News this Week

Science  06 Mar 2015:
Vol. 347, Issue 6226, pp. 1048
  1. This week's section

    Asia's cities swell as population surges

    China's Pearl River delta is now the world's largest urban area.


    Over the past decade, East and Southeast Asia have experienced an urbanization boom unlike any the world has ever seen. From China and Japan to the Philippines and Indonesia, the urban population of 17 countries in East and Southeast Asia increased from 738 million people in 2000 to 969 million in 2010. But the rate of expansion of urban land area—2% annually, on average, over that period—did not keep up with the rate of population change, which was about 2.8% per year, according to a 4 March report in Environmental Research Letters. Instead, Asia's teeming metropolises are cramming ever more humanity within existing city limits—confounding predictions that the cities will greatly expand their footprints as migrants flood in. “The assumption from past research has been that cities of all sizes will eventually decline in density,” says author Annemarie Schneider, a geographer at University of Wisconsin, Madison. “This study reveals the opposite.” The trend may seem obvious to Asian cities straining to provide basic services for burgeoning populations. But for urban planners, the findings, Schneider says, could change “how officials plan and adapt to urbanization in the future.”

    Seeing a virus in 3D

    Physicists can take pictures of tiny things from chemical nanostructures to proteins to living cells. But 3D biological particles, like viruses, have proved elusive. To take a 2D image, scientists send pulses of high-energy x-rays through the particle and record the resulting diffraction patterns. Theoretically, they could stitch together multiple 2D images, each taken at a different angle, to create a 3D picture—but they'd need to know how the particle was oriented in space when each picture was taken. Now, researchers working with the Linac Coherent Light Source at SLAC National Accelerator Laboratory in Menlo Park, California, have devised an algorithm that can figure out how hundreds of such diffraction patterns fit together to form a complete 3D image of a sample—a technique, they reported this week in Physical Review Letters, that can reveal both the external shape and internal structure of a single particle. They tested their technique by imaging mimivirus (shown), a rather large virus that is probably not infectious. But the algorithm should be able to handle much smaller and more dangerous viruses, including influenza, herpes, and HIV.


    Rise in U.S. lab animals

    Mice have become the world's most used mammal in research.


    The number of animals used by the top U.S.-funded biomedical research institutions has risen 73% over 15 years, a “dramatic increase,” according to an analysis by People for the Ethical Treatment of Animals (PETA). Although federal law requires that research labs report their use of cats, dogs, and nonhuman primates, smaller vertebrates—including rodents—are exempt. To get a sense of the trends, PETA obtained data from inventories submitted to the National Institutes of Health (NIH) every 4 years. The top 25 NIH-funded institutions housed a daily average of 74,600 animals from 1997 to 2003; that leaped to an average of 128,800 a day by 2008 to 2012, a 73% increase, PETA reports in the Journal of Medical Ethics. Most of the animals were mice. This parallels a rise in the use of transgenic mice internationally, PETA says. NIH cautioned that using the inventory data to track animal numbers is “inappropriate” because the data don't show usage, but are only a “snapshot” that NIH uses to make sure institutions have adequate veterinary care.

    “Long before being nerdy was cool, there was Leonard Nimoy.”

    President Barack Obama, in a tribute to Nimoy, who played Star Trek's beloved Mr. Spock. Nimoy died last week at age 83.

    By the numbers

    $41.5 million—Amount dedicated last week by the National Institutes of Health to the Human Placenta Project to study the mass of tissue that sustains a developing fetus.

    4.1—Average number of Oriental rat fleas—known to carry plague and typhus in the past—per New York City rat in a Journal of Medical Entomology survey. Values below 1 indicate minimal risk of epidemic disease spread.

    1—Number of physicists now on the U.S. House of Representatives' science committee as of last week, when Representative Bill Foster (D–IL) joined.

    Around the world


    Push for E.U. energy bloc

    The European Commission announced a plan on 25 February to create a unified energy market, where “energy flows freely across borders,” according to the so-called Energy Union proposal. The plan calls for more research and innovation on energy efficiency and renewable energy technologies to help transform energy systems, maintain Europe's technological leadership, and boost export prospects. This would help wean the bloc from fraught gas imports, hitting Russian President Vladimir Putin “where it hurts most,” says Guy Verhofstadt, a liberal member of the European Parliament from Belgium. But green groups have criticized the plan for putting too much emphasis on fossil fuels and nuclear energy—“yesterday's instead of tomorrow's technologies,” says Rebecca Harms, a Green member of the European Parliament from Germany. The proposal will next be discussed by the European Parliament and member states.

    Minneapolis, Minnesota

    Trials under scrutiny

    A damning report released last week on how the University of Minnesota protects volunteers in its clinical trials charged the university with inadequate review of research studies and failure to sufficiently protect the most vulnerable subjects. Examining protocols from 20 active trials and meeting minutes from the institutional review board (IRB), the reviewers found “little discussion of the risks and benefits” to volunteers, and noted that there were often no IRB members with expertise in a protocol present during its review. The report comes after years of complaints by academics inside and outside the school, who claimed the school failed to protect 27-year-old Dan Markingson, who died by suicide in 2004 while enrolled in a psychiatric drug trial. At press time, the Faculty Senate was preparing to meet with University President Eric Kaler and the authors of the report. Senior administrators say they hope to develop a plan to respond to the report within 60 days.

    Greenwich, Connecticut

    New database for oldest fossils

    The 160-million-year-old Juramaia sinensis is the earliest known ancestor to placental mammals.


    Hoping to help scientists understand the origin and evolution of life on Earth, a new repository of data about the world's oldest fossils was launched last week. The Fossil Calibration Database (, funded by the National Evolutionary Synthesis Center, will offer scientists a reliable anchor point from which they can accurately date new fossils and determine when species branched off from their family tree. New fossils are discovered all the time, but until now there was no centralized list of the oldest, so many estimates of evolutionary change rely on “really outdated information,” says paleontologist Daniel Ksepka of the Bruce Museum in Greenwich, Connecticut. He co-led the team of more than 20 paleontologists, molecular biologists, and computer programmers behind the project. To ensure the new resource remains a gold standard, new finds will be regularly added after careful vetting by specialists.

    Argonne, Illinois

    Ask A Scientist shuts down

    One of the Internet's oldest sources of science information for the public is closing its virtual doors. Argonne National Laboratory announced last month that they will be discontinuing their Newton – Ask A Scientist program on 1 March. Argonne created the service in 1991 as a way for students and teachers to connect with scientists. Volunteer scientists have answered 20,000 questions over the years, from “Why does steel rust?” to “What happens to light in a black hole?” But the website was outdated and its use was declining, says Meridith Bruozas, Argonne's manager of educational programs and outreach. “As technology has advanced … it kind of doesn't serve its purpose anymore.” Instead, the lab has shifted to using Twitter, Facebook, reddit, and Google Hangouts to give students a way to quiz scientists.


    HIV researcher admits fraud

    In an unusual turn for a scientific misconduct case, a former HIV researcher at Iowa State University (ISU) has pleaded guilty to federal fraud charges. Dong-Pyou Han resigned in 2013, shortly before the federal Office of Research Integrity (ORI) found he had faked data in a rabbit study of an HIV vaccine for a National Institutes of Health (NIH) grant proposal. ORI barred Han from seeking grants for 3 years, but Senator Chuck Grassley (R–IA) complained that the punishment was too light for a study that cost taxpayers millions of dollars. ISU later returned $500,000 and NIH withheld a $1.4 million award. Han faces up to 10 years in prison on two felony counts of making false statements; his sentencing is set for 29 May.

    Three Q's


    After 42 years at the Massachusetts Institute of Technology in Cambridge, including 16 years as an administrator, physicist Marc Kastner knows the value of basic research—and how to convince rich people to support it at a premier research institution. Last week he announced he was leaving to become the first president of the Science Philanthropy Alliance—a job that will give him the chance to make the case on a national scale.

    Q:How will the alliance operate?

    A:It will not raise any money for itself. Instead, we're trying to increase gifts to universities or help create new foundations that will fund basic research.

    Q:Why is that so important today?

    A:There's been a tilt in federal funding toward things that are more applied and more translational. My task is to explain to potential donors the enormous opportunities for doing exciting things in basic science and the satisfaction they will get out of that.

    Q:Is it OK if the well-endowed universities simply get richer?

    A:Absolutely. If foundations choose to be concerned about geography, that's their business. But my experience with these foundations is that they really want to fund the best people to do the best research. And that's fine with me.

  2. To catch a wave

    1. Adrian Cho*

    After decades of work, physicists say they are a year or two away from detecting ripples in spacetime.

    The twin 4-kilometer arms of LIGO Livingston embrace a working forest, where logging generates vibrations that the instrument must damp out.


    This patch of woodland just north of Livingston, Louisiana, population 1893, isn't the first place you'd go looking for a breakthrough in physics. Standing on a small overpass that crosses an odd arching tunnel, Joseph Giaime, a physicist at Louisiana State University (LSU), 55 kilometers west in Baton Rouge, gestures toward an expanse of spindly loblolly pine, parts of it freshly reduced to stumps and mud. “It's a working forest,” he says, “so they come in here to harvest the logs.” On a quiet late fall morning, it seems like only a logger or perhaps a hunter would ever come here.

    Yet it is here that physicists may fulfill perhaps the most spectacular prediction of Albert Einstein's theory of gravity, or general relativity. The tunnel runs east to west for 4 kilometers and meets a similar one running north to south in a nearby warehouselike building. The structures house the Laser Interferometer Gravitational-Wave Observatory (LIGO), an ultrasensitive instrument that may soon detect ripples in space and time set off when neutron stars or black holes merge.

    Einstein himself predicted the existence of such gravitational waves nearly a century ago. But only now is the quest to detect them coming to a culmination. The device in Livingston and its twin in Hanford, Washington, ran from 2002 to 2010 and saw nothing. But those Initial LIGO instruments aimed only to prove that the experiment was technologically feasible, physicists say. Now, they're finishing a $205 million rebuild of the detectors, known as Advanced LIGO, which should make them 10 times more sensitive and, they say, virtually ensure a detection. “It's as close to a guarantee as one gets in life,” says Peter Saulson, a physicist at Syracuse University in New York, who works on LIGO.

    Detecting those ripples would open a new window on the cosmos. But it won't come easy. Each tunnel contains a pair of mirrors that form an “optical cavity,” within which infrared light bounces back and forth. To look for the stretching of space, physicists will compare the cavities' lengths. But they'll have to sense that motion through the din of other vibrations. Glancing at the pavement on the overpass, Giaime says that the ground constantly jiggles by about a millionth of a meter, shaken by seismic waves, the rumble of nearby trains, and other things. LIGO physicists have to shield the mirrors from such vibrations so that they can see the cavities stretch or shorten by distances 10 trillion times smaller—just a billionth the width of an atom.

    IN 1915, Einstein explained that gravity arises when mass and energy warp space and time, or spacetime. A year later, he predicted that massive objects undergoing the right kind of oscillating motion should emit ripples in spacetime—gravitational waves that zip along at light speed.

    For decades that prediction remained controversial, in part because the mathematics of general relativity is so complicated. Einstein himself at first made a technical error, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge. “Einstein had it right,” he says, “but then he [messed] up.” Some theorists argued that the waves were a mathematical artifact and shouldn't actually exist. In 1936, Einstein himself briefly took that mistaken position.

    Even if the waves were real, detecting them seemed impossible, Weiss says. At a time when scientists knew nothing of the cosmos's gravitational powerhouses—neutron stars and black holes—the only obvious source of waves was a pair of stars orbiting each other. Calculations showed that they would produce a signal too faint to be detected.

    By the 1950s, theorists were speculating about neutron stars and black holes, and they finally agreed that the waves should exist. In 1969, Joseph Weber, a physicist at the University of Maryland, College Park, even claimed to have discovered them. His setup included two massive aluminum cylinders 1.5 meters long and 0.6 meters wide, one of them in Illinois. A gravitational wave would stretch a bar and cause it to vibrate like a tuning fork, and electrical sensors would then detect the stretching. Weber saw signs of waves pinging the bars together. But other experimenters couldn't reproduce Weber's published results, and theorists argued that his claimed signals were implausibly strong.

    Still, Weber's efforts triggered the development of LIGO. In 1969, Weiss, a laser expert, had been assigned to teach general relativity. “I knew bugger all about it,” he says. In particular, he couldn't understand Weber's method. So he devised his own optical method, identifying the relevant sources of noise. “I worked it out for myself, and I gave it to the students as a homework problem,” he says.


    Weiss's idea, which he published in 1972 in an internal MIT publication, was slow to catch on. “It was obvious to me that this was pie in the sky and it would never work,” recalls Kip Thorne, a theorist at the California Institute of Technology (Caltech) in Pasadena, California. Thorne recorded his skepticism in Gravitation, the massive textbook that he co-wrote and published in 1973. “I had an exercise that said ‘Show that this technology will never work to detect gravitational waves,’” Thorne says.


    Take an aerial tour of LIGO at

    But by 1978 Thorne had warmed to the idea, and he persuaded Caltech to put up $2 million to build a 40-meter prototype interferometer. “It wasn't a hard sell at all,” Thorne says, “which was a contrast to the situation at MIT.” Weiss says that Thorne played a vital role in winning support for a full-scale detector from the National Science Foundation in 1990. Construction in Livingston and Hanford finally began in 1994.

    Now, many physicists say Advanced LIGO is all but a sure winner. On a bright Monday morning in December, researchers at Livingston are embarking on a 10-day stint that will mark their first attempt to run as if making observations. LIGO Livingston has the feel of an outpost. Roughly 30 physicists, engineers, technicians, and operators gather in the large room that serves as the facility's foyer, auditorium, and—with a table-tennis table in one corner—rec room. “Engineering run 6 began 8 minutes ago,” announces Janeen Romie, an engineer from Caltech. It seems odd that so few people can run such a big rig.

    But in principle, LIGO is simple. Within the interferometer's sewer pipe–like vacuum chamber, at the elbow of the device, a laser beam shines on a beam splitter, which sends half the light down each of the interferometer's arms. Within each arm, the light builds up as it bounces between the mirrors at either end. Some of the light leaks through the mirrors at the near ends of the arms and shines back on the beam splitter. If the two arms are exactly the same length, the merging waves will overlap and interfere with each other in a way that directs the light back toward the laser.

    But if the lengths are slightly different, then the recombining waves will be out of sync and light will emerge from the beam splitter perpendicular to the original beam. From that “dark port” output, physicists can measure any difference in the arms' lengths to an iota of the light's wavelength. Because a gravitational wave sweeping across the apparatus would generally stretch one arm more than the other, it would cause light to warble out of the dark port at the frequency at which the wave ripples. That light would be the signal of the gravitational wave.

    In practice, LIGO is a monumental challenge in sifting an infinitesimal signal from a mountain of vibrational noise. Sources of gravitational waves should “sing” at frequencies ranging from 10 to 1000 cycles per second, or hertz. But at frequencies of hundreds or thousands of hertz the individual photons in the laser beam produce noise as they jostle the mirrors. To smooth out such noise, researchers crank up the amount of light and deploy massive mirrors. At frequencies of tens of hertz and lower, seismic vibrations dominate, so researchers dangle the mirrors from elaborate suspension systems and actively counteract that motion. Still, a large earthquake anywhere in the world or even the surf pounding the distant coast can knock the interferometer off line.

    To boost the Hanford and Livingston detectors' sensitivity 10-fold, to a ten-billionth of a nanometer, physicists have completely rebuilt the devices. Each of the original 22-kilogram mirrors hung like a pendulum from a single steel fiber; the new 40-kilogram mirrors hang on silica fibers at the end of a four-pendulum chain. Instead of LIGO's original 10 kilowatts of light power, researchers aim to circulate 750 kilowatts. They will collect 100,000 channels of data to monitor the interferometer. Comparing the new and old LIGO is “like comparing a car to a wheel,” says Frederick Raab, a Caltech physicist who leads the Hanford site.

    The new Livingston machine has already doubled Initial LIGO's sensitivity. “In 6 months they've made equivalent progress to what Initial LIGO made in 3 or 4 years,” says Raab, who adds that the Hanford site is about 6 months behind. But Valery Frolov, a Caltech physicist in charge of commissioning the Livingston detector, cautions that machine isn't running anywhere close to specs. The seismic isolation was supposed to be better, he says, and researchers haven't been able to keep the interferometer “locked” and running for long periods. As for reaching design sensitivity, “I don't know whether it will take 1 year or whether it will take 5 years like Initial LIGO did,” he warns.

    Still, LIGO researchers plan to make a first observing run this year and hope to reach design sensitivity next year. “We will have detections that we will be able to stand up and defend, if not in 2016, then in 2017 or 2018,” says Gabriela González, a physicist at LSU and spokesperson for the more than 900-member LIGO Science Collaboration.


    That forecast is based on the statistics of the stars. LIGO's prime target is the waves generated by a pair of neutron stars—the cores of exploded stars that weigh more than the sun but measure tens of kilometers across—whirling into each other in a death spiral lasting several minutes. Initial LIGO could sense such a pair up to 50 million light-years way. Given the rarity of neutronstar pairs, that search volume was too small to guarantee seeing one. Advanced LIGO should see 10 times as far and probe 1000 times as much space, enough to contain about 10 sources per year, González says. However, Clifford Will, a theorist at the University of Florida in Gainesville, notes that the number of sources is the most uncertain part of the experiment. “If it's less than one per year, that's not going to be too good,” he says.

    The hunt will be global. As well as combining data from the two LIGO detectors, researchers will share data with their peers working on the VIRGO detector, an interferometer with 3-kilometer arms near Pisa, Italy, that is undergoing upgrades, and on GEO600, one with 600-meter arms near Hannover, Germany. By comparing data, collaborators can better sift signals from noise and can pinpoint sources on the sky. Japanese researchers are also building a detector, and LIGO leaders hope to add a third detector, in India (Science, 14 February 2014, p. 717).

    FOR THEORISTS—if not for the rest of the world—seeing gravitational waves for the first time will be something of an anticlimax. “We are so confident that gravitational waves exist that we don't actually need to see one,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland. That's because in 1974 American astrophysicists Russell Hulse and Joseph Taylor Jr. found indirect but convincing evidence of the waves. They spotted two pulsars—neutron stars that emit radio signals with clockwork regularity—orbiting each other. From the timing of the radio pulses, Hulse and Taylor could monitor the pulsars' orbit. They found it is decaying at exactly the rate expected if the pulsars were radiating energy in the form of gravitational waves.

    Rainer Weiss of the Massachusetts Institute of Technology laid out the basic plan for LIGO 43 years ago.


    LIGO's real payoff will come in opening a new frontier in astronomy, says Robert Wald, a gravitational theorist at the University of Chicago in Illinois. “It's kind of like after being able to see for a while, being able to hear, too,” Wald says. For example, if a black hole tears apart a neutron star, then details of the gravitational waves may reveal the properties of matter in neutron stars.

    All told, detecting gravitational waves would merit science's highest accolade, physicists say. “As soon as they detect a gravitational wave, it's a Nobel Prize,” Kamionkowski predicts. “It's such an extraordinary experimental accomplishment.” But the prize can be shared by at most three people, so the question is who should get it.

    Weiss is a shoo-in, many say, but he demurs. “I don't want to deny that there was some innovation [in my work], but it didn't come out of the blue,” he says. “The lone crazy man working in a box, that just doesn't hold true.” In 1962 two Russian physicists published a paper on detecting gravitational waves with an interferometer, as Weiss says he learned long after his 1972 work. In the 1970s, Robert Forward of the Hughes Aircraft Company in Malibu, California, ran a small interferometer. Key design elements of LIGO came from Ronald Drever, project director at Caltech from 1979 to 1987, who, Thorne says, “has to be recognized as one of the fathers of the LIGO idea.”

    But to make that prize-winning discovery, physicists must get Advanced LIGO up and running. At 8 a.m. on Tuesday morning, LIGO operator Gary Traylor comes off the night shift. “Last night was a total washout,” he says in his soft Southern accent, swiveling in a chair in the brightly lit control room. “There's a low pressure area moving over the Atlantic that's causing 20-foot waves to crash into the coast,” Traylor says, and that distant drumming overwhelmed the detector. So in the small hours, LIGO did sense waves. But not the ones everybody is hoping to see.

    • * inLivingston, Louisiana

  3. Einstein's milestones

    1. Emily Conover

    In the century since Einstein formulated general relativity, scientists have built on his theory, with developments that were groundbreaking, tragic, and just plain strange.

    Milestone: 1914

    Relativity and the Great War

    Albert Einstein developed his intellectual bombshell, general relativity, against a backdrop of all-too-real bombs. World War I profoundly limited scientists' ability to share ideas and perform crucial experiments to test the theory.

    The physicist Karl Schwarzschild managed to contribute while serving as an artillery officer with the German army on the Russian front. Schwarzschild's work described the curvature of spacetime outside a spherical, nonrotating massive object—a result that later proved key to studying black holes. In a 1915 letter to Einstein, he wrote, “[T]he war treated me kindly enough, in spite of the heavy gunfire, to allow me to get away from it all and take this walk in the land of your ideas.” Schwarzschild succumbed to disease not long afterward.

    Other researchers had similarly bad luck. In August 1914, the German astronomer Erwin Freundlich led an expedition to the Crimea to take measurements during a total solar eclipse. He and colleagues hoped to test Einstein's prediction that the sun's gravity would deflect nearby starlight. But when war broke out before the eclipse, Russian officials seized the scientists' equipment and detained them.

    The British astronomer Arthur Eddington performed the experiment 5 years later on the island of Principe, but he too felt the shadow of the war. Eddington was a conscientious objector who nearly landed in prison for refusing conscription. His colleague, Astronomer Royal Frank Dyson, was able to obtain an exemption for Eddington on the condition that he participate in the expedition. The measurement—made in 1919, after the war ended—verified Einstein's theory, a result Eddington saw as a tool for peace. He wrote to Einstein, “It is the best possible thing that could have happened for scientific relations between England and Germany.”

    Milestone: 1915

    Mercury delivers good news about a newborn theory

    General relativity hit the ground running—and thrilled Albert Einstein—by explaining a decadesold puzzle regarding Mercury's orbit. According to Newtonian gravity, an isolated planet would follow exactly the same elliptical path on each orbit around its star; only the influence of neighboring planets would cause the ellipse to gradually shift, or precess, about the sun. In 1859, however, astronomer Urbain Le Verrier pointed out that Mercury was precessing slightly more than purely Newtonian gravity predicted.

    Scientists proposed a slew of possible but unlikely explanations. Some insisted there must be a new planet between Mercury and the sun, which became known as Vulcan. Others proposed bands of dust near the sun, or an unseen moon around Mercury. Some tried to solve the issue by tweaking Newton's gravity. But none of these explanations withstood scrutiny.

    In November 1915, Einstein finally had the solution: His new theory fully explained Mercury's extra precession. Einstein later said that the thrill of this discovery had given him heart palpitations. “For a few days, I was beside myself with joyous excitement,” he wrote.

    The result immediately boosted the theory's credibility. Mathematician David Hilbert wrote to congratulate Einstein and praised him on the speed of his calculations, which Einstein had performed in only a week. What Einstein didn't let on was that his speed was the result of practice: He had done the calculations once before with an incorrect version of his theory.

    Milestone: 1935

    Battle erupts over black holes

    In the 1930s, an up-and-coming physicist clashed with a distinguished member of the old guard over the cosmic implications of general relativity. Their fiery dispute enlivened scientific meetings for years.

    The young rising star was the Indian-born physicist Subrahmanyan Chandrasekhar, known as Chandra; his opponent, the astronomical powerhouse Arthur Eddington (see p. 1085). Their bone of contention: the fate of aging stars. Eddington and most other scientists thought that after stars used up their fuel, they simply faded away into inert stars known as white dwarfs. But, building on the equations of general relativity and quantum mechanics, Chandra calculated that very massive stars were unstable and would collapse into nothingness at the end of their lives, producing black holes—a name coined decades later.

    Chandra, then at the University of Cambridge, presented his surprising conclusions at a meeting of the Royal Astronomical Society in London in 1935. Eddington spoke immediately afterward, mercilessly pillorying Chandra's black holes as mathematical oddities that wouldn't hold up in real-world situations.“stellar buffoonery,” as he later put it. “I think there should be a law of Nature to prevent a star from behaving in this absurd way!” he said. The sniping went on for years.

    Eddington was well known for viciously laying into prominent physicists. The British scientists James Jeans and Chandra's mentor, Edward Arthur Milne, had both suffered similar tongue-lashings. Yet Eddington's rejection hurt Chandra both personally and professionally. He left Britain for the United States and began working on other topics.

    Chandrasekhar's radical conclusions eventually did gain acceptance, as other physicists followed up on his results and astronomers began seeing hints of black holes in exotic corners of the universe. The ultimate vindication came in 1983, when he won the Nobel Prize in physics for his work.

    Milestone: 1936

    Einstein eschews peer review

    Albert Einstein was not infallible, and sometimes his pride made him slow to acknowledge mistakes. A notable example took place in 1936, when he butted heads with the editor of the journal Physical Review over a process that modern scientists take for granted: peer review.

    Einstein, then at the Institute for Advanced Study in Princeton, New Jersey, and collaborator Nathan Rosen had submitted an article titled “Do Gravitational Waves Exist?” Their answer, surprisingly, was “no.”

    At the time, peer review by anonymous outside experts was beginning to take hold among journals in the United States. Einstein, however, wasn't used to it: Until he left Germany 3 years earlier, he had regularly published in German journals without external peer review. He was indignant when he learned that his paper had received a critical review, and he withdrew it in a huff. “We (Mr. Rosen and I) … had not authorized you to show it to specialists before it is printed,” he wrote to the editor. “I see no reason to address the—in any case erroneous—comments of your anonymous expert.” He and Rosen submitted the paper to another journal, the Journal of the Franklin Institute, without change.

    Yet before it was printed, Einstein revised the manuscript, retitling it “On Gravitational Waves.” It now came to the opposite conclusion: that gravitational waves were possible. The unidentified referee had pointed out a legitimate flaw in the original paper. Historians have recently confirmed that the referee was Howard Percy Robertson of Princeton University. After his anonymous criticisms were ignored, Robertson had delicately approached Einstein and convinced him of his error.

    Even though peer review had helped Einstein save face, he stuck to his guns and never published another scientific paper in the Physical Review.

    * Research by Daniel Kennefick of the University of Arkansas uncovered the historical events detailed in this anecdote. You can read the whole story in his article in the September 2005 issue of Physics Today.

    Milestone: 1959

    Bringing general relativity down to Earth

    General relativity mostly reveals itself on cosmological scales, but its effects also show up closer to home—even in our pockets. The GPS that so many smart phones use to orient and guide users would be useless if the system did not account for relativity.

    According to general relativity, time slows in a gravitational field; as a result, clocks closer to a gravitational mass run slower than those farther from it—an effect known as time dilation. Time dilation results in a subtle reddening of light moving up from Earth's surface, as the weakening gravity causes the light's electromagnetic fields to oscillate at a lower frequency.

    Researchers first definitively detected that “gravitational redshift” in 1959, in an experiment at a 23-meter tower at Harvard University. Physicists Robert Pound and Glen Rebka set up a source of light with a known frequency at the bottom of the tower and a detector at the top. The photons changed frequency in transit by an amount that agreed with Albert Einstein's theory. In 1977, scientists laying the foundation for GPS navigation confirmed the underlying effect, time dilation, by launching a satellite with a highly precise cesium clock. Sure enough, the clock quickly went out of sync with its Earth-bound counterparts, in agreement with Albert Einstein's theory.

    For GPS to function, clocks on satellites and on the ground have to stay in sync, allowing your smart phone to measure the exact travel time of radio signals from multiple satellites. The relative timing of the signals allows the phone's GPS receiver to calculate position. If engineers failed to account for gravity's time dilation, the weaker gravity in orbit would nudge the clock in each GPS satellite ahead of ground-based clocks by tens of microseconds per day—an error that would quickly make the navigational system useless.

    Milestone: 1974

    Neutron stars show effects of gravitational waves

    Forty years ago, a pair of stars locked in a cosmological danse macabre gave cosmologists a vivid glimpse of general relativity in action. One key prediction of the theory is that massive, accelerating objects send out ripples in spacetime. Scientists haven't detected such gravitational waves directly, but the orbiting stars showed that they exist.

    Astrophysicist Joseph Taylor Jr. and his doctoral student Russell Hulse were surveying the galaxy for pulsars: collapsed stars, or neutron stars, that sweep the universe with tight, lighthouselike beams of energy. Using the 305-meter-wide dish of the Arecibo Observatory in Puerto Rico, Hulse and Taylor could see those beams as regular pulses of radio waves. One of their finds, known as PSR B1913+16, raised eyebrows. The intervals between pulses, about 59 milliseconds, were oddly irregular—sometimes tens of microseconds longer than expected, sometimes shorter. Apparently the pulsar was orbiting another neutron star, causing its signals to vary as it moved toward and away from Earth.

    More tantalizing was what happened in the following years: The orbit of the pulsar contracted. It was shrinking exactly as Albert Einstein's equations predicted it should if the stars were dissipating energy in the form of gravitational waves. Hulse and Taylor's observations won them the 1993 Nobel Prize in physics. Since then, several other binary pulsars have told the same story.

    In a few hundred million years, PSR B1913+16 and its companion will collide and merge, emitting a new, more powerful burst of gravitational waves. Detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory may soon detect such signals from other pairs of perishing stars—finally observing the gravitational waves that physicists are already sure are there.

    Updated (17 June 2015): Source information has been added for “Einstein eschews peer review.”

  4. The dark lab

    1. Daniel Clery

    To put general relativity to the acid test, researchers are looking inward—toward the supermassive black hole at the center of the Milky Way.

    A black hole distorts the image of a disk of dust and gas around it, courtesy of the special effects team for the film Interstellar.


    Like an Olympic athlete, the general theory of relativity has passed many tests in its century-long career. Its string of successes began in 1915, when Albert Einstein's picture of gravity as curved spacetime neatly explained shifts in the orbit of Mercury that had vexed astronomers for more than half a century. In recent decades it has faced more exotic and extreme tests, such as explaining why pairs of superdense neutron stars whirling around each other appear to be gradually spiraling toward collision. Here, too, general relativity triumphed: The stars are losing energy at exactly the rate expected if, as the theory predicts, they emit gravitational waves (see p. 1096).

    Yet physicists remain unsatisfied. The tests so far have been too easy, they say. The gravitational fields involved have been fairly weak, coming from single stars and bending or slowing light only very slightly. If the theory is going to show cracks, it will be under more extreme, high-field conditions. That matters because—on paper, at least—general relativity isn't the only game in town. Theorists have put forward alternative models for gravity, but in low fields they look identical to Einstein's theory. In strong fields, they begin to change.

    Now, searching for a tougher test, researchers are looking toward the center of our galaxy. There, shrouded in dust, lurks a bright, compact source of radio waves known as Sagittarius A* (Sgr A*) for its position in the sky, near the edge of the constellation Sagittarius. Because of the way stars move in its vicinity, astronomers think that Sgr A* marks the dark heart of the Milky Way: a supermassive black hole weighing as much as 4 million suns but crammed into a space smaller than the distance between the sun and Mercury. That black hole produces the most intense gravitational field in our galaxy and so provides a unique laboratory for testing the predictions of general relativity. Over the next few years, using a range of new instruments tuned to infrared light and radio waves—radiation capable of penetrating the clouds of dust and gas around the galaxy's core—astronomers are hoping to see whether Sgr A* is bending relativity beyond the breaking point.

    Two teams of astronomers—one led by Andrea Ghez of the University of California, Los Angeles (UCLA), and the other by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany—are staring at the center of the galaxy more intently than anyone before them. They are tracking a handful of stars that swoop close to the center—one of them to a distance equal to that between the sun and the edge of the solar system. Meanwhile, a unique new radio telescope array—still being assembled—is gearing up to carry the scrutiny right up to the edge of the putative black hole itself. In each case, the mission is the same: to spot discrepancies that Einstein's formulae cannot explain.

    By peering through the glowing gas and dust that hides the galactic center, the Atacama Large Millimeter/submillimeter Array in Chile may help image the black hole and find pulsars around it.


    General relativity has “never before been tested at the high-field limit,” says astrophysicist Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts. Elsewhere in the galaxy, astronomers have observed stars apparently caught in the grip of smaller black holes. But the stars close to Sgr A* “are 100 times closer to the event horizon [the boundary of a black hole] and the mass scale is a million times greater,” Ghez says. “Does general relativity work down at scales 100 times closer? You're getting into the realm of basic physics: What is gravity? That's why people care.”

    Testing general relativity in this distant laboratory isn't easy. The black hole at Sgr A* is a small object, by galactic standards, and it emits no light. What radiation we do see is from superheated dust and gas falling in toward the event horizon. Once material passes that boundary, no trace of it remains. All astronomers can observe is the effects of the black hole's gravity on things around it. The UCLA and MPE teams aim to do just that.

    Both groups began work in the early 1990s, when the current generation of 8- to 10-meter optical and infrared telescopes was coming online. At first, it was very difficult to pick out the movement of individual stars. The teams first determined that the stars were moving very fast (consistent with orbits around a very large mass) and then that they were accelerating around something. In 2002, the brightest of the near-in stars appeared to make its closest approach to the black hole and swing away again, essentially allowing the researchers to calculate its full orbit. It was following an ellipse so tight and so fast—5000 kilometers per second at closest approach—that it had to enclose an enormous, compact mass. “Then the community began believing in the supermassive black hole,” says astronomer Stefan Gillessen of the MPE team.

    Observations stepped up a gear during the 2000s, thanks to adaptive optics: systems that rapidly deform a telescope's mirror to compensate for the blurring effect of Earth's atmosphere. The sharper images that resulted enabled the teams to see more stars and to track them more accurately. Now the researchers could start looking for signs that relativity was making the stars' orbits deviate from a classical Newtonian course. So far, the effects of relativity have not emerged.

    Both teams expect that to change starting in 2018, when that same bright star from 2002—known as S2 in Europe and S0-2 in North America—has its next close encounter with the black hole and the gravitational field it experiences is at a maximum. By then, both the W. M. Keck telescope in Hawaii, which the UCLA team uses, and Europe's Very Large Telescope in Chile, used by the MPE team, will have been upgraded. “We're trying to line up all the tools and methods ready for 2018,” says UCLA's Gunther Witzel.

    The teams will be looking for two telltale relativistic effects during and after the close approach, Ghez says. First, they expect to see the star's light shift toward longer, redder wavelengths as the photons strain against the black hole's intense gravity.

    A more subtle effect they hope to see is precession. A star moving in a Newtonian orbit would trace out an unchanging elliptical path through space, so long as no other object perturbs it. General relativity, however, predicts that after S2/S0-2's closest approach, warped space will make the star overshoot its previous orbit very slightly, shifting the axis of its ellipse by 0.2°. The change should become apparent gradually, as the star diverges from its earlier orbit. “By 2019 we should start to see the difference,” Ghez says.

    With two teams after the prize, there's bound to be a race. “Everyone will be trying to get it. It's a question of when do you believe your own measurements,” says Ghez, who adds that systematic errors could easily swamp the effect. But she welcomes the competition from Germany. “It's good for getting confirmation of your results. We push each other.”


    SOME OBSERVERS would like an even more stringent test of relativity. The orbiting stars don't get that close to the galactic center, after all. S2/S0-2's nearest approach is still four times the distance between the sun and the planet Neptune. If general relativity is correct, the galactic black hole's event horizon stretches only 1/1500 that far out. An international team of researchers is preparing to look right to that edge, beyond which no photons can escape, by building a telescope array as wide as Earth itself.

    The Event Horizon Telescope (EHT), as the array is called, will use short-wavelength radio waves to peer through the dust veiling the galactic center. Conventional radio telescopes can't get a detailed image of Sgr A* because their centimeters-long wavelengths limit their resolution. But shorter radio waves, with wavelengths measured in millimeters or less, yield sharper images. Combining waves from far-apart radio telescopes can further boost the resolution. About 15 years ago, astrophysicists calculated that by combining signals from millimeter-wave observatories separated as widely as Earth allows, they could image the area around Sgr A*. Then scientists could tackle three basic questions: Do the black hole and its event horizon really exist? If so, are they shaped the way that general relativity says they should be? Or does some other theory give a better description?

    There are only a handful of millimeter-wave observatories around the world, but the EHT team is attempting to link as many as possible into a single array. The technique used, known as very-long-baseline interferometry, involves making observations with the different scopes simultaneously and recording the data with very accurate time stamps. Later, a computer can merge the separate observations as if they were all taken at once by one huge dish. To create the planet-wide array, the EHT team has had to equip some of the individual telescopes with better receivers, recorders, and highly accurate atomic clocks. Early this year a team was doing so at the South Pole Telescope in Antarctica. “This is what gets me out of bed in the morning: fashioning a new type of telescope out of a few bits and pieces,” says team leader Shep Doeleman of the Massachusetts Institute of Technology's Haystack Observatory in Westford and CfA.

    Over the past few years, the EHT team has been testing the system with just a few dishes—in Hawaii, Arizona, and California—and has seen structures at the galactic center of about the right size but not with enough detail to probe relativity. Later this month they will try again after adding new, more distant dishes to the array: Mexico's Large Millimeter Telescope and the Atacama Pathfinder Experiment in Chile. With this extra receiving area and longer baselines, the team hopes to see the first definitive sign of the black hole: its shadow.

    The black hole should block out light from stars behind it, casting a visible shadow. Its intense gravity should also bend—or “lens”—light from stars behind it, producing a ring of distorted starlight around the edge of the shadow. That starlight is too faint to be seen from Earth. But theorists say the EHT should see a bright ring of lensed radio waves from another source: the glowing, superheated gas and dust swirling around the black hole. A dark circle—the black hole's shadow—should blot out the very center of the glow.

    Detecting the shadow, just outside the black hole's event horizon, will be a major validation of general relativity. “Just seeing the shadow as an image will be proof of the existence of a black hole,” says astrophysicist Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany. It would finally give astrophysicists something more than circumstantial evidence of these objects—pure creatures of general relativity. “There is no direct evidence that [a black hole] exists; everything is from theory. First we must show it is there, and then does it deviate from general relativity,” says EHT collaborator Heino Falcke of Radboud University in Nijmegen, the Netherlands.


    The shadow “would look different if there was no event horizon,” Kramer says. Theorists say that if general relativity holds, the shadow should be roughly circular; alternate theories of gravity predict slightly different shapes, such as prolate, like a cigar, or oblate, like an M&M. EHT might be able to tell the difference when it reaches full power, researchers say. The array will really come into its own when other key instruments are added in the next few years, in particular the South Pole Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA is the world's largest observatory at millimeter wavelengths; adding its 66 dishes will double EHT's resolution and boost its sensitivity 10 times, Doeleman says.

    As radio astronomers sharpen their scrutiny of the galactic center, they might stumble on something that could give Einstein's theory the most stringent test of all: a pulsar, a spinning neutron star that emits clocklike radio pulses, orbiting close to the black hole. It would amount to a precise clock, probing the structure of spacetime around the black hole with undreamed-of precision, says theorist Norbert Wex of MPIfR. By tracking variations in the pulsar's timing, Wex says, researchers could measure the black hole's mass to better than one part in a million and its spin. From those two quantities, they could calculate a quality of its gravitational field known as the quadrupole moment, predicted by general relativity. Using the pulsar again, they can then directly measure the quadrupole moment to see if relativity got it right.

    For theorists, that raises a tantalizing possibility: Even a slight difference in the two values would imply that the black hole is nonspherical. But according to general relativity, the shape of a black hole is forbidden knowledge. According to the oddly named “no-hair” theorem of relativity, the only things that it is possible to know about a black hole are its charge, its mass, and its spin. All other information about it (its “hair”) has disappeared below the event horizon, never to be seen again.

    Unfortunately, despite a couple of decades of searching, the galactic center seems to be devoid of pulsars. “There should be thousands. We're completely puzzled,” Falcke says. In 2013, European radio astronomers did find one magnetar—a rare type of high–magnetic field pulsar—orbiting Sgr A*, but not close enough to probe the black hole's spacetime. The finding did raise hopes, though, because “it shows the pulsar mechanism can work [at the galactic center] and that they are being made,” Falcke says. Bigger telescopes, like the upcoming Square Kilometre Array, or higher frequencies, such as those used by ALMA, might pierce the gloom and spot the coveted natural probe.

    One way or another, researchers are looking forward to exploiting the extraordinary laboratory at the galactic center. “The next decade will be very exciting. We'll get much more data … and hopefully an image of a black hole,” Loeb says. Says Falcke: “It's about space and time. It can't be more fundamental than this.”

  5. General Relativity: The comic book

    1. Adrian Cho

    A superquick, superpainless guide to the theory that conquered the universe.


    Einstein's theory of general relativity turns 100 this year! Want to know how it works? Here's an interactive comic that you don't need superpowers to understand.

  6. Drop test

    1. Adrian Cho

    Physicists are challenging Einstein's theory in new versions of the most famous experiment that never happened.

    Mark Kasevich of Stanford University plans to recreate Galileo's famous tower experiment with atoms.


    About 425 years ago, legend has it, Galileo Galilei climbed the Leaning Tower of Pisa. Before a throng of scholars and students, the savant dropped pairs of balls of different weights and materials—say, wood and lead—to show that regardless of their weight or composition, all objects accelerate at the same rate under gravity's pull. About a year from now, a satellite will blast into orbit to perform the legendary test more precisely than Galileo could have imagined.

    Rather than dropping things to the ground, the Drag-Compensated Micro-Satellite for the Observation of the Equivalence Principle (MicroSCOPE) will contain two free-floating weights of different materials and will monitor whether one feels a stronger tug from Earth's gravity than the other. If so, the result would sink general relativity, Albert Einstein's theory of gravity. After more than 15 years of development, “the instrument is done, definitely done,” says Pierre Touboul, a physicist at the French aerospace laboratory, ONERA, in Chatillon. “Now we cross our fingers.”

    Funded primarily by the French National Center for Space Studies, MicroSCOPE will test a key assumption of general relativity called the equivalence principle, which marries two conceptions of mass. Inertial mass determines how much an object resists moving when pushed by a force—as when you shove a car. Gravitational mass determines how strongly gravity pulls on the object. According to the equivalence principle, the two masses are one and the same, regardless of how heavy a thing is or what it's made of. That explains Galileo's experiment: If the two types of mass are identical, then for all objects the pull of gravity varies in strict proportion to the resistance to motion, ensuring that all things fall at the same rate. MicroSCOPE aims to test whether the two masses are the same with a precision 100 times better than any previous experiment, and other efforts could go even further.

    According to general relativity, the equivalence principle must hold exactly, as acceleration and gravity are essentially the same thing. But general relativity may not be the last word on gravity, because so far it cannot be reconciled with quantum mechanics, which governs physics on the smallest scales. Efforts to bridge that gap often violate the equivalence principle, says Clifford Will, a theorist at the University of Florida in Gainesville. Spotting a violation “would definitely mean that there is some sort of physics beyond Einstein's theory,” he says.

    IRONICALLY, THE MOST FAMOUS TEST of the principle, Galileo's demonstration at Pisa, probably never happened. “It's a fiction,” says Alberto Martínez, a historian at the University of Texas, Austin. The first account of the event was penned long after Galileo died by his assistant Vincenzo Viviani, who said the great man wanted to show that Aristotle was wrong when he contended that heavier objects fall faster than lighter ones do.

    Galileo did write about such tests in 1638 in his Dialogues Concerning Two New Sciences: “[T]he variation of speed in air between balls of gold, lead, copper, porphyry, and other heavy materials is so slight that … I came to the conclusion that in a medium totally devoid of resistance all bodies would fall with the same speed.” But Galileo likely inferred the result by timing balls rolling down ramps, says John Heilbron, a historian emeritus at the University of California (UC), Berkeley. “He had a clear idea that it didn't matter what he made the ball out of,” Heilbron says. “I think he was too lazy” to actually drag weights up a tower.

    Although Galileo's analysis jibes with the equivalence principle, he wouldn't have understood it that way, says Domenico Bertoloni Meli, a historian of science at Indiana University, Bloomington. The concepts of inertial and gravitational mass were invented later by Isaac Newton. Newton proved that the two types of mass were equal by showing that pendulums of equal lengths but different materials swing at the same rate, as he described in Philosophiæ Naturalis Principia Mathematica in 1687.

    The equivalence principle proved key to Einstein's invention of general relativity. Einstein deduced that gravity arises when energy and mass bend spacetime. In that warped spacetime, freefalling objects follow the straightest possible paths, or geodesics, which to us appear as the parabolic arc of a thrown ball and the elliptical orbit of a planet. The change of the object's speed and direction is its acceleration, which depends on the amount of warping of spacetime. If such warping is all there is to gravity, then in a given situation all things must accelerate at the same rate as they fall. That's because for any starting position and velocity, there is only one straightest path in spacetime.

    But gravity could be more complicated, says Thibault Damour, a theorist at the Institute of Advanced Scientific Studies (IHES) in Bures-sur-Yvette, France. According to Einstein's famous equation E = mc2, an object's inertial mass measures the energy trapped inside it. So a sliver of an atom's mass comes from the electromagnetic force that binds the electrons to the nucleus. Much more comes from the energy of the strong force that binds particles called quarks inside the nucleus's protons and neutrons. In general relativity, all energy has the same effect regardless of its source, Damour says.

    However, in some theories that aim to unify gravity and quantum mechanics, it matters how such energy arises. For example, string theory posits that every fundamental particle is an infinitesimal string rippling through a complex 10-dimensional space. In string theory a “dilaton field” acts like an additional form of gravity but pulls on different types of particles with different strengths. So, two objects with the same internal energy and inertial mass may have different gravitational masses, violating the equivalence principle. The ratio of a nucleus's inertial and gravitational masses could depend on the tally of protons and neutrons in it or the difference in the numbers of protons and neutrons, Damour says.


    PHYSICISTS HAVE ALREADY TESTED the equivalence principle to exquisite precision. The best test comes from Eric Adelberger, a physicist at the University of Washington, Seattle, and colleagues in the Eöt-Wash Group. “They're the gold standard right now,” Will says. Eöt-Wash researchers don't drop things, but instead follow an approach pioneered in the 1800s by Hungarian physicist Loránd Eötvös, after whom the group is named.

    Eötvös used a small dumbbell of weights of different materials suspended horizontally from a thin fiber. Gravity pulls each weight toward the center of Earth. But Earth also spins, so the inertia of the weights creates a tiny centrifugal force that flings them away from the planet's axis. The sum of the two forces, which align only at the equator, defines the direction “down” for each weight. If the equivalence principle holds, then the centrifugal force on each weight is locked into proportion to the gravitational one, so down is the same for both weights. Then, the dumbbell will rest pointing in any direction.

    The MicroSCOPE satellite is scheduled for launch in April 2016.


    But if inertial and gravitational mass are different, then the flinging will affect the weights differently and the net force on each one will point in a slightly different direction. “If the equivalence principle is violated, then every material has its own down,” Adelberger says. That difference would cause the dumbbell to twist toward a particular orientation. In 1889, Eötvös saw no such sign and confirmed the equivalence principle to one part in 20 million.

    For 25 years, Eöt-Wash researchers have refined this test. Their latest rig consists not of a dumbbell but of a nearly cylindrical shell studded on either side with weights of different materials. Instead of looking for a static twist, they slowly rotate the entire rig and look for a periodic twisting of the cylinder. Using beryllium and titanium, they found gravitational and inertial mass equal to one part in 10 trillion, as they reported in Physical Review Letters in 2008. That's not quite precise enough to test string theory predictions. “In principle, we could get another order of magnitude,” Adelberger says. “There are difficulties, but that's the goal.”

    Now the MicroSCOPE team aims to probe the equivalence principle to one part in a quadrillion. “When you have the ability to do such a test, you have to do it,” says project leader Touboul. MicroSCOPE will carry aloft two cylindrical shells, one the size of a toilet paper roll and made of titanium and a smaller one inside it made of platinum-rhodium. If the equivalence principle holds, both will glide on precisely the same orbit. If not, one should slip Earthward relative to the other.

    In practice, MicroSCOPE researchers will apply electrostatic forces to counteract any motion and use the force as their signal. As an extra test, they'll periodically flip the satellite so that if one cylinder does tend to circle closer to Earth, researchers will have to switch the direction of the force at the same time. MicroSCOPE will also carry a second pair of cylinders, both made of platinumrhodium, to act as a control.

    For such an ambitious experiment, MicroSCOPE is relatively cheap. The instrument cost about €20 million, Touboul says, and the entire mission less than €200 million. Nevertheless, Damour says that according to some models MicroSCOPE has a shot at seeing a violation of the equivalence principle. “There is no sharp prediction,” he says, “but there are models that say MicroSCOPE should see a strong signal.”

    If that sensitivity isn't enough, Mark Kasevich, a physicist at Stanford University in Palo Alto, California, thinks he can do 100 times better still. He is working on an atomic version of Galileo's drop test that will compare two different atoms: rubidium-87, which has 37 protons and 50 neutrons, and rubidium-85 which has two fewer neutrons. His 10-meter-tall vacuum chamber looks nothing like the Leaning Tower. Instead of dropping atoms from the top, Kasevich's team will toss them up from the bottom and watch them fall back down 2.3 seconds later.

    It's a tough experiment. To keep the atoms from spreading throughout the chamber, researchers must cool them to a fraction of a degree above absolute zero. “They start out as a little ball of atoms, they go up and come back down, and they're still a little ball of atoms,” Kasevich says. To track the atoms, the researchers employ a technique called atom interferometry, in which pulses of laser light split the quantum wave describing the atoms in two and then bring the pieces back together, using the light waves as a ruler to measure the atoms' fall. The physicists ran the experiment with rubidium-87 alone last year and plan to run with both atoms soon. “We're optimistic that within the next year we will have our first result,” Kasevich says.

    Others caution that Kasevich has been promising results for some time. Reaching the sensitivity goal may be difficult, Adelberger says, as researchers will have to compensate for tiny variations in gravity throughout the tower caused by massive objects nearby. Damour also notes that the two isotopes of rubidium are not that different, which limits the experiment's ability to test the effects predicted by string theory models.

    REGARDLESS OF THE SETUP—towers, pendulums, satellites—most experiments assume the test objects' gravity is too weak to contribute to their mass, so they test the so-called weak equivalence principle. But for a body the size of Earth, gravitational energy accounts for a half a billionth of its inertial mass. The strong equivalence principle states that gravitational and inertial mass remain equal even when such self-gravitation is included. To see if it holds, physicists are using the Earth and moon as test masses.

    Since 1970, physicists have precisely tracked the distance to the moon by bouncing pulses of laser light off reflectors left on the surface by the Apollo astronauts and Russia's robotic rovers. The data make it possible to study how Earth and the moon—two masses of different size and composition—“fall” toward the sun. Any violation of equivalence would produce a tiny shift in the moon's orbit around Earth, toward or away from the sun, says Thomas Murphy, a physicist at UC San Diego. He heads the APOLLO project, tracking the moon with a 3.5-meter telescope at the Apache Point Observatory in New Mexico.

    To search for a signal, researchers must account for many other factors that influence the Earth-moon distance, which varies from 356,000 to 406,700 kilometers, including effects as small as the atmosphere pressing on Earth's surface. “Some people run in terror from the number of things we have to contend with,” Murphy says. Researchers have shown that any shift in the orbit must be smaller than 4 millimeters, confirming the strong equivalence principle to one part in 10,000, Murphy says.

    Jens Gundlach (left), Eric Adelberger (with torsion balance), and Blayne Heckel of the field-leading Eöt-Wash group.


    Scientists have other tests in mind. Ernst Rasel, an atomic physicist at Gottfried Wilhelm Leibniz University of Hannover in Germany, is developing a satellite known as the Space-Time Explorer and QUantum Equivalence Principle Space Test (STEQUEST) that would run a cold-atom experiment like Kasevich's in space. There it could compare rubidium and potassium for longer times, says Naceur Gaaloul, a physicist at Hannover. As Science went to press, STEQUEST researchers were awaiting the European Space Agency's selection of projects to compete for its next midsized mission, to be chosen in 2018.

    Then there is the Satellite Test of the Equivalence Principle (STEP), a mission proposed in the 1970s that some researchers say could cost $300 million. STEP, conceived by Stanford's Francis Everitt and Paul Worden, is a more ambitious version of MicroSCOPE that would have 1000 times greater sensitivity. A mission like STEP is scientifically justified, says the University of Florida's Will, “but it isn't clear that such a mission is possible in the current funding climate.” That climate could change, IHES's Damour notes, if MicroSCOPE spots a sign that Galileo was wrong and, after all, some balls really do fall faster than others.

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