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Chinese gravitational-wave hunt hits crunch time

The pressure is on to choose between several proposals for space-based detectors.

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Gravitational waves from the binary star system HM Cancri (artist’s impression) would be the target of TianQin, one of China’s proposed space-based detectors.

In the wake of last month’s historic detection of gravitational waves by a US-led collaboration, a range of Chinese proposals to take studies of these ripples in space-time to the next level are attracting fresh attention.

The suggestions, from two separate teams, are for space-based observatories that would pick up a wider range of gravitational radiation than ground-based observatories can. The most ambitious plan could give China an edge over the leading European proposal to detect gravitational waves from space, but whether a single country can achieve that on its own is unclear. Also under consideration are a possible collaboration between Chinese researchers and the European effort, and a cheaper Chinese plan.

Although an Earth-based detector — the US Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) — was the first to confirm a prediction made by Albert Einstein a century ago, launching the field of gravitational-wave astronomy, such detectors can pick up only limited frequencies. Advanced LIGO compares laser light beamed along two perpendicular detector arms to reveal whether one beam has been compressed or stretched by gravitational waves.

Each LIGO arm measures 4 kilo­metres, but picking up the frequencies that are richest in gravitational waves requires distances of hundreds of thousands of kilometres or more. This can be achieved only in space, where spacecraft equipped with lasers can be positioned at these distances. Space-based detectors also avoid fluctuations in Earth’s gravitational field, which can obscure signals.

With such considerations in mind, the European Space Agency (ESA) is pursuing a space-based gravitational-wave detector. One of the Chinese proposals, Taiji, meaning ‘supreme ultimate’, is to create a more ambitious version of the leading proposal for the European project, which is called eLISA (Evolved Laser Interferometer Space Antenna).

Like eLISA, Taiji would consist of a triangle of three spacecraft in orbit around the Sun, which bounce lasers between each other (see ‘China’s choices’). The distance between eLISA’s components is still under discussion, but current plans suggest it could be 2 million kilometres, says eLISA member Karsten Danzmann of the Max Planck Institute for Gravitational Physics in Hanover, Germany. Taiji’s spacecraft would be separated by 3 million kilometres, giving the detector access to different frequencies. Taiji would launch in 2033, slipping in a year ahead of eLISA’s current schedule. “If Taiji produces a Chinese version of eLISA, then it will bring China to the frontier,” says Yanbei Chen, a gravitational-wave physicist at the California Institute of Technology in Pasadena, who works on LIGO.

SOURCES: eLISA/Wu Yue-Liang/Luo Jun

Gerhard Heinzel, an eLISA physicist also at 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 in terms of cost but also resources in terms of scientists and experts in the presence of competing science projects”, he says.

Taiji project leader Wu Yue-Liang, a particle physicist at the Chinese Academy of Sciences’ Institute of Theoretical Physics in Beijing, estimates that the project will cost 14 billion yuan (US$2 billion), roughly twice as much as ESA is budgeting for its gravitational-wave detector.

Second string

A second Chinese proposal, led by Luo Jun, a physicist at the Sun Yat-Sen University campus 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 project has three satellites that orbit Earth at a distance of about 150,000 kilometres from each other. It would cost 2 billion yuan, says Luo.

TianQin would be more limited than Taiji in terms of what it could detect: rather than acting as an observatory for the waves emitted by myriad objects including black holes and neutron stars, it would mainly target a particular pair of orbiting white dwarf stars, called HM Cancri. TianQin’s simplicity makes it cheaper and more certain of success, Luo says. The 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, but says that TianQin could lay the groundwork for a Taiji-like project in the future.

Wu Ji, director-general of the Chinese Academy of Sciences’ National Space Science Center, says that the TianQin and Taiji teams should merge. “If China decides to have a space gravitational mission, there should be an integrated one, with a new name probably. There is no way to support two missions at the same time.”

Both Wu Yue-Liang and Luo are confident that their proposals will move forward to the concrete design phase in the next five years. Taiji currently receives money from the Chinese Academy of Sciences and TianQin from the city of Zhuhai — but both need much more cash. The LIGO discovery could increase their chances of success. The “government will know more the importance of fundamental research” in gravitational waves, says Wu Ji. “China should catch up in this area,” he adds.

On 5 March, the Chinese central govern­ment released a draft list of 100 strategic projects that will be emphasized in the country’s next five-year plan, which includes “a new generation of heavy launch vehicles, satellites, space platforms and new payload” and a “deep-space station”.

Chinese researchers could also end up collaborating with Europe. As well as its main project, the Taiji group has outlined the possibility of a direct collaboration with eLISA: it would either contribute 1.5 billion yuan directly or develop its own scaled-down, 8-billion-yuan version of eLISA that would coordinate closely with the European effort, sharing data. Heinzel recommends that a united Chinese group work on one of these less ambitious options.

The direct contribution from China in particular could be a boon for eLISA. Originally, NASA collaborated with ESA on a planned space-based gravitational-wave observatory, named LISA. But the United States pulled out of LISA five years ago and ESA had to pare down the mission, resulting in the eLISA proposal. China’s entry into the project could fill that hole, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology in Cambridge, who is credited as the chief inventor of LIGO. This would perhaps allow Europe to pursue a design closer to that of LISA, which was better equipped than the eLISA proposal and would have had a longer mission lifetime.

A decision is needed soon if China is to achieve a launch date around 2030, cautions Heinzel. “Now is the time to do very serious technology development,” he says. “It is time to start making decisions.”

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