In the mid-nineteenth century, Scottish physicist James Clerk Maxwell discovered that electric and magnetic forces are unified in the electromagnetic field. Maxwell calculated that light moves at a fixed speed: it always travels at the speed of light. Visible light is just another kind of electromagnetic wave, since stationary light doesn’t exist.
This formulation troubled young Albert Einstein. What happens, he wondered, if we chase after a beam of light at light speed? After a decade of contemplating Maxwell’s definitions of light and motion, in June 1905 Einstein found a way of understanding how the world appears to observers who are moving relative to each other. He concluded that the moving observer experiences time more slowly than a stationary observer does. This concept is called time dilation; the shorter length of the moving observer is called the Lorentz contraction. This answer, formulated when Einstein was twenty-six years old, upended all traditional understandings of space and time.
The discrepancy between the moving observer and the stationary observer underlies Einstein’s theory of special relativity, which says that if you want to measure speed accurately, you must always specify who is doing the measuring. Why? Because, as Einstein showed, the concept of motion is always relative. There is no such thing as an absolute frame of reference when it comes to objects moving in space. Force-free motion has meaning only when compared to other motions, and the same is true for accelerated motion. At the crux of relativity is the idea that simultaneous observations by no means yield identical viewpoints.
With a series of helpful examples, Greene shows that relativity is a difficult concept to understand on an intuitive level. People must give up the notion that all observers, regardless of their state of motion, can see things simultaneously. According to the special theory of relativity, things that are simultaneous to one observer need not be simultaneous to the other, depending on both observers’ state of motion. Relativity hinges on a complete symmetry between observers.
One important exception to relativity is the constancy of the speed of light. Light travels at 670 million miles an hour (186,000 miles per second) no matter what. The importance of this discovery cannot be overemphasized. It answered Einstein’s adolescent question: no matter how fast you chase after a light beam, it will still retreat at light speed. The discovery of this constant led to a complete overhaul of physicists’ understanding of the universe and, in time, to the undoing of Newtonian mechanics.
Time is measured by clocks, which undergo motion at a constant velocity. But because motion influences the passage of time, a “universal clock” cannot exist. Time passes more slowly for an individual in motion than it passes for an individual at rest. This principle applies not just to ticking clocks, but also to human activity and the decay of the body. Muons moving at high speed disintegrate slower than those at low speed, but—and here is the paradox—both particles experience exactly the same quantity of life. To understand this concept, think of a person who lives for 500 years and reads ten times more slowly than a person who lives for fifty years. Although the slow reader lives much longer than the fast reader, both read exactly the same number of books.