Redefining Reality - The Intellectual Implications of Modern Science

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grounds. First, the principle of relativity on which the theory is
based is limited to observers who are moving at a constant speed in
a straight line. Second, the theory touched on every known force in physics
except for one, gravitation. Then, Einstein had what he termed his “happiest
thought”: He realized that gravity and acceleration were different ways of
describing the same thing. With this new principle of equivalence, Einstein
began work on a generalized theory of relativity. In the end, the result was
a picture of a space-time that curves and warps around mass and energy, an
active space-time that reacts to what’s in space.
Einstein saw his original work—the theory of special relativity—
as descriptive of space, time, and motion in a universe without
gravitation. The problem, of course, is that as soon as we have an
object with mass in the universe, we have gravity; thus, the theory
would need to be reformulated.
Einstein struggled with this reformulation until he had what he
called his “happiest thought.” Looking at some window washers
high up on a building, he imagined one of them falling off
the ladder.
Einstein realized that although the man is falling because of
gravity, while he is falling, he would not experience any of the
usual gravitational effects. Objects around him would seem
When accelerating in freefall, it is as if there is no gravity at all.
This thought simultaneously answered both of Einstein’s
problems with special relativity: the fact that accelerating
reference frames were ruled out and the lack of gravitation.
Thinking about the man falling off the ladder showed him
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