## Celebrating Richard Feynman at TEDxCaltech

February 4, 2011 | Posted by Microsoft Research Blog

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One of the responsibilities for us as researchers is to have the courage to challenge accepted “truths” and to seek out new insights. Richard Feynman was a physicist who not only epitomized both of these qualities in his research but also took enormous pleasure in communicating the ideas of physics to students. Feynman won the Nobel Prize for his computational toolkit that we now call Feynman Diagrams. The techniques he developed helped the physics community make sense of Quantum Electrodynamics (QED) after the war, when the entire community was in a state of confusion about how to handle the infinities that appeared all over the place when one tried to make a perturbative expansion in the coupling.

Feynman was the subject of a recent TEDxCaltech conference, fittingly called, “Feynman’s Vision: The Next 50 Years.” The event was organized in recognition of the 50-year anniversary of Feynman’s visionary talk, “There’s Plenty of Room at the Bottom,” in which he set out a vision for nanoscience that is only now beginning to be realized. It is also 50 years since he gave his revolutionary “Feynman Lectures on Physics,” which educated generations of physicists.

I had the honor of speaking about Feynman’s contributions to computing, from his days at Los Alamos during the war, his Nobel Prize winning computational toolkit (Feynman Diagrams), and his invention of quantum computing, By striving to think differently, he truly changed the world. The following are some highlights from my presentation.

**Parallel Computing Without Computers**

Feynman worked on the Manhattan Project at Los Alamos in the 1940s with Robert Oppenheimer, Hans Bethe, and Edward Teller. In order to make an atom bomb from the newly-discovered trans-uranic element, Plutonium, it was necessary to generate a spherical compression wave to compress the Plutonium to critical mass for the chain reaction to start. It was, therefore, necessary to calculate how to position explosive charges in a cavity to generate such a compression wave; these calculations were sufficiently complex that they had to be done numerically. The team assigned to perform these calculations was known as the “IBM team,” but it should be stressed that this was in the days before computers and the team operated on decks of cards with adding machines, tabulators, sorters, collators, and so on. The problem was that the calculations were taking too long, so Feynman was put in charge of the IBM team.

Feynman immediately discovered that because of the obsession with secrecy at Los Alamos, the team members had no idea of the significance of their calculations or why they were important for the war effort. He went straight to Oppenheimer and asked for permission to brief the team about the importance of their implosion calculations. He also discovered a way to speed up the calculations. By assigning each problem to a different colored deck of cards, the team could work on more than one problem at once. While one deck was using one of the machines for one stage of the calculation, another deck could be using a different machine for a different stage of its calculation. In essence, this is a now-familiar technique of parallel computing—the pipeline parallelism familiar from the Cray vector supercomputers, for example.

The result was a total transformation. Instead of completing only three problems in nine months, the team was able to complete nine problems in three months! Of course, this led to a different problem when management reasoned that it should be possible to complete the last calculation needed for the Trinity test in less than a month. To meet this deadline, Feynman and his team had to address the more difficult problem of breaking up a single calculation into pieces that could be performed in parallel.

**Feynman Diagrams**

My next story starts in 1948 at the Pocono Conference where all the great figures of physics—Niels Bohr, Paul Dirac, Robert Oppenheimer, Edward Teller, and so on—had assembled to try to understand how to make sense of the infinities in QED. Feynman and Schwinger were the star speakers, but Feynman was unable to make his audience understand how he did his calculations. His interpretation of positrons as negative energy electrons moving backwards in time was just too hard for them to accept. After the conference, Feynman was in despair and later said, “My machines came from too far away.”

Less than a year later, Feynman had his triumph. At an American Physical Society meeting in New York, Murray Slotnick talked about some calculations he had done with two different meson-nucleon couplings. He had shown that these two couplings indeed gave different answers. After Slotnick’s talk, Oppenheimer got up from the audience and said that Slotnick’s calculations must be wrong since they violated Case’s Theorem. Poor Slotnick had to confess that he had never heard of Case’s Theorem and Oppenheimer informed him that he could remedy his ignorance by listening to Professor Case present his theorem the following day.

That night, Feynman couldn’t sleep so he decided to re-do Slotnick’s calculations by using his diagram techniques. The next day at the conference, Feynman sought out Slotnick, told him what he had done, and suggested they compare results. “What do you mean you worked it out last night?” Slotnick responded. “It took me six months!” As the two compared answers, Slotnick asked, “What is that Q in there, that variable Q?” Feynman replied that the Q was the momentum transfer as the electron was deflected by different angles. “Oh,” Slotnick replied. “I only have the limiting value as Q approaches zero. For forward scattering.” Feynman said, “No problem, we can just set Q equal to zero in my formulas!” Feynman found that he had obtained the same answer as Slotnick.

After Case had presented his theorem, Feynman stood up at the back of the audience and said, “Professor Case, I checked Slotnick’s calculations last night and I agree with him, so your theorem must be wrong.” And then he sat down. That was a thrilling moment for Feynman, like winning the Nobel Prize—which he did much later—because he was now sure that he had achieved something significant. It had taken Slotnick six months to do the case of zero momentum transfer while Feynman had been able to complete the calculation for arbitrary momentum transfer in one evening. The computational toolkit that we now call Feynman Diagrams have now penetrated to almost all areas of physics and his diagrams appear on the blackboards of physicists all around the world. This toolkit is undoubtedly Feynman’s greatest gift to physics and the story perfectly illustrates Feynman’s preference for concrete, detailed calculation rather than reliance on more abstract theorems.

**The Physics of Computation**

At the invitation of his friend Ed Fredkin, Feynman delivered a keynote lecture at “The Physics of Computation” Conference at MIT in 1981. Feynman considered the problem of whether it was possible to perform an accurate simulation of Nature on a classical computer. As Nature ultimately obeys the laws of quantum mechanics, the problem reduces to simulating a quantum mechanical system on a classical computer. Because of the nature of quantum objects like electrons, truly quantum mechanical calculations on a classical computer rapidly become impractical for more than a few 10’s of electrons.

Feynman then proceeded to consider a new type of computer based on quantum mechanics: a quantum computer. He realized that this was a new type of computer: “Not a Turing machine, but a machine of a different kind.” Interestingly, Feynman did not go on to explore the different capabilities of quantum computers but simply demonstrated how you could use them to simulate true quantum systems.

By his presence at the conference, Feynman stimulated interest both in the physics of computation and in quantum computing. At this conference 30 years later, we heard several talks summarizing progress towards actually building a quantum computer. In the last five years of his life, Feynman gave lectures on computation at Caltech, initially with colleagues Carver Mead and John Hopfield, and for the last three years by himself.

I was fortunate enough to be asked by Feynman to write up his “Lectures on Computation.” The lectures were a veritable *tour de force *and were probably a decade ahead of their time. Feynman considered the limits to computation due to mathematics, thermodynamics, noise, silicon engineering, and quantum mechanics. In the lectures, he also gave his view about the field of computer science: He regarded science as the study of natural systems and classified computer science as engineering since it studied man-made systems.

**Inspiring Later Generations**

Feynman said that he started out very focused on physics and only broadened his studies later in life. There are several fascinating biographies of Feynman but the one I like best is *No Ordinary Genius* by Christopher Sykes. This is a wonderful collection of anecdotes, interview, and articles about Feynman and his wide range of interests—from physics, to painting, to bongo drums and the Challenger Enquiry. Feynman was a wonderful inspiration to the entire scientific community and his enjoyment of and enthusiasm for physics is beautifully captured in the TV interview, “The Pleasure of Finding Things Out,” produced by Christopher Sykes for the BBC. Feynman is forever a reminder that we must try to think differently in order to innovate and succeed.

—*Tony Hey**, corporate vice president of the External Research Division of Microsoft Research*

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