
2019 redefinition of the SI base units - tony
https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units
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kevin_b_er
So I'm glad they fixed the problems of depending on kilogram sample object for
anything, but the definitions are now bizarre if you want to learn them in
reference to real-world things. How will they be taught now?

The Ampere went from being the thought experiment of two infinite wires 1m
apart creating a certain force to being something nearly unintelligible to the
average person.

The Kelvin was just turning 0C into absolute units, carefully formalized as
the triple point of water which was actually 0.01C, and making it into
absolute units. Now its something nearly intelligible to the public.

Another problem is I'm not entirely sure the atomic mass is quite defined
anymore. That number at the bottom of each box of the periodic table of
elements may not have a valid unit anymore??

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anticensor
Charge of an electron is a concrete fact, unlike infinitely long imaginary
wires inducing an electromagnetic pull.

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droithomme
> Charge of an electron is a concrete fact

It's only a fact in one's imagination, as concrete as a fact about properties
of any other thing that can only be measured indirectly.

We have no way to directly measure the charge of one electron, so it's a
fanciful claim, or at best an approximation, and not a concrete fact.

Claims about wires can be verified.

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joshuamorton
> We have no way to directly measure the charge of one electron, so it's a
> fanciful claim, or at best an approximation, and not a concrete fact.

Are you suggesting the oil drop experiment was invalid?

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Willson50
"The kilogram is the mass of a body at rest whose equivalent energy equals the
energy of a collection of photons whose frequencies sum to
[1.356392489652×10^50] hertz."

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effie
Glad that is a deprecated formulation. One never has collection of photons of
exact frequency of net mass 1kg. Also 1kg weight can't be realistically
entirely turned into photons.

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saagarjha
Your sarcasm was a bit hard to distinguish ;)

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effie
There wasn't any sarcasm intended.

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Aardwolf
Why is ampere a base unit instead of charge? Charge seems the more fundamental
one to me, like mass is, and ampere feels more derived, as if volumetric flow
rate were made a base unit instead of mass.

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kriztw
Because the standards are made for people who makes measurement devices. It is
much easier to measure electric current than charge.

They also did make charge more fundamental, as they derive the Ampere by
fixing the charge of an electron.

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anonu
Seems like a positive change. But somehow (perplexingly) we still use inches,
feet and miles in a major part of the world.

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atonse
My argument is mostly for things like day-to-day practical measurements,
there's no difference between the two units, so it doesn't affect people.

Temperature – absolutely no difference in _practicality_ in communicating
about the daily weather in C or F. People that claim otherwise are usually
just arguing for the system they grew up with.

Weight – no difference in measuring most larger weights in pounds vs kgs.
(weighing people, furniture, cars, trucks, comparing the weight of laptops,
etc)

Distance – Whether DC to NY is 220 miles or 352 kilometers makes no
difference.

In most daily use, the differences in units probably don't make any difference
to most people, which is why the cost of switching (so that it looks good on
paper) just hasn't caught on.

However, in the sciences, in cooking, areas where you need precise
measurements, grams, KG, etc all give you nicer units to work with.

When measuring things for DIY and home improvement, God, I wish I could do
everything in meters and centimeters than 1/8 or 1/6 of an inch (mainly for
doing calculations).

And when cooking, I cringe with "cups" and "tbsp" – I wish it were all grams
and ml.

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XCSme
Temperature - at what temperature does the water freeze? 0C, 32F. At what
temperature does the water boil? 100C, 212F. Which ones are easier to remember
on a day-to-day basis?

Weight, Distance - conversion is a lot easier in the metric system

You just mentioned that there were no differences, than why would you use
units which have strange relations between each other instead of units that
are easy to remember, easy to convert are based on math instead of
historical/common knowledge?

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atonse
How often are you boiling and freezing water when deciding what clothes to
wear?

Ok so when measuring water freezing, you have a nice 0. But how do you measure
hot weather? 30C? 35C? Meaningless numbers, correct?

I am talking about daily weather. Celsius is no more intuitive than Fahrenheit
when discussing it.

Even with cooking, when you are around a certain system, you're just used to
it.

I bake a cake at 350F, I fry things at 350-400F. 175C is no more practical
than 350F. I need to cook meat to at least 140F-160F to kill the bacteria.

The equivalents in Celsius are no easier to remember from a practical
standpoint.

Even body temperature, from my understanding, almost everyone uses 98.5F,
correct? (At least in India, where everything was KG and KM, people still used
to measure body temperature in F).

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tigershark
Seriously you don’t see any difference in defining hot weather as 1/3 of the
boiling temperature of water? Try to do that in Fahrenheit and you have to do
something crazy like (212 - 32) / 3 + 32, and I’m not even 100% sure that is
correct... And I guess that for you is much better to remember that at 32 F is
very dangerous to drive outside rather than at 0. I really can’t understand
how can someone would advocate for maths gimmicks when you have a much easier
solution _that is even an international standard_ (or to be precise it’s
extremely easy to convert to and from that standard)

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likpok
In Fahrenheit, hot is 100 and cold is 0.

In Celsius, hot is 30 and cold is -10.

When talking about it for what you're wearing and subjective experience,
Celsius is a little more awkward and compressed.

Sure, scientifically celsius is more useful, but how often does that overlap
with the weather?

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tigershark
No, cold is 0 in Celsius. You risk your life in icy roads at 0.

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apricot
Can anyone help me understand why the base SI unit of mass is the kilogram and
not the gram? Why is the ×1000 prefix included in the base unit of mass, but
not in any other units?

When I asked that question to my science teacher, a long time ago, he said
that was because it was easier to manufacture a precise prototype kilogram
than a prototype gram, and that made sense at the time, but what's the reason
now?

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droithomme
For an international standard of universal constants they sure change a lot.

Starting to get like the problem of OS updates changing everything, except
what if it was the foundations of all scientific measurement constantly
changing instead of some OS you don't even have to use.

Some might say oh they haven't really changed. Well if they haven't really
changed why change them.

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zokier
What does candela still do there? It is extremely unfundamental imho, and sort
of vaguely based on human physiology which seems out of place for SI unit. It
is not even fully defined because you need also a definition for luminous
efficacy. Of course candela is utterly useless outside the visible spectrum
which should be a huge alarm bell against it.

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spindle
Someone will save money by no longer having to store the prototype artefacts
... unless we're in any danger of running out of caesium atoms.

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zzo38computer
I am glad the definition of kilogram is corrected; I always thought it was no
good, and now it is good.

