Recently I published my first biographical book, *Quantum Man: Richard Feynman’s Life in Science.* This was a departure for me—I usually write about science, not the people who do science. But I realized that being able to explore Feynman’s understanding of his own work, through his papers, would help me to give a clearer popular explanation of his physics.

Most people don’t realize that in science nowadays—in physics, anyway—we rarely turn to the original literature to understand scientific concepts. That’s because scientific ideas invariably get distilled and refined, with the result that they become easier to understand, and often more general. Ideas get reinterpreted, on the basis of new discoveries and new mathematical formalisms, to such an extent that often the content of original papers is almost unrecognizable. While the results of a lifetime of work on electromagnetism during the 19th century can famously be written on a T-shirt today in the form of what are known as Maxwell’s four equations, these bear little resemblance to the formulation of his ideas as he originally expressed them.

So I knew that working with Feynman’s papers would help me better translate the evolution of his ideas.

I also felt that I owed it to both Feynman and the public to present a more balanced picture of his contributions than was generally available. He created a mythology about himself that others have happily latched on to. He became known primarily as a prankster, a bongo-playing practical joker, and a womanizer, all combined with an almost mystical intellect. But Feynman’s real legacy was none of that. His legacy was his science, and its contribution to our understanding of the universe. When it came to the details of science, he was deadly serious.

What made Feynman special was not so much his nonconformity, but the genuine weirdness of the quantum universe that he so personally and successfully assaulted. The wildness of the man was a match for the wildness of the quantum universe. He was willing to break all the rules to tame a theory that breaks all the rules.

Quantum mechanics has captured the public’s imagination precisely because it defies classical logic, while at the same time appearing to make the universe strangely accessible. Quantum objects seem to be able to do many mutually inconsistent things—an electron, for example, projected at a barrier with two slits, can go through both slits at the same time. But the properties of the system that one measures appear to depend crucially on *how* one chooses to measure that system.** **Thus, after I shoot a series of electrons through the slits, the pattern of spots that can be observed on a fluorescent screen placed behind the barrier vary, depending on whether or not you measure the electrons’ positions at the barrier at one slit or the other, or neither.

Alas, that fact has led to misunderstandings and new-age myths. Programs based on such misconceptions, like those about the “law of attraction” described in the best-selling book *The Secret,* falsely suggest that quantum mechanics supports the notion that we can literally change external phenomena just by thinking about them. In the example I’ve given here, thinking about the electrons is irrelevant. One must make observations and take active measurements of the electrons to affect their behavior.

The work that led to Feynman’s Nobel Prize was based on a fundamental re-examination of the quantum universe—one that led to a change in the way physicists now picture quantum processes. Driven by the need to reformulate quantum theory to deal with a vexing problem associated with the quantum properties of fundamental electric charges, Feynman trained us to think of quantum processes as occurring in both space and time, even though Schrödinger’s conventional, wave-mechanics theory taught us that we cannot pinpoint elementary quantum objects at specific locations in space or time.

How can one have one’s cake and eat it, too? Feynman was able to argue that elementary particles like electrons do not trace single classical trajectories, as baseballs and cannonballs appear to do. Rather, one can imagine them tracing many such trajectories at the same time. With each trajectory, we can associate a quantum “weighting” factor. The probability that an electron starting out at one point and at one time will be measured to be at a distant point at some time later** **can be determined by combining contributions from all the possible paths that an electron could take between those points, with each path’s contribution to the sum total being proportional to its weighting factor.

This notion even explains the existence of exotic “antiparticles.” These are not merely creations of *Star Trek* but actually exist in the real world. Every type of elementary particle in nature has an antiparticle with opposite electric charge but precisely the same mass. Feynman showed that this was required by the theory of relativity as applied to the quantum universe.

If, for short times, elementary particles could follow trajectories in which their speed exceeds the speed of light—impossible in the classical universe, but allowed among the possible quantum trajectories in Feynman’s view of quantum mechanics—then they can appear, by relativity, to be briefly going backward in time. But a positively charged particle moving backward in time resembles a negatively charged particle moving forward in time. The trajectory of a particle moving forward in time, then briefly backward in time, then forward in time implies that during some period, three different particles appear to be existing at once—the original particle and a pair of particles with equal and opposite charges, or, as we now call them, a particle-antiparticle pair. Since one starts with a single particle and ends up with a single particle, a particle-antiparticle pair must be both spontaneously created and also annihilated during this period. Simple!

The techniques that Feynman developed do not apply merely to esoteric objects like antiparticles. They have played a central role in allowing physicists to do things like handling liquid helium and building the atomic clocks at the heart of modern global positioning systems. Indeed, Feynman’s “path integral” formalism has become a staple of essentially every area of modern physics.

Feynman’s own love of physics was not restricted to the esoteric. In 1960, well before the invention of microchips, Feynman wrote a beautiful essay speculating on the seemingly “infinite possibilities” of engineering machines on smaller and smaller scales—what we now refer to as nanotechnology. He laid the groundwork for what may be one of the most exciting future applications of quantum mechanics to the macroscopic world—quantum computing. By exploiting the fact that quantum systems can do many things at the same time (as long as you don’t measure them during the process), we may one day be able to create quantum computers whose multitasking characteristics might allow them to vastly outperform today’s computers, making possible in real time the completion of calculations that now would take longer than the age of the universe.

Feynman was driven to his remarkable insights about nature not because he loved to dwell in idle speculation, but because the requirements of understanding various experiments drove him to think outside the box. He once said that science is imagination in a straitjacket, and his willingness to force his creative imagination to bend to the will of nature reflects one of the most important characteristics of successful science.

It takes a certain kind of courage to accept the world as it is, whether one likes it or not, despite one’s most profound dreams and hopes about the way the world should be.** **The risk is outweighed, however, by the gift of understanding. Feynman personified the scientific integrity and bravery required to follow nature wherever it leads. That may be his greatest legacy.