The theory of special relativity is built upon the assumption that the speed of light in vacuum, denoted as "c," is constant and independent of the motion of the source or the observer. This assumption has been extensively tested and confirmed by numerous experiments and observations.
If we were to consider a scenario where the speed of light is not constant, it would imply a violation of this fundamental postulate of special relativity. In such a case, the theory would no longer be applicable, and the consequences predicted by special relativity, such as time dilation, length contraction, and the equivalence of mass and energy (E=mc²), would no longer hold true.
Moreover, many other physical theories, such as quantum mechanics and electromagnetism, are consistent with the constancy of the speed of light. Changing the speed of light would require a major reworking of these theories as well.
It's important to note that experimental evidence from a wide range of phenomena, including particle accelerators, cosmic rays, and high-precision measurements, consistently supports the constancy of the speed of light. These experiments have repeatedly verified the predictions of special relativity, reinforcing the notion that the theory accurately describes the behavior of objects moving at speeds approaching the speed of light.
Therefore, as long as experimental evidence continues to confirm the constancy of the speed of light, special relativity remains a valid and robust framework for understanding the behavior of objects in motion.