A team led by B Phillips and Paul Lett at the National Institute of Standards and Technology, Maryland, US, have observed the retardation effect, that is, the small but finite delay between the time an atom sends out an electromagnetic field and another feels it. They used a small sample of sodium atoms which were cooled to almost absolute zero temperature using laser cooling techniques. These ultra cold atoms were then bombarded with photons at very specific energies to see if the atoms bind together at large distances. This is the first report of the detection of retardation effect.
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