The understanding that gravity distorts space and is not a force in the traditional sense is based on the general theory of relativity, which was developed by Albert Einstein in the early 20th century.
In Einstein's theory, gravity is not viewed as a force that acts between masses, but rather as the curvature of spacetime caused by the presence of mass and energy. According to general relativity, objects with mass or energy curve the fabric of spacetime around them. This curvature affects the motion of other objects, causing them to move on curved paths.
The concept of spacetime curvature can be difficult to visualize, but an analogy often used is to imagine a rubber sheet stretched out. When you place a heavy object, such as a bowling ball, on the sheet, it creates a depression or curvature in the sheet. If you were to roll a smaller object, like a marble, nearby, it would naturally move toward the bowling ball along a curved path due to the curvature of the sheet. In this analogy, the bowling ball represents a massive object like the Sun, and the marble represents a smaller object like a planet.
The predictions of general relativity have been extensively tested and confirmed through various experiments and observations. For example, the bending of light by gravity, known as gravitational lensing, has been observed and measured. The precise predictions made by general relativity for the motion of planets, the deflection of light, and the behavior of clocks in strong gravitational fields have all been confirmed with high accuracy.
Additionally, the discovery of gravitational waves in 2015 provided direct evidence for the existence of gravitational waves, which are ripples in spacetime caused by the acceleration of massive objects. The detection of these waves supports the predictions of general relativity.
In summary, our understanding that gravity distorts space and is not a force in the traditional sense is based on the comprehensive and successful framework of general relativity, which has been supported by numerous experiments and observations.