# Redefining the unit of mass

• I
Science Advisor
in May we'll indeed define the underlying mass unit kg, precisely by just counting particles, but that's another story
Can you give details?

## Answers and Replies

Science Advisor
Gold Member
2022 Award
Very probably becoming effective on May 20 this year the kg will be redefined formally through fixing the value of Plancks constant, ##h##, based on the definition of the second (unchanged via the atomar hyperfine transition of Cs) and the meter (unchanged via fixing the speed of light in vacuo). The actual value is fixed forever. To guarantee continuity the value has been fixed in two competing experiments in various national institutes of standard. One is related to fixing the Avogadro number (thus providing the new definition of the mole) via the counting of Si atoms in ultraprecise crystalline spheres of silicon (the Avogadro Project). The other is based on the Watt balance and related thus also to the redefinition of the Ampere.

Details of the redefinition can be found here:

https://www.bipm.org/utils/en/pdf/si-revised-brochure/Draft-SI-Brochure-2018.pdf

and for the realization ("mise en practique") of the new kg definition:

https://www.bipm.org/utils/en/pdf/si-mep/MeP-kg-2018.pdf

Here you find also details for the entire set of base units and their interdependency:

https://www.bipm.org/en/measurement-units/rev-si/#communication

sophiecentaur, Dale, anorlunda and 2 others
Mentor
mise en practique
I personally think that these mise en pratique are a fantastic idea. The definition of the SI no longer depends on the specific experiment, but as better experimental techniques become available we can simply update the mise en practique as a new recognized method of realizing the standard to a specified and known accuracy! I had heard about them, but I did not know that drafts were already published.

Last edited:
russ_watters
Mentor
and for the realization ("mise en practique") of the new kg definition:
I personally think that these mise en practique are a fantastic idea.
Please, that's pratique, without a c.

Dale
Staff Emeritus
Science Advisor
Education Advisor
to the redefinition of the Ampere

Thereby making 4π a measured quantity.

weirdoguy
Science Advisor
Is there a reason why the new standard defined $h$ to be a fixed rational rather than the much more frequently used ##\hbar=h/2\pi##? The convention makes the quantum of spin, ##\hbar/2## an exactly known but transcendental number.

vanhees71
Science Advisor
Gold Member
2022 Award
That's a good question. Maybe it's for historical reasons? I don't know. My QM 1 professor told us that you shouldn't trust anybody using ##h## instead of ##\hbar## ;-)).