Einstein’s famous tenet of special relativity — that time slows down on a moving clock — has been verified 10 times more precisely than ever before. The result comes from physicists in Germany and Canada, who have timed the “ticking” of lithium ions as they hurtle around a ring at a fraction of the speed of light.

Sit two clocks side by side and, if they are accurate, they will always show the same time. But if one clock is moving rapidly, it will appear to an observer standing next to the stationary clock to be ticking too slowly. This “time dilation” effect, which was predicted by Einstein in his special theory of relativity in 1905, has been verified many times — first to within 1% of predictions in an experiment by Herbert Ives and G R Stilwell in 1938, and more recently by comparing the times of atomic clocks on Earth with those of orbiting global-positioning-system (GPS) satellites.

Such measurements haven’t stopped scientists from suggesting deviations from special relativity, however. For instance, those that are looking for explanations why there is much more matter than antimatter in the universe often invoke a violation of “CPT theorem”, which says that the laws of physics remain the same if the charge, parity and time-reversal properties of a particle are inverted together. CPT violation can justify the observed excess of normal matter, but it might also imply the equations underlying the Standard Model of particle physics, which are based on special relativity, are incomplete.

Testing times

Experiments by Gerald Gwinner from the University of Manitoba in Canada, together with colleagues from various German institutions, give no hint of such deviations from special relativity and thus physics beyond the Standard Model. To test Einstein’s theory, they improved on a technique called laser saturation spectroscopy to measure the time dilation of groups of lithium-7 ions injected at high speed into a magnetic storage ring, based at the Max Planck Institute for Nuclear Physics in Heidelberg (Nature Physics advance online publication).

It means that at the sensitivity level of our experiment, and all others that look for evidence of new physics beyond the Standard Model, is not high enough yet to see anything

When at rest with respect to an observer, lithium-7 ions have an electronic transition between energy levels that always takes place at a frequency close to 546 THz — effectively a “ticking clock”. In principle, the amount time dilation changes this frequency for speeding lithium-7 ions could be found by illuminating them with a laser from behind and noting the laser frequency that incites the transition — shown by the ions “fluorescing” or absorbing and re-emitting photons in all directions. In practice, a group of ions in a storage ring have a distribution of velocities, which limits the measurement precision.

Two observers

The researchers avoid this limitation by aiming a second laser into the beam of ions. Although this laser also makes all the ions fluoresce, those in the centre of the velocity distribution receive so many photons that their fluorescence saturates causing a local dip in the spectrum so that ions of only one velocity are “marked”.

Gwinner and colleagues then take the product of the two lasers’ frequencies, which — according to special relativity — should be equal to the square of the transition frequency when the lithium-7 ions are stationary. But because this transition frequency isn’t known accurately enough for their needs, the researchers repeat the experiment for lithium-7 ions travelling at both 3% and 6.4% of the speed of light and check the products are the same.

As expected, the products were indeed the same. But the accuracy of Gwinner and colleagues’ experiment, which is quantified by a “Mansouri-Sexl parameter” of less than 8.4 × 10-8, is over 10 times better than the GPS tests of special relativity. “It means that at the sensitivity level of our experiment, and all others that look for evidence of new physics beyond the Standard Model, is not high enough yet to see anything,” Gwinner told physicsworld.com.