YOUR watch runs a tiny bit faster at the top of Everest, where Earth鈥檚 gravity is slightly weaker, than it does at sea level. This difference is dubbed the 鈥済ravitational red shift鈥� (GRS) and is one of the trickiest predictions of general relativity to measure because the effect is so small. Now the accuracy of measurement has been improved by a factor of 10,000.
Holger M眉ller at the University of California, Berkeley, decided to reanalyse a decade-old experiment. In the 1990s, a team led by Nobel laureate Steven Chu made an 鈥渁tomic fountain鈥� of caesium atoms, launching them 30 centimetres into the air. A pulse of laser light struck the atoms as they neared their zenith, which kicked them into a two-state quantum superposition. One of the states was given extra momentum, causing it to rise to a slightly higher altitude than the other state before falling.
M眉ller realised the atoms and their very rapid oscillations could be treated as tiny 鈥渃locks鈥� and so could be used to measure GRS. The team compared the difference between the two states and discovered that the state that climbed slightly higher had oscillated ever-so-slightly faster than the lower state. With an accuracy of 7 parts in a billion, this measurement is 10,000 times as accurate as the previous one (Nature, ).
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General relativity has been questioned by some theories, 鈥渟o it鈥檚 exciting when people can do such high-precision measurements on relativity鈥�, says Jun Ye at the University of Colorado in Boulder.
