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Time is a concept inextricably linked to nature and human experience; it is placed, along with the concept of space, at the foundation of man’s constructed models of the universe and the phenomena that occur within it.
The concept of time and its measurement are essentially based on cyclic processes: the first proposal for the use of linear processes, more congruent with the philosophical concept of time, was made in 1715 by E. Halley, who indicated in the degree of salinity of the sea, increasing with time, an index of time. 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 for half of the element’s nuclei to decay. Linear time scales proved particularly useful in the development of methods for measuring large time intervals (e.g., the carbon-14 method). Implicit in the use of radioactive natural clocks and the development of ultraprecise clocks is the notion that atoms obey the same physical laws in all places and at all times: the possibility that physical laws vary over time remains to be tested.
Understanding time in physics
In physics, it is considered a fundamental physical quantity, defined only by the method used to measure it. The problem of measuring time is one of the most important problems in science and technology.
The concept of time, from the metrological point of view, should be studied in the two aspects of the time scale and the unit of time; a time scale is an uninterrupted succession of phenomena that makes it possible 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 coincide to form the scale, dates can be expressed in uniform time, which in turn can be used to interpret 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 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 the knowledge of the theory behind the phenomena forming the scale. For a long time, these phenomena were astronomical, related to the rotation of the Earth and its orbit around the Sun.
The units of time (day, second, year, etc.) were derived by comparing numerous observations. The unit of time, however, can be directly available if there is a reproducible duration at any place and any time: conditions of this kind are met today with the use of clocks, in particular atomic clocks, which use as the unit of time the period of a suitable atomic transition, chosen as the sample duration (the inverse of the sample duration is the sample frequency: for practical purposes it is actually more convenient to use frequencies rather than time). With the latter, it is possible to define a time scale, called atomic time, which is independent of the set of astronomical phenomena commonly considered for the evaluation of time.
The precision with which one operates today with an astronomical time scale, i.e. the dating of astronomical phenomena, is limited only by observational errors. However, it should be emphasized that the theory of astronomical phenomena, especially of the Earth’s motion, is still imperfect, so the resulting unit of time is imprecise.Therefore, for the sake of homogeneity and convenience, conventional time scales are used to date astronomical phenomena. The most common ones are based on sidereal time and solar time: the former has as its unit the sidereal day, the latter the solar day, defined by the value of the hour angle of the stars (or an appropriately chosen star) and the Sun, respectively. Thus, both are local times, that is, their value at the same moment depends on the position of the observer on the Earth (longitude); this variability gave rise to the need to introduce time zones, 24 zones on the Earth, in each of which the same time conventionally applies.
The philosophical notion of time
In philosophy, the concept of time varies according to whether it is considered from an objectivist point of view, in which time is seen as something real and absolute in itself, independent of relations to the external world and the human subject, or from a subjectivist-idealist point of view, in which the origin of time is located in the subject. The concept of time has a special place in contemporary existentialist thought. Fundamentally realist and objectivist, though in different approaches, is the concept of time that the Greek thinkers, from the Pythagoreans to Plato, had, who saw in time the image – in movement, but cyclical movement, always returning, as in the cycles of the years, the seasons, the regular movements of the stars – of the eternity and immutability of being.
Aristotle calls time the “measure of motion,” that is, the measurable expression of the regular and constant movements of the life of the cosmos. This concept was taken up in a different form by the great post-Aristotelian schools, as well as by the leading thinkers of the Christian Middle Ages, but was nevertheless neglected by the religious thought of the late ancient world. Plotinus, in fact, identified time with the very life of the soul, with its passing from one moment of its inner existence to another; St. Augustine, relying on the three-dimensionality of time, asserted that the future is “expected,” the past is “remembered,” only the present is authentic temporality, though it always flows between the other two dimensions. However, the Aristotelian concept of time remained dominant in philosophy until I. Kant, who instead made a real revolution by defining time as a “pure a priori intuition”, the “form of internal sense”. Far from conceiving of it as an absolute dimension, Kant sees in it a fundamental condition of the possibility of perception and thus of knowledge itself.
The Kantian concept of time, interpreted in a one-sided way, as in fact occurred in German idealism, undoubtedly leads to subjectivist reductions that betray the genuine thought of Kant, whose analysis of time must be integrated with those pages of the Analytic of Principles where he identifies the order of temporal succession with the causal order of phenomena: a thesis that has been reproposed in modern times by H. Reichenbach and that is also applied 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 contrasted with the “spatialized” time of contemporary science, in many spiritualist currents beginning with H. Bergson; and even in Husserlian phenomenology, albeit on a very different background, we see an interpretation of time as a current of lived experience. On the other hand, a very distinctive philosophical conception of time emerges 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 lies that authentic temporality that the philosopher contrasts with the inauthentic temporality of datable and measurable time.
The sociological approach to time
Two main areas of time analysis can be distinguished in sociology. The first goes back to G. Friedmann‘s research on natural time and mechanical time as the opposition between the world of nature – governed by the rhythms of species, the organism, the seasons – and the world of technology, subjected to the dictatorship of productivity, the minute chronological organization of life and everyday life (even echoed in the notion of the sports record).
The second declension of time refers, for the social sciences, to the now traditional and somewhat hackneyed notion of leisure. This term has been used to translate the richer and more precise English concept of leisure (or the French loisir) to denote non-work opportunities for leisure, recreation, and various sporting, recreational, or generally cultural activities.
Some scholars, however, prefer to distinguish at least between leisure (as simple non-work, although this does not adequately take into account certain social conditions, from that of the unemployed to that of the housewife or retiree) and liberated time, as a living space that is removed from the rigid organization of work through individual or collective strategies of time reconquest. The theme of liberated time has been brought to the attention of the social sciences by women’s emancipation and liberation movements, as part of a critical analysis of the division of social roles and of work itself in contemporary technologically advanced societies.