What is time?

Everyone knows what is time but nobody knows what is the nature of time, yet we always talk of time as metaphors or tautologies. We often confuse time with temporal phenomena and so we create as many definitions of time, that's what Etienne Klein said in his conference "Le temps existe-t-il ?".
"If no one asks me, I know, but if you ask me and I want to explain, so I do not know anymore" in Book XI of the Confessions of St. Augustine of Hippo, or Saint Augustine (354 - 430).
Time is linear, so it is impossible to revive things the same way, it never returns twice the same point. However, it seems that we all have a different way of feeling the time because every moment of our life is marked by a continued succession of unique events. Time is what passes when nothing passes, said Jean Giono (1895-1970). Like all objects in the universe are in motion, is a way of saying that time does not exist?
But around us things move, change, grow age and therefore there is something passing.
For a long time, the rotation of the Earth on its axis represented the most accurate measure of the "passage of time". But since Einstein we know that time is space and our Earth is "tossed" on the four-dimensional fabric of space-time, as on the surface of a modified gravity of the planets giant trampoline, the sun, the stars and the galaxy. Thus the Earth is moving in the vast Universe according to the fluctuations of space-time, leaving perceive no trace of the entities that conduct. We fall with it into infinity, drawing a series of points along spirals endlessly different. We never pass back by this point of space-time where we are currently. This unique place in space-time belongs to the present and immediately disappears into the past, replaced by a new present moment, a new point on the trampoline of space-time. It's thus we still continue our path toward an uncertain future.

In addition, since 1905 we know that there is no universal reality of time, for each observer, all moments are unique. Albert Einstein relativized the absolute nature of space and time, the movement causes a slowing of time. Thus space and time are related, the space is a bit of time and the time, a bit of space.
We move on Earth that travel (30 km / s) around the Sun that moves (230 km / s) around the Milky Way. The Milky Way falls (90 km / s) to his turn to the Andromeda galaxy, each of these galaxies are rushing (45 km / s) toward the center of the Local Group, our galaxy cluster. The Local Group is attracted (600 km / s), by cluster of galaxies in the Virgo supercluster and of the Hydra and Centaurus, which in turn fall to another, large agglomeration of clusters of galaxies called the Great Attractor ...
Planets, stars, galaxies undergo the course of time. The universe expands, the past can never be replayed and our days are a succession of causes and effects from the "beginning" of the Universe. Never contradicted by experience, the causality principle states that if an event (the cause) produces another event (the effect), then the effect can not precede its cause. This is so that time is linear and not cyclical. Cyclical time is impossible because even if the initial conditions of the early universe is replayed, because it is a dynamic system any small change would cause unpredictable results in the long term (the work of Henri Poincaré on the phenomenon sensitivity to initial conditions). It is therefore impossible to relive things in the same way because any phenomenon is the effect of a cause that precedes it. With the principle of causality, the past is unalterable, it will be eternally true, it can not be changed. We never pass twice by the same time. In other words, buffeted by gravity, for each of us, every day is unique.

Image: The movement of our planet pace the passage of time, but the nature of time is one of the greatest mystery of physics. There is no definition of the time, yet we use it in many metaphors. Scientists wonder more and more about the nature of time, "What is time? "And even" is time exists? ". Image taken by Apollo 8, the first mission to be carried men beyond Earth orbit between 21 December 1968 and 27 December 1968.

We are not inert but alive and we travel in the middle of this great cosmic ballet as conscious observers.
On board our spacecraft, our body continuously receives information to analyze, a lot of information, hundreds of billions of information per second. Our bodies and especially our brains keep only a few thousand of information, those interested us most, those that will allow us to feed us to stay alive, to grow in our environment and to reproduce ourselves through time. Selfishly our sense filter reality and information that we record are those concerning our bodies, our environment, our space and time. To pass the time and evolve toward greater complexity, our conscious brain death needs benchmarks, so it introduces temporality, and for that it does need the past, present and future.
Besides, is it not our brain that creates time, as some scientists think?
Maybe when we are immortal, we will get rid of the time, but for now we age by using our memory, we need to feel the time, to hear the ticking of our clocks, to remember the past, to understand the present and to perceive the future. Because the nature of time escapes us, it is not time that we perceive but entropy variations, i.e. change, become things, aging, wear, but no time itself.
Entropy is a thermodynamic function, this state variable is used to characterize the disorder in a system.

Image: Living beings are thermodynamic open systems, they exchange matter and energy with the external environment. The total entropy of a system will only grow indefinitely, this is the second principle of thermodynamics. The system will never go back. Credit image astronoo.com