Inconstant Constants (June 2005,S.A. Magazine)
Do the inner workings of nature change with time?
By John D. Barrow and John K. Webb
(edit:The equations of physics are filled with quantities such as the speed of light. Physicists routinely assume that these quantities are constant: they have the same values everywhere in space and time.Over the past six years, the authors and their collaborators have called that assumption into question. By comparing quasar observations with laboratory reference measurements, they have argued that chemical elements i n the distant past absorbed light differently than the same elements do today. The difference can be explained by a change in one of the constants, known as the fine-structure constant, of a few parts per million.
Small though it might seem, this change, if confirmed, would be revolutionary. It would mean that the observed constants are not universal and could be a sign that space has extra dimensions)
Over the past six years, the authors and their collaborators have called that assumption into question. By comparing quasar observations with laboratory reference measurements, they have argued that chemical elements i n the distant past absorbed light differently than the same elements do today. The difference can be explained by a change in one of the constants, known as the fine-structure constant, of a few parts per million.
Small though it might seem, this change, if confirmed, would be revolutionary. It would mean that the observed constants are not universal and could be a sign that space has extra dimensions
Some things never change. Physicists call them the constants of nature. Such quantities as the velocity of light, c, Newton's constant of gravitation, G, and the mass of the electron, me, are assumed to be the same at all places and times in the universe. They form the scaffolding around which the theories of physics are erected, and they define the fabric of our universe. Physics has progressed by making ever more accurate measurements of their values.
And yet, remarkably, no one has ever successfully predicted or explained any of the constants. Physicists have no idea why they take the special numerical values that they do. In SI units, c is 299,792,458; G is 6.673 X 10-11; and me is 9.10938188 X 10-31--numbers that follow no discernible pattern. The only thread running through the values is that if many of them were even slightly different, complex atomic structures such as living beings would not be possible. The desire to explain the constants has been one of the driving forces behind efforts to develop a complete unified description of nature, or "theory of everything." Physicists have hoped that such a theory would show that each of the constants of nature could have only one logically possible value. It would reveal an underlying order to the seeming arbitrariness of nature.
In recent years, however, the status of the constants has grown more muddled, not less. Researchers have found that the best candidate for a theory of everything, the variant of string theory called M-theory, is self-consistent only if the universe has more than four dimensions of space and time--as many as seven more. One implication is that the constants we observe may not, in fact, be the truly fundamental ones. Those live in the full higher-dimensional space, and we see only their three-dimensional "shadows."
Meanwhile physicists have also come to appreciate that the values of many of the constants may be the result of mere happenstance, acquired during random events and elementary particle processes early in the history of the universe. In fact, string theory allows for a vast number--10500--of possible "worlds" with different self-consistent sets of laws and constants [see "The String Theory Landscape," by Raphael Bousso and Joseph Polchinski; Scientific American, September 2004]. So far researchers have no idea why our combination was selected. Continued study may reduce the number of logically possible worlds to one, but we have to remain open to the unnerving possibility that our known universe is but one of many--a part of a multiverse--and that different parts of the multiverse exhibit different solutions to the theory, our observed laws of nature being merely one edition of many systems of local bylaws [see "Parallel Universes," by Max Tegmark; Scientific American, May 2003].
No further explanation would then be possible for many of our numerical constants other than that they constitute a rare combination that permits consciousness to evolve. Our observable universe could be one of many isolated oases surrounded by an infinity of lifeless space--a surreal place where different forces of nature hold sway and particles such as electrons or structures such as carbon atoms and DNA molecules could be impossibilities. If you tried to venture into that outside world, you would cease to be.
Thus, string theory gives with the right hand and takes with the left. It was devised in part to explain the seemingly arbitrary values of the physical constants, and the basic equations of the theory contain few arbitrary parameters. Yet so far string theory offers no explanation for the observed values of the constants.
A Ruler You Can Trust
Indeed, the word "constant" may be a misnomer. Our constants could vary both in time and in space. If the extra dimensions of space were to change in size, the "constants" in our three-dimensional world would change with them. And if we looked far enough out in space, we might begin to see regions where the "constants" have settled into different values. Ever since the 1930s, researchers have speculated that the constants may not be constant. String theory gives this idea a theoretical plausibility and makes it all the more important for observers to search for deviations from constancy.
Such experiments are challenging. The first problem is that the laboratory apparatus itself may be sensitive to changes in the constants. The size of all atoms could be increasing, but if the ruler you are using to measure them is getting longer, too, you would never be able to tell. Experimenters routinely assume that their reference standards--rulers, masses, clocks--are fixed, but they cannot do so when testing the constants. They must focus their attention on constants that have no units--they are pure numbers--so that their values are the same irrespective of the units system. An example is the ratio of two masses, such as the proton mass to the electron mass.
One ratio of particular interest combines the velocity of light, c, the electric charge on a single electron, e, Planck's constant, h, and the so-called vacuum permittivity, 0. This famous quantity, = e2/20hc, called the fine-structure constant, was first introduced in 1916 by Arnold Sommerfeld, a pioneer in applying the theory of quantum mechanics to electromagnetism. It quantifies the relativistic (c) and quantum (h) qualities of electromagnetic (e) interactions involving charged particles in empty space (0). Measured to be equal to 1/137.03599976, or approximately 1/137, has endowed the number 137 with a legendary status among physicists (it usually opens the combination locks on their briefcases).
If had a different value, all sorts of vital features of the world around us would change. If the value were lower, the density of solid atomic matter would fall (in proportion to 3), molecular bonds would break at lower temperatures (2), and the number of stable elements in the periodic table could increase (1/). If were too big, small atomic nuclei could not exist, because the electrical repulsion of their protons would overwhelm the strong nuclear force binding them together. A value as big as 0.1 would blow apart carbon.
The nuclear reactions in stars are especially sensitive to . For fusion to occur, a star's gravity must produce temperatures high enough to force nuclei together despite their tendency to repel one another. If exceeded 0.1, fusion would be impossible (unless other parameters, such as the electron-to-proton mass ratio, were adjusted to compensate). A shift of just 4 percent in would alter the energy levels in the nucleus of carbon to such an extent that the production of this element by stars would shut down.
Nuclear Proliferation
The second experimental problem, less easily solved, is that measuring changes in the constants requires high-precision equipment that remains stable long enough to register any changes. Even atomic clocks can detect drifts in the fine-structure constant only over days or, at most, years. If changed by more than four parts in 1015 over a three-year period, the best clocks would see it. None have. That may sound like an impressive confirmation of constancy, but three years is a cosmic eyeblink. Slow but substantial changes during the long history of the universe would have gone unnoticed.
Fortunately, physicists have found other tests. During the 1970s, scientists from the French atomic energy commission noticed something peculiar about the isotopic composition of ore from a uranium mine at Oklo in Gabon, West Africa: it looked like the waste products of a nuclear reactor. About two billion years ago, Oklo must have been the site of a natural reactor [see "A Natural Fission Reactor," by George A. Cowan; Scientific American, July 1976].(continued in next post)
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