(22b) The Theory of Relativity

By Newton's laws, two frames of reference moving with constant velocity relative to each other observe exactly the same physical behavior. There exists no way to tell, which of them is moving and which one is at rest: there is no "absolute rest frame" and no "absolute (constant) velocity. " Everything is relative, and either frame can be chosen as reference benchmark.

In the 19th century the laws of electricity and magnetism were discovered, and they suggested that light itself was a related phenomenon, an electromagnetic wave (as is further discussed in the sections about the Sun). For a while, however, it seemed as if, certain subtle electromagnetic phenomena did allow one to distinguish whether a frame was in motion of not. Those effects were hard to verify, and when they were finally tried out, they did not work--they could not tell which system was in motion.

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. Einstein's alternative formulation also changed the concept of time, predicting that intervals measured in different moving frames of reference did not always agree--that time became "relative."

The modifications suggested by Einstein only became significant near the velocity of light, and in day-to-day phenomena they 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, which is indeed an upper limit that cannot be passed. The relativity of time was demonstrated when it was found that muons produced by fast atomic nuclei ("cosmic rays") high in the atmosphere--particles with a lifetime of about 2 microseconds--survived much longer and generally reached the Earth's surface, because in the frame of reference of the Earth, their lifetime was actually longer.