According to the theory of special relativity, as an object approaches the speed of light, its mass does not increase or decrease in the way classical Newtonian physics would suggest. Instead, its mass remains constant, and other properties, such as energy and momentum, change.
In classical physics, mass is considered to be an invariant quantity, meaning it does not change with velocity. However, in special relativity, the concept of mass is better understood as relativistic mass or rest mass. The rest mass of an object is the mass it has when it is at rest, and it remains the same regardless of its motion.
As an object accelerates and approaches the speed of light, its energy and momentum increase. This is described by the equation:
E = mc^2 / sqrt(1 - (v^2/c^2))
Where: E is the relativistic energy of the object, m is the rest mass of the object, c is the speed of light in a vacuum, v is the velocity of the object.
As the velocity (v) of the object gets closer to the speed of light (c), the denominator of the equation approaches zero, resulting in a larger energy value for the object. However, the rest mass (m) remains constant throughout.
It's worth noting that the concept of relativistic mass is not commonly used in modern physics. Instead, physicists prefer to work with rest mass and consider energy and momentum as the fundamental quantities.