The h Bar

Danu

20:00

No, the connection induces such a split

ACuriousMind

@Danu It's all equivalent.

Danu

@0celo7 I'll allow it ;)

Slereah

No, you can do the split before the bundle

Well

Danu

@ACuriousMind Perhaps but I find the opposite direction rather unnatural-sounding.

0celo7

the Duffers is in here...

watching

ACuriousMind

20:00

Defining it in terms of horizontal and vertical subspaces is sometimes called an Ehresmann connection

Slereah

You can define the vertical part

Danu

How does one randomly come up with a split?

0celo7

@Danu AoC

Balarka Sen

too much geometry for me

Danu

@ACuriousMind Ah, right, I saw those pass by sometime ago

@BalarkaSen So, what kind of stuff are you interested in then? More algebraically-oriented?

Slereah

20:02

The vertical split is done using the projection IIRC?

Balarka Sen

I am interested in algebraic algebra, geometric geometry and more generally mathematical mathematics.

Danu

@BalarkaSen hurr durr

0celo7

does algebraic algebra have any applications?

Balarka Sen

yes, it can be used to do algebro-algebraic algebra

0celo7

what does that do

Danu

20:03

Or higher algebraic algebra

Balarka Sen

what does higher remind me of

...

of course.

Danu

Eventually, you might get to $\infty$-algebro-algebras

Balarka Sen

lol, that does it.

0celo7

algebroi

Balarka Sen

nlab.

Danu

20:04

Algebroski

Slereah

Do an integral over the space of algebras

Balarka Sen

@Slereah you mean coend

0celo7

is there an algebraic definition of integration?

Danu

@BalarkaSen Physics using $(\infty,1)$-cohesive topoi!

Balarka Sen

lolol

Danu

20:04

ABSTRACTION INTENSIFIES

ACuriousMind

@Danu No joke, considering higher category theory eventually leads to the notion of $\infty$-categories.

Danu

^I know that

ACuriousMind

Good.

Danu

Also Urs Schreiber actually wrote a book on $(\infty,1)$-cohesive topoi in physics

0celo7

chat is completely detached from reality

Danu

20:05

Ain't no jokez

0celo7

@JohnDuffield how's life?

Danu

Or something like it, anyways

ACuriousMind

@Danu I know. Everytime I think I understand a lot I try to read it.

Balarka Sen

@ACuriousMind Do you really care for \infty-stuff? I have only ever seen them in Lurie's TQFTs, and have never tried to understand it again.

Danu

Abstraction for the sake of abstraction, woohoo

skill patrol

20:06

@0celo7 he's still suspended

ACuriousMind

@BalarkaSen No, until now, it's just a funny little fact I keep at the back of my mind.

0celo7

@skillpatrol indeed

skill patrol

};-)

ACuriousMind

@Danu Some call it beauty ;)

Danu

I think the amount of effort that goes into the enormous amount of abstraction that e.g. Schreiber does is better spent otherwise unless you're purely looking for a most-abstract way of writing stuff.

0celo7

20:06

@ACuriousMind I call it a waste of funding...

Danu

@ACuriousMind But many disagree.

Balarka Sen

The stuff's cool because of Grothendieck's homotopy hypothesis, but I have never really grokked it.

@Danu \infty-stuff is good.

Danu

@BalarkaSen I meant this only as far as applications to physics are concerned.

Balarka Sen

Has lots of concrete application.

@Danu can be applied in physics

Danu

If you're interested in pure mathematics then go for it.

0celo7

20:07

> Most treatments of GR begin with the assumption that spacetime is a pseudo-Riemannian manifold (or, sometimes, that it is a more general manifold).

What else could it be

Danu

@BalarkaSen I have yet to see any application to physics that is not much simpler done by more elementary means.

Balarka Sen

look up Jacob Lurie's Cobordism hypothesis and Topological Quantum Field Theories

Slereah

Manifold with boundaries

Conifold

Danu

@BalarkaSen I know it

Slereah

pseudo manifold

Orbifold

ACuriousMind

20:08

@Danu I find myself less and less concerned with actual physics, I must admit

Danu

(as in heard/read about it)

Slereah

whatever

Danu

@ACuriousMind Me, too.

0celo7

@ACuriousMind sad

Slereah

also sometimes it is discrete

0celo7

20:08

@Danu also sad

@Slereah really

Danu

I'm also super interested in that crazy-abstract stuff. But that doesn't mean I have the illusion that it is really the best way of progressing in physics.

Balarka Sen

oh, sure

Slereah

@0celo7 I dunno

0celo7

@Slereah any references for that?

Balarka Sen

yeah, I agree Schreieber is a bit hopeful about all this

Danu

20:09

Even this TQFT stuff---it's not really that useful at all in doing physics.

ACuriousMind

@0celo7 a smooth space, or perhaps a diffeological space.

Slereah

Not that I know

I can give you ref for the rest tho

Danu

It's super awesome though

Balarka Sen

@Danu really?

0celo7

@ACuriousMind smooth space?

Balarka Sen

20:10

all I know is that TQFTs are useful in low dimensional topology

Danu

And I fully intend on misrepresenting its usefulness in order to obtain funding to work on it :D

Balarka Sen

I don't even know where the field theory jumps in

ACuriousMind

@Danu lol

0celo7

I must say I've never seen a book that does anything other than pseudo-Riem

Danu

...just like I suspect most people do.

Bass

20:10

@ACuriousMind do you have more "Morse theory gems", like $f$-has-two-critical-points-$\implies M\simeq S^n$? (if that was Morse theory)

Slereah

@0celo7 : HE does manifolds with boundaries, I think?

ACuriousMind

@BalarkaSen Yeah, topological field theories rarely, if ever, describe actual physical systems.

0celo7

@Danu that's...you feel comfortable doing that?

Danu

@BalarkaSen The high-brow mathematics is just not needed in any real applications that we've found so far, as far as I can tell.

@0celo7 Yup.

0celo7

@Slereah check their def of manifold

Danu

20:10

Completely.

0celo7

@Danu I could never steal like that

oh well

Danu

I do not care much for contributing to society

Balarka Sen

@Bass How about the Morse lemma itself?

Danu

@0celo7 You're still a child in your mind ;)

Balarka Sen

@ACuriousMind ah

0celo7

20:11

@Danu No, realizing that tax money is not yours is not at all childish.

Danu

Morse theory can be applied---but again it seems like the applications just make simpler situations unnecessarily complicated.

@0celo7 Having such a black-and-white sense of morality is.

0celo7

@Danu How is taking coerced money for your personal gain not immoral in any sense?

ACuriousMind

@Bass Given I just began reading about it, not as of yet

Danu

@0celo7 In time you'll know... :)

Hah, I like the edit on that one

TAXES ARE COERCION

It's all thaaaat simple ^^

0celo7

@Danu Not an argument.

Danu

20:13

@0celo7 I'm not trying to convince you.

ACuriousMind

@Danu I'd not actually say that. Plenty of people with black-and-white morals running around which aren't children, and calling black-and-white morals childish might be an insult to children :P

Balarka Sen

I should stop coming to this chat lest people in the math chat finds out I lurk here. That'd be pretty embarrassing :P

Danu

@ACuriousMind Ehh, maybe.

@BalarkaSen Lol

Are you mathematicians that snobbish?

I always had the impression that they were more willing to share ideas, open etc.

Maybe I'm wrong.

Balarka Sen

I was joking, of course

ACuriousMind

Jul 30 at 21:27, by 0celo7

I'm not even an adult.

Balarka Sen

20:14

Physics is good stuff.

ACuriousMind

I've gotten quite a mileage out of that quote :P

Danu

Mathematics is good stuff

Balarka Sen

I just don't understand it well enough.

0celo7

I'm off for today.

Ban incoming.

Danu

That's a bit of a lame edit, @0celo7. I removed that message.

Also, getting so frustrated in internet conversations is also a sign of...

ACuriousMind

20:15

@Danu ...being human.

Danu

@ACuriousMind Or a good robot :)

Balarka Sen

I think I should learn some higher algebra from Lurie and join the homotopy chat.

Danu

@BalarkaSen ...I just don't understand it well enough :(

Balarka Sen

\infty-topoi

0celo7

@Danu I get frustrated because you're so god damn smart and shouldn't be acting like this

I wish I could do the things you guys do

Danu

20:16

@0celo7 I don't think that that's why you get frustrated.

0celo7

@Danu Then what is it

Danu

@0celo7 Not sure :P

0celo7

@ACuriousMind How is reposting it over and over not trolling

I said that when I was 17

It was in reference to me being 17

ACuriousMind

@0celo7 I tend to think of it more as a running gag

0celo7

@ACuriousMind Will you stop if I ask you to stop?

ACuriousMind

20:20

But if you want me to stop posting it, I'll comply.

Geez, astronomer, get your game straight :P

0celo7

@ACuriousMind Please stop.

ACuriousMind

@0celo7 Will do

skill patrol

Like an "in joke"

0celo7

Also, how is @ACuriousMind allowed to call me a troll? Does that not violate "be nice"?

skill patrol

Can I start :P

0celo7

20:23

@skillpatrol Don't.

ACuriousMind

@0celo7 This (starred) time around it really was just a joke meant to imply that you're usually not pedantic

skill patrol

He's being selectively sensitive again :P

ACuriousMind

We've said multiple times before that if you feel certain things are too mean, you should just say so.

skill patrol

^

If you can't take it. Don't dish it out pal.

Bass

@BalarkaSen looks interesting. If a function $f\to\mathbb{R}$ has four critical points, does that imply that $M\simeq T^2$?

ACuriousMind

20:31

@Bass Only if $M$ is two-dimensional, obviously, and in 2D it's really not hard to find out the type of surface something is.

Balarka Sen

@Bass no

you need information about the index of the critical points

0celo7

@ACuriousMind We've already established that the SE model does not consider context.

@skillpatrol I'm asking about it in the context of SE rules.

If it really bothered me I'd block him.

ACuriousMind

@0celo7 What? The validation of a rude/offensive flag in chat does not, but you are only expected to raise a flag if you are actually offended.

0celo7

@ACuriousMind Someone could come in and flag as offensive.

Lord knows that's happened to me.

ACuriousMind

@0celo7 And your point is?

I'm not sure what this conversation is about if you are not actually offended.

skull petrol

20:40

Potential offense :P

0celo7

@skullpetrol Exactly.

skull petrol

Aka hyper-sensitive

ACuriousMind

@0celo7 Yes, some things taken out of context might seem offensive, and the chat flag system is not properly equipped to deal with that. I thought we'd already established that.

skull petrol

If you're offended leave the computer.

0celo7

@skullpetrol No, I just block the offensive individual.

skull petrol

20:43

Problem solved.

ACuriousMind

I'm not getting what you're aiming for.

skull petrol

He wants to block me.

0celo7

@skullpetrol There are times, yes.

skull petrol

If you gonna block then block

0celo7

I'm not going to block you.

ACuriousMind

20:45

@skullpetrol Not getting your goal in this converation either, tbh.

0celo7

@ACuriousMind Trying to annoy me.

skull petrol

Just wait till I get your redskins on the grid iron, you gonna see what it's like to be in the black hole with the the black beast @0celo7

I'll show you trash talk like you never heard before.

ACuriousMind

I have no idea what that means, but it sounds somewhere between ridiculous and vaguely threatening.

0celo7

@skullpetrol Peace and quiet.

@skullpetrol I mute the enemy.

@ACuriousMind Mostly the first. But I have no idea if he's actually from Oakland. If he is, it's probably an actionable threat.

@ACuriousMind It's regarding handegg.

user54412

If I serially downvote (and flag, and VTD) a user's answers, am I going to get in trouble?

ACuriousMind

20:55

13

Q: Downvoting serial answers without serial downvoting

In the past days I've encountered a particular user who has made a "serial answering": he has answered several different (even uncorrelated) question with almost the same answer and the same sketches. The problem is that some things in those answer are misleading, and some others plainly wrong. ...

0celo7

If I go through a certain user's answer list and downvote everything I think is a poor answer, am I violating a rule?

user54412

yes

ACuriousMind

@0celo7 Answer also there. If you use their profile and actually look through their answers specifically, yes.

user54412

of course that's exactly what I just did

skull petrol

:O

user54412

20:58

in my defense, this user just posted a bunch of answers in the last few minutes ranting about how they've been inappropriately silenced by the powers that be

0celo7

@ACuriousMind Why?

I'm asking for the rationale.

@ChrisWhite Links?

ACuriousMind

@ChrisWhite Hm, well, in that case, I think if they actually are all ending up deleted it's nothing to worry about

user54412

@0celo7 they're already being deleted

0celo7

@ChrisWhite That's a shame.

Slereah

the proof that static compact spacetimes are totally vicious is p. long

0celo7

21:00

I no longer understand my former understanding of strong causality.

ACuriousMind

@0celo7 Because you should judge content as you encounter it, not judge users. I guess, I'm not fully convinced this is a sensible rule, but I guess from a pragmatic viewpoint it's pretty impossible to have the automatic serial voting detection distinguish between "good" serial votes and bad serial votes, so if you don't want your votes to be useless, you just shouldn't do that

skull petrol

Well said^

Let's not take it personally.

0celo7

@skullpetrol Huh?

skull petrol

Serial down voting.

Judge content as you encounter it.

0celo7

21:27

Great Matlab doesn't like my for loops

is something like

for i < 235

stuff

end

is that not valid?

TanMath

I think it is..

0celo7

it says < is "unexpected"

hmm, it works when I do it with a while loop

TanMath

@0celo7 oh.. Yeah.

0celo7

derp

I'm just bad at ~~math~~ matlab

TanMath

@0celo7 technically you have for I in range(1,236)

0celo7

21:32

@TanMath yes

but the while loop works just as well

maybe not for code golf

ACuriousMind

@0celo7 That's because a for and a while loop are different things.

0celo7

@ACuriousMind YES

i get it

:(

ACM is a computer scientist as well

this is scary

TanMath

@ACuriousMind well, C++ for loop is like a while loop...

0celo7

I think

for loop $\cong$ while loop

Slereah

All loops are the same thing

For loop is a while loop with some initialization and incrementation

ACuriousMind

21:39

@TanMath Well, kind of, but it's bad style to use them interchangably. The point of a for-loop should be that the loop variable runs through a certain range, a while-loop should ensure the condition it checks is satisfied. While you can use one to do the other, it's not what they're meant to do.

0celo7

turns out ACM is bad at CS

that's one thing

Slereah

Well both are the same thing in a way

BUT

Try using a for loop if a for loop is to be used

The compiler might implement it in a more efficient way

0celo7

@ACuriousMind tell that to my code

it has multiple while loops that count an index :)

I don't see the issue

I'm gonna make a summation machine using an index and while

I think while loops look prettier than for loops

ACuriousMind

Fortunately, I have learned that "I don't see the issue" is code for "I'm joking".

0celo7

@ACuriousMind uh

not all the time

@ACuriousMind seriously

now you're never gonna believe me when I don't see the issue

alarge

21:51

@0celo7 If you're coding Matlab and you have many loops, you're going to have a bad time. Usually you can get rid of most of them if you think hard enough.

ACuriousMind

@0celo7 Then I have officially no way to judge when you are being serious and when not.

0celo7

@alarge I need to sum a bunch of numbers in a vector

subject to a condition

what's wrong with loops in matlab

alarge

@0celo7 Make it a matrix and use sum().

0celo7

@alarge you can use sum() on a vector

alarge

@0celo7 It's an interpreted language, loops are sloooooow.

0celo7

21:52

but what about the condition

@alarge interpreted language?

alarge

@0celo7 You'll have an array with the condition, and you can mask with it. Again, depends what you're doing.

0celo7

@ACuriousMind that's an issue

alarge

@0celo7 As opposed to compiled.

0celo7

@alarge ...mask?

alarge

@0celo7 sum((x > 0) .* x)

0celo7

21:53

what the heck

@alarge I have no clue what that means

alarge

@0celo7 Well, try it. (x > 0) will be an array of 1's and 0's. .* is elementwise multiplication.

so (x>0) will mask out all the elements that are less than or equal to zero.

0celo7

@alarge interesting

suppose I have a vector which is N long

and I want to add each odd-numbered element

how would I do that

alarge

sum(x(1:2:len(x))) maybe

0celo7

len?

I would use rem

alarge

It's been like 7 years since I last touched Matlab

0celo7

21:56

Oh I see

hmm, that's cool

wonder if I should try that on the final

we're just doing very basic stuff

alarge

Well those are still the very basics of Matlab syntax.

0celo7

VERY basic

this class only meets once a week

I would take a dedicated MATLAB course next semester but you have to be a sophomore

alarge

I used to TA in some signal processing classes for freshmen and I'm quite sure .* etc were covered there.

You should have a look at your course materials, it probably has more information than you need to know in a test.

0celo7

@alarge I mean, I'm not surprised that it works like that

But they never taught us to use logic thingies like that

I know why it works

alarge

Cool. Now go write all of your assignments without loops (and profile to see what the speed difference is)

0celo7

22:06

@alarge too busy doing all my homework I should have done earlier but was playing Fallout

And when I'm done with that I'll play Fallout

:)

0celo7

22:19

@StanShunpike Wonder what it was...

@ACuriousMind Does Morse Theory have applications in GR?

ACuriousMind

@0celo7 Don't know, don't care

0celo7

link.springer.com/article/10.1007%2FBF01608493#page-1

hmm

There are some articles on it

Slereah

Morse means seal in french

(the sea critter)

0celo7

seal theory

I don't get it

that, on the other hand

makes sense

aww

0

Q: Operational Definition of Reference Frame in General Relativity

Most treatments of GR begin with the assumption that spacetime is a pseudo-Riemannian manifold (or, sometimes, that it is a more general manifold). But this entails quite a few tacit assumptions about the nature of space and time. I would like to approach things from a much more fundamental poin...

general-relativity differential-geometry reference-frames coordinate-systems observers

@Slereah Is he asking for Fermi normals?

0

Q: What are the definitions and the differences between string "background" and string "vacuum"?

In cosmology one studies perturbations around FRW metric classically (pure GR, we say that we perturbe the FWR "background"). In QFT we have perturbation theory quantistically (we expand around a vacuum, defined as that state such that all field expectation values minimize the Hamiltonian). 1) I...

quantum-field-theory general-relativity cosmology string-theory compactification

> quantistically

@ACuriousMind Is this a word?

ACuriousMind

22:39

@0celo7 Not in my world.

0celo7

@ACuriousMind But "quantumly" is ;)

ACuriousMind

Yes, of course :)

0celo7

Ah jeez, which of my billion books cover Fermi normal coordinates...

that's American billion

having a German billion would be a bit much

0celo7

22:55

RIP

literally

Should I send it in to Springer?

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