Qiu Chenyun, 35, took up a new position in Nanjing, far from family and friends after relocation from Suzhou. Reading may be a good hobby, but like many she had little quality time for herself on the helter-skelter of modern life.

One of her friends recommended the radio adaptation of The Three-Body Problem, the best-selling science fiction novel by Liu Cixin.

She hesitated at first, because she had actually tried to read the novel, but found it difficult to follow the loosely narrated story with its many characters and strange scientific concepts. However, when she tried the radio play, which launched on Nov 30, she was immediately hooked.

"The adaptation is excellent. Compared with other audio books I am listening to, this one is better because it is played by many different people, rather than read by one person. The sound effects and background music are vivid, too," she says.

The radio play adaptation of The Three-Body Problem has been running for nearly four months with 16 episodes released so far on Ximalaya, which have been broadcast 15 million times. Each Wednesday, the platform releases a new episode that lasts about 30 minutes, and as planned the 1,500-page trilogy (The Three-Body Problem, Dark Forest, and Death's End) will be contained in a total of 80 episodes. It was produced by 729 Voice Studio, a company set up by professional voice performers in late 2016, which started with the production of the radio play, Kill Zone.

The Three-Body Problem has been the most popular radio play on Ximalaya, says Jiang Feng, vice-president of the online platform. The most common comments on the app are "too short" and "new episodes released too slow".

"On average, we can produce one episode in one to two weeks. If we increase the length to one hour, the production cycle will have to double," says Zi Tangsu, CEO of 729 Voice Studio.

Despite its popularity around the world, The Three-Body Problem trilogy would be difficult, with its imaginative plot, to adapt for screens. Comparatively, radio presents the story with just voices and can avoid these difficulties, says Cheng Han, scriptwriter of the radio play.

However, it does not mean that the radio play adaptation is easy. It took Cheng three months to write the first episode.

"I was not satisfied with my work, so I kept starting over, again and again," says Cheng. As a fan of Liu and the trilogy, Cheng says she felt a pressure to do her best.

One of the key principles for radio play adaptation is to fully respect the original work and change or delete as little as possible. "It's because most of the audience are fans of the books or those who decide to listen due to the books' fame, so they want the authentic story," Cheng says. However, she deletes visual descriptions, such as those about views and clothing, if it does not influence the plot.

To attract the attention of listeners, within each episode, scriptwriters need to create a kind of psychological rhythm by setting up small conflicts, big conflicts and the climax, she says.

There are other difficulties. One is how to use sound to represent spectacular scenes. Another is how to explain, in a simple but interesting way, the complex scientific concepts that stretch over pages in the novel.

"Because listeners can only concentrate for a limited time, it's no good to explain the concepts through monologues or dialogues. Instead we usually use sound effects to achieve that purpose," Cheng says.

"Luckily, Liu uses a lot of metaphors to explain those concepts, which I will keep."

Although the radio play adaptation needs to be authentic, compared with the first book, the second book of the trilogy, Dark Forest, contains too many characters and a complicated narrative, in which Liu hides the clues.

"It's impossible to do the play according to the plot of the novel. So I created an Excel sheet to map out all the characters, all the page numbers and paragraphs about them, as well as the content," Cheng says.

"We will focus on a certain character in one or two successive episodes to thread all the clues about him or her, so that our audience can understand clearly what happens to different characters."

Compared with image, sound can give listeners bigger scope for imagination, "which means a radio play might be one of the best forms to present science fiction", Liu says in the trailer of the radio play.

Apart from script writing, another difficulty in making this radio play was to find the right voice performers. There are more than 40 voice actors and actresses in the first season.

"When interviewing performers, we need to see whether their voices can match the image of the roles and whether they can properly represent the characters," says Liu Cong, director of the play, and also the performer of Da Shi, the policeman.

The most difficult thing is how to use voice to show people's spiritual state in a different time, like during the "cultural revolution" (1966-76), he says.