Nuclear Transitions and New Standards of Length and Time

Modern standards of length and time are based on wavelength measurements of laser light sources, whose relative stability is limited by the temperature dependence of molecular or atomic transition frequencies and cannot be better than 10-15. The cardinal way of improvement is to find nuclear transitions coincident with any laser line. This short review is devoted to the present status of investigations and available projects of using the 7.6 eV transition in 229Th. Some new ideas are proposed and discussed. (authors)