Metric vs. other measurement systems - chart

[mklotz]

For once, I am looking forward to a TV programme later this week, to do with the “internationally accepted prototype” of a kilogram. This has shed, I read, a fraction of its weight. This is described as the “equivalent of an eyelash”. Really? That seems quite a lot, and since it is locked in a vault near Paris, it’s not likely to be due to wear and tear. Hopefully the programme will explain why, although if it’s quantum physics, that will in itself be enough. The metrologists are apparently “racing” to invent a new standard kilogram. What are the likely candidates? I suggest 703 coffee beans.

According to this article...

https://www.nbcnews.com/mach/science...ns-ncna1007731

it's lost only 50 micrograms, the weight of a few fingerprints; an eyelash would weigh much more.

Science has advanced enough that we can now define the fundamental quantities (meter, kilogram, second) of the metric system in terms of physical constants not subject to the vagaries of nature as the prototypes of the past were. The meter is already defined in terms of the speed of light in a vacuum, a universal constant. The second is defined in terms of the radiation emitted by cesium. The kilogram was the last fundamental to be defined in terms of a physical artefact.

Now the kilogram has been defined in terms of Planck's constant, h = 6.62607015E-34 kg·m²/sec so the physical standard in France is no longer needed; it can rust and abrade away as much as it wishes.

Defining in terms of universal physical constants not only removes the dependence on objects subject to change, it allows users anywhere to generate the standard rather than having to travel to a prototype, measure, and then attempt to carry that measurement back unchanged.