I think the SI second is no longer defined as oscillations of Cesium atoms in
an atomic clock, but rather as the length of time it takes a beam of light to
travel a specified distance.
Gavin, you are confusing the definition of the second with the definition of the metre. The second is still defined in terms of the oscillations of the Cesium atom*. It is the metre that is defined as the distance light travels in a second.
Regards, Bill Hooper Fernandina Beach, Florida, USA
*"The second is the duration
of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium atom" (at rest and at a temperture of 0 K).
Section 2.1.1.3 "Unit of time (second) The International System of Units (SI) 7th Ed. (1998) and year 2000 Supplement
