Passing the alcohol test: Fundamental properties of molecules have not changed during the past seven billion years

Measurements are the only way that physicists can find out about fundamental universal constants such as the proton-to-electron ratio. Although all terrestrial experiments produce the same value for this ratio, it would theoretically be possible for the constant to have a different value in different regions of the universe or at different times in its history. The methanol molecule is a suitable sensor for detecting such deviations.

A number of lines in the radio spectrum of this molecule would exhibit a significant frequency shift if the proton-to-electron mass ratio changed, while other lines would not be affected by this shift. A group at the VU University of Amsterdam only recently found out which property makes methanol such a sensitive sensor: ultimately it is a quantum tunnel effect which occurs if the internal rotation of the molecule is impeded. This effect leads to very high values for the sensitivity coefficients of the corresponding spectral lines, which can all be calculated individually.

"This makes the methanol molecule an ideal test case in order to discover a possible change in the proton-to-electron mass ratio over time," says Wim Ubachs, Professor at the VU University of Amsterdam and head of the physics department. "We therefore proposed a search for line radiation from methanol in the distant universe in order to compare the structure of the molecules thus found with the structure of today's methanol in laboratory experiments."

The team observed a galaxy where a number of different molecules had already been observed. The galaxy, which is in the line of sight of a high-intensity radio source called PKS1830-211, is about seven billion light years away from Earth. The scientists aimed for four different line transitions in the radio spectrum of the methanol molecule with their search program. And they were actually able to see all four lines with the aid of the 100-metre radio telescope in Effelsberg.

"As an optical astronomer it has been an interesting experience to carry out observations at the large wavelengths which occur in the radio range," says Julija Bagdonaite, doctoral student at the VU University of Amsterdam and lead author of the publication. "The methanol molecule had absorbed these radio waves seven billion years ago, and the waves have carried along its fingerprint from the distant past on their passage to Earth."

By analysing the quantum structure of the methanol molecule the researchers were able to deduce that two of its spectral lines which they observed at frequencies around 25 GHz were influenced hardly at all by a change in the proton-to-electron mass ratio. The other two lines reacted much more sensitively to a modification of this parameter.

"The source we investigated is by far the most suitable of all our observational objects for investigating the validity of our local physics even in very distant exotic environments," says Christian Henkel from the Max Planck Institute for Radio Astronomy. "It would be fantastic if we could find more sources of this type which we could use to look even further back into the past."

The scientists also included systematic effects of the observations in the evaluation of the data and thus obtained the following result: in the course of the past seven billion years the mass ratio of protons and electrons has changed by a factor of 10^-7 at most and is therefore rightly deemed to be a universal constant. This result can definitely be interpreted to mean that the structure of the molecular matter as derived from spectral observations agrees very accurately with the structure seven billion years ago. Possible deviations amount to a mere one hundred thousandth of a percent or even less.

"If we were really to find deviations in this fundamental constant, we would have a problem with our understanding of the foundations of physics," concludes Karl Menten, Director at the Max Planck Institute for Radio Astronomy. "Most importantly, this would violate Einstein's equivalence principle, the cornerstone of the general theory of relativity."