Redefining the Mass Unit: Counting Particles for Precision

In summary, the kg will be redefined on May 20 this year to have a precise value for Plancks constant, ##h## based on the definition of the second (unchanged) and the meter (unchanged).
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A. Neumaier
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vanhees71 said:
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?
 
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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
 
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vanhees71 said:
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.
 
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vanhees71 said:
and for the realization ("mise en practique") of the new kg definition:
Dale said:
I personally think that these mise en practique are a fantastic idea.
Please, that's pratique, without a c.
 
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vanhees71 said:
to the redefinition of the Ampere

Thereby making 4π a measured quantity. o0)
 
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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.
 
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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## ;-)).
 

1. What is the current unit of mass and why is it being redefined?

The current unit of mass is the kilogram, which is defined by a physical object known as the International Prototype Kilogram (IPK). It is being redefined because the IPK is subject to variations in mass over time, making it an unreliable standard.

2. How will the unit of mass be redefined?

The unit of mass will be redefined using a fundamental constant of nature, known as the Planck constant. This constant relates the energy of a photon to its frequency and will be used to define the kilogram in terms of a specific number of photons.

3. Will this change affect everyday measurements of mass?

Yes, the redefinition of the unit of mass will have an impact on everyday measurements. However, the change will be very small and only noticeable in highly precise measurements.

4. What are the benefits of redefining the unit of mass?

Redefining the unit of mass will provide a more stable and accurate standard for measurements. It will also allow for greater consistency and reproducibility in scientific experiments and industrial processes.

5. Will other units of measurement be affected by the redefinition of the unit of mass?

Yes, the redefinition of the unit of mass will have a ripple effect on other units of measurement, such as the units of force and energy. These units are currently defined in terms of the kilogram, so their definitions will also need to be adjusted to align with the new definition of the kilogram.

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