Albert Einstein then proposed, in 1905, the "principle of relativity" as a fundamental property of the universe. No matter what physical process was used, absolute motion at a constant velocity was undetectable. No loophole existed, not even through the laws of electricity and magnetism.
The trouble was, changing those laws (to plug the apparent loophole) would have upset the electromagnetic theory of light, for which ample evidence existed--e.g. radio waves. Einstein therefore suggested that those laws were correct and instead, Newton's laws were the ones needing to be modified--even though those laws already did hold that absolute motion was undetectable. Furthermore, time intervals measured in different moving frames of reference did not always agree--time became "relative."
The modifications suggested by Einstein only became significant near the velocity of light, and in day-to-day phenomena it could be ignored. As the velocity of light was approached, however, inertia (i.e. mass) increased, making it harder and harder to accelerate any matter and setting that velocity as an absolute limit, which no material object could exceed.
All those predictions have been amply confirmed by experiments, and particle accelerators in particular have left little doubt that particles get more massive as they approach the velocity of light, and that velocity is indeed an upper limit which cannot be passed. The relativity of time was demonstrated when it was found that muons--particles with a lifetime of about 2 microseconds--produced by fast atomic nuclei ("cosmic rays") high in the atmosphere, survived much longer and generally reached the Earth's surface, because in the frame of reference of the Earth, their lifetime seemed longer.
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