Cyclic Time and Linear Time

Stephen Hawking

In an article published in 1999, in volume 879 of the Annals of the New York Academy of Sciences, Pier Luigi Luisi speaks about the two traditional models of time that have been considered by traditional philosophy and the mythologies of various historical civilizations. They must not be confused with the two philosophical models originated in the twentieth century, the time A and time B of which I spoke in another post of this blog.

• Cyclical time, predominant in Asian civilizations and the Greco-Roman world until the Christian world view took root there. The origin of this model is evident, for many natural phenomena are cyclical: sunrise and sunset; the phases of the moon; the annual movements of the stars, synchronized with the seasons and with many biological phenomena...
• Linear time, prevailing in the three religions who consider themselves descendants of Abraham: Judaism, Christianity and Islam. Linear time can be compared with the course of the life of a living being, which begins at birth, goes on with changes during a certain period, and ends with death.

Stephen Hawking, in his book A Brief History of Time, distinguishes three different time arrows:

1. The thermodynamic arrow, incorporated in the second principle, which states that in a thermodynamically isolated system, entropy (a measure of energy disorganization) always increases.
2. The cosmological arrow, due to the fact that the universe has its origin in the Big Bang and from there evolves in a linear way.
3. The psychological arrow, which allows us to distinguish the past from the future.

Roger Penrose

The linear cosmological arrow could become cyclical if the universe were to contract from a certain point in time, ending in a Big Crunch. There could be a rebound that would lead to a new Big Bang and another cosmic cycle. When Hawking wrote this book, he clearly favored the cyclic universe. Later this model has been abandoned, when it was discovered in 1998 that the universe is in accelerated expansion (see another post in this blog), although Roger Penrose is recently trying to recover it.

Luisi, who has researched in the field of synthetic biology, adds two other arrows of time to Hawking’s three, both related to biology:

4. The arrow of the origin of life: when the Earth started to exist, there was no life. About one billion years later, the first remains of living beings appear. What happened before? At some point, life must have appeared, because there was no life before. That gives rise to a temporary asymmetry that we can consider as an arrow of biological time.
5. The arrow of evolution. Living beings on the Earth have changed with time. As I explained elsewhere, the amount of information that a living being can process increases over time. Since the apparition of man, it has grown enormously.

Luisi suggests that the life of any living being cannot be represented as a simple cyclical phenomenon of nutrient acquisition, its assimilation and the elimination of waste, but rather as a spiral that changes slowly until it dissolves in death. Luisi also addresses the problem of what happens after death, which depends on the philosophy applied. For materialistic atheism, everything ends in death. For some religions, especially Eastern, most living being that die are reincarnated again in this world. For others, such as Christianity, we are moved to the next life in a different world. Of course, science has nothing to say about this.

Regarding the arrow of evolution, Luisi describes three possibilities arising in this context:

Stephen Jay Gould

• That the tree of life is contingent, so that the evolution on another planet, or an alternative evolution on our Earth, would have reached a totally different result. In such case, humanity cannot be the goal of evolution. This is Stephen Jay Gould’s position in his book Wonderful Life.
• That the tree of life is not contingent, since evolution converges, as shown by the successive apparition of very similar body structures in very different branches, such as sharks, ichthyosaurs and dolphins. According to this theory, held by Simon Conway Morris in his book The Crucible of Creation, written in answer to Gould’s book, a species very similar to man would have appeared on Earth even though evolution had been different, and it should also appear in any other place in the universe where life arises. Conway Morris comes back to this idea in his more recent book Life's Solution: Inevitable Humans in a Lonely Universe.
• That the apparition of man is the goal of evolution, which has been directed by God, as John Polkinghorne asserts in his book Belief in God in an Age of Science.

The first two positions are scientific theories, because they can be shown to be false. To choose between those two options, it would be enough if we find extraterrestrial life and compare it with terrestrial life. I am afraid that the third option is beyond the reach of science, for I do not share the opinion of the defenders of Intelligent Design, who assert that it can be proved scientifically.

The same post in Spanish
Thematic thread on Time: Preceding Next

Manuel Alfonseca