12:00 AM
@DanielSank: The problem with modern physics are the people who don't speak our lingo. They take things that we don't mean literally... well... literally.

@CuriousOne Like what
I am the least literal person alive
literally :D

@DanielSank: Even I still talk about quanta as if they were particles... even while I know that it's wrong.
@DanielSank: But I can't help it... my electron gun is a pea shooter, not an"electrically charged fermionic quantum wave source". :-)

12:51 AM
@Ocelo7 And, how do you do ?

interesting. I know what Euler classes are, but only defined on simple vector bundles.

The construction is hairy
Working on a circle bundle for now

I believe.

What definition do you know?

12:56 AM
One dimension lower-huh ? ;)

hmm?
Sphere as in $S^n$

Yeah, because you are working on $S^1$ instead of $S^2$.
I know the definition of Euler class on an n-dimensional real vector bundle.

in terms of curvature?

and the using in differential geometry for defining the Euler-class
I did not learn about curvatures of bundles yet.
Or did you not mean that ?

well then how are you defining it?

1:04 AM
Im not familiar with the PSE notation, but I am defining it as the element of the vecbundle mapped from a section of the bundle. Said in bad english.
Or, I am defining the Euler class that way.

what

the Euler class

I have no idea what you mean by what you just said

My english is bad, but I try:
Let V be an n-dimensional real vector bundle E---->X with a not negilible section s:Y ------>E on a (possibly empty) subset Y (subset of) X. Now we map an element to it e(E,s) (element of) H^n (X,Y, Z) (or e(E) (element of) H^n (X,Z) if Y is empty)

What is $H^n(X,Y,Z)$

1:13 AM
I am also not sure if this definition is true.
Its in my book.

what book

German book tha I used once.

name?
Are you German

Yes

Ich kann Deutsch wenn es mir gefällt

1:15 AM
Das ist super.

Welches buch?

"Differentialgeometrie, Topologie und Physik"

By Jost?

von Mikio Nakahara

Oh, that book exists in English too

1:18 AM
Thats good.

Actually I'm pretty sure it's an English book
I have it, what page
I'm looking, he defines it as the Pfaffian of the curvature

the "Pfaffian" ?

square root of the determinant

Ok, then I dont know what you are talking about. I quoted the definition from my book.

Not Nakahara
I can't find what you said in there

1:22 AM
But mine is from nakahara

Page?

239, Chapter 10 "Vector bundles, tensor bundles, spinor bundles and applications in physics"

which edition

How do I know ?

look at the front
does it say which edition?
if not, it's probably the first

1:24 AM
There is nothing :/

one sec

1989

I can find a first edition in the library

Hello, its from 1989, it stands there.

what?
I got a first edition

1:27 AM
And ? 1989 ?

page 239 is about killing vectors
and chapter 10 is 100 pages later
and titled differently.

Are you sure, we are talking about the same book ?

Geometry, Topology and Physics by Nakahara

:/
Yeah.

can you give a link to the German version?

1:29 AM
I dunno where to find.

2:12 AM
@0celo7 Du bist Muttersprachler?

Bin dort geboren und aufgewachsen, aber bin Ami

@0celo7 Wow! What a high level club is it. :-)

Was meinst du

@0celo7 Was für eine exotische Community haben wir hier im PSE :-)

Indeed

2:20 AM
@CuriousOne As I know, string theory requires a quite high level math, how can they do this without super-excellent math skills?
@0celo7 A "League of extraordinary gentlemen" :-)

2:32 AM
@peterh Hand waving

@CuriousOne exactly; who said it?

3:34 AM
do we really consider the KE of electrons while solving nuclear physics problems? -,-
nvm

4:16 AM
I'm surprised this never got a good answer
0

Suppose I want to know what the universe looks like from the perspective of a frame of reference moving at $c$ relative to my current frame. As discussed at length in various other questions on this forum, from the perspective of the current established theory of special relativity, the notion of...

Everyone went off assuming I was asking something other than what I was asking, and making weird statements about how I should get a new "research project".

Hi Daniel. Is "raise eyebrows" such an obscure phrase?

@JohnRennie Well, I raise my eyebrows when I'm skeptical or surprised.
I don't think it is correct to say that we wish to exclude any answers which if found in a physics textbook would raise eyebrows.

I think you're taking my comment excessively literally

@JohnRennie Well, online communication happening without facial cues makes that somewhat necessary at times.
Also no vocal cues.

@JohnRennie I learned about a new kind of cohomology today
It's scarier than de Rham, but easier to compute

4:27 AM
@DanielSank in this case the lack of visual cues leaves me unsure if you really don't see what I am getting at or whether you're just criticising my way of saying it.

@JohnRennie Something curious: the Ricci tensor and scalar curvatures satisfy $R_{ij}R^{ij}\ge \mathrm{scal}^2/n$.

I saw your statement of this earlier but didn't quite get it. Isn't $R_{ij}R^{ij}$ just the contraction of the Ricci tensor i.e. the scalar curvature?

No?
$\mathrm{scal}=R^i{}_j=R_{ij}g^{ij}$
The proof fails for Lorentzian manifolds, naturally
For a Riemannian manifold we can put an inner product on the space of 2-tensors via $(A,B)=g_{ij}g_{kl}A^{ik}B^{jl}$
Then $(\mathrm{Ric},g)=\mathrm{scal}$
apply Cauchy-Schwarz to this.

@JohnRennie I think I know what you mean, but the language is vague enough and the subject touchy enough that I'd recommend more precise language.
Note that my comment on your meta post simply asked for clarified meaning.

@JohnRennie It's a generalization of the inner product $(A,B)=\mathrm{tr}(AB)$ on matrices

4:36 AM
@DanielSank Is it really a touchy subject? Why is it surprising that I think answers here should be of textbook quality? That's probaby more of an ideal than a realistic target, but it seems a good thing to strive for.
@0celo7 I suspect I don't understand the notation well enough to know what's going on here.

@JohnRennie There is a symmetric, positive definite bilinear form on $\mathfrak{gl}(n)$ given by $\langle -,-\rangle:\mathfrak{gl}(n)\times\mathfrak{gl}(n)\to\Bbb R, (A,B)\mapsto \mathrm{tr}(AB)$
This can be generalized to any tensor space using the metric
And to any bundle over a compact base by using integration
In particular, we have $\langle A,B\rangle \le\sqrt{\langle A,A\rangle}\cdot\sqrt{\langle B,B\rangle}$
$\langle \mathrm{Ric},g\rangle=g_{ij}g_{kl}R^{ik}g^{jl}=R^{ij}g_{ij}=\mathrm{scal}$
$\langle\mathrm{Ric},\mathrm{Ric}\rangle=R_{ij}R^{ij}$
$\langle g,g\rangle=n^2$
whoops, too used to GR :)
hmm, did I screw up the squares
No, it's $\langle g,g\rangle=g_{ij}g_{kl}g^{ik}g^{jl}=\delta^k_j\delta_k^j=\delta^i_i=n$
better
So $\mathrm{scal}\le \sqrt{R_{ij}R^{ij}}\sqrt{n}$
@JohnRennie make sense now?

Well I now see what you're doing, but understanding it all is a different matter :-)
Sorry if I seem inattentive. I'm reindexing some SQL databases - which is why I'm up at 5 a.m. on a flipping Sunday!

@JohnRennie The only issue is proving that $\langle-,-\rangle$ is actually an inner product
which is something I trust Dr. Petersen on :)
that's probably a folklore theorem
@JohnRennie do you know what those are?
@JohnRennie No clue what that is

4:53 AM
Jesus Christ I have four server down alerts :-(

oh, you have to expand the tensors into a basis
and then do...something
not hard, just tedious

Phew, false alarms :-)

Ahoy

let's calculate $\langle A,A\rangle$
$A^i_jA^j_i$
hmm
That's probably just a sum of squares

BTW I listened to the 2Chainz track you mentioned a couple of days ago, but it wasn't really my thing. I'll give the 2Chainz mixtape a miss.

5:06 AM
Birthday song?
Fantastic song
Just put it on
She got a big booty so I call her Big Booty.

I have a theory that the music you listened to in your teenage years moulds your musical preferences and it's hard to escape that in later life.

Probably the most profound thing...ever
I'm probably being stupid
why is the trace of $A^2$ positive

What is the best book for learning/improving logical reasoning?

As a teen in the 70a I was listening to a combination of hard rock (Sabbath, Zeppelin etc) and prog rock (Pink Floyd, Yes, Genesis). So that's what I'm stuck with.

$\mathrm{tr}(A^2)=\sum_{ij}A^i_jA^j_i$
????

5:08 AM
@JesterTran I think we'd need a bit more context. What sort of things are you trying to reason about?

in the symmetric case it's trivial
what am I missing :/

@JohnRennie Drawing conclusions from complex information

@JesterTran that doesn't really help. For example if you're asking about understanding international politics it's unlikely anyone here can help.

Listen to Trump and Farage speeches @JesterTran
...what am I doing wrong

Back
Oh God, don't listen to Trump speeches

5:12 AM
$A^i{}_jA^j{}_i$
ARGH

@0celo7 Listening to Trump and Farage speeches?

To improve critical thinking

What would Trump know about mathematics?
Or politics?

More than you, probably

5:13 AM
.-.

Oh look at this wise guy

Walked right into that one...

@0celo7 BITCHSLAP!

@JohnRennie currently failing at matrix algebra :P

Anyway, it's down to him and Hillary
Honestly, not sure what to pick
Probably leaning towards Hillary

5:15 AM
Hillary has brain damage
Look it up

@SirCumference have you seen the latest polls? Clinton will stroll into the White House.

@JohnRennie 100 days out

@0celo7 Don't all of us though?
I don't know what I'm saying

@SirCumference No, she had a stroke

I'm tired
@0celo7 Still preferable over Trump

5:16 AM
@JohnRennie If only the GOP wasn't actively sabotaging the God Emperor

The guy who's against net neutrality, supports the TPP, etc.
He manages to be on all the wrong sides of debates

What's really damaged him is going after the family of a muslim solder who was killed in action.

Damn
Then there's the people who think he'll actually build the wall

Trump hates the TPP you idiot

Source?

5:17 AM

Fuck

If you even bothered to listen to his message, you would know that

Just realized my source isn't super good for that

He's a populist

It says "New studies link autism with vaccines"

5:18 AM
You just said the populist supports a super elitist trade deal

and "astrologer explains how moon is made of cheese"

@JohnRennie Yeah

Oye, still against net neutrality

I don't know why he did that

@0celo7 and that's the point. You really want someone capable of that level of misjudgement as your president?

5:20 AM
@JohnRennie This. This right here.

@JohnRennie It's an uncharacteristic misjudgement, which is why it's so curious.

Sigh...

I'm thinking he was expecting some information to surface and it didn't

0celo, what actually makes you want to vote for Trump?
His terrorism "plan"? The wall?

I want to make America great again, what kind of question is that

5:22 AM
@0celo7 What kind of answer is that

Help me with linear algebra and I'll answer

Goddammit man this is our country on the line

$A^i{}_jA^j{}_i$
hmm
I should work it out in 2D

What is this for? College?

$A^i_1A^1_i+A^i_2A^2_i$
$A^1_1A^1_1+A^2_1A^1_2+A^1_2A^2_1+A^2_2A^2_2$
...

5:25 AM
Have ya tried helping him out?

unless things are symmetric, that's not good

@JohnRennie Site policy and culture is touchy.

@SirCumference 0celo7's grasp of maths is way ahead of mine. I've occasionally helped by acting as a sounding board, but that's it.

Huh, wow

this is linear algebra :/

5:26 AM
Particularly when we're talking about how to simultaneously maintain quality standards while not pushing away would-be new users.
As you can see in the meta post, people's feelings come into play. So yes, "touchy" is an adjective I think appropriate.

@0celo7 Congrats
You're a prodigy

not really

But anyway, the main point was about raising eyebrows and textbook style answers.

@DanielSank that isn't the point of Emilio's post though. He's talking about dealing with the most egregious level of crap.

Actually, maybe it needs to be $T_{ij}T^{ji}$

5:28 AM
@JohnRennie Unfortunately, "most egregious" and "crap" only make sense when someone passes judgement.

Right, and the point of peer review is that it dilutes away the extremes of opinion.

I think most of what Emilio said made sense (although he went about it so round-about-ly that I almost couldn't tell what the hell he actually wanted), but the changes he actually suggested have nothing to do with levels of crappiness.
@JohnRennie Indubitably.

On a side note, I got a job at the Museum of Natural History

Undubiously <-- that should be a real word.
His post could be re-written as follows:
1. We have some pretty awful content, both questions and answers. Some times this stuff survives too long.
2. We should exercise our flagging and down-voting more to deal with this.
3. We can also make a change to how flags are routed so that mods don't have to deal with low quality posts as much.
4. What do y'all think?

no, that seems to be correct with the first ordering

5:31 AM
I have no idea why there was so much wasted space going on and on about how bad some of the content is. That's totally irrelevant to the community effort he asked for and the slight workflow change he proposed.

@DanielSank I think I agree with all of the above

@JohnRennie k

I also can't see any evidence from the comments or votes in that question and answers for the subject being touchy.

@JohnRennie I am still confused about which part of Emilio's so-called "proposal" you like.

@DanielSank all of it

5:33 AM
@JohnRennie How about the part where Emilio talks about drawn out debates that fatigue everyone?
@JohnRennie oh

So, uh, Hohmannfan sent me this
Anyone know what that paper is about?

But if I'm honest my answer was more of a response to the other answer than a direct answer to Emilio. I wanted to provide a counteranswer to see how people voted.

it's a meme paper
we've been over this

@0celo7 I know you've said that

@JohnRennie Unfortunately there are never enough votes in meta to really know much.

5:35 AM
But it looks like a real paper

Rarely, in any case.

I'm going to assume it's real

I find that somewhat frustrating.

@DanielSank the voting patterns are clear though :-)

Relatedly, I think a major problem with our site is that people don't vote enough, up and down.
Me included.

5:36 AM
@0celo7 Plus it's from the Research Institute for Mathematical Studies
A legit site

For me the problem is at least partially that I dislike the -1 from down votes, and also that my reaction to most of the question I read is "what the hell is this person asking?".
I comment a lot asking people to focus their question more.

Well, this is going to sound soppy, but I really believe this site can make the world a better place - well, the world of physics at least. And being (partially) retired I have the free time available to devote to this end.
But you won't make the world (of physics) a better place by posting answers that will get students into trouble if they repeat them in their exams.

@JohnRennie I think this site makes the world better too. That's an objective assessment, and not soppy :-)
@JohnRennie Agreed...

I need to sleep

That's... what down votes and flags are for.

5:39 AM
doing algebra at 1:40 isn't working
cheerio

Well that's all my answer to Emilio's post says

@0celo7 'Night

Uh, yeah... but most of Emilio's post is (at least for me) almost impossible to understand.
For flagging: use the Very Low Quality flag for answers which are complete nonsense, such as

incoherent gibberish
anything not parseable as English text
as well as answers that are wrong, or based on non-mainstream physics, that you think are flat-out incorrect, so completely incorrect that no amount of editing can make it useful or constructive, and so completely incorrect that it is actively harmful to the site and its standing.

Do not use this flag for:
followup questions

@SirCumference I was doing it completely wrong

Wait what
Did that just hit you

5:41 AM
I figured it out when I laid down
yeah

So uh, what the hell does he mean to not use the flag for followup questions and comments?
Is his point just that we're misusing the flags?

I raised an index wrong lol

Yes, I agree that it could have been more concisely written e.g. your summary above. But he and I are in agreement about the way the PSE should be operating.

Ironic. When you stop thinking about it, it comes to you

@JohnRennie you're the GR head, you should have pointed that out

5:41 AM
Why did it take him two screens tall to say "guys, use the flags in the following way"?

Well there's some history to this.

What, no dude that message I just posted is a copypasta from Emilio.

It stems from arguments about using the Low Quality flag. And that's an argument that has been simmering for a while.

@JohnRennie Emilio's post didn't propose a way the site should be operating. It told me that there's are many low quality posts, told me to use flags, and suggested changing the timeout for mod visibility on flags.
@JohnRennie I didn't get that from the meta post.
I think what I'm trying to say is that your answer says "I agree" but I have no idea what you agree to because Emilio's post is so hard to understand.
It's like there's a secret communication between you two that I'm not getting.
So I have no idea what you guys are agreeing on, or how the site is going to change, if at all.

Emilio is basically saying flag crap as low quality then vote to delete it.
I'm saying flag crap as low quality then delete it.
I just provide an impassioned argument to support that point of view.

5:45 AM
I'm just so confused.
When I go to a question or answer and click "flag", there is not a "low quality" option.
So what is everyone even talking about?
I feel like I'm in the twilight zone.
Jeez, maybe the most useful part of Emilio's post is the one explaining how the low quality queue works. It would be nice to have that laid out more explicitly with screenshots, etc.
Right now, I simply have no idea what his post actually means.

@JohnRennie I don't have that option.

"99 flags remaining"
How long have you been here?

It must be a rep issue, in which case Emilio's post just became turbo-confusing because most of the readers can't even see the options he's talking about.

5:49 AM
@DanielSank There's no special significance to that answer, I just selected it to show the LQ flag option.
@SirCumference you get 100 flags a day. I've only flagged once today so I have 99 left today. Tomorrow it goes back to 100.

@JohnRennie Yes, did you read my message saying that I don't get that option?

@JohnRennie Dude, you'd have to have been here forever to get 100 flags...

@DanielSank Look at:
-3

Case 3: When the axes of the bar magnet and the coil are the same, magnetic flux enters the magnet from south pole and flux comes out from the north pole. When we keep the magnet stationary and rotate the coil about its vertical axis, which is perpendicular to the axis of the bar magnet,the flux...

and see if you get that flag option

@JohnRennie Yes, I have the "low quality" option, so it's about votes?

@SirCumference ah right. The number of flags increases with your rep.

5:52 AM
@JohnRennie IIRC, it increases depending on your approved flags:declined flags ratio
Although the algorithm itself is weird

@DanielSank The LQ option doesn't always appear for me either. I don't understand the rules for when you do and don't get that option but presumably they are documented somewhere.

Jesus, 1700 flags?
._.

@JohnRennie I see.

Now does it all make sense? :-)

5:53 AM
-11

There are a lot of crazy quests in astrophysics, one of them is searches for dark matter. How someone could observe dark matter while it is far away?

This question is amazing

I think I'm annoyed that Emilio is trying to give us a path forward on something important but the post is so wandering that I can't tell what's actually going on.
It's like reading a textbook on a subject you love but finding the wording or notation so convoluted that you don't learn anything. I always really hated that feeling.

But all Emilio wants is to make this site a better place for the world of physics, and that's what I want too and I'm sure what you want as well.

@JohnRennie Oh for... of course. That doesn't help clarify what he's actually proposing in the meta site.
Come on, dude, that's like me asking for details on your plan to save puppies, and instead of answering you say "I love puppies, don't you love puppies? Don't you want to help puppies too?".

He's proposing flagging the most egregious crap as LQ and voting to delete it.

Yes, I want to help puppies.
@JohnRennie Ok... but I can't flag eggregious crap as LQ...
The options isn't there, sometimes, for some reason.
So I don't know what to do.

5:58 AM
I suspect the answer must have attracted some downvotes for the option to appear. I find that in practice the option has always been there when I've wanted it.
Maybe the answer has also to have been posted some minimum time ago.

@JohnRennie I see. Well, if the whole point of Emilio's post is to say what you just said in the chat room, perhaps you'd suggest he edit?
I can't be the only one who is mystified by that wall of text.

@DanielSank Hmm, I must admit that I've chatted with Emilio before about this so I knew what he was getting at. I hadn't stopped to think that his post would have been incompressible to non-initiates but I have to concede that you have a point.
But his concrete proposal is that from Monday we start flagging wrong answers as low quality (assuming that option appears).
And that's a procedural chance because at the moment the LQ flag is supposed to mean only that a post is incomprehensible.

@JohnRennie Ok, now I know.
But other people reading that meta post won't.
Most people won't even read it.

I'm a bit surprised that no mod has commented because this will affect what appears in their review queues. Maybe they are waiting to see what the consensus view is.

@JohnRennie Ah.
Thanks for explaining what's going on.
~Sigh~, I'm turning into a type 4 user.
:(

6:09 AM

@JohnRennie Great.
I care about site quality, but I decided to not care about the issue raised by Emilio until someone explains it clearly.

user147690
6:39 AM
What are these type 1 and type 4 in reference to?

@AlexClark see:
Jul 31 at 7:33, by peterh
558

Why is Stack Overflow so negative of late? I hate to pile on, but I couldn't resist summarizing the problem. And I apologize ahead of time for any potentially offensive language that I might use. Basically there are 4 camps of users on Stack Overflow: The "caretakers" who want to keep the...

1 hour later…
7:42 AM
@JohnRennie that's a really good post you've written on meta - although I was quite surprised to read, probably for the first time, that PSE is mainly educational

7:56 AM
@Sanya : thanks. I've used gimp, and I've got Microsoft office 2013 on this laptop, it includes Publisher. The sort of pictures I want to do are physics illustrations. For example the other day I was trying to do a picture like this but with the grid lines bulging out.

@peterh Because there are absolutely no formal criteria for correctness. Physical derivations are usually nothing but an imaginative use of mathematical techniques. Very often it takes decades to clean up the physicist's mess and often the cleanup is not even needed because the resulting theory is not even correct. I take that back, the resulting theory, so far, has never been correct. That's not how mathematicians work.

@vzn : I share your sentiment.