The h Bar

12:17 AM

@DanielSank I keep missing you... I have a question, how do I determine the timescale of the Lindblad Equation in the problem? like in my plot, in the time axis, how do I figure out the units, like if it is femtoseconds or picseconds for example? I wasn't sure to ask it on Physics.SE because i thought maybe it was too trivial of a question or it was to broad...

also, can anybody help me understand Christoffel symbols and covariant derivatives?

where is @ACuriousMind ? I bet he could help...@0celo7 could you help?

Steven Stewart-Gallus

1:01 AM

Given that the gravity inside in an uniformly dense shell is zero would we notice if the universe was surrounded with a giant ball of dark matter?

DanielSank

1:28 AM

@TanMath probably. What do you want to know?

3075

1:59 AM

Bad news guys I'm in the hospital because my left lung collapsed. :'(

0celo7

@ACuriousMind He's on vacation.

@TanMath Why would I be able to help you?

@3075 that sucks.

TanMath

@DanielSank I don't understand the concept at all...

I understand they are correction terms for the covariant derivative..

@0celo7 because you are a big relativity expert, aren't you?

Along with @ACuriousMind

You guys are always talking about general relativity...or so I thought...

3075

2:22 AM

@0celo7 It occurred randomly when I was brushing my teeth at 7:30 am.

It felt like a sword went through the left side of my chest.

Feels bad... :'''(

Bernard Meurer

@3075 WAT

THE FUCK

DOOD

0celo7

@3075 I told you to stop smoking crack...

Bernard Meurer

@3075 How hardcore are you at brushing teeth?

@3075 Are you alright?

0celo7

I know I like rap and expensive shoes but you didn't have to go full G on us...

3075

@0celo7 Jokes aside I may have to do a minor surgery to fix it if it doesn't fix itself in 3 days.

0celo7

2:26 AM

@TanMath @ACuriousMind knows literally nothing about GR

But I can tell you about Christoffel symbols...

3075

@BernardMeurer I'm ok, it feels a bit better but I have to spend 3 days and nights in the hospital.

Bernard Meurer

@0celo7 Can you tell me abotu Jacobi symbol?

0celo7

@BernardMeurer Is that related to the Jacobi theta function?

Bernard Meurer

@3075 Mother of Schrödinger

@0celo7 IDK it just says jacobi symbol

0celo7

@BernardMeurer I only know a little number theory

And I don't plan to take the grad course on it

Bernard Meurer

2:29 AM

@0celo7 dang it

3075

I'm working through David tong's electrodynamics notes while lying down on the hospital bed.

I should be done in 3 days

Since I have all day

TanMath

@0celo7 seriously? Then what do you guys talk about?

0celo7

@TanMath not GR

What do you want to understand

TanMath

What is its purpose, how it works in covariant derivatives, and how to do covariant derivatives...

3075

@TanMath every GR book should explain that

TanMath

2:34 AM

@3075 I know, but I am not understanding it...

it got a little too much for me...

3075

I would help you but I forgot all the GR I knew

Refer to my profile description for details

TanMath

It is so easy to forget GR...

@3075 I noticed.

0celo7

@TanMath well how are you defining the connection

3075

Yeah I only learned up to deriving the Einstein field equations in the course I took

Forgot it though

0celo7

are you defining it by a horizontal distribution (hopefully not) or by a special derivation of the vector bundle?

3075

2:37 AM

@0celo7 I learned covariant derivatives without knowing about those words

TanMath

@0celo7 wat????

0celo7

just tell me how you're defining the connection

/covariant derivative

I can start rambling if you want...

TanMath

You can start rambling...

0celo7

It would help if you told me how you're defining stuff, dude

TanMath

I just know there should be a correction since we need to differentiate the basis vector...

Bernard Meurer

2:41 AM

wtf are y'all going on about

0celo7

I think that's an unhelpful way of thinking about things

It's too analytic

Not very geometric

user54412

@BernardMeurer unicode. &#)@#* unicode

0celo7

Of course the most geometric picture requires tons of bundle crap

Bernard Meurer

@ChrisWhite Lol, you getting some good encoding love?

0celo7

@ChrisWhite why is your name in italics

user54412

2:42 AM

good question

Bernard Meurer

Because he's a room admin

0celo7

@ChrisWhite lol you have no power

you're like @Danu

3075

@TanMath basically taking the partial derivative of a tensor does not give you a tensor, so you need the partial derivative plus some correction term to make it a tensor and the correction is he Christoffel connection term

0celo7

ahhh

only a physicist thinks like that

it's not really clear why that should work

3075

The correction term is basically a linear transformation

0celo7

2:44 AM

@3075 yes that's true

it's a bundle endomorphism-valued form

Bernard Meurer

@ChrisWhite I bloody hate unicode too, so let's rejoice in our hatred

3075

Cool I'm remembering stuff but I'm on my phone so no formulas sorry @TanMath

0celo7

@TanMath Ok, let's do this

user54412

@BernardMeurer I realized that innocent, hypen-looking thing in my .tex file was something more sinister when it started rendering as cthulhu-speak in my pdf.

Bernard Meurer

LOL

0celo7

2:46 AM

So the covariant derivative/connection is a generalization of the directional derivative on $\mathbb R^n$

3075

Oh yeah

Bernard Meurer

@ChrisWhite When you realize all your semicolons are actually a weird greek char because unicode just played ya

0celo7

The directional derivative is an operator $\nabla _v$ associated to a vector $v$ that is basically the gradient dotted with $v$

now you can differentiate all sorts of stuff easily

user54412

@BernardMeurer I think I'll change my vimrc to do something when it sees anything outside \x00-\x7f -- maybe I can get it flash red and set off alarm bells.

0celo7

this is because the directional derivative of the unit vectors is zero (you already pointed this out)

DanielSank

2:48 AM

Hi everybody

0celo7

@DanielSank I'm trying to be pedagogic

Bernard Meurer

@ChrisWhite If I recall correctly there's a script that fixes that

You could even make on in Python with relative ease

0celo7

@TanMath So here is how I like to think of covariant differentiation

Bernard Meurer

But that's a cool .vimrc addition, if you do implement it send it to me :)

0celo7

the fundamental concept is parallel transport

3075

2:49 AM

@0celo7 I don't need rigorous gr for st right?

0celo7

no, what kind of question is that?

Bernard Meurer

@0celo7 Geodesics have that!

I remember this

0celo7

@BernardMeurer yes my child

@TanMath are you paying attention

Bernard Meurer

@DanielSank C.W. Is getting the unicode love

3075

Did you guys see the new Dell xps laptop because it has a cool screen.

user54412

2:51 AM

^ I'm guessing it's blue

Bernard Meurer

Lol

0celo7

@ChrisWhite why would you think that

Bernard Meurer

@3075 Nope, link plox

3075

Lol I wish

TanMath

@0celo7 I was gone for a bit...

Bernard Meurer

2:51 AM

@0celo7 Because @3075 loves blue

3075

Yeah

Bernard Meurer

EVEN HIS SHOES ARE BLUE

0celo7

@BernardMeurer huh

I missed that

TanMath

So I understand all this...

0celo7

@TanMath good

TanMath

2:52 AM

I just don't get how the christoffel symbols were derived...

3075

@BernardMeurer my whole wardrobe is blue with no exception

0celo7

Yes, wait a little.

Bernard Meurer

@3075 PICS

0celo7

@TanMath Wait...are you talking about the Riemannian stuff?

Bernard Meurer

@3075 DOUBT IT

Not even I'm that obsessed

and I am

very peculiar

3075

2:53 AM

It's simple, I wear the same blue pants everyday with blue socks, blue shoes and one of 5 different blue shirts

It's a small wardrobe but it works

0celo7

Are you asking why $\Gamma^i{}_{jk}=\frac{1}{2}g^{il}(\partial_jg_{kl}+\partial_j g_{kl}-\partial_lg_{jk})$?

Bernard Meurer

@3075 How many equal pants have you got?

0celo7

or why $\Gamma$ is even there

DanielSank

@BernardMeurer what's cw?

3075

@BernardMeurer only one but I wash it every weekend

Bernard Meurer

2:55 AM

@DanielSank It's this weird creature they make at Princeton. I've hear they feed exclusively on chocolate covered coffee beans

@3075 EVERY WEEKEND?! And you wear that every day??

DanielSank

@BernardMeurer wat?

Bernard Meurer

@DanielSank It's @ChrisWhite :p

3075

@BernardMeurer yeah

Bernard Meurer

@3075 Dude, when's your birthday? I'm getting you at least one more pair of blue pants

0celo7

@3075 dude I bought some sweet blue shoes the other day

They might be the best shoes I've ever had

So comfy

3075

2:57 AM

@BernardMeurer January, and come to waterloo first xD

@0celo7 link?

Bernard Meurer

@3075 WORKING ON IT

@3075 Working real hard :'(

0celo7

@3075 footlocker.com/product/model:240714/sku:24010401/…

TanMath

@0celo7 I didn't learn riemannian stuff yet...

user54412

@BernardMeurer I have a kilo of them sitting next to me, and a dissertation to write.

0celo7

ok, @TanMath

Bernard Meurer

2:58 AM

I like the blue ones I just bought

0celo7

Writing an essay, one sec.

Bernard Meurer

@ChrisWhite Mail me some man, don't be greedy

TanMath

@DanielSank so how do I figure out the tinescale?

3075

The doctor said my lung will probably collapse again in the future. So I'm really sad now.

Bernard Meurer

They're hard to find down here

3075

2:59 AM

@0celo7 awesome I like them

Bernard Meurer

3075

You guys are really cool with your blue shoes.

0celo7

@TanMath Let $\pi:E\to M$ be a vector bundle with fiber $V$, $\mathrm{dim}\,M=d,\mathrm{dim}\,V=n$. Then a connection is a map $D:\Gamma(TM)\times\Gamma (E)\to \Gamma(E), (V,\sigma)\mapsto D_V\sigma$ with the following properties: (i) $D$ is tensorial in $V$, (ii) $D$ is $\mathbb R$-linear in $\sigma$, (iii) product rule $D_V(f\sigma)= V(f)\sigma+fD_V\sigma,f\in C^\infty(M)$. Let $x\in M$ and let $U$ be a chart domain for $M$ and thus for $E$. Thus we have coordinate vector fields ...

$\partial_1,\dotsc,\partial_d$ on $U$ and using $E|_U\cong U\times V$ we get a basis $\mu_1,\dotsc,\mu_n$ of sections of $E|_U$.

We then define the Christoffel symbols by $D_{\partial_i}\mu_j=\Gamma^k{}_{ij}\mu_k$.

So let's calculate a covariant derivative.

Bernard Meurer

@0celo7 WTF is a slushie

3075

@BernardMeurer you don't know?

Bernard Meurer

3:07 AM

@3075 No? Why'd I ask if I did :)

3075

It's a drink with ice blended with colouring and flavour

Bernard Meurer

@3075 Is it good?

3075

Depends

It can be, but it's bad for you duh

0celo7

Let $\mu\in\Gamma(E)$. Then, locally, $\mu=a^k\mu_k$ and $$D_{\partial_i}\mu=D_{\partial_i}(a^k\mu_k)=\partial_i(a^k)\mu_k+a^k\Gamma^{l}{‌}_{ik}\mu_l=(\partial_i a^k+\Gamma^{k}{}_{il}a^l)\mu_k$$

3075

The old people in my room are snoring in sync it's harmonious.

0celo7

3:11 AM

@TanMath does that make sense

@TanMath Also note that $(\Gamma^k{}_{ij})\in\Gamma(\mathfrak{gl}(n)\otimes T^*M|_U)$

i.e. it's a matrix-valued one-form

kevin Tah N.

omg the shoe talk was quite serious

Bernard Meurer

@3075 Do you like it?

3075

Like what?

Bernard Meurer

@3075 Lol, I don't snore

Slushies

3075

I like the blue slushies

Bernard Meurer

3:25 AM

You gotta be shitting

0celo7

@BernardMeurer you don't know what a slushie is?

Bernard Meurer

@0celo7 Now I do, it's a colored drink apparently

0celo7

sadly @TanMath did not like my explanation

@BernardMeurer no it's syrup over crushed ice

Bernard Meurer

@0celo7 You can be a tough teach sometimes

Like with R. and you teaching German

0celo7

dude

Bernard Meurer

3:26 AM

@0celo7 That sounds nasty

0celo7

no like a starbucks syrup

not maple

Bernard Meurer

Starbucks syrup? Wat

I need a north america culture immersion

0celo7

like sugar water

condensed sugar water

Bernard Meurer

Still sounds nasty

3075

It can be syrup or powder

0celo7

3:29 AM

powder?

that's some Candian thing

3075

It's delicious if it's blue

@0celo7 like coloured powder with flavour and sugar in it mixed with ice

Bernard Meurer

Apparently if you burn propolis to a right degree it makes a nice, lasting, honey smell

3075

It will look the same as syrup

Bernard Meurer

Just found that out

because I thought burning it was a good idea

0celo7

@BernardMeurer How many times I said how many times?

Bernard Meurer

3:31 AM

@0celo7 2 now

0celo7

Have you not checked your snap?

Bernard Meurer

@0celo7 Let me

@0celo7 3 times

DanielSank

@TanMath write the equations of motion in a dimensionless form.

0celo7

@DanielSank but $c=G=\hbar=k=1=m=q$

Bernard Meurer

@0celo7 Wat

0celo7

3:37 AM

what is unclear my Brazilian friend

TanMath

@0celo7 not at all!

@DanielSank huh?

Bernard Meurer

@0celo7
1. What are those
2. Why would you have so many different variables if they're all equal
3. Wat

0celo7

@TanMath :(

What is unclear

Bernard Meurer

3 because the function wat(x) is always an unbounded recursion

0celo7

@BernardMeurer 1. physical constants + mass and charge 2. there's lots of physical constants 3. you set them equal to get rid of pesky shit in your equations

Bernard Meurer

3:38 AM

@0celo7 "you set them equal" But can you just set stuff like that? Ain't that cheating

I set my GPA to 4

0celo7

I think @DanielSank thinks it's cheating

Bernard Meurer

Wouldn't that be sweet

3075

Lol

0celo7

It's fine if you know how to dimensional analysis in the end

@BernardMeurer I set mine to 4...oh wait

Bernard Meurer

@0celo7 It's already 4?

3075

3:39 AM

@BernardMeurer but everyone else's GPA scales as well

0celo7

:)

Bernard Meurer

@3075 Ah, lol, so it's pesky cheating

TanMath

@0celo7 everything!

like what is a vector bundle?

math.haifa.ac.il/ROVENSKI/rovenski/McM.pdf

chapter 4...

This is what I am reading...

0celo7

@TanMath A fiber bundle with each fiber a vector space.

Bernard Meurer

@0celo7 What's a fiber?

0celo7

3:41 AM

@BernardMeurer Inverse image of the projection.

Bernard Meurer

@0celo7 Yeah, alright, let's leave this one for a couple years in the future

0celo7

@BernardMeurer I'm not trying to be an ass, that's what it really is.

Bernard Meurer

@0celo7 I know you're not! It's just that when the explanation of something requires an explanation it throws a stackoverflow in my head

0celo7

@BernardMeurer Let me find a picture...

OFF TO THE PDF FOLDER

Bernard Meurer

Lol

0celo7

3:44 AM

Internet points if anyone can name the book...

@BernardMeurer so $B$ is the fiber bundle over the manifold $M$

$x$ is a point in $M$

Bredon?

no

@0celo7 you so funny

@0celo7 Is $B$ the area described by that rectangle?

Wait, the manifold is just that line?

0celo7

@BernardMeurer the fiber bundle is equipped with a map $\pi$, the projection, that maps a point $p$ in $B$ into a point in $M$

@BernardMeurer yes on both counts

@BernardMeurer so the fiber over $x$ is all $p\in B$ that get projected to $x\in M$

Bernard Meurer

3:47 AM

@0celo7 So the fiber bundle $B$ Will map a point in itself into another point in $M$?

Does every point in $B$ only have one mapping onto $M$?

0celo7

hmm?

One mapping?

Bernard Meurer

@0celo7 Like, one point in $B$ will yield one point in $M$, and not more than one ever

0celo7

@BernardMeurer well yes that's what a function does

Ok imagine a vector bundle like this

Let $M$ be a manifold, like a circle

Bernard Meurer

Wait, I hadn't read your message about the fiber over $x$, I get it now

Alright

0celo7

@BernardMeurer I have a good example

Let's look at the trivial line bundle over the circle, it's easy to visualize.

Bernard Meurer

3:50 AM

Note: That propolis smell thing will actually give you a bad headache after a while

0celo7

Take a circle, and attach a real line at every point of the circle

What do you get?

Bernard Meurer

@0celo7 Idk how to name this, it looks like the 2D projection of a cylinder in my head

0celo7

Yes, it's a cylinder

Bernard Meurer

Damn, I got my brain vision on point

0celo7

So the fiber over one of the points of the circle is just the line at that point

Because the projection maps all of the lines into the point they're attached at

Bernard Meurer

3:52 AM

Processing...

Ah

I think I get it

3075

I'm drifting to sleep because of these meds, night guys

Bernard Meurer

@3075 Night man, get better!

0celo7

dude get some codeine cough syrup

look up "lean"

check Soulja Boy's snap if you need info

Bernard Meurer

@0celo7 NSA HALP

0celo7

...what?

DeNiSkA

4:23 AM

@BernardMeurer do you know what is the expression for changing data type of a variable? (in C)

0celo7

@yuggib up early?

Bernard Meurer

@DeNiSkA You can't change the type of a variable per-se you can cast one variable into another one by doing something like this int i = 10;float foo = (float) i;`

So we're casting the variable i of type int into the variable foo of type float

yuggib

@0celo7 yep; I have to go to work and I have a three hours' train ride

Bernard Meurer

of course this will yield data loss in many cases, such as converting a float to an int

DeNiSkA

i see!

thanks @ber

Bernard Meurer

4:28 AM

@DeNiSkA No worries mate

0celo7

@BernardMeurer my car wasn't making that weird noise today

Bernard Meurer

@0celo7 that's always good

vzn

5:36 AM

@DanielSank your latest meta post has zero "feedback" by anyone other than me outside of votes. and you say people "care"? there is not much sign of that in meta so far after 2 posts, and significant pushback by EP. have participated substantially on meta in other SEs and this latest experience is not uncommon for me, nor do think it is atypical of metas in general... :|

@DanielSank think there is a big difference between feedback & criticism. this "project" is in very early/ "embryonic" stage and its not fair/ realistic to ask more of it than is feasible/ practical & so far there are very few volunteers etc. lets keep this all in perspective; there is no record of any invited guests anywhere on any SE afaik...

@DanielSank we have different ideas of what constitutes "increments" eh?

Bernard Meurer

> your latest meta post has zero "feedback" by anyone other than me outside of votes. and you say people "care"?

Dude, you're being kind of a dick

@vzn

vzn

5:54 AM

← speechless

DanielSank

6:30 AM

@vzn I don't know what you're talking about. There are 19 votes on that post already.

@BernardMeurer <3

Bernard Meurer

@DanielSank <3

@DanielSank I started getting cappy working on Py2 & 3

DanielSank

@BernardMeurer Sweet.

Bernard Meurer

but then I remembered I had popcorn and I got distracted

So I'll be done tomorrow

DanielSank

Every time I talk to you you have some particular food.

dmckee

7:42 AM

Comment not posted:

> ::Gets out green grading pen and carefully marks off 1 point for failure to use units throughout the work::

Slereah

8:04 AM

Hey hey

Slereah

8:23 AM

0

Q: Questions for the june 14th AMA

Hey I'm the scheduled guest for the june 14th Physics AMA and I was told to maybe make a little presentation and asks for any questions for the AMA people might have, so here it is. So I am Samuel Lereah, got a Master degree in particle physics from the university of Nantes. My master thesis was...

discussion

there

Physics Meta

0

Q: Questions for the june 14th AMA

Hey I'm the scheduled guest for the june 14th Physics AMA and I was told to maybe make a little presentation and asks for any questions for the AMA people might have, so here it is. So I am Samuel Lereah, got a Master degree in particle physics from the university of Nantes. My master thesis was...

discussion

yuggib

it is not so clear to me what qualifies people to be suitable to this AMA experiment

Slereah

well in my case i was the only one to volunteer

yuggib

I see...you're the guinea pig

Slereah

Plus I don't think we can really get big name scientists right now :p

yuggib

8:37 AM

well, there are very few and not so active big name scientists on the whole PSE

Slereah

Not quite sure how to do the presentation, tho

Dunno the level of the audience

I should probably keep it somewhat informal

Slereah

8:49 AM

@yuggib would you like to volunteer too? :p

yuggib

I have nothing interesting to say

;-P

Slereah

Well what do you work on

FrancescoS

Hi everyone!

yuggib

mostly proving how things really work when physicists wave their hands babbling about intuition

but that's not interesting to physicists

Slereah

It is pretty interesting!

Anything in particular?

yuggib

9:00 AM

sometimes I have the immense pleasure of proving their intuition wrong :-D

but that happens only sometimes...

FrancescoS

When you work with Heavy quark effective field theories, why do you calculate loops diagrams without traces and gamma matrices? Look here arxiv.org/abs/hep-ph/0307297 pag. 18 for example

Slereah

which equation?

FrancescoS

for example, 3.7

Slereah

Hm

dunno

Is the diagram just two quarks?

When I did effective field theory it was directly with mesons

FrancescoS

In the diagram the double line is the heavy quark i suppose

Slereah

9:08 AM

Well the Dirac propagator is something like the scalar propagator * some other term

Maybe in the heavy mass limit you can neglect the second term?

$$\tilde{S}_F(p) = \frac{(\gamma^\mu p_\mu + m)}{p^2 - m^2 + i \varepsilon}. $$

FrancescoS

This for sure, you neglect the gamma matrices in the numerator, and this is ok. But the trace? There is no constant term arising from this approssimation?

Slereah

For $m \gg 1$ you have $\gamma^\mu p_\mu + m \approx m$, I suppose

FrancescoS

@Slereah yes i know, but do i have to forget also typical feynman rules for fermion loops? Like the minus sign for each loops and the trace?

Slereah

Not a clue

DeNiSkA

9:47 AM

@ber How did you learn MySQL ?(i am not able to get some good resource)

ACuriousMind

@dmckee Green is too friendly. Red. Grading has to be done in dark, ominous red.

@FrancescoS Read page 17 and 18. They a) write the HQET Lagrangian without the $\gamma$ to begin with and b) explicitly say that the $\gamma$ is replaced by $v$.

yuggib

::moment of self-advertisement:: I like my answer here. It seems a rather elegant solution...

FrancescoS

@ACuriousMind Hi! Yes, I understood this. So, I can forget any traces and minus signs?

ACuriousMind

@FrancescoS Well, the traces are over the gamma matrices, if there are no gamma matrices, I don't see what there is to trace over...

I have no clue about HQET, I just looked at the paper

ACuriousMind

10:08 AM

Why are people so often unable to write "Schrödinger", but then write "Nöther"???

FrancescoS

DO you mean Nöether?

I don't know

ACuriousMind

Well, Nöether is equally wrong, but no, people really write Nöther instead of Noether.

yuggib

And you don't like Schroedinger?

or do people write Schrodinger?

ACuriousMind

Well, if you don't have an 'ö' on your keyboard, Schroedinger is fine. It's 'Schrodinger' that annoys me.

yuggib

I see

I hadn't find it so often

and, well, only germans have ¨ characters on their keyboards, it does not mean those characters are not easily accessible with a couple of keystrokes ;-P

ACuriousMind

10:16 AM

Well, but not everyone knows how to get those

I, for one, have no idea how you generated the dots without a sign underneath them :P

yuggib

:-D

in an american (mac) keyboard, it's alt+u

on windows/linux, no idea

ACuriousMind

Yeah, unsurprisingly, that doesn't work on Windows

Slereah

10:51 AM