
Why scientists are redefining the kilogram - mileycyrusXOXO
https://www.engadget.com/2017/10/24/why-scientists-are-redefining-the-kilogram/
======
hydrox24
I have always had one question about defining measurements in terms of
universal constants. Perhaps it is a silly one.

What if universal constants are, in the long run, variable? What if over the
course of 500 years the speed of light, or the strength of gravity, changes?

If the measurements used to define these constants are defined in terms of the
constants themselves, how would we see change? Perhaps this is a silly
question in practice. If so, I would be interested in the 'theoretical' or
hypothetical answer for a world where traditional measures of the kilogram and
second have been forgotten.

~~~
lmm
If the universe was just a sequence of random arrangements of random
phenomena, then no physics would be possible - there would be no reproducible
experiments, and so no way to predict the future based on the present.

Specific things may be variables rather than constants, but we can only ever
define that in relation to some other constant. If the hyperfine transition of
the cesium-133 atom starts taking longer, how could we measure that, except in
relation to some other periodic activity, e.g. the hyperfine transition of
hydrogen atoms (or, if we go back to Galileo, human heartbeats)? So we could
equally say that the hydrogen transition has got faster or the cesium
transition has got slower. Now if 111 elements "agreed" on how fast time was
passing and one other element differed, it would probably make sense to say
that it was the one element whose transitions were slowing down rather than
the 111 elements whose transitions were speeding up. But ultimately there is
no single objective measure of time in the universe; rather there are a number
of periodic patterns in the universe that are mostly consistent with each
other, and most easily explained by there being a single, consensus concept of
time passing. If one particular pattern starts going against that consensus,
it makes sense to say that that particular pattern is changing, and since we
want our definition of the passage of time to agree with the consensus, if the
pattern that changed happened to be the "official" definition of time
(hyperfine transitions of cesium-133) then we would almost certainly just
switch to a different pattern in nature that followed the consensus. But if
the consensus itself broke down, well, at that point it no longer makes sense
to talk about a single concept of "time" \- rather we'd have to talk about the
specific patterns we were talking about. And if the patterns went away
entirely, physics itself would become impossible.

~~~
balabaster
In relative terms I agree. However, what if some outside force were causing
the 111 elements transitions to speed up, but that outside force had no effect
on the 1 who was immune to that cause? How can we ever be sure of anything? If
we go into things assuming that the 111 elements are correct and the 1 was
incorrect, we could be tying ourselves to assumptions that may be invalid.

Think to Geordie's epiphany: "It never occurred to me that space was the thing
that was moving."

~~~
lmm
> what if some outside force were causing the 111 elements transitions to
> speed up, but that outside force had no effect on the 1 who was immune to
> that cause? How can we ever be sure of anything?

If two models generate the same predictions, they are the same model, and the
simpler (in the Kolgomorov complexity sense) is best understood as the truth
(in so far as that concept is meaningful at all). A force acting on everything
in the universe except one atom, or a (equal and opposite) force acting on
that one atom, is the same thing[1], just as a universe where all distances
are multiplied by two is indistinguishable from the current universe. Again,
there's no way to make an observation from outside the universe; we can only
make relative measurements, observe patterns, and use them to make
predictions.

[1] ultimately, this comes down to: they generate the same sense-impressions
that I subjectively experience - but for simplicity let's assume an objective
universe exists, other humans exist, and we can observe the same things - I
believe this assumption is justified as the most parsimonious explanation of
the patterns I perceive in the sense-impressions I experience

~~~
gowld
> is best understood as the truth

No! It's _preferred as an explanation_ because it is more elegant and
convenient, NOT because it is more true. It is the same true!

~~~
lmm
Elsewhere in the thread someone gave the example of heliocentrism versus
geocentrism. You can form a heliocentric model that makes accurate
predictions, you just have to add epicycles to everything. But I would say
that heliocentrism is not merely more elegant and more convenient, but also
more true. Wouldn't you? For a more extreme example, in the story from
[http://lesswrong.com/lw/i4/belief_in_belief/](http://lesswrong.com/lw/i4/belief_in_belief/)
I think we would say that it isn't true that there's a dragon in the garage.

~~~
avar
In that case however we have other data than just the model to suggest that
geocentrism is false, whereas the topic of this thread is models in physics
mathematics that merely differ in complexity, with no apparent difference in
"truth".

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ajross
The most interesting takeaway from this article for me isn't the kilogram,
which is obvious and makes sense, but that the _mole_ is being redefined.

A "mole" is a unitless quantity. Literally it's just a number. I'm just
dumbfounded that when they defined it, they didn't just pick an actual number
but referenced an experiment to define it. Now it's a number.

~~~
kazinator
I guess that the problem with Avogadro's number (and others such, like the
Coulomb) is that they are approximations and not specific counts.

The difference between the Avogadro Number and the new Avogadro Constant is
that they will define it as a concrete integer value: 6.02NNNN... x 10^23.
When the digits are agreed upon, I think the idea is that all subsequent
digits are going to be defined as zero, making it a precise integer.

So then if it is a precise integer, we can then try construct objects with
exactly that many particles (silicon atoms) and then weigh them, and so forth:
masses can be tied to precise counts.

It's kind of analogous to declaring that an inch is exactly 2.54 mm:
2.54000000... not 2.5400XXX for some unknown X that is somehow empirically
measured.

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pandler
So if I understand correctly, this is saying that we would be formally
redefining the kilogram, mole, ampere and Kelvin units (or prototypes?) as
some quantifiable relation to immutable, measurable constants instead of the
arbitrary and self-referential "this is a kg because it's how much this block
of metal weighs and we decided that's what a kg is". Is that correct?

Not being a scientist myself, I'm struggling to understand the impact of these
changes being made. Is it just a matter of increasing the precision of base
units?

Say they redefine the mole to be based on that 1kg sphere of silicon-28. What
is the real-world impact of doing so? Presumably one mole will still be one
mole and not all of a sudden e.g. 6.03x10^23.

Does anyone have an example of where a change like this would be important?

~~~
lmm
The big issue is that the standard reference kilogram (the block of metal) is
losing mass. If we take the SI standards literally, this means e.g. distant
galaxies now mass more kilograms than they did last year. Which seems
obviously undesirable.

The impact should ultimately be to make ultra-precise physics - and
eventually, engineering - easier, because people reproducing an experiment in
a different lab will have a closer consensus on exactly how much 1kg is.

~~~
pdpi
> The big issue is that the standard reference kilogram (the block of metal)
> is losing mass.

As per the article, there's several of them. Some are losing mass, others are
gaining.

~~~
danbruc
But only one of them, Big K, defines the kilogram. So the ones losing mass
might just gain mass more slowly than Big K. Or they might even all be losing
mass but some more slowly than Big K making them apparently gain mass. I don't
know if we know how the masses actually changed.

~~~
falcolas
We don't. And that's the problem. If it was a consistent loss, it would be
much easier to work around.

------
ourmandave
_" For the scale that's in your grocery store or bathroom, nothing's going to
change," Dr. David Newell of NIST said._

Was _really_ hoping my Intent to Deliver charges would drop to simple
possession, but I guess that's not going to happen...

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neaden
While this is a good thing, I can't help but be a little bit sad. I liked the
idea that while meters and seconds were defined by some natural process or
law, the Kilogram was something that physically existed, that you could see or
even theoretically hold in your hand.

~~~
Bromskloss
> even theoretically hold in your hand.

Yeah, if you were that strong.

~~~
dpark
So, if you aren’t an infant and don’t have a severe disability?

~~~
Bromskloss
Somehow, I feel that the joke got lost.

~~~
dpark
Clearly

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venturis_voice
Interesting read I saw a similar post about this from July:

[https://www.washingtonpost.com/news/speaking-of-
science/wp/2...](https://www.washingtonpost.com/news/speaking-of-
science/wp/2017/07/05/scientists-are-about-to-change-what-a-kilogram-is-thats-
massive/?utm_term=.d8dff516db82)

Redefining mass based on an idea as opposed to a physical object took me some
time to get my head around.

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dpcx
IANAS... Is it possible that these adjustments could mean the difference
between someones work needing a "magic number" to be correct and not? I've
heard stories about physicists who needed to add some "correcting" variables
to make their math totally correct, which is why I thought of this.

~~~
fixermark
I think this is unrelated.

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nercht12
You know what's kinda funny: In order to make the changes to "define" these
physical constants, we first make equipment using the old definitions. XD (Not
that it changes the outcome, mind you.)

