Einstein's dissatisfaction with Newton's explanation of gravity stemmed from a few key issues:
Inconsistency with the Theory of Electromagnetism: Newton's theory of gravity, known as classical mechanics, treated gravity as a force acting instantaneously at a distance. However, at the end of the 19th century, it was becoming increasingly clear that electromagnetic phenomena, such as light, were governed by a different set of rules known as Maxwell's equations. These equations described the propagation of electromagnetic waves, suggesting that the speed of light was constant. This inconsistency between the behavior of gravity and electromagnetism bothered Einstein.
Problems with Newtonian Mechanics in Extreme Conditions: Another source of dissatisfaction for Einstein was that Newtonian mechanics could not fully explain certain phenomena observed in extreme conditions. For example, the behavior of light near massive objects like stars and black holes, as well as the motion of Mercury's orbit, deviated slightly from what Newton's laws predicted.
Motivated by these issues and a desire to reconcile the laws of physics, Einstein embarked on his revolutionary path towards the theories of relativity:
Special Theory of Relativity: In 1905, Einstein introduced the special theory of relativity, which revolutionized our understanding of space, time, and motion. This theory challenged the concept of absolute space and time and introduced the notion of the constancy of the speed of light in all inertial frames of reference. Special relativity showed that space and time were interconnected and could no longer be considered separate entities.
General Theory of Relativity: Building upon the foundations of special relativity, Einstein spent a decade developing the general theory of relativity, which he published in 1915. This theory provided a new understanding of gravity. Instead of treating gravity as a force, Einstein proposed that it arises from the curvature of spacetime caused by the presence of mass and energy. In the general theory of relativity, the motion of objects is influenced by the curvature of spacetime, resulting in the familiar effects of gravity.
Einstein's dissatisfaction with Newton's gravity, combined with his deep curiosity and relentless pursuit of fundamental truths, motivated him to challenge conventional wisdom and develop groundbreaking theories that transformed our understanding of the universe. His insights into relativity revolutionized physics and remain fundamental pillars of modern physics to this day.