Hey I noticed that guy's "papers" on vixra last week. Quite the example of quality vs quantity

hellooo

Just wonderin’ if anyone else has the feeling that the “review to close” queue is now systematically longer than some months ago, i.e. the number of posts flagged for closure is now habitually 50+.

@ZeroTheHero the number of items the site shows you is personalised i.e. it's the number available to you to review. If you keep on top of the reviewing the number stays low - for me it hovers around 10-20 posts to review. But if you stop reviewing for a few days it escalates alarmingly quickly.

@ZeroTheHero we do have a shortage of close vote reviewers. The total queue once rarely grew longer than 30, now it often hovers around 100. The current reviewers are barely enough, and it would be much better if more >3k reviewerd

So after looking at the wikipedia page for the hieroglyph block of unicode

turns out the fonts I have to display it display almost every characters

Except the three penis characters

This is an outrage

Wasn't it you got banned for posting those particular unicode characters a while back?

I don't think so

There's no shame in writing

I recall someone getting banned for it ...

@Blue ah yes, that rings a bell :-)

@Blue ask the room owner :P

chat.stackexchange.com/users/168812/arun-debray

what if weekends are not designed for rest? Why should 2 days of a week be designed as holidays? I recall in my childhood, we only had 1 and a half day in weekends for holidays--Saturday was still a half school day.

We had school on Saturday mornings as well, and lectures on Saturday morning at uni. But the youth of today are feckless layabouts who don't know they're born :-)

in my childhood, I mainly used weekends holidays to make up lost sleeping during school days because it's like I never slept enough on a school day; sometimes in school I was just too tired to open my eyes and had to hide myself somewhere to doze during course breaks.

@JohnRennie in university, only in the first year I had class on Saturday every two weeks, but it's a light class: English conversation.

I seldom get high scores in English Conversation (I do't know why) but I always get high scores in Englishig Reading and English Composition.

We know that in an atom, the electrons will have negative charge, the protons positive and the neutron won't have charge. Well, if we have the same number of protons and electrons, does that mean that the atom doesn't have charge (is neutral), and won't be attracted?

I've never heard of schools here having Saturday classes other than an occasional special thing. People do seem to spend half of their Sunday at church around here

What is weird to me is the megachurch that we’ve got around here @danielunderwood

I can understand going to a small church a lot better than a megachurch

they say religious devotion in the US is higher than compared to Europe

I think my town is small enough to not have megachurches here. But evidently the small churches get pretty political (people political, not political political). There are 4 or so churches within a mile of my house though

And of course the Sunday morning tv church

We do have some churches that are quite good for the community, though they seem to be matched by a similar number of wacko cult churches

@NovaliumCompany isn't it? atoms are meant to be neutral. Charged atoms are called ions. In my impression of chemistry the way of naming is this way, but I haven't studied chemistry for long. Also, my main knowledge of chemistry was gained in high school, which was not taught in English.

@danielunderwood From elementary to high school, I always had classes on Saturday morning and in university, I only had two-hour class English Conversation on Saturday morning every two weeks in the first year.

hmmm that's interesting. I usually just slept or worked Saturday mornings. I don't remember any classes on Saturday in college, though I think more people went to visit family on weekends a long time ago

Do you guys (I guess anyone outside of the US) refer to university as anything after high school or is it something distinct from college?

We have large universities and small colleges here, but anyone rarely makes the distinction

It’s irritating when I run into a piece of physics/math which I knew really well at one point but can’t recall properly now

In the present case, how to derive the way ladder operators work for angular momentum

@danielunderwood In high school, on Saturday every two weeks we had an exam "week exam", a big exam across the whole school designed to evaluate our study achievement.

@Semiclassical yeah I've written that up so many times :\

Last time I phrased most of it in pure Lie algebra terms and it made the most sense finally

Stoopid normalization factors

It's very irritating when I want to think, but my brain doesn't function so well, like after a meal.

@CaptainBohemian was that just your school or a typical thing in your country? We had standardized tests every year in primary school, but nothing standardized in high school aside from people who took SAT/ACT and our school having everyone take the pSAT

Who needs normalization?

Someone who’s trying to properly derive Clebsch-Gordan crapofficients

> crapofficients

slight frustration

lol

I mean, deriving it by hand for small spin isn’t hard

Are those normally covered in undergrad QM? We talked about them in an "introductory QM" that not everyone had to take, but not in our standard required QM course

but i'm trying to recollect how to do it for generic spin without looking it up

@danielunderwood The regulation that Saturday is a school/work day is across the whole country, as for that exam on Saturday most were across only my school but there were several ones which were across all high schools in the whole city--here high schools are government by the city's government they belong to. In primary school, our exams tend not to be important, But from middle to high school, they were very important because they were for preparation for entrance exam to high school and university.

Entrance exam to high school? Do you have to apply to high schools like you do to university?

but the regulation has changed to that Saturday is no longer a school/work day.

@danielunderwood there once was a distinction between universities and "expert high schools" (Fachhochschulen) here, where the latter were somewhat easier to get into and didn't offer doctorates, but this distinction is slowly eroding

I don't even remember an entrance exam for university. I guess we did submit SAT or ACT scores, but that was it

@danielunderwood you have to pass some entrance exam to enter high school.

Does that just determine which high school you go to or is it common for people not to go to high school?

The latter are also more Praxis-oriented, typically offering more engineering courses than pure science

like a tech school?

Would it be the equivalent of a community college here in the states?

Or yeah a tech school. Though some of our community colleges are referred to as tech schools, so I'm not sure if there's a difference

@danielunderwood it determines which high school you go, the higher score you get in entrance exam, the better high school you go.

most people go to high school, even they can't go to comprehensive high schools, they can go to vocational schools, some of which can be entered even you perform very poorly academically.

hmmm interesting. Most of the people here don't have multiple schools to go to unless they decide to/can afford to go to a private school. I was fortunate enough to be able to choose a free charter school, but I still don't think it was a great high school education

actually, the T in MIT does stand for tech :P

but vocational schools are not designed to prepare students to prepare for entrance exam for university.

Yeah there's Virginia Tech too

@CaptainBohemian what country are you in?

but due to there are many universities here, most people can go to university, though not always good university.

Does everyone in your country enter university on more or less the same level? I remember it being a bit of an issue when I started university since some of the entering students knew a bit of calculus and some didn't, so we couldn't do calclus-based physics for a while

@ACuriousMind why is it "eroding"?

there are more and more universities here, but most of them aren't really academic universities, such as those which have physics department. Most of them are transformed from technical colleges so they only have vocation-oriented departments.

@user1732 Because some of them now also offer doctorates and many universities are no longer so strict about their formal admission requirements - it used to be that certain kinds of qualifications qualified you only for the schools but not for a university

@danielunderwood Certainly not, but at least at my uni they weren't shy to set the bar high. If you hadn't seen a Taylor series before, tough luck - you had to learn it fast, then.

interesting erosion process

actually I don't know how to translate well in English those vocational schools following middle school or high school; college may not be a correct translation.

@ACuriousMind ours were pretty lenient during the first couple semesters. Though I was also ahead of a fair number of the others going in, so I may just not have experienced the difficulty. I think it actually got worse as time went on since some professors would assume we had mastery of prereqs and some seemed to assume we didn't learn them at all

@bolbteppa trying to decide what the best starting point is for such a derivation

in a textbook you'd usually start with orbital angular momentum $L_i = \epsilon_{ijk}x_j p_k$ and go from there

But I'd like to emphasize symmetry as much as possible

why don't textbooks emphasize symmetry as much?

because they're aiming for clarity, not elegance

(for instance, I definitely want to mention that $[L_i,A_j]=i\epsilon_{ijk}A_l$ should hold for any operator $A_j$ that transforms like a vector under rotations. but I'm forgetting how one motivates that)

isn't clarity easier to remember?

for undergrad student purposes, yes

@danielunderwood when I was in high school, we were taught calculus, but it's like afterwards calculus was not taught in high school, so when I was a master student serving as a teaching assistant for first year undergraduate students, the professor asked his students to ask calculus from me and my classmate who served as a teaching assistant for the same course as mine.

but i'm not an undergrad student anymore

and these notes are for me more than they are for someone else

are you going to post it on your blog?

wasn't planning to

which blog? is it on WordPress?

yeah. i haven't actually done any posts yet tho

One way to motivate the transformation law is like how you motivate tensor transformations the old way, a vector is any quantity that transforms like the 'position vector', similarly since we know the position vector satisfies the angular momentum commutation relations, a vector is any quantity that also satisfies the commutation relations

can WordPress be able to post math symbols? I have not posted anything in WordPost for long. It's like when I last posted something there was in 2012 or 2013.

But I have not gone into Clebsch-Gordan stuff tbh, it's group rep theory I've been meaning to do properly :(

@CaptainBohemian yeah. there's a some built-in latex commands but they're pretty annoying to use tbh

there's also plug-ins which work better for that purpose apparently

@bolbteppa gotcha.

@Semiclassical I haven't posted any technical stuffs there. I posted them in Facebook since I started to use it in 2013, but I found Facebook is a very bad place to post things containg math symbols

among my list of technical things I don't actually know how to show is the Clebsch-Gordan coefficient identity $\langle 00|j\,m_j\,j\,{-m_j}\rangle=\frac{(-1)^{j-m_j}}{\sqrt{2j+1}}$

which is such a simple little result, but it's one which is utterly awful to derive if you proceed as one usually does for C-G coefficients

i.e. start from the highest-weight state $|jj\rangle = |j_1\,j_1\rangle\otimes |j_2\,j_2\rangle$ with $J=j_1+j_2$, apply lowering operators repeatedly, then use orthogonality to get the state with $j=j_1+j_2-1=M$, etc.

the problem is that you'd get to the $|00\rangle$ state last in that progression

which...not a great idea

Landau gives that and basically says it's obvious from the normalization and the sequence of signs should be obvious :\

lol

also, I find citeulike can be posted blog with math symbols using some commands like here but some commands can't be rendered there.

He refers to some spinor formula as well so maybe that explains it

it should suffice to show that $\langle 00 | j\,m_j,j\,{-m_j}\rangle = -\langle 00 | j\,{m_j-1}, j\,{-m_j+1}\rangle$

man, I had forgotten how annoying the notation could be

(that one needs $m_1+m_2=0$ is just the requirement that $J_z|00\rangle = 0$)

which in turn makes me think I should look at $\langle 00 | J^-_1 J^+_2 |j\, m_j,j\,{-m_j}\rangle$

except that there's the stoopid normalization factor

@bolbteppa do you know which one that is?

I wouldn't do it with ladder operators

I think I understand the normalization when you write it out with wave functions since the sum is over 2j + 1 functions with opposite $m_j$'s and the $(-1)^?$ thing is supposed to follow from a general spinor formula I vaguely get

sure. the issue is showing that all the states in $|00\rangle$ are equally probable

once you've got that, then symmetry and normalization give you the actual coefficients

But it's not really obvious to me how to establish that

Ah, he says - "The coefficients in the sum must all have the same absolute value, since all values of the components $m$ of the angular momenta of the particles are equally probable"

...okay, but why? :/

Hmm, I guess - why wouldn't they be?

that's not an argument

I mean, it's not as though the various $m$ values represent different directions in space

for spin-0, for instance, it's clear enough to me why $(m_1,m_2)=(+1,-1)$ and $(-1,+1)$ should be equally probable: it just amounts to swapping the labels on the two particles

but why are they each equally as probable as $(m_1,m_2)=(0,0)$?

This is why I skipped this stuff :p

It's probably obvious from rotational invariance but I am not sure

@ACuriousMind what was the class size?

best algebraic starting point I can see is the ladder operator identity $J_1^+ J_2^- +J_1^- J_2^+ = 2(J^2-J_1^2-J_2^2)$

the nice thing there is that $(J_1^+ J_2^-+J_1^- J_2^+)|j\,j,j\,{-j}\rangle = J_1^- J_2^+ |j\,j,j\,{-j}\rangle$

so we have $$\langle 00|J_1^- J_2^+|j\,j,j\,{-j}\rangle = \langle 00|2(J^2-J_1^2-J_2^2)|j\,j,j\,{-j}\rangle = -4j(j+1)\langle 00|j\,j,j\,{-j}\rangle$$

looking up the normalization factors, one has $J^-|j j\rangle = \sqrt{2j}|j\,j-1\rangle$ and $J^+|j\,{-j}\rangle = \sqrt{2j}|j\,{-j+1}\rangle$

which would seemingly give $\langle 00 |j\,{j-1},j\,{-j+1}\rangle =-(j+1)\langle 00|j\,j,j\,{-j}\rangle$

Which is bad, since that factor of $j+1$ shouldn't be there :/

oh, i see it: should be $2(J^2-J_1^2-J_2^2-J_1^z J_2^z)$

correction to the correction: $2\vec{J}_1\cdot \vec{J}_2 = J_1^+ J_2^- + J_1^- J_2^+ + 2J_1^z J_2^z = J^2-J_1^2-J_2^2$

@Slereah lol such a clown had to look this up because a lot is riding on it! looks like they are indeed egyptian heiroglyphs of body parts but not specifically named in the unicode docs... maybe not added in rev1... who is the guilty party? guess youll just have to deal with the traditional alternatives :P en.wikipedia.org/wiki/Egyptian_Hieroglyphs_(Unicode_block) unicode.org/charts/PDF/U13000.pdf

dazeddigital.com/artsandculture/article/31372/1/…

@user1732 Between 5 and 600, depending on the course :P

@bolbteppa to be clear, I'd love an explanation like this

but it doesn't seem obvious at all

Understanding why they are equally probable on general principles seems to be really important since you need to use it when you go beyond two particles e.g. the whole 3-j symbol stuff

right

that was the other approach I was thinking to attempt

I'd say it's obvious but yeah I need to do some stuff before I could sit down with this properly

it's certainly an appealing property

There's the whole finding an irrep of direct product of irreps aspect to this which complicates it for me at least

yeah

you're trying to combine two identical irreps and get the trivial subrep

I can validate the result well enough by using ladder operator methods

might as well recurse that all the way back to 'my brain'

I don't know whether it has a defined name

I don't particularly care

Nah he doesn't use his brain. Citations end at his hands

it's close to, but not quite, the Physical Review citation style

ah, so one places the Heisenberg citation cut there

@Semiclassical not quite. it's a spinal-cord function.

@Semiclassical In group theoretic terms, the $\lvert 00\rangle$ state is the trivial subrepresentation of the tensor product of the two representations. When the two representations are on the same space,then the tensor product is the space of 2-tensors/matrices on that space, and the trivial subrep is the trace. And the trace weighs each basis element contributing to it equally.

@Chair if you mean for papers, though, then I do have a custom bibtex style file

initially produced using makebst but then subsequently modified to make it great

hmm

i remember too little of representation theory for that to be too helpful, alas

that's it in action

I can put it on github if you're interested

@EmilioPisanty now I'm disappointed they didn't / weren't able to call that fourth paper "ribbons of darkness", since that's the name of a song I know

@Semiclassical they make up for it in the sheer coolness of the paper

cool

dx.doi.org/10.1038/432165a

coooool

there's a ton more in subsequent work

for a good selection see refs. 12-15 of my recent preprint (link)

also references therin and thereto

particularly ref. 15

though I'm still not clear on why it hasn't appeared as a full publication

@Blue zenodo.org/record/1345139

Mathematica, for the overwhelming majority.

@Blue no

everything is in Mathematica except figure S1

which was made in Matlab

and figure 3a and 3b, which were made in Blender

and obviously the photographs in figure S5

it is... not an easy software tool to learn.

It can do 3d models and animations...I'll second that it's not easy to learn

I think it can do stuff with materials too, but I don't know how much people do that in blender vs photoshop and such

So I'm looking at PhD offers in France

One of them is by Rovelli

@Slereah wow! I have once checked his publications.

amubox.univ-amu.fr/index.php/s/ug34vNaz63VvKmX#pdfviewer

@Slereah wow

quite unfortunate that Lille is a wasteland for theoretical physics

as is LQG :p

apc.univ-paris7.fr/APC_CS/sites/default/files/…

Hm

This one looks interesting, but otoh he uses the (+---) metric

heathen

I can kind of read the start of that :p

or would that be heretic, hmm

The (+,-,-,-) metric is the Landau metric

But really anything to get back into physics

I'll even do GDP increasing research if I have to!

Is funding guaranteed

What about that list I sent a while back

By law, yes

That's why I never got a thesis

They can't let you work for free

I would go for the Sorbonne just for the history

Wait they want someone going into a PhD program to have solid knowledge of GR and LQG? Wow

@Blue I said research

@danielunderwood Well I'm 32 so by now that should work

though tbh my LQG is a bit shaky

s n a p

The R6 thing looks more my speed

Also it's not in Marseilles

Which is a bit far

@Slereah did you do undergrad and take a break?

@danielunderwood that break was called THE REAL WORLD

Wooo I have hope!

there is no hope

Hm, maybe I should look into the england physics stuff, too

It's not too far

o.O

O.o

@Slereah edpif.org/en/recrutement/prop.php

thx

though I did find that site

Someone from France going to school in England would be like me going to school in Ohio lol

I looked and thought that was the best page for France PhD's

I also found some cosmology/machine learning thesis

The competition is scary, let it be known

Which is probably a thing where I can boast some creds

Do people just apply for a thesis from the beginning there?

@bolbteppa Well it's not like I don't have a job in the meanwhile

32 is nothing as well

impossible is nothing

plz

I'm basically a skeleton now

me currently

№ 13 is clearly the best of them all there

and it's available

"Applicants for research must hold a Licence degree and Master's degree at Assez Bien or better, from a university or a Diplome d'Ingenieur degree from a Grande École."

I guess I won't be applying at the Imperial College of Science!

Those jerks

edpif.org/fr/recrutement/prop2018/Prop_LPTHE_ANTONIADIS.pdf

Saw that one yeah

putting it in the maybes

[Random]

::Explosion::

apc.univ-paris7.fr/APC_CS/fr/deep-learning-find-clusters

Also interesting

One you could chance sending an e-mail to and see if you get past the first stage and see if it can go somewhere and cancel if another is good

Well

First I'm gonna read the bibliography

Gotta do it srs

wait a moment...

That thesis is by Bartlett!

Wasn't he in that video

on that GR conference CD

The great thing about the one I linked to is it says "During the first months, the candidate should rather study the literature in this highly competitive subject, in order to improve her/his level and to be able to choose the precise research direction depending on personal interests"

Oh wait

I was thinking of Bartnik

Not Bartlett

why are all the GR thesis always in stuff I don't study

it's always CMB and neutrinos and early universe

LPTHE is right beside the Eiffel Tower too, well walking distance I think

@Slereah I have the same feeling, too. Most open GR positions are not so interesting.

I lived in Paris for like 3 years and still I've never went to the Eiffel tower

Hunk of junk

Is it part of the Sorbonne

All the cool GR stuff was mostly done in the 70's

maybe I should look in the math thesis, too

Physics-math sort of stuff

it does seem hard to find a research position these days if you're not within some obvious pipeline

If you're nearby could go in person which is a huge bonus

Well Paris it's doable to go in person

@Semiclassical what is an obvious pipeline?

@CaptainBohemian being in the school itself

And it's true

I've tried finding a thesis outside of a master degree

it's hard

or part of some collaboration, etc.

What is a thesis outside of a master degee?

what part do u not understand