4

How is the speed of light measured by the most current method i.e. using lasers and atomic clocks? Alternatively, is there a better method?

Qmechanic
  • 201,751
Nighthawk
  • 71
  • 3

1 Answers1

1

The speed of light can't be measured anymore (in SI units) because it has had a defined value since 1983. See Why do universal constants have the values they do?

Before 1983, the meter was defined in terms of the wavelength of a certain emission line of krypton 86. The second is also defined in terms of an atomic standard (the frequency of a transition in cesium 133). Therefore before 1983 it was possible to measure the speed of light, and such a measurement would have consisted of comparing these two atomic standards.

Community
  • 1
  • We still can measure the time it takes for a light pulse to travel a certain distance. It doesn't matter if we use the result to determine c or the length of our meter or the accuracy of our clock. If I read the question correctly it asks for current experimental methods employed. – guntbert Nov 10 '14 at 19:02
  • 1
    @guntbert: To do what you propose, you would have to have length and distance standards. The length and distance standards are such that c has a defined value. –  Nov 10 '14 at 19:22
  • Maybe it is a language problem on my side (determining vs measuring?): I thought I proposed to describe an experiment to confirm that my "meter" is really "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second". – guntbert Nov 10 '14 at 20:26
  • 1
    @guntbert: What you propose sounds like a method of checking a secondary standard for length against the primary standard for length. That's fine, but it's not a determination of $c$. –  Nov 10 '14 at 21:12