Can physical constants change over time? Some scientists think so. Others are struggling to explain how
Open the inside-back cover of any physics textbook and you will find a list of the fundamental constants of nature: the speed of light, the charge of the electron, and so on. As physicists concoct ever more bizarre theories to explain the universe, these numbers are their faithful, unchanging accomplices. So some Australian researchers caused quite a stir when they announced last year that one of these constants seems to have changed. The group has now sent a representative to tour the United States and explain this finding. At a recent seminar at the Harvard-Smithsonian Centre for Astrophysics, Michael Murphy, the scientists' spokesman, presented the team's evidence for this extraordinary claim.
At the heart of the controversy is the number known as alpha, a quantity that can be calculated by combining several fundamental physical constants. To get alpha, square the charge of the electron and divide it by the speed of light times Planck's constant, a key value used in quantum theory. Then multiply the result by two times pi, the ratio of a circle's circumference to its diameter. When the right units of measurement are chosen for each component, they cancel out. All that remains is alpha, a pure number.
Scientists who nursed Platonic ideas about physics once suspected that alpha's value was an exact fraction, one over 137. Today, physicists reckon the denominator of this fraction is actually closer to 137.036. Yet the mystery persists: why is this seemingly arbitrary figure stitched into the fabric of reality? The work performed by John Webb, Victor Flambaum and Mr Murphy at the University of New South Wales, and their colleagues elsewhere, has only fuelled this enigma. If they are correct, it is pointless to ascribe significance to the measured value of alpha, because it has grown with time: by 0.0006% over the past 9 billion years.
Alpha can be calculated by making precise measurements of physical objects. One way to compute its value is to shine light through a gas-filled container. The atoms in the gas will absorb certain colours of light, each of which corresponds to a particular wavelength. The spectrum of light that emerges from such a container will show a series of dark lines interrupting what would be a continuous rainbow. The spacing of these lines, their so-called "fine structure", depends on alpha, which is known as the fine-structure constant.
The Australian scientists applied essentially the same technique to starlight in order to obtain an estimate of what alpha's value was several billion years ago. Using Keck I, a telescope on a mountain-top in Hawaii, the physicists analysed light emanating from very bright but distant galaxies called quasars. Because light takes so long to reach the earth from distant galaxies, astronomers were able to study the value of alpha in light from the distant past. On its route to earth, light from some quasars passed through a cloud of gas. Just as in a container, molecules in an inter-galactic gas cloud absorb certain wavelengths and leave a pattern of dark lines on a quasar's spectrum.
Because the universe is expanding, any wave of light emitted from a far-flung source is stretched out. That lengthens the wavelength of the light and shifts it towards the red end of the colour spectrum. Dr Webb's team found that, even after correcting for this red shift, the dark lines in the quasar spectra did not appear at the same wavelengths as they do in laboratories on earth. In the quasar spectra, some of the dark lines were shifted slightly towards the red end of the colour spectrum and others towards the blue end. Only if alpha were a smidgen smaller when the light passed through the gas cloud could such a pattern have been produced.
The deviations are too small to be seen in any single quasar, but by averaging the results for 72 quasars the team claims to have secure statistical evidence. Mr Murphy's tour is intended to publicise this finding and to convince other scientists to perform independent measurements. He has generally been met with unease from theoretical physicists and with scepticism from astronomers. Theorists, for their part, have been worrying about what might cause alpha to change, and what change would imply for the stability of its components. Has the charge of the electron or the speed of light altered over time, or has some combination of effects occurred?
For most theories of the universe, such variability would prove disastrous. There is one theory, though, that may allow constants to vary over time. Though it came into vogue as string theory, this idea is now known as M-theory. It supposes that the universe contains 11 dimensions: the four familiar ones of time and space, plus seven that have shrivelled up and become inaccessible since the Big Bang.
The constants that physicists measure in their laboratories are only four-dimensional shadows of more fundamental entities that exist in 11 dimensions. These shadows would shift with time if the hidden dimensions changed scale. Unfortunately, nobody knows how to use M-theory to calculate whether alpha should be going up or down, or by how much.
Other theorists, such as Georgi Dvali of New York University and Joao Magueijo of Imperial College, London, say a changing alpha might come from a hitherto unknown force of nature that could modify the law of gravity. The new force would cause objects made of different materials to fall at different rates. However heretical that may sound, this notion has the advantage over M-theory in that its predictions might be testable in future experiments.
Astronomers, who despite their profession tend to be more down-to-earth than theoretical physicists, are withholding judgment until the measurements are repeated. Chris Carilli, of the National Radio Astronomy Observatory in Socorro, New Mexico, points out that the quasar-based method to measure the ancient value of alpha is at least 30 years old, and has never before resulted in a value different from today's alpha. Dr Webb invented a new method of analysis that is potentially more sensitive to changes in alpha, but is also more susceptible to subtle errors.
Dr Carilli and his colleagues are also suspicious because the new measurement disagrees with a different way of determining alpha in the remote past. About 2 billion years ago, uranium rocks at a site in Oklo, Gabon, experienced such compression that they underwent fission reactions much like those that occur in nuclear reactors. Physicists know this because of the unusual amounts of different types of uranium and other elements in the area, which was used by France as a uranium mine. By studying these deposits, scientists have deduced that the value of alpha during the ancient nuclear reactions was the same as it is today.
If the evidence from the mine and the evidence from the quasars are both correct, alpha would have to have changed between 2 billion and 9 billion years ago, and then stayed constant. That would be still more perplexing. Most physicists would be happier if the whole problem just went away. It will not be easy for them to surrender the beloved constancy of their constants of nature.
Apr 4th 2002 | CAMBRIDGE, MASSACHUSETTS
From The Economist print edition