Einstein passed by the clock towers morning on his way home, a route he often took with his closest friend, Michele Besso. The two men regularly discussed science and philosophy—including the nature of time. After one such discussion, Einstein came to a sudden realization: Time is not absolute.

In the Special Theory of Relativity, Einstein determined that time is relative—in other words, the rate at which time passes depends on your frame of reference.

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.

You don't have to be the size of a planet to do some space-time warping. Large objects such as the Sun and planets aren't the only masses that warp the fabric of space-time. Anything with mass—including your body—bends this four-dimensional cosmic grid.

The theory, which revolutionized our understanding of time and space, is based on Einstein's astonishing recognition that light always travels at a constant speed, regardless of how fast you're moving when you measure it.

Albert Einstein reinterpreted the inner workings of nature, the very essence of light, time, energy and gravity. His insights fundamentally changed the way we look at the universe--and made him the most famous scientist of the 20th century.

Einstein went on to present his findings mathematically: energy (E) equals mass (m) times the speed of light (c) squared (2), or E=mc 2. The secret the equation revealed—that mass and energy are different forms of the same thing—had eluded scientists for centuries.

Explore the life and times of Albert Einstein: The Early Years, Family Roots, Einstein's Escapes, Career Scientist. Albert Einstein's lifelong passion for physics was sparked at the age of four or five when his father showed him a small compass.

During his life, Einstein used his fame to draw attention to things he believed in. He spoke about peace between Jews and Arabs and about finding homes for both Jewish and political refugees. He opposed all forms of racial and ethnic discrimination, such as segregation.

According to Einstein, you need to describe where you are not only in three-dimensional space —length, width and height—but also in time. Time is the fourth dimension. So to know where you are, you have to know what time it is.