You could hear a quantum particle drop as academics and journalists waited in eager anticipation for Stephen Hawking's first public address since arriving in Hong Kong on Monday. The silence in the Hong Kong University of Science and Technology's lecture theatre, as Professor Hawking programmed his voice synthesiser, contrasted starkly with the noisy melee that had greeted him at the airport the day before. He was introduced to an audience of more than 150 people by HKUST president Paul Chu Ching-wu, ahead of his lecture at the university tomorrow afternoon. The questions, submitted by members of the audience ahead of the meeting, dealt with science and economics, life on other planets, Professor Hawking's children, and accident victim Tang Siu-pun, 37 - also known as Ah Bun or Bun Tsai. A quadriplegic who once pleaded to be allowed to die, Ah Bun is among several disabled people chosen to attend Professor Hawking's lecture tomorrow. Professor Hawking said that although research often led to commercial developments, 'science has to be directed by scientific considerations, not economic ones'. He pointed out that his predecessor at Cambridge University, Paul Dirac, had paved the way for the computer by facilitating the development of the transistor. Asked whether he thought humans could live in space, he said that we would have to 'spread out . . . for the survival of the species'. But mankind would first have to survive threats such as nuclear weapons, engineered viruses and global warming. Answering a question about Ah Bun and legalised euthanasia, Professor Hawking, who has been in a wheelchair since developing motor neurone disease when he was 21 at Cambridge University, said that although he should be able to end his life if he wanted to, it would be 'a grave mistake ... While there's life, there is hope.' Asked what message he would send students interested in science and cosmology, he said there was 'nothing like the thrill of discovery, when you find something that no one knew before'. Saying that he was 'very close' to his children, Professor Hawking revealed that he and his daughter Lucy were working on a children's science book. Lucy Hawking, who is accompanying her father, added it would be a bit like Harry Potter meets A Brief History of Time. Professor Hawking's lecture tomorrow will be on 'The Origin of the Universe'. Later this week he will head for Beijing, where he will take part in an international mathematics conference on string theory. The conference will be chaired by Yau Shing-tung, the director of the Chinese University of Hong Kong's Institute of Mathematical Sciences.