The h Bar

NeuroFuzzy

18:05

@Slereah lol

Physics Meta

0

Q: Surge of reputaiton ... don't oppose but just why

I've just got a surge of 100 "zorkmids" on every of my accounts. I don't object, but I can't find any information why that happened. As to my experience any meta-question will be punished by lots of downvotes, please answer only via comments, so I can shed off the downvotes by simply deleting th...

discussion

0celo7

@Slereah can one do GR on Kahler manifolds

Slereah

who knows

Probably

0celo7

Well not all interesting manifolds are Kahler

0celo7

18:29

@Slereah you should find out what the condition is for a manifold to be Kahler

Slereah

"In mathematics and especially differential geometry, a Kähler manifold is a manifold with three mutually compatible structures; a complex structure, a Riemannian structure, and a symplectic structure."

0celo7

and then see if interesting GR manifolds satisfy it

Slereah

I guess it has to be even dimensional, for a start

0celo7

+1 GR

Slereah

19:12

Hey the Dolbeault operator

It was that kind of shit I read about when preparing for my thesis

That never came :,(

Qmechanic

19:25

I would appreciate it if someone third could explain-to-OP/mediate/step-in/vote-to-reopen/vote-to-close here.

ACuriousMind

@Qmechanic Commented with a link to a sufficient duplicate (which one could close with the same reason of "unphysical premise", but the actual question about the propagation of gravity doesn't actually rely on spontaneous creation/disappearence, I'd say)

Qmechanic

19:45

@ACuriousMind : Agree.

FenderLesPaul

@0celo7 wtf

6

I thought you were an aspiring physicist

6

why are you asking questions like that?

6

0celo7

20:08

wtf +5

@FenderLesPaul I have no clue what your "wtf" comment is about

also how did you get +5

@FenderLesPaul explain!!!

Stan Shunpike

@0celo7 what exactly is a "perturbation"?

0celo7

@StanShunpike depends on the system...but typically it's some sort of small variation of the parameters of the system

and perturbation theory is then seeing how the system behaves under such a change

Stan Shunpike

Is that a sufficient definition? So its just like a fluctuation in whatever parameters I am considering. So, for exampls, a system in thermo equilibrium with fixed energy could be "perturbed" by fluctuating the energy level by some small amount.

0celo7

20:23

@StanShunpike well...sure?

I've never seen thermodynamic perturbation theory before

Stan Shunpike

Lol

0celo7

but let's take something I do know

Stan Shunpike

Ok let me try a different example

0celo7

GR!

Stan Shunpike

LOL

Ok lets do gr

0celo7

20:24

we can study perturbations around minkowski space by writing $g=\eta+h$ and expanding the einstein equations in $h$

Stan Shunpike

so you are perturbing flatness then?

0celo7

we can also do it around some fixed curved background by writing $g=\bar g+h$

@StanShunpike we are making a small variation of the parameters around flatness

Stan Shunpike

Does $\eta$ have to be a tensor?

0celo7

in the GR context?

$\eta$ is the minkowski tensor

Stan Shunpike

Yeah

Oh lol better review havent done that since march

0celo7

20:26

you separate the metric into some "background part" and some "fluctuation" part $h$ and expand GR as a power series around the background in terms of $h$

Stan Shunpike

Ok

So then

Does h have to be a tensor?

0celo7

well...yes

how else will $g$ be a tensor

but that's hardly relevant

we could do this in QFT as well

or QM

Arnold does perturbation of classical mechanical systems as well

Stan Shunpike

Yeah so in Griffiths QM book, he describes on 224 a one dimensional infinite square well. He says $H^0\psi_n^0 = E^0\psi_n^0$ and suppose we have solved this TISE for that potential. He then suggests a new hamiltonian $H = H^0 +\lambda H'$, ie with perturbation. He then proceeds to write $E_n$ and $\psi_n$ as power series in $\lambda$. Why might power series be useful in a context like that?

@0celo7 ^

0celo7

because you can calculate each term in the power series but can't sum it

the sum is not possible in closed form

but you can approximate it

and there are various sum methods to get info from such series

Huy

just keep reading

0celo7

20:37

amazon.com/Advanced-Mathematical-Methods-Scientists-Engineers/…

@Huy huh?

Huy

@0celo7: iirc it gets pretty obvious why he writes it as power series

0celo7

am I correct or not

I don't have that book

Huy

maybe

what's h^{-1}(0)

0celo7

at least not in Munich

@Huy I have no clue. Seriously.

Huy

troll

I got the book for free

found it rather handy as a first intro

for a total noob

wtf man people who got their PhD are now doing AMAs and get 200+ upvotes

0celo7

20:42

@Huy can you please just explain what $h^{-1}(0)$ is

Huy

$h: X \to Y$, $h^{-1}(0) = \{x \in X| \, h(x) = 0\}$

0celo7

yeah no shit

Huy

well you couldn't

0celo7

but what's the answer to your question?

Huy

which one

0celo7

20:44

to write down $h^{-1}(0)$

Huy

I said "write it down as a set"

and for some reason you didn't

and said you didn't know how

and now you say no shit

-> troll

0celo7

7 hours ago, by 0celo7

@Huy $f^{-1}(A)=\{x\in X|f(x)\in A\}, f:X\to Y, A\subset Y$ IIRC

Seriously?

Huy

you didn't proceed

$h = f - g$ and $A = \{0\}$

0celo7

well I didn't know if that's what you wanted and you never said

Huy

cuz you seemed like you were just trolling

0celo7

20:47

@Huy are you serious

Huy

yes

0celo7

how do you do a reverse $\subset$

Huy

$\supset$

0celo7

ah, thnx

Huy

that actually makes sense

I find $\in$ vs $\ni$ funny

0celo7

20:48

yup

@Huy I knew that one

Huy

does this one work $\unlhd$

0celo7

wtf is that symbol

Huy

yeah no idea what that's supposed to stand for

normal subgroup

but idk why "unlhd"

0celo7

normal subgroup has no line

Huy

it's like $\subseteq$

idk how to do the one without the line tbh

0celo7

20:51

$\triangleleft$

Huy

Let $H \triangleleft G$ be a normal subgroup.

that just looks wrong

Let $H \unlhd G$ be a normal subgroup.

the triangle should be the same size as in the second one.

0celo7

are you studying lie groups

Huy

sometimes, yes

do you know this one

$\rtimes$

0celo7

yes

Huy

do you use that in physics

or why do you know it

I haven't used it since my algebra course

only now in Lie groups again

0celo7

20:56

the Poincare group is the semidirect product of the Lorentz group and translations

Huy

ok

do you know a lot of group theory

or just like what they are

0celo7

@Huy no, but I want to

Huy

ok

0celo7

@Huy pretty much

Huy

we can study together

0celo7

20:57

no?

Huy

I'll restudy algebra at some point

0celo7

you call me a troll and refuse to play GTA with me

Huy

why you no like me?

0celo7

why would I help you

Huy

well yeah after the Steam disaster yesterday

and I think everyone in this room would agree with me that you are a troll

0celo7

20:58

what steam disaster?

FenderLesPaul

I disagree

Huy

nobody asked you

@0celo7 reddit.com/r/Steam/comments/3y7r0b/…

0celo7

issue has been resolved

I just turned on Steam

am I screwed?

Huy

so what

anyways

algebra is cool

but I don't understand it

FenderLesPaul

nah

Huy

21:00

nobody asked you

@0celo7 it was supposed to help you btw

0celo7

@FenderLesPaul so what did I do that's so bad that it warrants 5 stars

fine, ignore me

whatever

Huy

mad

are you Ghost

yuggib

22:53

@0celo7 yes they do if you are logged in...the discount is already there in the shopping cart

FenderLesPaul

23:08

@0celo7 nuthin

foeggabout it

0537

23:32

it's kind of difficult to forget when it's 5 starred on the side.

lol.

Joshua Lin

http://i.imgur.com/j4KkZCj.png
Why is there so much hate against John Duffield?
I don't really understand anything about the topics he answers about (quantum mechanics or general relativity), but in my non-knowledgeable eyes he usually provides answers directly to the question (whereas other people seem to skirt over some issues or provide half answers). Does he get downvoted so much because he is wrong all the time? (I'm not sure if he is or not anymore)

FenderLesPaul

He's flat out wrong

about most of the things he answers

Joshua Lin

oh ok

FenderLesPaul

not all but more than a fair amount

Transcript for