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Physical Constants in the Cosmos

Have the laws of physics stayed the same throughout the history of the cosmos? It’s an interesting question because even minute changes to physical constants could imply the existence of extra dimensions, of the sort posited by string theorists. But that’s a big ‘could’, because despite earlier controversial findings, at least one cornerstone constant — the ratio of a proton’s mass to that of an electron — looks to be exactly the same in a galaxy some 6 billion light years away as it is when we measure it on Earth. A study led by Michael Murphy (Swinburne University) presents the result in a recent issue of Science.

The constant, known as mu, determines the value of the strong nuclear force, so it has everything to do with how atomic nuclei hold themselves together. No one can say why the mass of a proton should be 1836 times that of an electron. All we know is that it is. To be more precise, the value is 1836.15. The recently published research studied light from the quasar B0218+367, examining how it was partially absorbed by ammonia gas in an intervening galaxy on its way to Earth-based astronomers. It’s a useful measurement because the wavelengths at which ammonia absorbs energy from the quasar turn out to be quite sensitive to mu.

Take a look at the image below to get an idea of how key gravitational lensing has proven to be in this work. The quasar B0218+367 is about 7.5 billion years away. Two things are happening to its light as it moves toward us. First, its wavelength is being stretched, making it redder the farther it travels. And usefully for us, the light is being gravitationally lensed by the intervening galaxy six billion light years away. The result: Two quasar images and one extremely helpful set of data.

Quasars in lensing image

Image: Radio contour map of the quasar B0218+367 at about 7.5 billion light years distance. The galaxy containing absorbing ammonia molecules lies about 6 billion light years away and, though it is not seen in this radio map, gravitationally lenses the background quasar light to produce two bright quasar images on the sky (big red circles). The physical size of the image (at the distance of the absorbing galaxy) is about 19,000 light years across. Credit: Andi Biggs (MERLIN Image).

Christian Henkel (Max Planck Institute for Radio Astronomy) sees a clear result: “By comparing the ammonia absorption with that of other molecules, we were able to determine the value of the proton-electron mass ratio in this galaxy, and confirm that it is the same as it is on Earth.”

While we tend to assume that the laws of physics are the same everywhere, it’s an assumption that has to be verified by observations of different times and places in the cosmos. For that matter, the four fundamental forces of nature — gravity, electromagnetism, and the strong and weak nuclear forces — can’t be predicted from our theories, but can only be measured by experiment. That points to a major hole in our understanding of how physical constants govern the universe.

Finding out whether these constants remain the same is a prerequisite for deepening our understanding of how they emerge. And despite the earlier claim (in a Dutch study of 2006) that small differences in mu were observable, this new work is the first to use ammonia molecules, which turn out to be ten times more sensitive than any previous method (the Dutch team used molecular hydrogen). What’s next is to move the investigation beyond the confines of a single galaxy to weigh the value of mu in other eras, but as the details of the lensing involved in this experiment make clear, finding the right targets is a significant challenge.

The paper is Murphy et al., “Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe,” Science Vol. 320. No. 5883 (20 June 2008), pp. 1611-1613 (abstract).

Comments on this entry are closed.

  • tacitus June 23, 2008, 12:06

    Yay! It’s nice when we find that the Universe makes sense once in a while!

  • James M. Essig June 23, 2008, 15:57

    Hi Paul and tacitus;

    This is an interesting finding.

    One can imagine how the strength of the physical constants vary within various other universes especially the relative strengths.

    Since the mathematical interval that extends the range of possible relative strengths between that of the strong nuclear force and that of the electromagnetic force presumable can include any positive real number over a limited number line interval, and perhaps any positive real number period, I would assume that the number of choices for the ratio that nature could choose for this value is equal to the cardinality of infinity of Aleph 1.

    If by chance, nature has somehow produced a hidden relative strength to the ratio of the strength of the strong nuclear force to that of the electromagnetic force or some sort of hidden force variable that can vary independently or partially independently from the known relative force strength difference, in other words, an additional hidden variable of relative strength between these two forces, then the number of choices mother nature has is +R x +R.

    If it turns out that our universe has 100 fundamental constants with positive values, then the set of possible values of these constants is equal to +R EXP 100 where +R is the set of positive real numbers.

    If the Bulk in p-brane theory ultimately turns out to be infinite dimensional in the Aleph 0 sense, then the number of geometries possible including curvature of each dimension, relative curvature of each dimension to the other dimensions and the like wherein each dimension extends the cardinality of infinity of Aleph 0 light-years is astounding, at least equal to R EXP (Aleph 0). If there are Aleph 1 dimensions and each dimension extends the cardinality of Aleph 0 light-years, then the number of possible choices is at least equal to R EXP Aleph 1. I am assuming continuously variable degrees of geometry variations for each infinitesimal differential element of each dimension in the above assumption.

    In reality, because each differential element of each dimensionality can have in mathematical theory, an Aleph 1 number of degrees of curvature, the number of possible geometries is way, way beyond even R EXP Aleph 1. Who knows? Perhaps the Bulk has (Aleph 1) EXP (Aleph 1) dimensions

    The point I am trying to make is that the physical cosmos will always have surprises in store for us, not just in terms of possible physical laws and characteristics variations, but also in terms of possible bodily ETI life forms and civilizations.



  • ljk June 25, 2008, 9:46

    Galaxy map hints at fractal universe

    New Scientist Space June 25, 2008

    New data from the Sloan Digital Sky
    Survey shows that the galaxies
    exhibit an explicitly fractal
    pattern up to a scale of about 100
    million light years, say physicists
    at Enrico Fermi Centre in Rome,
    Luciano Pietronero of the University
    of Rome, and St Petersburg State


  • nate June 25, 2008, 12:47

    there are some studies that also look at the light from distant quasars in researching the strength of the fine structure constant and though debated some have shown that it has varied in stength, all be it by tiny amounts, none the less it seems to have a varied history and that history seems to run parallel to when the universe’s acceleration began to increase mid-way in cosmic history, it is very curious indeed. See John D. Barrow’s paper on the fine structure constant.

  • ljk June 25, 2008, 14:48

    Is the Universe Actually Made of Math?

    Cosmologist Max Tegmark says mathematical formulas create reality. 06.16.2008


  • Tissa Perera June 26, 2008, 18:36

    Yes the proton to electron mass ratio or better still the baryon to electron mass ratio should be well stable. I derive a formula for baryon to electron mass
    ratio which gives these values to better than 0.3% accuracy.
    see cosmicdarkmatter.com for a hint.

  • ၾMR.KYAWHTIKE September 2, 2008, 7:14

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