According to the theory of relativity, as an object with mass accelerates and approaches the speed of light, its mass increases. This phenomenon is known as relativistic mass. The increase in mass is a result of the object's energy increasing as its speed approaches the speed of light.
However, it's important to note that the concept of relativistic mass is not commonly used in modern physics. Instead, physicists prefer to use the concept of rest mass or invariant mass, which is the mass of an object as measured in its own rest frame.
In the theory of relativity, the rest mass of an object remains constant regardless of its velocity. So if a particle were to be accelerated to the speed of light and somehow maintained at that speed, its rest mass would remain unchanged. However, this scenario is purely hypothetical as it would require infinite energy to accelerate a particle with mass to the speed of light, as discussed earlier.
It's also worth mentioning that particles with rest mass, such as electrons or protons, cannot reach or exceed the speed of light due to the relativistic effects discussed previously. Massless particles, like photons, always travel at the speed of light and have zero rest mass.