Scientific Method / Science & Exploration

Cosmic alcohol, vintage seven billion years, measures physical constant

Confirms the proton-electron mass ratio has been constant since.

The 100-meter Effenberg radio telescope in Germany, the second-largest fully steerable telescope in the world.
Paul Jansen

While quantum physics and general relativity don't always play well together, they agree on important points, including that of the spectra of atoms. To wit: the spectrum of hydrogen shouldn't depend on where the atom is located in the Universe, or whether it emitted its light 10 billion years ago or 10 seconds ago.

But to say that the behavior of the Universe has been consistent throughout its entire age is a strong claim, so it's essential to test it against observational data. In the era of precision cosmology, astronomers have the tools to measure spectra out to a significant fraction of the age of the Universe—distances of billions of light-years.


The Standard Model of particles and interactions contains many fundamental constants: the masses of the lightest quarks and electrons, the strengths of the various forces, and so forth. The constancy of physical constants is a reasonable assumption, but it isn't a necessary feature of the Universe. In fact, our theories don't currently tell us why these constants take the values they do—meaning calculations must be "fine-tuned".

As a result, some physicists have postulated the existence of slowly varying parameters instead of constants. This would mean that atoms in distant galaxies might interact in a slightly different way. In fact, interactions in two galaxies equally far from Earth might differ from each other if the parameters change both in space and time. Once the constants are no longer constant, many bets are off, though large variations can be ruled out by the apparent similarity of hydrogen's spectrum across space and time.

Enlarge / The structure of methanol, where the white spheres are hydrogen, the black sphere is carbon, and the red is oxygen. The junction between the hydrogen and carbon is a swivel, allowing the "head" of the structure to rotate.

Methanol (CH3OH) provides a particularly sensitive measure of the ratio of proton mass to electron mass. (This ratio also plays a role in the hydrogen spectrum; previous observations measured its constancy to less than one part in 100,000 in galaxies 12 billion years ago.) The reason for the sensitivity is the structure of the molecule, which is like a tripod (the CH3 portion, known as the methyl group) with a rotating head (the OH). The hydrogen atoms repel each other, so the "head" can't rotate arbitrarily: it meets resistance if it aligns with the "feet" of the tripod.

As with any quantum system, the possible configurations of the molecule are quantized: separated by fixed energy differences. The transitions between these configurations is what gives an atom or molecule its distinct spectrum. Methanol's torsional spectrum depends directly on the relationship between the proton and electron mass.

The current study examined light from a very bright, distant supermassive black hole (a type of system called a blazar) as it passed through an intervening galaxy 7 billion years from us. Some of the light exhibited the telltale signs of absorption by methanol, allowing a direct comparison of its spectrum with laboratory measurements. The difference between the measurements was incredibly small: less than one part in 107, or roughly 0.00001 percent.

Seven billion years is more than half of the Universe's present age of 13.7 billion years. If the proton-electron mass ratio hasn't changed measurably in that time, it's likely to be a true constant of nature. While this result gets us no closer to solving the fine-tuning problem, it supports one of the central principles of general relativity: that physics doesn't depend on location in space and time.

Science, 2012. DOI: 10.1126/science.1224898  (About DOIs).

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