Text of Laurent Pericone published in french in 2013 in Les Cahiers Science & Vie, no 134, “Comprendre la nature du temps” and complemented by other sources.
Since man began to observe and measure time, he has tried to understand the mysteries of his nature. Notion with blurred contours, thinkers and scientists bring their knowledge to better apprehend it.
How to define time? It is one of the strangest phenomena in existence: its reality seems clear to us, but no one knows how to demonstrate it. At the beginning of Christianity, Saint Augustine (354-430) questioned himself in this way: “What then is time? If no one asks me, I know what it is. If I wish to explain to him who asks, I do not know.” The exercise is difficult because time covers many concepts: simultaneity, succession and duration. The same word can be used to mean change, evolution, repetition or even becoming… Can we wonder about the nature of the present moment, for example? For centuries, this questioning has agitated thinkers and philosophers. Aristotle (384-322 BC) had made this observation: “As the past is no more, as the future is not yet, as the present itself has already finished being as soon as it began to exist, how could there be a creature of time?”. The present passes as it is not always the same and it does not pass as we leave a present moment only to find another one. “The present is a paradox because it is both persistent and ephemeral : always there but not always identical to himself, it contains permanence and change”, observes the physicist Etienne Klein.
Awareness of duration
For physicists, it is an objective fact, independent of human reality; for philosophers, it is an innate notion existing only in the minds of men. René Descartes (1596-1650), then Emmanuel Kant (1724-1804), theorized this subjective representation of time. The philosopher Henri Bergson (1859-1941) proposes to adopt the subjective time, lived and felt by each human being. He associates time with the notion of duration, because man has an intimate awareness of living in the duration. But how to represent it – by a line or a circle? Does time pass or regenerate? For centuries, it was the shape of the circle that seduced the great civilizations because it represents perfection, like the cycles of the Sun, the Moon or the seasons.
With the Stoics, the world disappears and then regenerates identically: what we call the future is just the past coming back. In cultures where the divine is less distant from man, as in India, it is the cyclical nature of time that dominates. Linear time was nevertheless imposed because it responds to the principle of causality: the cause of a phenomenon is necessarily anterior to the phenomenon itself, and once an event has taken place, it is impossible to return to it. The “time arrow” has one direction and one direction only… This expression was introduced in 1928 by the astrophysicist Arthur Eddington (1882-1944). Astronomers tell us that the history of the Universe has been going in the same direction since its birth, 13.7 billion years ago, until the Earth was formed… (4.5 billion years) and the appearance of life (3.5 billion).
The departure is given by the Big Bang, January 1st at 0 o’clock, and our present is represented by December 31st at midnight. The actual duration of this condensed year is 13.7 billion years, the age of the Universe. Each day of the calendar represents 37.5 million years, every minute 26 millennia and every second 438 years.
The Cosmic Calendar is a method to visualize the chronology of the universe.
The concept was popularized by Carl Sagan in his book The Dragons of Eden (1977).
Man perceives this linear time first of all because he feels aging. “We never stop thinking about time in relation to our bodies. We feel older every day, we are our own clocks”, notes the writer Umberto Eco (1932-2016). But the human being is also sensitive to psychological or subjective time. It does not flow the same way for each one of us and blurs the boundaries between the recent past, the present and the near future. It seems to vary according to age and circumstances. Working time seems slower than leisure time; the time of waiting seems to stretch while the time of reunion or happiness is often very short… To live in society, people have observed nature, the alternance of day and night, the return of the seasons and the movement of the stars… At the same time, they created calendars and clocks to determine the length of a working day or the speed of an athlete. For the sociologist Norbert Elias (1897-1990), time is above all a “social symbol” that comes from a desire for standardization throughout society.
Vision of experimental science
This “invented” time has not contributed to a better understanding of its nature. Until the 17th century, time was centred on human communities and allowed them to live together, but it was not used to study natural phenomena. Experimental science will give it a new status. Galileo (1564-1642) considered time as a quantifiable quantity that allows us to measure motion. He uses time rather than space travelled as the fundamental parameter of his experience of falling bodies. The acquired speed is proportional to the duration of the fall.
With the increasing precision of clocks, time is becoming a reliable unit of measurement. The physicist Isaac Newton (1642-1727) developed his theory of absolute time, which dominated science for two centuries. “Newton believed that you could measure the time interval between two events and that the interval would be the same regardless of who was measuring it, as long as you used a good clock”, says physicist Stephen Hawking (1942-2018).
But absolute time does not exist. This is one of the contributions of Albert Einstein‘s (1879-1955) theory of relativity which, at the beginning of the 20th century, links space and time. Time is relative and varies according to the speed at which we move. It slows down as you get closer to the speed of light, 300,000 km/s. The physicist Paul Langevin (1872-1946) illustrated this theory with the paradox of twin brothers. Let’s imagine that one of the brothers stays on Earth and the other leaves in a rocket for a journey into space at very high speed. When he returns to Earth, he finds his brother several years older than him. Ten years elapsed on his calendar, but the time lasted longer for his twin who remained motionless (in the movie Interstellar (Christopher Nolan, 2014), we see the father becoming younger than his daughter).
Consisting of hundreds of Hollywood film clips depicting time in real-time as it plays, Christian Marclay’s The Clock was quite revolutionary at its release. The 24-hour montage of film and TV clips featuring clocks and watches was designed to be functional, in that it actually told the time. As a result, visitors were led through a long and enduring history of cinema.
This hypothesis was tested with the Hafele-Keating experiment in October 1971, with two jet planes flying around the Earth. The atomic clocks they were carrying had a delay of a few billionths of a second compared to the identical clock that remained stationary on the ground at the United States Naval Observatory. This phenomenon must be taken into account so that the clocks of the GPS system satellites are perfectly synchronized with those on Earth. Quantum physics, which studies atoms and particles, teaches us that on a microscopic scale, paradoxical phenomena exist. Thus, when very few elements interact, “reversible” events are observable. This is the example of billiards: two balls collide; after the collision, they go back in opposite directions. If you film this action and project the sequence upside down, no one will be able to detect the reversal. “The reason for this ambiguity is that the second collision is governed by the same dynamic laws as the first. It is just as feasible as the original collision”, says Etienne Klein. Conversely, if you film the fall of a cup of coffee, the manipulation will be immediately visible on the film upside down. Scientists are wondering: is time really irreversible? How can we explain the irreversibility observed at the macroscopic scale (the broken cup of coffee will not be able to return to its former state), when physical laws ignore it at the microscopic scale? An element of response is provided by the second law of thermodynamics, which postulates that any transformation of a thermodynamic system takes place with an increase in entropy, characterized by its degree of disorganization, or unpredictability of the information content of a system.
The mysteries of time will continue to question human intelligence…
References
Pericone, L. (2013). Comprendre la nature du temps. Dans L’invention du temps, Les Cahiers Science & Vie no 134, pp. 9-12.
Biémont, E. (2016). Structure et images du temps. Dans Le règne du temps. Des cadrans solaires aux horloges atomiques, p. 38. Bruxelles, Belgique: Académie Royale de Belgique.
Wikipedia (links in the text)