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Time Travel for Nonscientists
Paul Johnson's popular history of the twentieth century opens thus: "The Modern World began on 29 May 1919 when photographs of a solar eclipse, taken on the island of Principe off West Africa and at Sobral in Brazil, confirmed the truth of a new theory of the universe."1 The new theory of the universe was Einstein's General Relativity, a radical, mysterious, new explanation of gravity, destined to replace the more intuitive and accessible theory of Isaac Newton that had inspired and sustained the Enlightenment.
The emergence of the new theory also signaled the arrival of what cultural anthropologist Chris Toumey has called "Old Testament Science," a science "respected without comprehension" and revered because of "awesome signs," like the God of the Old Testament.2 This kind of science could not be popularized in a way that could be understood by the educated person. General Relativity was a highly mathematical description of space, time, matter, and their interactions. Time and space lost their straightforward Newtonian identities and dissolved into an abstraction called "spacetime." What we used to think of as time became another dimension, somehow similar to the more familiar dimensions of space, and perhaps something we could travel through instead of merely ride along. The formerly clear-cut notions of "space" and "time" were replaced by "spacelike intervals" and "timelike intervals," themselves abstractions from the physical structure known as "spacetime."
Spacetime is a four-dimensional matrix that can be twisted and distorted by massive bodies like stars in ways that are highly counterintuitive and even bizarre. Light, for example, no longer travels in "straight" lines; a planet in an orbit close to a star, like Mercury, does does not retrace its path each time around like its more distant companions; clocks run at different rates depending on the strength of the local gravitational field. Perhaps the most bizarre element of the theory is the prediction of black ...