There are other things, known as fundamental constants, that are not meant to change either. These include the speed of light and the charge on the electron, and their values have been measured to an accuracy of many decimal places. In recent years, however, evidence has emerged that some constants might not be constant after all.
To check if a fundamental constant is changing, it is necessary to measure it extremely accurately at two different times and compare the results. One of the best ways of doing this is to measure how light is emitted and absorbed by various atoms and molecules. All atoms and molecules have unique patterns of emission and absorption, "lines" at different wavelengths, much like the products in a supermarket have barcodes. If one of the fundamental constants is changing with time, then these barcodes will also change in a distinctive way. A growing number of physicists and astronomers have started to make these very demanding measurements.
Some of these experiments look for small changes in the barcodes over short periods. To put it simply, they measure the barcode, wait a few months or years, and measure it again. Other researchers take advantage of the vastness of the universe to compare the barcodes they have measured in their laboratories with corresponding barcodes from billions of years ago. This is possible because the light now reaching the Earth from distant stars and galaxies was stamped with the relevant barcodes when the universe was much younger.
The experiments are complicated by the fact that the wavelengths of all the lines in the barcode are "red-shifted" by the expansion of the universe. (This is analogous to the Doppler effect that causes the pitch of a fire-engine siren to rise as it approaches you, and then fall as it races off into the distance.) However, this red shift also tells researchers how old the barcode is.
The first fundamental constant to look vulnerable was the fine-structure constant. This number, also known as alpha, is of interest to physicists because it is a measure of the strength of the electromagnetic force that holds atoms together. An international team of astronomers led by John Webb of the University of New South Wales in Australia have measured the barcodes of various atoms in the light from more than 100 distant quasars and are convinced that alpha was slightly smaller billions of years ago. However, these results remain controversial and other groups have failed to detect any change in alpha using a similar approach.
Now a second constant - the mass of the proton divided by the mass of the electron - looks as if it might have changed. Wim Ubachs of the Free University of Amsterdam and co-workers have just published a paper which claims that this constant has decreased by 0.002% over the past 12bn years.
If these results hold up, physicists will have to revise some of their most basic models of the behaviour of matter and forces. But the fundamental constants should be of interest to everyone. If the value of these constants had been different in the past, then life would not have evolved on Earth, either because the molecular structures needed for life would not have been stable, or because there would have been no stars.
· Peter Rodgers was until recently editor of Physics World magazine