Steven V Johnson wrote:
Can someone refresh my memory about the precise time measurements conducted with atomic clocks positioned at different elevations on the surface of Earth.
Gravity or acceleration slow down time. They are one and the same in general relativity theory.
If you start with 2 atomic clocks synchronized together, and you move one up 10 m to another floor, that causes it speed up slightly, and diverge from the one below. It is amazing that they can measure such small differences in time. Of all the fundamental units, time can now be measured most precisely, which is why distance is now a function of time (1 m = distance light travels in a vacuum during the interval of 1/299,792,458 s).
<http://physics.nist.gov/cuu/Units/meter.html>http://physics.nist.gov/cuu/Units/meter.html
The least satisfactory unit is the kilogram. Various new methods of defining it are now being developed. NIST and other standards agencies will eventually pick the best one.
- Jed
