The evidence for gravity acting between non-celestial and non-atomic objects, such as the two bowling balls scenario you mentioned, comes from several sources:
Newton's Law of Universal Gravitation: Sir Isaac Newton formulated the law of gravity, which states that every particle with mass attracts every other particle with mass through a force called gravity. This law has been extensively tested and validated through experiments and observations. The force of gravity between two objects depends on their masses and the distance between them, as described by Newton's equation.
Free Fall and Acceleration: When objects are dropped or fall freely under gravity, they accelerate at the same rate regardless of their mass (neglecting air resistance). This principle was famously demonstrated by Galileo Galilei by dropping objects of different masses from the Leaning Tower of Pisa. The consistent acceleration observed in free fall is a direct consequence of gravity acting on all objects equally.
Cavendish Experiment: In 1798, Henry Cavendish conducted an experiment using a torsion balance to measure the gravitational attraction between two small lead spheres. By carefully measuring the small twist in the balance, Cavendish was able to determine the gravitational constant and confirm the existence of gravity as a force between objects.
Satellite and Spacecraft Orbits: The behavior of satellites and spacecraft in orbit around the Earth or other celestial bodies provides strong evidence for the existence of gravity. These objects follow elliptical orbits determined by the gravitational attraction of the massive bodies they orbit. Precise calculations and observations of satellite motions have consistently confirmed the predictions of gravitational theory.
While gravity is less noticeable in everyday scenarios compared to celestial or atomic scales, its effects are still present and measurable. The force of gravity between two bowling balls on a table is relatively weak compared to, say, the gravitational force between the Earth and the Moon, but it is still there. However, the gravitational force between everyday objects is often overshadowed by other forces, such as friction or the normal force from the table, making it less noticeable in those situations.
Overall, the evidence for gravity acting between objects beyond celestial or atomic scales is supported by centuries of scientific study, empirical observations, and experimental measurements.