
Ampere to get rational redefinition - feelthepain
http://www.nature.com/news/ampere-to-get-rational-redefinition-1.14512
======
Steuard
This sort of change is a _great_ idea on a fundamental level: define our units
in terms of fundamental constants or processes that are absolutely
reproducible. But as the article explains, any such definition has to be at
least as precise _in practice_ as the less fundamental definitions that they
replace.

Hence the challenge (mentioned in the article) of defining the kilogram in a
sensible way. The obvious method would be to say "1 kg = the mass of [###] of
carbon atoms" or something like that. But counting 10^23 individual atoms is
impossible for all practical purposes: it's still less precise than just
weighing that reference lump of metal (despite the flaws of that approach).

The remarkable thing described in this article is that scientists are
evidently at the verge of being able to count one electron at a time _fast
enough_ for use in calibrating actual, practical current measurement devices.
That's pretty cool.

~~~
VLM
"The obvious method"

Using a cesium time standard and an atomic force microscope figure out the
half life of individual atoms of 14C (or something more fun/shorter) then
define a kilo of 14C as a pile of 14C generating X decay events per whatever
interval calibrated by our cesium time standard.

(edited to add the point is you can watch individual atoms with the AFM and
see which ones disintegrated in the past since the last scan, and its no great
trick for a century or so to detect individual decays using a geiger counter.
Also 14C is no fun for this task. Try a shorter halflife)

~~~
bcbrown
The half-life is statistical, not mechanistic, so there would be an inherent
variability in that definition. The larger the radioactive mass and the longer
the time period, the less variability, but it's still variable.

~~~
jjoonathan
The fact that it's variable isn't a valid criticism on its own: there is
uncertainty in every standard we use, and the objective is to minimize that
uncertainty. It's quite possible that the practically achievable error bars
from this method would be thinner than the practical error bars of another
method. Besides, the fact that you can arbitrarily increase the precision of
this method without hitting any hard physical limitations is a solid selling
point.

~~~
bcbrown
You are right that there's always going to be uncertainty. Perhaps it turns
out that in practice, the uncertainty with this method is less than other
approaches. The point I was trying to make was about the different between
incidental uncertainty due to imprecise equipment, and more fundamental
uncertainty due to physical laws. For example, other intrinsic uncertainties
are the Planck resolution for time and space, and the Heisenberg Uncertainty
Principle for certain combinations of information. Similar to those,
radioactivity has a fundamental limit. We cannot predict when an atom will
emit radiation, all we can do is make a statistical prediction.

------
ta43434

        ...almost as much of an embarrassment as...
    

Why is it embarrassing? It's the best we've got so far, and it's been pretty
useful. We're working on making it more intrinsic to the universe. Chill.

------
VLM
Ohms law will not be denied and if the volt from a Josephson junction
apparatus combined with the ohm from a quantum Hall effect apparatus don't
match up with the new amp, it'll be interesting to see who gets adjusted to
match.

The scientific soap opera of metrology (not meteorology) is interesting to
read about. Computer science is hardly the only discipline to suffer from
multiple standards.

One standard defined an amp as a certain mass of plated silver in a certain
amount of time, so if that was still "cool" then you'd have mass defined
indirectly solely from the quantum hall effect and Josephson junction effect
and time aka cesium vibrations or whatever it is, which wanders into the whole
"a kilo is a lump of metal in Paris" situation.

------
chris_wot
I feel quite stupid, but I just can't parse the following:

"At present, an ampere is defined as the amount of charge flowing per second
through two infinitely long wires one metre apart"

If the wires are one metre apart, then how are they infinitely long?

~~~
thegoleffect
Imagine two identical, parallel wires that are infinitely long. The two wires
are spaced one meter apart (still in parallel). When current is passed through
the wires they would exert force on each other proportional to current.

The infinitely long part ensures the force is uniform through all parts of the
wire.

Hope that helps.

~~~
chris_wot
Ah... That makes total sense now - thanks!

------
nmc
This also relates to the loss of weight [1] of the _" 125-year-old platinum-
and-iridium cylinder"_ defining the kilogram.

[1]
[https://news.ycombinator.com/item?id=6617039](https://news.ycombinator.com/item?id=6617039)

------
mynegation
I remember that there is a problem with a better definition of a kilogram:
unlike metre and second that are defined through measurable physical
constants, we use metallic prototype for kg. There were some attempts to
define it as the mass of the precisely cut silicon sphere, but we still use
the prototype.

Now the question to someone more knowledgeable in physics than me: if the
described effort pans out, does that mean we can define kilogram as the mass
that would accelerate at 1 m/s^2 if we apply the same force that affects two
infinite wires one meter apart, with 1 ampere flowing through them?

~~~
rprospero
To clarify slightly, the proposal wasn't to switch from defining the kilogram
as the mass of one particular lump of platinum to one particular lump of
silicon. Instead, the idea was to define the kilogram as the mass of a certain
number of Silicon 28 atoms and use the silicon sphere to get that number as
close as possible to our old kilogram.

This would then allow independent scientists to make their own spheres,
calculate the number of silicon atoms in the sphere, and have their own
reference benchmarks for the kilo without needing to check in with Paris.

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XaspR8d
It appears the new definition is going to be something like "the current (in
the direction of flow) of 6.2415093*10^18 elementary charges passing a
boundary in 1 second", replacing that number with a specific 19-digit value.
[1]

[1]
[https://en.wikipedia.org/wiki/Ampere#Proposed_future_definit...](https://en.wikipedia.org/wiki/Ampere#Proposed_future_definition)

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gaius
What science could you do with the "new" Amp that you couldn't now, with the
old?

~~~
noiv
I think it is more about doing same science with same results in different
places. The infinite long wires part is not exactly reproducible.

