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.

The young aspiring mathematician took notes with impeccable care. He knew his German friend would also appreciate it. He didn't know it yet, but that classmate sitting next to him in geometry class would eventually become one of the most brilliant minds of the 20th century: Albert Einstein. Sherlock Holmes had John Watson, Don Quixote had Sancho Panza, and Einstein always had Marcel Grossmann by his side, the faithful companion who not only shared his notes but also became a key figure in the development of a theory that would change our understanding of the universe. "Those notes were my salvation," Einstein explained.

The Universe is expanding, and it's doing so ever faster, although astronomers don't quite know why. To find out, a team of cosmologists from the University of Toulouse III - Paul Sabatier and the University of Geneva have put the current model of the Universe predicted by Einstein's own theories to the test. The results, published in the magazine Nature Communications, indicate that the theory of general relativity, which the German physicist developed more than a hundred years ago, may be insufficient to accurately describe the evolution of the universe. "We have enough evidence to believe that Einstein's theory is not definitive," says Isaac Tutusaus, a Catalan cosmologist at the University of Toulouse III - Paul Sabatier and co-author of the study.

The dining room of the Hotel Métropole in Brussels gave off the usual aroma of morning coffee. The most brilliant minds of the early 20th century gathered around the tables, deep in intense and profound conversations, commenting on and refuting the presentations given the day before during the final session of the Fifth Solvay Conference. This series of meetings between eminent scientists was initiated by the Belgian chemist Ernest Solvay and, since 1911, was dedicated to delving into the most complex problems of physics and chemistry.