According to the theory of special relativity, as an object approaches the speed of light, its relativistic mass increases. This phenomenon is often referred to as "relativistic mass."
The increase in mass is not due to the object gaining more matter, but rather it is a consequence of the energy associated with the object's motion. As an object's velocity increases, its kinetic energy also increases. The relationship between an object's mass (m), its velocity (v), and its relativistic mass (m_r) is given by the equation:
m_r = m / sqrt(1 - (v^2 / c^2))
In this equation, c represents the speed of light. As the object's velocity (v) approaches the speed of light (c), the denominator of the equation approaches zero, resulting in an increase in the object's relativistic mass (m_r).
It's important to note that the concept of relativistic mass is not commonly used in modern physics. Instead, the concept of "rest mass" or "invariant mass" is preferred. The rest mass of an object remains constant regardless of its velocity. Therefore, as an object approaches the speed of light, its relativistic mass increases, but its rest mass remains the same.
In summary, as an object approaches the speed of light, its relativistic mass increases, but its rest mass remains unchanged.