While defining what time is presents a difficulty, measuring it does not. Scientists themselves do not explain what time is, but confine themselves to the measurement of time. From the mixing up of these two concepts endless confusion arises. Thus, Feynman:

Maybe it is just as well if we face the fact that time is one of the things we cannot define (in the dictionary sense), and just say that it is what we already know it to be: it is how long we wait! What really matters anyway is not how we define time, but how we measure it.[38]

The measurement of time necessarily involves a frame of reference, and any phenomenon that entails change with time – e.g., the rotation of the earth or the swing of a pendulum. The earth’s daily rotation on its axis provides a time scale. The decay of radioactive elements can be used for measuring long time intervals. The measurement of time involves a subjective element. The Egyptians divided day and night into twelfths. The Sumerians had a numerical system based on 60, and thus divided the hour into 60 minutes and the minute into 60 seconds. The metre was defined as one 10 millionth of the distance from the earth’s pole to the equator (although this is not strictly accurate). The centimetre is 100th of a metre, and so on. At the beginning of this century, the investigation of the subatomic world led to the discovery of two natural units of measurement: the speed of light, c, and Planck’s constant, h. These are not directly mass, length, or time, but the unity of all three.

There is an international agreement that the metre is defined as the distance between two scratches on a bar kept in a laboratory in France. More recently, it has been realised that this definition is neither as precise as would be useful, nor as permanent or universal as one would like. It is currently being considered that a new definition be adopted, an agreed-upon (arbitrary) number of wavelengths of a chosen spectral line. On the other hand, the measurement of time varies according to the scale and life span of the objects under consideration.

It is clear that the concept of time will vary according to the frame of reference. A year on earth is not the same as a year on Jupiter. Nor is the idea of time and space the same for a human being as for a mosquito with a life span of a few days, or a subatomic particle with a life span of a trillionth of a second (assuming, of course, that such entities could possess a concept of anything at all). What we are referring to here is the way time is perceived in different contexts. If we accept the given frame of references the way in which time would be seen would be different. Even in practice this can be seen, to some extent. For example, normal methods of measuring time cannot be applied to the measurement of the life span of subatomic particles, and different standards must also be used for measuring “geological time”.

From this point of view, time can be said to be relative. Measurement necessarily involves relationships. Human thought contains many concepts that are essentially relative, for example relative magnitudes, such as “big” and “small”. A man is small compared to an elephant, but big in comparison to an ant. Smallness and bigness, in themselves, have no meaning. A millionth of a second, in ordinary terms, seems a very short length of time, yet at the subatomic level it is an extremely long time. At the other extreme, a million years is an extremely short time on the cosmological level.

All ideas of space, time and motion depend on our observations of the relations and changes in the material world. However, the measurement of time varies considerably when we consider different kinds of matter. The measurement of space and time is inevitably relative to some frame of reference – the earth, the sun, or any other static point – to which events of the universe can relate. Now it is clear that matter undergoes all kinds of different change: change of position, which, in turn, involves different velocities, change of state, involving different energy states, birth, decay and death, organisation and disorganisation, and many other transformations, all of which can be expressed and measured in terms of time.

In Einstein, time and space are not regarded as isolated phenomena, and indeed it is impossible to regard them as “things in themselves”. Einstein advanced the view that time depends on the movement of a system and that the intervals of time change in such a way that the speed of light in the given system does not vary according to the movement. Spatial scales are also subject to change. The old classical Newtonian theories are still valid for everyday purposes, and even as a good approximation of the general workings of the universe. Newtonian mechanics still applies in a very wide branch of sciences, not only astronomy, but also practical sciences such as engineering. At low speeds, the effects of special relativity can be ignored. For example, the error involved in considering the behaviour of a plane moving at 250 miles an hour would be about ten billionth of one per cent. However, beyond certain limits it breaks down. At the kind of speeds that we find in particle acceleration, for example, it is necessary to take into account Einstein’s prediction that mass is not constant, but increases with velocity.

From the point of view of our normal everyday notion of the measurement of time, the extremely short life span of certain subatomic particles cannot be adequately expressed. A pi-meson, for instance, has a life span of only about 10-16 of a second, before it disintegrates. Likewise, the period of a nuclear vibration, or the lifetime of a strange resonance particle, is 10-24 second, approximately the time needed for light to cross the nucleus of a hydrogen atom. Another scale of measurement is necessary. Very short times, say 10-12 second, are measured by an electron beam oscilloscope. Even shorter times can be calibrated by means of laser techniques. At the other end of the scale, very long periods can be measured by a radioactive “clock”.

In a sense, every atom in the universe is a clock, because it absorbs light (that is, electromagnetic rays) and emits it at precisely defined frequencies. Since 1967, the official internationally recognised standard of time is based on the atomic (caesium) clock. One second is defined as 9,192,631,770 vibrations of the microwave radiation from caesium-133 atoms during a specified atomic rearrangement. Even this highly accurate clock is not absolutely perfect. Different readings are taken from atomic clocks in about 80 different countries, and agreement is reached, “weighting” the time in favour of the steadiest clocks. By such means it is possible to arrive at accurate time-measurement to one millionth of a second per day, or even less.

For everyday purposes, “normal” time keeping, based on the rotation of the earth and the apparent movements of the sun and stars, is sufficient. But for a whole series of operations in the field of modern advanced technology, such as certain radio navigational aids in ships and aeroplanes, it becomes inadequate, leading to serious errors. It is at these kinds of levels that the effects of relativity begin to make themselves felt. Experiments have shown that atomic clocks run slower at ground level than at high altitudes, where the gravitational effect is weaker. Atomic clocks, flown at an altitude of 30,000 feet, gained about three billionth of a second an hour. This conforms to Einstein’s prediction to within one per cent.

  1. Feynman, R. op. cit., chapter 5, p. 2.