100 years later, Einstein's masterpiece still stands
Paul Halpern is a University of the Sciences physics professor and the author of "Einstein's Dice and Schrödinger's Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics"
Paul Halpern
is a University of the Sciences physics professor and the author of "Einstein's Dice and Schrödinger's Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics"
It is not often that a centenarian is just as spry and vital as the day she was born, but that's the case with the general theory of relativity.
This month we are celebrating exactly 100 years since the meeting in which Albert Einstein announced the completion of his masterly theory of how gravity works. His grand theory relating the geometry of space and time to the matter and energy within it represents an extraordinary triumph of the human imagination.
Before Einstein, there was Isaac Newton, who had offered a simpler but much less satisfying theory of gravity.
Contrary to popular myth, Newton did not discover gravity. From earliest times, no one could mistake the fact that things fall down if released from a certain height. Rather, Newton demonstrated that gravity is a universal force with certain predictable properties. This force could act over immense distances, linking, for instance, the sun and Earth as if an invisible string tied them together.
Although Newton's theory was very successful, it contained a logical flaw: the idea that gravity could be transmitted instantly. If the sun suddenly disappeared, his theory predicted that Earth would immediately sense its absence, as if a thread were cut, and start traveling in a straight line through space.
Yet Einstein had showed in his special theory of relativity (the predecessor to the general theory), space has a speed limit. No signal in empty space can travel faster than the speed of light. The sun's light takes eight minutes to reach Earth. Therefore, Earth could not possibly respond to the sun's disappearance in less than that time period - certainly not instantly.
Einstein's general relativity beautifully recasts gravitation as a local, rather than long-distance, phenomenon. The equations he announced in November 1915 show precisely how it is the fabric of space and time - known in tandem as space-time - that conveys gravity.
Much like a turbulent wild river with eddies and currents that change the course of boats, space-time's bumps and ripples compel planets and stars to alter their paths. In the case of Earth, a gravitational well compels Earth to travel in an elliptical orbit rather than in a straight line. The cause of that indentation in space-time is the mass of the sun. Or, as often expressed by the late gravitational physicist John Wheeler (who came to know Einstein well), space tells matter how to move and matter, in turn, tells space how to curve.
How Einstein's theory was tested offers an important lesson in the value of international cooperation.
Einstein predicted that the paths of light rays would be diverted by the presence of massive objects. He estimated the angle by which starlight would bend in the vicinity of the sun and pointed out that this distortion could be measured during a solar eclipse.
To accomplish this task, Einstein found a friend in the British astronomer Arthur Eddington, who organized expeditions in spring 1919 to West Africa and South America, where a solar eclipse could readily be seen. The remarkable thing about the collaboration between Einstein and Eddington was that it took place shortly after World War I, when their two native countries (Germany and the United Kingdom) had been enemies. Both thinkers were pacifists and hated nationalism, which made such cooperation easier.
During the summer of 1919, the eclipse results were analyzed and compared with predictions based on Newtonian physics. Although the data were sparse, the results fell much closer to Einstein's forecast than one based on Newton's theory of gravitation. Thus it came to pass that in November 1919 the British Royal Academy announced that Einstein's theory was triumphant.
The confirmation of general relativity generated startling headlines around the world. The firmament of the heavens was no longer stable and secure; rather, it was an ever-changing canvas. Nothing in science was stable. In fact, 10 years later, results by the American astronomer Edwin Hubble showed that the universe itself was expanding - another prediction of general relativity.
For various reasons, including its lack of correspondence with quantum mechanics, the other major theory developed in the early 20th century, physicists have tried to modify general relativity. Nevertheless, despite a century of effort, Einstein's masterpiece still stands strong.
Let's offer a toast to the beauty of the venerable equations describing the cosmos, and wish them well for decades to come - unless, that is, a quantum version comes along.