I was reading a document when an article about atomic clock passed by. There is statement that I don't understand

The second is 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 cesium 133 atom.

"This definition refers to a cesium atom at rest at a temperature of 0 K"

Could I ask what does it mean by an atom at rest ? Does it mean that the atom is not moving ? If it means "not moving"; how can it be possible?


1 Answer 1


In this context, "at rest" means in a reference frame centered at the atom.

In practice, you can't get any system to exactly 0 K, nor can you make an observation from directly on the atom.

But, you can plot the periods of radiation relative to temperatures that are very low and to make any necessary special relativistic tweaks (which would be tiny, but we are talking about ten significant digit precision here, so even a tiny tweak might be relevant) to the observations based upon average velocity at the temperature at which the a cesium atom would be moving at that temperature. You can then extrapolate from this data to the value it would have in theory if the data were analytically extrapolated to O K and the rest frame of the atom based upon your measurements at various very low temperatures.

  • $\begingroup$ Oh by "In practice, you cant..... atom". You mean that the temperature is just very very close to zero kelvin but not 0 kelvin ? $\endgroup$ Commented Mar 13, 2017 at 2:11
  • $\begingroup$ This is exactly what I mean. $\endgroup$
    – ohwilleke
    Commented Mar 13, 2017 at 2:21
  • $\begingroup$ Thank you ohwilleke. In the statement, is there any understandable reason why "transition between the two hyperfine levels of the ground state of the cesium 133 atom". From what I understand from this statement. It should mean "It take one second for Cs to go from ground state to excited state". I mean after the "transition between two hyperfine levels of the ground state", the atom is no longer in its ground state, right ? $\endgroup$ Commented Mar 13, 2017 at 3:32
  • $\begingroup$ In terms of the reference frame, from what you say, I think that you mean an atom is never at rest (not moving). It's always moving unless the temperature is 0 K. And by "centered at the atom." and "at rest", it means that the atom "slightly moves" but just around the center and not far away from the center. The atom cannot stop moving because 0 K is still impossible in practice $\endgroup$ Commented Mar 13, 2017 at 3:38
  • $\begingroup$ @ProtonUpUpDown It is a category error to say that a particle is moving or is not moving without saying that it is moving or not with respect to what reference frame. It can be moving relative to Jupiter even if it was at a theoretically impossible o K. One reference frame you can always choose is the reference frame of the atom itself, and in that reference frame there is always zero kinetic energy. $\endgroup$
    – ohwilleke
    Commented Mar 23, 2017 at 22:01

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