
The Most Symmetrical Objects in the World - dnetesn
http://nautil.us/blog/the-most-symmetrical-objects-in-the-world
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bcoates
I think the part about the kilogram is missing the point -- it's not that a
pure silicon crystal sphere is a better artifact to be _the_ kilogram than the
platinum-iridium cylinder. The idea is that they are defining a reproducible
process to manufacture an object of known mass without reference to some
artifact at all. They are proving by demonstration that anyone can manufacture
such an object and then test it for chemical purity, perfection of crystal
lattice, size, and sphericity. This allows you to form a counting argument
based only on math and the physical laws of the universe that the produced
object has within some small error exactly n silicon atoms in it, which will
then be the definition of the mass unit ( _any_ sphere of n silicon atoms, not
any particular one).

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

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bostik
On a more pedestrian scacle...

There are some extremely smooth and round objects available for ornamental
purposes. Finnish company Sorvikivi [0] has been making such granite balls for
20 years, and their products are so accurately manufactured that the balls can
be supported by a hydroplane between the ball and an equally smooth support
bowl.

I remember seeing one of their products at the lobby of a hotel. Because the
ball was effectively floating on water, you could rotate it by hand. And we
did.

[0]:
[http://www.sorvikivi.com/eng/index.html](http://www.sorvikivi.com/eng/index.html)

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lpmay
I had a friend who worked on a very deep sea drone. He told me that it was
better to flood as much of the drone as possible to minimize the strength of
the pressure vessels they had to build. The one exception to this was the
buoyant components, which had to be hollow to work. These were made of hollow
ceramic spheres of exacting sphericity. A sphere provides maximum enclosed
volume and strength for a minimum amount of structure, so was a good choice
for the pressure vessel shape. They had to be so perfectly spherical to force
the pressure to be balanced across the sphere and distribute the compression
evenly.

Not sure if this was the exact drone he worked on, but the spheres look like
what he described.

[http://www.whoi.edu/oceanus/feature/floating-without-
implodi...](http://www.whoi.edu/oceanus/feature/floating-without-imploding)

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cnvogel
The incredible precision with which the two spheres mentioned in the article
have been manufactured sure is mind-boggling. But I think the article itself
is a good example for what I find pretty annoying in popular presentation of
quantitative results in science.

\- "off by anything larger than one one-hundred-billionth of a degree"

\- [normal gyroscopes] "are over ten-million times less accurate"

...and to a lesser extent, because people in mechanical engineering or metal-
working are actually used to measure in micrometers:

\- "three ten-millionths of an inch away"

\- "only 25 nanometers away from being perfectly round"

The precision is presented as a very impressive sounding, but to the typical
reader pretty meaningless number, because in our normal lives we don't have
any reference for hundred-billionths of anything, and frankly many people have
difficulties writing up 100-billionths correctly, anyway.

I'd really like to see the general notion of "order of magnitude" be used for
these kinds of achievements, which can be presented as a "number of digits"
figure easily:

\- To measure the relativistic frame-dragging, Gravity Probe B had to measure
the degrees of rotation of its gyroscopes with a precision of 11 digits after
the decimal point whereas (military gyroscopes) only are accurate up to 7
digits.

\- three ten-millionth of a inch -> Diameter (in inch, let's assume it's 3
inches) specified with 8 digits of accuracy, similarly for the "25 nanometers
away from being round)

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JadeNB
> The problem is that the current IPK has lost a tiny bit of weight, in
> comparison with 40 similar cylinders held in other countries, which is a
> significant flaw in an object used to define a unit of mass. So the Avogadro
> Project has created two softball-sized, near-perfect spheres, made entirely
> of silicon-28 atoms, that should remain exactly one kilogram in perpetuity.

I seem to remember reading a while ago that no-one was quite sure _why_ the
reference kilogram was losing mass. Why should one therefore be confident that
the new spheres will not do so?—or is this another 'distortion for
convenience' of the article, like:

> Lie group E8, for instance, is a set of 248 different forms of symmetry that
> apply to a theoretical 57-dimensional object.

E_8 as a Lie group is 'continuous', and therefore infinite, and E_8 as a Weyl
group has size much, much, much bigger than 248. The number 248 presumably
comes from the dimension of the Lie group E_8 (and I suppose that one could
twist words to accept that the dimension of the Lie group is somehow a
reflection of the "number of different kinds of symmetries" that it contains);
I'm not sure (but am not an expert on this, and so might just not know) whence
comes the 57.

~~~
micampe
The sphere would not be defining the kilogram: it will be used to count the
number of atoms it contains, which would allow to redefine the Avogadro number
without reference to the kilogram, which would then define the kilogram as the
mass of a fixed number of Si28 atoms. See the video I posted in another
comment.

~~~
JadeNB
I see. Indeed, this is the impression that I got from the last paragraph:

> Now that the spheres are done, researchers in different countries will try
> to discern the exact number of atoms they contain, to get universal
> agreement on what exactly is the mass of a kilogram.

However, when it is understood this way, I find the previous word choice
confusing:

> two softball-sized, near-perfect spheres, made entirely of silicon-28 atoms,
> … _should_ remain exactly one kilogram in perpetuity

(emphasis mine). If they are made entirely of silicon-28, and the kilogram is
defined as the mass of a certain number of silicon-28 atoms, then they _will_
remain one kilogram in perpetuity (by definition), right?

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lutusp
> According to the Guinness Book of World Records, these are the roundest
> objects ever made.

What a shame they didn't specify that these are the most _spherical_ objects
ever made. "Round" can, and usually does, refer to two dimensions, while
"spherical" refers to three.

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IvyMike
Guess what?

[http://www.guinnessworldrecords.com/world-
records/3000/most-...](http://www.guinnessworldrecords.com/world-
records/3000/most-spherical-manmade-object)

~~~
lutusp
So the OP got it wrong. Good for Guinness, and thanks for the heads-up.

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3rd3
They forgot to mention this beauty:
[http://en.wikipedia.org/wiki/Red_Square_Nebula](http://en.wikipedia.org/wiki/Red_Square_Nebula)

