What is the magnitude of the gravitational field inside a hollow, spherical planet?

The magnitude of the gravitational field inside a hollow, spherical planet is indeed zero N/kg. This result stems from the application of Newton's law of universal gravitation and the shell theorem.

The shell theorem states that a uniform spherical shell of mass exerts no net gravitational force on an object located inside it. Therefore, if you are anywhere within the hollow sphere, all the mass of the shell surrounding you exerts equal and opposite forces, resulting in a net gravitational field of zero.

This principle highlights how the distribution of mass affects gravitational interactions. For example, if one were to analyze the gravitational effects at the surface of a hollow sphere, where the mass is distributed evenly about the center, the gravitational field would be non-zero, pointing towards the center. However, once you are inside the hollow section, that influence cancels out entirely—hence the gravitational field inside is zero.

Understanding this concept is crucial for appreciating how gravitational fields work in non-uniform mass distributions and can help inform further studies in gravitational physics.