The h Bar

12:05 AM

@KyleKanos :)

@0celo7 I noticed that your answers don't really get many upvotes, even the ones that are very well written. Why do you think it is like that?

0celo7

@Icosahedron Beats me.

Icosahedron

@0celo7 Either way you're not buying the books.

ACuriousMind

@0celo7 That's the curse of technical answers. Hawking's "The amount of sold books halves with every formula" applies well to answers here, in my observation.

Icosahedron

Though this is supposed to be a technical site.

ACuriousMind

12:19 AM

@Icosahedron I think the same :) In reality, most technical questions don't accrue many views (and consequently, many votes) because only a tiny fraction of the population is interested in them.

Marcel

@ACuriousMind: that German.SE post made my day :D

"geiel!"

Icosahedron

What happened to TheoreticalPhysics.SE, and what is the dispute with PhysicsOverflow?

ACuriousMind

@Icosahedron meta.physics.stackexchange.com/q/2609/50583 and meta.physics.stackexchange.com/q/6196/50583

@Marcel I immediately thought "Someone must've misheard geil" when reading the title. Nevertheless, the comments there are really funny^^

0celo7

My answers aren't even technical.

ACuriousMind

@0celo7 physics.stackexchange.com/a/173864/50583, physics.stackexchange.com/a/174990/50583, physics.stackexchange.com/a/173505/50583, physics.stackexchange.com/a/174197/50583

Icosahedron

12:28 AM

heh

ACuriousMind

All qualify as what I'd call technical, and they're 4 out of five I clicked: P

0celo7

Well I can cherry pick too.

Also physics.stackexchange.com/questions/174982/… got a lot of upvotes for me.

physics.stackexchange.com/questions/173859/… is a homework question.

ACuriousMind

I...really don't want to be belittling, but...six upvotes is not "a lot" :P

It's "decent", or something like that

0celo7

@ACuriousMind for me

Icosahedron

"Not bad for a high school student.. bla bla" etc

0celo7

12:31 AM

@Icosahedron ?

ACuriousMind

Also, you mostly answer GR and related stuff, which generally is "technical" in the sense that most people neither understand nor care what the question is about

Icosahedron

Nevermind.

0celo7

@Icosahedron Poe's law.

I have no clue what that was supposed to mean.

ACuriousMind

Loosely related to the topic: meta.physics.stackexchange.com/q/6549/50583

I'd really like to see more people post answers to that

If only so that more people have to battle with getting Chris' script to work :P

@0celo7 My uncharitable reading is that it was meant to imply that I look down on you because you're a high school student :P (which I don't)

0celo7

@ACuriousMind I took it to mean he thinks I'm fishing for compliments or something like that.

But I'm not sure if he's being sarcastic or what.

Icosahedron

12:40 AM

Do I really have to explain it

I've seen you say similar things before, not that it's bad or anything.

That comment was supposed to be sarcastic.

0celo7

I said that when I hit 3k rep.

Icosahedron

You were being sarcastic too.

Of course it's not bad, it's outstanding

(It was not a ridicule if that is what you thought)

0celo7

Assume that no one can tell sarcasm on the internet.

ACuriousMind

::recalibrates sarcasmometer::

0celo7

Don't bother.

There's a theorem which says that your calibration is wrong.

ACuriousMind

12:51 AM

@0celo7 :(

0celo7

@ACuriousMind The proof involves spin structures :P

ACuriousMind

@0celo7 Ah, because of the different spins that can be put on words? :D

2

0celo7

HA

Didn't even think of that

Marcel

I think there's a "no-go theorem" out there on that topic

usually that's a good bet ^^

ACuriousMind

@0celo7 I've got a deal with some fellow students where we pay a few cents for each such math/physics pun. We'll use all the money to eat/drink very well someday.

We'll feast like gods :D

0celo7

12:59 AM

:D

Icosahedron

@0celo7 You did not complete a study in electrodynamics?

0celo7

@Icosahedron Nope.

Icosahedron

I'm not entirely finished with Griffiths book, and I was thinking of starting Jackson to complete my knowledge before I do GR.

Are you saying that I shouldn't?

0celo7

I'm not sure what good it will do.

Icosahedron

You've finished griffiths level at least though?

Where are you at?

0celo7

1:02 AM

I know Maxwell's equations.

Uh

What else is there?

Icosahedron

All those stuff.

Boring stuff.

0celo7

@Icosahedron This.

Jackson is regarded as a very difficult textbook.

Marcel

@0celo7: solving poisson's equations with boundary constrains

Icosahedron

The problems I heard, the text itself is pretty accessible.

(I mean the first parts)

0celo7

I have little interest in using a lot of precious time reading electrodynamics.

Alfred Centauri

1:04 AM

@Icosahedron "to complete my knowledge"... heh

Icosahedron

heh

0celo7

I know the general concepts, specific results are not used frequently at all.

Icosahedron

I mean I want to be well rounded.

Marcel

I'd say some basic calculations of solutions in EM-dynamics is quite usefull

at least if you get in touch with green's functions etc. for the first time

Alfred Centauri

@Icosahedron no need to explain - at the ripe old age of (almost) 55, I hope to never complete my knowledge.

0celo7

1:06 AM

@Icosahedron How far are you in Shankar?

Icosahedron

Fair enough, I don't think it's possible to.

I didn't mean it that way.

@0celo7 Not very far.

I'm reading about topology from munkres book.

(I'm taking a course at uni now)

ACuriousMind

The worst thing that could happen to a scientist would be if there was nothing left to learn.

Alfred Centauri

@Icosahedron I know... thus the 'heh'. I can't imagine a greater hell than having completed one's knowledge. I hope to never stop learning something new.

@ACuriousMind I once asked a wise older philosopher who claimed his was the search for the ultimate truth "are you certain you want to discover it?"

Icosahedron

Turtles all the way. en.wikipedia.org/wiki/Turtles_all_the_way_down

ACuriousMind

@AlfredCentauri And what did he answer?

Marcel

1:12 AM

Zombie Kid Likes Turtles

►

Alfred Centauri

@ACuriousMind I believe his answer was "hmmm..."

ACuriousMind

@Icosahedron Nah, just Great A'Tuin swimming through space

Icosahedron

@AlfredCentauri You would not want to know the truth even at death?

I think that is reasonable. Though I may be wrong.

ACuriousMind

What is truth? What is love?

2

(Finally a chance to post that again :D)

Marcel

ding ding ding dong

Icosahedron

1:17 AM

@0celo7 I don't want to be be lost in this. I will have to quicken the pace.

0celo7

@Icosahedron Lost in what?

ACuriousMind

@Marcel Can't decide on the numbers of dings? :D

(Perhaps better than not being able to decide on the number of dongs...)

::hides::

Marcel

:D

Icosahedron

@Marcel looking at the edit history is funny.

Marcel

it's the groove

Icosahedron

1:20 AM

@0celo7 Lost in the topics that I'm studying.

ACuriousMind

@AlfredCentauri At least an honest answer

Marcel

[] dong

that edit block :/

ACuriousMind

This bar is beautifully weird tonight :)

Icosahedron

$\lim_{n \to []} ding = dong$

ACuriousMind

Your ding doesn't depend on n.

Alfred Centauri

1:27 AM

@ACuriousMind YouTube truth? Very well: You can run but you can't hide: youtube.com/watch?v=FZfVaL_faEQ

Icosahedron

@ACuriousMind well I tried.

$\lim_{ding \to \inf} ding = [] dong$

better?

0celo7

@Icosahedron inf?

what's the []?

also use \text or \mathrm

ACuriousMind

@AlfredCentauri From long before my time, but nice

@0celo7 Ahhh, let the typography nerd flow through you ;)

0celo7

@ACuriousMind I fantasize about sitting down and copying whole books just because they use some stupid notation or don't \mathrm.

ACuriousMind

@0celo7 I...can't say I don't do the same.

0celo7

1:38 AM

@ACuriousMind Just think how beautiful The Quantum Theory of Fields could be...

All three in one...nice modern TeX font...\mathrm...actually use $S$ for the action...

ACuriousMind

::drools::

Icosahedron

Springer doesn't seem to agree.

Marcel

:D ^this

Alfred Centauri

@0celo7 is there a Rosetta stone for that one? I find it baffling.

0celo7

@AlfredCentauri It doesn't help that he writes the dependence of every object on every variable.

The notation is very cluttered.

Alfred Centauri

1:42 AM

@0celo7 OK, good, so I'm not the only one that finds it distracting.

0celo7

@AlfredCentauri Have you looked at his book Cosmology? (Not to be confused with G&C.)

There are equations which span 6 lines.

Alfred Centauri

@0celo7 I haven't. Does it have similar flaws? Oh... I see your edit now. Dang.

@Icosahedron Do you mean, by truth, understanding Weinberg's notation?

Icosahedron

@AlfredCentauri I haven't read weinberg yet. Though in spivak's calculus on manifolds book (1971) in some proofs he has staircase equations.

Marcel

Weinberg has notation?

ACuriousMind

@0celo7 Nitpick: It's the Computer Modern font, not the "TeX font" :P

0celo7

1:48 AM

This is what comes to mind when I hear "Weinberg"

ACuriousMind

I can haz notation?

Icosahedron

How would you suggest that he formats it?

Marcel

actually he should use more spaces between symbols, this would make it .... longer

0celo7

Eq. (H.37) in Cosmology. (Yes, I have the print version too.)

Icosahedron

@0celo7 where do you buy all of your books?

ACuriousMind

1:50 AM

@0celo7 All these indices...arrrgh

0celo7

@Icosahedron Amazon.

@ACuriousMind Haha

Marcel

buy ... books?

Alfred Centauri

@0celo7 isn't there some way to just wrap all that into some nice component free bold faced thingy and call it a day?

0celo7

@ACuriousMind Check out Sect. 8.3 in Straumann. Derivation of the Kerr metric. It's a masterful combination of indices and coordinate free. He never uses indices unless he has to.

Then we have Wald...who managed to make a coordinate free notation with indices.

Icosahedron

How does Straumann's new book compare to his old one?

0celo7

1:54 AM

I only have the new one.

Icosahedron

print?

0celo7

Both.

Icosahedron

I can't find it online

It's the only GR book I can't find.

ACuriousMind

@0celo7 $*$ instead of $\star$...nggggh :D

0celo7

@ACuriousMind YES. $*$, $d$ and \var capital Greek letters are my biggest complaints about that book.

Marcel

1:56 AM

libge*n

0celo7

@ACuriousMind How do you define the Ricci tensor without referring to coordinates at all?

(Wald's notation is cheating.)

Marcel

maybe something similar to the gauss-curvature on a surface: product of eigenvalues of the weingarten map

0celo7

@Marcel I mean as the contraction of the Riemann tensor.

Icosahedron

What is meant by protected for "me too!" "thanks" responses?

Marcel

well this coordinate representation just is a way to express it

you can define the contraction of tensors without coordinates

0celo7

2:03 AM

How do you write that in index free notation?

ACuriousMind

@0celo7 Let the linear map $R(X,Y) : T_p M \to T_p M$ be the Riemann tensor that has eaten two vectors. Then, $\mathrm{Ric}(X,Y) := \mathrm{Tr}(R(X,Y))$.

Marcel

somthing like a trace

yeah @ACuriousMind

0celo7

@ACuriousMind Are you contracting the right indices there?

$$R(X,Y)\rightarrow R^i{}_{jkl}X^kY^l$$

You need to contract the $i$ and $k$ indices.

Also don't you need to introduce a basis to do contraction?

(The result is basis-independent of course.)

Marcel

trace is acutally a coordinate independent thing

you can define it by determinat-functions

0celo7

I know that it is independent, but how is it calculated without using coordinates?

Marcel

2:11 AM

let $\Delta(x_1,...,x_n)$ be a determinat functions (anti-symmetric and multi-linear)

let $\tau$ be a map from the vector space to itself

(a endomorphism)

than:

$\sum_i^n \Delta(x_1, ... , \tau(x_i), ..., x_n) = trace(\tau) \Delta(x_1, ... x_n)$

for any set of linear independant vectors $x_i$

0celo7

Aha!

You needed a set of vectors, i.e. a basis!

Marcel

no

actually any set will do

but let me think

0celo7

Yes, any set that can form a basis :P

@ACuriousMind I need your wisdom.

Marcel

no, sry. think i messed this up

this will hold for any set of $x_i$

think of $\Delta(x_1, ...,x_n)$ of a function

and of $\Delta(x_1, ... , \tau(x_i), ..., x_n)$

as another function

0celo7

Well any set that isn't parallel can be a basis.

Marcel

2:15 AM

trace is the constant that relates those two

independant of basis or whatever

0celo7

And if two are parallel, then it vanishes by antisymmetry.

I never said it was basis dependent.

Marcel

sure

0celo7

I just don't think it is possible to do without having a basis at some point during the calculation.

Marcel

still they are proportional by the trace

trace is this unique proportionaly constant of those two maps

ACuriousMind

@0celo7 Trace is (up to multiplicative constant) uniquely defined to be a map on endomorphisms that is: 1. additive for the product of matrices 2. linear for scalar multiplication 3. invariant under cyclic permuation

0celo7

2:19 AM

Additive for the product of matrices?

Marcel

the point is that on a vector space there are only determinat-functions that are proportional to each other

ACuriousMind

@0celo7 damn, additive for the sum of matrices

0celo7

Oh.

ACuriousMind

I.e. $\mathrm{Tr}(A+B) = \mathrm{Tr}(A)+\mathrm{Tr}(B)$

0celo7

Well that one I know :P

Marcel

2:21 AM

this proportionalty can be used to define coordinate independent constants for a map

like the trace and the actual determinat

0celo7

@ACuriousMind Ok, time for the weekly "clear up some more confusions" time

Marcel

i gotta go sleep

0celo7

Suppose we have a 2-dimensional manifold. Let's keep this simple.

Marcel

gn8

0celo7

night

Take a 2-form $\alpha$ and a 1-form $\beta$. Then $\alpha\wedge \beta=0$.

Suppose, however, we have the interior product $\iota_X$.

@ACuriousMind For instance, $\iota_X\alpha\wedge\beta\ne0$, right?

So can we exploit the antiderivation property of $\iota_X$ to move the interior product onto $\beta$?

ACuriousMind

2:26 AM

@0celo7 Yes, $\iota_X\alpha \wedge \beta = \alpha \wedge \iota_X\beta$

modulo a sign, possibly :P

0celo7

Yes, maybe I'm having a brain melt, but I don't see how that works out in coordinates :P

The minus was correct.

The $(-1)^p$ in the antiderivation is positive, but that term gets moved over, so it's a minus.

ACuriousMind

2:42 AM

@0celo7 I...see what you mean

One moment

Kyle Kanos

I believe I'm ready for the barrage of downvotes now.

santiago

2:59 AM

@KyleKanos i'd upvote it if I could

Kyle Kanos

@santiago Well as long as you've got 10 or 15 rep on the site, you can

santiago

@KyleKanos i have 11, let me try

nah, it says i need 15

excellent post though!

ACuriousMind

3:30 AM

@0celo7 This is the ugliest thing I've ever done, but it works out, if you use the Levi-Civita properties in 2D cleverly enough.

I don't recommend doing that, though. My calculation is a horrible mess.

@KyleKanos You are completely right. +1

ACuriousMind

3:45 AM

Well, sun's coming up, time to sleep...

9

NeuroFuzzy

@ACuriousMind what have you done?!

0celo7

3:58 AM

@ACuriousMind Hmm, I need some help. On the right we have $\alpha\wedge\iota_X \beta$. By definition, $\iota_X\beta=\beta_i X^i$. Thus $$(\iota_X\alpha)\wedge\beta=-\beta_i X^i\alpha$$

Apply this to the vectors $Y,Z$. Then $$(\iota_X\alpha)(Y)\beta(Z)-(\iota_X\alpha)(Z)\beta(Y)=-\beta_iX^i\alpha(Y,Z)$$ To me, this is obviously wrong. What am I doing wrong?

Stan Shunpike

Anyone played Far Cry 4?

Just played it and it was pretty awesome. The animal fighting is pretty realistic. They must have done research. Its pretty crazy.

NeuroFuzzy

5:06 AM

@StanShunpike interesting! I was going to buy it, I was a fan of far cry instincts back when I was in, like, middle school, and I'm a HUGE fan of the third one. But when I found out in the fourth one that you couldn't just walk to the top of the real mountains I was really disappointed...

Stan Shunpike

5:29 AM

@NeuroFuzzy its pretty awesome, despite that. But now you make me wanna go look at the other ones

I've never played it before. Are there animals in the other ones?

I still have a 360 but my brother upgraded to a One

NeuroFuzzy

6:02 AM

@StanShunpike Yes! Well, 12 year old me rates far cry instincts very highly :p youtube.com/watch?v=u6t557jZz0U

usukidoll

6:16 AM

@DavidZ yes I meant rho for matlab... and now I need to know what gamma, kappa, and sigma is for matlab too.

David Z

I don't know if Matlab has any special meaning for them

usukidoll

well I tried writing the words out just like in latex but uh my graph became weird. I'm supposed to replicate some graphs for my project

Stan Shunpike

6:34 AM

@NeuroFuzzy nice! Well, I'm convinced. I just bought a packaged deal. Fry Cry 3, Far Cry 2 and one other one all for $25

ACuriousMind

12:19 PM

@0celo7 Why would it be obviously wrong?

0celo7

12:36 PM

@ACuriousMind I obviously can't do it :P

Do I need one of those fancy identities?

ACuriousMind

No idea. I'm asking why it's "obviously wrong", I haven't checked it explicitly (since it would be correct by the antiderivative property of the interior product, I have no reason to believe it is false).

0celo7

What approach did you take?

I don't see how I can switch Y and Z in the first term and Z and Y in the second term.

ACuriousMind

You should replace some summations over indices with $\delta$s, replaces these with $\epsilon$s, and then combine these differently to get other kinds of $\delta$s.

If you choose the right way, it should come out, if you choose a wrong term, you just get back what you started from, it's maddening

And not enlightening at all

0celo7

Eww

Oh well, time to get some volunteer hours

ACuriousMind

@0celo7 What are "volunteer hours", exactly?

I mean, I gather that these are hours where you volunteer for something, but "getting" them implies you can do something with them.

Danu

2:56 PM

Hi peepz

ACuriousMind

Hey Danu

Danu

My brother was visiting for a few days so I haven't been on

ACuriousMind

@Danu I was wondering if you'd gone Phonon's way and disappeared ;)

Danu

NEVAR

Plus, the semester hasn't even started yet

also, I'll be having 8 AM classes errrryday. FML

ACuriousMind

Oh, that sucks.

Earliest here is 9am, and I'll be having nothing on Friday :)

Marcel

3:10 PM

are there really that good lectures that one would visited at 8/9 am? I think you are almost always better of with a book on selfstudy @Danu @ACuriousMind

Danu

@Marcel I'm not sure if I wanna follow any physics advice by you ;)

I might end up all classical and stuff

Anyways, I'm definitely gonna be attending all classes.

Marcel

as long there is no logic contradiction that leads you to classical stuff

i don't see the problem to that ;)

ACuriousMind

@Marcel Yes, there are such good lectures.

Marcel

I definitely have to switch universtiy I guess ^^

Danu

LMU is pretty nice, at least as far as learning stuff goes. Hate the (lack of) organization

Marcel

3:13 PM

lectures always feel like a compressed version of things i can read up in books with more detail

and if I have to read them up in the book anyway to get the full concept

what's the deal with lectures anyway?

Danu

Most lecturers don't follow books here.

So you'll never see the same stuff in a book---or at least not in a single one.

And I almost never study "on the side". Attending lectures is enough for me to get it (as well as doing the problem sets of course)

So, for me it's actually where I learn the subject lol

Marcel

hm. I agree with problem sets in math-lectures. but physics problem sets are 80% about: "can you push that whole letters around on 2 pages such that you don't make a mistake?"

@Danu do you know Prof. Dürr?

Danu

No, I don't. I think someone asked it before. Wasn't he one of these quantum foundation guys?

Marcel

yeah. Prof. Dürr is working at the institute of mathematical foundations of physics

(he's doing bohmian mechanics :-p )

Danu

@ACuriousMind Hey, that was my "obligatory link"

@Marcel That's probably why I haven't heard of him. I have no interest in that stuff.

Marcel

3:19 PM

:(((((

Danu

:))))

HDE 226868

Has anyone else had an issue like this:

Danu

What exactly is the problem?

HDE 226868

It's happened twice, then never again.

@Danu The active tab is not in order.

ACuriousMind

I don't get it

Danu

3:20 PM

Okay. It doesn't look like that for me.

I never use the active tab though :P

@Marcel I think it's a great way to be forgotten ;)

HDE 226868

@Danu I just went to physics.stackexchange.com. Clicking through the tabs fixed it.

Ah well, seems it's just my computer.

Danu

Hmkay...

Marcel

@Danu: which way?

ACuriousMind

@Danu Get a new one :P

Danu

4:03 PM

You already took all the other ones, too!

Danu

4:36 PM

@ACuriousMind How is it that the sun's coming up at 3:45 AM? o_0

ACuriousMind

4:49 PM

@Danu That was, uh 5:45 here, you know?

dmckee

Lots of good stuff at APS.

David Z

PSA: if you think a question is off topic and should be migrated to another site, please make sure to work the words "off topic" into your custom flag reason. (Standard off-topic close flags are also okay even if the question would fit on another site.)

innisfree

What's the policy on copying diagrams from papers of the arXiv (with citation)? I did it in my answer because the diagram is helpful but time-consuming to reproduce
http://physics.stackexchange.com/questions/175513/neutral-k-and-b-mesons-decay-to-2-photons

santiago

I can see why a lot of the homework questions are irritating

ACuriousMind

@DavidZ Uh...doesn't the custom close reason contain off-topic by default?

David Z

5:00 PM

@ACuriousMind nope

I'm talking about when you cast a custom flag for moderator attention

(I mean, not just you specifically, just anyone who does this)

ACuriousMind

Ah, I thought you were talking about the off-topic custom close reason

nvm, then

Danu

5:20 PM

@ACuriousMind I guess it's the time-stamps messing up.

@innisfree I recently saw a paper that straight-up copied 3-4 figures from a different one without citation, lol

ACuriousMind

@Danu The timestamps are UTC, and we are two hours different from UTC

No messing up involved

Danu

Not all the stamps though. The ones I'm looking at right now agree with my clocks.

That's why it confused me

ACuriousMind

Ah, those in the transcript are UTC

Those in ordinary chat are "real time"

Danu

Yeah, I know now ;)

I think it should be possible to change it, but maybe not useful :P

Icosahedron

8:20 PM

@ACuriousMind I am thinking of reading a math text alongside GR, would you recommend smooth manifolds by lee or riemannian geometry by do carmo? (or other?)

ACuriousMind

@Icosahedron Have you forgotten that I've read almost no books? ;)

Icosahedron

Then where do you get all of your knowledge from?

ACuriousMind

I'm having Deja-vu, but: From lectures, mainly.

Icosahedron

Deja-vu

innisfree

@acurious lectures and lecture notes on the web? or at a university?

ACuriousMind

8:35 PM

@innisfree At a university (Heidelberg, to be precise)

innisfree

they must have advanced courses - did you take courses from the math department on mathematical physics etc?

ACuriousMind

I did take a lot of math courses, but there isn't one on mathematical physics. Most of my lecturers in theoretical physics had a very mathematical inclination, though.

innisfree

is heidelberg considered the best university in germany?

ACuriousMind

In physics (and some other fields) along with Munich, yes

Though, for those with a more practical inclination, Aachen is a good choice, too

innisfree

interesting that it varies by field - in some countries, particular universities are considered the best, period.

ACuriousMind

8:46 PM

No, that's definitely not the case here. Though there is a formal title of being an "university of excellence" (which about a dozen unis carry, I think, but don't quote me on that), you might well find that, for some subjects, a university which is generally considered poor is outstanding there.

E.g. Heidelberg lacks most kinds of engineering degrees, and its sociology, economics and similar things are not considered the best, but for law, medicine, or physics, it is definitely among the best.

innisfree

have differences between east and west german universities survived?

ACuriousMind

No idea, really, but I don't feel as if there are many differences. The only thing I can think of is that you should go to the East if you want to study Russian ;)

innisfree

thanks for the info

HDE 226868

11:13 PM

Has anyone else noticed a lot of really poor posts today? I've flagged about 6 separate items.

^ Apologies to the mods for all the work, but the point of flagging is to make it easier for you guys, right?

ACuriousMind

I don't think today is any worse than usual

0celo7

11:34 PM

@ACuriousMind I need to log volunteer hours for the leadership seminar I'm attending.

Got 4 today, was pretty interesting.