Atomic Fountain reveals 'Gravitational Red Shift'

Atomic fountain reveals 'gravitational red shift' - New Scientist

YOUR watch runs a tiny bit faster at the top of Everest, where Earth's gravity is slightly weaker, than it does at sea level.

This difference is dubbed the "gravitational red shift" (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 Müller 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 "atomic fountain" 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.

Müller realised the atoms and their very rapid oscillations could be treated as tiny "clocks" 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, DOI: 10.1038/nature08776).