Who in the generations of scientists after Einstein has shown such a clear understanding of what to do with the bully pulpit of public acclaim? Look at the most famous living scientists—Stephen Hawking, for example, or James Watson. They have all contributed wonderful ideas to the world, but few interact meaningfully with today's major political and social issues.
Beneath the symbolic Einstein lies the scientific Einstein—the man whose theoretical breakthroughs justified the fame and all that flowed from it. Here, too, he was more than just a superficial radical. Physicist Banesh Hoffman, Einstein's collaborator and biographer, aptly described him as "creator and rebel." Einstein destroyed classical certainty, but only so he could uncover a deeper kind of certainty. Isaac Newton had envisioned a universe built on absolute space, an invisible metric against which all motion can be measured. Einstein replaced Newton's universe with one built on absolute law, meaning that the speed of light and other fundamentals of physics remain the same from all perspectives. The alternative—that physical laws should vary with the observer's motion relative to some undetectable, unknowable frame of reference—now seems absurd. Yet every natural philosopher before Einstein, going back to Aristotle and beyond, accepted some version of that proposition.
Einstein arrived at special relativity almost purely from an examination of logical flaws in the then current theories of physics, flaws that were evident for all to see. Speaking recently at the Aspen Center for Physics, physicist Murray Gell-Mann marveled at Einstein's ability to take James Clerk Maxwell's equations of electromagnetism more seriously than Maxwell himself did and track the full implications of Galileo's idea of relative motion and Newton's model of gravity. Here was the glorious payoff of Einstein's fierce commitment to freethinking. He insisted on examining the workings of the world at a more rigorous level than even the most illustrious of his predecessors, until he was totally certain that the system made sense. His requirement of total consistency forced him to take seriously the problems that his predecessors and colleagues alike had swept aside as trivialities or unanswerable matters of metaphysics.
Einstein's triumphs were guided not by strange new experiments but by rigorous logic, not by the most esoteric questions of the day but by the most basic ones. He famously argued that "all physical theories, their mathematical expressions notwithstanding, ought to lend themselves to so simple a description that even a child could understand them." Similarly, he described the questions that motivated his theories as fundamentally childlike ones that he had carried with him into adult life.
Often Einstein framed these questions in terms of thought experiments that highlighted the universal nature of his thinking. In one conceptual exercise, he wondered what a person would see if he could catch up with a beam of light. Newton's theory of space says that such a thing should be possible; Maxwell's theory of light says that it should not. Special relativity showed how to explain what we observe (Maxwell's laws) by discarding what we cannot observe (Newton's absolute space). These mental pictures convinced Einstein that the cosmos had to operate on a more consistent and fundamentally simple basis than his colleagues believed. He addressed these shortcomings by inventing his own physics and finding new ways to measure space and time.
One of Einstein's greatest inspirations, as he later recalled, occurred during a flash of insight in 1907: "For an observer falling freely from the roof of a house there exists—at least in his immediate surroundings—no gravitational field." (A note of caution: This may be the symbolic Einstein at work, retroactively fabricating an event to make his concepts clear. What matters, however, is that his ideas can be expressed in such terms and that he chose to do so.) In other words, the acceleration caused by gravity exactly erases the force exerted by gravity. Einstein cast the situation another way. A man in an enclosed elevator cannot in principle say whether he is motionless on Earth's surface and feeling the pull of gravity or moving through space, being pushed upward at an identical rate of acceleration. This became Einstein's principle of equivalence, which states that a uniform acceleration is equivalent to, or indistinguishable from, a uniform gravitational field.
Einstein developed those ideas into general relativity, a theory of gravity that subsumed and expanded on Newton's. General relativity far surpassed special relativity in redefining space and time—not just how they are measured but how they are linked together—to find a clearer, more comprehensive description of reality. The warping of space-time is an exotic-sounding concept that is actually a part of everyday experience. It is what holds together giant clusters of galaxies, but it is also what I experience every time I sit down in a chair or take a step. The symbolic Einstein showed that physicists need not be remote and detached from the real world; the scientific Einstein showed that their research need not be, either. No wonder he makes so many surprise appearances in my life.
Relativity—general relativity in particular—leads me directly from the scientific Einstein to the philosophical Einstein. With general relativity, Einstein completed a program begun in ancient Greece to determine the scope of natural law and thereby define the relationship between us and the universe. In the Greek conception, the flawed earthly laws and elements are distinct from those of the heavenly spheres, which follow perfectly circular motion and consist of aether—the perfect "fifth element." The perceived split between heaven and earth lived on in diminished form, all the way through Newton's absolute space (which he described as "the sensorium of God") and the nineteenth-century reconception of the aether as an invisible, all-pervasive medium that transmits light through empty space and provides the background reference frame of all motion.
In Einstein's universe there is no fifth element, which means that there is no escaping the authority of science. To me, this is one of the most disruptive aspects of Einstein's entire rebellious vision. Shortly after completing general relativity, he published a paper that expressed these ideas in rigorous terms, essentially inventing the field of cosmology, the study of the cosmos as a whole. He also established the terms for a new relationship between science and religion.
