HN Theater @HNTheaterMonth

Also relevant: https://www.youtube.com/watch?v=ZMByI4s-D-Y

Mole was just redefined as exactly 6.02214076e23 — the number of Si-28 atoms in a perfect 1 kg sphere. This was an intentionally check against the kibble balance measurement.

More info from an older Veritasium video: https://www.youtube.com/watch?v=ZMByI4s-D-Y

⬐ mherdeg

Yeah, I had totally missed that a change was in the works to redefine Avogardo's number as an exact number with no uncertainty. Pretty cool.

As Veritasium explains [0], all of the reference kilograms have drifted, and some appear to weigh less than they used to. So even if we know the precise mechanism of action of the drift, it doesn't help with the fact that our measurements are less reliable than they used to be.

Of course, what's probably happened is that our measuring tools have gotten more accurate!

[0] https://www.youtube.com/watch?v=ZMByI4s-D-Y

⬐ cmurf

The kilogram is a measure of mass, not weight which is a measure of force. In order to really figure out mass, I'd think the localized gravity has to be accounted for. e.g. GRACE maps gravity variation around the earth by measuring the acceleration difference between a pair of satellites.

⬐ mattashii

The measurements were taken at the same place at around the same time, so fluctuations in local gravity should have been negligible compared to actual mass fluctuations.

One cubic centimetre of water at 4 degrees Celsius used to define one gram (or more precisely one litre -> one kilogram, which comes out to be the exact same thing) [1], [2].

Although 1 cm^3 of water is still pretty damn close to 1g.

[1] https://en.wikipedia.org/wiki/Grave_%28unit%29

[2] https://www.youtube.com/watch?v=ZMByI4s-D-Y (around 1:50, and actually the whole video is excellent)

silicon-28 sphere(s?): https://www.youtube.com/watch?v=ZMByI4s-D-Y (yep, they let him palm it)

watt balance: https://www.youtube.com/watch?v=VlJSwb4i_uQ

⬐ jahnu

Surprised that the cotton gloves or particles that might float onto them or be coughed or breathed onto them aren't abrasive.

I stumbled upon a very interesting short documentary about the metric system and the world's roundest object. Well worth 10 minutes of your break today. http://www.youtube.com/watch?v=ZMByI4s-D-Y

Interesting video with some historical background about the kilogram and that sphere http://youtu.be/ZMByI4s-D-Y

Fantastic video by Vertiasium on this: https://www.youtube.com/watch?v=ZMByI4s-D-Y

(side note: that video is about a sphere, but the current standard kilogram and its copies are actually shaped like little bells)

⬐ avoid3d

To clarify: that sphere is not a 'standard' in a sense that it is going to definite a kilogram. It is a tool which is going to be used to measure the correct numbers to define the kilogram in terms of universal constants. In this sense it is very different from the 'bells' you refer to, and I think the way you stated the link was misleading.

The scientific community is working on the replacement:

Known as the Avogadro Project, the plan is to bring together enough atoms of one substance – silicon – to make a kilo.

Attention has focused on silicon because:

- its characteristics are very well understood

- a single crystal of the right size can be grown

- its atomic structure is extremely uniform

- its widespread use in the computer industry means it can be obtained with relative ease at high purity and resonable cost.

A spherical shape was chosen because a sphere has no edges that might get damaged and only one dimension has to be measured in order to calculate its volume.

---

http://www.csiro.au/content/ps35k

http://www.youtube.com/watch?v=ZMByI4s-D-Y

⬐ ISL

The Avogadro project isn't the only horse in this race. The electromechanical approach, known as a Watt balance, is underway in several national standards labs around the world.

⬐ Shish2k

> A spherical shape was chosen because a sphere has no edges that might get damaged and only one dimension has to be measured in order to calculate its volume.

Surely every dimension needs to be checked, to ensure that it's a perfect sphere? (Which I assume is harder than checking a cuboid has six flat sides joined at right angles)

⬐ ISL

If you build an apparatus for measuring a sphere's diameter, it's possible to measure it in any orientation. If their test mass were cylindrical, they'd need the ability to measure both the diameter and the length to the requisite precision. I believe it is in this sense that 'dimension' was used.

Check out this veritasium video https://www.youtube.com/watch?v=ZMByI4s-D-Y - It goes into a little detail on how the kilogram got it's name, it is also very interesting in it's own right.

For anyone interested in the Planck and Avogadro projects to redefine the kilo, this video is an imperative watch: https://www.youtube.com/watch?v=ZMByI4s-D-Y

⬐ blowski

Interesting, but what's with all the weird flirting?

"Weirdly, it’s not even known if the IPK is getting lighter, or if the national prototypes are getting heavier — but either way, something is causing these kilos to change weight, by around 50 micrograms every 100 years."

Well they COULD always do that thing with the water where they heat it to 4 degrees celcius and measure its volume and weight. Then they'd know which it is.

That said, there's a nice video on this: http://youtube.com/watch?v=ZMByI4s-D-Y

⬐ xenophonf

Yeah, that's actually a lot more difficult than you make it sound. See, for instance, http://en.wikipedia.org/wiki/Vienna_Standard_Mean_Ocean_Wate....

⬐ fragsworth

> measure its volume and weight

Except they'd have to measure its weight (mass), which is in... Kg...

⬐ EGreg

Exactly. They'd COMPARE it to the existing physical object used as the reference for the kg. Assuming water allover the world hasnt changed its properties over the last few centuries they'd know whether it became heavier or lighter.

⬐ emhs

Well, they're using the static, known density at 4 degrees Celsius to produce water with a known mass, and then test what the measurement of that mass comes to. Basically they're using a known mass of water as a calibration aid. But I don't know that this level of precision could compete with the efforts underway.

⬐ None

None

⬐ yetanotherphd

According to the video, that process is a way to measure Plank's constant (indirectly), which is precisely what they refer to in the article.

^This. It's well explained in this YT video:

https://www.youtube.com/watch?v=ZMByI4s-D-Y

The world's roundest object helps solve the longest running problem in measurement -- how to define the kilogram. A kilogram isn't what it used to be. Literally. The original name for it was the 'grave', proposed in 1793 but it fell victim to the French Revolution like its creator, Lavoisier. So begins the tale of the most unusual SI unit. The kilogram is the only base unit with a prefix in its name, and the only one still defined by a physical artifact, the international prototype kilogram or IPK.

But the problem with this definition has long been apparent. The IPK doesn't seem to maintain its mass compared to 40 similar cylinders minted at the same time. The goal is therefore to eliminate the kilogram's dependence on a physical object. Two main approaches are being considered to achieve this end: the Avogadro Project and the Watt Balance.

The Avogadro project aims to redefine Avogadro's constant (currently defined by the kilogram -- the number of atoms in 12 g of carbon-12) and reverse the relationship so that the kilogram is precisely specified by Avogadro's constant. This method required creating the most perfect sphere on Earth. It is made out of a single crystal of silicon 28 atoms. By carefully measuring the diameter, the volume can be precisely specified. Since the atom spacing of silicon is well known, the number of atoms in a sphere can be accurately calculated. This allows for a very precise determination of Avogadro's constant.

Special thanks to Katie Green, Dr. David Farrant, the CSIRO, and the National Measurment Institute for their help. Thanks also to Nessy Hill for filming and reviewing earlier drafts of this video.

Music by Kevin McLeod (incompetech.com) Decision, Danse Macabre, Scissors