The law of conservation of mass and energy, which is a fundamental principle in physics, states that the total mass-energy of a closed system remains constant over time. This principle is encompassed by the more general concept known as the law of conservation of mass-energy.
When considering the increase in mass of an object as it approaches the speed of light, it is important to clarify a few points. According to Einstein's theory of relativity, specifically the theory of special relativity, mass is not a fixed quantity but rather depends on the relative motion between the observer and the object. The term "relativistic mass" is sometimes used to describe this phenomenon.
As an object accelerates and approaches the speed of light, its relativistic mass appears to increase from the perspective of an observer who is at rest relative to the object. However, this increase in mass does not imply the creation or destruction of matter or energy. Instead, it is a consequence of the object's kinetic energy.
In special relativity, the relationship between an object's rest mass (also called invariant mass or simply mass) and its relativistic mass is given by the famous equation:
E = mc^2,
where E represents the total energy of the object, m is its rest mass, and c is the speed of light in a vacuum.
As the object's speed increases, its kinetic energy increases, which in turn contributes to the observed increase in mass. However, the total mass-energy of the system, including both the rest mass and the relativistic mass, remains conserved.
It's worth noting that the concept of relativistic mass is not commonly used in modern physics. Instead, physicists tend to focus on the rest mass of objects and work with the equations and principles that are based on it. The idea of relativistic mass was a historical stepping stone to the development of special relativity but is now considered less useful for describing physical phenomena.