Sean Carroll Brings Physics to the Masses

Sean Carroll, a senior research associate in physics at Caltech, has won the 2014 Andrew Gemant Award from the American Institute of Physics. The award honors "significant contributions to the cultural, artistic, or humanistic dimensions of physics"—in Carroll's case, "extraordinary public outreach on particle physics and cosmology . . . and for his pioneering work communicating with a variety of international audiences using social networking."

"Science should be part of the regular conversation that people have over dinner, along with sports and movies and politics," Carroll says. "The universe belongs to everybody, and we can all share in the quest to understand it."

Between blogging, tweeting, appearing on The Colbert Report and NOVA, and writing books chronicling the hunt for the Higgs boson or exploring why time only flows in one direction, Carroll spends his days as a theoretical physicist coming up with alternative theories of gravity and constructing mathematical models of the interactions between the ordinary matter that we perceive and the unseen "dark matter" and "dark energy" that, taken together, are believed to account for 95 percent of the universe's mass.

Previous awardees include Edwin Krupp, director of Los Angeles's Griffith Observatory; Paula Apsell, senior executive producer of NOVA; and Stephen Hawking.

Carroll will receive the award on January 5, 2015 at the Reuben H. Fleet Science Center in San Diego's Balboa Park and deliver a public lecture.

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50 Years of Quarks

A Milestone in Physics

Caltech's Murray Gell-Mann simplified the world of particle physics in 1964 by standing it on its head. He theorized that protons—subatomic particles as solid as billiard balls and as stable as the universe—were actually cobbled together from bizarre entities, dubbed "quarks," whose properties are unlike anything seen in our world. Unlike protons, quarks cannot be separated from their fellows and studied in isolation; despite this, our understanding of the universe is built on their amply documented existence.

These days, the subatomic particle catalog has hundreds of entries. Back in the 1920s, there were only two—the massive proton, which had a charge of +1 and was found in the atom's nucleus; and the electron, which had very little mass, a charge of –1, and orbited the nucleus. Every proton occupied one of two possible "spin states" in relation to the surrounding space. These spins could easily be flipped in a behavior described by a mathematical construct called the SU(2) symmetry group. "Quantum spin states do not have a familiar analog in everyday experience," says Caltech's Steven Frautschi, professor of theoretical physics, emeritus. "However, they can be turned into one another by 180-degree rotations in ordinary space, which is what SU(2) does." 

In 1932, the neutron was discovered. This new particle appeared to be the proton's close relative—even its mass was the same, to within 0.2 percent—but the neutron had no electric charge. SU(2) symmetry in ordinary space could not account for the neutron's existence, but quantum mechanic Werner Heisenberg fixed the problem by declaring that the two particles were indeed fraternal twins . . . if you took SU(2) from another point of view. Frautschi explains: "Like rotating a physical object in ordinary space, Heisenberg extended SU(2) by rotating the symmetry group in a 'space' that quantum theorists made up." 

Heisenberg gave his rotation a quantum number, now called isospin, which described the particle's interaction with the so-called strong nuclear force. (The strong force overcomes the mutual repulsion between positively charged protons, binding them and neutrons to one another and allowing stable atomic nuclei to exist.) The mathematical treatment of isospin in Heisenberg's theoretical space was identical to that of the proton's spin in ordinary space, allowing neutrons to turn into protons and vice versa. In the physical world, Heisenberg's version of SU(2) is like a slowly spinning roulette wheel after the ball has come to rest—if the white ball (a proton) could transmute itself into a black ball (a neutron) and then back again to a white ball once every revolution.

A comprehensive theory of the strong force was published three years later by Hideki Yukawa of Osaka University. Quantum-mechanical forces need particles to carry them, and Yukawa calculated that the strong-force carriers would be much more massive than electrons but not nearly as massive as protons. Soon after, in 1937, Caltech research fellow Seth Neddermeyer (PhD '35) and Nobel laureate physics professor Carl Anderson (BS '27, PhD '30) stumbled upon a likely candidate: a new particle with about 200 times the electron's mass and about one-ninth the mass of the proton. 

Although it was widely assumed that Neddermeyer and Anderson had found the force-carrying particles that would prove Yukawa's theory, the paper announcing the discovery merely described them as "higher mass states of ordinary electrons." This proved to be the case—the new particles, now called muons, did not behave as Yukawa had predicted but instead behaved exactly like electrons. This offered the first inkling that otherwise identical particles came in multigenerational "families" of very different masses. 

The search for Yukawa's strong-force carriers did not bear fruit until 1947, when particles dubbed pions finally turned up—as did kaons, the massive second-generation members of the pion family. These kaons, however, were oddly long-lived, lasting a quadrillion times longer than expected. ("Long-lived" is relative, as the average kaon decayed into other particles in less than a millionth of a second.) 

Then, in 1953, Murray Gell-Mann, then at the University of Chicago, and Kazuhiko Nishijima (also at Osaka University) independently demystified the kaons' strange longevity by proposing yet another new quantum number to explain it. This number, imaginatively called "strangeness," permits particles possessing it to decay—but only by shedding one strangeness unit at a time. This relatively slow process created stepwise cascades of successively less-strange particles, ultimately ending in particles whose strangeness is zero.

Unfortunately, strangeness and SU(2) did not mesh mathematically. The theorists remained at an impasse; meanwhile, the experimentalists built ever-more-powerful machines that created ever-more-massive, ever-more-exotic particles whose ever-briefer existences could only be inferred by working backward from the collections of mundane particles into which they decayed. 

The mushrooming catalog of discoveries defied all attempts at organization until 1961, when Gell-Mann—who had moved to Caltech in 1955—and Israeli physicist Yuval Ne'eman independently proposed sorting particles into mini-periodic tables organized by electric charge and strangeness number. Gell-Mann dubbed his version the "Eightfold Way," after Buddhism's Eightfold Path to enlightenment, because the tables tended to contain eight members each. 

The Eightfold Way brought physicists full circle, as it proved to be a rotating SU(3) symmetry group. Just as charge had driven the isospin axis in Heisenberg's SU(2) symmetry, strangeness provided a second, perpendicular rotation. In other words, SU(2) spun only around the y axis, as it were, but SU(3) spun on both the x and y axes simultaneously. It was as if the roulette wheel had morphed into a globe spinning around the poles while the polar axis itself spun around two points on the equator. Relationships between particles could be represented as rotations in isospin, in strangeness, or in both. 

Although the Eightfold Way solved one problem, it created another. Whereas SU(2) manifests itself through doublets—the proton-neutron dichotomy—SU(3)'s hallmark is the triplet. "Nature is likely to use this fundamental representation," says physics professor Frautschi, "but there was no sign of triplets in the data." Triplets could be conjured into existence, however, if the rock-solid proton could be broken apart. In that case, SU(3)'s fundamental triplet could be a menu of three hypothetical entities, each with its own unique set of quantum numbers. 

If the menu choices were truly independent—much like allowing a diner to order an enchilada with all beans and no rice on the side, for example—a fundamental triplet offered enough possibilities to build every massive particle known, and then some. Intermediate-mass pions and kaons would contain two menu selections; protons, neutrons, and a slew of more massive particles would be three-item combos. "It's all about making patterns," Frautschi explains. "You write down sets of quantum numbers, add them up, and see what fits." 

However, the numbers refused to add up. Both the two-piece kaon and a three-piece particle called the sigma came in positive, negative, and electrically neutral versions. But if the only charges available to the triplet's members were –1, 0, and +1, no conceivable combination of choices allowed all the other quantum numbers to come out right. 

This should have been the end of the story. Robert Millikan, Caltech's first Nobel laureate, had won his prize for showing that electric charge came only in whole-numbered units. But in 1964, Gell-Mann and George Zweig (PhD '64) independently flew in the face of all that was known by proposing that the fundamental triplet had one member with a +2/3 charge and two members with charges of –1/3

Gell-Mann called the members of his triplet "quarks," after the sentence "Three quarks for Muster Mark!" in James Joyce's Finnegan's Wake. Everything found in the old SU(2) symmetry group could be fashioned from +2/3 "up" quarks and –1/3 "down" quarks, both of which had a strangeness number of zero. A proton was up-up-down, for example; a neutron was down-down-up. The other –1/3 quark had a quantum of strangeness; adding these "strange" quarks to the mix took care of the particles that SU(2) couldn't handle. Since this proposal was so heretical, Gell-Mann presented quarks as no more than an expedient accounting system, writing, "It is fun to speculate about the way quarks would behave if they were physical particles of finite mass (instead of purely mathematical entities . . . )." 

Zweig, meanwhile, called his theoretical constructs "aces," as they were put together into "deuces" and "treys" to make pions and protons. He was also less circumspect than Gell-Mann. "The results . . . seem somewhat miraculous," Zweig wrote. "Perhaps the model is . . . a rather elaborate mnemonic device. [But] there is also the outside chance that the model is a closer approximation to nature than we may think, and that fractionally charged aces abound within us." Sadly, Zweig's paper met a very different fate than Gell-Mann's. Since Zweig was working as a very junior postdoctoral fellow at CERN, the European Center for Particle Physics, all his manuscripts had to be reviewed by his superiors before publication. The senior staff considered Zweig's ideas too outré, and his paper got sent to a file room instead of a journal. He returned to Caltech soon after, joining the faculty. 

Gell-Mann went on to win the Nobel Prize for Physics in 1969—although not for the quark model per se, which was still on thin ice. (The very first experiments demonstrating that protons might contain something else had been run at the Stanford Linear Accelerator the preceding year.) Instead, he was cited "for contributions and discoveries concerning the classification of elementary particles and their interactions."

Quarks have since been shown to be physical particles with finite masses. The up quark has been found to have about half the mass of the down, while the strange quark has been shown to be some 50 times more massive—a sure sign that it represented a second generation of quarks, just as muons had turned out to be second-generation electrons. In 1974, the other second-generation quark turned up—the "charm" quark—followed three years later by the third-generation "bottom" quark. It then took nearly two decades to find what is called the "top" quark—which, as far as we know, completes the quark family tree. 

Gell-Mann was named the Robert Andrews Millikan Professor of Theoretical Physics in 1967—a fitting irony that the man who showed that fractional electric charges are necessary holds the chair named for the man who showed that electric charge is indivisible.

Gell-Mann's paper introducing the quark was all of two pages long; what has been written about quarks since then would fill warehouses. This half-century of discoveries was celebrated at a conference in the 84-year-old Gell-Mann's honor, hosted by Caltech's theoretical high-energy physics group in December, 2013.

Douglas Smith
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Ed Stone Honored

Voyager Project Scientist

Ed Stone, Caltech's David Morrisroe Professor of Physics, has won the aerospace equivalent of an Oscar. On Wednesday, July 16, the American Astronautical Society (AAS) presented him its sixth Lifetime Achievement Award for his "sustained and extraordinary contributions to America's space programs, including innovative planetary missions."

From 1991 to 2001, Stone was the director of Caltech's Jet Propulsion Laboratory, America's lead center for the robotic exploration of the solar system. (The Lab was transferred to NASA at the space agency's birth in 1958.) Among the firsts during Stone's 10-year tenure, JPL launched a successful mission to explore Saturn, its rings, and its moons; dispatched a spacecraft to visit a comet and return samples of its dust cloud to Earth; and went off-roading on Mars. The toaster-oven-sized Sojourner rover covered 100 yards in 80 days, and its six-wheeled body plan has been the standard for all Mars rovers since.

Stone is also the project scientist for JPL's Voyagers, a role he assumed during the mission's detailed design phase in 1972—five years before the twin spacecraft were launched. The two ships parted ways after setting similar courses for Jupiter and Saturn and giving us the first good look at those gas giants and their unexpectedly exotic moons. Voyager 2 continued on the Grand Tour of the outer solar system, visiting Uranus and Neptune, while Voyager 1 steered up and out of the plane of the solar system. In 2012, nearly 35 years to the day after its launch, Voyager 1 sailed beyond the reach of the solar wind and became the first man-made object to enter the interstellar void. "It's a special honor to receive this award and a privilege to have been part of Voyager's exploration of the solar system, a 37-year journey of discovery that now extends into the space between the stars," Stone says.

The AAS's Lifetime Achievement Award is presented in Washington, D.C., on every tenth anniversary of the society's founding. The first award, in 1964, went to Wernher von Braun, who created the Saturn V rocket that would take the Apollo astronauts on the moon. The second award went to JPL director William H. Pickering (BS '32, MS '33, PhD '36), who in 1958 put America's first satellite, Explorer 1, into orbit atop another of von Braun's rockets.

Douglas Smith
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Kip Thorne Discusses First Discovery of Thorne-Żytkow Object

In 1975, Kip Thorne (BS '62, and the Richard P. Feynman Professor of Theoretical Physics, Emeritus) and then-Caltech postdoctoral fellow Anna Żytkow sought the answer to an intriguing question: Would it be possible to have a star that had a neutron star as its core—that is, a hot, dense star composed entirely of neutrons within another more traditional star? Thorne and Żytkow predicted that if a neutron star were at the core of another star, the host star would be a red supergiant—an extremely large, luminous star—and that such red supergiants would have peculiar abundances of elements. Researchers who followed this line of inquiry referred to this hypothetical type of star as a Thorne-Żytkow object (TŻO).

Nearly 40 years later, astronomers believe they may have found such an object: a star labeled HV 2112 and located in the Small Magellanic Cloud, a dwarf galaxy that is a near neighbor of the Milky Way and visible to the naked eye. HV 2112 was identified as a TŻO candidate with the 6.5-meter Magellan Clay telescope on Las Campanas in Chile by Emily Levesque (University of Colorado), Philip Massey (Lowell Observatory; BS '75, MS '75, Caltech), Żytkow (now at the University of Cambridge), and Nidia Morrell (also at the University of Cambridge).

We recently sat down with Thorne to ask how it feels to have astronomers discover something whose existence he postulated decades before.

When you came up with the idea of TŻOs, were you trying to explain anything that had been observed, or was it a simple "what if?" speculation?

It was totally theoretical. We weren't the first people to ask the question either. In the mid-1930s, theoretical physicist George Gamow speculated about these kinds of objects and wondered if even our sun might have a neutron star in its core. That was soon after Caltech's Fritz Zwicky conceived the idea of a neutron star. But Gamow never did anything quantitative with his speculations.

The idea of seriously pursuing what these things might look like was due to Bohdan Paczynski, a superb astrophysicist on the faculty of the University of Warsaw. In the early 1970s, he would shuttle back and forth between Caltech, where he would spend about three months a year, and Warsaw, where he stayed for nine months. He had a real advantage over everybody else during this era when people were trying to understand stellar structure and stellar evolution in depth. Nine months of the year he didn't have a computer available, so he had to think. Then during the three months he was at Caltech, he could compute.

Paczynski was the leading person in the world in understanding the late stages of the evolution of stars. He suggested to his postdoctoral student Anna Żytkow that she look into this idea of stars with neutron cores, and then Anna easily talked me into joining her on the project, and came to Caltech for a second postdoc. I had the expertise in relativity, and she had a lot better understanding of the astrophysics of stars than I did. So it became a very enjoyable collaboration. For me it was a learning process. As one often does as a professor, I learned from working with a superb postdoc who had key knowledge and experience that I did not have.

What were the properties of TŻOs as you and Żytkow theorized them?

We didn't know in advance what they would look like, though we thought—correctly it turns out—that they would be red supergiants. Our calculations showed that if the star was heavier than about 11 suns, it would have a shell of burning material around the neutron core, a shell that would generate new elements as it burned. Convection, the circulation of hot gas inside the star, would reach right into the burning shell and carry the products of burning all the way to the surface of the star long before the burning was complete. This convection, reaching into a burning shell, was unlike anything seen in any other kind of star.

Is this how you get different elements in TŻOs than those ordinarily seen on the surface of a star?

That's right. We could see that the elements produced would be peculiar, but our calculations were not good enough to make this quantitative. In the 1990s, a graduate student of mine named Garrett Biehle (PhD '93) worked out, with considerable reliability, what the products of nuclear burning would be. He predicted unusually large amounts of rubidium and molybdenum; and a bit later Philipp Podsiadlowski, Robert Cannon, and Martin Rees at the University of Cambridge showed there would also be a lot of lithium.

It is excess rubidium, molybdenum, and lithium that Żytkow and her colleagues have found in HV 2112.

Does that mean TŻOs are fairly easy to recognize with a spectrographic analysis, which can determine the elements of a star?

No, it's not easy! TŻOs should have a unique signature, but these objects would be pretty rare.

What are the circumstances in which a TŻO would develop?

As far as we understand it, the most likely way these things form is that a neutron star cannibalizes the core of a companion star. You have a neutron star orbiting around a companion star, and they spiral together, and the neutron star takes up residence in the core of the companion. Bohdan Paczynski and Jerry Ostriker, an astrophysicist at Princeton University, speculated this would happen way back in 1975 while I was doing my original work with Żytkow, and subsequent analyses have confirmed it.

The other way a TŻO might develop is from the supernova explosion that makes the neutron star. In a supernova that creates a neutron star, matter is ejected in an asymmetric way. Occasionally these kicks resulting from the ejection of matter will drive the neutron star into the interior of the companion star, according to analyses by Peter Leonard and Jack Hills at Los Alamos, and Rachel Dewey at JPL.

Is there anything other than peculiar element abundances that would indicate a TŻO? Does it look different from other red supergiant stars?

TŻOs are the most highly luminous of red supergiant stars but not so much so that you could pick them out from the crowd: all red supergiants are very bright. I think the only way to identify them is through these element abundances.

Are you convinced that this star discovered by Żytkow and her colleagues is a TŻO?

The evidence that HV 2112 is a TŻO is strong but not ironclad. Certainly it's by far the best candidate for a TŻO that anyone has seen, but additional confirmation is needed.

How does it feel to hear that something you imagined on paper so long ago has been seen out in the universe?

It's certainly satisfying. It's an area of astrophysics that I dipped into briefly and then left. That's one of the lovely things about being a theorist: you can dip into a huge number of different areas. One of the things I've most enjoyed about my career is moving from one area to another and learning new astrophysics. Anna Żytkow deserves the lion's share of the credit for this finding. She pushed very hard on observers to get some good telescope time. It's her tenacity more than anything else that made this happen.

What are you working on now that you are retired?

I'm an executive producer of the film Interstellar, directed by Christopher Nolan and based in part on the science I've done during my Caltech career. Greater secrecy surrounds Interstellar than most any movie that's been done in Hollywood. I'm not allowed to talk about it, but let's just say that I've been spending a lot of my time on it in the last year. And I've recently finished writing a book about the science in Interstellar.

The other major project I'm wrapping up is a textbook that I've written with Roger Blandford [formerly a professor at Caltech; now on the faculty at Stanford]: Modern Classical Physics. It's based on a course that Roger or I taught every other year at Caltech from 1980 until my retirement in 2009. It covers fluid mechanics, elasticity, optics, statistical physics, plasma physics, and curved space-time—that is, everything in classical physics that any PhD physicist should be familiar with, but usually isn't. This week we delivered the manuscript to the copy editor. After 34 years of developing this monumental treatise/textbook, it's quite a relief.

I'm also working with some of my former students and postdocs on trying to understand the nonlinear dynamics of curved space-time. For this we gain insights from numerical relativity: simulations of the collisions of spinning black holes. But I've had to shelve this work for the past half year due to the pressures of the movie and books. I hope to return to it soon.

Cynthia Eller
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