Time is an abstract entity (as well as a physical quantity), useful for quantifying and measuring the flow of events. Any measurement of time involves measuring a change in some physical quantity, hence: time is the change or the interval in which a change occurs. It is impossible to know that time has passed unless something changes.

Time is the continuous sequence of existence and events that occurs in a seemingly irreversible succession from the past, through the present, into the future. It is a component quantity of various measures used to sequence events, to compare the duration of events or the intervals between them, and to quantify the rates of change of quantities in material reality or conscious experience. Time, in physics, is referred to as a fourth dimension, along with the three spatial dimensions.

The amount of time or change is calibrated against a standard. The unit of measurement in the International System for time is the second. This allows us not only to measure the amount of time, but also to determine a sequence of events.

Time has long been an important object of study in religion, philosophy, and science, but defining it in a way that is applicable to all fields without circularity has always eluded scholars. However, various fields such as business, industry, sports, science, and the performing arts all incorporate some notion of time into their respective measurement systems.

The physical nature of time is addressed by general relativity with respect to events in space-time. Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a spaceship. Each event can be assigned four numbers representing its time and location (the coordinates of the event). However, the numerical values are different for different observers.

In general relativity, the question of what time is now has meaning only in relation to a particular observer. Distance and time are intimately related, and the time it takes light to travel a given distance is the same for all observers, as first demonstrated publicly by Michelson and Morley. General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. At present, there is no generally accepted theory of quantum general relativity.

Time is a concept inextricably linked to the nature and experience of man; it is placed, together with the concept of space, at the foundation of the models, built by man, of the Universe and the phenomena that occur in it. Physically it is a relative quantity, defined only through the method used for its measurement. The problem of measuring time is one of the most important faced by science and technology.

The notion of time, from the metrological point of view, should be examined in the two aspects of the time scale and the time unit; a time scale is an uninterrupted succession of phenomena that allows to establish a chronology, that is to assign a date to every other event; knowing the mechanics, that is the set of physical laws that concur to form the scale, dates can be expressed in uniform time, which in turn can be used for the interpretation of any other natural phenomenon.

The unit of time is the duration of the time interval separating two phenomena chosen once and for all along the time scale. The history of time measurement shows that the phenomena chosen to form the time scale were periodic natural phenomena; multiples or submultiples of the period of the phenomena themselves were adopted as the units of time.

The precision with which the units of time are determined, that is, the uniformity of the time scale, is a function of knowledge of the theory behind the phenomena forming the scale. For a long time, such phenomena were astronomical ones, related to the rotation of the Earth and its revolution around the Sun. The units of time (day, second, year, etc.) were derived through the comparison of numerous observations.

The time unit can be directly available, however, when there is a duration reproducible in every place and time: conditions of this kind are now satisfied with the use of clocks, in particular atomic clocks, which use as time unit the period of an appropriate atomic transition, chosen as a sample duration (the inverse of the sample duration is the sample frequency: for practical purposes is more convenient the use of frequencies, rather than time). With the latter it is possible to define a time scale, called atomic time, which is independent from the series of astronomical phenomena commonly considered for the evaluation of time.

The precision with which today we operate using an astronomical time scale, i.e. we date astronomical phenomena, is limited only by observation errors. It should be noted, however, that the theory of astronomical phenomena, in particular the movements of the Earth, is still imperfect, so the unit of time that derives from it is inaccurate: for homogeneity and convenience, in the dating of astronomical phenomena are therefore used conventional time scales.

The most commonly used are based on sidereal time and solar time: the first has as its unit the sidereal day, the second the solar day, defined by the value of the hour angle, respectively, of the stars (or a star appropriately chosen) and the Sun. They are both local times, that is, that their value, at the same time, depends on the position of the observer on Earth (longitude), from this variability arose the need to introduce time zones, 24 areas of the Earth in each of which is in force, conventionally, the same time.

Notion of time

The notion of time and its measurement are based essentially on cyclical processes: the first proposal for the use of linear processes, more congruent with the philosophical concept of time, was advanced in 1715 by E. Halley, who indicated in the degree of salinity of the sea, increasing over time, an index of . Similarly, W. T. Kelvin indicated in the cooling of the Earth, and H. Helmholtz in the contraction of the Sun, indices of time.

The discovery of radioactivity provided the most precise linear process: the corresponding unit is the half-life of a radioactive element, that is, the time required by half of the nuclei of the element itself to disintegrate. Linear time scales proved particularly useful in devising methods for measuring large time intervals (e.g. carbon 14 method).

Implicit in the use of radioactive natural clocks and in the development of ultraprecise clocks is the notion that atoms obey, in every place and time, the same physical laws: the possibility that the physical laws vary over time is, however, still subject to verification.

Cosmic time

The cosmic time is a criterium of definition and measurement of time adopted by cosmologists, which takes as reference the local conditions of some parameters such as the rate of expansion of the chronotope, the local density of galaxies, the temperature of the microwave background radiation, the gravitational red shift etc. in order to establish a time scale marked by the progressive evolution (local) of the parameters themselves.

An observer will note identical cosmic times if the measurement of cosmological parameters related to two different points of the Universe gives identical results.

Relativistic time

The measurement of time represents in relativity (special and general) an operation strictly dependent on the kinematic and/or gravitational conditions in which the observer performs it.

For example, μ particles, or muons, present in the swarms of particles produced by cosmic ray impacts in the atmosphere, hit the Earth’s surface at speeds very close to that of light after a flight time of the order of 200 microseconds. Muons are unstable particles that – in the laboratory – spontaneously decay in only 2 microseconds. The increase of their average lifetime in atmospheric swarms is justified by the time dilation provided by special relativity for moving observers.

Time dilation phenomena also occur for the theory of general relativity. In this theory, Einstein effect consists in a red shift manifested by electromagnetic radiation radiated by celestial bodies with considerable gravitational field (gravitational red shift).

Near the Sun, this shift reaches 0.01 Å, equivalent in frequency – and therefore in the local time scale – to a time dilation of 2×10-6 seconds per second. However, the time dilation would increase to 15 times larger values if the star were a compact white dwarf with very strong surface intensity of the attractive field, such as Sirius B. Finally, the complete zeroing of the time course (infinitely large dilation) would occur at the “event surface” surrounding a black hole.

Thermodynamic time

Even if apparently reversible processes in time can be easily imagined (the oscillation of a pendulum, the motion of planets, the trajectory of a marble bouncing between the sides of a billiard table), the microscopic analysis shows that, in reality, even these processes are irreversible thermodynamic processes that define a unique way of flowing of time, that is always in the same direction.

In fact, it is known that, of the energy that is given to a system to induce any transformation, an ineliminable fraction must still be dispersed in a non-recoverable way in the form of disordered energy such as, for example, heat.

The decreasing amplitude of pendular oscillations due to friction (heat generators), the heating of the ball as a result of collisions, the dissipation of kinetic energy in the form of gravitational radiation by the planet in orbit provide as many keys to define the direction in which time flows. In general, all real systems, not interacting with each other, in implementing thermodynamic transformations, tend to reach states of increasing internal disorder.

In strict terms, the laws of thermodynamics require that the real transformations of isolated systems take place in the sense of increasing entropy. Irreversibility and increase of entropy are synonyms that, in the same way that they denote the characteristics which events obey in their becoming, also contain the essential property that gives meaning to time, since the latter would lose all meaning if there were no production of events, or there were no means to determine the universal direction of flow.

Time in philosophy

The concept of time differs according to whether it is considered under the objectivist aspect, in which time is seen as something real and absolute in itself, independent of relations with the external world and the human subject, or under the subjectivist-idealist view, which places the origin of time in the subject. The conception of time in contemporary existentialist thought has a separate place.

The Greek thinkers, from the Pythagoreans to Plato, had a fundamentally realist and objectivist concept of time. They saw in time the image – in movement, but a cyclic movement, always returning, as in the cycles of the years, of the seasons, of the regular movements of the stars – of the eternity and immutability of being.

In his Physics, Aristotle defines time as the “measure of movement”, that is, the measurable expression of the regular and constant movements of the life of the cosmos. Taken up in a different form by the major post-Aristotelian schools, as well as by the main thinkers of the Christian Middle Ages, this concept was however neglected by the religious thought of the late antique world.

Plotinus, in fact, identified time with the very life of the soul, with its passing from one moment to another of its inner existence; Saint Augustine, basing himself on the three-dimensionality of time, asserts that the future is “expected”, the past “remembered”, only the present is authentic temporality, even if always flowing between the other two dimensions.

The Aristotelian concept of time, however, remained dominant in philosophy until I. Kant. Kant, who instead operated a real revolution by defining time as a “pure a priori intuition”, the “form of internal sense”. Far from conceiving it as an absolute dimension, Kant sees in it rather a fundamental condition of the possibility of perception, and therefore of knowledge itself.

Now, Kant’s concept of time, interpreted in a unilateral way as in fact occurred in German idealism, undoubtedly leads to subjectivist reductions that betray the genuine thought of Kant, whose analysis of time should be integrated with those pages of the Analytic of Principles where he identifies the order of temporal succession with the causal order of phenomena: thesis proposed again in modern times by H. Reichenbach and applied also to Einstein’s theory, which always sees in time a value of causal succession, denying only the uniqueness and absoluteness of such an order.

A “conscientized” time is then again opposed to the “spatialized” time of contemporary science, in many spiritualist currents starting from H. Bergson; and also in Husserlian phenomenology, even if on a very different background, there is an interpretation of time as a current of lived experiences.

A very peculiar philosophical conception of time was born with modern existentialism, and especially with M. Heidegger, in his work entitled Being and Time. In his interpretation of “being” in terms of possibility, project and anticipation, Heidegger affirms the existential primacy of the future, in which consists that authentic temporality that the philosopher contrasts with the inauthentic temporality of datable and measurable time.

Time in business economics

Time, along with space, is one of the coordinates that define the context in which to place and evaluate any business event. In this sense, economic time differs from physical time, in that it is measured by taking as its unit the degree and type of connection existing between past management, present management and future management. It is, therefore, the interweaving of the investment-realization cycle that marks the rhythms of economic-business time.

It follows that, in business economics, time assumes a relevant importance, so much so that situations of equilibrium are examined by considering the action of exogenous and endogenous variables which, without solution of continuity, erode, if negative, the positions reached, or, if positive, allow their restoration, maintenance and improvement; it is not by chance that the conditions of business equilibrium are typically prospective, that is, “valid over time”.

This approach is not, on the other hand, taken into account in the field of political economy, where the temporal dimension is generally not considered, since the equilibrium situations are examined statistically, assuming that the variations occur and act instantaneously.

From a more strictly operational point of view, the consideration of time, understood in an economic-business sense, requires that all planning and programming activity be developed according to periods dictated by the influences existing between present, past and future management.

It is called time and method calculation the technical analysis of the way of producing under conditions of maximum economy. In an industry, the number of people dedicated to this analysis varies according to both the size of the company and the type of production. The time and method calculation has a twofold task: the first is to check whether the current production method is economically viable, the second is to allow a cost balance to be made for the possible replacement of production processes. In order that a method realizes the greater profit it must tend to the contraction of the costs of all the necessary items in order to reach the finished product (materials, machines or systems, staff).

If there are no reasons of aesthetic or functional order for the manufactured article, a method is reputed to be better when it provides a product at a lower cost than another. For manufacturing industries, the cost of the raw material has a limited influence on the cost of the finished product, while the technological process directly linked to the transformation of the raw material has an ever-increasing importance: this is due to the continuous increase in the cost of the time required to employ manpower and machinery. Therefore, the method that employs plants and personnel for a shorter time tends to prevail; this gives rise to the need for an in-depth analysis of times.

The time taken by a machine to produce an automatic process is closely linked to the type of work and decreases only with technological improvements; on the other hand, the time taken by operators to feed the machine and remove pieces at the end of the process is the object of analysis.

Natural difficulties are encountered in evaluating with precision the average time necessary to carry out a series of operations considering an ideal operator who works with capacity, commitment and average fatigue, when even just the measurement of time, during an operation, becomes an element of disturbance for the operator himself who, feeling observed and controlled, reacts negatively.

In order to obtain these data, different methods are used: the simplest is direct timekeeping, carried out by specialized people (timekeepers), who perform a certain number of measurements of the time taken to carry out the same operation at different times during the day and with different operators. In addition, it is appropriate that the survey is also carried out by different tempists to make the method of detection objective. But if such system allows to analyze a production process already in progress with modest expense, it proves expensive to analyze a method in phase of project, in how much it is necessary to prepare a system or to simulate the operation or at least to mimic the movements that are necessary in order to carry out the production: in this way however the naturalness of the operations is notably altered.

Currently, direct timekeeping is always valid for a posteriori control of the expected times, but during the project the times are determined with the MTM system (Methods Time Measurement) which consists in breaking down the actions necessary to carry out the operation into elementary movements of every single part of the body and in determining the times corresponding to each operation by means of special tables, where the times necessary to carry out every single movement appear, calculated on a large number of surveys carried out also by means of cameras.

The times thus determined must be increased in percentage both to take into account the unforeseen events that always compromise the linearity of operations, such as the accidental fall of a piece from the operator’s hands, and because an operator cannot work continuously for eight hours. Even for break times, diagrams have been drawn up to indicate the optimum balance between activity and rest times so that performance at the end of the day is higher.

While respecting the human dimension of the worker, there are consequently limits to the full application of the results of an analysis of the work cycle of a product based only on the concepts of calculating times and methods. It follows that the reduction of the costs cannot go down, for this way, under of a value limit. Therefore, other analysis techniques are spreading which, respecting these limits, have the same aim of reducing costs.