In late 2018 the kilogram's definition switched from a physical object (the International Prototype Kilogram) to Planck's Constant; a grand move in my opinion.

However, I haven't heard of the consequences resulting from this change. Is 1 kg still 1 kg? Or do we need to now convert pre-2019 masses to the new definition?

  • $\begingroup$ They fixed the value of Planck's constant in such a way that $1 kg$ remains $1 kg$. It's just redefined because having a physical object is inconvenient. $\endgroup$ – harshit54 Feb 26 at 18:43
  • $\begingroup$ It's worth noting that the physical artifact is known to have been changing in any case. Those changes were small, but they were annoyingly unpredictable. $\endgroup$ – dmckee Feb 26 at 18:59
  • $\begingroup$ @dmckee True. It was like 50 micrograms lighter or something. Plus kg was the last physical unit not to be based on a constant, no? $\endgroup$ – Lux Claridge Feb 26 at 19:04

There are no practical consequences because the changeover was done in a way that ensured no disruption.

For example, before the change, Planck’s constant was a measured value, $6.626070150(81)\times10^{-34}$ Joule-seconds. Now it is a defined value, exactly $6.62607015\times10^{-34}$ Joule-seconds. It “changed” to the mean measured value.

This means that any change to the kilogram was at the level of less than one part in a billion. The old kilogram artifact was not stable to that level anyway.

  • $\begingroup$ This might be a separate question, but if the prototype kilogram was not known to one part in a billion, how could Planck's constant be determined to that accuracy? At some point you must calibrate against a physical prototype kilogram surely? $\endgroup$ – jacob1729 Feb 26 at 22:46
  • $\begingroup$ Good question! Unfortunately, I don’t have an answer for you. $\endgroup$ – G. Smith Feb 27 at 0:43

Indeed, the definition of 1kg has changed. This means that what was "1kg" pre-2018 is not what 1kg is in the new definiton. However, this change is extremely small, and therefore does not affect our daily lives. Some very precise measurements must be converted, of course, but we are talking about the order of a few micrograms.


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