[Paleopsych] SW: On the Fundamental Constants over Time
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Theoretical Physics: On the Fundamental Constants over Time
http://scienceweek.com/2004/sc041119-5.htm
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:
ASTROPHYSICS: ON THE FINE STRUCTURE CONSTANT
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
years(4).
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:
GENERAL PHYSICS: ON THE VALUES OF THE FUNDAMENTAL CONSTANTS
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
tested.
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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