What does passive gravitational mass refer to?

Passive gravitational mass refers to the measure of the strength of an object's interaction with the gravitational field. This concept is grounded in how mass experiences gravitational forces exerted by other masses. When you consider Newton's law of universal gravitation, passive gravitational mass determines how much gravitational force an object will experience when it is placed in a gravitational field produced by another mass.

For example, a more massive object will experience a stronger gravitational force compared to a less massive object when placed in the same gravitational field, illustrating the idea that passive gravitational mass is directly related to the interaction strength between masses under gravity. This explanation aligns with the principles of both classical mechanics and general relativity, where gravitational mass determines how objects respond to gravitational fields and ultimately influences their motion.

The other options do not accurately describe passive gravitational mass. The total energy in a gravitational field pertains more to potential energy considerations, rather than mass itself. The measure of mass in relation to light speed involves concepts from relativity that are not directly about gravitational interactions alone. The density of an object, while related to mass, does not pertain specifically to gravity's effects on the object but rather to its physical characteristics.