OUR most accurate clocks are probing a key tenet of 贰颈苍蝉迟别颈苍鈥檚 theory of relativity: the idea that time isn鈥檛 absolute. Any violation of this principle could point us to a long-sought theory that would unite 贰颈苍蝉迟别颈苍鈥檚 ideas with quantum mechanics.
Special relativity established that the laws of physics are the same for any two observers moving at a constant speed relative to each other, a symmetry called Lorentz invariance. One consequence is that they would observe each other鈥檚 clocks running at different rates. Each observer would regard themselves as stationary and see the other observer鈥檚 clock as ticking slowly 鈥� an effect called time dilation.
贰颈苍蝉迟别颈苍鈥檚 general relativity compounds the effect. It says that the clocks would run differently if they experience different gravitational forces.
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For two decades, comparing atomic clocks aboard GPS satellites with those on Earth have helped test the effect 鈥� and always confirmed it. But since any deviation from relativity would be very subtle, we might need a more precise instrument to find it.
Most atomic clocks rely on the frequency of the microwave radiation emitted when electrons in caesium-133 atoms change energy states. Next-generation clocks that use strontium atoms have at least three times the precision, barely gaining or losing a second over 15 billion years.
鈥淎 violation of Lorentz invariance could point to a way to combine relativity and quantum mechanics鈥�
Now, of the Paris Observatory and his colleagues have used strontium clocks to test time dilation. Two optical fibre links, one between London and Paris and another between Paris and Braunschweig, Germany, were used to compare devices in these locations.
These clocks are moving at different velocities because of their position on the Earth鈥檚 surface, and relativity makes precise predictions about the extent of time dilation they experience. For example, a clock closer to the equator should tick more slowly than one closer to the North Pole. After one day, clocks in Paris and London should show a difference of 5 nanoseconds.
To compare them, the team synchronised lasers to the frequency of the radiation from each clock鈥檚 strontium atoms. Then they transmitted the beams over the fibre-optic links, allowing them to superimpose the lasers to detect any telltale differences in frequency indicating one clock ticking faster than the other.
With the measurements, the team calculated a parameter called alpha, which should be zero if there is no violation of Lorentz invariance. The latest results show that alpha is less than 10-8 鈥� a result two orders of magnitude better than from experiments using caesium clocks, and twice as accurate as the previous best limit, obtained by studying electronic transitions in lithium ions moving at one-third the speed of light ().
Letting the experiments run for longer will improve accuracy even further, says team member of the UK鈥檚 National Physical Laboratory in Teddington.
So far so good for relativity. But how would physicists react if a violation of Lorentz invariance is ever measured? 鈥淭he immediate consequence would be that nobody would believe it,鈥� says , a theorist at the Frankfurt Institute for Advanced Studies in Germany.
However, if a violation is ever confirmed, the implications would be huge. 鈥淨uantising gravity, [the nature of] dark matter and dark energy 鈥� these are three big questions for which Lorentz invariance violations would be an extremely important hint as to the nature of the underlying theory,鈥� she says.
This article appeared in print under the headline 鈥淣etworked atomic clocks seek untimely behaviour鈥�
