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By Root 960 0

light has gained energy and thereby increased its frequency, it follows that Rebka finds that the peaks are arriving more frequently than they would if the light source were beside him. But the peaks are synchronized to his colleague’s heartbeat. That means that according to Rebka down in the basement, Pound’s heart would be beating faster and so he would age more quickly. The effect is a tiny one, corresponding to a speeding up of one second every 13 million years. It is testament to the skill and ingenuity of Pound and Rebka that they managed to devise an experiment capable of detecting the effect. This speeding up of time is precisely what is happening with the GPS satellite clocks. They are at a much higher altitude than the 22.5 meters of the Harvard laboratory but the basic idea is just the same: Clocks run faster in weaker gravitational fields.

Einstein’s general theory of relativity, confirmed beautifully by experiment, has led us to view spacetime not as a forever-fixed blend of space and time but instead as a more dynamical entity—one that can be manipulated by the presence of matter and, since through E = mc2 we know that mass and energy are interchangeable, energy too. In turn, the dynamical structure of spacetime controls the way objects move through it. No longer are we to think of space as an inert arena within which things happen and of time as the immutable and absolute ticking of a giant clock in the sky. Perhaps the most important lesson to learn in the face of this radical revision is that it is not wise to extrapolate experience beyond its realm. Why should fast-moving things behave according to the same laws as the slow-moving things we encounter in everyday life? Likewise, why should we have a right to infer the behavior of very massive objects by studying only the lighter ones?

Certainly our everyday experiences prove to be a pretty poor guide and, as Einstein has shown us, the deeper level of understanding is so much more elegant. Bringing together as it does such disparate concepts as mass and energy, space and time, and ultimately gravity, Einstein’s special and general theories will stand forever as two of the greatest achievements of the human mind. In the years to come, new understanding built upon new observations and experiments may well lead to a revision in the ideas we have presented here. Indeed many physicists are already anticipating a new order in their quest for more accurate and more widely applicable theories. This humbling lesson not to extrapolate beyond the evidence is not confined to relativity— the other great leap forward in twentieth-century physics was the discovery of the quantum theory, which underpins the behavior of all things at atomic scales and smaller. Nobody ever would have figured out how nature works at small distances based purely on everyday experience. To human beings, whose direct observations are confined to the “big things,” the quantum theory is ridiculously counterintuitive, but in the twenty-first century it underpins so much of our modern lives, from medical imaging to the latest computing technologies, that we must accept it whether we feel comfortable about it or not.

Today physicists are faced with a dilemma. Einstein’s general relativity, our best theory of gravity, cannot be meshed with quantum theory. Either one or both must be revised. Does spacetime “break up” at tiny distance scales? Maybe it does not really exist at all but is instead only an illusion formed by the ever-increasing set of “things that happen.” Are the fundamental objects in nature tiny vibrations of energy known as strings? Or does the solution lie in some other theory yet to be uncovered? This is the frontier of fundamental physics, and those standing on the edge are both thrilled and inspired to be looking out into the unknown.

At the end of a book on Einstein’s theories of relativity, it is all too easy to contribute to an unfortunate cult of personality surrounding the great man, and this is not our intention. Indeed, such a cult probably inhibits future progress