It has always been a source of inspiration for artists to play with time and the impossibility of returning to the past. In In Search of Lost Time, a long, slow novel with exquisite prose, Marcel Proust recalls his childhood. It is the evocation that makes the story attractive, that makes it unique, that transports the reader to a past that no longer exists, but which can still be imagined and dreamed of.Let's think of a long line. At one end are subatomic particles, and at the opposite end, the universe, distances measured in light-years, and black holes – concentrations of mass with such gravitational pull that not even light can escape them. Until the last century, only a part of the reality represented on this line was known to us: we did not have the technical means to observe the very small or the very large.Now, however, we even know that time slows down in the presence of mass. It is an experimentally proven fact: for the man who lives in the mountains, further from the mass of the Earth, time passes more quickly than for the one who lives on the plain. When we move at high speed, time also changes: it slows down. If we could travel in a spaceship at the speed of light, we could explore the universe without time passing for us. The comparison of two clocks—one at rest on Earth and the other orbiting at high speed, between 20,000 and 30,000 km/h, in an artificial satellite—shows that time shrinks and stretches. It has ceased to be a fixed and universal constant.Albert Einstein formulated it in the theory of relativity years before being able to prove it; the ability to understand before seeing constitutes the essence of scientific thought. The physicist postulated that the only universal constant is not time or space, as was believed until then, but the speed of light. And he raised a problem that was more philosophical than physical: what is the meaning of now and later if for each of us time is different?Until the 18th century, there were two predominant views regarding time. For Aristotle, it was the measure of change: without events, time did not exist. For Isaac Newton, time was immutable, independent, and constant: without changes, time flowed at a constant speed. The principles that Newton formulated in the 17th century, in his theory of universal gravitation, made it possible to explain the movement of the planets. But Newton was not able to explain more than the part of reality that he could see.