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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.
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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.
