In early formulations of special relativity, the concept of relativistic mass was introduced to describe how an object’s mass appears to increase as its speed approaches the speed of light. According to this idea, the faster an object moves, the more massive it becomes, requiring more and more energy to accelerate it further. This explains why no object with mass can reach the speed of light—it would require infinite energy.
However, modern physics prefers the concept of invariant mass (also called rest mass), which remains constant regardless of the object’s speed. Instead of saying the mass increases, physicists now describe the increase in energy and momentum through Einstein’s famous relation:
Total energy² = (momentum × c)² + (rest mass × c²)²
This approach keeps mass as a fixed property of a particle and attributes the effects of motion to changes in energy and momentum, not mass itself. The use of invariant mass simplifies many aspects of relativistic physics and aligns better with the framework of quantum field theory and particle physics.