The h Bar

What's the difference in the Riemann tensor and the hessian in terms of the curvature information they measure? Is the riemann tensor contains curvature information at a point that is from the hessian already (that is is the curvature information from the hessian a subset of that in the riemann tensor?

FenderLesPaul

Hessian tells you about extrinsic curvature of embedded submanifolds

0celo7

@FenderLesPaul please make that more precise

I was just talking to an engineer buddy of mine about Hessians

FenderLesPaul

idk why I changed it to extrinsic curvature

I meant Gaussian curvature

@0celo7 determinant of Hessian gives gaussian curvature of the surface

whereas its trace gives you the mean curvature

0celo7

Hessian of?

FenderLesPaul

the embedding map

0celo7

2:47 AM

is that the function $f=0$?

FenderLesPaul

yes

0celo7

gaussian is intrinsic, mean is extrinsic?

FenderLesPaul

yeah

The gauss-codacci equations relate the two

0celo7

yeah, I feel like I know this

I could read that section in Straumann

buuuuut, too tired

humblebundle.com/store/promo/rockstarweek

@PC gamers, Rockstar sale

dmckee

Coming to chat to avoid posting a not-constructive comment that I'm just itching to issue.

4

Q: Can tin foil hats block anything?

This is joked about all the time, but... Can tin foil hats actually block anything? If they can, what frequencies? Is there any research into tin or aluminum foil and radio blocking or amplifying abilities when shaped into a hat? If they really don't do anything, what would be better? Radio blo...

Answer: Respectons. They block 100% of respectons.

0celo7

2:56 AM

100% of womenons

obe

I think they're cool.

bolbteppa

5:16 AM

PDF Page 446 http://www.itp.phys.ethz.ch/research/qftstrings/archive/13HSStrings/StringsHS13Notes.pdf
"$[J_3,J^+] = J_3, [J_3,J^{-}] = - J_3$, In intuitive terms, this tells us the charges of $J^{\pm}$ with respect to the $U(1)$ subgroup generated by the Cartan $J_3$."
Huh?

What is this trying to say?

Also: "$|J^{\pm}>$ tranform with charges $\pm$ under the $U(1)$ generated by $J_3$."

bolbteppa

6:03 AM

@0celo7 @ACuriousMind I remember asking you about the $1/2\pi i$ in the CFT charge $Q = \dfrac{1}{2\pi i}\int dz T(z) \varepsilon(z)$, think it's partially explained on page 55, section 7.5, here itp.phys.ethz.ch/research/qftstrings/archive/13HSStrings/… if you grok 7.46, point is there is an explanation, wahoo!

Needed that delta function to justify it as far as I can see

Danu

6:30 AM

Does anyone know any textbooks on phase space quantization (Moyal product etc.)?

David Z

@0celo7 if it's a woman wearing the tin foil hat, does it then block men?

Danu

Sigh

I think the most upvoted answer to that question should be edited to get rid of the jokey parts

David Z

I don't think so. IMO it's fine for an answer to have some humor as long as it does also constitute a proper answer.

yuggib

8:14 AM

@Danu There is a pedagogical introduction on chapters 22 and 23 of the book by Hall:

link.springer.com/book/10.1007%2F978-1-4614-7116-5

In the nlab page on geometric quantization there are many other refs...

Slereah

8:28 AM

@Danu I have a paper about the link between phase space quantization and path integrals, if you want?

Hm, let's see

José Enrique Moyal, Quantum Mechanics as a Statistical Theory, Proceedings of the Cambridge Philosophical Society, vol. 45 (1949)

Aku Valtakoski, Mathematical Aspects of Functional Integration, University of Helsinki (2000)

also

sciencedirect.com/science/article/pii/0370157380901118

Danu

@Slereah Oh, that's from Moyal himself? Neat.

@yuggib Thanks, I have this book

Slereah

I think it's the original paper

basically you replace operators with regular functions but a weird multiplication

Danu

I'm currently at a conference about noncommutative geometry

It's all about this stuff :P

yuggib

non-commutative geometry and physics or non-commutative geometry in general?

Alain Connes / van Suijlekoom stuff?

I would like to know better these things one day or another...not sure it is worth it though :P

Danu

In physics

There are also a lot of talks about nonassociative quantum mechanics

why.jpg

Slereah

8:44 AM

I wonder if all formalisms of QM have a method to switch from one to the other directly

Or if in some cases you need intermediate steps

yuggib

I would like to understand the "quantum mechanical" formulation of the Riemann conjecture given by Connes...but then I would have to know all about Adeles and so on o.O

Danu

@dmckee Democracy is sacred

Slereah

George Washington died for your sins

skill patrol

9:56 AM

"Freedom" is just another word for nothing left to lose...

Physics Meta

11:55 AM

0

Q: how about a what-if tag?

Just musing -- there are a measurable number of questions which, while more or less physics-related, are a much better fit for what-if.xkcd. Is it worth creating a "what-if" tag for these? The discussion at Are outlandish XKCD style What If questions welcome here? seems to suggest at least som...

feature-request

0celo7

12:22 PM

Good morning

Or whatever time it is in the third world

Slereah

Hello

skill patrol

bonjour

ACuriousMind

@0celo7 'Morning' fits, I just woke up ;)

0celo7

I know I'm getting better but these coughing fits are really annoying

@ACuriousMind so is there any hope of you reading page 212 ff in BBS and ELIE (explain like I'm an engineer)

Slereah

12:39 PM

Hm

That question about pair production from graviton

Feynman had a student who did a thesis on electrons in covariant gravity

I think the answer should be in there

But I can't remember the name of it

Also it was awful

Typewriter thesis

genealogy.ams.org/id.php?id=91222

Ah, the internet

0celo7

My thesis is gonna be

Slereah

Dissertation: Quantum mechanics of the interaction of gravity with electrons: theory of a spin-two field coupled to energy

That's the one

0celo7

Multieyed Cyrillic demon letter thesis

Slereah

physics2000.com/PDF/HugginsPhDThesis1962.pdf

As awful as I remember it

nothing about pair production though

blogs.umass.edu/grqft/files/2014/11/…

p. 21 has it, though

0celo7

@ACuriousMind read BBS...

Think of all the things I've done for you

ACuriousMind

12:56 PM

@0celo7 lol, I just looked at that. There's no way they develop the machinery to understand that in the hundred pages between where I stopped and where that is. :D

This book seems depressingly useless

0celo7

They seem to just throw out random facts with very little motivation

Read the section on anomalies. Totally useless too IMO

Maybe not if you have more experience with anomalies in QFT

@ACuriousMind have you tried polchinski?

ACuriousMind

@0celo7 Not yet

FenderLesPaul

1:18 PM

one of my professors, who is a string theorist, isn't very fond of BBS either

he says they just state things out of nowhere in passing with no justification

and you're left saying "what the hell?"

Slereah

BBS.

?

Is that the Big Book of Strings

FenderLesPaul

isn't that BBoS?

yuggib

Becker$^2$ Schwartz

or something like that...

0celo7

Indeed.

Apparently GSW is really good.

Slereah

too many beckers

I only have one string book

It's the one like

introduction to the bosonic string

or something

I forget the author

FenderLesPaul

1:32 PM

Is string theory an instrument?

Slereah

booo

Sean

Is anyone on here a member of a professional physics society like APS or AAPT?

0celo7

@Slereah sounds like polchinski

Slereah

that's the one yeah

FenderLesPaul

So someone posted a paper on the arXiv today that does exactly what I'm working on right now :(

0celo7

1:38 PM

Sucks to be you

Slereah

Exactly exactly?

Or can you expand on it

(PUBLISH OR PERISH)

ACuriousMind

@Sean I seem to recall @dmckee is

FenderLesPaul

I can expand upon it yeah

they only did a holographic calculation

so I can just expand on the CFT calculation

it just sucks because the correlator in the CFT that I thought would characterize computational complexity of a black hole is exactly what they used too

but I scrapped the idea 3 months ago because for a 2D CFT with a local quench the CFT calculation became a bit too annoying to do :p

0celo7

My laptop is dead. Physics lecture is gonna be so fuuuun

Slereah

You're gonna have to actually listen and learn!

Sean

1:46 PM

I'm just wondering if it's actually worth while to join one

0celo7

Do they charge dues?

I wouldn't think so unless you're doing research and need the connections

Sean

oh yeah, and they aren't cheap

I thought particularly that AAPT might be good, because i'm a physics teacher

0celo7

I'm the only resource you need to be a better teacher

Sean

plus it would be nice to have access to the journals, since it's not like i'm tied to a university that has access

0celo7

I can tell you what to do

Rule 1: it's never too early to introduce tangent bundles

Sean

1:51 PM

but I know with APS, the journals are still $50 bucks per online subscription even with a membership

yeah, i don't know what a tangent bundle is

ostensibly related to a fiber bundle

0celo7

Rule 2: it's never too early to tell them that electrons are 511keV photons in a Dirac's belt configuration

Slereah

@0celo7 : Introduce it when you do Newton's law

0celo7

Rule 3: quantum fluctuations are real

Slereah

None of that "vector in R^3" business

0celo7

Exactly.

If you're not learning mechanics from Arnold you're doing it wrong.

Sean

1:53 PM

hahaha yeah i teach real physics. none of this "electrons are light" business

@ACuriousMind gets so pissed at the phrase "quantum fluctuations"

0celo7

Another popsci cargo cult intellectual garbage man denying bona fide physics, Einstein and the Evidence

ACuriousMind

@0celo7 Well...$\langle A^2 \rangle - \langle A \rangle^2$ is real, that's correct...

FenderLesPaul

quantum fluctuations are responsible for Donald Trump's success

Sean

anyway, i just didn't know if joining APS was something that everyone whos anyone did, or if it was more restricted to people doing major research who are publishing

0celo7

@FenderLesPaul people who drag politics into chat deserve death >:(

@ACuriousMind no, I mean in space there are virtual particles popping in and out of existence

ACuriousMind

2:00 PM

Not taking the bait :P

0celo7

There's no bait, Zee says that

Are you telling me a physicist would just lie

@Sean can only physicists join?

David Z

@Sean probably many of us are APS or equivalent members

0celo7

Nuclear engineers can end up doing physicsy things

Slereah

@0celo7 No you fool

Think of Einstein and the evidence

David Z

@Sean no, there are no restrictions. Anyone can join. Only a fraction of the membership (30% maybe?) are research physicists.

0celo7

2:04 PM

On mobile, no clue what statement makes me a fool

Is evidence the same thing as The Evidence?

Sean

2:16 PM

@DavidZ do you feel like it's worth the dues for a high school physics teacher?

I'd like to do AAPT for sure, but i didn't know if APS would also be valuable

David Z

eh, not really. Actually even for researchers, the only reason I think APS is worth it is that if you're not an APS member everyone else in the community will wonder what's wrong with you ;-)

(this is for Americans, of course, not foreigners)

0celo7

I think ACM should become a high school physics teacher.

David Z

That being said, the APS meetings do often have some interesting presentations on education research, which might make it worthwhile for you. And I think (though not sure) you get a substantial discount on meeting fees for being a member.

Sean

do you go to a lot of the meetings?

I know @DanielSank has said that he goes to some

Slereah

2:32 PM

really saying that virtual particles aren't real is fine, but then you have to remember that real particles aren't either :p

0celo7

What did Einstein have to say on the matter?

Slereah

not sure

How old is the notion of virtual particle

40's?

0celo7

No clue.

One can only hope Bajoran writes a QFT saga and sets us all straight on the matter.

Feynman

3:14 PM

Hi guys, I am your old Godparticle (now Feynman), can you all support me in this proposal of creating polymath.stackexchange community:

area51.stackexchange.com/proposals/91400/…

0celo7

God, now I have a runny nose. I feel like a 5 year old 😩

ACuriousMind

10

Q: What are the funerary rites of Gnoll worshipers of Erythnul?

I'm playing a Gnoll Cleric who worships Erythnul in a D&D 3.5e game. Sadly one of my party members died in our last session (he's not a Gnoll). I would like to do something with his body (not just leave it where it fell). What are the funerary rites of an Erythnulian Gnoll? As a lore question, ...

dungeons-and-dragons religions-and-deities lore

Role-playing Games is great. :D

0celo7

Nerrrrrrd

ACuriousMind

Yes :)

Feynman

3:30 PM

@ACuriousMind: Can you support my proposal?

See above..

ACuriousMind

@Feynman I don't see a need for it. What kind of questions could you ask there that you cannot ask on any of the individual science sites? There are no example questions or definitions of scope yet.

Feynman

Okay, I am editing the questions now.. One of the example would be physics.stackexchange.com/questions/201530/…

that connects...

math, physics, history..

that is polymathic...

@ACuriousMind: I hope you are here...

I hope to see the site to be a home of questions which used to cross-posted...

@ACuriousMind: Can you support me now?

ACuriousMind

@Feynman But...that question is on topic here, and possibly also at History of Science and Mathematics

Why do you want a site that would only siphon questions away from other sites?

Feynman

Polymathic questions need attention of all the experts not just an physics, or math or history expert, so I see the requirement...

Can you support me now? :)

ACuriousMind

3:49 PM

No, because I do not see the need for such a site. Almost all questions which actually "need" an expert in multiple fields are too broad for the SE model, anyway.

Feynman

Okay...

@ACuriousMind: But, don't you think polymathic questions to be not broad for a polymath?

Can you support me now? :)

ACuriousMind

"Too broad" is not a characterization that depends on the answerer. It means a good answer is necessarily almost a book, since it needs to take so many things into account. If it doesn't need to, then the question could as well be answered on one of the existing science sites.

Also, I don't think "polymaths" still exist, we have accumulated far too much knowledge for that

What exists is people knowing things from more than one subject, but that's far from being a polymath, i.e. an actual scholar of multiple subjects.

David Z

@Sean sorry, missed that - I've been to three of them

They tend not to be particularly important

at least, not compared to specialist conferences

Feynman

4:12 PM

Sorry, I see faults in your reasoning, don't you think good answer to be not necessarily a book, if the answerer is really a polymath (atleast who is trying to become a polymath)? Atleast answer to the question which linked need not need a book...

My aim is to have community who wants to become polymath...

It is a motivation...

0celo7

@ACuriousMind Kerbal Space Program?

Feynman

Add the letter "I" In between "which" and "linked" in my previous comment..

ACuriousMind

@0celo7 That's not a question ;)

Feynman

Okay, I am leaving...

0celo7

@Feynman bye

Finally got GTA4 to work properly.

@ACuriousMind See the question mark? Do you even English?

ACuriousMind

4:59 PM

@0celo7 Yeah, but...what do you want to ask? "Why do you play it?" "Is it good?" "What is it?" Something else?

0celo7

@ACuriousMind Yes, read my mind (I knew you would).

ACuriousMind

@0celo7 I did not ask a Yes/No question, what kind of answer is that?

Sean

5:14 PM

Do you stop getting rep points for edits after 2k rep?

0celo7

@Sean Yes.

@ACuriousMind One that makes you think.

(All of the above)

Sean

that's no fun

i guess now i just keep editing out of altruism

NeuroFuzzy

@Feynman I'd be worried people would confuse polymath for things like the Terry Tao polymath projects.

@Feynman and I really don't think that question is a polymath question... it's exactly what HSM is for I think. The kind of people who read Newton oand related with a modern eye.

I could be wrong about that last bit as I'm not involved in the site at all.

Feynman

6:09 PM

@NeuroFuzzy: Sorry, the question I asked was more alligned towards math, but it does require expertise in math and history along with some physical intuition. We can find many questions which require knowledge of different fields of science...

Have a look at this

0

Q: Connecting different sites

Proposal: Polymath I have seen questions being cross-posted. I want to make a site for those type of questions which requires polymathic folks. If the questions posted in poymath site is tagged with different subjects (and SE exists for that subject), say physics, math and history; we can progra...

science

@ACuriousMind: Have a look at the above question..

@0celo7: You may also look at the question, and I will be glad if you support my proposal..

0celo7

@ACuriousMind In the plane, are all closed forms exact?

Qmechanic

6:47 PM

@0celo7 : Zero-forms are never exact but could be closed.

0celo7

@Qmechanic I meant one-forms.

Because in differential equations, we have exact equations, but the method for checking that they are exact is really just computing the exterior derivative...

So really we're just checking that they're closed...

ACuriousMind

7:13 PM

@0celo7 You should know that. :P Which object tells you whether closed-but-not-exact forms exist?

Glen The Udderboat

"You can observe a lot by watching."

"Nobody goes there anymore. It's too crowded."

"Always go to other people's funerals, otherwise they won't go to yours."

Yogi Berra

Lawrence Peter "Yogi" Berra (May 12, 1925 – September 22, 2015) was an American professional baseball catcher, manager, and coach who played 19 seasons in Major League Baseball (MLB) (1946–63, 1965), all but the last for the New York Yankees. An 18-time All-Star and 10-time World Series champion as a player, Berra had a career batting average of .285, while compiling 358 home runs and 1,430 runs batted in. He is one of only four players to win the American League Most Valuable Player Award three times. Widely regarded as one of the greatest catchers in baseball history, he was elected to the Baseball...

user54412

@ACuriousMind Is it good? I've heard it's fun, but I've also heard it really fakes the dynamics in a number of ways.

0celo7

7:29 PM

@ACuriousMind ::mumbles something about co/homology::

0celo7

7:51 PM

@ACuriousMind D: I've forgotten what exactly I need

ACuriousMind

@0celo7 It's the cohomology - since deRham cohomology is precisely the closed modulo the exact forms. All closed k-forms are exact if and only if the k-th cohomology vanishes.

0celo7

I knew that

ACuriousMind

@ChrisWhite Well...given the weird contraptions I've brought into flight, I'd guess at least the aerodynamics are not that realistic...but realistic enough that I've killed about three dozen kerbonauts trying to get one into a stable orbit :P

0celo7

I've forgotten how to calculate it D:

ACuriousMind

@ChrisWhite So, I think it's very fun, but you'll probably find lots of small things to complain about if you know a lot about aerodynamics and orbital dynamics.

@0celo7 Hint: The plane (any $\mathbb{R}^n$ really) is contracible.

Feynman

7:58 PM

Guys, I have edited the description of the proposal..

See here..

1

Polymath

Proposed Q&A site for polymathic community. The site covers CRITICAL questions which can be answered only by a partially or completely polymath field expert, and which may not be answerable by a unique/non-polymath field expert.

Currently in definition.

Do follow, if you are interested...

Must follow if you are a polymath :)

0celo7

@ACuriousMind like "I have no clue where to start" forgot

@ACuriousMind Do I use the de-Rham theorem and then calculate the homology?

And is there something about the homology of contractible spaces?

John Duffield

8:16 PM

@0celo7 : LOL, you got it kid. Right after the pair production and the electron diffraction and the annihilation.

Glen The Udderboat

@Feynman That's not going to fly. I'm genuinely surprised that you got this far.

ACuriousMind

@0celo7 Why do you want to use something about the homology? Use something about their cohomology! (Namely, unpack what "contractible" means, and then use that (co)homology is homotopy invariant)

Then all you need to know is the (co)homology of a point.

Feynman

@GlenTheUdderboat: I didn't get you..

Glen The Udderboat

@Feynman No worries. Good luck!

0celo7

@ACuriousMind well I don't know that either D:

for shame

how do you even have 1-forms on a point

D:

Feynman

8:30 PM

@GlenTheUdderboat: Thank you!!!

0celo7

if I have rope over multiple pulleys (assuming a perfect rope and ignoring the mass of the pulleys), is the tension in the rope the same along each free part?

Glen The Udderboat

@0celo7 Ignoring gravity, I'd say yes. Assuming that "free" means between pulleys.

0celo7

@GlenTheUdderboat yes, that's what I mean

why ignoring gravity

that's just another force on the stuff the pulleys are connecting

ACuriousMind

@0celo7 You...don't.

0celo7

@ACuriousMind what do I do then!!?

Glen The Udderboat

8:37 PM

@0celo7 Well, in general, for pulleys it matters whether they are fixed or not. And if you're going to ignore mass, why not ignore gravity?

ACuriousMind

@0celo7 Well, there aren't any forms except 0-forms on a point. So its deRham cohomology vanishes in all but the zeroth degree, no?

0celo7

@GlenTheUdderboat you don't ignore gravity on the masses connected by the ropes

@ACuriousMind ffs

Glen The Udderboat

@0celo7 OK

0celo7

so that works in all R^n?

Glen The Udderboat

@0celo7 Still yes.

0celo7

8:39 PM

well now I'm confused

ACuriousMind

@0celo7 Yes. All $\mathbb{R}^n$ have vanishing cohomology except for the zeroth degree, where it is $\mathrm{R}$ (or $\mathbb{Z}$ or whatever you're computing the coefficients in).

0celo7

ok, what about that one vector calc thing $\nabla\cdot B=0$

so we can have $B=\nabla\times A$ globally?

or am I just confusing things

what is happening

Glen The Udderboat

The (co)homology of the pulleys determines the invariant tension of the massless rope.

0celo7

or is it curl grad f =0 that I'm thinking of

@ACuriousMind halp

@GlenTheUdderboat D:

this was not discussed in class