- #1

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## Summary:

- I was thinking about the measurement instruments used to find the wavelength and frequency of EM waves. I was looking at the problem in historical perspective.

## Main Question or Discussion Point

Hi,

How were such huge number of periods per second measured originally? Was an instrument such as Michelson interferometer used? I think it was Albert Michelson who invented the interferometer to make calculations of wavelength and frequency of EM waves.

Another related question is that why the temperature of 0 K was chosen since 0 Kelvin couldn't be achieved practically. I understand that the atoms must be at complete rest while emitting the radiation but the problem is that absolute rest state is not possible so how do they go around this problem practically?

I'd really appreciate if you could help me with the queries above. Thank you.

Source: https://en.wikipedia.org/wiki/Second#"Atomic"_secondSince 1967, the second has been defined as exactly "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (at a temperature of 0 K). This length of a second was selected to correspond exactly to the length of the ephemeris second previously defined. Atomic clocks use such a frequency to measure seconds by counting cycles per second at that frequency. Radiation of this kind is one of the most stable and reproducible phenomena of nature. The current generation of atomic clocks is accurate to within one second in a few hundred million years.

How were such huge number of periods per second measured originally? Was an instrument such as Michelson interferometer used? I think it was Albert Michelson who invented the interferometer to make calculations of wavelength and frequency of EM waves.

Another related question is that why the temperature of 0 K was chosen since 0 Kelvin couldn't be achieved practically. I understand that the atoms must be at complete rest while emitting the radiation but the problem is that absolute rest state is not possible so how do they go around this problem practically?

I'd really appreciate if you could help me with the queries above. Thank you.