According to relativity theory, mass is equivalent to what?

Mass is equivalent to energy according to Einstein's theory of relativity, which is articulated in the well-known equation E=mc². This equation implies that mass can be converted into energy and vice versa, establishing a profound relationship between the two quantities. Energy can be thought of as a measure of a system's ability to do work or produce change, and mass is an intrinsic property of matter that quantifies the amount of material an object contains.

In the context of relativity, mass and energy are interchangeable; for instance, when an object is accelerated to near the speed of light, its relativistic mass effectively increases, requiring more energy to continue accelerating it. Thus, this equivalence is a fundamental concept in physics that reveals how mass and energy influence the behavior of objects in the universe, particularly at high speeds or in strong gravitational fields.

This understanding also underpins many practical applications, such as nuclear reactions, where a small amount of mass is converted into a significant amount of energy. Therefore, recognizing mass as equivalent to energy is pivotal in grasping the broader implications of relativity in both theoretical and applied physics.