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ASTROPHYSICS Chinese gravitational-wave hunt hits crunch time The pressure is on to choose between several proposals for space-based detectors.

BY DAVID CYRANOSKI gravitational-wave astronomy, such detectors ultimate’, is to create a more ambitious version can pick up only limited frequencies. Advanced of the leading proposal for the European pro- n the wake of last month’s historic LIGO compares laser light beamed along two ject, which is called eLISA (Evolved Laser detection of gravitational waves by a perpendicular detector arms to reveal whether Interferometer Space Antenna). US-led collaboration, a range of Chinese one beam has been compressed or stretched by Like eLISA, Taiji would consist of a trian- Iproposals to take studies of these ripples in gravitational waves. gle of three spacecraft in orbit around the space-time to the next level are attracting Each LIGO arm measures 4 kilo­metres, Sun, which bounce lasers between each other fresh attention. but picking up the (see ‘China’s choices’). The distance between The suggestions, from two separate teams, “If China decides frequencies that are eLISA’s components is still under discussion, are for space-based observatories that would to have a space richest in gravita- but current plans suggest it could be 2 million pick up a wider range of gravitational radiation gravitational tional waves requires kilometres, says eLISA member Karsten than ground-based observatories can. The most mission, there distances of hundreds Danzmann of the Max Planck Institute for ambitious plan could give China an edge over should be an of thousands of kilo- Gravitational Physics in Hanover, Germany. the leading European proposal to detect gravi- integrated one.” metres or more. This Taiji’s spacecraft would be separated by 3 mil- tational waves from space, but whether a single can be achieved only lion kilometres, giving the detector access to country can achieve that on its own is unclear. in space, where spacecraft equipped with different frequencies. Taiji would launch in Also under consideration are a possible collab- lasers can be positioned at these distances. 2033, slipping in a year ahead of eLISA’s cur- oration between Chinese researchers and the Space-based detectors also avoid fluctuations rent schedule. “If Taiji produces a Chinese European effort, and a cheaper Chinese plan. in Earth’s gravitational field, which can obscure version of eLISA, then it will bring China to Although an Earth-based detector — the signals. the frontier,” says Yanbei Chen, a gravitational- US Advanced Laser Interferometer Gravita- With such considerations in mind, the Euro- wave physicist at the California Institute of tional-Wave Observatory (LIGO) — was the pean Space Agency (ESA) is pursuing a space- Technology in Pasadena, who works on LIGO. first to confirm a prediction made by Albert based gravitational-wave detector. One of the Gerhard Heinzel, an eLISA physicist also at Einstein a century ago, launching the field of Chinese proposals, Taiji, meaning ‘supreme the Max Planck Institute in Hanover, cautions against a single country going it alone on such a large project. It “is definitely too big — mainly CHINA’S CHOICES in terms of cost but also resources in terms of Chinese researchers have proposed several ways to detect gravitational waves in space. scientists and experts in the presence of com- TAIJI peting science projects”, he says. eLISA spacecraft The most ambitious proposal uses Taiji project leader Wu Yue-Liang, a particle ~2 million km apart three spacecraft in a triangle that physicist at the Chinese Academy of Sciences’ orbits the Sun and detects Earth gravitational waves from a range Institute of Theoretical Physics in Beijing, esti- of objects, like Europe’s eLISA mates that the project will cost 14 billion yuan proposal. The spacecraft are farther (US$2 billion), roughly twice as much as ESA apart than in eLISA, giving Taiji

access to di erent frequencies. Ea is budgeting for its gravitational-wave detector. JUN ELISA/WU YUE-LIANG/LUO SOURCES: rth ’s o rb it SECOND STRING Sun A second Chinese proposal, led by Luo Jun, a Taiji spacecraft physicist at the Sun Yat-Sen University cam- 3 million km apart pus in Zhuhai, would lower the bar in terms of cost and resources. Called TianQin, a name that refers to the metaphor of nature playing a stringed instrument (a zither) in space, the TianQin spacecraft project has three satellites that orbit Earth at a ~150,000 km apart TIANQIN distance of about 150,000 kilometres from each Earth A cheaper proposal puts three other. It would cost 2 billion yuan, says Luo. craft in orbit around Earth, and TianQin would be more limited than Taiji in much closer to each other than Sun in Taiji. This would target the terms of what it could detect: rather than acting gravitational waves emitted as an observatory for the waves emitted by myr- by HM Cancri, a pair of white dwarf stars. iad objects including black holes and neutron Earth ’s orbit stars, it would mainly target a particular pair of orbiting white dwarf stars, called HM Cancri. TianQin’s simplicity makes it cheaper and

150 | NATURE | VOL 531 | 10 MARCH 2016 © 2016 Macmillan Publishers Limited. All rights reserved IN FOCUS NEWS more certain of success, Luo says. The REPRODUCIBILITY spacecraft could launch in 15–20 years, he adds, around the same time as the Taiji group says that it could launch. Luo thinks that a simpler project is more realistic now, Statisticians issue but says that TianQin could lay the ground- work for a Taiji-like project in the future. Wu Ji, director-general of the Chinese warning on P values Academy of Sciences’ National Space Sci- ence Center, says that the TianQin and Taiji teams should merge. “If China decides to Statement aims to halt missteps in the quest for certainty. have a space gravitational mission, there should be an integrated one, with a new BY MONYA BAKER cannot indicate the importance of a finding; name probably. There is no way to support for instance, a drug can have a statistically sig- two missions at the same time.” isuse of the P value — a common nificant effect on patients’ blood glucose levels Both Wu Yue-Liang and Luo are confi- test for judging the strength of sci- without having a therapeutic effect. dent that their proposals will move forward entific evidence — is contributing Giovanni Parmigiani, a biostatistician at the to the concrete design phase in the next Mto the number of research findings that cannot Dana Farber Cancer Institute in Boston, Mas- five years. Taiji currently receives money be reproduced, the American Statistical Asso- sachusetts, says that misunderstandings about from the Chinese Academy of Sciences ciation (ASA) warned on 8 March. The group what information a P value provides often crop and TianQin from the city of Zhuhai — has taken the unusual step of issuing principles up in textbooks and practice manuals. A course but both need much more cash. The LIGO to guide use of the P value, which it says can- correction is long overdue, he adds. “Surely if discovery could increase their chances of not determine whether a hypothesis is true or this happened twenty years ago, biomedical success. The “government will know more whether results are important. research could be in a better place now.” the importance of fundamental research” This is the first time that the 177-year-old in gravitational waves, says Wu Ji. “China ASA has made explicit recommendations on FRUSTRATION ABOUNDS should catch up in this area,” he adds. such a foundational matter, says executive direc- Criticism of the P value is nothing new. In 2011, On 5 March, the Chinese central govern­ tor Ron Wasserstein. The society’s members had researchers trying to raise awareness about false ment released a draft list of 100 strategic become increasingly concerned that the P value positives gamed an analysis to reach a statisti- projects that will be emphasized in the was being misapplied, in ways that cast doubt on cally significant finding: that listening to music country’s next five-year plan, which statistics generally, he adds. by the Beatles makes undergraduates younger includes “a new generation of heavy launch In its statement, the ASA advises research- (J. P. Simmons et al. Psychol. Sci. 22, 1359–1366; vehicles, satellites, space platforms and new ers to avoid drawing scientific conclusions or 2011). More controversially, in 2015, a set of payload” and a “deep-space station”. making policy decisions purely on the basis documentary filmmakers published conclu- Chinese researchers could also end up of P values (R. L. Wasserstein and N. A. Lazar sions from a purposely shoddy clinical trial — collaborating with Europe. As well as its Am. Stat. http://doi.org/bc4d; 2016). Research- supported by a robust P value — to show that main project, the Taiji group has outlined ers should describe not only the data analyses eating chocolate helps people to lose weight. the possibility of a direct collaboration that produce statistically significant results, the (The article has since been retracted.) with eLISA: it would either contribute society says, but all statistical tests and choices But Simine Vazire, a psychologist at the Uni- 1.5 billion yuan directly or develop its own made in calculations. Otherwise, results may versity of California, Davis, and editor of the scaled-down, 8-billion-yuan version of seem falsely robust. journal Social Psychological and Personality Sci- eLISA that would coordinate closely with Véronique Kiermer, executive editor of the ence, thinks that the ASA statement could help the European effort, sharing data. Heinzel Public Library of Science journals, says that the to convince authors to disclose all of the statis- recommends that a united Chinese group ASA’s statement lends weight and visibility to tical analyses that they run. “To the extent that work on one of these less ambitious options. longstanding concerns over undue reliance on people might be sceptical, it helps to have stat- The direct contribution from China in the P value. “It is also very important in that it isticians saying, ‘No, you can’t interpret P values particular could be a boon for eLISA. Origi- shows statisticians, as a profession, engaging without this information,” she says. nally, NASA collaborated with ESA on a with the problems in the literature outside of More drastic steps, such as a ban on pub- planned space-based gravitational-wave their field,” she adds. lishing P values in articles instituted by at least observatory, named LISA. But the United P values are commonly used to test (and dis- one journal, could be counter-productive, says States pulled out of LISA five years ago and miss) a ‘null hypothesis’, which generally states Andrew Vickers, a biostatistician at Memorial ESA had to pare down the mission, resulting that there is no difference between two groups, Sloan Kettering Cancer Center in New York in the eLISA proposal. China’s entry into the or that there is no correlation between a pair of City. He compares attempts to bar the use of project could fill that hole, says Rainer Weiss, characteristics. The smaller the P value, the less P values to addressing the risk of automobile a physicist at the Massachusetts Institute of likely an observed set of values would occur by accidents by warning people not to drive — Technology in Cambridge, who is credited as chance — assuming that the null hypothesis is a message that many in the target audience the chief inventor of LIGO. This would per- true. A P value of 0.05 or less is generally taken would probably ignore. Instead, Vickers says haps allow Europe to pursue a design closer to mean that a finding is statistically significant that researchers should be instructed to “treat to that of LISA, which was better equipped and warrants publication. But that is not neces- statistics as a science, and not a recipe”. than the eLISA proposal and would have had sarily true, the ASA statement notes. But a better understanding of the P value will a longer mission lifetime. A P value of 0.05 does not mean that there is not take away the human impulse to use sta- A decision is needed soon if China is to a 95% chance that a given hypothesis is correct. tistics to create an impossible level of certainty, achieve a launch date around 2030, cautions Instead, it signifies that if the null hypothesis is warns Andrew Gelman, a statistician at Colum- Heinzel. “Now is the time to do very seri- true, and all other assumptions made are valid, bia University in New York City. “People want ous technology development,” he says. “It is there is a 5% chance of obtaining a result at least something that they can’t really get,” he says. time to start making decisions.” ■ as extreme as the one observed. And a P value “They want certainty.” ■

10 MARCH 2016 | VOL 531 | NATURE | 151 © 2016 Macmillan Publishers Limited. All rights reserved