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Naty1
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from the current Physicsworld newsletter:
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The kilogram is currently defined by a lump of metal in Paris – but now researchers in the UK, France and Sweden have confirmed a key assumption of a new method of defining the standard based on fundamental constants. ...
The kilogram standard is made from platinum and iridium, and is housed at the International Bureau of Weights and Measures (BIPM) in Paris. Over the past 60 years, several comparisons of this kilogram with identical copies suggest that its mass is changing. As a result, scientists have been looking for a new way to define the kilogram using just fundamental constants.
Beyond the simple wear that check standards can experience, the mass of even the carefully stored national prototypes can drift relative to the IPK for a variety of reasons, some known and some unknown. Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they gain mass through adsorption of atmospheric contamination onto their surfaces. Accordingly, they are cleaned in a process the BIPM developed between 1939 and 1946 known as “the BIPM cleaning method” that comprises lightly rubbing with a chamois soaked in equal parts ether and ethanol, followed by steam cleaning with bi-distilled water, and allowing the prototypes to settle for 7–10 days before verification.[Note 12] Cleaning the prototypes removes between 5 and 60 µg of contamination depending largely on the time elapsed since the last cleaning. Further, a second cleaning can remove up to 10 µg more. After cleaning—even when they are stored under their bell jars—the IPK and its replicas immediately begin gaining mass again. The BIPM even developed a model of this gain and concluded that it averaged 1.11 µg per month for the first 3 months after cleaning and then decreased to an average of about 1 µg per year thereafter.
What has become clear after the third periodic verification performed between 1988 and 1992 is that masses of the entire worldwide ensemble of prototypes have been slowly but inexorably diverging from each other. It is also clear that the mass of the IPK lost perhaps 50 µg over the last century, and possibly significantly more, in comparison to its official copies.[12][14] The reason for this drift has eluded physicists who have dedicated their careers to the SI unit of mass. No plausible mechanism has been proposed to explain either a steady decrease in the mass of the IPK, or an increase in that of its replicas dispersed throughout the world.[
The kilogram standard mass is changing because the international prototype kilogram, which has been used as the standard since 1889, is losing mass over time due to environmental factors and the handling of the object. This has led to inconsistency and uncertainty in measurements, so a more precise and stable definition was needed.
The new definition of the kilogram is based on the Planck constant, a fundamental constant of nature that relates a particle's energy to its frequency. This provides a more accurate and stable definition of the kilogram, as it is not dependent on a physical object that can change over time.
This change will have a minimal impact on most scientific research, as the difference in measurement between the old and new definition is very small. However, it will provide a more precise and consistent standard for measurements, which can improve the accuracy of experiments and reduce uncertainty.
For the average person, this change will have no noticeable impact on everyday life. The kilogram is still defined in terms of its mass in everyday units, so there will be no change in how we use or think about the kilogram in our daily lives.
The decision to change the kilogram standard was made by the General Conference on Weights and Measures (CGPM), an international organization responsible for maintaining the International System of Units (SI). The decision was based on years of research and collaboration among scientists and measurement experts, and was officially adopted in 2018.