According to the theory of relativity, as an object with mass accelerates towards the speed of light, its mass appears to increase from the perspective of an outside observer. This concept is known as relativistic mass.
The increase in mass is a consequence of the energy required to accelerate an object. According to Einstein's famous equation, E = mc², energy (E) is directly proportional to mass (m) and the square of the speed of light (c²). As an object accelerates, its kinetic energy increases, and therefore, its mass increases as well.
This effect becomes more noticeable as the object approaches the speed of light. As the object's velocity approaches c, the increase in mass becomes significant, requiring increasingly more energy to continue accelerating. The object's inertia also increases, making it harder to further increase its velocity.
It's important to note that from the perspective of an observer traveling with the object, the object's mass remains constant. The concept of increasing mass is a relativistic effect observed by observers in different reference frames.