A (nearly) perfect law
Newton’s Law of Gravitation was, and to a very good approximation still is, a triumph. It predicts the gravitational force of apples falling towards the Earth, rockets and spacecraft trying to leave it, the moon and comets orbiting it. On a larger scale , it predicts the motion of planets around the sun and other stars moving as part of galaxies. Over 20-plus orders of magnitude, it gets things pretty much correct, packaged up in one neat little equation. It allowed us to send spacecraft and astronauts to the moon and was even used to predict where Neptune should be, even before it was ever observed.
But what it doesn’t do is tell us why gravity exists or what causes it. It is excellent at predicting the effects of gravity; in that respect it is an ideal tool. Gravity is the mutual force of attraction between two objects. But why is that, and what causes it? Newton knew of the law’s limitations and said that the secrets of why and what lay in the realm of ’a supreme being’.
It wasn’t until Einstein formulated his General Theory of Relativity in 1915 that some light was shed on the ’why?’ question. (The justification for General Relativity comes from the The Principle of Equivalence.) In GR, grativational acceleration arises from the curvature of space-time (a 4-d arena in which all physical events take place). Spacetime is disturbed by massive objects distorting its ’smoothness’; the heavier the object, the greater the distortion and the the ’stronger’ the gravitational acceleration that results. As Einstein put it in his original paper :
"matter tells space how to bend; space tells matter how to move".
But even GR doesn’t solve all the problems. There are major issues in trying to apply GR on the lengthscale of atoms and sub-atomic particles. This is the realm where Quantum Mechanics is king and offers a description of the other three fundamental forces that act on these small lengthscales. Gravity can be integrated into quantum mechanics with the introduction of a hypothetical elementary particle called a graviton; this is then an answer to our question of ’what?’ The graviton mediates the force of gravity, in an analagous way to the electromagnetic force being mediated by the photon. The problem is that it is impossible (on a practical level, not a theoretical one) to detect one of these particles. Attempts to detect coherent states of many gravitons – so-called gravitational waves – have likewise not been successful.
Gravity - despite everyone’s familiarity and firsthand experience of it in the world around us - is still far from fully understood.