What does the Galilean equivalence principle state?

The Galilean equivalence principle states that inertial mass is equal to gravitational mass. This principle underpins our understanding of gravity and motion, indicating that the mass of an object, which determines how it resists acceleration when a force is applied (inertial mass), is the same as the mass that determines the strength of the gravitational force acting on it (gravitational mass).

This means that when considering a freely falling object, the acceleration due to gravity acts on it regardless of its mass, leading to the conclusion that all objects will fall at the same rate in the absence of air resistance. This principle was crucial for the development of Newtonian mechanics and laid the groundwork for further theories, including Einstein's theory of relativity, which expanded upon the concepts of mass and gravity.

In contrast, the other options either introduce concepts not directly related to the Galilean equivalence principle or pertain to different principles in physics, such as the constancy of mass across various gravitational fields or the behavior of light in different frames of reference. The essence of the principle focuses specifically on the equality of inertial and gravitational mass.