Sir Isaac Newton developed the theory of gravity and relativity. He decided that there was a force that drew objects toward the centre of the earth but at the time he made his observations he didn't know what the power of this force was. In August 1684, when Newton was 41, Halley, a good friend and the man Halley's Comet is named after, put a question to Newton: supposing an inverse square law of attraction toward the sun, what curve would a planet make? Newton then said an ellipse: he had calculated this a long time ago (when he was 23). He started working again on it and several months passed before providing Halley with a proof.

He had great difficulties because there was no vocabulary associated with this work. So, he invented it: he defined Quantity of matter as mass, and Quantity of motion as the product of velocity and mass. He then send Halley a 9-page treatise "On the motion of bodies in orbit". Halley was excited and asked Newton for more pages, but Newton was not finished. Eventually he established that this mysterious force was gravity.

Albert Einstein picked it up from there. He was 21 years old at the turn of the 20th century, 1900. By then Newton's theory of motion gravity had dominated physics, providing the tools to explain the everyday phenomena in our world and explaining any almost physical problem imaginable. However, there were tiny discrepancies that did not fit in the theory. This was a typical case were in order to explain these small discrepancies a major breakthrough is required; a huge jump in our understanding of the world.

In terms of experiments, the astronomers had found that the orbit of Mercury around the sun is not a fixed ellipse; it moved slightly as the years passed. In Newton's theory, this result was not predicted since all the ellipses should be fixed. It was then thought that because Mercury is so close to the huge mass of Sun, it might be affected in a gravitationally different way. Einstein managed at 1916 to find a satisfying theory that would explain this fact, but in order to get a glimpse of it we should go a few years back and look at his theory of special relativity first.

In 1905, at the age of 26, Einstein managed to extend Newton's theory for the motion of bodies at high speeds; by assuming that the speed of light is constant no matter how one is traveling (at a constant speed) he derived an extension to Newton's equations which holds true for every speed, not only for low ones. Newton was still correct in that limiting case. These results however were valid only if the objects are moving at constant velocity; a replacement of Newton's send law which describes what happens when an object is accelerated was yet to be found. Einstein was the first person responsible of extending his theory of zero acceleration to a theory of non-zero acceleration.

He stated that the gravitational field is not actually a force as Newton has described, but instead a curvature in space. To put it in simple words, the bodies are affected by gravity not because of a force directly exerted on them but because space is curved and therefore they have to follow space's grid. The presence of mass or energy does not affect the bodies directly; it affects the space first, and then the bodies move in this curved space. Earth always moves in a straight line (not in the Euclidian sense though). The presence of sun curves space, and therefore curves this straight line and forces earth to appear to be moving in an ellipse.

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