The h Bar

Slereah

11:22 AM

I like how the flat torus has a specific name

WAIT!

Apparently there is an existing shape for a plane with a handle!

The Dupin parabolic cyclide!

for $k < 1$

Or... is it?

I'm not sure

I mean apparently it's made from parabolas, so I guess it's not compact

Slereah

11:50 AM

"By displaying the circles there appear gaps which are caused by the necessary restriction of the parameters u,v."

Hm

Oh wait

I think it's another case where the coordinates may not be injective in some places

dtic.mil/dtic/tr/fulltext/u2/p012051.pdf

Qmechanic

12:07 PM

3

Q: Scalar Curvature of a Conformally Flat Metric

Suppose that you have a metric $g_{\mu\nu}=\phi^2\eta_{\mu\nu}$ for some function $\phi$. There is a standard formula for what the scalar curvature $R$ looks like in terms of $\phi$, which is given by (in dimension 4) $$ R=-\frac{6}{\phi^3}\eta^{\mu\nu}\partial_\mu\partial_\nu\phi. $$ In trying...

Temporarily locked question to avoid migration cf. meta discussion about on-topicness of math questions. Input to how we proceed in this question is welcomed. ...[June 15th, 2017: Unlocked.]

Slereah

books.google.fr/…

Possibly of note

Secret

12:20 PM

4D Clifford Torus Hypersurface

►

actually, I am not sure if they are the same thing

.

user116211

@DanielSank just by clicking on it, we could check the number of upvotes and downvotes ;/

user116211

Hallo!

user116211

@yuggib We could invite big names but first at least start AMA with our guinea pig ;P

user116211

@Slereah: Is your thesis in French?

Slereah

Yes

Hence the french words

user116211

12:36 PM

@Slereah Okay, so.... okay ;_;

user116211

How can i make it to English T__T

Slereah

Eh

It's not the most original work

I'd advise you to read Demichev's book on path integrals if you want most of what's in it

user116211

@Slereah Merci beaucoup

Slereah

Also Grosche, Kleinert

Errr... Marinov, I think?

Or something

Look at the bibliography, really

Basically the original conclusion of the thesis is that the Ito integral for the action gives the only covariant result

Slereah

12:57 PM

Which is probably due to the fact that the measure is $\prod \sqrt{g}(x_i) dx_i$

Danu

@Qmechanic The post you linked in your comment on that locked question leads me to an unrelated post on Mathematics.

Qmechanic

@Danu : Ups. Thanks. Copy & paste error. Corrected.

0celo7

@Slereah what

Slereah

1:15 PM

The propagator is $$\approx \int \prod \sqrt{g(x_i)} dx_i e^{g(x_i + \lambda \delta x) \delta x^2}$$

0celo7

for what

what is that

Slereah

Point particle in a curved space

0celo7

ohhhhhh

what is that exponential

Slereah

The action

0celo7

hmm?

Slereah

1:16 PM

$$e^{\frac{i}{\hbar} S}$$

0celo7

what are all those terms

Slereah

cf the thesis

0celo7

are you sure that's right

Slereah

Or read... "K.S. Cheng, Quantization of a General Dynamical System by Feinman's Path Integration Formulation, Journal of Mathematical Physics, vol. 13, Issue 11 (1972)"

I think that's the one

0celo7

1:38 PM

@Slereah what is $g$

and what is $\lambda$

and what is $\delta x$

Slereah

$g$ is a Riemannian metric

$\lambda$ is the parameter of the stochastic integral

0celo7

no, what exactly is $g$

the determinant?

Slereah

It is shorthand here

The correct form is

0celo7

wtf

Slereah

It's like @0celo7 doesn't even know I'm a lazy shit that will not write out things in full

Gee get a clue guy

0celo7

1:44 PM

I still don't know what any of that means

Slereah

It's a lattice form of the integral

0celo7

if you need coordinates this is only a local result

but path integrals are global, no?

so this is wrong

Slereah

$$\int dt g_{ab}(x) \dot{x}^a \dot{x}^b \rightarrow \lim_{\varepsilon \rightarrow 0}\sum \varepsilon g_{ab}(x_i) \frac{x^a_{i+1} - x^a_i}{\varepsilon} \frac{x^b_{i+1} - x^b_i}{\varepsilon} $$

0celo7

what is $\epsilon$

Slereah

A small increment of time

0celo7

1:47 PM

yea that's coordinate bullcrap

some back when you have a global result

Slereah

$t$ is not a coordinate in non-relativistic QM

0celo7

why is there a metric in NRQM

Slereah

I told you

It's a RIEMANNIAN metric

0celo7

I thought you didn't know the proper terminology

Slereah

Well obviously you thought wrong

0celo7

1:48 PM

but you are still using coordinates

Slereah

For I am never wrong

0celo7

so how is this correct?

Slereah

b/c it is

ACuriousMind

@0celo7 Have you considered reading the thesis to find out? :P

Slereah

Although

There is a trick

0celo7

1:49 PM

@ACuriousMind no

IT'S IN FRENCH

Slereah

because $$\dot x \not \to \frac{x_{i+1} - x_i}{\varepsilon}$$

ACuriousMind

@0celo7 I have been told that reading French papers without speaking actual French is a skill a mathematician has to acquire

Slereah

You must use a Clever Ruse to get the discretized form

0celo7

@ACuriousMind I'm not a mathematician

@Slereah why is that not true

Slereah

Because

yuggib

1:51 PM

@ACuriousMind of course it is; so many crucial papers are only in french

ACuriousMind

@0celo7 Did you drop your math studies? oO

0celo7

@ACuriousMind no

ACuriousMind

@0celo7 So you're a mathematician-in-training. That needs to learn to read French.

0celo7

@ACuriousMind but I don't want to be one

Slereah

$${x} (t + \varepsilon) = x(t) + \varepsilon \dot x(t) + \frac{\varepsilon^2}{2} \ddot x(t) + ...$$

user116211

1:52 PM

@yuggib Many Bourbaki books are still only in French T__T

Slereah

BUT

In a stochastic integral

The $\varepsilon^2$ doesn't go away

yuggib

@MAFIA36790 yes, but you'll probably find some of that things in english papers/books as well

0celo7

wtf my level 56 Alakazam only knows confusion

Slereah

and $\ddot x = -\Gamma \dot x \dot x$

yuggib

the problem is that there are research papers, even recent ones, only in french

Slereah

1:53 PM

To discretize the position you have to expand in geodesic terms

user116211

@yuggib even German books are translated in English; so yeh they are mean ;/

Slereah

That's why the real expression is

0celo7

holy crap Jolteon only knows tackle

Slereah

0celo7

damn mystery dungeon

yuggib

1:54 PM

@MAFIA36790 they're not mean, simply french

0celo7

why does the $\epsilon^3$ term go away

user116211

@yuggib yeh :((

yuggib

and for a frenchman, speaking english is like throwing punches at its own head very hard

Slereah

The action is $\approx \sum \varepsilon \dot x^2$

So the lowest $\varepsilon^3$ term will be

$$ \sum \varepsilon \frac{\Delta x}{\varepsilon} \varepsilon^3 (\frac{\Delta x}{\varepsilon})^3$$

Which is gonna be $\sum (\Delta x)^4$

which doesn't converge

Well, it does, to 0

user116211

BTW, @yuggib after completing a major part of the book (Grillet) I would ask you about recommending me books on functional analysis. I'm also going through Axler's Linear Algebra.

user116211

1:59 PM

The later of course deals with finite-dimensional vector space.

Slereah

Tho maybe ask a math guy if you want a rigorous demonstration of this :p

This is a very physicky proof

yuggib

@MAFIA36790 ok; but there are a lot of good functional analysis books out there

user116211

@yuggib Yeah; that's why I asked you :)

Slereah

Usually even the $\varepsilon$ term would go away, since the action for that would be at most $$\sum \varepsilon \frac{\Delta x}{\varepsilon} \varepsilon \frac{\Delta x}{\varepsilon^2} \approx \sum \frac{\Delta x^3}{\varepsilon}$$

0celo7

I have no idea what you're talking about.

Slereah

2:07 PM

But in the case of a stochastic integral, you have $\Delta x^2 \approx \varepsilon$, so it is something like $\sum \Delta x$

So something of the form $\int dx$

vzn

@DanielSank distributed over only a few users, no Q/A added by anyone else :| ... and maybe nobody else noticed but the chat ad dropped to 5v lately, needs more votes. we cant ignore/ "sweep under rug" active opposition which is already non-negligible :(

Slereah

Well read the paper I pointed to you

user116211

@vzn Hmm; it's still getting downvotes ;/

vzn

@MAFIA36790 yikes it got another -1 since last nite! aieee o_O

user116211

@vzn You need to concentrate on why the downvotes.

vzn

2:11 PM

@MAFIA36790 lol downvoters have nothing to say, just need more voters :)

user116211

@vzn okay, this maybe frustrating but Emilio aired his opinion greatly. I see other users also; but yeh if you are not going to comment how we could know ;//

lucas

@dmckee Is this one complete answer too?

May someone (rep>10000) help me to know if this answer is complete? physics.stackexchange.com/questions/258518/…

ACuriousMind

2:28 PM

I...actually wouldn't say that that answer is "complete" in any sense. It's a picture whose relation to the question is unclear and three formulae without any explanatory text.

Slereah

If you want the "serious" version btw

Itô calculus

Itô calculus, named after Kiyoshi Itô, extends the methods of calculus to stochastic processes such as Brownian motion (see Wiener process). It has important applications in mathematical finance and stochastic differential equations. The central concept is the Itô stochastic integral, a stochastic generalization of the Riemann–Stieltjes integral in analysis. The integrands and the integrators are now stochastic processes: where H is a locally square-integrable process adapted to the filtration generated by X (Revuz & Yor 1999, Chapter IV), which is a Brownian motion or, more generally, a s...

The particle position is a stochastic process $X_t$

lucas

@ACuriousMind So, should it be deleted?

ACuriousMind

I would not have flagged it for deletion, but I would have downvoted it because it's unclear what it's saying and because it tried to answer a blatant homework question.

I will not vote to undelete it because it doesn't add anything useful.

lucas

You think those formulas cannot solve the problem?

ACuriousMind

@lucas No, I think the answer as written is a) not an answer to the question b) not useful to anyone who didn't already know these formulae.

lucas

2:37 PM

@ACuriousMind If he already knows the formula, he should solve the problem.

But he couldn't.

ACuriousMind

Yeah, and if he doesn't know them, it's completely unclear what these formulae are supposed to tell him. There is no text explaining what these formulae pertain to, or how they are supposed to answer the question.

Slereah

columbia.edu/~ks20/FE-Notes/4700-07-Notes-Ito.pdf

^for more details

ACuriousMind

Also, I don't know what your issue is. You have already been told that we do not like people giving answer to blatant homework questions because that encourages more of the same. Go find some better questions to answer.

lucas

@ACuriousMind If I was explaining the answer, then they were saying that my answer is a complete one and it should be deleted.

ACuriousMind

@lucas Yeah. So the conclusion is: Just don't answer homework questions!

Simple as that.

Slereah

2:41 PM

@Wouter why is your gravatar a swastika

lucas

@ACuriousMind I think that is not only homework question.

vzn

@JohnRennie thx much for your interest in the guest session, did you see/ +1 the chat ad? :)

Wouter

@Slereah It isn't. It's still my original gravatar automatically generated by PSE.

But thanks for pointing that out, now I can't look at it anymore without seeing a swastika. :P

lucas

@ACuriousMind I think he has not seen that $r$ and $\theta$ are expressed like this one before.

ACuriousMind

@lucas The question just poses a problem and asks us how to solve it (or rather for "any help"). It's the most blatant homework(-like) question possible

@lucas You may well think that. How is that relevant?

lucas

2:46 PM

@ACuriousMind OK. So I shouldn't answer any question here. Because all of my answers are in contrast with our policy.

0celo7

@lucas If you say so

lucas

@ACuriousMind May you please check my other answers and flag them for deletion?

@0celo7 What does this sentence mean?

0celo7

@lucas this kind of behavior does not work

ACM has literally no feelings

lucas

@0celo7 What kind?

0celo7

@lucas throwing a tantrum, saying stuff like "all of my answers are in contrast with our policy"

lucas

2:48 PM

@0celo7 What does mean "ACM"?

ramsay

$f(x)=2,\,\,g(x)=3\implies f'(x)=g'(x)$ and then integrating this we get $f(x)=g(x)$ which is contradiction! (*this is the method my teacher did but functions were different*)
is this correct?

ACuriousMind

@lucas What? I have never looked at "all" of your answers. We were talking about your one answer there that dmckee deleted.

0celo7

ACM has never been frustrated so he has no empathy for it

Wouter

For those not currently in the business of discussing homework questions: I'm looking for a simple model for interatomic interactions. My chemistry students would have to show that it leads to harmonic motion for small deviations from equilibrium.

ACuriousMind

@ramsay what? Never forget the integration constant!

lucas

2:50 PM

@0celo7 Sorry, your sentences are hard. I am using dictionary at the moment.

ramsay

@ACuriousMind but he didn't use any constant that's why i got damn confused

Wouter

I should say I'm looking for something else than the Morse potential or the Lennard-Jones potential.

ACuriousMind

I was just about to write "Lennard-Jones" :P

lucas

@0celo7 If it is possible, use simple sentences please.

ACuriousMind

I think that's the only model for interatomic interactions that I know :D

Wouter

2:51 PM

@ACuriousMind :D Thanks anyway :)

0celo7

@lucas I've been told my vocabulary/sentence structure is not very complex

lucas

@0celo7 I believe it. My English is so terrible.

The problem is my English not yours.

0celo7

The real issue is that ACM is the modern-day embodiment of the German war machine

But in a physics form

He feels no pain, sympathy or empathy.

Slereah

My word speaking not so good :(((

ACuriousMind

@0celo7 What happened to "mother duck"? :P

2:56 PM

oh lord

lol

@ACuriousMind dammit why is Einstein screaming at me

ACuriousMind

@0celo7 How am I supposed to know? Remember, I don't have empathy!

0celo7

he's squirming on the floor like a larva

@ACuriousMind for humans!

ACuriousMind

2:57 PM

Did you feed him recently? :P

0celo7

of course, I feed him every morning

and he has dry food out

ACuriousMind

Does he need to be brushed?

0celo7

Always

ACuriousMind

Perhaps something's itching him if he's squirming like a larva

Could just be the aliens hatching, though

Slereah

Is Einstein the cat

0celo7

2:58 PM

@Slereah No

@ACuriousMind he need to be brushed perpetually

the moment you stop brushing he needs to be brushed again

ACuriousMind

There's your answer, then!

0celo7

long haired cat: never again

Slereah

Get yourself one of those hairless cat

HISSSSS

So very pruny

ramsay

@Slereah eiiyukk

0celo7

no thanks...

@ACuriousMind white, gray, blue or tan shoes today

oh or darker gray

ACuriousMind

3:03 PM

white

0celo7

3:21 PM

@ACuriousMind good choice.

@ACuriousMind I'll have a larger selection for you when I'm back at school and have all my stuff ;)

ACuriousMind

Oh. Good.

0celo7

@ACuriousMind Not very enthusiastic!

lucas

4:00 PM

@JohnRennie: Hi John. How are you?:-)

John Rennie

@vzn Just upvoted. Still only at +3 though ...

@Wouter I'm sure I answered a question on that topic a little while back ...

lucas

@JohnRennie: Why don't you talk with me? We had a scientific discussion. Just this. Why do you hate me just because of a scientific discussion? Although I apologized several times.

John Rennie

@Wouter It doesn't matter what the potential is because in the immediate vicinity of the minimum it looks parabolic and therefore results in simple harmonic motion.

DanielSank

4:21 PM

@vzn If you read the comments on the community ad you'll see that multiple people had the same problem: the link doesn't work. There are six down votes. I think you should take the ad down and put it back when we're actually ready to deploy it.

lucas

@JohnRennie: I like the Trafalgar Square. If I could, I came to see that.

Slereah

4:37 PM

And home

So I sorted out a bit my firefox tabs

I put all the science papers in a window

Almost 300 tabs

vzn

@DanielSank read all the comments & responded. the link works. the ad was already "deployed" briefly. cant plz all the perfectionists & naysayers. downvote reasons do not nec match comments. presumably there is some silent opposition for arbitrary reasons. its good enough for now

DanielSank

5:09 PM

@vzn It links to the general chat page.

Did you read innisfree's comment?

Slereah

@DanielSank Is the meta thread I posted alright btw

DanielSank

@Slereah Link?

Slereah

1

Q: Questions for the june 14th AMA

Hey I'm the scheduled guest for the june 14th Physics AMA and I was told to maybe make a little presentation and asks for any questions for the AMA people might have, so here it is. So I am Samuel Lereah, got a Master degree in particle physics from the university of Nantes. My master thesis was...

discussion

lucas

Money generates money. Reputation generates reputation. Viva the Capitalism. Down with the logic.

DanielSank

@lucas Wut?

@Slereah Thanks for doing that.

Slereah

5:15 PM

I didn't even say any swears

DanielSank

@Slereah Give me a minute. I'm going to do something nice.

DanielSank

5:34 PM

Ok, @Slereah here's a meta post with the schedule and other info.

'sup @ChrisWhite?

Loong

@DanielSank You might want to explain what AMA means.

user54412

@DanielSank fun fact: it's apparently a holiday outside my department

DanielSank

@Loong Good point. Done.

@ChrisWhite But not inside?

Loong

@DanielSank thanks

user54412

we don't believe in work/life balance

DanielSank

5:37 PM

@ChrisWhite We? You got a mouse in your pocket?

Physics Meta

0

Q: Physics Stack Exchange "Ask me anything" (AMA)

Purpose The user base of Physics Stack Exchange is rather diverse in terms of cultural background, career progress, field of expertise, and personality. While the discussion in hbar is lively, it is mostly focused on physics, with some side conversations on a variety of topics from cooking to fa...

discussion

user54412

@DanielSank No I'm just happy to see you :p

DanielSank

@vzn see meta. I also fixed the link in the ad to point at the meta post so people will find the schedule etc.

Bernard Meurer

Howdy

DanielSank

@BernardMeurer Hi.

Bernard Meurer

5:43 PM

@DanielSank <3

DanielSank

<3

Bernard Meurer

Nice work on getting the AMA going

ACuriousMind

Jeez, get a room you two

DanielSank

@BernardMeurer ?

You mean the meta?

Bernard Meurer

@ACuriousMind Want to join?

@DanielSank Yeah the meta posts and all

ACuriousMind

5:44 PM

@BernardMeurer Nah, I'm fine

DanielSank

@ACuriousMind awwww, why not?

I'm nice.

Hello @EmilioPisanty.

Thanks for finding that typo.

Emilio Pisanty

No worries

Bernard Meurer

@ACuriousMind He's nice I can attest to that

He bought me a burrito once

ACuriousMind

@DanielSank I fear the fury of your wife :P

DanielSank

@ACuriousMind Not married yet.

Bernard Meurer

5:48 PM

Oh yeah, soon enough I'll have to ask permission to send Dan <3 :O

DanielSank

@BernardMeurer Nah it's cool.

She understands.

Bernard Meurer

@DanielSank You sure? I have nice calves, people get jelly

DanielSank

@BernardMeurer It's cool. Trust me. You're not 10% as hot as my fiance and she knows it.

heh

Bernard Meurer

</3

DanielSank

@EmilioPisanty For whatever it's worth, I appreciate your timely and precise feedback on the community ad. I was rather dismayed that it was posted so prematurely and without proper linking, etc.

Bernard Meurer

5:50 PM

:)

Well, I'm going outside today for the first time in a week

I'll catch you folks later

DanielSank

@vzn Take a look at the comments under the community ad. I think it's worth while to understand why EP was critical at the beginning. He was trying to help. Because the ad was released before there was a meaningful link, the post now has six down votes. It's hard to turn that around because there's no mechanism for people who have down-voted to get notifications of improvement.

This is an example of preparation being important.

Emilio Pisanty

@DanielSank No worries.

I don't know if it shows, but I rather care about the community ads.

(grin)

user54412

It shows, and it's a good thing

DanielSank

@EmilioPisanty I figured you just cared about content being of high quality, which is obviously good.

Emilio Pisanty

@DanielSank Yeah, that's right on the money.

DanielSank

5:54 PM

@ChrisWhite @EmilioPisanty so what do you guys think? Now that I fixed the link should we delete the ad and start over?

user54412

We don't ask you to do our work, so please don't ask us to do your homework for you. — Jon Custer 3 hours ago

user54412

^ New HW close reason

DanielSank

I'm afraid of those 6 down votes lingering because people won't re-check what they've down-voted.

Emilio Pisanty

@DanielSank I'd say don't worry about the downvotes

DanielSank

@ChrisWhite :( That's not a helpful message. That kind of sarcasm does not breed better users.

Emilio Pisanty

5:55 PM

The exact score is not important

What matters is that it's >6

DanielSank

@EmilioPisanty orly?

Emilio Pisanty

After it clears the bar it goes live and there isn't any mechanism I'm aware of that will show it less often for having a lower score

(though there might be)

DanielSank

Ah ok.

Emilio Pisanty

It's at score 3 now

So all you need is three more votes, which I don't think should be that hard

user54412

@DanielSank Call me a pessimist, but I feel anyone who just dumps their assignment on us verbatim is morally bankrupt enough that they'll never contribute to a community anyway.

Emilio Pisanty

5:57 PM

@DanielSank I'll try and up the graphics in the next few days if I have time

DanielSank

@EmilioPisanty Thank you.

What's posted there was a five minute sketch I made and never intended anyone to post.

Emilio Pisanty

@ChrisWhite Off topic, but I'd like to show off my ←↓→↑ arrowing capabilities direct from keyboard, really pays off.

user54412

@EmilioPisanty Are those standard, or do you use arrows so much you made a macro for it?

Emilio Pisanty

@ChrisWhite I use a custom software keyboard