The h Bar

gonenc

11:00

This guy seems to be promoting his own paper, which is maybe remotely related to the question.

Slereah

I can't get mad at that moustache

gonenc

For the reference:

0

A: Have we figured out how to analyze turbulent fluids?

This is a very funny discussion here! The trick in understanding turbulence is idealization. It is an old trick, used by Newton, but obviously forgotten? He idealized Sun and Earth as point (!) masses, i.e. no volume at all, only mass, no inner degrees of freedom, mass only. That's the way one sh...

0

A: What is the mystery of turbulence?

One of the problems in discussions about turbulence during the last 60 or 70 years is a matter of culture - the departure of math from physics. Were Newton and Leibniz mathematicians or physicists? Some of today's mathematicians have a weak relation with reality, ie. with observations, measuremen...

-1

A: About turbulence modeling

possibly the article below gives you some insight: https://www.researchgate.net/publication/235437361_Universal_equations_and_constants_of_turbulent_motion Regards, Helmut

Slereah

I'd rather not read fluid mechanics!

The horror

"It's entirely possible to be downvoted more than upvoted and still get away with positive rep."

Ah, that would explain things.

gonenc

@Slereah whose quote is that? but regardless it is somewhat obvious that this is the case. 1 upvote=5 downvotes

Slereah

Reading the Meta SE

gonenc

11:08

@Slereah the mother meta or phy meta?

Slereah

physics

gonenc

question to all: do you know why a downvote is -2 whereas an upvote is +10? it seems a bit out-of-balance

Slereah

Perhaps they thought that people would be more likely to downvote than to upvote

In the way that people prefer to criticize than to approve

Danu

11:33

I've got a silly question:

Slereah

Ask away

Danu

Slereah

Someone might have a silly answer

Danu

Am I silly to wonder "how does one find such a least-norm element"? Can there not be any problems with this?

Slereah

Not a clue!

Danu

11:37

The set can definitely be infinite, but I think it should somehow be okay since Euclidean domains are modeled after $\mathbb{Z}$, so there is some kind of discreteness in the norm.

But I wouldn't know how to systematically find a lowest-norm element in finitely many steps...

Slereah

Perhaps you should enquire at the math SE

Danu

The chat there is kind unresponsive, generally.

They didn't reply when I said "hi" :P

gonenc

@Slereah I bet either ACM or Ocelot will answer faster than math se

Slereah

Perhaps!

@ACuriousMind, where are you!

Danu

I'm counting on ACM (never trust Ocelot, haha!)

@gonenc This issue has been raised many times (on e.g. Meta Stack Exchange)

gonenc

11:41

@Danu that wasn't physics was it? it looked more like a maths question :D

Slereah

why is it never resolved, though

Danu

@gonenc Not at all physics, agreed.

@Slereah Because the developers think it shouldn't change.

Slereah

But for what reason

Also should it be the same across all SE

I'm not sure it should

Some fields attract more people with undue confidence in their knowledge of the topic than others

gonenc

@Danu you were in mathematical physics right?

Slereah

You won't find a lot of LaTeX cranks, but physics or history will be a lot more delicate

Danu

11:45

@gonenc Am, yes. But I spend most of my free time learning pure mathematics.

gonenc

@Danu I'm quasi afraid of lemma, theorem proof type of books :D :P

Danu

It's a fear only some of us ever overcome ;)

Slereah

If you are gr8 on mathematical physics, btw

Do you happen to know if the frame bundle is a ~special bundle~

As it varies with the coordinate chart, which is not a bundle at all

Or is that a generic feature of bundles

The question is a bit vague I know

Danu

The frame bundle is just the bundle of all possible frames of a vector bundle

Slereah

I haven't thought about it in a while

Danu

11:48

So the structure is: 1) Base manifold 2) Vector bundle over base manifold 3) Frame bundle over vector bundle

Slereah

What's the link between the bundle and the chart atlas, I guess is what I'm sayin

coordinate chart

I forget the proper term

Of the base manifold

Also is there a proper term for manifolds with coordinate charts that aren't open sets of R^n

gonenc

@Danu but the community kind of does

269

A: Should the weight of downvotes be increased?

Yes, definitely -5 for a down-vote for the down-votee, but then I would also "up" the cost to -2 for the down-voter. The higher cost will reduce the "just feels like it" down-vote behaviour some people are displaying.

Slereah

Like when spherical coordinates are used

Danu

It seems clear to me that every coordinate chart (locally) induces a frame on a vector bundle and thereby picks out an element of the frame bundle.

@Slereah ...but those are defined everywhere except at $r=0$, so it's an open subset

Slereah

What about using spherical coordinates on the manifold of a sphere, though

Instead of using the proper chart of stereographic projection

Danu

11:51

@gonenc Does it really? There are many, many thousands of users. Millions, probably, who haven't seen it.

Slereah

Is there a specific name for this

Danu

It's not a chart.

Slereah

Manifolds are always defined as having charts of open sets of R^n but in actual use it is rarely the case

I know, but is there a term for such a usage

Danu

What do you mean, it is rarely the case?

I totally disagree

gonenc

@Danu I cannot see the number of downvotes on that question but 269 upvotes looks like an approval rate to me

Danu

11:53

@gonenc My point is that an almost negligible part of the community is even aware of this question.

Slereah

Well in physics you will often define the metrics and such using spherical coordinates and such, with the assurance that it is fine

gonenc

@Danu that is true

Slereah

Because we are aware of what the singularities mean

I was wondering if there was a mathematical framework for that kind of thing

Danu

What is your problem with the spherical coordinates?

Slereah

Nothing

Danu

11:54

So why do you think they do not constitute a coordinate chart?

Hi ACM!

Slereah

It's just that books always start out as saying "The manifold must use coordinate charts of subsets of R^n because so and so reasons"

Danu

Answer my algebra question above (scroll up 'till you see a picture posted), pliz

Slereah

And then they just kinda go "We'll use spherical coordinates but it's fine"

Danu

What would be the problem with spherical coordinates? I don't get it

ACuriousMind

@Slereah You rang?

Slereah

11:55

I just wonder if there is a mathematical framework in detail for it?

@Danu was looking for your help!

Danu : No problem, just that it's not in the definition of the manifold, yet it is always used

I just wonder why

Danu

You keep on implying that there is, at least naively, some kind of problem.

What is it?

Slereah

No problem?

Books usually just tell about problems with the coordinate singularities

As the reason why we should not

But they still end up using them

Danu

Polar coordinates are just not defined everywhere

Slereah

I am aware, yes

Danu

...but that's not a problem because they're defined on an open subset and that's all we ask for

one never claims that they must be defined on the entire $\mathbb{R}^n$

ACuriousMind

12:00

@Danu The norm is a function from the ring to the non-negative integers, so the image of any ideal has a minimum. Take the preimage of the minimum and choose any of its elements, and you got an element of least norm.

What problems do you think could occur there?

Danu

Something something infimum

ACuriousMind

@Danu They are the non-negative integers. Every subset of them has a minimum.

Danu

Yeah, that's what I said back there too. This integer structure should give what we need.

ACuriousMind

BTW, it is not required to find the element in "finitely many steps", as you said there

It just has to exist, this is algebra, where we worship Zorn's lemma ;)

Danu

Basically, what I was missing is that the function is actually to $\mathbb{Z}_+$ and not to $\mathbb{R}_+$

Zorn's lemma gives you a finite way to establish existence :P That's kind of the point of it, I guess

ACuriousMind

12:04

Yeah, in $\mathbb{R}_+$, we could have problems with the infimum not being a minimum, but here, it's fine.

Danu

Is it me, or are rings just a lot more interesting than (finite) groups?

ACuriousMind

They are

Slereah

Well they have more features

So more things can be said about them

Danu

I just found myself skipping through some sections on this algebra book because the stuff on groups is so tedious.

Or less, @slereah. I guess in this case, though, they hit the sweet spot of the interesting-impossible trade-off.

Slereah

Just wait until you read about fields!

ACuriousMind

12:08

Rings (and modules over them) are basically the "richest" algebraic structure there is. (which may be formalized by the embedding theorem that every Abelian category is a subcategory of the category of modules over some ring)

@Slereah Field extensions are just groups again :P

Danu

@Slereah Fields are too tame. They have too many rules, almost no interesting structure left

Slereah

Then why not just sets

Sets are wild and free!

Danu

Too free to be of interest.

It's all about balance

Slereah

Well thanks for your insight prince Gautama!

Danu

^^

Slereah

12:15

One day, I will learn all the math in the world

And I will finally be able to finish Rovelli

Danu

This summer, I will learn algebraic topology.

Why do you wanna know that LQG stuff?

Slereah

Why not

Danu

Big investment.

ACuriousMind

This user reminds me of this.

Slereah

Less so than string theory

We are out of Bort license plates

Danu

12:17

@Slereah But fewer people apparently thought it was worthwhile ;)

Slereah

Well they had different goals

String theory is a big thing that aims to solve various problems

LQG is just canonical QG

Danu

@Slereah I attended some presentations on it, and it came across as quite unconvincing. Basically, it appears that they keep on trying to quantize using certain variables, then discover some impossible-to-solve problem with them, switch variables and repeat.

Slereah

Basically yeah

Canonical QG was pretty intractable

Then they switched to Ashtekar (spelling?) variables and due to math things it was somewhat tractable

Danu

Correct spelling

Slereah

Then again, string theory too comes from a long line of previously failed theory

It takes root in covariant quantum gravity

And various previous iterations of string theory have failed as well

Danu

12:21

@Slereah It comes from an attempt at a theory of strong interactions

Slereah

As well as supergravity which is somewhat related

I know

Danu

In his lectures at LMU, professor Mukhanov often strays off-topic and starts bashing all attempts at quantum gravity. It's fun to watch :)

Slereah

Quantum gravity is a very....

Parochial thing?

Physicists get very emotional about it

Danu

In any case, I'm under the impression that the problem with string theory is that it's too rich so nobody knows where to make progress, rather than that no progress is made.

Slereah

I don't care too much

Danu

12:22

I'm interested in learning string theory for the purely mathematical aspects :)

ACuriousMind

::looks up parochial:: ::nods::

Danu

Geometry = best

Slereah

I don't have invested feelings in a theory being right or wrong

As long as it is interesting

Danu

Completely agree

I don't care for working on something "real"

Slereah

I know that things look bad for LQG, due to the measures of Lorentz invariance breaking

But still, it's a nice theory to look into

It's one of the simplest and most natural extension of GR into the quantum realm

Danu

12:24

It's not so much geometric though. It focuses more on the Hamiltonian/QM aspects, which I've always found very unattractive.

Slereah

Along with covariant QG and semiclassical gravity

ACuriousMind

@Danu Thou shalt worship the Hamiltonian!

Slereah

Really people should invest more in semiclassical gravity, because it is probably the first theory that will give any kind of results :p

simply because it's low energy

Danu

Semiclassical stuff is cool. Totally non-rigorous and ill-defined, but it works so well!!!

Slereah

Well semiclassical is somewhat rigorous?

Danu

12:25

Yeah, I agree, semiclassical is awesome. Just completed a course on it (again, by Mukhanov :D)

Slereah

Like IIRC it corresponds to tree-level QG

ACuriousMind

It's semi-rigorous :P

Danu

@Slereah QFT in curved spacetimes? It is semi-rigorous for physicists :P

QFT is not even defined, right... :P

Slereah

Not sure how many QG theories simplify to semiclassical gravity in the low energy limit

Well QFT has plenty of rigorous definitions

It's just that nobody uses them

Because who wants that hassle

ACuriousMind

@Slereah Too bad we can't show our favourite QFTs fulfill these axioms :D

Slereah

12:26

Do you really want to solve exactly an interacting theory

Danu

Nobody can even contruct any theory

ACuriousMind

@Slereah Yes.

Slereah

Well yes, but CAN YOU

Danu

Never mind exact solutions. Existence!

ACuriousMind

At least, I'd like to know that the thing I write down is well-defined.

The actual solution is not that interesting after that.

Slereah

12:27

2D QFT is mostly well defined, I think?

Danu

I honestly don't care too much in the case of QFT

2D QFT is mostly non-existent, also :P

Slereah

It's not plagued with as many problems

ACuriousMind

@Slereah 2D, and 3D to some extent, are fine in some cases.

Slereah

1D QFT then!

ACuriousMind

But what we really need is 4D Yang-Mills

Slereah

12:27

Pretty easy

ACuriousMind

@Slereah That's my favourite.

Danu

@Slereah That's called QM :P

Slereah

1D GR is also nice

Because 1D metrics are all diffeomorphic to flat space

Danu

Lol

Slereah

2D GR is independant of matter content, which is also nice

Except

There's a Cheating Action

That you can use to pretend that GR works in 2D

It's pretty transparently cheating

It's unfortunate because a good 2D GR would be nice to have

2D manifolds are pretty easy to deal with

ACuriousMind

12:32

Gauge theories in 2D are nice. No pesky bosons, just non-particle global field configurations.

Surprisingly, the theory contains no particles, but is not trivial if the manifold is not trivial

Slereah

You don't even have the spin statistic theorem exactly!

You have those weird conitnuous transformations between bosons and fermions

Kyle Kanos

12:44

@Danu Yeah, I really thought I was going to lose from move like 30 until end game. My brother said he was very angry with himself for losing that one

Danu

@KyleKanos You wanna play a quick game? :P

Kyle Kanos

I got some work to do :/

Danu

aight

ACuriousMind

@KyleKanos You have work?

Kyle Kanos

The data challenge thing ends on Friday

I'm basically done, just need to make my presentation more spiffy or something

0celo7

12:56

@Danu lol Saw much?

Slereah

"This paper investigates semiclassical backreaction of a conformally coupled massless scalar field
on the geometrical background of a nearly spinning cosmic string"

Be still my beating heart

Not sure what "nearly spinning" means

Do they have trouble making it spin

They should take it to the mechanic

0celo7

13:29

Semiclassical = semiheresy

Slereah

What do you not like in QM, Ocelot

0celo7

It's god damn weird

Slereah

Well so is GR

0celo7

No

GR is perfectly logical

Slereah

But still weird

It's not even deterministic, either

0celo7

13:34

Within a globally hyperbolic region it is

Slereah

Oh sure, just pick the spacetime you want!

Why not Minkowski space, it's nice

0celo7

Is your mother in Minkowski space

Slereah

My mama's so far she is a Schwarzschild metric :(

0celo7

Speaking of that

ACuriousMind

@0celo7 lol, no

Slereah

13:36

Really the non-determinism of QM isn't the weirdest part to me

It's more the collapse thing

0celo7

I started to edit a clothing mesh for my female body mod

ACuriousMind

::shrugs:: Just don't care about ontology, and you'll be fine

0celo7

Then the program crashed after 20 minutes of hard work

Can't find the save file

Slereah

yeah, but

Different ontologies may hide actual physical differences

0celo7

@ACuriousMind on mobile, what is lol no

ACuriousMind

13:38

@0celo7 "GR is perfectly logical" :D

Slereah

Well the nice thing about GR is that it is mathematically well defined

0celo7

@ACuriousMind go read Straumann

ACuriousMind

@Slereah I have yet to see actually different physical predictions from the people fighting about all these interpretations and ontologies.

0celo7

Shoo

Slereah

@ACuriousMind : Well in the past some interpretation got pushed off because of this

Nowadays it is harder because we have axed off most of the easy to check interpretations

0celo7

13:39

@Slereah removing the singularity from the manifold helps

ACuriousMind

Yeah, and now they construct them carefully not to predict anything different from "ontology-free" QM :D

Slereah

Did you know you can do GR without singularity shenanigans by using distributions

But it is even worse

Non-linear distributions are some awful business

0celo7

I hate distributions in QM, I don't want to ruin GR with them

Should I stream my modding?

HDE 226868

Aww, I missed the Dr. Who conversation!

Slereah

Apparently it is for dumb babies, though

HDE 226868

13:48

My sister started to watch the 5th series, and I got dragged in to liking it. So I suppose I'm a dumb baby, and a Matt Smith fan.

John Duffield

@Slereah : GR isn't weird, not when you read the original material. @ACuriousMind: the different physical predictions relate to black holes, see mathpages.com/rr/s7-02/7-02.htm which refers to two GR interpretations.

Kyle Kanos

Best Doctor ^

HDE 226868

@KyleKanos But . . . but . . . bow ties are cool!

Kyle Kanos

Have you seen the scarf?

HDE 226868

Have you see the fez?

Kyle Kanos

13:51

Nope. Tennant was the last Doctor I saw

Slereah

Have you seen WILLIAM RIKER?

Kyle Kanos

And he still had Martha

Slereah

(I have still not seen Doctor Who!)

Well I saw the first doctor I guess

Grumpy old man

Kyle Kanos

There's been like 3-4 seasons between or something crazy like that

HDE 226868

I really wanted to watch Tennant, but I don't think the library has his episodes yet.

Kyle Kanos

13:52

I thought Eccleston did a great job with the Doctor in his one season

Slereah

Hello I am old

HDE 226868

I don't think the Doctor will ever be played by a remotely old guy ever again. Besides Capaldi.

0celo7

Never watched Dr Who

No clue what it is

ACuriousMind

@0celo7 Tsk.

0celo7

And everyone talking about it makes me want to watch it less

Slereah

13:54

It does have terrible fans

HDE 226868

Hey.

0celo7

@HDE226868 Face it, you're terrible.

Slereah

You know what bothers me?

HDE 226868

@Ocelo7 Exterminate! Exterminate!

Slereah

In the first episode of Sliders

0celo7

13:56

@Slereah the fact that my female character's butts stick out of clothes because Bethesda likes PG13 women?

Slereah

Professor Arturo asks "the symmetry group of the Dirac field" or somesuch

And the answer is

0celo7

Is that just me

Slereah

U4

The hell

I don't think U(4) is even related to any physics

0celo7

Does the straight Dirac field even have a symmetry group

HDE 226868

A bigger question is why do things that don't actually work need explaining with astrophysical mechanisms? Any credible news source on the issue explains exactly why this machine doesn't work. — Kyle Kanos 1 hour ago

That should be the EM Drive close reason.

Slereah

13:58

Prof. Maximilian Arturo: The answer to the question is U-4, not U2, Mr. Bennish! [pointing to Conrad Bennish, Jr., who was listening to noisy music without paying any attention to the class].

Dammit Arturo

I would have thought you knew your stuff

ACuriousMind

@Slereah Ooooh, Sliders.

0celo7

More stuff I've never heard of before

Slereah

Ah, there was the question :

" Join the revolution or suffer the consequences! Thanks for the warning! As even the most intellectually impoverished physicist knows, the largest symmetry group of a single Dirac field is? The silence is deafening."

Dammit Arturo, whi U(4)

It should be like SL(2,C)

I dunno

If you use U(4) you're gonna mix the left and right spinors willy nilly!

Kyle Kanos

TV show physics needs to sound impressive, it doesn't actually need to correct

Slereah

Sure but it bothers me that they took the trouble of asking something complicated like the symmetry group of a Dirac field and not have it correct

You can even do it still with the pun for U2

SU(2), mister Bennish, not U2!

0celo7

14:02

SL(2,C) is the Weyl group, no?

Maybe, maybe not

Can't into physics now

Slereah

It's the spin group of SO(3,1)

ACuriousMind

@0celo7 It's (almost) the Möbius group. The Weyl group is something of a root system, it's not a specific group AFAIK.

Danu

Awwyis, confirmed going to Corfu 2015 workshop/summer school

0celo7

@ACuriousMind I mean that Weyl spirits transform under the fundamental rep of SL(2,C)

Danu

@ACuriousMind It's the group of reflections of the roots

0celo7

14:07

Spirits = spinors

Slereah

Popov ghosts transform under spirits, though

Danu

Gotta love that elite funding :D

ACuriousMind

@0celo7 Still, that's not the Weyl group :P

@Danu Sell-out ;)

Danu

@ACuriousMind Hey, the thing I'm going to is on non-commutative geometry! Doesn't get much more useless than that :D

Kyle Kanos

14:25

Hmm. There's a question that is a HW that is actually a dupe of another HW question that's closed as Too localized.

Slereah

HW?

Kyle Kanos

HW = Homework

0celo7

Lol @ secret service guys texting

Slereah

Mythology SE asks the hard questions

"Why does Inanna rejoice at her wondrous vulva?"

Well, wouldn't you

HDE 226868

Gee, thanks.

0celo7

14:36

@Slereah lol

Slereah

14:50

I wish there was a crazy stack exchange

Ancient aliens stack exchange or somesuch

although quite amusing : meta.mythology.stackexchange.com/questions/279/…

Kyle Kanos

16:03

I was literally typing something close to this out when I got scooped by Rennie:

The EM drive doesn't work, so the maximum speed is zero. In the studies you link the measured thrust is so small it could easily be an unsuspected systematic error. No-one has unambiguously demonstrated thrust from an EM drive. — John Rennie 1 min ago

dmckee

16:30

@JohnDuffield As a matter of vocabulary "serial voting" (the thing we have a automated process to catch and reverese) means a single user casting a series of votes on several posts of another user. That's clearly not what you have here.

In principle a series of votes on a single one of your posts could represent sock-puppet voting, but there isn't anything that pops out on that front. In any case, we have tools to help us catch this.

Nor is it my position--as a moderator--to judge the correctness of either votes or posts. I vote just like any other user, but I don't have the power to reverse another user's vote.

I'll say this, the use of vocabulary and the consensus on the est way to present material in physics changes in time.

I'm not a relativist, but I am a particle physics experimenter, and you seem to be insisting that there is only one right way to talk about thing and that other ways are not merely less than optimal but actually wrong. And you do it while advocating for language that seems dated to me: not in keeping with the presentation used today.

Generally I can see where that version is going, but it clearly would require a cascade of changes in presentation and emphasis. I assume that modern author have a reason for not liking the resulting semantics.

This is very like the "debate" over "relativistic mass".

It's not about one way giving right results and the other giving wrong results. It's about one way fostering misconceptions and difficulties as students try to go forward and the other causing less of that.

And the simply appeal to authority "But Einstien worte..." is absolutely useless. This is not a religion and Einstein is not a prophet.

dmckee

So here is the run: I won't and can't declare your posts correct. And the other mods also can't do that.

dmckee

As far as closing goes, you're barking up the wrong tree. I've always been in favor of more and faster closing and higher standards.

Even for first time users. That is the way we establish a highly professional mode of discourse.

If you see people breaking the "Be nice." rule in their closing comments, then flag that instance.

dmckee

16:54

@gonenc When Stack Exchange was in beta the rule was +10 for upvates and downvotes were -2 to the poster and -1 to the asker for all posts.

Later two changes were made to the treatment of question. The upvote value was reduced to +5 and the cost tot he downvoter was eliminated.

Slereah

So anyway apparently chronology protection (from the vacuum explosion) seems a pretty common feature of CTCs

dmckee

So, why the difference in treatment at all? meta.stackexchange.com/questions/62130/… meta.stackexchange.com/questions/15863/…

Why the change to the upvote value on questions? meta.stackexchange.com/questions/7322/…

Slereah

It seems to be due to closed timelike curves generating some lim_x->y 1/s(x,y)^4 term in the stress energy tensor

That is pretty badly divergent

dmckee

Why the change to the downvote treatment on questions? meta.stackexchange.com/questions/90324/…

Slereah

its only saving grace is that the value can be made arbitrarily small before the Cauchy horizon

So some people are vaguely hoping to pass that problem to quantum gravity

Somewhat sad

dmckee

16:58

I'm sure I've missed a couple of meta questions that have important bits of the debates in them. But the short version is: Downvotes are important but should be used too freely, questions were reduced in value and downvotes on questions were made free to give the crowd a stronger hand in discouraging endless barrages of really bad questions.

Slereah

Also hello @dmckee

dmckee

Hi.

Kyle Kanos

17:22

So apparently c=0

Slereah

.Dammit

You broke the universe again

2

dmckee

@gonenc This would be a good thing to flag. We have a rule concerning overt self promotion.

BTW--Am I the only one who is starting to assume that a researchgate link implies the thing linked probably isn't published in a reputable journal?

gonenc

@dmckee should I flag it for mod attention?

dmckee

Please. I'm not sure how to respond right now and am running out of time. Maybe one of the other guys will field it.

Slereah

I'm not too fond of Research Gate because you have to register to save pdfs

gonenc

17:30

@dmckee consider it done should I flag all of them or just one linking the other ones in the flag description?

dmckee

One is enough. Just say he's been posting the same link a lot.

gonenc

17:47

@dmckee done

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