In a bold hypothesis for its time, **Isaac Newton **(1643-1727) stated in 1687 that the force that pulls the apple to the ground is the same one that keeps the Moon around the Earth. It unifies the heavens and the Earth in a single theory he calls 'gravity', the word "weight" (gravitas) in Latin. However he has no idea how it works. In the 19th century, there is one side the supporters of a mechanical world obeying the laws of Newton and other, the partisans of a world based on electricity and magnetism. At that time we talk about **ether**, present in all space, which can not be empty, in which propagates electromagnetic and gravitational phenomena. Einstein's theory of relativity will return to this debate since 1905. So in the early 20th century a small office employee of Swiss technical inventions will solve the mystery of the nature of gravity. At the age of 26 Albert Einstein (1879-1955) will make a startling discovery. The **speed of light** is the speed limit, that nothing in the universe can not exceed. | | But if nothing can travel faster than light, then it goes against the vision of Newtonian gravity. Yet Newton's equations allowed to calculate the trajectory of planets perfectly round the sun. Newton's theory predicts that if destruction of the sun, the planets would rush out of their orbits to drift into space. He envisioned gravity as a force acting instantaneously at any distance. Einstein knew that the light does not spread instantaneously. At 299 792 km/s, it takes 8 minutes for sunlight to travel 150 million miles, which separate them from Earth. Given that nothing, not even gravity, can travel faster than light, how the Earth could leave its orbit before the darkness resulting from the disappearance of the sun, is reached before our eyes? Einstein would solve this dilemma by giving a picture of the universe in which gravity does not exceed the cosmic speed limit. While continuing to study the patent the day it launches into an epic quest to solve this mystery. | | ### Image: **Einstein** was born March 14, 1879 in Ulm, Württemberg in Germany, and died April 18, 1955 in Princeton, New Jersey United States. Einstein gave a **picture of the universe** in which gravity does not exceed the cosmic speed limit. |

Einstein binds the three dimensions of space and the single dimension of time, in the same tissue of Spacetime. This four-dimensional tissue of space-time looks like the surface of a trampoline, stretched by the planets and stars. It is this distortion or curvature of Spacetime, which creates what it feels like gravity. A planet like Earth is in orbit because it simply follows the curves of the tissue space, distorted by the presence of the Sun and planets. With this new understanding of gravity we know now that if the sun disappears, as in our cosmic catastrophe, the gravitational disturbance that results, will form a wave that propagates through the tissue space in the same way as ripples spread on the surface of the water when you throw a pebble into a pond. So we will not feel any change in our orbit around the sun until the wave reaches the Earth. Einstein calculated that most of these gravity waves travel at the exact **speed of light** 299,792 km/s. Einstein will give the world a new image of what is really the force of gravity. | | These curves and distortions in the tissue of space and time. He calls this new picture of gravity, general relativity. Despite its extraordinary global recognition, Einstein set another objective immensely high, the unification of gravity with the only other known force at the time, **electromagnetism**. The electromagnetism was unified as a few decades ago (1864), by Scotsman **James Clerk Maxwell**, through 4 mathematical equations. This unified theory of electromagnetism, explains the behavior of charges and electric currents, magnets or electromagnetic waves, such as light or radio waves. Einstein was convinced that if he can unify his theory of gravity with electromagnetism, it will be able to formulate an equation that can describe the whole Universe. But it will not succeed, for already a group of young scientists in the 1920s, would devise a new theory, string theory, a new way of thinking and very eccentric physics. | | ### Image: This four-dimensional tissue of Spacetime, like the surface of a trampoline, stretched by the planets and stars. It is this distortion or curvature of space time, which creates what it feels like gravity. |