The constants are still constant

Researchers at the National Physical Laboratory (NPL) in the UK, as well as the Physikalisch-Technische Bundesanstalt (PTB) in Germany, have improved the constraints on time-variation of fundamental constants by making measurements of two optical clock transitions in the same atom (ytterbium). Their experiments have shown that one essential fundamental constant - the mass ratio of protons to electrons - can have changed only by a maximum of one part in a million over the age of our solar system.

Fundamental constants of nature are the pillars of modern physics, and measurement scientists are in the process of redefining the standard units of measurement (SI units) to be directly related to these fundamental constants. Recent developments in the measurement of time and frequency are allowing scientists to test these constants and see if they live up to their name.

The current definition of the second is based on a microwave frequency in caesium atoms. New types of atomic clock operate at optical frequencies using laser light, rather than microwaves, and have been demonstrated to have improved levels of stability and accuracy over microwave clocks. NPL is developing optical atomic clocks using a number of different atoms and ions, one of which is the ytterbium ion.

The tick rate of an atomic clock is known as a ‘transition frequency’ - the frequency of electromagnetic radiation absorbed or released as electrons orbiting the atom move between two different energy states. The ytterbium ion is unique in that it has two of these transitions that are being used as optical frequency standards. These two transitions also have very different sensitivities to variation in the fine structure constant, which relates to the strength of the electromagnetic force.

If the fine structure constant were changing over time, as has been proposed by some cosmologists and astrophysicists, one of these transition frequencies would get smaller and one would get larger. Thus, by repeatedly measuring the frequencies, scientists can test the theory that the fine structure constant is changing over time. NPL has now made measurements of the optical clock transitions against the SI second, along with the first ever measurement of the ratio between the two optical clock transitions in the same atom.

Meanwhile, PTB checked the constancy of the mass ratio of protons to electrons by comparing an optical clock with a trapped ytterbium ion and caesium atomic clocks over seven years. The mass of the electron determines the frequency of the optical atomic clocks, while the mass of the protons shows in the frequency of the caesium clock (via the properties of the atomic nucleus). The researchers concluded that the mass ratio of protons to electrons shows no detectable change up to a relative uncertainty of only a few parts per 10^-16 per year.

Combining these results with the NPL data, as well as a wide variety of other optical clocks worldwide, has allowed a new limit to be placed on present-day time-variation of the fine structure constant at -0.7 (2.1) x 10^-17 year^-1. This means scientists can rule out changes at the level of two parts in 100,000,000,000,000,000 per year. These measurements have also resulted in a near-threefold improvement in the constraint on the present-day time-variation of the proton-to-electron mass ratio at 0.2 (1.1) x 10^-16 year^-1. The results have been published in the journal Physical Review Letters.

As well as testing the laws of physics, this new measurement is vital for a future redefinition of the second. In addition, the improvements in atomic clocks and frequency measurement also have practical applications including improved navigation, timing and synchronisation.