
Counting Down to the New Ampere (2016) - throwaway000002
https://www.nist.gov/news-events/news/2016/08/counting-down-new-ampere
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wohlergehen
Does anyone know why these apparatuses are usually "inverted", i.e. hang from
the ceiling. I've seen the same thing for quantum computers. Is it related to
the way they are cooled? Or is it easier to work with somehow?

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pohl
I'm sharing this prior to reading it:

[https://en.m.wikipedia.org/wiki/Dilution_refrigerator](https://en.m.wikipedia.org/wiki/Dilution_refrigerator)

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miduil
Thank you for sharing.

I've started searching for "dilution refrigerator" and found this video,
explaining a little bit how ³He–⁴He mixture cooling works and what the
applications are. I think the video also helps putting the Wikipedia article
in a perspective and vice versa.

Quantum Cooling to (Near) Absolute Zero (2013):
[https://www.youtube.com/watch?v=7jT5rbE69ho](https://www.youtube.com/watch?v=7jT5rbE69ho)

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averagewall
More exciting is that the mol and Avogadro's constant will be pushed off to
the side on their own. Hopefully an even more future update will remove them
entirely. They're really quite redundant and pretty much only exist to
facilitate the needless presence of the non-SI mass unit, the unified atomic
mass unit, which thankfully will now be defined in terms of the kg instead of
having two independent mass units like we have now.

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Pulcinella
The mole and amu's (similar concepts) will never go away. It's a fundamental
concept in chemistry. Reactions happen between individual particles but
counting out individual atoms or molecules to supply for a reactions is
impossible or ridiculously impractical at best. So chemicals have to be
measured by mass (which we can measure) and then converted into number of
particles. Until a molecular counting device exists, the mole will remain.

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averagewall
It won't go away for the same reason pounds and miles won't go away - people
who know it can't be bothered learning something new, and people who are
learning it aren't influential enough to cause change. Avogadro's constant is
not in any way fundamental. It exists to reconcile the two different mass
units that chemists use - gram and amu.

With the 2018 SI change, Avogadro's constant will be defined as an arbitrary
number without any physical basis, and the amu will be a constant multiple of
the kg. No more 1/12 the mass of carbon 12.

Of course we'll still need a way to represent large numbers, but there's no
fundamental reason it has to be such a complicated number. It could be exactly
10^24, for instance. Again, I agree this isn't going to happen because of
legacy inertia.

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arjo129
Why is an ampere a fundamental unit but,not a coulomb?

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URSpider94
If you go back in history to when the SI units were defined, there would have
been no way to measure a Coulomb of electrons, since in nature you'd somehow
have to distinguish the electrons you are counting from all the other
electrons sitting around. With the invention of single electron counting, we
could do it now, but we aren't going to change the SI system a century in.
And, the idea was that the seven primary units are both independent (you can
not derive one from any of the others) and sufficient to derive all of the
other units of measure. Once you have the Ampere, you can define a Coulomb as
a relative quantity.

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perlgeek
A mostly unrelated question, in case any metrology geeks are around: why is
the Kelvin an SI unit?

Naively, there seem to be multiple approaches to derive temperature from
other, more fundamental units. Like using the thermodynamic definition, 1/T =
dS/dE, or using Boltzmann's law to approach temperature from the mean kinetic
energy of gas particles. Are none of them suitable for precise measurement?

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Thorondor
It's not really possible to derive temperature from other more fundamental
units. For example, you can't define entropy as an absolute number; it needs
to be assigned a unit, and the standard way of doing this is to multiply it by
the Boltzmann constant, which depends on a unit of temperature. The mean
kinetic energy of gas particles (3/2 kT) also depends on the Boltzmann
constant.

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amluto
> For example, you can't define entropy as an absolute number; it needs to be
> assigned a unit,

You can define entropy directly without reference to other units, although
it's a bit awkward. Entropy is the log of the number of microstates that
correspond to a system's macrostate. Concretely, if you put n mols of ideal
gas molecules in a box of volume V at a pressure P and temperature T, there is
some large number of microstates corresponding to all those parameters.
Entropy is the log of this number.

In classical mechanics, there's a normalization problem if you try to get an
actual number out of this type of problem -- the microstates and all the
macroscopic parameters are continuous. In quantum mechanics, though, this
issue is solvable, although it's still awkward.

I can imaging a different type of system in which entropy really can be
calculated, though. Imagine a particle that can be in exactly one of two
states that are macroscopically identical. Now try to _cool_ the system so
that the particle is in one of those states of your choice. To do so, you will
need to dump exactly 1 bit of entropy.

1 bit of entropy is tiny, but adiabatic demagnetization refrigerators work
kind of like this, albeit in reverse, and I could imagine an experiment that
would use a device like an adiabatic demagnetization fridge to remove a
calibrated number of bits of entropy from some object. From this, you could,
in principle, define entropy directly.

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URSpider94
While you are right that this is the statistical definition of entropy, there
are two issues: first, entropy is a unitless quantity, so defining it exactly
doesn't actually move the ball forward in terms of defining units of measure.
Second, outside of its theoretical underpinnings, physicists and chemists
hardly ever talk about absolute values of entropy, they almost always use
differences in entropy -- which factors out the need to define the exact
number of microstates present before and after.

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amluto
> first, entropy is a unitless quantity

The parent post asked about defining temperature in terms of more fundamental
units. Entropy is typically written in J/K. If you treat it as dimensionless,
then you get a definition of temperature in terms of energy for free. My point
is that you can, in principle, actually do an experiment to make this useful.

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vanjoe
Why is the kilogram the base unit instead of gram?

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kazinator
The kilogram has a suitable scale for physical events that matter in common
engineering situations.

Acceleration due to gravity is a nice number between 0 and 10 if measured in
"kilogram-meters per second squared".

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toptal
So does this mean that quantum computing will become more viable? Since in
quantum computing, calculations are accomplished by measuring the spin of an
electron, I would imagine this would increase the throughput to a measurement
instrument since this is allowing one electron to pass at a faster pace. While
electron measurement instruments still need to be advanced significantly, I
would imagine an innovation like this would further advance the reality of
true quantum computing. Is this an accurate assessment?

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FabHK
I think you'd also need to redefine the meter and second to speed up light.
Unfortunately, the footnote specifies that we can expect only "major changes
in the kilogram, ampere, kelvin, and mole" in the new SI 2018, so it seems
we'll have to wait for the release after that.

On the bright side, the changes to the kilogram might begin to address the
obesity crisis.

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pavpanchekha
Meter is already specified by fixing the speed of light. Seconds are defined
in terms of Cesium atom vibrations.

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pdonis
_> Seconds are defined in terms of Cesium atom vibrations._

Not vibrations of the atoms themselves. The second is defined in terms of the
period of the radiation corresponding to a particular hyperfine transition of
the Cesium atom.

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Filligree
What does "hyperfine" actually mean, in this context?

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pdonis
It's a reference to a particular kind of splitting of the energy levels of
electrons in atoms, due to interactions between the electrons and the nucleus.

Basically, as more and more precise measurements of the energy levels of
electrons in atoms were made in the 1920s, 30s, and 40s, physicists kept
finding that energy levels that were thought to be degenerate (i.e., multiple
states with the same energy) were actually split into multiple, closely spaced
levels. The original quantum model was the non-relativistic Schrodinger
equation as applied to the atom. Then it was found that electron energy levels
that were degenerate in that model were actually split into multiple levels
because of the effects of electron spin and certain relativistic corrections;
this splitting was called "fine structure". Then it was found that there was
even further splitting, of energy levels that were degenerate in the fine
structure model, due to interactions between the electron and the nucleus;
this further splitting was called "hyperfine structure".

More here:

[https://en.wikipedia.org/wiki/Hyperfine_structure](https://en.wikipedia.org/wiki/Hyperfine_structure)

