#SI units

[oliviabutsmart]

Physics "Friday" #11 [Opinion]: Okay so I have a few more problems with SI Units

Preamble: Now what Missy?

Education Level: Primary School (Y5/6)

Topic: Measuring Systems (Metrology)

Ok so this was going to be my necessary follow up to my first opinion post, Is Fahrenheit the better temperature scale?

I was meant to save it for the next time I had too much stuff to deal with on Friday and needed to write something easier. In fact, that's going to be how I do opinion posts - when I don't need to do as much a-thunkin'.

Regardless, there are of course other problems with the SI units system that many people point out. This is basically me doing it myself.

What are the SI Units?

During the french revolution, there were several attempts to metricise the existing unit system. Generating standardised units on length, volume, weight, temperature ... they even tried to decimalise time.

These units were based off physical constants:

The metre was one 10 millionth the distance from the north pole to the equator

The litre was the volume taken up by one kilogram of water

Celsius was based on the boiling/melting points of water

A kilogram was based on the weight of a specific Platinum-Iridium Alloy called the IPK (International Prototype Kilogram)

Now obviously, these weren't perfect constants. The earth's radius changed, the boiling point of water depends on the pressure of the environment, and the IPK can vary based on surrounding conditions.

This was fixed in 2019, when every SI measurement unit was defined using a universal constant rather than physical objects like a mass or the earth:

Hyperfine transition frequency of Caesium

Speed of light

Planck Constant

Charge of the Electron

Boltzmann Constant

Avogadro's Constant

Luminous Efficacy of 540 THz radiation

This gives our seven SI units:

Time (Second)

Distance (Meter)

Mass (Kilogram)

Charge (Columb)

Temperature (Kelvin)

Amount (Mole)

Luminous Intensity (Candela)

Problem #1: Unit-less units

There are several units that come from mathematics that appear to be unitless, like:

Angle (Radians)

Solid Angle (Steradians)

Amount (Number count)

We use this all the time like in lumens (candela steradians), angular velocity (radians per second), and number density (particles per cubic metre).

Wait ... amount was already listed as an SI quanity - it was a mole wasn't it?

That's exactly it. A mole is just defined as 6.022×10²³ objects of something. This is where we experience our first problem.

Why is radian and steradian not considered a unit in it's own right but a mole is?

Radians are defined as being one metre per metre. The angle required to create an arc with a length of 1 metre given a radius of one metre.

Image Credit: BYJU's

This comes from the formula S=Rθ where circular arc length (S) is equal to the radius (R) multiplied by angle in radians (θ).

The problem with this, is that the radian is still an arbitrary dimension. We could've chosen our standard angle unit to be a degree, and all we need to do is change the definition of the sine function and the above arc length definition:

S=kθR were kθ is dimensionless and k = 1 rad⁻¹

It's almost as if, while the radian is dimensionless, it still behaves and acts like a unit. Much like how we can redefine everything and make the unit for length be a foot instead, we can do the same with angle.

You may object, as S=Rθ looks a lot more fundamental than adding an extra unneeded constant. But it's like if we were to use natural units, where the speed of light is equal to 1, making E = mc² actually E = m.

This doesn't mean that m is somehow 'fundamental' when we define it in terms of energy. All we've done is redefine our unit system in a way that makes a few constants equal to one.

The same thing goes for radians.

A little side-note on Intensity

You could argue that this also applies to relative intensity units like Magnitude or Bels. However, I'd argue that they don't count as their unit systems are defined as being relative. The strength of a signal or luminosity is just choosing a unit system of power.

For example, relative magnitude is based on the luminosity of the sun. Sound/Signal amplitude is based on watts or volts.

Problem #2: Redundant Units

Many of the quantities in the SI unit system are technically redundant. Temperature can just be redefined as energy density, mass can be redefined as energy, etc.

What I'm concerned with is one unit in particular: the Candela - what exactly is it?

I often find that a lot of people attempting to explain the SI unit system often brush over this unit, even though it feels rather important given it's called 'Luminous Intensity'.

But hold on, isn't luminosity just about energy production? After all, radiation is just energy, and it's emitted as a form of energy.

A candela is equal to one lumen per steradian, where a lumen is the total luminous intensity of an object that emits light everywhere. Candelas concern luminous intensity from a specific solid angle view of the object.

Watts, the measurement of power and bolometric luminosity, measure the total (or bolometric) luminosity of an object at all wavelengths of light.

Lumens, on the other hand, represent the power produced by an object after each wavelength is passed through some filtering function that accounts for how the human eye sees light. This weighting function is determined by the ISO.

Of course, our eyes only see light within a particular range, and inside of that range, different wavelengths appear more intense to us because we pick it up more.

But this fails to recognise that this is still just a glorified unit of power. Just because it's weighted based on some function, shouldn't change what unit is necessarily uses. Hence it is technically redundant as it can be defined as a combination of other SI units - similar to radians.

Problem #3: When the unit system isn't used

I mentioned it in the last post, but there are a lot of occasions where the standard SI units aren't used. The worst culprit I feel is Astronomy, with Physics only having issues in lieu of SI vs. Natural units vs. Electron/Atomic Units. Again here's the list:

My beloved SI units

CGS Units

Whatever the fuck a Jansky is

Don't even start with natural units I can't live without big G

"Ampere in CGS units is g1/2 cm3/2 s−2"

Solar Luminosity/Mass of Sun

Angstroms (like please can we just use nanometers?)

How many Jupiters or Earths fit into this cloud of gas?

The vomit of parallax units i.e. AU, pc, Mpc, arcseconds, radians

Steradians (Solid angles can be finicky)

Logarithms, logarithms everywhere!

Hubble's constant being in km/s/Mpc but then having to turn that into Hz or per year - like can someone please acknowledged how cursed this is?

When you do Kepler's 3rd law on Mercury and realise it doesn't work (because you forgot Einstein existed) ... so no units end up working

ADUs and/or whatever you get when you deal with telescope outputs

Sidereal time, J2000, etc.

Sky Coordinates (it always depends on the telescope mount)

(the last two are new entries into the list I forgot!)

A lot of these units are very, very, annoying to handle. Often at times because they are just so unnecessary. We have scientific notation for a reason - so why are we using Jansky? Why ever use the mass of planets unless if we are talking about specifically exoplanets?

It can especially be annoying when in astronomy course subjects, unit conversions make up like 50% of the work and 50% of the error in calculations.

And the biggest problem, of course, is the CGS units system. I hate it. So much. From what I know CGS is used simply because it appears as more correctly "scaled" for a lot of astronomical processes.

However, the problem is that it just adds extra conversion factors into every equation. Now I have to remember big G in both CGS units as well as in SI or solar units etc. And it doesn't jive with a lot of other astronomical units.

J2000 is also rather annoying - why are we using the Julian calendar at all? Shouldn't we try and strive for using a more accurate year instead? Because what happens is that every four years the calendar shifts by a day.

The "Ampere in CGS units is g1/2 cm3/2 s-2" is a direct quote from one of my professors. And it also makes very little sense - because it says that the coulomb and ampere units are actually redundant.

However, CGS units aren't usually the only thing we use. Sometimes we put things in Gaussian CGS units, where we define the coulomb's constant (kₑ) as being 1. Thus, similar to natural units, we can define a statCoulomb in terms of our three base units:

1 statC = 1 g1/2⋅cm3/2⋅s−1.

Now this is where we find our ampere definition! But wait ... this is materially different to the real ampere. In fact, we can determine the difference between coulombs and statcoulombs:

So they are different! And thus, after several paragraphs, my lecturer is wrong and they shouldn't've used the Ampere but a different stat ampere.

So what would I do?

Well, the big problem with all unit systems is that they are just equivalent to eachother dimensionally.

Because of all the seperate equations surrounding each of the SI units, we can define any SI unit in terms of any other SI unit combination just by setting certain constants to one.

So there's quite a lot of units that we could me missing.

But here's how I would do it ...

There are five unique SI units:

Distance - Metre

Time - Second

Mass - Kilogram

Charge - Coulomb

Temperature - Kelvin

There are also three dimensionless/special units:

Amount - Mole

Angle - Radian

Luminosity - Lumen

These three units are special as they can be expressed as dimensionless constants or in terms of other units, however we want them to be expressed in our own specific units.

Solid Angle is just determined from square radians where 1 sr = π/4 rad² (ratio of square and circle of same diameter).

The constants which define these unique units are:

Hyperfine transition frequency of Caesium

Speed of light

Planck Constant

Charge of the Electron

Boltzmann Constant

For the non-unique units we have:

Avogadro's Constant

Pi

Luminous Efficacy of 540 THz radiation

Conclusion

This post did end up taking longer than I expected ... much longer. Nevertheless it was fun to do!

As stated in the start, opinion posts are for those Fridays where things aren't as easy as spending 2-3 hrs writing a post. Because when you want to argue a point you have a little more passion behind it.

Anywho. Did you know that only 5% of people who read this post actually follow this channel?. SMASH THAT FUCKING LIKE AND SUBSCRIBE. Brought to you by Raid Skillspace VPN™.

Feedback's always welcome. Come and debate me you cowards. Follow if you like it ... or don't.

[sukimas]

cold as balls out but it's gonna be 14 degrees later today. how am i supposed to fucking dress for this

[er-cryptid]

[bcb-and-b-368-lindsay-road-40]

Historians will be puzzled by 1970-onward

An older writing prompt... a la Neem.

The question: What world events do I remember? This is a fun question. I was born in 1970 and here’s what I remember: 1979-1981, The USA hostages in revolutionary Iran. US president Billy Carter being incapable of dealing with the issue (or at least that was how it was editorialized to me as a kid). 1968-1972. The Apollo Moon landings? Nope. No memory of that on the news (I would have been…

[ryanoftinellb]

When I graduated with a degree in physics, the biggest and smallest SI prefices were ‘yocto’ and ‘yotta’. Now, because the total amount of information available to humans is growing so quickly, we've had to expand upwards to ‘ronna’ and ‘quetta’. It's only the physicists love of symmetry that has given us ‘ronto’ and ‘quecto’.

I have 47 yoctobytes of Star Trek :D .

[tenth-sentence]

While the SI unit of ∆rH° is J/mol, values are usually quoted in kJ/mol.

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[whats-in-a-sentence]

Table 2.3 lists the most common prefixes.

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[ignemia]

Assuming an average Karen complains once per establisment visited we can make Average Karen, Karen for short (k) a unit.

Now then if a person complains 3 times in a single establishmrent they have power of 3 k.

Assuming this in metric system we can extrapolate existence of scary unit kk (kilo-karen) and absolutely terrifying Mk (Mega-karen).

[krazycat6167]

So @somerandomdudelmao made a version of their sona in a dystopia (inspired by @tapakah0 doing the same to theirs) and the person in this ask named the robot C.A.S.5 and I thought, ‘well then there’s at least four other C.A.S. units out and about in the world’ leading to this being the end result! It was a lot of fun to come up with the different customizations each C.A.S. unit has.

also, the design for C.A.S.4 (Cash) was partially inspired by @mobiitez post.

Doodles:

[odinsblog]

Five days before Donald Trump became president in January 2017, a manager at a bank branch in Cairo received an unusual letter from an organization linked to the Egyptian intelligence service. It asked the bank to “kindly withdraw” nearly $10 million from the organization’s account — all in cash.

Inside the state-run National Bank of Egypt, employees were soon busy placing bundles of $100 bills into two large bags, according to records from the bank. Four men arrived and carried away the bags, which U.S. officials later described in sealed court filings as weighing a combined 200 pounds and containing what was then a sizable share of Egypt’s reserve of U.S. currency.

Federal investigators learned of the withdrawal, which has not been previously reported, early in 2019. The discovery intensified a secret criminal investigation that had begun two years earlier with classified U.S. intelligence indicating that Egyptian President Abdel Fatah El-Sisi sought to give Trump $10 million to boost his 2016 presidential campaign, a Washington Post investigation has found.

Since receiving the intelligence about Sisi, the Justice Department had been examining whether money moved from Cairo to Trump, potentially violating federal law that bans U.S. candidates from taking foreign funds. Investigators had also sought to learn if money from Sisi might have factored into Trump’s decision in the final days of his run for the White House to inject his campaign with $10 million of his own money.

Those questions, at least in the view of several investigators on the case, would never be answered, The Post found.

Within months of learning of the withdrawal, prosecutors and FBI agents were blocked by top Justice Department officials from obtaining bank records they believed might hold critical evidence, according to interviews with people familiar with the case as well as documents and contemporaneous notes of the investigation. The case ground to a halt by the fall of 2019 as Trump’s then-attorney general, William P. Barr, raised doubts about whether there was sufficient evidence to continue the probe of Trump.

(continue reading)

[accio-victuuri]

just wang yibo things: responding to pick up lines 😂😂😂

[ugartecoco]

first goal of the season

[rose022]

hi~ im rose! (among other names lol, ask for them or find them) welcome to my intro!

- dont feel bad about blocking or unfollowing me, just wanna say this. i may make posts abt being confused when i see people leave but curate ur own experience idc be happy

- you can call me whatever you want idc, just lemme know. like genuinely any nickname. i do have two others tbh but ehh

- yeah yeah im an adult so dont block me if i like ur post and u say mdni ight? also for any minors if that makes u uncomfortable u can leave. i dont tend to post anything nsfw here, just sometimes suggestive stuff and both nsfw and suggestive are tagged just like that

- pronouns are whatever is funniest in the moment. or just use your own. neos are ok

- asks and dms always open for anything. i won't really reach out first but i promise you can always talk to me about anything. if you don't have anyone else or just want to talk about a particular thing or want to vent, etc. like genuinely pretty please talk to me about your interests i will gladly listen!!! <3

- also you can always tag me in anything!! i love tag games (i may take a bit) and if theres anything you think id like or reminds you of me id love to see it!!

- don't really have a dni but like i will block you if you're a bigot. im a fag and autistic if you don't like that go away idc. also if you are pro cop or military stuff fuck off

- if we're mutuals please tag stuff with cw rose for anything with touya or akito being shipped with anyone else or TBHK nene and tsukasa as a ship, self harm images, stuff about spiders (or any bugs kinda) mostly pictures of them or them on someone (bees moths and butterflies exempt), needles if theyre detailed not cartoony, real life gore of animals or humans (not sfx or art just real stuff), sa, stiff about veins, and please use tone tags but mostly just for joking or sarcasm as i tend to assume everything is serious.

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- !!current things i tag are agere, homestuck, eye strain, saiouma, bsd, religion, body dysmorphia, religion, marlo dont look (for nsfw stuff, tbh im not sure what counts but anything with mentions of sexual stuff that isn't just like a joke), elie shoo (csm saiouma gore), suggestive (anything relating to anything nsfw like jokes or anything else), adrien no look (alcohol, crickets, girl interrupted, hospital innuendos), clowns

- tags

#rose rambles - my posts

#rose tagged ramblings - i made a separate one for when i talk a lot in tags

#rose art - quite simply rose's art

#rose ocs - the sillies from my brain

#ask rose! - asks

#rose pics - look i don't get creative with naming these i think you can tell

#rose cosplay - ^^^ yeah

#helpful - things that could be helpful to anyone

#save - similar to above but more so personal things

#important - maybe not technically helpful but i think people should see

#favs - self explanatory, i believe

#sillies - similar to above but not really

#luv - mutual appreciation mostly

#heart! - things made by mutuals!

#liebe - things made for me!

#mecore - woah thats so me frfr

#rose writing - idk man take a guess

(i wanna like fix all my posts to have tags but i reblog so much and itll take me a million years to get back to the start of my account atp. maybe one day.)

- commission info

art:

writing: $2 per 100 words (?)

i also take requests for these!! or other stuff but like it will almost definitely take me a bajillion years to finish so if you wanna make sure i do something, gimme money. but u dont gotta its ok ily anyways

i have a k-fi rose226 and dm for p-ypal

- sideblogs

@nightcord-luka-official

@luka-spotted most active

@daily-akiyama-mizuki hiatus!

@daily-asahina-mafuyu hiatus!

@kokomi-sangonomiya-official

@mafuyu-asahina-official

@wansho-luka-official

@junior-high-enanan-official

@fischl-official

@rosemusictime lol no followers

@oz-the-bird-official (to translate my Fischl)

@ako-udagawa-official this and above in progress

@daily-purple-character send reqs

@rose-poll-account

@aoi-shirosaki-official pjsk oc rp blog

@hikari-itsuki-official ^

@yuki-jigoku-official ^

@kei-makura-official ^

@roseswonderland things i make

@yuki-spotted

@niigo-va-updates

(i may remove u as a follower depending on if i trust you enough for these next few but if we're mutuals ur pretty much safe)

@moonlit-thoughts22 vent

@selfless-lvr selfship/kin

@softrosebud agere

@shhh-its-rose moots only one mentioned at the top of this

and ask for the nsfw one... cus im too scared to just add it

(guys i think I have too many sideblogs help)

im not active on many other social media but if we're mutuals you can ask for like anything and ill probably have an account. somehow im most active here tho (and in one discord server but u guys cant join)

[intersectionalpraxis]

And the fact that after he found out someone posted these photos, he wanted them taken down for his privacy -and ironically considering what he did recently.

And of course I know all escorts/those doing sex work/etc have the right to bodily autonomy, but this specific context makes me wonder because wtf is this grown 51 year-old man doing with 20 year olds like this -especially when you know about the darker sides of the entertainment industry in South Korea and how 'sponsorships' work -and I'm not saying that's what's happening here because I don't know how they connected and how transactional it is, but the amount of power and influence this man does give me an ick.

[stopitlight]

you can tell a lot about a person depending on whether they headcannon jacobi as mark midland after returning to earth or before joining goddard futuristics.

does daniel jacobi blow up teller and radio bob before joining si-5, or does he get reminded by si-5 in all the little things bomb squad do? what brand of "oh my god daniel jacobi is TRAUMATIZED" are you?

[waywardwizzard]

"Why is it that every time we go to a do-si-do, we end up captured?" Mal asked, ignoring the man behind him who was gripping his shoulder hard enough to bruise.

" 'Cause you have no common sense. Sir," Zoë said.

"Because you thought it would be a good idea to bring a known fugitive to an Alliance Ball," Simon deadpanned.

Mal glared at the two of them.

"It was a rhetorical question."

"Didn't sound that way, sir."

"Bi zui."