**RICHARD FEYNMAN** WAS not your typical scientist. The American theoretical physicist had a penchant for picking safes, was a capable bongo player and made no secret of his fondness of strip clubs where he scribbled equations on napkins between performances.

But Feynman wasn’t just a wacky academic, generating inconsequential theories. Now, 25 years after his death on 15 February 1988, Focus takes a look at his incredible scientific legacy. He’s helped us to understand the inner workings of atoms, inspired a whole branch of technology and provided an insight into the earliest moments of the Universe.

Whatever he did, Feynman did in his own way. Called on to investigate the Challenger Space Shuttle disaster, he used his charm and scientific logic to get to the bottom of what went wrong. It’s a story that will soon be told in a BBC Two drama.

Feynman’s philosophy towards his work was summed up best when he said: “Study hard what interests you the most in the most undisciplined, irreverent and original manner possible.” It’s a philosophy that led him to touch many fields of science.

## Quantum Electrodynamics

**In his late 20s, he developed a theory that spans much of physics and also reaches into the heart of chemistry.**

NOT LONG BEFORE his 30th birthday, in early 1948, Richard Feynman spoke at a meeting of some of the brightest minds in quantum physics in Pocono, Pennsylvania. Teaching theoretical physics at Cornell Universir at the time, he presented his early ideas on quantum electrodynamics (QED) – a theory that describes all interactions involving charged particles. Since the behaviour of atoms and molecules depends on interactions between these particles – things such as electrons and nuclei – QED explains all of chemistry.

The trouble was. nobody understood what Feynman was getting at because he used a new approach to depicting the behaviour of subatomic particles and which later developed into his famous ‘Feynman diagrams’.

At the same meeting at the Pocono Manor Inn, Harvard physicist Julian Schwinger presented another version of QED. Again, no-one understood it – this time because it was so mathematically complicated. And soon after, news came from Japan that another physicist, Sin-ltiro Tomonaga, had independently developed a third version of QED.

Other physicists remained baffled until British-born Freeman Dyson published a paper in 1949 on The Radiation Theories Of Tomonaga, Schwinger And Feynman. which showed that all three theories were equivalent to one another and, crucially, made it clear that Feynman’s version was the easiest one to work with. Dyson had translated Feynman into language that ordinary physicists could understand, and Feynman’s own complete account of QED was published in three major papers over the next three years.

The impact of QED can’t be overestimated. It explains everything that isn’t explained by gravity. It’s also the most accurate theory ever tested by experiments on Earth. One of these involved measuring a property known as the magnetic moment of the electron – a magnetic effect caused by an electron’s spin. Feynman was fond of pointing out that the agreement between the theory and this experiment is better than 0.00000002 per cent – equivalent to measuring the distance from Los Angeles to New York to the thickness of a human hair.

Tomonaga, Schwinger and Feynman shared the Nobel Prize for physics in 1965, but by then Feynman’s version of QED had long been established as the way to tackle problems in quantum electrodynamics. At a meeting of the American Physical Society soon after, Dyson said: “We have the key to the Universe. Quantum electrodynamics works and does everything you wanted it to. We understand how to calculate everything concerned with electrons and photons. Now all that remains is merely to apply the same [ideas] to understand weak interactions, to understand gravitation and to understand nuclear forces.”

He was mostly right. The theory of weak interactions – which explains the radioactive decay of subatomic particles – was developed as the electroweak theory, an extension of QED. The theory of nuclear forces, called quantum chromodynamics or QCD, is closely modelled on QED, as the name suggests. It explains how subatomic particles are held together to form the likes of protons and neutrons. Only gravity stubbornly refuses to come into the fold. But it is fair to say that QED has played a central role in theoretical physics. By his early 30s, Feynman had become the leading physicist of his generation. (Quantum Electrodynamics (QED))

## Nanotechnology

**An advocate of technology on tiny scales, Feynman helped to usher in an era of microscopic engineering.**

FEYNMAN WASN’T ‘JUST’ a theorist. He was interested in practical applications of science and technology. In 1959, just after Christmas, he gave what turned out to be an influential talk to the American Physical Society on the subject ‘There’s Plenty of Room at the Bottom’. He threw out two challenges, each with a USD 1,000 prize. The first was to build an electric motor that would fit inside a cube 1/64th of an inch on each side. To his astonishment, the prize – which he paid himself – was claimed in November 1960 by the engineer William McLellan. McLellan turned up at Feynman’s office at the California Institute of Technology (Caltech) carrying a large wooden box. Feynman suspected it was time to get his chequebook out when the box was opened to reveal a microscope McLellan had brought along so he could see the motor.

The second prize was for the first person to find a way of writing small enough to fit the entire Encyclopedia Britannica on the head of a pin. On that scale, every book ever written would fit on a pamphlet you could carry in your hand, he said. The prize was claimed in 1985 by Tom Newman, a graduate student at Stanford University, who wrote the first page of A Tale Of Two Cities to the required scale, actually on the head of a pin, using an electron beam.

By the mid-1990s, scientists at Los Alamos National Laboratory in New Mexico were writing whole books on steel pins measuring 25mm by 2mm, each storing two gigabytes of information in a permanent form. This, rather than vulnerable electronic media, is the best way to preserve information for posterity.

But Feynman’s key insight was not so much the hardware he envisaged, but the emphasis on storing information as efficiently as possible. Today, physicists manipulate individual atoms and electrons as ‘on-off’ switches to store information in binary code, the ultimate expression of what Feynman envisaged more than half a century ago. (What is Nanotechnology Used for Today?)

## Quantum computing

**Feynman envisioned harnessing the strange world of quantum physics to create super-powerful computers.**

THIS IS A hot topic today, with the prospect of building machines as far in advance of classical computers as classical computers are in advance of the abacus. At a meeting at the Massachusetts Institute of Technology (MIT) in 1981, Feynman gave a talk titled ‘Simulating Physics With Computers’, in which he tackled two questions: is it possible to simulate quantum physics with a quantum computer – a computer that harnesses the power of atoms to process data? And, is it possible to simulate quantum physics with a classical computer?

He gave an example of how a ‘universal quantum simulator’ might work, and said: “I therefore believe it’s true that with a suitable class of quantum machines you could imitate any quantum system, including the physical world.” This is the root of the idea that our whole Universe could be a simulation running inside a computer.

Feynman thought further about computation in general and about quantum computers in particular He gave a course on computation at Caltech from 1984-86 and another in Anaheim in California in 1984, in which he described the basis of a quantum mechanical computer. In that talk, he came up with another of his memorable comments. “It seems that the laws of physics present no barrier to reducing the size of computers until bits are the size of atoms and quantum behaviour holds dominant sway,” he said.

Such machines have now been built. But he never seems to have put two (from his 1981 lecture) and two (from his 1984 lecture) together and realised that such a computer would be fundamentally different from a classical computer not Just in terms of its physics, but in terms of the kinds of problem it could solve. (What is quantum computing?)

## Worlds Within Worlds

**Richard Feynman was the originator of the idea of particles existing inside other particles.**

It Is a cornerstone of the standard model of physics that entities such as protons are not point-like particles, but contain other entities known as quarks. The standard model is the theory concerning the electromagnetic, the weak and the strong nuclear interactions. Quarks interact with one another by exchanging gluons, equivalent to the way charged particles interact by exchanging photons. But it is not always appreciated that this whole package of ideas about particles within particles was developed by Feynman in the 1960s, at a time when the existence of quarks was regarded as a wild idea.

Feynman used the term ‘partons’ to include what are now known as quarks and gluons, and he was instrumental in encouraging the experiments that proved the existence of quarks. American physicist Murray Gell-Mann, Feynman’s sometimes bitter rival and the man credited with the idea of quarks, used to sneer at what he referred to as Feynman’s ‘put-ons’. Like Feynman’s diagrams, this made particle physics accessible even to people without the brain power of a Feynman or a Gell-Mann.

But Feynman had the last word. His ideas were picked up by experimenters at the Stanford Linear Accelerator Center (SLAC), a new particle accelerator built near the university’s main campus. In August 1968, Feynman visited SI-AC and looked over its data. He quickly realised that the results matched the predictions of his parton theory and his ideas spread through the team like wildfire.

Further experiments inspired by this proved the existence of quarks and also of the other kinds of partons predicted by Feynman. The experimenters – Jerome Friedman, Henry Kendall and Richard Taylor – received the Nobel Prize in 1990, two years after Feynman died.

## Superfluidity

**Feynman described the bizarre behaviour of fluids near absolute zero.**

WHAT DO YOU do after you have solved the puzzle of quantum electrodynamics? That would have been enough for most physicists, allowing them to settle down into a quiet life of gentle research. But in the 1950s, still in his 30s, Feynman turned his attention to another great puzzle – superfluidity, the frictionless flow of a fluid.

He tackled the question of how and why liquid helium becomes superfluid at temperatures close to absolute zero (-273″C) in a series of 10 papers – more than he wrote on QED – published between 1953 and 1958. In doing so he devised a satisfactory theory of superfluidity.

The Soviet physicist Lev Landau independently came up with an explanation of the behaviour of liquid helium, essentially the same as Feynman’s but couched in different mathematical language, and received the Nobel Prize for the work in 1962. If it hadn’t been obvious by then that Feynman would soon get the Prize for his work on QED, the Nobel Committee might well have given him a share of Landau’s Prize. He certainly deserved it.

## Feynman diagrams

THE KEYTO Feynman’s way to tackle problems in quantum electrodynamics (QED) and other problems in quantum mechanics – the physics of the very small – are the diagrams that now bear his name. A very basic Feynman diagram represents two electrically charged particles – they might be electrons – which get closer together until they exchange a photon, the ‘particle’ of electromagnetism, before moving apart. This is the QED version of the idea that like charges repel.

Many people think that this is all there is to a Feynman diagram – a kind of pictorial representation to give you an image of what is going on. But the true power of these diagrams is that each line and each meeting of lines actually represents a mathematical expression that reflects the behaviour of subatomic particles. To a trained physicist, a Feynman diagram can be read like a page of equations, but much more quickly.

There’s more. For a start, the wiggly line linking the two electrons doesn’t really represent a single photon, but a sum of all the possible ways the photon could have gone from one electron to the other – a so-called ‘path integral’. It doesn’t even specify which way the photon goes, which is why there is no arrow on it, and why physicists use the word ‘exchange’.

Most Feynman diagrams are much more complicated than this simple example. An electron interacting with the field of a magnet, for example, can emit a photon and then re-absorb the same photon, called a ‘virtual’ photon, after it has interacted with a photon from the magnet. Or it can emit two virtual photons one after the other and re-absorb them. And so on. The calculations get harder as you go on.

In 1975, Feynman bought a new Dodge Tradesman Maxivan. He had a personalized licence plate fitted which spelled QANTUM – only six letters were allowed at the time so there was no space for the first ‘u’. He also had his beloved Feynman diagrams painted on the outside. The van was usually driven by Feynman’s wife, Gweneth, as well as transporting the family on camping holidays. Ralph Leighton, the biographer, film producer and friend of Feynman, who now owns the van, says it’s a symbol of his free spirit, showing his love of exploring the everyday world and of his quest to understand physics.

## Inflation

**Feynman postulated how the Universe was born from nothing.**

IN LECTURES ON gravitation given at Caltech in the early 1960s, Feynman suggested the now fashionable idea that the Universe could have emerged literally from nothing (Big Bang pictured, inset). To do that, the total energy content of the Universe must add up to precisely zero. Feynman realised that while all the energy tied up in the form of mass is positive, the gravitational energy within it is negative, so the two could cancel each other out. For that to happen, the average density of matter in the Universe had to have a very specific value of around one atom per cubic metre known to astronomers as the ‘critical density’. But at the time, observations suggested the density was well below this. That didn’t worry Feynman, who said: “The critical density is just about the best density to use in cosmological problems.”

Unaware of Feynman’s insight, in the 1970s and 1980s cosmologists developed the theory of inflation, which depends on the idea of ‘a Universe from nothing’, and 40 years after Feynman’s lectures, the WMAP satellite proved that the Universe does have the critical density. Few attended Feynman’s gravitation lectures, but in the audience were two students – James Bardeen and James Hartle – who went on to make major contributions to the theory of gravity.