Newton's Gravity vs Einstein's Gravity

Newton's gravity

Why does an apple fall from a tree towards the ground and why does the moon orbit around the Earth? In the 17th century, mathematician and physicist Isaac Newton answers these two questions with a single answer “gravitation”. So what is gravitation? In the universe all objects that possess a mass attract each other. For example the moon attracts the earth and the earth attracts the moon. Now let us imagine a first object A and a second object B separated by a certain distance. Because of gravitation, these two objects attract each other; in this way A will get closer to B and B will get closer to A. If A was more massive then the gravitational pull would be stronger inversely if A was less massive and the gravitational pull would be weaker, this is equally true for B. Following on, if the two objects were more distant the gravitational pull would be weaker and if they were close the gravitational pull would be stronger as such the gravitational pull of one object on another goes away when the two objects are very far apart. Let's now take the example of an apple. The Apple possesses a certain mass and so does the earth so the Apple attracts the earth and inversely the earth attracts the Apple. This is why the Apple and the earth will get closer one to the other up until the Apple touches the ground. Now let's imagine that we throw the Apple in a direction with a certain speed like before the Apple will be attracted to the earth but it will also move in the direction in which it was thrown depending on its speed. The faster the Apple is thrown, the further the Apple falls. Now if we could throw the Apple with a high enough speed we would see that this velocity would counter the gravitational pull and that the Apple would never fall down on the ground this is what happens with the moon and that is why the moon orbits around the earth as well as why the earth orbits around the Sun.

Einstein's gravity

As we stated earlier, in the 17th century, Newton discovers universal gravitation with this he manages to predict the movement of an apple falling from a tree or the orbit of the moon around the earth. However, Newton does not manage to explain why gravity exists nor how it acts. To answer these questions it is necessary to wait for the early 20th century with Albert Einstein's theory of “general relativity”. For Einstein, the three dimensions of space in which we move and time in which we evolve are just two parts of a same thing space-time. In space-time, objects exist in a given place at a given time. Let us try to conceptualise space-time as a grid on which we could place an object A. when A is placed on this grid it distorts it according to its mass. Indeed, the more massive the object the more it will distort space-time. Inversely the less massive the less space-time will be disturbed. Now let's imagine that we place a second object B on this grid. B distorts space-time, as well, since it possesses a mass but less than a since its mass is not as great. What happens if we release a marble in a bowl? because of the bowls curvature, the marble will slide along the inside of it until it comes to rest at the center. In a similar way, when we think about space-time in distorting the grid, A creates a sort of bowl around itself consequently just like the marble, B will slide along this bowl gradually edging closer to A. If we look at this scene from above we observe that B is attracted by a just like with Newton's model of gravity. If we were to release B in this bowl with a certain speed it would go around the bowl while getting closer to A. Finally, if we could release B with a sufficiently great speed B would begin to go around a without ever getting closer to it it is in this way that planets orbits around the Sun. Just like with Newton, Einstein's theory predicts the movement of planets around the Sun. However, being more thorough this theory has also enabled the discovery of many other scientific novelties like the fact that a very important density can lead to the appearance of what we call a black hole.