The Day We Found the Universe - Marcia Bartusiak [75]
For years success eluded him as he struggled to figure out how to make his equations truly universal and still reproduce Newton's law of gravity for the simplest cases, when gravity was weak and velocities were low. After all, Einstein couldn't just throw out a law that had been time-tested for more than two centuries. His new theory had to agree very closely with Newton's in the everyday realm where physicists had long been conducting experiments, a place where space-time distortions were too small to be overt. But then the theory would have to merge smoothly into either the intense gravity or high-velocity regimes in which the strange effects of relativity at last become obvious. “In all my life I have labored not nearly as hard,” he wrote a colleague in the midst of his deliberations. “… Compared with this problem, the original relativity is child's play.”
The breakthrough for the thirty-six-year-old physicist finally came in November 1915. Over that month Einstein reported weekly to the Prussian Academy of Sciences on his final progress toward a new theory of gravitation. A key moment arrived in mid month, when he was able to successfully explain a small displacement in the orbit of Mercury, a nagging mystery to astronomers for decades. Einstein later remarked that he had palpitations of the heart upon seeing this result: “I was beside myself with ecstasy for days.”
Complete success arrived on November 25, the day he presented his concluding paper. In this culminating talk, Einstein presented the decisive modifications that allowed him to secure a comprehensive theory. Written in the terse notation of tensor calculus—shorthand for a larger set of more complex functions—the general theory of relativity looks deceptively like a simple algebraic equation. It fits on one line and is the embodiment of mathematical elegance:
Ruv − ½guvR = −kTuv
On the left side are quantities that describe the gravitational field as the geometry of space-time. In fact, the Rs denote how much spacetime is curved. On the right side is a representation of mass-energy and how it is distributed. The equal sign sets up an intimate relationship between these two entities. As Princeton physicist John Archibald Wheeler liked to put it, “Spacetime tells mass how to move and mass tells spacetime how to curve.”
Einstein showed that the three dimensions of space and the additional dimension of time join up to form a real, palpable object. While it's impossible for us to visualize these four dimensions, it can be pictured in three. Think of space-time as a boundless rubber sheet. Masses, such as a star or planet, then indent this flexible mat, curving space-time. The more massive the object, the deeper the depression. Planets thus circle the Sun not because they are held by invisible tendrils of force, as Newton had us think, but because they are caught in the natural hollow formed by the Sun in four-dimensional space-time, much as a rolling marble would circle around a bowling ball sitting in a trampoline. With this image in mind, the pull of gravity could now be easily explained; it's merely matter sliding like a downhill skier along the undulations of space-time. When Einstein's younger son, Eduard, later asked his father why he was so famous, Einstein singled out this elegant and lucid illustration of gravity as curving space-time. “When a blind beetle crawls over the surface of a curved branch, it doesn't notice that the track it has covered is indeed curved,” he explained. “I was lucky enough to notice what the beetle didn't notice.”
This realization was why Einstein was so excited by his successful result regarding the planet Mercury. It was clear evidence of this fantastic new image of gravity, its geometric representation. His insight centered on the fact that planets do not orbit the Sun in perfect circles but