As we well know, humankind has always wondered why things fall and why the planets stay in orbit. For centuries the answer was Newton’s: an invisible force, gravity, that pulls bodies toward one another. Then, in 1915, Albert Einstein arrived, and that explanation — brilliant as it was — was no longer enough. In its place he proposed a theory with truly unique features: general relativity. The idea is as simple to state as it is revolutionary to accept. Space and time are not a fixed stage on which objects move: they are a single fabric, spacetime, which matter and energy curve. That curvature is gravity. A planet is not “pulled” by the Sun: it follows the dip the Sun has left in spacetime, like a marble rolling on a stretched sheet. No invisible ropes, no magic: geometry. The theory accounts for phenomena that classical gravity left unresolved. Light bends near an enormous mass: that is the effect confirmed during an eclipse in 1919, the one that made Einstein a worldwide celebrity. Time runs more slowly where gravity is stronger: without this correction, GPS systems would be off by kilometres. It predicts black holes, objects so dense that they close spacetime in on itself. It predicts gravitational waves, ripples in the cosmic fabric, first measured in 2015 and now a routine tool of astronomy. It is not a laboratory accessory, and you cannot wear it. It is a map of the world: it serves those who build satellites, those who observe the universe, and anyone who wants to understand why an apple falls and why a galaxy turns. It is more than a century old and it continues to pass every test. Few human “inventions” can say the same. If you are interested in publishing one of your own (and wish to protect it properly first), contact us at the telephone numbers given at the bottom of the page, or write to us using the ELECTRONIC FORM. Visitors are reminded that the texts published in this section are protected by copyright, which forbids their total or partial reproduction. The scientific theories presented here belong to the heritage of knowledge; their presentation on these pages is protected as an original work.