National Physical Laboratory

Fundamental constants and tests of physical theories header Fundamental constants and tests of physical theories

Advances in 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

Atomic Clock Endcap

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 participating in the ACES (Atomic Clock Ensemble in Space) mission.

The Rydberg constant and the proton size

Rydberg constant experiment

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 recent measurements of the proton size differ significantly from the currently accepted value.

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