A US$1 million (HK$7.8 million) cash prize is on offer for the solution to a decades-old problem in particle physics first formulated by Chinese University physicist and Nobel Prize winner Yang Chen-ning. The Clay Mathematics Institute, based in Cambridge, Massachusetts, has put up a prize of US$1 million for the solution of each of seven problems that have eluded mathematicians and physicists during the past century. There is no time limit for the prizes. One of the seven problems is called the Yang-Mills Theory, jointly formulated by Professor Yang, 78, and US physicist Robert Mills in the early 1950s when they were working together in the United States. The theory is a geometrical account of the fundamental forces between elementary particles. However, according to Hong Kong University physics professor Cheng Kwong-sang, a satisfactory mathematical solution to the Yang-Mills equations has never been discovered. 'It would take a real genius to work it out and that person would probably have to come up with new concepts in physics or mathematics which we don't have now,' Professor Cheng said. 'The Yang-Mills theory is very important in particle physics, but there has not been a complete mathematical solution. 'Mathematicians demand a higher level of proof and rigour; physicists can work around the problem with assumptions and approximations as long as the theory does its job.' Professor Yang is professor-at-large at the Chinese University. He is currently in the United States where he was Albert Einstein Professor of Physics at the State University of New York for more than three decades until his recent retirement. He shared the 1957 Nobel Prize in physics with Lee Tsung-dao for their work on particle decay that violates what is called parity, once thought to be an inviolable physical law. The seven problems cover both pure mathematics and physics. They include the century-old Riemann hypothesis on the distribution of prime numbers, the unsolved Navier-Stokes equations in fluid dynamics and a classic polynomial problem with applications for computer code-breaking. Details about the problems are available at www.claymath.org .