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Comment by jelder | Hacker News Reader

The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.

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As covered in the article, the reference object loses some of it's atoms every time it is handled. The smaller the reference object, the larger the relative error induced this way.

Not just that, it's probably just the surface (or near surface) that sublimes. The surface only increases with the square of the radius while the volume increases with the cube. so multiplying the volume by 27 only multiplies the surface by 9.

It was easier and more reliable to manufacture the reference kilo in the past. With the new redefinition, you can make your own reference gram, nanogram, megagram, etc.

Your intuition is correct -- it is possible to build more-precise intercomparison tools at the gram scale than the kilogram scale, but until next Monday, our units are referenced to Le Grand K, and any gram-reference must subdivide the absolute standard. After Monday, you're free to build your own gram-scale Kibble balance and call it an absolute standard.

The reference kilogram is estimated to have lost 50 micrograms over it's lifetime. I think it's safe to say that a reference gram would have lost a similar amount of mass. This would have caused a larger change in the definition of a gram/kilogram over time because the ratio of mass lost in the reference would have been greater.

Not much, but still.

Why wouldn't the mass loss scale with the mass of the object? Why would it always be the same mass loss in absolute terms?

It's hard to say because the mechanisms for the mass loss are not fully known. In fact, we are just guessing that mass loss actually occurred, and how much did occur. How would we have measured the mass change of the reference kilogram? By definition the reference kilogram was the most stable quantity of mass we could have generated at the time, and the most well protected quantity of mass for the duration of it's service as the reference kilogram.

I think mass loss is probably proportional with either the surface area or the surface area of the handled portion. This is just a hypothesis based on what I suspect the mechanism for mass loss is (friction from air resistance and being touched). Now that we have a more accurate way of measuring mass we could repeat the experiment of trying to preserve a quantity of mass, even doing different variations based on quantity, material, conditions, etc. Such an experiment would be relatively expensive and take a long time for good results, but maybe someone curious with funding will do it.

EDIT: The idea that we could have preserved a gram of material, while measuring it's mass every 40 years, for the last 130 years while it only lost 5 picograms (1/1000th of the estimated mass of the reference kilogram lost) is so crazy I can't believe it.

Is the concept of a half-life similarly crazy to you?

No, and radioactive decay doesn't account for the variances between the reference kilogram and it's sister kilograms we've observed.

So it's an issue of "an event happens uniformly randomly across a mass" that seems unusual to you, and only applicable to radioactive decay?

Basically, it's part of the lie of the mythology of the metric system.

The metric system did some great things (eliminated the use of measures that depended on the substance being measured, dry pints vs wet pints, bushel of wheat versus bushel of oats; simple relationship between units of length, area and volume), some things that were already common at the time (eliminated regional definitions of units; related the volume and mass of water), and some stupid things (metric prefixes).

But at the end of the day, the size of the meter and kilogram were chosen to be very nearly 3 Parisian feet and 2 Parisian pounds, because that made it easier to adopt.

This reply doesn't seem to address the comment it's replying to at all...

The answer to the question "why is the unit weight the kilogram instead of the gram" is "because the kilogram is about 2 Parisian pounds, which is what everyone at the time actually cared about".

As a scientist I've gotta say...how are metric prefixes bad? I couldn't do my job without them every day.

As a scientist, use scientific notation off the base units; you eliminate opportunities for confusion. The prefixes don't really add anything.

The Sun is 1.5 x 10^11 meters from the Earth.

Calling it 1.5 x 10^8 kilometers doesn't help you visualize the distance better, nor calling it 150 gigameters. Now you have three numbers that mean the same thing floating around, and if you accidentally write 1.5 x 10^8 m somewhere instead of km, or read 1.5 x 10^11 km as m, you've just introduced a thousand-fold error.

Common units (eg, Angstrom) make sense when you do not need to convert between them; it's convenient if they are easily convertible to your standard unit, but having mg and ug and g and kg floating around is just an unnecessary headache, you inevitably accidentally interpret a microgram dose as a milligram dose from time to time and poison a patient or eight.

Unless you need to make a dial on a physical piece of equipment. In that case it's helpful not to have to append x10^N to all of your numbers.

Or if you need to write it down anywhere at all; that's why programmers use eN or e+N. Although we really need to get around to standardizing a hexadecimal equivalent.

Personally, I find the prefixes helpful when memorizing approximate values, eg for back-of-the-envelope estimates (proton mass is 1GeV, electron mass is .5MeV, room temperature is 25meV, h_bar c is 197 MeV fm, ...)

A valid defense of the metric prefixes (I mean, I still think they're on the balance a mistake, but you do have a point), but ironically, electron-volts are not an SI unit.

Why 1 GeV instead of 160 picojoules?

If you don't mind I'll keep using milligrams and micrograms, micromolar, nanomolar etc. instead of writing all that out.

I love that everyone took this as a serious comment.

It's either the best unintentionally punny question or the best unintentionally serious pun in history.

Either way, I'm loving this.

Apparently they were going to go a different way, but this saved them a ton.

Likely a lot harder to get the tolerances down on something much smaller.

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