Re: 6.022136736e23 Message #12 Posted by Dave Shaffer on 15 Apr 2003, 12:46 p.m., in response to message #11 by Ellis Easley
Sorry to say: no conspiracies.
For reasons of ease of use (i.e. you don't have to go to Paris), the speed of light is now a defined standard: 299792458 meters/second and the time definition is set by some number of cycles (I don't remember the value, but you can look it up) of a certain transition of the Cesium atom (as used in cesium frequency standards/clocks). The meter is then derived from these two values.
The remaining fundamental quantity (mass) is still defined by a piece of something somewhere (Paris or Boulder, Colo.). However, folks are working on relating it to fundamental physical quantities, too. I think this would be by relating force (through electromagnetic interactions) to mass and using Newton's F=ma (where time and distance are also needed, for the acceleration term, but we took care of those quantities in the preceding paragraph).
It is changes like these that cause "changes" in things like Avagadro's number. Every few years, there are complete recalculations of all the physicals constants, in a big least-squares adjustment that takes into account the latest in measurement precision.
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