Quantum mechanics underlies current attempts to account for the strong nuclear force (see Quantum Chromodynamics) and to develop a unified theory for all the fundamental interactions of matter (see Physics: Modern Physics: Unified Field Theories). Nevertheless, doubts exist about the completeness of quantum theory. The divergence difficulty, for example, is only partly resolved. Just as Newtonian mechanics was eventually amended by quantum mechanics and relativity, many scientists—and Einstein was among them—are convinced that quantum theory will also undergo profound changes in the future. Great theoretical difficulties exist, for example, between quantum mechanics and chaos theory, which began to develop rapidly in the 1980s. Ongoing efforts are being made by theorists such as the British physicist Stephen Hawking, to develop a system that encompasses both relativity and quantum mechanics.