[Paleopsych] SW: On the Fundamental Constants over Time

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Theoretical Physics: On the Fundamental Constants over Time

    The following points are made by K.A. Olive and Y-Z. Qian (Physics
    Today 2004 October):
    1) Are any of nature's fundamental parameters truly constant? If not,
    are they fixed by the vacuum state of the Universe, or do they vary
    slowly in time even today? To fully answer those questions requires
    either an unambiguous experimental detection of a change in a
    fundamental quantity or a significantly deeper understanding of the
    underlying physics represented by those parameters.
    2) At first glance, a long list of quantities usually assumed to be
    constant could potentially vary: Newton's constant G(subN),
    Boltzmann's constant k(subB), the charge of the electron e, the
    electric permittivity epsilon-0, and magnetic permeability mu-0, the
    speed of light c, Planck's constant h-bar, Fermi's constant G(subF),
    the fine-structure constant alpha = e^(2)/(h-bar)c and other gauge
    coupling constants, Yukawa coupling constants that fix the masses of
    quarks and leptons, and so on. One must, however, distinguish what may
    be called a fundamental dimensionless parameter of the theory from a
    fundamental unit. Dimensionless parameters include gauge couplings and
    quantities that, like the ratio of the proton to electron mass, are
    combinations of dimensioned quantities whose units cancel. Their
    variations represent fundamental and observable effects.
    3) In contrast, variations in dimensioned quantities are not
    unambiguously observable.(1) To point out the ambiguity is not to
    imply that a universe with, say, a variable speed of light is
    equivalent to one in which the speed of light is fixed. But no
    observable difference between those two universes can be uniquely
    ascribed to the variation in c. It thus becomes operationally
    meaningless to talk about measuring the variation in the speed of
    light or whether a variation in alpha is due to a variation in c or
    h-bar. It is simply a variation in alpha.
    4) Lev Okun(2) provides a nice example, based on the hydrogen atom,
    that illustrates the inability to detect the variation in c despite
    the physical changes such a variation would cause.(2) Lowering the
    value of c lowers the rest-mass energy of an electron, E(sube) =
    m(sube)c^(2). When 2E(sube) becomes smaller than the binding energy of
    the electron to the proton in a hydrogen atom, E(subb) =
    m)sube)e^(4)/2(h-bar)^(2), it becomes energetically favorable for the
    proton to decay to a hydrogen atom and a positron. Clearly, that's an
    observable effect providing evidence that some constant of nature has
    changed. However, the quantity that determines whether the above decay
    occurs is the ratio E(subb)/2E(sube) = e^(4)/4(h-bar)^(2)c^(2) =
    alpha^(2)/4. Therefore, one cannot say which constant among e, h-bar,
    and c is changing, only that the dimensionless alpha is.(3-5)
    References (abridged):
    1. M. J. Duff, L. B. Okun, G. Veneziano, J. High Energy Phys.
    2002(03), 023 (2002).
    2. L. B. Okun, Sov. Phys. Usp. 34, 818 (1991)
    3. B. Bertoti, L. Iess, P. Tortora, Nature 425, 374 (2003)
    4. J. K. Webb et al., Phys. Rev. Lett. 82, 884 (1999); M. T. Murphy et
    al., Mon. Not. R. Astron. Soc. 327, 1208 (2001) ; J. K. Webb et al.,
    Phys. Rev. Lett. 87, 091301 (2001); M. T. Murphy et al., Mon. Not. R.
    Astron. Soc. 327, 1223 (2001)
    5. M. T. Murphy, J. K. Webb, V. V. Flambaum, Mon. Not. R. Astron. Soc.
    345, 609 (2003)
    Physics Today http://www.physicstoday.org
    Related Material:
    The following points are made by L.L. Cowie and A. Songaila (Nature
    2004 428:132):
    1) The physical constants might not be so constant. If any variation
    in their values were measured, it could give us profound constraints
    on a long-sought quantized theory of gravity. So it is no surprise
    then that a claim(1,2) to have measured a variation over time in the
    value of the fine-structure constant, alpha, has led to a spate of
    papers incorporating this result into a wide range of theories. Chand
    et al(3) have used the same technique of measuring radiation from
    distant quasars but reach the opposite conclusion -- there is no
    significant variation in alpha . What should we believe?
    2) The fine-structure constant was originally uncovered in studies of
    the closely spaced spectral lines of atoms such as hydrogen and
    helium. This "fine structure" reflects the quantization of electron
    energies within the atom. The constant is defined as the product
    2(pi)e^(2)/hc (where e is the charge of the electron, h is Planck's
    constant and c is the speed of light) but is perhaps more familiar in
    its numerical form: alpha = 1/137.
    3) Curiously enough, the most stringent limits on the variation of
    alpha come not from laboratory or astronomical measurements but from a
    natural nuclear reactor in Africa. About 1.8 billion years ago, in
    what is now the Oklo mine in Gabon, a spontaneous chain reaction
    began, involving the fission of the uranium isotope U-235. The capture
    of thermal neutrons from the fission process by other elements can be
    related to alpha . The best recent analysis of data from Oklo shows
    that any fractional change in the fine-structure constant
    (delta-alpha/alpha) has been less than 10^(-7) over nearly two billion
    4) Although less sensitive, cosmological measurements can nevertheless
    probe much longer periods, encompassing much of the 14-billion-year
    life of the Universe. Gas in and between galaxies that lie along the
    line of sight from the Earth to quasars scatters the light from these
    enormously distant sources. Atoms and molecules in the gas absorb
    certain wavelengths, imprinting absorption lines in the radiation
    spectra of these objects. Often the lines of many elements, in many
    ionization states, might be seen from a particular patch of gas. The
    wavelengths at which the lines occur depend on the distance of the
    absorbing gas from the observer, because the radiation becomes
    "redshifted" to longer wavelengths, owing to the expansion of the
    Universe, as it travels from its source.
    5) If there had been any variation in the fine-structure constant over
    the billions of years of the light's journey, that would have affected
    the energy levels in the atoms and would therefore have shifted the
    wavelengths of the absorption lines. We cannot measure absolute shifts
    in these wavelengths, because we have no way of independently knowing
    the distance to the source and hence the redshift that the radiation
    has undergone. But we can measure relative shifts of the wavelengths
    from all the absorption lines seen for a particular system. Absorption
    lines have been detected for quasars so distant that the radiation we
    see from them corresponds to a time when the Universe was only 6% of
    its present age(5).
    References (abridged):
    1. Webb, J. K. et al. Phys. Rev. Lett. 87, 091301 (2001)
    2. Murphy, M. T., Webb, J. K. & Flambaum, V. V. Mon. Not. R. Astron.
    Soc. 345, 609-638 (2003)
    3. Chand, H., Srianand, R., Petitjean, P. & Aracil, B. Astron.
    Astrophys. (in the press); preprint at
    http://arxiv.org/abs/astro-ph/0402177 (2004)
    4. Damour, T. & Dyson, F. Nucl. Phys. B 480, 37-54 (1996)
    5. White, R. L., Becker, R. H., Fan, X. & Strauss, M. A. Astron. J.
    126, 1-14 (2003)
    Nature http://www.nature.com/nature
    Related Material:
    Notes by ScienceWeek:
    In physics, the term "fundamental constants" (universal constants)
    refers in general to those constants that do not change throughout the
    Universe. For example, the charge on an electron, the speed of light
    in a vacuum, the Planck constant, the gravitational constant, are some
    of the constants considered as "fundamental constants".
    In 1931, the physicist F.K. Richtmyer (d. 1939), author of a textbook
    well-known to an entire generation of physics students, remarked: "Why
    should one wish to make measurements with ever increasing precision?
    Because the whole history of physics proves that a new discovery is
    quite likely to be found lurking in the next decimal place." The
    essential basis for this view is that accurate values of the
    fundamental constants are required for the critical comparison of
    theory with experiment, and it is only such comparisons that enable
    our understanding of the physical world to advance. A closely related
    idea is that by comparing the numerical values of the same fundamental
    constants obtained from experiments in the different fields of
    physics, the self-consistency of the basic theories of physics can be
    The following points are made by P.J. Mohr and B.N. Taylor (Physics
    Today March 2001):
    1) The authors point out that the values of the fundamental constants
    are determined by a broad range of experimental measurements and
    theoretical calculations involving many fields of physics and
    measurement science (metrology). The best value of even a single
    constant is likely to be determined by an indirect chain of
    information based on seemingly unrelated phenomena. For example, the
    value of the mass of the electron in kilograms is based mainly on the
    combined information from experiments that involve classical
    mechanical and electromagnetic measurements, the highest precision
    optical laser spectroscopy, experiments involving trapped electrons,
    and condensed matter quantum phenomena, together with condensed matter
    theory and extensive calculations in quantum electrodynamics.
    2) Two additional features of the values of the fundamental constants
    are not evident from a table of numbers: a) The numbers form a tightly
    linked set -- very few of the values are independent of the others. In
    general, a change in a single item of the data on which the constants
    are based will change many of the values. b) The numbers are based
    only on the information available at a particular time. Therefore, the
    recommended values change over time, and the type of information from
    which the values are obtained changes as well. For example, in the
    distant past, the charge of the electron was determined by the classic
    oil-drop experiment, but that method is no longer competitive. Now the
    electron charge is determined indirectly from other constants.
    3) The author points out that the basic approach to finding a
    self-consistent set of values for the fundamental constants is to
    identify the critical experiments, determine the theoretical
    expressions as functions of the fundamental constants that make
    predictions for the measured quantities, and adjust the value of the
    constants to achieve the best match between theory and experiment. The
    idea of making systematic study of potentially relevant experimental
    and theoretical information in order to produce a set of
    self-consistent values of the constants dates back to Raymond T.
    Birge, who published such a study in 1929 as the very first article in
    what is now the _Reviews of Modern Physics_. The Task Group on
    Fundamental Constants, established by the Committee on Data for
    Science and Technology in 1969, has published three sets of
    recommended values of the fundamental constants, one set in 1973, one
    set in 1986-1987, and the latest in 1999-2000. The most recent set is
    termed the "1998 recommended values", because it is based on the
    information available as of 31 December 1998.
    Physics Today http://www.physicstoday.org

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