According to the theory of special relativity, as an object with mass accelerates towards the speed of light, its mass appears to increase. This effect is often referred to as "relativistic mass."
The concept of relativistic mass is a bit nuanced and can be misleading if not properly understood. In modern physics, it is more common to use the term "rest mass" or "invariant mass" to describe an object's intrinsic mass, which is the mass it possesses when it is at rest or moving at speeds much lower than the speed of light.
As an object approaches the speed of light, its relativistic mass increases according to the following equation:
m = m₀ / √(1 - v²/c²)
Where: m is the relativistic mass, m₀ is the rest mass (invariant mass), v is the velocity of the object, and c is the speed of light in a vacuum (approximately 299,792 kilometers per second or 186,282 miles per second).
As the velocity (v) approaches the speed of light (c), the denominator of the equation approaches zero, and the relativistic mass becomes infinite. However, it is important to note that this does not imply that an object with mass can reach or exceed the speed of light. According to our current understanding of physics, objects with mass require infinite energy to accelerate to the speed of light, and it is impossible for them to reach or exceed it.
In conclusion, the mass of an object appears to increase as it approaches the speed of light, but it cannot actually reach or exceed the speed of light due to the fundamental limitations imposed by the theory of special relativity.