This could be physicist Stephen Hawking's last trip to China, the man who invited him to Hong Kong has warned. Professor Yau Shing-tung, who is chairing the conference that Professor Hawking is due to address next week, said he feared the scientist's ailing health may rule out long-distance travel. He last invited the theoretical physicist to the mainland in 2002 to address an earlier string theory conference. 'I don't want to say it, but I am afraid his health may prevent him from coming here again. I don't think his physical situation is as good as it was four years ago. I was really impressed that he accepted this offer,' he said. Although he set up Professor Hawking's trip, he said the Hong Kong University of Science and Technology paid for airfares and accommodation. Dr Cheng Shiu-yuen, dean of science at HKUST, where Professor Hawking delivered the Institute of Advanced Studies' inaugural lecture on 'The Origin of the Universe' on Thursday, said Professor Hawking had not charged an honorarium. Professor Yau, who counts Professor Hawking among his personal friends, said the physicist invited him to Cambridge in 1978 after he [Yau] and a colleague had solved a mathematical problem - the positive energy conjecture - that had implications for theoretical physics. 'He was the first person to understand our argument, better than all other physicists who are less mathematically oriented. Each year there was a big conference in general relativity and they had a session just to discuss this problem. When we solved it many physicists were puzzled because it was too mathematical for them. They didn't know what to say, but because I was reasonably famous at that time they didn't want to discount it,' Professor Yau said. 'He was rather weak but was still able to speak a little. His students understood what he was talking about so I communicated through them. I stayed there for about three days and was amazed by his energy. He could discuss theories from morning to night.'