Diagram of the methanol molecule

Measuring Fundamental Constants with Methanol

Article written: 13 Jun , 2011
Updated: 24 Dec , 2015



Key to the astronomical modeling process by which scientists attempt to understand our universe, is a comprehensive knowledge of the values making up these models. These are generally measured to exceptionally high confidence levels in laboratories. Astronomers then assume these constants are just that – constant. This generally seems to be a good assumption since models often produce mostly accurate pictures of our universe. But just to be sure, astronomers like to make sure these constants haven’t varied across space or time. Making sure, however, is a difficult challenge. Fortunately, a recent paper has suggested that we may be able to explore the fundamental masses of protons and electrons (or at least their ratio) by looking at the relatively common molecule of methanol.

The new report is based on the complex spectra of the methane molecule. In simple atoms, photons are generated from transitions between atomic orbitals since they have no other way to store and translate energy. But with molecules, the chemical bonds between the component atoms can store the energy in vibrational modes in much the same way masses connected to springs can vibrate. Additionally, molecules lack radial symmetry and can store energy by rotation. For this reason, the spectra of cool stars show far more absorption lines than hot ones since the cooler temperatures allow molecules to begin forming.

Many of these spectral features are present in the microwave portion of the spectra and some are extremely dependent on quantum mechanical effects which in turn depend on precise masses of the proton and electron. If those masses were to change, the position of some spectral lines would change as well. By comparing these variations to their expected positions, astronomers can gain valuable insights to how these fundamental values may change.

The primary difficulty is that, in the grand scheme of things, methanol (CH3OH) is rare since our universe is 98% hydrogen and helium. The last 2% is composed of every other element (with oxygen and carbon being the next most common). Thus, methanol is comprised of three of the four most common elements, but they have to find each other, to form the molecule in question. On top of that, they must also exist in the right temperature range; too hot and the molecule is broken apart; too cold and there’s not enough energy to cause emission for us to detect it. Due to the rarity of molecules with these conditions, you might expect that finding enough of it, especially across the galaxy or universe, would be challenging.

Fortunately, methanol is one of the few molecules which are prone to creating astronomical masers. Masers are the microwave equivalent of lasers in which a small input of light can cause a cascading effect in which it induces the molecules it strikes to also emit light at specific frequencies. This can greatly enhance the brightness of a cloud containing methanol, increasing the distance to which it could be readily detected.

By studying methanol masers within the Milky Way using this technique, the authors found that, if the ratio of the mass of an electron to that of a proton does change, it does so by less than three parts in one hundred million. Similar studies have also been conducted using ammonia as the tracer molecule (which can also form masers) and have come to similar conclusions.

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5 Responses

  1. Anonymous says

    Well, that’s a relief! I imagine that if physical constants can vary it would be bad news for complex structures that involve countless intricate physical and chemical processes. Like, you know, human beings.

    • it still may be plausible on huge distances, like between superclusters. milky way is just our neighborhood, so this finding was expected. but in a galaxy far far away…

    • it still may be plausible on huge distances, like between superclusters. milky way is just our neighborhood, so this finding was expected. but in a galaxy far far away…

  2. Wezley Jackson says

    When I first read this article I thought it said “Menthol.” I wish it was Friday 😉

  3. Anonymous says

    Other measurements of physical constants have been conducted, such as those which measure fine structure splitting and ? = (1/4??)e^2/?c from distant sources, have found similar null results. This does indicate the vacuum structure of the universe is constant. Physical constant do vary, but with transverse momentum in interactions. The electric charge polarizes the vacuum which consists of e e^+ pairs, where a bare electric charge that is negative forces these dipoles to align. This has the effect of changing the effective charge of an electron as one penetrates close. It requires fairly high energy to detect this. The fine structure constant ? =~ 1/137, and at the TeV energy range ? =~ 1/128. Such changes then occur in the momentum-energy domain, not in the space and time domain.

    There is in association with this something called the renormalization group (RG) and this gives a scaling of coupling constants with respect to a “flow” in momentum space. The flow has analogues to fluid dynamics, and has the elementary Callin-Symanzik form of the equation is

    (E?/?E + ?(g)?/?g)G(x_1, x_2, … x_n, E, g) = 0

    for E = energy, g the coupling constant, and G(**) the propagator function of fields.. The ?(g) is the Euler-beta function, which occurs in various forms in quantum field theory, including the vertex algebra in string theory. This equation is identical in form to the Navier-Stokes equation for fluid flow, and the change in coupling constants, g or in the case of quantum electrodynamics e or ?, changes according to an abstract type of fluid flow.

    So far there is no clear data to suggest such changes or “flows” occur with respect to space or time at zero interaction energy, or zero transverse momentum. This does suggest the zero point of the above flow, at E = 0, is a fixture of the quantum vacuum configuration which is not dependent on spatial or time variables. How the end point is set is an aspect of the so called “fine tuning” problem. However, that gets us into another question that is in my opinion well formulated at this time.


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