Chapter 2: Space, Time, and the Eye of the Beholder
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 symmetrybetween 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.
