The question of gravity has been of interest to scientists since antiquity. But a definite explanation of gravity is yet to surface. Even its most accomplished expression, that of Einstein’s general relativity, does not explain everything.
Why do we fall? This question is quite unexpectedly one of the most fundamental questions in the history of science. Many answers have been provided throughout the ages with, almost every time, a completely new way of understanding the world around us. Today, despite our many discoveries and advanced technology, the debate around gravity is far from settled. To understand where we are, we must know where we come from. And the history of gravity is closely linked to that of cosmology.
As often in science, we must go back to Aristotle, in the fourth century BC, to find one of the first interpretations. For the Greek philosopher, matter is composed of four elements of a different heaviness. Earth, the heaviest of the four, then water, air, and finally fire, which does not fall but rises. According to Aristotle, the nature of heavy bodies is to seek to reach the center of the Earth. Hence the latter’s spherical shape. This rule, however, applies only to the sublunary world, which extends to the moon. The superlunary world responds to other rules. But one thing is certain: the Earth is at the center of the universe, and all the celestial bodies revolve around it. A model for the least rudimentary, which will continue in more and more complex versions for fifteen centuries.
However, the problem with this model where celestial objects rotate in circular rotation around the Earth is that it does not withstand the test of astronomical observations. Around 140 AD, Ptolemy described a complex epicycle system, still centered on the Earth. In his model, the trajectory of the stars is built from several circles nested within each, which he used to correctly predict their position. It wouldn’t be until Copernicus, or rather his death in 1543 since he didn’t publish his model during his lifetime due to the stern opposition of the Catholic Church on the subject, to discover a simpler system, centered on the sun.
At the beginning of the next century, Kepler also adopted a heliocentric model with elliptic trajectories, which is even better. But it is of course Galileo and his famous astronomical telescope that definitively buries the model of Aristotle. Galileo is also the one who established for the first time, at the beginning of the seventeenth century, the non-intuitive principle of equivalence according to which the speed in free fall is identical for all bodies.
Gravity and the movement of the planets will finally meet with Isaac Newton and his publication in 1687 of Principia mathematica along with his famous universal law of gravitation. He established that the force acting between two objects is proportional to the product of the two masses and inversely proportional to the square of the distance that separates them. The formula works for a falling apple and for the moon that turns around the Earth. Well, there is little Mercury who likes to deflect a tiny bit of the predicted trajectory, but we will not bother for so little, right?
Well, not so fast! At the beginning of the twentieth century, Albert Einstein began from the principle of equivalence and these bodies in free fall that do not feel acceleration to lead to this incongruous but so powerful idea of space-time curvature in the presence of a field of gravitation. The resulting equations of general relativity, much more complex than those of Newton, allow predictions of an unprecedented accuracy, including that of the trajectory of Mercury.
Physicist John Wheeler later summarizes Albert Einstein’s theory as follows: “Space-time tells the subject how to move. Matter tells space-time how to bend.” However, there are still some areas of darkness on the issue of gravity, particularly the theory that is currently developing and which attacks the infinitely small world of quantum physics.
This new theory is particularly effective in predicting what happens on the atomic scale. While it and Einstein’s theory cohabit rather well most of the time, it is not at all the case in the extreme situations predicted in black holes and at the very beginning of the universe, just after the big-bang. For several decades, physicists in the field have been searching for a unifying theory that proposes a quantum approach to gravitation. There are several candidates, but we continue here to fail like by fault of gravity: none to date has been experimentally validated.