• Member Since 28th Oct, 2012
  • offline last seen 57 minutes ago

Pineta


Particle Physics and Pony Fiction Experimentalist

More Blog Posts440

  • 6 weeks
    End of the Universe

    I am working to finish Infinite Imponability Drive as soon as I can. Unfortunately the last two weeks have been so crazy that it’s been hard to set aside more than a few hours to do any writing…

    Read More

    6 comments · 143 views
  • 9 weeks
    Imponable Update

    Work on Infinite Imponability Drive continues. I aim to get another chapter up by next weekend. Thank you to everyone who left comments. Sorry I have not been very responsive. I got sidetracked for the last two weeks preparing a talk for the ATOM society on Particle Detectors for the LHC and Beyond, which took rather more of my time than I

    Read More

    1 comments · 130 views
  • 10 weeks
    Imponable Interlude

    Everything is beautiful now that we have our first rainbow of the season.

    What is life? Is it nothing more than the endless search for a cutie mark? And what is a cutie mark but a constant reminder that we're all only one bugbear attack away from oblivion?

    Read More

    3 comments · 188 views
  • 12 weeks
    Quantum Decoherence

    Happy end-of-2023 everyone.

    I just posted a new story.

    EInfinite Imponability Drive
    In an infinitely improbable set of events, Twilight Sparkle, Sunny Starscout, and other ponies of all generations meet at the Restaurant at the end of the Universe.
    Pineta · 12k words  ·  48  0 · 822 views

    This is one of the craziest things that I have ever tried to write and is a consequence of me having rather more unstructured free time than usual for the last week.

    Read More

    2 comments · 137 views
  • 15 weeks
    Secrets of Auroras

    Read More

    3 comments · 148 views
Feb
22nd
2015

The Problem of Weighing the Kilogram · 4:23pm Feb 22nd, 2015

The next chapter of Time on Their Hooves is now out. Continuing this saga of timekeeping in Equestria.

This gives me an opportunity to write a post on one of my many nerdy obsessions: measurement standards and precision metrology. Okay, this is not quite as sexy as space stations and particle accelerators, but it is still pretty cool to think about how you can measure anything to parts-per-billion precision and the sometimes bizarre philosophical problems you have to think about when measuring anything at this level.

This story begins in revolutionary France in the 1790s. In between storming the Bastille and dispatching the royals to the guillotine, those inventive Parisians were hard at work developing a radical new set of measurement units, designed to sweep away the confusion of the thousands of different ‘standards’ in use across the eighteenth-century world.

The metric system was envisaged as a universal standard, accessible to anyone. Just as the measurement of time was set by the rotation of the Earth, with no bias to any country; the unit of length would be set by its size. Thus was born the idea of the metre as one ten millionth of the distance from the North Pole to the Equator. The kilogram would be the mass of one thousandth of a cubic metre of water.

But the difficulties of pacing out the distance from central Africa to the Arctic, meant that the traditional method of keeping a solid rod and weight prevailed as a practical realization. For years the metre was formally defined as the distance between two points on a platinum alloy rod. By the 1950s, the accuracy of atomic clocks had reached the point that they could track tiny changes in the period of the spinning Earth. Thus it made sense to redefine the second as 9,192,631,770 oscillations of a caesium 133 atom. As lasers emerged as the most accurate way to measure length, it made sense to redefine the metre as the distance travelled by light in 1/299,792,458 of a second.

Thus both time and distance have a simple definition, fixed to an atomic standard, which can be measured by any well-equipped national standards laboratory.

But the kilogram has proven to be a weightier problem. Measuring mass at the parts-per-billion level can be done, but you are balancing one mass against another, so you need a standard mass to calibrate your scales. You calibrate your standard against another standard, and ultimately everything is set against the International Prototype Kilogram (or Le Grand K). An artefact cast in London in the 1880s and stored at International Bureau of Weights and Measures in Paris. Replicas were made, polished to the same mass, and distributed to laboratories worldwide, and hence used, by a chain of cross-calibrations, to calibrate all official scales.

As the kilogram is defined as the mass of Le Grand K, every forty years or so, the replica kilograms are brought back to Paris, carefully cleaned, and weighed. The last time it was done in 1989, it appears the replicas were all gaining weight. Why? No one knows for sure. Maybe they were acquiring extra atoms from atmospheric pollution. It would seem more likely the No. 1 was shedding a little mass, maybe during the cleaning process, but by definition, it can't. It always weighs one kilogram. So, if it has lost a little material, then everything in the universe now weighs a little more than in 1890.

This crazy conclusion is the fundamental flaw with a system of defining mass in terms of an actual object. But it's still the most accurate method we have—for now—it is widely expected that the kilogram will be redefined sometime soon.

How will this be done? This is to be decided. One option would be to define it in terms of the mass of a fixed number of atoms. We can't actually count a kilogram worth of atoms directly, but it is possible to determine the number of atoms in a crystal of silicon by using x-rays to measure the spacing between the atoms, and laser to measure its volume. An alternative method is to use a Watt balance to equate the weight of a mass to an electrical force balancing it. Combine this with a precise measurement of the local acceleration due to gravity, a Josphson Voltage Standard, and a quantum Hall effect resistance standard, and you have an accurate, but rather complicated, way of fixing the kilogram to the value of the Planck constant.

This would then allow any well-equipped lab to measure the absolute mass of an object, without reference to the IPK. Want to try it at home? NIST have published details on how to build your own out of LEGO bricks (with a precision of 1%).

So one day we will be able to sleep sound, knowing that we will not be gaining weight due to a lump of platinum in a French vault shedding atoms.

Back in Equestria, my story explores the problem of defining the fundamental unit of time using a water clock.

Comments ( 12 )

That is easily one of the coolest things I have ever seen Lego bricks do. Glad to know there are ways to declare independence from the tyranny of the nobility standard kilogram. Truly, the original spirit of the metric system lives on.

This was addressed briefly a few years ago, I remember. There was this big hubbub about the world's roundest object being a potential to replace Le Grand K as the base, but nothing ever came of it.

Oh, and:

But kilogram has proven to be a weightier problem.

Not sure if that was intended, but it made me smile briefly.

Every now and then I see a blog post and I have to say to myself, "I don't recall why I started following this person, but I'm really glad I did."

In economics we solve this problem by not having anything objective to measure, ha ha....

Very interesting, did not know this.

There was an article somewhere about how the most accurate clock in the world was invented, and then the researchers working on it found out that it was so accurate that differences in height changed its ticking rate, ironically making it useless at synchronizing.

I would like to just say how incredible it is that the LEGO company builds their toys to such a level of quality that this is possible.

It's a big reason I don't mind them being a bit pricey as toys go. You easily get what you pay for.

2821005
If you're nerdy enough to shell out for it you can buy old, decommissioned atomic clocks for well below cost. They're an order of magnitude or three less accurate than the modern standard, hence the clearance sale, yet still accurate to within tiny fractions of a second. (I believe you're still talking about a four-figure price.)

I mention this because I heard on a podcast once about a guy who obtained two such clocks and calibrated them to each other. He then demonstrated relativity to his kids by taking them on a camping trip in the mountains with one clock in tow and showing that the clocks had gone out of sync on their return.

2820725

the world's roundest object being a potential to replace Le Grand K as the base, but nothing ever came of it.

This one?
wired.com/images_blogs/wiredscience/2010/10/silicon-sphere-avogadro-project.jpg
It's still a candidate.

2820861

In economics we solve this problem by not having anything objective to measure, ha ha....

Maybe you could give us a story or blog post on the gold standard and why it didn't work

2821005

There was an article somewhere about how the most accurate clock in the world was invented, and then the researchers working on it found out that it was so accurate that differences in height changed its ticking rate, ironically making it useless at synchronizing.

Indeed. At this level of accuracy you have to remember that relativity means the time between two events depends on the position where you measure it. The second is defined so it can be measured by a single clock, but setting coordinated universal time requires a network of atomic clocks across the globe.

2821776
And maybe some in space so that they are less affected by gravity.

I wish I could write sciencey stuff this well... When I'm in physics mode I can barely even write code.... When I'm in engineering mode, on the other hand...

I knew most of this already, and yet I loved hearing you tell it

I always imagined that ponies use PlanckUnits, as should we.

Login or register to comment