Four base units of measure in the metric system about to be changed

a reply to: OccamsRazor04

One of the ways we measure a beam of light is through its angular momentum - a constant quantity that measures how much light is rotating. And until now, it was thought that for all forms of light, the angular momentum would be a whole number (known as an integer) multiple of Planck's constant - a physical constant that sets the scale of quantum effects.

But researchers led by Trinity College Dublin have now demonstrated that a new form of light exists, where the angular momentum is only half of this value.

sciencealert.com, May 20116 - Physicists Just Discovered a Totally New Form of Light.

I was being kind of facetious when I made the comment (I get it. Lots of people around the world doing science use SI). I think this is what I was thinking and was hoping somebody would remember when Plank's Constant was not found to hold (as in the "new form of light" quoted above). When a constant is not constant and can behave differently than you were taught, is it really such a great idea to base one of the most used measurement units on something you still do not understand?

Part of me can understand a cylinder sitting in a vault, even if I never see it, but hand over a formula to calculate the mass of an object based on Plank's Constant in the total sample??

a reply to: TEOTWAWKIAIFF

I'm from that generation that uses some Imperial measurements and some Metric.
About sums me up.

But....how can absolute zero be anything other than absolute zero?

a reply to: Freeborn


I do not know!

But we are pushing things to micro Kelvin units and lower (IIRC). I have not seen any updates on that aspect. It is the all mighty Kilo that has all the headlines.

Still digging!

The end result of all this hard work is an instrument known as the Kibble balance. This was invented in 1975 by British physicist Bryan Kibble, and has been optimized since to reach new levels of accuracy. Despite its complications, the Kibble balance works like a traditional set of scales or beam balance, just like those you might use to weigh groceries. But while these scales usually weigh one mass against another, the Kibble balance weighs mass against an electromagnetic force which can be measured extremely accurately.

This electromagnetic force is generated using a coil of wire surrounded by permanent magnets. This setup can create two different methods of weighing. In the first, you run a current through the coil of wire to generate electromagnetic pull. In the second, you physically move the coil up and down like a piston, which has the same effect. Due to a number of recent discoveries (including those Nobel Prizes we mentioned), we can measure some of the forces involved in both of the weighing modes with incredible precision. And by combining this knowledge, we can measure the mass on one side of the Kibble balance using Planck’s constant. This is what allows scientists to create a new definition for the kilogram: measuring the fundamentals of the physical world down to what is essentially the smallest possible physical action.

theverge.com - The Kilogram Is Dead; Long Live The Kilogram.

The article says they have take into account the force of gravity in which the measuring device sits! And there are only two places in the world that have a Kibble Balance accurate enough to measure the new definition: Canada and the US (all info, same source). The new definition will not go into effect until May of next year (if adopted).

Explain it a bit, and I am not as freaked out as I had been last Friday with the OP. That is pretty cool, if rather over complicated, device!

Still no word on what is being changed in Lord Kelvin's scale (the above article mentions it in passing choosing to focus on the kilogram).

There have been several good posts about this in other threads. At a minimum this change will better suit applications that run under "local conditions". Wouldn't be the first time technical constrains caused rethinking the problem. As Kelly Johnson once said "I'd sell my grandmother for 6 more inches".

a reply to: Freeborn

I finally have an answer to several of my questions... a week later!

The new definition of the kilogram uses a measurement from another fixed value from nature, Planck's constant (h), which will be defined as 6.62607015×10−34 joule seconds. Planck's constant can be found by dividing the electromagnetic frequency of a particle of light or "photon" by the amount of energy it carries.

The constant is usually measured in joule seconds but this can also be expressed as kilogram square metres per second. We know what a second and a metre is from the other definitions. So by adding these measurements, along with an exact knowledge of Planck's constant, we can get a new, very precise definition of the kilogram.

phys.org, Nov. 15, 2018 - The kilogram is being redefined – a physicist explains.

When they changed over from expressing it in joule seconds, they can work backwards using the other terms (solve the equation for kilogram), with the precise definition of "meter" in terms of C, as is a second, then I see what they did.

Armed with a new definition of kilogram (in terms of Plank's constant), the other changes have to follow.

The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10^–23 when expressed in the unit J K^–1, which is equal to kg m^2 s^–2 K^–1, where the kilogram, metre and second are defined in terms of h, c and ∆ν(sub Cs).

BIPM.org (pdf, p. 30) - Convocation of the General Conference on Weights and Measures (26th meeting).

First, the SI constants are "abrogated" [repealed of their current definition], then the kilogram is redefined. The other SI constants have to follow suit with the new definition of Kilogram (in terms of Plank's Constant).

Thanks physicist for explaining the math and reasoning behind these changes. Oh, and for actually linking in the proceedings notes! It is easy to see the math involved and makes a heck of a lot more sense rather than what I posted in the OP where it was just "decreed to be so."

Existential crisis averted! Although I did use it as an excuse to stay drunk and watch F1 early on Sunday morning!

We're In!!

Scientists, for whom the update represented decades of work, clapped, cheered and even wept as delegates gathered in Versailles one by one said "yes" or "oui" to the change, hailed as a revolution in how humanity measures and quantifies its world.

The redefinition of the kilogram, the globally approved unit of mass, was the mostly hotly anticipated change. For more than a century, the kilogram has been defined as the mass of a cylinder of platinum-iridium alloy kept in a high-security vault in France. That artefact, nicknamed "Le Grand K," has been the world's sole true kilogram since 1889 .

Now, with the vote, the kilogram and all of the other main measurement units will be defined using numerical values that fit handily onto a wallet card. Those numbers were read to the national delegates before they voted. The update will take effect May 20.

Nothing left but to pop some champagne and eat snails!

I guess 129 years is a pretty good run for Le Grand K. Not as cool as what they used on the Pyramids of Giza but a pretty good run none the less.

*sigh* another doom porn date flies past...