Is the fine structure constant changing?
(August 2001)


An international team of astrophysicists from the University of New South Wales and others from Britain and the USA have raised the possibility that the fine structure constant a might have changed slightly as the universe got older. This is the conclusion they have reached from spectroscopic studies of gas clouds that lie between us and distant quasars.

The value of a(alpha) is equal to e2/hc, where e is the value of the charge on an electron (or a proton), h (more correctly, 'h-cross', the letter h with a line through it) is one way of stating Planck's constant*, and c is the speed of light. For this constant to be changing, one of the values would also need to change, and most of the comparatively few media accounts tended to follow the lead of the New York Times, which suggested that it was the speed of light which was changing, but physicists say this is the least likely of the three 'constant' values to change.

But first, a quick look at the technical details: the researchers examined ancient light being re-emitted from ancient (and more distant) gas clouds, and compared what they saw with light being re-emitted from nearby and more recent gas clouds. They then corrected their data to allow for the redshifts caused by the movement of the gas clouds, and examined the spacing of doublets of absorption lines in the spectra of a variety of different atoms in the dust clouds. These gave a steady result: a consistent shift in alpha with increasing redshift, especially where z>1.

The news broke almost two weeks before the official date of the print publication in Physical Review letters, but raised remarkably little interest, in part, perhaps, because a number of physicists suggested that there might be other explanations (which they left carefully unspecified). The authors identify 13 potential systematic errors that could have produced the result, and indicate that most of the possible false causes may be ruled out. The only two that cannot be ruled out should have produced smaller values of alpha, not the larger values that are being detected.

For example, one possible systematic error, caused by a wrong alignment of the spectrograph slit would, if it had occurred, have reduced the observed effect, not increased it, and they comment that:

''We summarise and stress the following important points regarding potential systematic effects: (i) a thorough investigation reveals no systematic effect which can produce the results we report. (Furthermore, applying either of the 2 significant corrections would enhance the significance of our results.''

Putting it another way, the value of alpha is very close to 1/137, and the measured change is about 1 part in 100,000, while there remains a chance of around 1 part in 10,000 that the observed effect is a statistical fluke. That means the probability that the effect is real is 99.99%, fairly good odds.

So whatever they have captured here, it would appear to be significant, and that probably means that one or more of the values that physicists regard as constant does in fact change overtime. So how do people go about looking into the past, to assess what the value of the fine structure constant should be?

Quasars shine with the sort of intensity you expect from a whole galaxy, but they appear to be 'tiny' - about small enough to fit inside the orbit of Mercury, and that makes them an extremely small and bright point-source of light, when they are seen from any distance. The other key feature of a quasar is that they have a continuous spectrum, emitting at all wavelengths.

When you shine a continuous spectrum through a cloud of gas, some wavelengths are absorbed and as Robert Bunsen and Gustav Kirchhoff knew almost a century and a half ago, the wavelengths that are absorbed are exactly the same wavelengths that would be emitted if the same atoms were 'excited', given extra energy. The effect is slightly varied if the gas is whizzing away from us, due to the effects of redshift.

Now the lines are also divided up, with small variations either side, and the extent of this shifting is proportional to the fine structure constant. (Note that this is a deliberately non-mathematical account of a complex issue.) The redshift of radiation depends on how old it is, so we can determine the age of each quasar, and we can also look at the extent of the shift away from the central line. So if the extent of the shifting differs with age, this would be a good reason to argue that the fine structure constant was changing. It may not be a sufficient reason, but it is certainly a good reason.

The sample looked at the quasars covering a period from 23% of the age of the universe up to 87% of the age of the universe, using measurements made on the Keck telescope's HIRES spectrograph at Mauna Kea on Hawaii, with observations being made on frequencies associated with iron, zinc, magnesium, chromium and other metals in the clouds, and the results for all the measurements showed a consistent trend.

The fine structure constant is the fundamental constant of electromagnetism, and may be thought of as a measure of the inherent strength of the electromagnetic force. In the past, others have explored the possibility of what are known as secular changes in fundamental constants, but the previous cases have all been explained away by other means - which is perhaps part of the reason why physicists have not been carrying on too much about the result so far.

So what does it mean if the value of alpha has been shifting slowly with time? The fine structure constant explains how electromagnetic forces hold atoms together. There are certain stable orbitals around atoms, and there is a very precisely known amount of energy that an electron needs to absorb when it jumps from one orbital to one with higher energy, and the same amount of energy is emitted when the electron drops back down again. Now it seems that, in the past, this amount of energy may have been slightly different.

This means that physicists would need to go back and revise the 'standard model', if, as the evidence seems to suggest the value of alpha has increased by even a tiny amount. The people who will gain most from this are those who hold that string theory is valid, because this theory holds that there are many more dimensions than time and the three dimensions of space that we can see, and if these are real, they could account for the value of alpha increasing.

It is lucky for us that the increase, if it is real, is small, because if the value of alpha had increased too much, carbon atoms could not be stable, and life forms like us could not exist.

*Note: h-cross or h-bar is actually Planck's constant divided by twice the value of pi. The value of the fine structure constant alpha is normally given in units which need not concern us here, because if h-cross is changing, then so is h-bar.

Key names: John K. Webb, Michael Murphy, V. V. Flambaum and V. A. Dzuba of the university of NSW, John Barrow (Cambridge), Chris Churchill (Penn State),J. X. Prochaska (Carnegie Observatories) and A. M. Wolfe (UCSD). It is worth noting here that Michael Murphy is just six months into his Phd studies, and already has his first publication in the world's leading specialist journal.

Our source for this is Peter Reece, also at the same stage in his Phd studies, and second author in a paper which appeared in Nature for August23, a story described in The glowing photovoltaic cell, this month. Peter and Michael have known each other since they were 12, when they were at the same school, St. Columba's High School at Springwood, in the Blue Mountains to the west of Sydney. Is there something in the mountain air, or is it special teaching? All we can ascertain from the school is that both students were always keen on their mathematics and science, and there was an enthusiastic staff.

To see an aftermath of this story,see Is the speed of light changing?, August 2002.

©WebsterWorld Pty Ltd/contributors 2002


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