The basic constants of nature aren鈥檛 called constants for nothing. Physics is supposed to work the same way across the universe and over all of time. Now measurements of the radio spectra of a distant gas cloud hint that some fundamental quantities might not be fixed after all, raising the possibility that a radical rethink of the standard model of particle physics may one day be needed.
The evidence comes from observations of a dense gas cloud some 2.9 billion light years away which has a radio source, the active supermassive black hole PKS 1413+135, right behind it. Hydroxyl radicals in the gas cloud absorb the galaxy鈥檚 radio energy at certain wavelengths and emit it again at different wavelengths. This results in so-called 鈥渃onjugate鈥� features in the radio spectrum of the gas, with a dip in intensity corresponding to absorption and an accompanying spike corresponding to emission.
The dip and spike have the same shape, which shows that they arise from the same gas. But Nissim Kanekar of the National Centre for Radio Astrophysics in Pune, India, and colleagues found that the gap in frequency between the two was smaller than the properties of hydroxyl radicals would lead us to expect.
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The gap depends on three fundamental constants: the ratio of the mass of the proton to the mass of the electron, the ratio that measures a proton鈥檚 response to a magnetic field, and the fine-structure constant, alpha, which governs the strength of the electromagnetic force. The discrepancy in the size of the gap thus amounts to 鈥渢entative evidence鈥� that one or more of these constants may once have been different in this region of space, Kanekar says.
The change in these constants, if genuine, is tiny. For example, if a change in alpha were solely responsible for the discrepancy, the measurements suggest alpha would have been just 0.00031 per cent smaller 3 billion years ago than today (). But even such a small effect would require 鈥渁 new, more fundamental theory of particle physics鈥� to explain it, says of Swinburne University of Technology in Melbourne, Australia.
Measurements by Murphy and colleagues of visible light from distant quasars absorbed by intervening gas clouds have also hinted alpha was smaller in the past. But it was never certain that the light measured all came from the same region. 鈥淭hat鈥檚 a critical assumption,鈥� says Murphy.
鈥淩adio measurements currently appear to be the most promising avenue for a secure detection of fine-structure constant evolution,鈥� says of the University of Pittsburgh, Pennsylvania. 鈥淚 wouldn鈥檛 call this more than a hint, though. It鈥檚 the first application of a new technique.鈥�
The subtle discrepancy found by Kanekar鈥檚 team might be caused by 鈥渃ontamination鈥� from light from another patch of gas. Last month, the team began using the Arecibo radio telescope in Puerto Rico to rule this out.
The nuclear option for clocking change
Physical constants could be measured with unprecedented accuracy if atomic clocks go nuclear.
Atomic clocks traditionally rely on the frequency of light needed for electrons to make transitions between different energy states. Measurements of these frequencies have also been used to make ultra-precise determinations of the physical constants, showing that any change in the fine-structure constant, alpha, which governs the strength of the electromagnetic interaction, is no bigger than one part in 1017 per year.
Now Wade Rellergert of the University of California, Los Angeles, and colleagues say a clock that uses transitions between energy levels in the nuclei of thorium-229 atoms could potentially improve on that limit by a factor of 100 (). Unlike other atomic nuclei, thorium-229 nuclei boast a transition that can be used to make a clock. This transition is more sensitive to any changes in the fundamental constants, so any shifts in its frequency could reveal changes in alpha or place more stringent limits on any change.
Team members are now working on growing crystals doped with atoms of thorium-229. With these crystals they will be able to make simultaneous measurements on 10 billion more thorium-229 nuclei than using other methods, which could help pin down any deviations in the transition frequency over time, Rellergert says.
