While it is true that the gravitational interaction is the weakest of the fundamental forces, it still plays a significant role in governing the motions of celestial bodies like planets and stars. The reason why gravity dominates these large-scale motions despite its relative weakness can be attributed to a few key factors:
Cumulative Effect: Although gravity is weak compared to other forces like electromagnetism, it has an additive or cumulative effect over large distances. When you have a vast number of particles, such as in a planet or a star, the collective gravitational pull from all those particles can become significant and dominate the motion of the object as a whole.
Long-Range Interaction: Gravity is a long-range force, meaning it extends over large distances without significant decay. Unlike other forces, such as electromagnetism, which rapidly diminish with distance, gravity has an infinite range. This property allows gravity to exert its influence over vast distances, even though it weakens with distance.
Lack of Cancellation: In systems like planetary or galactic motion, the gravitational forces from different components tend to add up rather than cancel each other out. For example, in a planetary system, each planet's gravity contributes to the overall motion, resulting in a combined effect that governs the system's dynamics. Similarly, in a galaxy, the cumulative gravitational pull from all the stars within it dominates the motion of those stars.
Absence of Repulsive Forces: Unlike electromagnetism, which can have both attractive and repulsive components, gravity is purely attractive. The absence of repulsive forces allows gravity to act as the driving force for the aggregation of matter. This is particularly evident in the formation of large-scale structures like galaxies, where the mutual attraction of matter leads to the gravitational collapse and subsequent motion of stars within the galactic system.
So, while gravity may be the weakest of the fundamental forces, its cumulative effect, long-range nature, lack of cancellation, and the absence of repulsive forces make it the dominant force in governing the motions of planets around the sun and stars within galaxies.