Physics has made in the 19th century progress and striking the community of physicists from the beginning of the 20th century, in a heated debate on the fundamental laws of the Universe. On one side, supporters of a mechanical world obeying the laws of Newton and the other advocates a world based on electricity and magnetism. At this time there is talk of ether present in all space, which spreads the electromagnetic and gravitational phenomena. Einstein's theory of relativity will return to this debate since 1905. In 1665 a man was sitting under a tree when suddenly he saw an apple fall in front of him. With the fall of the apple Isaac Newton revolutionized the whole image of the universe. In a bold assumption for his time, he asserts that the force that pulls the apple to the ground is the same one that keeps the Moon around the Earth. | | Suddenly he unifies heaven and Earth into a single theory he called '**gravity**'. Gravity was the first force that is scientifically understood but 3 other forces would eventually follow and what is that Newton discovered the law of gravitation, there are over 300 years, these equations describing this force are so accurate predictions that we continue to use today. It is they who have allowed scientists to calculate the trajectory of a rocket that took man to the moon.
*NB: In Greek "**ether**" is a philosophical concept whose function is to fill the space that can not be empty. * | | ### Image: Thanks to the seriousness that the sun has captured 99.86% of the total mass of dust and gas of the primordial nebula. Jupiter, the largest planet in the system, has captured 71% of the remaining mass. Other planets have shared the residue of the gravitational evolution, i.e. 0.038% of the total mass. |

With Newton there was a problem, if its laws describe the force of gravity with great precision, yet Newton hides an embarrassing secret, he has no idea how gravity works. Not until the early 20th century a small office employee of Swiss technical inventions will bring a new view of gravity. While studying patent applications, Albert Einstein meditates on the behavior of light, there is no idea that his reveri.e. on the light led him to solve the mystery of the nature of gravity. At age 26, Einstein made a startling discovery. The speed of light is a kind of cosmic speed limit, a speed that nothing in the universe can not exceed. The trouble is that if nothing can travel faster than light, it goes against the Newtonian view of gravity. If a cosmic catastrophe the sun suddenly disappeared completely, the effect on the planets according to Newton is immediate. His theory predicts that if destruction of the sun, the planets would rush out of their orbits to drift into space. Newton envisioned gravity as a force acting instantaneously at any distance. | | Immediately you would feel the effect of destroying the sun, but Einstein saw a huge problem in the theory of Newton. He knew that the light is propagated not instantaneously, it takes sunlight 8 minutes to travel 150 million miles that separates the Earth. But he has demonstrated that nothing, not even gravity can not travel faster than light, how the Earth would leave its orbit before the darkness resulting from the disappearance of the sun is reaching our eyes? For the young scientist's Swiss office nothing can exceed the speed of light and thus the Newtonian view of gravity was wrong. If Newton is wrong why the planets do not they fall? Newton sought to explain why the planets in orbit. But Newton's equations are used to calculate their trajectory. Einstein had to resolve this dilemma. Einstein must imagine a picture of the universe in which gravity does not exceed the cosmic speed limit. In a solitary quest to solve this mystery and after 10 years of reflection, Einstein finally found the answer in a new kind of unification. | | ### Image: Newton's theory predicts that if destruction of the sun, the planets would rush out of their orbits to drift into space. Newton envisioned gravity as a force acting instantaneously at any distance. |

**Einstein** ended up considering the 3 dimensions of space and the single dimension of time, as linked in the same “fabric” of space-time. Planets and stars simply move along the distorted surfaces of this four-dimensional space-time. The fabric of space-time is like the fabric of a "trampoline" distorted by heavy objects. This distortion is what gravity feels like. The image of a trampoline being distorted by objects is often used, however, this is not a correct representation as it is a 2-dimensional image. Celestial objects orbit in a 4-dimensional space-time, including 3 space dimensions and 1 time dimension. We must therefore imagine this deformation in a 4-dimensional space, which is very difficult because it is an abstract and multidimensional concept. However, the presence of mass and energy curves space-time, and this curvature affects the trajectory of objects moving in it. The direction and magnitude of the deformation depends on the distribution of mass and energy in the region. The more concentrated the mass and energy, the greater the deformation. Celestial objects simply follow the curves of the space fabric.
| | Furthermore Einstein calculates that these gravity ripples travel at exactly the speed of light. So with this new approach, Einstein resolved the conflict with Newton over how fast gravity propagates. Einstein will give the world a new picture of what the force of gravitation really is. They are curves and distortions in the very fabric of space and time. He calls this new image of gravity, general theory of relativity. Despite his extraordinary success, the physicist was not yet satisfied and he immediately set another, even higher goal, the unification of gravity with the only other force known at that time, electromagnetism. Electromagnetism had only been unified a few decades earlier by the Scottish James Clark Maxwell (1831-1879). Albert Einstein is convinced that if he manages to unify his new theory of gravity with Maxwell's electromagnetism, he will be able to formulate the famous master equation, the one that can describe the entire universe. He will not succeed, because a group of young scientists, from the 1920s, began to formulate a new theory, quantum mechanics where other forces would appear. | | ### Image: With his new approach, Einstein resolves the conflict with Newton. It will give the world another picture of what the force of gravitation really is. |