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Physicists' Golden Jubilee

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Amid the bustle of active research -- the swirl of preprints, grant proposals, conference presentations, and all the rest -- it can be difficult for scientists to keep the long view in focus. New ideas and half-ready suggestions tumble forth at the speed of a Twitter feed. During 2013, physicists around the world posted new scientific papers to the central preprint server, arXiv.org, in just one area of specialization (astrophysics), at a rate of about 1.5 new papers every hour of every day (nights and weekends included). New papers on high-energy physics appeared at a comparable rate, covering everything from string theory to the features of the Higgs boson.

Sometimes these papers can cause a stir, as Stephen Hawking's recent ruminations did on whether black holes really exist, with news stories, blog posts, and retweets trending like the latest Kardashian rumors. But most scientific papers pass by with little fanfare, their impact on the overall course of research lost in the noise.

Fifty years ago, physicists also often felt overwhelmed by the fast pace of research and publication in the field, even in the days before email, websites, and social media. In 1964, Physics Abstracts recorded more than 30,000 articles in the physics literature from around the world, across all subfields and specialties. (Climbing fast, the annual total surpassed 90,000 a dozen years later, as I have written about here.)

Most of the papers in 1964 have long since been forgotten, just as many from last year will be. But a handful of those physics articles from 50 years ago have become central to the way physicists think about matter and the cosmos today -- so central, in fact, that we can sometimes forget that these foundational ideas even have a history.

First on my "greatest hits list" from 1964 is the notion that protons and neutrons, the tiny particles that make up atomic nuclei, are themselves made of smaller particles called "quarks." Two physicists, working independently, introduced the idea early in 1964; they were trying to make sense of curious patterns in the masses and interactions of dozens of newly discovered particles. Murray Gell-Mann submitted a brief paper on January 4, 1964, in which he duly credited James Joyce's Finnegan's Wake for the whimsical term "quark." (Joyce's novel appears as Ref. 6 in Gell-Mann's brief references list.) Gell-Mann's two-page article was published in the February 1 edition of the journal Physics Letters. At nearly the same time, George Zweig at CERN wrote a lengthy paper introducing the same basic idea; he called the proposed new entities "aces" instead of quarks, first in a preprint dated January 17, 1964, and expanded in a longer version dated February 21, 1964.

Gell-Mann and Zweig were concerned with the "strong force," the force that binds protons and neutrons within atomic nuclei. Other physicists were thinking about the "weak force," responsible for phenomena like radioactive decay. Experiments suggested that the weak force obeyed specific symmetries, and several theorists recognized that such a force could be modeled if, as in electromagnetism, the weak force arose when sub-atomic particles exchanged special force-carrying particles.

The symmetries demanded that the hypothetical force-carrying particles have no mass at all, just like the massless photons that give rise to electromagnetic forces. Unlike electromagnetism, however, the weak force has a very short range: it is only effective when particles are very close to each other (such as packed tightly within an atomic nucleus), which implied that the force-carrying particles should be very massive. Hence the conundrum: theorists could model the symmetries of the weak force or its short range, but not both.

In short order, several physicists proposed a clever solution: what if the force-carrying particles of the weak force really were massless, but were always trudging through some medium -- a medium that fills all of space and slows the force-carriers' motion, like marbles rolling through molasses? Several versions of that idea appeared in the journal Physical Review Letters over the summer and fall of 1964, in short papers by Francois Englert and Robert Brout (received at the journal on June 26 and published on August 31), by Peter Higgs (received on August 31 and published on October 19), and by Gerald Guralnik, Carl Hagen, and Thomas Kibble (received on October 12 and published on November 16). Higgs's paper noted that the molasses-like medium implied that there should exist another type of massive particle, now known as the "Higgs boson." (Sean Carroll nicely sorts through the details of who did what when in his recent book.) Decades (and billions of dollars) later, two huge teams discovered the Higgs boson in experiments at CERN, as they announced on July 4, 2012.

Particle physicist John Bell was enjoying a sabbatical from CERN in the autumn of 1964 when his thoughts returned to a topic that his advisors had counseled him to forget: the counterintuitive notion of "quantum entanglement." Measuring a property of one particle, the equations of quantum theory seemed to suggest, could affect measurements on a separate particle, even if the two particles were arbitrarily far apart. "Spooky action at a distance," Einstein had written caustically to a friend.

Bell returned to the notion and derived a remarkably elegant expression, quantifying exactly how much the measurements of the two particles could be correlated if the particles behaved independently of each other. He further demonstrated that quantum theory required a stronger connection: the behavior of the two-particle system was indeed greater than the sum of its parts, by a specific, quantitative amount. That meant, Bell realized, that this fundamental feature of quantum theory could be tested in a laboratory, not just taken on faith. He submitted his article to a quirky, short-lived journal entitled simply Physics on November 4, 1964, where it was published later that year.

While all those theorists were thinking about exotic notions like quarks, Higgs particles, and quantum entanglement, two radio astronomers at Bell Labs in New Jersey were struggling to get their huge instrument to work properly. Robert Wilson and Arno Penzias were re-tooling a 20-foot horn-reflector antenna that had been designed for early satellite communications, decking it out to measure emissions from various objects in our galaxy. While working painstakingly to calibrate the instrument, Penzias and Wilson kept finding a stubborn, residual hum: a small signal that simply would not go away, no matter which direction they pointed the antenna.

Desperate to eliminate the noise, they climbed inside the huge antenna to "evict" a pair of pigeons, who had "covered the inside with a white material familiar to all city dwellers," as Wilson later recounted. They also determined that the excess signal could not have been left over from a high-altitude detonation of a nuclear bomb, before such tests were driven underground by the 1963 Limited Test Ban Treaty. Physicists at nearby Princeton University eventually convinced them that their signal derived neither from pigeon droppings nor nuclear blasts: rather, they had detected the "cosmic microwave background radiation," the remnant glow from the hot Big Bang. Penzias and Wilson published a short paper in 1965, reporting data that had been collected throughout the summer and fall of 1964.

With hindsight we can identify these four great triumphs of 1964, little gems buried in physicists' bulging journals. Few did at the time. The idea of quarks, for example, flew in the face of prevailing theoretical fashions. (As a typo, a jab, or a Freudian slip, Gell-Mann described the front-runner model of his day as "a highly promised approach" at the start of his paper, rather than "a highly promising" one.) Zweig's paper on aces was rejected and never appeared in a peer-reviewed journal, though the preprint is still available. Physicists accumulated compelling evidence in favor of the quark hypothesis slowly, piecemeal, over the next decade.

Higgs's paper was also initially rejected by journal referees, and experimental support took far longer to materialize. Bell's elegant paper received almost no attention for years; it fired the imagination of several physicists on the margins long before it entered the mainstream. And Jim Peebles, one of the young Princeton physicists who helped Penzias and Wilson make sense of their strange cosmic signal, had his paper on the effect rejected not once but twice from the Physical Review.

Fewer still could recognize any connections among those ideas at the time. Specialization in science follows a twisting, often unpredictable path, buffeted as much by shifts in funding and priorities as by the total flux of new research articles. It seems that no one at the time thought that matter like Higgs particles could have anything to do with patterns in the cosmic microwave background radiation, or that quantum entanglement might be relevant to churning cosmic events like the death of stars. Yet connections like those -- many of them still speculative -- drove Hawking's latest pronouncements (that caused all the ruckus), and similar ideas fill new papers on arXiv.org every day.

Intellectual fashions change, and peer review is certainly not perfect. Nonetheless, here's hoping that a few of the papers among the torrent in 2014 will impress scientists 50 years from now the way these landmarks from 1964 still resonate today.