Most of us think we understand time. Time is what clocks measure, ticking off the steady beat of seconds, minutes and hours. Although our wristwatches may run fast or slow because of mechanical flaws, we believe that there is some "master clock" for the universe to which, in an ideal world, all clocks could be synchronized. The notion of the regular passage of time is so ingrained in human consciousness that our languages have developed special ways to distinguish events that occur in the past, present or future. And until Einstein, most physicists also accepted the idea of universal time without question.
A mathematical dilemma
When Einstein proposed that the speed of light is constant for all observers, he introduced a conundrum. How could different observers measure the same speed for light when the observers themselves were moving at different speeds? Speed is a measure of distance divided by time (for example, kilometers per hour or miles per hour). Einstein realized that for speed to remain constant, intervals of time and distance would have to change in a way that kept their ratio exactly the same.
Einstein's answer overturned long-held ideas about the nature of time as a steady, continuous progression of events from past to present to future. Although it's hard to believe, there is no single "master clock" for the entire universe. Time does not progress at the same rate for everyone, everywhere. Instead, Einstein showed that how fast time progresses depends on how fast the clock measuring time is moving. The faster an object travels, the more slowly time passes for that object, as measured by a stationary observer. Perhaps even more astonishing, one person's past could theoretically be another's future--which is why Einstein described the past, present and future as "persistent illusions."