New limits on variations in the fundamental constants of nature - used to test a range of theories in physics - are presented in this week’s Nature Communications. The findings place new limits on any possible changes in some constants between the high density environment of Earth and the low density environment of interstellar space.

Our standard theories of physics all assume that the fundamental constants like the electron mass or the atomic fine structure constant are fixed and unvarying in time and space. However, other theories suggest that they may change based on time, location, or local matter density. Testing for such variations would then allow us to understand whether these models could help explain outstanding problems in physics. Michael Tarbutt and colleagues present one such test, which looks at the possible influence of local matter density on the fundamental constants. They develop a method to produce short pulses of ultracold molecules of CH and measure the frequency their atomic transitions to very high precision. They are then able to compare this with measurements of the same transitions taken from astronomical observations of the interstellar medium, where the local matter density of the CH molecules is extremely different. This allows the team to place new limits on the size of possible variations in both the fine structure constant and the electron-to-proton mass ratio due to the local environment.

This method provides new constraints for any theory that requires the fundamental constants of nature to change. With improved sensitivity in astronomical measurements, these constraints may be improved and extended to different sources.