Astronomers Peer Inside Stars, Finding Giant Magnets

Astronomers have for the first time probed the magnetic fields in the mysterious inner regions of stars, finding they are strongly magnetized.

Using a technique called asteroseismology, the scientists were able to calculate the magnetic field strengths in the fusion-powered hearts of dozens of red giants, stars that are evolved versions of our sun.

"In the same way medical ultrasound uses sound waves to image the interior of the human body, asteroseismology uses sound waves generated by turbulence on the surface of stars to probe their inner properties," says Caltech postdoctoral researcher Jim Fuller, who co-led a new study detailing the research.

The findings, published in the October 23 issue of Science, will help astronomers better understand the life and death of stars. Magnetic fields likely determine the interior rotation rates of stars; such rates have dramatic effects on how the stars evolve.

Until now, astronomers have been able to study the magnetic fields of stars only on their surfaces, and have had to use supercomputer models to simulate the fields near the cores, where the nuclear-fusion process takes place. "We still don't know what the center of our own sun looks like," Fuller says.

Red giants have a different physical makeup from so-called main-sequence stars such as our sun—one that makes them ideal for asteroseismology (a field that was born at Caltech in 1962, when the late physicist and astronomer Robert Leighton discovered the solar oscillations using the solar telescopes at Mount Wilson). The cores of red-giant stars are much denser than those of younger stars. As a consequence, sound waves do not reflect off the cores, as they do in stars like our sun. Instead, the sound waves are transformed into another class of waves, called gravity waves.

"It turns out the gravity waves that we see in the red giants do propagate all the way to the center of these stars," says co-lead author Matteo Cantiello, a specialist in stellar astrophysics from UC Santa Barbara's Kavli Institute for Theoretical Physics (KITP).

This conversion from sound waves to gravity waves has major consequences for the tiny shape changes, or oscillations, that red giants undergo. "Depending on their size and internal structure, stars oscillate in different patterns," Fuller says. In one form of oscillation pattern, known as the dipole mode, one hemisphere of the star becomes brighter while the other becomes dimmer. Astronomers observe these oscillations in a star by measuring how its light varies over time.

When strong magnetic fields are present in a star's core, the fields can disrupt the propagation of gravity waves, causing some of the waves to lose energy and become trapped within the core. Fuller and his coauthors have coined the term "magnetic greenhouse effect" to describe this phenomenon because it works similarly to the greenhouse effect on Earth, in which greenhouse gases in the atmosphere help trap heat from the sun. The trapping of gravity waves inside a red giant causes some of the energy of the star's oscillation to be lost, and the result is a smaller than expected dipole mode.

In 2013, NASA's Kepler space telescope, which can measure stellar brightness variations with incredibly high precision, detected dipole-mode damping in several red giants. Dennis Stello, an astronomer at the University of Sydney, brought the Kepler data to the attention of Fuller and Cantiello. Working in collaboration with KITP director Lars Bildsten and Rafael Garcia of France's Alternative Energies and Atomic Energy Commission, the scientists showed that the magnetic greenhouse effect was the most likely explanation for dipole-mode damping in the red giants. Their calculations revealed that the internal magnetic fields of the red giants were as much as 10 million times stronger than Earth's magnetic field.

"This is exciting, as internal magnetic fields play an important role for the evolution and ultimate fate of stars," says Professor of Theoretical Astrophysics Sterl Phinney, Caltech's executive officer for astronomy, who was not involved in the study.

A better understanding of the interior magnetic fields of stars could also help settle a debate about the origin of powerful magnetic fields on the surfaces of certain neutron stars and white dwarfs, two classes of stellar corpses that form when stars die.

"The magnetic fields that they find in the red-giant cores are comparable to those of the strongly magnetized white dwarfs," Phinney says. "The fact that only some of the red giants show the dipole suppression, which indicates strong core fields, may well be related to why only some stars leave behind remnants with strong magnetic fields after they die."

The asteroseismology technique the team used to probe red giants probably will not work with our sun. "However," Fuller says, "stellar oscillations are our best probe of the interiors of stars, so more surprises are likely."

The paper is entitled "Asteroseismology can reveal strong internal magnetic fields in red giant stars." In addition to Fuller, Cantiello, Garcia, and Bildsten, the other coauthor is Dennis Stello from the University of Sydney. Jim Fuller was supported by the National Science Foundation and a Lee A. DuBridge Postdoctoral Fellowship at Caltech.

This work was written collaboratively on the web. An Open Science version of the published paper can be found on Authorea, including a layperson's summary.

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Hitlin Awarded Prestigious Prize for Particle Physics

Professor of Physics David Hitlin has been awarded the 2016 W.K.H. Panofsky Prize in Experimental Particle Physics from the American Physical Society. The $10,000 prize, established in 1985, is given annually to recognize achievements in high energy particle physics. The 2016 prize is shared with Jonathan Dorfan, Stephen Olsen, and Fumihiko Takasaki "for leadership in the BABAR and Belle experiments, which established the violation of CP symmetry in B meson decay, and furthered our understanding of quark mixing and quantum chromodynamics," according to the prize citation.

Hitlin was the founding spokesperson of the BABAR collaboration, and Dorfan directed the construction of the PEP-II asymmetric B factory and was also the technical coordinator during construction of BABAR. Dorfan is the director emeritus of the SLAC National Accelerator Laboratory and president of the Okinawa Institute of Science and Technology. BABAR is a particle detector designed to study unstable elementary particles called neutral B mesons, composed of a third-generation bottom quark and a down quark.

BABAR, and a similar experiment called Belle, carried out simultaneously at the High Energy Accelerator Research Organization in Tsukuba, Japan, led by Stephen Olsen and his colleague Fumihiko Takasaki, made the first measurements of the time-dependent Charge-Parity (CP)-violating asymmetry in the rare decay of neutral B mesons to a J/psi meson and a short-lived neutral K meson. These results established the validity of the model of Makoto Kobayashi and Toshihide Maskawa, who showed that if there were three families of quarks, there would be a CP-violating phase. This phase could cause a difference in the decay rate of neutral B mesons and their antiparticles to this final state. This CP-violating phase could help explain why our universe is dominated by matter, while at the time of the Big Bang there were equal amounts of matter and antimatter.

In 2001, BABAR and Belle convincingly measured this CP-violating phase, finding its value to be consistent with the predictions of the model. Kobayashi and Maskawa subsequently shared the Nobel Prize in Physics in 2008 for "the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature."

"This is an award that is long overdue," says Mark Wise, Caltech's John A. McCone Professor of High Energy Physics. "The measurements by BABAR and Belle of CP non-conservation in B decays showed that the CP non-conservation observed in weak decays is intimately connected with the generation of mass for the quarks. It didn't have to be that way, and this discovery was a milestone achievement in particle physics."

While BABAR ended data taking in 2008, the analysis of the data set continues and so far has led to 550 published papers. BABAR has subsequently refined and extended its measurements to many other B meson decays as well as charm meson and tau lepton decays, that have provided compelling evidence for the validity of the Standard Model of elementary particle physics, which unifies into a single theory the smallest building blocks of matter and three of nature's four forces.

"This award recognizes the contributions of more than 600 physicists and engineers from 75 institutions in 13 countries," Hitlin says. "I was fortunate to have the opportunity to lead this extraordinary collaboration."

Hitlin arrived at Caltech in 1979 as an associate professor and became a full professor in 1986. He was the principal investigator of the Caltech High Energy Physics group from 1994 to 2010 and is a Fellow of the American Physical Society.

The name BABAR is not an acronym, as are the names of most high energy physics experiments. Rather, it is derived from the name of the elephant Babar, the main character of a set of children's books, and refers to the B and anti-B (Bbar) mesons, which are at the heart of the experiment. 

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Physicist David Hsieh Wins Packard Fellowship

Caltech physicist David Hsieh, who devises and builds new laser-based instruments in order to identify, understand, and manipulate novel phases of matter, has been awarded a Packard Fellowship for Science and Engineering. These fellowships, awarded annually by the David and Lucile Packard Foundation, "provide early-career scientists with flexible funding and the freedom to take risks and explore new frontiers in their fields," according to the foundation.

Hsieh, an assistant professor of physics, is one of 18 new Packard Fellows who will receive individual grants of $875,000 distributed over five years.

Hsieh received the news that he had been selected as a fellow at a casual meeting with Fiona Harrison, the new chair of the Division of Physics, Mathematics, and Astronomy, at the Red Door Café on campus. He was completely caught off guard. "I thought we were just meeting to talk about how things were going," he says. "Then partway through our conversation, Fiona's phone rang, and I asked if I should leave to give her some privacy. She said, 'No, you should stay. This person wants to talk to you.'"

On the other end was Xiao-Wei Wang, Packard Fellows program manager at the Foundation. Hsieh recounts that Wang asked if he was sitting in a comfortable place and then told him the good news.

Hsieh says he is honored by the recognition and excited about the freedom that the grant will provide. "Both the sum and the flexibility with which these funds can be used gives me a very unique opportunity to pursue the riskiest branches of physics that I'm interested in," he says.

He plans to explore the possibility of using light to alter the electronic phase of materials in order to manipulate their macroscopic properties. For example, he says, "Can I use electromagnetic radiation to change something from a metal to an insulator, or from a magnet to a nonmagnet?" If such tailoring of materials properties with light is possible, it could open the door to the creation of more powerful and more versatile electronic devices, potentially at a lower energy cost.

"I'm thrilled that David has won this prestigious award. It will give him the flexibility to pursue high-risk, high-reward projects," says Fiona Harrison, the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics, and Astronomy and the Benjamin M. Rosen Professor of Physics. "Being there when he heard the news was definitely a highlight of my job as division chair so far."

Hsieh earned his BS in physics and mathematics in 2003 from Stanford University and his PhD from Princeton University in 2009. He was a Pappalardo Fellow at MIT before joining the Caltech faculty in 2012.

He joins 23 other current Caltech researchers who have been named Packard Fellows since the program's inception in 1988. To date, the Packard Foundation, a private family foundation created in 1964 by Hewlett-Packard Company cofounder David Packard and his wife, Lucile, has awarded $362 million to support 541 scientists and engineers from 52 national universities. Each year, participating universities are invited to nominate two faculty members for consideration by the 12-member Fellowship Advisory Panel of internationally recognized scientists and engineers, which recommends nominees for approval by the Packard Foundation Board of Trustees. 

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Robotic Astronomer Finds New Home

When it comes to telescopes, bigger is almost always better. Larger telescopes collect more light, enabling them to see the farthest and faintest objects in the sky. But older and smaller telescopes can still do important science—and, when a less-in-demand, smaller telescope is equipped with an autonomous, robotic, state-of-the-art laser adaptive optics system to erase the atmosphere's blurring effect, the data that can be obtained give us a unique window on the universe around us.

For the last three years, the world's only robotic adaptive optics system, Robo-AO, has been making new discoveries with the 60-inch telescope at Caltech's Palomar Observatory north of San Diego. One of the major projects that Robo-AO undertook was a comprehensive study of stars that were identified by NASA's Kepler space telescope as likely to host planets. Now, the instrument, developed at Caltech, is moving to a new home: the 2.1-meter telescope at Kitt Peak National Observatory in Arizona.

The hallmark of Robo-AO is its efficiency. Whereas other adaptive optics systems must be monitored and set up by several people, Robo-AO is fully robotic, which makes it 10 times more efficient than any other adaptive optics system in the world. Robo-AO is a joint project of the Caltech Optical Observatories (COO) and the Inter-University Center for Astronomy and Astrophysics (ICUAA) in Pune, India, guided by principal investigator Christoph Baranec (BS '01), a former Caltech postdoc (now a professor and Sloan Research Fellow at the University of Hawaii), and co-principal investigator Professor A. N. Ramaprakash of the IUCAA.

Budget cuts forced the National Optical Astronomy Observatory, which operates the Kitt Peak Observatory for the National Science Foundation, to seek new operators for the observatory's 2.1-meter telescope. "We immediately sent a proposal," says Shri Kulkarni, Caltech's John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science and director of the COO. "The larger size of the telescope and the excellent site made for a perfect match between Robo-AO and the 2.1-m telescope." The proposal won. Funding for three years of operation was provided through a $1 million dollar donation from Rohan Murty—an entrepreneur based in Boston and Bangalore—and his family. "Automating the discovery of the universe impacts one of mankind's oldest activities: to look up at the heavens and wonder about our place in the universe," says Murty. "This activity has been the source of inspiration for religion, poetry, literature, science, and culture in general. Hence, I am pleased to support a dedicated telescopic facility for Robo-AO, which will serve as a new frontier for what computer science can do for astronomy and for mankind, in general."

"What Robo-AO is best in the world at doing is large surveys at high angular resolution," says Reed Riddle, the project scientist for Robo-AO's Kitt Peak operation. This makes it ideal for follow-up observations of the candidate planetary systems that Kepler continues to find. Kepler detects exoplanets when they pass in front of, or transit, their stars, causing a slight dip in starlight. This allows astronomers to both discover and measure the sizes of thousands of exoplanets. "However, if another star is close to the target star, it dilutes the depth of the transit dip, which means we measure incorrect sizes for the planet candidates," says Nicholas Law, a former Caltech postdoc who is now a professor at the University of North Carolina, Chapel Hill, and a Robo-AO team member. Astronomers must study these planets in more detail to confirm that they are real and not a spurious reading caused by nearby stars. To date, Robo-AO has evaluated about 90 percent of Kepler's targets—some 3,300 potential planetary systems. In comparison, astronomers using all other adaptive optics systems in the world put together have investigated far fewer of the Kepler planet candidates.

But Robo-AO can do much more than evaluate exoplanets, Kulkarni notes. "As a dedicated facility, Robo-AO on the 2.1-m telescope can undertake other long-term studies, such as monitoring weather on Neptune, and large surveys, such as tracking and resolving asteroids. Furthermore, it is an excellent opportunity for young people to have hands-on experience with a state-of-the-art facility," he says. Indeed, Caltech graduate student Rebecca Jensen-Clem will be using Robo-AO for her thesis project to study weather on brown dwarfs, and postdoctoral fellow Dmitry Duev, though trained in radio astronomy, plans to use the facility for an ambitious study of binarity in asteroids—whether an asteroid is part of a single or double system (observing a binary orbit allows for a measurement of the masses of both members of the pair).

Over the past two decades, adaptive optics has revolutionized astronomy, giving ground-based telescopes the ability to obtain pristine views that previously had been possible only from space. Adaptive systems, now found on all large telescopes, such as the Keck Observatory in Hawaii, send a laser beam into the sky, where the light scatters off air molecules and returns to the telescope, traveling through the same atmospheric turbulence that blurs the telescope's images. Once a detector measures this blur, a deformable mirror can then warp the light in such a way that the distortion is precisely canceled out.

"Don't get me wrong, the adaptive optics on big telescopes offer us the best angular resolution," says Baranec, who also serves as the Robo-AO instrument scientist.  Robo-AO, with its high on-sky efficiency, excels in its ability to observe a large sample rapidly. Using its data, astronomers are working to determine how the prevalence of giant gas planets close to their stars depends on the presence of a companion star. Robo-AO also has discovered one of only two known quadruple star systems containing planets. Twenty science papers have been published since the system was commissioned three years ago, and several more are in progress.

In this same vein, Robo-AO has surveyed 3,000 of the nearest stars searching for multiple star systems (our solar system has only one star but many others contain stars two, three, even more stellar companions). Robo-AO will continue this program, investigating tens of thousands of stars in a survey Kulkarni describes as undertaking "galactic geography."

And there will be more extrasolar planets discovered. The Kepler mission is winding down, but its successor, the Transiting Exoplanet Survey Satellite, scheduled for launch in 2017, is expected to find tens of thousands of new worlds—which Robo-AO will be uniquely suited to study. "The Robo-AO team is making one-sixth of its observing time available to the U.S. astronomical community. In addition we have had discussions with several colleagues at astronomy departments in the U.S., India, and Europe with the view of expanding the range of astronomical research that can be undertaken by Robo-AO," Kulkarni says.

So while it may not be the biggest system in use, Kulkarni says, Robo-AO does its job very well. "Not everything has to be big telescopes or big space missions," he says. "It's a little celebration of less-is-more."

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Patrick Rall: A Passion for Entanglement

SURF participant Patrick Rall and a summer of quantum information science.

For senior Patrick Rall, a native of Munich, Germany, the summer offers one of the year's few chances to visit home. But for the last two summers, Rall, a Caltech physics major, has been spending his summers on campus, drawn by another opportunity—the chance to conduct cutting-edge research while being mentored by John Preskill, the Richard P. Feynman Professor of Theoretical Physics, as part of the Institute's Summer Undergraduate Research Fellowships (SURF) program. Last year, Rall worked in the laser lab of Assistant Professor of Physics David Hsieh on a condensed matter physics experiment. This summer, he switched his attention to quantum information science, a new field that seeks to exploit quantum mechanical effects to create next-generation computers that will be faster and more secure than those currently available.

A key idea in quantum mechanics is superposition of states. Subatomic particles like electrons can be described as having multiple positions, or more than one speed or energy level. This is illustrated by the thought experiment developed in 1935 by Austrian physicist Edwin Schrödinger. In it, a cat is placed into an imaginary box containing a bottle of poison, radioactive material, and a radiation detector. If a radioactive particle decays and radiation is detected inside the box, the poison is released and the cat is killed. But according to quantum mechanics, the cat could be simultaneously alive and dead. Yet if one were to open the lid of the box, the cat would become alive or dead. By opening the box, we have destroyed the quantum nature of the state; that is to say, the observation itself affects the outcome, and yet that outcome is randomly determined.

"Where this gets really interesting is when more than one cat gets involved," Rall says. "Then we can have states where looking at one cat determines the outcome of looking at the other, even if they are on different continents or even different planets. For example, I cannot know if I will see a live or a dead cat upon opening either box, but I can know that the cats are either both alive or both dead."

This "spooky action at a distance"—as Einstein phrased it—is called entanglement, and an entangled state, physicists say, can store information. "When looking at systems with many cats, the amount of entanglement information is much larger than what I can obtain by looking at the cats individually," Rall says. "To harness the sheer quantity of information stored in these so-called many-body systems, we must better understand the structure of these spooky correlations. This is what I worked on this summer."

Quantum many-body systems are difficult to simulate on a computer, but by looking at small-enough systems and using mathematical tools, researchers can study complex entangled quantum states. Physicists have been studying many-body entanglement for a long time because of its importance in understanding certain semiconductors.

"This summer, I had the privilege to work under Professor Preskill, and that was an incredible experience," Rall says. A central interest of Preskill's lab is to design schemes for quantum computation. Modern computers use classical bits—ones and zeroes—to store data. A quantum computer would use quantum bits—or qubits—and use their superposition and entanglement to perform computation. Quantum computers, while still in the experimental stage (with heavy investment from companies like IBM, Microsoft, and Google), have been touted for their potential to generate unbreakable codes and to efficiently simulate many complex systems, with implications for computational chemistry and biology.

"The most interesting thing about the quantum computer is that we have no idea what it could be capable of," says Rall. "We know some quantum algorithms that are faster than the best-known classical algorithms. But what are the limits? Nobody knows."

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Alumnus Arthur McDonald Wins 2015 Nobel Prize in Physics

Arthur B. McDonald (PhD '70), director of the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, and Takaaki Kajita, at the University of Tokyo, Kashiwa, Japan, have shared the 2015 Nobel Prize in Physics for the discovery that neutrinos can change their identities as they travel through space.

McDonald and Kajita lead two large research teams whose work has upended the standard model of particle physics and settled a debate that has raged since 1930, when the neutrino's existence was first proposed by physicist Wolfgang Pauli. Pauli initially devised the neutrino as a bookkeeping device—one to carry away surplus energy from nuclear reactions in stars and from radioactive decay processes on Earth. In order to make the math work, he gave it no charge, almost no mass, and only the weakest of interactions with ordinary matter. Billions of them are coursing through our bodies every second, and we are entirely unaware of them.

There are three types of neutrinos—electron, muon, and tau—and they were, for many years, assumed to be massless and immutable. The technology to detect electron neutrinos emerged in the 1950s, and it slowly became apparent that as few as one-third of the neutrinos the theorists said the sun should be emitting were actually being observed. Various theories were proposed to explain the deficit, including the possibility that the detectable electron neutrinos were somehow transmuting into their undetectable kin en route to Earth.

Solving the mystery of the missing neutrinos would require extremely large detectors in order to catch enough of the elusive particles to get accurate statistics. Such sensitive detectors also require enormous amounts of shielding to avoid false readings.

The University of Tokyo's Super-Kamiokande neutrino detector, which came online in 1996, was built 1,000 meters underground in a zinc mine. Its detector, which counts muon neutrinos and records their direction of travel, found fewer cosmic-ray neutrinos coming up through the Earth than from any other direction. Since they should not be affected in any way by traveling through the 12,742-kilometer diameter of our planet, Kajita and his colleagues concluded that the extra distance had given them a little extra time to change their identities.

McDonald's SNO, built 2,100 meters deep in a nickel mine, began taking data in 1999. It has two counting systems. One is exclusively sensitive to electron neutrinos, which are the type emitted by the sun; the other records all neutrinos but does not identify their types. The SNO also recorded only about one-third of the predicted number of solar electron-type neutrinos—but the aggregate of all three types measured by the other counting systems matched the theory.

The conclusion, for which McDonald and Kajita were awarded the Nobel Prize, was that neutrinos must have a nonzero mass. Quantum mechanics treats particles as waves, and the potentially differing masses associated with muons and taus gives them different wavelengths. The probability waves of the three particle types are aligned when the particle is formed, but as they propagate they get out of synch. Therefore, there is a one-third chance of seeing any particular neutrino in its electron form. Because these particles have this nonzero mass, their gravitational effects on the large-scale behavior of the universe must be taken into account—a profound implication for cosmology.

McDonald came to Caltech in 1965 to pursue a PhD in physics in the Kellogg Radiation Laboratory under the mentorship of the late Charles A. Barnes, professor of physics, emeritus, who passed away in August 2015. "Charlie Barnes was a great mentor who was very proud of his students," says Bradley W. Filippone, professor of physics and a postdoctoral researcher under Barnes. "It is a shame that Charlie didn't get to see Art receive this tremendous honor."

A native of Sydney, Canada, McDonald received his bachelor of science and master's degrees, both in physics, from Dalhousie University in Halifax, Nova Scotia, in 1964 and 1965, respectively. After receiving his doctorate, he worked for the Chalk River Laboratories in Ontario until 1982, when he became a professor of physics at Princeton University. He left Princeton in 1989 and became a professor at Queen's University in Kingston, Canada; the same year, he became the director of the SNO. In 2006, he became the holder of the Gordon and Patricia Gray Chair in Particle Astrophysics, a position he held until his retirement in 2013.

Among many other awards and honors, McDonald is a fellow of the American Physical Society, the Royal Society of Canada, and of Great Britain's Royal Society. He is the recipient of the Killam Prize in the Natural Sciences; the Henry Marshall Tory Medal from the Royal Society of Canada, its highest award for scientific achievement; and the European Physics Society HEP Division Giuseppe and Vanna Cocconi Prize for Particle Astrophysics.

To date, 34 Caltech alumni and faculty have won a total of 35 Nobel Prizes. Last year, alumnus Eric Betzig (BS '83) received the Nobel Prize in Chemistry.

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LIGO's SURF Students Look for the Perfect Wave

As the Advanced LIGO Project geared up last summer, 27 undergraduates from around the world became full partners in one of the biggest, most complex physics experiment ever. Their contributions ranged from creating hardware and software for current use to helping design next-generation detectors.

LIGO, the Laser Interferometer Gravitational-Wave Observatory, is designed to detect the ripples in the fabric of space and time produced by such violent events as supernova explosions or the mergers of pairs of black holes trapped in a death spiral. Such waves were proposed by Einstein as a consequence of general relativity. His theory turns 100 this November, and all of its other predictions have been confirmed; LIGO, a joint project of Caltech and MIT, entered the search with its first science run in 2002.

Gravitational waves are so subtle that the hunt requires ingenuity on a grand scale—each of LIGO's twin observatories, located in Hanford, Washington, and Livingston, Louisiana, consists of a four-kilometer-long, L-shaped interferometer containing hanging mirrors designed to bob on such a wave as it passes through. Lasers measure the mirrors' motions down to one-thousandth the diameter of a proton. Advanced LIGO, a package of upgrades that became operational on September 18, 2015, is increasing that sensitivity by a factor of 10.

Thanks to Caltech's Summer Undergraduate Research Fellowships (SURF) program, students have been part of LIGO since the 1990s—more than 350 of them. Some have gone on to careers in LIGO, and some are now mentoring students themselves. Anamaria Effler SURFed with LIGO in 2004 and 2005 before graduating from Caltech with a bachelor's degree in physics in 2006. But, she says, "I wasn't sure I wanted to go to grad school, so I worked as an operator at LIGO Hanford for three years." The experience moved her to enroll at Louisiana State, "the closest school to a LIGO site." Effler is now a Caltech postdoc at LIGO Livingston, and last summer she mentored an undergraduate from the University of Michigan–Ann Arbor in a project to track down noise sources generated within the interferometer itself.

Noise comes from everywhere, at all frequencies—from stray high-energy photons jostling the mirrors to low-pitched rumblings from within the earth itself. LIGO Hanford feels waves lapping at the beach, for example, even though the Pacific Ocean is more than 250 miles to the west. Elaborate electromechanical systems and sophisticated mathematical filters compensate for such things, but battling noise is like peeling an onion. Removing a layer reveals a fresh one.

Advanced LIGO will have to conquer "Newtonian noise," a slow vibration caused by fluctuations in the earth's density. Explains Professor of Physics Alan Weinstein, head of Caltech's astrophysical analysis group for LIGO, "When the ground shakes, it shakes the suspensions, which shake the mirrors. A perfect suspension keeps that shaking from reaching the mirrors, but that shaking also changes the local gravitational field. You can screen out mechanical motion, but you can't screen out gravity." He expects LIGO will encounter Newtonian noise at frequencies below 10 Hz, or cycles per second, where it will mask the gravitational waves radiated by doomed black-hole pairs up until their last few seconds of life. Weinstein's colleague, Professor of Physics Rana Adhikari, is trying to develop what are essentially high-performance noise-cancelling headphones—feed-forward systems that use inputs from seismometers and geophones to compute the noise's ever-shifting waveforms and manipulate the mirror suspensions to negate the noise's effects in real time. Student contributions included working on a prototype seismometer and testing noise-filtering software.

Other students used Einstein's equations to simulate the waves that such a merger would generate. Says postdoctoral researcher Tjonnie Li, who mentored four students, "General relativity allows us to predict the waves' shapes quite precisely." But many physicists believe that, just as general relativity goes beyond Newton's laws, so will another theory go beyond relativity. "LIGO will be a good test, because every detail of the signal we see should match the predictions," Li adds. "One of my students was modeling how long it should take a pair of black holes to merge under various scenarios. If they merge faster than expected, it would mean that some energy is escaping in some way that general relativity did not predict. This might not point you to the right explanation, but it does tell you that there's something missing." Furthermore, Li says, by comparing their simulations to LIGO's actual data, researchers "can infer something about how the fundamental forces interact with one another. They're all equally strong when matter gets that dense."

Just as Advanced LIGO marks the next generation of detectors, LIGO's undergraduates embody the next generation of researchers. In the 2014–15 academic year, two of LIGO's three incoming graduate students had SURFed here while earning their bachelor's degrees elsewhere. Says Li, "It takes a lot of effort from all of the mentors, and in particular Alan for organizing all of it, but the benefits are immediately visible. If their interest has been piqued, they often do their PhDs in this field. They either come here, or go on to other institutions that do LIGO."

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