What is the relationship between the speed of light and the meter?

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Discussion Overview

The discussion revolves around the relationship between the speed of light and the definition of the meter, exploring how units of measure are derived and the implications of measuring the speed of light using a unit that is defined in relation to it. The scope includes conceptual clarification and historical context regarding the definitions of the meter and the speed of light.

Discussion Character

  • Conceptual clarification
  • Historical
  • Debate/contested

Main Points Raised

  • One participant notes that the meter is defined as the distance light travels in a specific time period, raising a question about the logic of measuring the speed of light using a unit derived from it.
  • Another participant states that the meter was originally defined based on more tangible references, and the speed of light was measured using predefined units, suggesting a historical evolution in definitions.
  • A third participant provides a specific definition of the meter adopted by NIST in 1983, indicating that it is based on the distance light travels in a vacuum in a precise time frame, which adds a technical detail to the discussion.
  • A later reply reiterates the initial question about the measurement of the speed of light, arguing that it does not make sense to measure it in terms of the current definition of the meter, which is fixed, leading to a result without experimental error. They also reference the historical definition of the meter prior to 1960, suggesting that it was more logical to measure the speed of light then.

Areas of Agreement / Disagreement

Participants express differing views on the implications of measuring the speed of light with the current definition of the meter. There is no consensus on whether this practice is logical or sensible, and the discussion remains unresolved regarding the philosophical implications of unit definitions.

Contextual Notes

The discussion highlights the evolution of the meter's definition and its dependence on historical context, as well as the implications of using a derived unit for measuring a fundamental constant. There are unresolved assumptions regarding the nature of measurement and definitions in physics.

TalonD
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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 length 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 ?
 
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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.
 
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.
 
TalonD said:
So we determine the length 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 ?

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.
 

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