Here's why:
* Relativistic Mass: The concept of "relativistic mass" is often used to explain the increase in energy and momentum of a particle as it approaches the speed of light. However, this concept is not universally accepted in modern physics.
* Rest Mass: The intrinsic mass of a particle (its "rest mass") remains constant regardless of its speed. This is the fundamental property of the particle and doesn't change.
* Length Contraction: What *does* change is the length of the particle along the direction of its motion. This phenomenon is called length contraction. However, this contraction is only observable by an observer in a different frame of reference. The particle itself does not experience any change in its size.
In summary:
* The mass of a particle does not change with its speed.
* The particle does experience length contraction along the direction of its motion, but this is a relativistic effect and not a change in its intrinsic size.
It's important to distinguish between the concepts of "mass" and "size" when discussing relativistic effects.
