Why does relativity matter for objects moving close to the speed of light?

Relativity is essential for understanding objects moving near the speed of light because, at these extreme velocities, the classical laws of physics—which assume absolute space and time—no longer accurately describe reality. To keep the speed of light constant for all observers, Einstein’s special relativity dictates that space and time must change relative to an observer's state of motion.12

When an object moves at speeds approaching $c$, several critical relativistic effects occur:

• Time Dilation: Time intervals for a moving body appear to lengthen, meaning clocks on that body run more slowly compared to a stationary observer.13
• Length Contraction: The object appears shorter along its direction of motion to an outside observer.1
• Relativistic Mass: The mass of an object increases with its speed. As the object approaches the speed of light, its mass approaches infinity, requiring infinite energy to accelerate further.4

Consequently, the speed of light acts as a universal cosmic speed limit that material objects can approach but never reach. These effects are confirmed by observing subatomic particles and high-precision experiments with atomic clocks.2543