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After Einstein formulated his special theory of relativity in 1905, he realized that it was incomplete in at least two ways. First, it held that no physical interaction can propagate faster than the speed of light; that conflicted with Newton’s theory of gravity, which conceived of gravity as a force that acted instantly between distant objects.
A person in a closed windowless chamber who feels his feet pressed to the floor will not be able to tell whether it’s because the chamber is in outer space being accelerated upward or because it is at rest in a gravitational field.
Likewise, a person who feels she is floating in the closed chamber will not know whether it’s because the chamber is in free fall or hovering in a gravity-free region of outer space.
Perhaps the bending of light meant that the fabric of space, through which the light beam traveled, was curved by gravity. The shortest path through a region of space that is curved by gravity might seem quite different from the straight lines of Euclidean geometry.
Math was nature’s playbook. “The central idea of general relativity is that gravity arises from the curvature of spacetime,” says physicist James Hartle. “Gravity is geometry.”9
Einstein’s goal as he pursued his general theory of relativity was to find the mathematical equations describing two complementary processes: 1. How a gravitational field acts on matter, telling it how to move. 2. And in turn, how matter generates gravitational fields in spacetime, telling it how to curve.
His head-snapping insight was that gravity could be defined as the curvature of spacetime, and thus it could be represented by a metric tensor.
“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 said. “I was lucky enough to notice what the beetle didn’t notice.”16
The most famous thought experiment along these lines was that described by Newton in the third book of his Principia. Imagine a bucket that begins to rotate as it hangs from a rope. At first the water in the bucket stays rather still and flat. But soon the friction from the bucket causes the water to spin around with it, and it assumes a concave shape. Why? Because inertia causes the spinning water to push outward, and therefore it pushes up the side of the bucket.
the visceral pacifism of Einstein, a man so gentle and averse to conflict that he even disliked playing chess. “Europe in its madness has now embarked on something incredibly preposterous,” he wrote Ehrenfest that month. “At such times one sees to what deplorable breed of brutes we belong.”40
The only method of containing such aggression, he argued, was a world organization that had the power to police member nations.52 It was a theme he would pick up again eighteen years later, in the final throes of his pure pacifism, when he engaged in a public exchange of letters with Sigmund Freud on both male psychology and the need for world government.
“Matter tells spacetime how to curve, and curved space tells matter how to move.”
Space and time become players in the evolving cosmos. They come alive. Matter here causes space to warp there, which causes matter over here to move, which causes space way over there to warp even more, and so on. General relativity provides the choreography for an entwined cosmic dance of space, time, matter, and energy.84
just as the fabric of a trampoline will curve and ripple as a bowling ball and some billiard balls roll across it, and in turn the dynamic curving and rippling of the trampoline fabric will determine the path of the rolling balls and cause the billiard balls to move toward the bowling ball. The curving and rippling fabric of spacetime explained gravity, its equivalence to acceleration, and, Einstein asserted, the general relativity of all forms of motion.
“One of the strongest motives that leads men to art and science is escape from everyday life with its painful crudity and hopeless dreariness,” Einstein said. “Such men make this cosmos and its construction the pivot of their emotional life, in order to find the peace and security which they cannot find in the narrow whirlpool of personal experience.”25
For the rest of his life Einstein would expound a democratic socialism that had a liberal, anti-authoritarian underpinning. He advocated equality, social justice, and the taming of capitalism. He was a fierce defender of the underdog. But to the extent that any revolutionaries edged over toward a Bolshevik desire to impose centralized control, or to the extent that a regime such as Russia’s struck him as authoritarian, Einstein’s instinctive love of individual liberty usually provoked a disdainful reaction.
“Socialism to him reflects the ethical desire to remove the appalling chasm between the classes and to produce a more just economic system,” his stepson-in-law wrote of Einstein’s attitudes during the 1920s. “And yet he cannot accept a socialist program. He appreciates the adventure of solitude and the happiness of freedom too much to welcome a system that threatens completely to eliminate the individual.”
With the gravitational field equations in his general theory of relativity, Einstein laid the foundations for studying the nature of the universe, thereby becoming the primary founder of modern cosmology.
“My passionate sense of social justice and social responsibility has always contrasted oddly with my pronounced lack of need for direct contact with other human beings and communities,” he reflected. “I am truly a ‘lone traveler’ and have never belonged to my country, my home, my friends, or even my immediate family, with my whole heart; in the face of all these ties, I have never lost a sense of distance and a need for solitude.”
“The value of a college education is not the learning of many facts but the training of the mind to think,”
“The undignified mania of trying to adapt and conform and assimilate, which happens among many of my social standing, has always been very repulsive to me,”
