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The second is defined as the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. So what is meant by a cycle of radiation?

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    $\begingroup$ A complete cycle of the sine wave. $\endgroup$ – PM 2Ring May 29 at 15:45
  • $\begingroup$ But won't any radiation of same frequency as that emitted due to the transition between the two hyperfine levels of the ground state of the cesium-133 atom take the same time? $\endgroup$ – bo habib Jun 4 at 9:44
  • $\begingroup$ Yes, by the definition of frequency. What EM source do you have in mind that has exactly the same frequency? ;) Note that the caesium atoms should be at 0K to produce the desired frequency, so a practical caesium clock has to make a compromise. $\endgroup$ – PM 2Ring Jun 4 at 9:58
  • $\begingroup$ See en.wikipedia.org/wiki/Atomic_fountain and en.wikipedia.org/wiki/Atomic_clock $\endgroup$ – PM 2Ring Jun 4 at 10:07
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Taken a point in the space, hit by the radiation, the electric field in that point will oscillate periodically, as $\sin(\omega t)$ (where $t$ is the time). The oscillation will repeat exactly equally after a period $T=2\pi /\omega$.

The part of oscillation taking place during a time interval of length $T$ is "one cycle". It is, for example, the part of oscillation between two minima (or between two maxima).

A second is defined as the time needed to perform 9,192,631,770 cycles, i.e. 1 s=9,192,631,770 $T$.

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  • $\begingroup$ But won't any radiation of same frequency as that emitted due to the transition between the two hyperfine levels of the ground state of the cesium-133 atom take the same time? $\endgroup$ – bo habib Jun 4 at 9:43
  • $\begingroup$ Any radiation with the same frequency will have the same period, thus a given number of cycles will take the same time. But I do not know any other physical process giving rise to the same frequency, spontaneously, without the need of an accurate tuning. $\endgroup$ – Doriano Brogioli Jun 4 at 10:16

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