Travelling to the past?

S.Augustin, by Louis Comfort Tiffany
Lightner Museum

In his Confessions (Book XI, chapter 14), St. Augustine wrote these words, still valid today:

What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.

In the current situation of our scientific and philosophical knowledge, we still don’t know what time is.

·         For classical philosophy and Newton’s science, time is a property of the universe. Therefore, time would be absolute.

·         For Kant, time is an a priori form of human sensibility (i.e. a kind of mental container to which our sensory experiences adapt).

·         For Einstein, time is relative to the state of repose or movement of each physical object. There is, therefore, no absolute time.

·         For the standard cosmological theory, there is the possibility to define an absolute cosmic time for every physical object, measuring the time distance since the Big Bang to the present.

·         For the A theory of time (using J. McTaggart’s terminology) the flow of time is part of reality. The past no longer exists. The future does not yet exist. There is only the present. If the A theory is correct, travel to the past is impossible, because you cannot travel to what does not exist.

·         For the B theory of time, the flow of time is an illusion. Past, present and future exist simultaneously, but for each of us the past is no longer directly accessible, and the future is not yet accessible. Einstein adopted the B philosophy of time. In a condolence letter written to someone who had lost a beloved person, he wrote the following:

The distinction between past, present and future is only a stubbornly persistent illusion.

Time A or Time B?

J.M.E. McTaggart, who in 1908 coined
the terms "A and B-theories of time."

Human beings seem to have an innate tendency to think that what we do not like or cannot explain does not exist. Thus in Hinduism and Buddhism, reality itself is considered an illusion (maya), something that must be discarded to achieve liberation. According to this philosophy, since it is an inseparable part of physical reality, time should also be considered as an illusion. In the Hindu Brahman and the Buddhist Nirvana, time does not exist.

In Western philosophy and science, the idea of ​​time has traditionally been quite different. Until the eighteenth century, nobody put in question the reality of reality. As an inseparable part of reality, time was absolute. In Newtonian mechanics, time plays that role. According to his theory of gravitation, the course of time is independent of the motion of the observer. Hence one can deduce the principle of relativity of classical mechanics: when several bodies are subjected to uniform rectilinear motion (at constant speed) it is impossible to distinguish which one is at rest and which is moving.