According to Albert Einstein's theory of special relativity, as an object with mass approaches the speed of light, its time dilation becomes significant. Time dilation means that time appears to pass slower for the moving object relative to a stationary observer.
However, it's important to note that for an object with mass, it is impossible to reach or exceed the speed of light. As an object with mass accelerates towards the speed of light, its relativistic mass increases, requiring more and more energy to continue accelerating. The amount of energy required to accelerate an object with mass to the speed of light becomes infinite, making it physically impossible to achieve.
Therefore, while it is interesting to explore the theoretical implications of time dilation at the speed of light, it does not apply to objects with mass in reality.
In regards to your question about the decaying process of an object, if we were to consider a hypothetical scenario where an object with mass could reach the speed of light, time would appear to stop for that object from the perspective of an observer at rest. However, it's important to note that this scenario is purely theoretical and not achievable in practice.
In such a scenario, if time were to stop for the object, it would imply that no further processes or changes, including decay, would occur from the object's perspective. However, from the perspective of an observer at rest, the object's decay would still occur at its normal rate.
It's worth emphasizing that these scenarios and discussions are highly theoretical and primarily serve as thought experiments to explore the implications of special relativity. In reality, objects with mass cannot reach or exceed the speed of light, and time does not stop for them.