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#Mon Aug 09 12:06:31 CEST 2010
Text__uIzqgKOdEd-WfYQ0GneCwQ_text=Atoms absorb and emit light with characteristic frequencies. This is why todays time standards are based on atomic clocks. <p></p><p>If the atoms move with respect to an external observer, a lower frequency is measured. How large the frequency is - that is how fast for emitted light one wave follows another - is a direct measure for the elapsing time in the reference frame of the atom. 
Text__uIzqgKOdEd-WfYQ0GneCwQ_title=Atoms are very precise clocks