How Long is a Meter? a Second?

Date: 09/06/2001 at 23:04:21
From: Dawn Britt
Subject: Meter determened

How is the length of a meter currently determined?

Date: 09/07/2001 at 14:04:00
From: Doctor Rick
Subject: Re: Meter determened

Hi, Dawn.

The standard of length has changed over the years as scientists' 
ability to make measurements has improved. Currently (since 1983) the 
meter has been officially defined to be the distance traveled by light 
in a vacuum in 1/299,792,458 second. See this article at the National 
Institute of Standards and Technology (NIST) Web site:

  Time Line for the Definition of the Meter (NIST)
  http://www.mel.nist.gov/div821/museum/timeline.htm   

The meter is defined this way because currently time can be measured 
more accurately than distance. Therefore distance is defined in terms 
of time, using the constant speed of light in a vacuum as the 
'conversion factor'. In other words, the speed of light is no longer a 
number to be found by experiment; it is defined to be 299,792,458 
meters per second, and this definition establishes a relation between 
the meter and the second.

So then, how is the second defined? It is currently defined as the 
time taken by exactly 9,192,631,770 oscillations of the radiation from 
the hyperfine transition of a cesium atom. You can read about the 
devices that measure this time to an accuracy of about 3 parts in 
10^14, an incredible measurement feat:

  Cesium Atomic Clocks (U.S. Naval Observatory)
  http://tycho.usno.navy.mil/cesium.html   

I have read that scientists measured the hyperfine transition to 3 
parts in 10^15, but that article is apparently no longer available on 
the Web.

- Doctor Rick, The Math Forum
  http://mathforum.org/dr.math/