An Old Man's Toy
“Things fall apart; the center cannot hold,” the poet William Butler Yeats wrote in “The Second Coming.” With Yeats, his reader tends to imagine that things cohere insofar as they have a center which holds them. He or she imagines that a sense of purpose can hold together the many aspects of a project or a lifetime, in much the same way that the sun’s gravitational attraction holds the planets in their orbits. The physical universe, too, most people suspect, must have a center.
This is what scientists term an “intuition” about the way things are. Most laymen expect things to be like this, they take it for granted, they believe they already know it, and seldom so much as give it a second thought. And indeed for most purposes, it is enough. Yeats’s poem speaks to his reader, who feels no need to question his hypothesis—and similarly most people seldom question the idea that our sense of identity gives coherence to our lives.
Intuitions in this sense—basic assumptions about the way things are—are very hard to shake, and yet many and perhaps all of the deeper insights of humankind, in realms that range all the way from the spiritual to the scientific, have arisen precisely where the accepted, the expected, the “intuitively obvious” has been challenged, questioned, doubted. For a Buddhist, for example, the intuitively obvious idea of personal identity is thrown in question. There is, the Buddha taught, no single and definable self standing at the center of a person’s being—merely the illusion of a self, a shadow thrown by the use of the word “I” to refer to a congeries of passions, pleasures, desires, thoughts, and griefs. To recite the Buddha’s words, however, is one thing; but to glimpse the flux of personality, and live with the sense that there is no clear center to give coherence—that is what the Buddha asks of his followers, and it is exactingly difficult to attain, precisely because it violates an intuitive sense of what is obviously the case.
For the Western mind, perhaps, this notion of a center around which everything revolves is most clearly seen in the assumptions people have about gravity. Call the way this center holds things “gravity,” the layman feels, and it is understood. Yet just as the Buddha throws in question our intuitive sense of identity, so the physicist throws in question our intuition that the universe has a center, and that the center somehow holds it all in place. An Old Man’s Thy: Gravity at Work and Play in Einstein’s Universe does not, however, simply debunk the layman’s intuitions. Rather, the author seeks to lead his readers through something of the same process of questioning and doubt by which the scientist comes to understand what is in fact known about gravity.
The physicist, A. Zee argues, actually has to learn to think counterintuitively, against the grain of assumption, in order to understand such seemingly crazy ideas as that there is no center, and that it is everywhere. In physics, indeed, only certain ideas are crazy enough to be likely—and ideas about what the center is and how it holds things together, those ideas which constitute our understanding of gravity, are among the craziest- sounding, the least obvious, the most counterintuitive, the most fascinating. Zee’s book, then, is not a sustained exposition of a single topic but rather a short and brilliant primer in counterintuitive thinking. In it, he unravels such mysteries as the curvature of space, the attempt to find a unified field theory, the Big Bang and subsequent expansion of galaxies, and why the sky is dark at night.
The “toy” of Zee’s title was given to the “old man”—Albert Einstein—by a neighbor and colleague on the occasion of his seventy-sixth birthday. It was a sophisticated variant on the traditional “cup and ball” toy, in which a small ball is attached by string to the stem of a small cup, the game being to toss the ball in the air and catch it in the cup. In the case of Einstein’s toy, the ball was attached by a string that passed through the inside of the cup to a weak spring, the whole thing being mounted on a broomstick. The trick in this case was not to get the ball in the cup by skillful catching, but to demonstrate that if the ball is hanging by the string outside the cup, in such a way that the spring is too weak to pull it in, it will nevertheless wind up inside the cup if the whole toy, or perhaps one should say “apparatus” in deference to the toy’s owner, is allowed to fall freely for a couple of feet.
The point is simple: While the apparatus is falling, there is no gravitational force acting on the ball, and the weak spring is therefore strong enough to pull the ball up over the lip of the cup. Einstein was delighted by the gift, because in illustrating the lack of gravitational effect on the ball, it also illustrated his “equivalence principle,” which states that gravity and acceleration are in effect equivalent to one another.
The equivalence principle is no longer counterintuitive: The so-called “g-forces” (for gravity-forces) of acceleration are routinely shown in films dealing with space- flight and even race car driving, and every schoolboy knows that gravitation draws falling objects toward the earth at a rate of 32 feet per...
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