Fundamental constants and tests of physical theoriesAdvances in optical frequency standards and metrology are opening up new horizons in fundamental physics, enabling scientists to measure fundamental constants and test the laws of physics with unprecedented precision.
Testing the Einstein Equivalence Principle
Most modern theoretical attempts to provide a unified description of the four fundamental forces of nature predict violations of the Einstein Equivalence Principle. These include the time- and space-dependence of fundamental physical constants such as the fine structure constant, which characterises the strength of electromagnetic interactions.
One of the best ways of searching for present-day variations of the fine structure constant is to compare the frequencies of different optical frequency standards over the course of several years. Although this period is short compared to cosmological timescales, such experiments complement astrophysical measurements due to the extremely high frequency resolution achievable. At NPL we are working on an experiment to compare two optical frequencies in a single trapped ytterbium ion.
Space-borne atomic clocks could also be used to search for violations of the Einstein Equivalence Principle by making measurements of the gravitational redshift with unprecedented precision. NPL is involved in European efforts to develop a new generation of optical atomic clocks for space, and is also exploring possibilities to participate in the ACES (Atomic Clock Ensemble in Space) mission.
The Rydberg constant and the proton size
The hydrogen atom is unique in that its transition frequencies can be calculated with a precision comparable to that of modern frequency metrology. This makes it possible to determine the Rydberg constant by comparing measured transition frequencies in hydrogen to the theoretical expressions for the energy levels.
An experiment underway at NPL aims to make an improved measurement of the Rydberg constant by laser spectroscopy on a hydrogen atomic beam. It may also play a role in understanding why a recent measurement of the proton size differs from the accepted value by almost five times the combined uncertainty of the two values.
Research Areas
- An experiment underway at NPL aims to reduce the uncertainty in the Rydberg constant by making accurate measurements of two-photon transition frequencies in atomic hydrogen.
- NPL is playing a prominent role in a Europe-wide effort that aims to advance optical clock and optical frequency comb technology, with a view to making it fit for use in space missions within 10 years.
- The singly charged ytterbium ion is unique among optical frequency standards in that the lowest-lying excited state with an estimated lifetime of around six years.