The world's time signals are now co-ordinated by the Bureau International de l'Heure (BIH) based on a world 'mean clock' that is the average of some eighty atomic clocks in twenty-four countries. It provides direct synchronization to within about a millisecond. Although this 'Co-ordinated Universal Time' (UTC), which has replaced GMT as the basis of civil time throughout the world, is now controlled from Paris, the world's prime meridian for longitude and time still passes
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through the old Observatory at Greenwich. In practice, the zero meridian is now defined by the adopted longitudes of the instruments that contribute to the determination of UTC. Since 1985 the contribution of the Royal Greenwich Observatory to the international determination of UTC and longitude has been through its observations of the artificial satellite Lageos by a laser-ranging system in use at Herstmonceux since the autumn of 1983. As from 1 January 1972 time signals have radiated atomic seconds, but just as there is not a whole number of days in a year, so there is not a whole number of atomic seconds in a solar day. This has led to the adoption of corrections, either positive or negative, of exactly one second. They are called 'leap seconds' and when required are on the last day of a calendar month, preferably on 31 December or 30 June.
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11. Rival Concepts of Time
Instant and duration
St Augustine appears to have been the first thinker to have carefully investigated the consequences of our actual experience of time being confined to the present instant. He came to the conclusion that our ideas of past and future must depend on our consciousness of memory and sense of expectation. In regarding time from this psychological point of view the primary concept is the instant rather than duration. Nevertheless, despite St Augustine's great influence on medieval theology, it was not until the humanistic Renaissance of the fifteenth century and the religious Reformation of the sixteenth century, followed by the Copernican revolution in astronomy and cosmology--all of which contributed to the dissolution of the timeless medieval world-picture with its hierarchical structure in which everything had its assigned place--that Western thinkers began to regard personal existence as being essentially based on the present moment.
The significance of the instant was presented in pictorial art by Hans Holbein ( 1497- 1543), for example in his famous painting of 1533 known as 'The Ambassadors'. In a pamphlet on this painting, published by the Trustees of the National Gallery in 1974, Alistair Smith has emphasized the sense of instaneity at the centre of Holbein's art. Fascinated by the nature of human mortality, his object was to depict the sense of personal existence at a definite instant. Smith draws attention to the way in which this moment of time was precisely indicated in the picture, the date (11 April) being registered on a cylindrical dial and the hour (10.30 a.m.) on a polyhedral dial.
One of the first to give literary expression to the notion of personal existence as based on the present moment was the famous French essayist Michel de Montaigne ( 1533-92). Even as a child he is said to have been greatly influenced by the Metamorphoses of Ovid, and his outlook throughout life was dominated by the conviction that the world in which we find ourselves is in a state of incessant change. Consequently,
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he believed that the assumptions on which our way of thinking is based are necessarily uncertain and defective.
This attitude of scepticism concerning human knowledge was turned to positive effect later by René Descartes ( 1596- 1650), in the philosophy that he developed on the basis of his famous axiom Cogito ergo sum. If, however, existence is to be identified with the transitory instant rather than with duration, how can the continued existence of the world be accounted for? Descartes's answer was that the world is recreated from instant to instant, conservation and creation differing only in respect of our way of thinking and not in reality, self-conservation being the unique prerogative of God.
In the eighteenth century, however, there was a general revolt against the idea of the instant as the basic temporal concept. Instead, it came to be appreciated that our experience of time is dualistic: intensity of sensation is associated with the instant, but our awareness of multiplicity of sensation depends on duration. This led to a new interest in the nature and significance of memory. For example, the French philosopher Denis Diderot ( 1713-84) in a famous passage ( Oeuvres, IX, p. 366) wrote:
I am led to believe that everything we have seen, known, perceived, heard--even the trees of a deep forest--nay, even the disposition of the branches, the form of the leaves and the variety of the colours, the green tints and the light; the look of grains of sand at the edge of the sea, the unevenness of the crests of waves, whether agitated by a light breeze, or churned to foam by a storm; the multitude of human voices, of animal cries, and physical sounds, the melody and harmony of all songs, of all pieces of music, of all the concerts we have listened to, all of it, unknown to us, exists within us.1
This remarkable claim, for which Diderot had no scientific evidence, has been supported this century by the experiments made by the Canadian neurosurgeon Wilder Penfield, who elicited 'flashbacks' by applying an electrode to the exposed cortex of patients undergoing brain surgery.2(See The Natural Philosophy of Time, pp. 103 ff.)
In the nineteenth century the idea of temporal succession came to assume greater importance in human life and thought than ever before. It not only gave rise to important developments in literature, including the evolution of the novel and a spate of autobiographies, but it also became of dominating importance in the natural sciences, as is exemplified by the oft-quoted statement of the geologist G. J. P. Scrope: 'The leading idea which is present in all our researches, and which accompanies every fresh
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observation, the sound of which to the ear of the student of Nature seems continually echoed from every part of her works, is--Time!-- Time!--Time!'3
As the century progressed, we find that truth itself tended to be regarded no longer as eternal and changeless but as time-dependent. Attention came to be focused on the historical process rather than on an eternally valid, unchanging order of things. In other words, interest was transferred from the 'thing completed' to the genetic process, that is, from 'being' to 'becoming'. This radically new point of view received its extreme formulation in the philosophy of the 'modern Heraclitus', Henri Bergson ( 1859- 1942), for whom ultimate reality was neither 'being' nor even 'being changed' but the continual process of 'change' itself, which he called la durée. An authoritative critical account of Bergson's eloquently expressed philosophy of la durée and its influence in the early decades of the present century has been given by the distinguished former Professor of the History of Philosophy in the University of Warsaw, who is now a Fellow of All Souls, Leszek Kolakowski in his book Bergson, published in 1985 by Oxford University Press, in the 'Past Masters' series. Bergson achieved the unique distinction of being both scathingly criticized by Bertrand Russell (in 1912) and having his books placed on the Index Prohibitorum by the Holy Office in 1914--the year in which he was elected a member of the Académie Française! A more scientifically orientated philosophy of change than Bergson's, but which owed something to his example, was developed between the wars by the British mathematician and philosopher A. N. Whitehead ( 1861- 1947), particularly in his book Process and Reality, based on his Gifford Lectures of 1928.
Relativistic and cosmic time
In view of the important role that time had come to play in modern life as well as in the scientific world-view, it was a great surprise when, in 1905, in a scientific paper that is now regarded as one of the most important published this century Albert Einstein revealed a previously unsuspected limitation of the current theory of time. According to that theory, for a given way of measuring time each event can have only one time associated with it. Events having the same time are said to be 'simultaneous'. The new point that occurred to Einstein was that, although the idea of simultaneity is perfectly clear for two events occurring at the same place as well
as at the same time, it is not equally clear for two events occurring at different places. Instead, the simultaneity of a
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distant event and one occurring in the observer's own experience depends on the relative position of the distant event and the mode of connection between it and the observer's perception of it. If the distance of the event is known and also the velocity of the signal (e.g. light) that connects it and the observer's perception of it, the observer can correlate the event with some previous instant in his own experience and can regard these two events as simultaneous. This calculation will, of course, be a separate operation for each observer, but until Einstein raised the question it had been tacitly assumed that, when such calculations are correctly performed, all observers will agree on the time of any given event. Einstein produced a successful theory in which this is not the case.
Einstein based his special theory of relativity, as it came to be called, on the principle that the laws of nature are expressible in the same mathematical form for all observers in uniform relative motion (including relative rest). This principle of relativity holds good in classical dynamics based on Newton's laws of motion, but Einstein believed that it should be extended to other branches of physics, in particular electromagnetism and the theory of light. In classical dynamics there is no velocity with special properties, but in electromagnetic theory the velocity of light (in empty space), which is about 300,000 kilometres a second, has special significance. Einstein believed that, if the properties of light are to be the same for all observers in uniform relative motion, they must all assign the same velocity to it. This additional condition, however, he found to be incompatible with the prevailing theory of time. Although according to his theory any two observers at relative rest will assign the same time to any given event, wherever it may occur, this is found not to be the case for any two observers in uniform relative motion in general. Consequently, the condition that each event has only one time associated with it no longer holds. Instead, its time depends on the observer.
Einstein's theory involves the assumption that no physical effect can be transmitted faster than the velocity of light (in empty space). Although neither Newton nor Leibniz imposed any such restriction, Einstein's theory is more in accord with Leibniz's concept of time than with Newton's. For, although Leibniz's idea that time is derived from events is compatible with Einstein's theory, Newton's concept of absolute time is not. Whereas for Newton time was independent of the universe and for Leibniz it was an aspect of the universe, the view that now prevails since Einstein's theory has come to be regarded as an essential part of physics is that time is an aspect of the universe which depends on the observer.
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An important consequence of Einstein's special theory of relativity is that a moving clock will appear to run slow compared with a similar clock at rest with respect to the observer, and the closer the velocity of the moving clock is to that of light the slower it will appear to run. This apparent slowing down of a moving clock is called 'time dilatation'. Of all the consequences of Einstein's theory this is the one that many people have found it most difficult to accept, since it clashes with our commonsense intuition of time. Nevertheless, there is now abundant experimental evidence, particularly that provided by high-speed particles, supporting this conclusion.
In his 1905 paper Einstein restricted the principle of relativity to observers in uniform relative motion and did not consider gravitational effects. In what he called the general theory of relativity, which he developed some ten years later in order to cope with gravitation, he extended the principle of relativity to include observers in any form of accelerated motion, special relativity being regarded as an important particular form of this more comprehensive theory. In this theory too the classical assumption that each event occurs at a unique time, the same for all observers, does not apply. In view of this, the idea of a unique cosmic time-scale for the physical universe as a whole might be thought to have no objective significance. Such a conclusion would, however, be mistaken, as has been shown by the developments that have taken place this century in cosmology.
In 1924 the astronomer E. P. Hubble, using the then recently installed 100-in. diameter telescope on Mount Wilson in California showed that the general background of the universe is formed not by the stars but by the galaxies, of which the Milky Way stellar system (that includes our own sun) is one. Five years later he found that the galaxies are receding systematically from one another. (The recessional motion of a galaxy is measured by its 'red shift', that is, the displacement of identifiable lines in its spectrum towards the red.) This discovery made almost as great a change in man's conception of the universe as the Copernican revolution four centuries earlier. Instead of an overall static model of the cosmos, it appeared that the universe is expanding, the rate of relative recession of the galaxies being proportional to their respective distances from one another. This is known as 'Hubble's law'.
Hubble's discovery stimulated much work in theoretical cosmology largely based on Einstein's general theory of relativity. As a result, there was a revival of the idea that there are successive states of the universe associated with a world-wide scale of time. This came about because in
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each of the world-models considered there was a definite set of particularly significant hypothetical observers, namely those located on the individual galaxies and moving with them. The local times associated with these observers fitted together to produce a world-wide time called 'cosmic time'.
An important assumption in the construction of the expanding world- models studied after Hubble's discovery of cosmical recession was that hypothetical observers on each galaxy would see themselves at a centre of isotropy (i.e. spherical symmetry) of the whole universe, so that its general appearance in each direction would be the same. Observational evidence in favour of this assumption can therefore be regarded as supporting the concept of cosmic time. Impressive confirmation of the assumption of cosmic isotropy has come from the discovery of what is often called the 'primeval fireball'. In 1965 A. A. Penzias and R. W. Wilson at the Bell Telephone Laboratory in New Jersey found that some unexpected radiation was leaking into the antenna of their radio-telescope. They soon discovered that this radiation was practically isotropic and at the wavelength at which they were working its intensity was equivalent to a temperature of about three degrees on the kelvin, or absolute, scale. This radiation has been interpreted as the relic of the primeval high temperature radiation associated with an explosive origin of the universe, a conclusion which has been accepted by most astronomers. The fact that the radiation is highly isotropic rules out the possibility of any local origin of its source. A source restricted either to the solar system, or our galaxy, or even to the local cluster of galaxies, could not produce radiation that would appear to us as isotropic. Moreover, large-scale departures from isotropy anywhere in the universe would affect the radiation and make it seem anisotropic to us. Consequently, the isotropy of the cosmic background radiation is powerful evidence that the universe is basically isotropic about each galaxy, and this is a strong argument for the existence of cosmic time.
The discovery of the expansion of the universe and the evidence for the existence of cosmic time have not only reinforced the tendency in recent centuries for time to become a major feature of the scientific world-view, but have thrown new light on the old problem of the total extent of past time. Although in the eighteenth and nineteenth centuries it was becoming clear to those who had discarded an out-of-date biblical chronology that the age of the universe must be reckoned in hundreds, and possibly thousands, of millions of years, it was not until the present century that more precise estimates could be made. As already mentioned (p. 157), the
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discovery of radioactivity and the development of nuclear physics have led us to assign ages to the earth and sun of nearly 5,000 million years. Moreover, astrophysicists have reason to believe that the ages of the old
est stellar dusters and our galaxy are between 10,000 and 16,000 million years. As for the universe as a whole, Hubble's law has been used to estimate its age. Observational evidence of the current rate of recession of the galaxies when applied to the simplest expanding world-models indicates that the universe may have had an explosive origin between 10,000 and 20,000 million years ago, the latter probably being the more correct estimate. Despite the uncertainties involved in obtaining this result, it is remarkable that it is consistent with the totally independent estimates of the ages of the oldest star clusters and our galaxy. In the present state of knowledge, it would seem to be the longest stretch of past time over which we can extend the existence of the physical universe as we know it.
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Time in History: Views of Time From Prehistory to the Present Day Page 23