Big Test Coming Up For Kilogram Redefinition (ieee.org)

szotz writes: Electromechanical balances have got to be better than an aged lump of platinum and iridium right? Teams are working to get kilograms measured and shipped to Paris in time for a test to see whether the technology (along with another that uses ultrapure silicon spheres) is now ready to redefine the kilogram. Why is this redefinition interesting? Because it's about using physics to overcome one problem with weight standards based on tightly held exemplars in standards bodies' inner sanctums: the mass of those exemplars can change, however subtly, introducing uncertainty and confusion. From the article: The world's metrologists aim to change this state of affairs in 2018 by fixing the kilogram to the Planck constant, a fundamental physical constant. That shift would, at least in principle, allow any laboratory to "realize" the kilogram from scratch with a series of experiments and specialized equipment. But for that scheme to work, the kilogram derived by one laboratory must be the same as those derived by others.

Comments

Measurements

[Okian Warrior]

Why the planck constant then? Why not e, or (pi), or any other constant, for that matter? If you're going to change the definition, isn't it just a matter of choosing the close enough factor?

By the way, I'm asking. I am ignorant about this.

The fundamental distinction between math and physics is measurement. We need to base the physical constants on something measured from the universe we're interested in.

As a simple example, mathematics defines and explores 3 basic forms of geometry: Euclidean, hyperbolic, and elliptic.

The distinction between these is based on the curvature of space as defined by the behaviour of parallel lines: if parallel lines eventually meet, then space has positive curvature like the surface of a sphere. If parallel lines diverge, then space has a negative curvature like the surface of a saddle. And if parallel lines stay parallel, then space has zero curvature and is Euclidean.

Three equally valid forms of geometry, but which one does the universe have? To choose the correct model, we have to measure the actual universe.

The same is true with the fundamental physical constants. There's any number of ways to base our measurements on pure math, but these don't necessarily reflect the reality we live in.

To do that, we need to take a measurement.

Re:Will a Litre be Redefined?

[alexhs]

I thought it was a stupid conversion mistake, but investigating on the topic :

One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice.

So, originally as I wrote.

From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density and standard pressure. The kilogram was in turn specified as the mass of a platinum/iridium cylinder held at Sèvres in France and was intended to be of the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3.

Oops. Not too bad, given that at that time the metre was wrong too:

it was later determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre.

And all is fine again:

In 1964, the definition relating the litre to mass was abandoned in favour of the current one.

The litre [...] is an SI accepted metric system unit of volume equal to 1 cubic decimetre (dm3), 1,000 cubic centimetres (cm3) or 1/1,000 cubic metre.

Sources:
Litre
Metre