We're a bit late with birthday greetings, but still wanted to weigh in with well wishes as the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity turns 50. It first appeared in a paper published in The Physical Review in July 1957, and is considered one of the most important milestones in 20th century physics. Small wonder that there have been so many honorary conferences organized this year to commemorate the occasion, most recently an APS-sponsored conference held October 10-13 at the University of Illinois, Urbana-Champaign. Eight Nobel Laureates were on hand to give talks, including both Leon Cooper (the "C") and Robert Schrieffer (the "S"). John Bardeen (the "B") missed the festivities; he died in 1991. The APS presented a bronze plaque marking the old Physics Building at UIUC as a "site of historic significance." And the university chose this occasion to announce its new Institute for Condensed Matter Theory, making it a truly golden anniversary in the field of condensed matter physics.
"What's all the fuss about?" the average non-scientist is probably wondering. Well, back in 1911, a Dutch physicist named Heike Kamerlingh Onnes was studying a variety of materials at ultra-low temperatures (i.e., close to absolute zero). He found that supercooled mercury lost its resistance completely to the flow of electricity and dubbed the phenomenon superconductivity. Later experiments revealed the same effect in tin, lead, and other pure metals. It was truly a momentous experimental discovery, but it lacked a theoretical underpinning. Try as they might, physicists couldn't explain the actual mechanism behind superconductivity.
Things got weirder the more they looked into this mysterious effect. For instance, in 1933, a physicist named Walter Meissner found that superconductors would expel a magnetic field, making it possible to levitate a magnet -- the "Meissner effect." And around 1950, physicists found that mercury isotopes with lower atomic weight became superconducting at a slightly higher temperature -- the "isotope effect." This seemed to suggest that the motion of atoms in a material, and not just the electrons, was involved in superconductivity.
Felix Bloch became so frustrated with the knotty problem that he postulated his own eponymous "Bloch's Theorem: Superconductivity is impossible" -- even though it was clearly possible, since it had been experimentally confirmed again and again, in an ever-growing number of materials. Richard Feynman admitted that he'd "spent an awful lot of time in trying to understand it... I developed an emotional block against the problem of conductivity." In fact, when he first learned about the seminal BCS paper, "I could not bring myself to read it for a long time." It took a lot to stump a scientist of Feynman's caliber, and he wasn't the only big-brained physicist mulling over the problem.
Technically, Bardeen was an electrical engineer by training, at least early on in his career. He was born in Madison, Wisconsin; his father, Charles, as a professor of anatomy and helped found the medical school at the University of Wisconsin, Madison (UWM). His academic brilliance showed up early: in third grade, his parents moved him up into junior high, and he started college at age 15, majoring in engineering at UWM. A bit surprisingly, considering his low-key temperament, he was a frat boy, a member of Zeta Psi. (Wikipedia tells me that he played billiards to raise the membership fees.) Yet he was also a member of the Tau Beta Pi engineering honor society. Maybe fraternities were different in those days. He ended up earning both a BS and a master's degree in his five years at UWM.
Bardeen worked for awhile at Gulf Research Laboratories in Pittsburgh, but quickly became bored with the work, and decided to earn his PhD in mathematical physics from Princeton University and embark on a research career. His thesis work was in solid-state physics, working with Eugene Wigner, among others, giving him experience that would come in handy years later when he found himself at Bell Labs, struggling to invent a working transistor with two colleagues, William Shockley and Walter Brattain. They finally achieved the first point-contact transistor on December 23, 1947. As most everyone knows by now, the transistor revolutionized the electronics industry. We owe our computers, our MP3 players, indeed, the entire online Information Age, to these three men toiling away in a Bell Labs laboratory during the holidays, when everyone else was drinking eggnog and singing Christmas carols.
Global recognition was not long in coming. The morning of November 1, 1956, Bardeen was scrambling eggs for breakfast while listening to the radio. That's how he learned that he'd just been awarded the Nobel Prize in Physics for inventing the transistor, along with Shockley and Brattain. Apparently he dropped the frying pan in his excitement to inform his wife. A few fun behind-the-scenes Nobel factoids: just before the ceremony, Bardeen found his white vest and white tie had turned green in the laundry, and had to borrow replacements from Brattain. The two men were so nervous before receiving their awards that they split a bottle of quinine to settle their stomachs.
By 1951, the University of Illinois had managed to lure Bardeen away from Bell Labs with the promise of letting him research whatever he wanted. When news of the isotope effect appeared, Bardeen turned his attention back to the problem of superconductivity. He didn't crack it right away, but he and his colleague, David Pines, did supply a critical missing piece. They showed that electrons -- which normally show a strong electrostatic repulsion for each other -- nevertheless could have a sort of indirect attraction, namely by creating vibrations among the lattice atoms, and those vibrations could in turn affect other electrons.
The breakthrough began in the mid-1950s, when Bardeen teamed up with Cooper (then a postdoctoral fellow) and Schrieffer, who was still a graduate student. Cooper supplied the "C" part when he figured out that electrons in a superconductor don't behave as if they were individual particles, but as pairs, now known as "Cooper pairs." Apply an electrical voltage to a superconductor, and you'll find that all those Cooper pairs move as a single entity, creating an electrical current. Cut off the voltage, and instead of gradually dissipating, the current will continue to flow indefinitely because the pairs encounter no resistance to their motion. It only works at ultra-low temperatures: the Cooper pairs separate into individual electrons as the material warms up.
Now for the "S" part: Schrieffer had his own breakthrough insight in early 1957 while riding on a NYC subway. (Based on my years in the Big Apple, most subway riders are probably too distracted by the advertisements for local celebrity dermatologist "Doctor Zee," or the presence of an incontinent homeless individual two seats away, to come up with revolutionary breakthroughs in physics, but Schrieffer beat the odds.) You could emulate Wikipedia and say he "figured out how to mathematically describe the enormous collection of Cooper pairs in a superconductor with one single wave function." Or -- if you're like me, and this makes your eyes glaze over in bewilderment -- you can think of it this way: Instead of crystallizing into a lattice like when water turns to ice, at those very low temperatures, the electrons were organizing and condensing into what amounted to a weird state of matter that permitted the free flow of electricity. Schrieffer himself later compared the concept to a popular dance of that time called the Frug, in which dance partners could be separated by other couples on the dance floor, yet still remained a pair. In the same way, the Cooper pairs in a superconducting material were oblivious to other electrons and the lattice, which meant they could move without hindrance.
Schrieffer's insight provided the final piece of the puzzle, causing Bardeen to observe, in his typically quiet manner, "Well, I think we've explained superconductivity" -- probably in much the same tone of voice as one would say, "Well, I guess it's time for lunch." Their theory explained both the isotope effect and the fact that magnetic fields below a certain strength couldn't quite penetrate superconductors. it also explained why the superconducting phenomenon could only be observed at very cold temperatures near absolute zero: any warmer, and the thermal jiggling would break up the Cooper pairs, disrupting their elegantly balanced quantum dance. In short, Bardeen later recalled, "All the hitherto puzzling features of superconductors fitted neatly together like the pieces of a jigsaw puzzle."
And thus it came to pass that Bardeen found himself the recipient of yet another Nobel Prize in physics -- at the time, he was the first person to win twice in the same field. (Marie Curie, Linus Pauling, and Frederick Sanger all won two Nobel Prizes, just not in the same field.) Another fun behind-the-scenes anecdote: When he won the prize the first time, Bardeen only brought one of three children to the ceremony in Stockholm because his sons were both at Harvard and he was reluctant to interrupt their studies. Sweden's King Gustav IV scolded him for doing so, and Bardeen solemnly assured the king that the next time he won the Nobel Prize, he would bring his entire family. I'm sure Bardeen never expected to make good on that promise, but when lightning did indeed strike twice for him, he made sure all three of his children attended the second ceremony.
It's a bit astounding that the BCS theory hasn't really been refined that much over the ensuing 50 years. Apparently, they got it right the first time. High-temperature superconductivity, discovered in 1986, remains a bit of a puzzle: the effect still relies on electron pairing, but the BCS theory doesn't quite apply. Still, it's only been 20 years, compared to the 50-year lapse between the original observation of superconductivity in metals and the development of BCS theory to explain it. High-Tc theory still has some wriggle room.
There has been some innovation shedding further light on the inner workings of superconductivity. For instance, last year a University of Arizona physicist named Andrei Lebed caused a few ripples in the physics community with his discovery that strong magnetism changes the basic, intrinsic properties of the flowing electrons -- an "exotic" kind of superconductivity. He's interested in the physical nature of the Cooper pairs. Whereas in the past, they have been treated as behaving like elementary particles, with correspondingly fixed properties. Lebed asserts that, in fact, "[S]uperconducting electron pairs are not unchanged elementary particles, but rather, complex objects with characteristics that depend on the strength of the magnetic field." And in the presence of super-strong magnetic fields, exotic Cooper pairs are created that follow the weird laws of quantum mechanics: the electron pairs are both rotating and non-rotating at the same time. Hmmm. Curiouser and curiouser.
Superfluidity is an extension of BCS theory, in that it describes a state in which liquid, like current in superconductors, can flow without resistance -- it literally has zero viscosity. Furthermore, BCS theory has provided a useful model for physicists working on everything from the behavior of subatomic particles to the inner workings of ultra-dense neutron stars. Too esoteric for you? Superconductivity, which the theory explains, is responsible for such life-altering technologies as MRI, radio telescopes, and superconducting quantum interference devices (SQUIDs) -- the latter used to make very sensitive geologic measurements,, among other things. High-temperature superconductivity is especially promising in power transmission: its ability to send current over longer distances with fewer losses could result in major energy savings, although to date such a system has yet to be implemented.
No wonder Bardeen appeared on LIFE Magazine's list of "100 Most Influential Americans of the Century" in 1990, one year before he died. Yet for all the accolades he received over the course of his stellar career, Bardeen never let that sort of thing go to his head. Almost every colleague, friend and biographer describes Bardeen as a most ordinary man, who didn't behave like the stereotypical "genius" physicist. He liked to golf and go on picnics. He hosted cookouts for friends and family, some of whom weren't even aware of his remarkable scientific accomplishments. What made his impact on physics extraordinary was his gift of pinpointing interesting problems in physics, selecting the right collaborators -- making sure to bring both experimentalists and theorists to the table -- and keeping his eye focused on the ball, worrying away at the problem until he arrived at a likely solution.
Alas, his trademark humility and insistence on bucking the "crazy genius scientist" stereotype meant that "the public and the media often overlooked him," according to University of Illinois historian Lillian Hoddeson, who wrote a book about Bardeen. And that's a shame. So in addition to wishing BCS theory a well-earned golden anniversary, here's to men like John Bardeen -- truly the people's physicist. We reap the benefits of his work every day, even if few of us know his name.