# Measuring the speed of light

1. Aug 12, 2008

### TalonD

Since the question of units of measure has come up recently, I have a question...

The length of the meter is derived from the speed of light. That is, it is the distance that light travels at speed c in a time period derived from the cesium atom.
The speed of light is commonly defined as how many meters it travels in a certain amount of time. 300 kilo meters per second approx.

So we determine the lenght of the meter in terms of the speed of light, but then we measure the speed of light by using the meter or.. kilo meter.

Why would you measure the speed of something by using a unit of measure that is derived from the speed of the thing you are measuring the speed of ?

2. Aug 12, 2008

### mathman

The meter (and other metric system units) was originally defined in terms of more prosaic things. The measurement of the speed of light was made in terms of predefined units. I believe that the current definition of meter is in terms of number of wavelengths of a specific energy transition, not in terms of a time period.

3. Aug 12, 2008

### Chronos

NIST adopted the definition of a meter as the distance traveled by light in a vacuum in 1/299,792,458 of a second in 1983. The second is defined with an uncertainty of 1 part in 10^14 by a Cesium clock. The iodine stabilized Helium-Neon laser at a wavelength of 632.99139822 nm is the recommended light source.

4. Aug 13, 2008

### rbj

your question is right but your premise is not. it makes no sense to measure the speed of light in terms of the meter as it is presently defined by SI. the result of the "measurement" would always be 299792458 m/s with no experimental error, no matter what.

but the meter was not always defined as such. before 1960, when the meter was the distance between two scratch marks on a platinum-iridium bar in France, then it made sense to measure c in terms of the second and meter of the day.