Fundamental constants and tests of physical theories
Advances in optical frequency standards and metrology are opening up new horizons in fundamental physics. In particular they provide powerful tools for scientists to make highly precise measurements of fundamental constants and to test the laws of physics, such as Einstein’s theories of special and general relativity.
The hydrogen atom is unique among optical frequency standards in that its transitions can be calculated with a precision comparable to that of modern frequency metrology. A collaborative experiment involving NPL and the University of Oxford aims to make an improved measurement of the Rydberg constant by laser spectroscopy of atomic hydrogen. This brings Oxford’s significant expertise in the spectroscopy of simple atomic systems together with NPL’s infrastructure for high accuracy optical frequency metrology. By providing input to CODATA this work will feed into the knowledge of other fundamental constants, which form the basis of international metrology.
Experiments are also underway to search for possible variations in fundamental constants with time. This issue is highly relevant to metrology, due to the long-term objective of defining SI units in terms of fundamental constants (e.g. the speed of light) or in terms of physical quantities that depend on fundamental constants (e.g. the hyperfine splitting of caesium).