin contracting at some point. Should this occur, the universe will collapse into a state of infinite density. In cosmology, this is referred to as the Big Crunch, which would correspond to the end of time in our universe. The general theory of relativity also models the Big Crunch as a later state of the universe that heralds the end of time.

General relativity and existing cosmological data imply the existence of black holes within the universe. Black holes can be thought of as small local “crunches.” Because of the many similarities of black holes to the Big Crunch and the Big Bang, they are extensively studied by astrophysicists and cosmologists who wish to answer questions about the beginning and the end of time.

The beginning and the end of time, and the relationships of time with space and matter, have also concerned philosophers and theologians for centuries. Among the notable philosophers who speculated and reasoned about such issues are Immanuel Kant, Leibniz, and Saint Augustine. When Saint Augustine was asked, what did God do before he created the universe? He replied that time was a property of the universe that God created and that time did not exist before the beginning of the universe. To the question, “Why did God not create everything a year sooner?” Leibniz responded that the question would be meaningful “if time was anything distinct from things existing in time.” In fact, this view of the intimate association of time and the universe can be traced back to Plato, the Greek philosopher who, in 400 BCE, believed that time was produced by the universe.

The theory of relativity and the concepts of time, space, and matter in relativity have allowed for the first time a reevaluation of such philosophical questions from scientific viewpoints. In particular, the model of the expanding universe, with the Big Bang and the Big Crunch phenomena, has brought the concepts of “genesis” and “revelation” into the realm of science in general, and theoretical physics in particular.

Context

Inquiries concerning the nature of time tend to become less speculative and philosophical and more scientific and practical as scientists develop better physical theories to predict its nature and make more refined instruments to verify such predictions.

The role of timekeeping devices is fundamental in this endeavor. With the sand clocks of the fourteenth century, with error rates of roughly one hour per day, it would have been meaningless to speak of the slowing or speeding of time by less than an hour in a day. With the advent of seventeenth-century pendulum clocks that were accurate within minutes per day, the time effects of minutes per day became measurable, and hence meaningful. Using atomic clocks in the twenty-first century, it is possible to measure the effects of matter and speed on time to within 1/15,000,000,000 of a second per year.

A major part of what is known about the characteristics of time as a physical entity is a result of the predictions of the theory of relativity. Based on various verifications of Einstein’s theory of gravity, it appears that this theory can explain the behavior of the universe at large. It is also known, however, that this theory cannot be used to describe the behavior of the physical world on subatomic scales. Fortunately, in the earlier part of the twentieth century, along with the theory of relativity, another branch of modern physics concerned with the study of matter and universe on small scales was developed—quantum mechanics.

According to quantum theory, for distances on the atomic scale, the behavior of physical objects cannot be modeled by large-scale theories, such as the general theory of relativity. In particular, the quantum effects become considerable close to the Big Bang, and general relativity is no longer applicable. In fact, close to the Big Bang, the quantum theory has its own implications concerning the state of the universe. A theory that could explain the universe on both a large scale and a small scale would provide better explanations concerning the nature of time than those now used.

Einstein spent his final thirty years looking for such a unified theory. At that time, however, there was little or no knowledge of the state of the universe on the large scale (expanding universe, black holes, and the Big Bang) or on the small scale (the nature of subatomic particles and nuclear forces). Despite decades of efforts by many physicists, there is no link between the (macro) theory of general relativity and the (micro) quantum theory. Theories like the loop quantum gravity and string theory have continually been used to attempt to reconcile quantum mechanics with relativity.

Scientists continue researching the nature of the universe in relation to time, and technological developments continue to support the evolution of this body of knowledge. Improvements in quantum mechanics and precision timekeeping in the twenty-first century enhanced scientific understanding of time. Devices like optical lattice clocks, considered the most precise time-keeping instruments in the twenty-first century, improve timekeeping accuracy, which supports space exploration, advances in global positioning systems, and discoveries in quantum mechanics. For example, in 2022, physicists at the Joint Institute for Laboratory Astrophysics used optical lattice clocks to detect gravitational time dilation at a millimeter scale for the first time. Their study showed that two atomic clocks just one millimeter apart in elevation tick at different rates because of Earth’s gravity. The atoms at the top of the lattice experienced slightly weaker gravity than those at the bottom, similar to the way astronauts experience lower gravity in orbit compared to people on Earth. By 2025, advances in optical atomic clocks supported ongoing efforts to redefine the second using optical frequencies. In early 2025, scientists performed an experiment using a complex entangled quantum system to research the concept of the arrow of time, or its one-way direction. Their findings suggested the existence of two opposing arrows of time within quantum systems. This suggests that, under specific conditions at the quantum level, physical processes may exhibit behavior consistent with opposing directions of time. This challenged modern concepts of time and thermodynamics.

Principal terms

ATOMIC CLOCK: an extremely precise clock that operates on the basis of natural vibrations within the atomic structure

BLACK HOLE: an object of incredibly high gravity, formed when a star with a large mass collapses into itself under its own gravity

COSMOLOGY: the study of the origin and structure of the universe

EVENT HORIZON: a zone around a black hole within which time stops, and from which nothing, not even light, can escape

INERTIAL OBSERVER: a stationary observer or an observer with a uniform (unaccelerated) motion

LIGHT SPECTRUM: the combination of component frequencies or colors in a ray of light (as in a rainbow)

SPACE-TIME: a four-dimensional system of coordinates whose points represent events in the physical world; three coordinates represent the location of the event and one represents the time of the event

WHITE DWARF: a massive star approximately the size of the Earth whose gravitation is tens of thousands of times greater than that of the Earth

Bibliography

Calder, Nigel. Einstein’s Universe. Penguin Books, 1979.

Davies, P. C. W. Space and Time in the Modern Universe. Cambridge UP, 1977.

Einstein, Albert. Relativity: The Special and the General Theory. 15th ed., Crown, 1952.

“General Relativity and the Nature of Spacetime.” NASA Science, science.nasa.gov/astrophysics/programs/physics-of-the-cosmos/general-relativity-and-the-nature-of-spacetime/. Accessed 24 Apr. 2026.

Good, R. H. Basic Concepts of Relativity. Reinhold, 1968.

Guff, Thomas, et al. “Emergence of Opposing Arrows of Time in Open Quantum Systems.” Scientific Reports, vol. 15, no. 1, 2025, p. 3658, doi:10.1038/s41598-025-87323-x. Accessed 24 Apr. 2026.

Hawking, Stephen W. A Brief History of Time. Bantam Books, 1988.

“Roadmap to the Redefinition of the Second.” BIPM, www.bipm.org/en/redefinition-second. Accessed 24 Apr. 2026.

Russell, Bertrand. The ABC of Relativity. 5th ed., New American Library, 1958.

Smart, J. J. C. Problems of Space and Time. Macmillan, 1979.

Strogatz, Steven. “What Is the Nature of Time?” Quanta Magazine, 29 Feb. 2024, www.quantamagazine.org/what-is-the-nature-of-time-20240229. Accessed 24 Apr. 2026.

Weinberg, Steven. The First Three Minutes. Basic Books, 1977.

Whitrow, G. J. The Nature of Time. Holt, Rinehart and Winston, 1971.