2:02 PM
@ACuriousMind do you have a reference for asymptotic series and all this stuff

@0celo7 The string perturbation series is also an expansion around the "classical free background" of string theory, which is essentially given by the precise choice of CFT that lives on the world sheets.

do your QFT notes talk about it

@0celo7 No. I only know what you can find by googling.
(or not, given your Google skills :P)

@ACuriousMind so?

For the "string background", see also landscape of string vacua

2:03 PM
So is the "classical free background" of string theory not the "metric on the bulk" or whatever

@0celo7 It's the "choice of target space" together with "choice of CFT"

what does "choice of CFT" mean

Where "target space" means choosing the compactification of the 10D space, I think

you're not explaining very well :/

@0celo7 lol, that's because I don't understand it very well
There's a reason my top tags are quantum-mechanics and quantum-field-theory, not string-theory :P

2:06 PM
well I don't think BBS will explain it either
what does "choice of CFT" mean??

Choosing the central charge and the conformal weights that are allowed to appear, at least in the range of minimal models :P
But since ST fixes the charge to 0 and the weights to 1 for the physical states, I'm not sure that's what is meant here

why :P
see, you think I don't know what that means
well screw you
I know some of those words

@0celo7 Because it is true, but not very helpful

damn, what are the conformal weights
sounds familiar
$h$ and $\bar h$
thank you BLT

@0celo7 Eigenvalues of $L_0$ and $L_0^\dagger$

2:10 PM
@ACuriousMind which are $h$ and $\bar h$

Or, yes, $h$ and $\bar h$ (hate that notation, btw, because $\bar h$ is not the conjugate of $h$)

so what does it mean "allowed to appear"

@0celo7 Well, one can define a CFT purely by the central charge and the conformal weights, since these fix the structure of the space of states (the space of states has to be a unitary representation of the Virasoro algebra, and generic ladder operator methods show those representations can be labeled by $c$ and the allowed conformal weights (and only certain weights are possible for certain charges to begin with)). But I'm not sure that's what the string people mean by choice of CFT
(Since charge and weights are fixed by ST AFAIK)
I'm also unsure how the fermions appear in this way.

@ACuriousMind hey I knew that first sentence
lol you've just read the nLab stuff
you don't really understand this :P

The CFT stuff I don't have from nLab. The string stuff...well, yes :D
@0celo7 I said that already

2:21 PM
@ACuriousMind now it's confirmed

@ACuriousMind T is not the conjugate of I either

I don't think BBS talks about this either...it might later on
I'm like half way through chapter 6 I think
@Slereah hah

@Slereah ::slow clap::

@Slereah : Reminds me of the saying There is no I in TEAM.

@ACuriousMind I never understood this part in BLT.

2:28 PM
@0celo7 ...and what exactly do you not understand?

@ACuriousMind how to get (9.3)
and (9.4)
@JohnDuffield If I ask a question about non-mainstream physics, it will get shut down.
As for the other stuff...I's not sure what you're saying. By what mechanism does light go "round and round"?
I don't think "round and round" is a null geodesic, is it?

@0celo7 Hm
@0celo7 Are $h,g$ phases, or arbitrary complex numbers?
(I hope they're phases, then I think I at least have the idea of whats happening)

@ACuriousMind Yes, that's what the footnote says.

2:46 PM
@0celo7 Okay, then $h$ and $g$ tell you how often you have to go around the torus until the boundary values are equal. The modular transformation just changes the size of the torus, but not its modular structure. So on a torus with a different size, the same field has to go around the torus a different number of times to be equal because the torus is larger/smaller.

Alright, but how do I derive (9.3)?
I get the concept.

That is, if we phrase the tori as lattices in the complex plane (spanned by $\lambda_1,\lambda_2$), then the function $\psi$ is always periodic with the period length $n_i\lambda_i$, where $n_i$ is given by solving $h^{n_i} = 1$.

Ok.

Now, you have to express the period lengths by $\lambda'_i$.

I see.
I'll figure that out...later.

2:52 PM
And that gives you some $h'$ and $g'$ after some manipulation, which will be the powers of $h$ and $g$ they write there.

3:25 PM
-1

Meow Meow Meow Meow Meow Meow.

Meow.

lol

3:38 PM
so hungryyy

I have candy bags waiting for me at home
Only 10 minutes left of work

are those for you or for kids o.o

>giving my candy to kids
No thx

What kind of candy?

sour candy stuff

3:50 PM
ew

THE BEST
Not sure what the non-french equivalent would be

The sour candy I know is like sour gummy bears or the like
Sometimes encrusted with sugar
It's awful either way :P

love those ones

Yeah, that's disgusting

3:56 PM
I love them but they are kind of awful for my mouth
It's basically sandpaper covered in acid

@NeuroFuzzy Why is the picture German?

not very good for the tongue or roof of the mouth

Does Ritter Sport only sell here?

@ACuriousMind Oh, no, it was just the first google result, they sell them at my school and the packaging is in english
I've thought about buying this before... amazon.com/Ritter-Sport-Chocolate-Marzipan-Ounce/dp/B000H27PB8

What about regular gummy bears?

4:01 PM
@obe Meh, they're alright, but nothing special

Blue gummy bears are.

4:17 PM
@Danu : not so, I give robust references to support the answers I give. There's nothing controversial about things like the speed of a light wave depends on the strength of the gravitational potential. But I see this sort of thing getting downvoted by mathematical kids whose arrogance is only exceeded by their ignorance, and who lap up popscience trash whilst dismissing Einstein and the evidence. I've seen similar re other posters. They give a rock-solid correct answer, but some patent blatant wrong answer gets the upvotes. — John Duffield 1 min ago
I just had to respond.

0

When an unclear or otherwise lacking question is posted on this site, it often happens that one of the more experienced users posts a comment asking for clarification. Fairly often, the asker responds in a comment to the original post, attempting to clarify or otherwise improve the question. With...

@0celo7 : the physics I talk about is is mainstream physics. I'm the one who refers to "Einstein and the evidence". You're the one who dismisses it. Sheesh, yesterday one of you was dismissing the wave nature of matter. :rolleyes:

@JohnDuffield Electron = photon going around a Dirac belt is mainstream?
@JohnDuffield For someone who asserts his intellectual superiority over many of the twentieth century's greatest physicists on a daily basis, I think are being very harsh about the "arrogance" and "ignorance" of the user base of this site. — Danu 11 mins ago

4:33 PM
@0celo7 I love this GIF.

@JohnDuffield I know many things better than Einstein.
Like, y'know, black holes.

@0celo7 they look like raider fans after we win a game :D

> A month ago, the loop quantum gravitist Carlo Rovelli wrote a somewhat surprisingly intelligent essay
Lumo v. Duffield when

@0celo7: Pair production is mainstream. The wave nature of matter is mainstream. Electron diffraction is mainstream. Spin ½ is mainstream. The Dirac wave equation is mainstream.

You still have not explained the mechanism behind your Dirac belt idea!
!!!

4:39 PM

How on Earth does a photon get trapped in a loop?

@0celo7 : no you don't. What you "know" about black holes is wrong. And puhlease, don't come out with phrases like "surpisingly intelligent" when talking about people like Rovelli.

What about the other leptons?

@Danu It seems that you got heavily misunderstood on meta...;-)

@yuggib Where?

4:40 PM
@Danu Gottlieb controversy, end of the comments to the answer

@JohnDuffield What exactly do I "know" that is wrong?
And that was a quote from Lumo.

@yuggib Oh god!!!!

I have no opinion on the fellow.

@Danu Yep...

@Danu he didn't accept the invitation to the room

4:41 PM
@Danu lol I saw that
Although the Caltech name drop is...meh
That's pretty defensive

Lumo formed and wrote his own wikipedia article.

@obe Really?

@Danu Who else on earth would write a wikipedia entry on lumo?
with all due respect...

@yuggib He did write one pretty significant paper.
And a number of medium-level significant ones
1.9k citation is nothing to sneer at, IMO.

@Danu I know people with hundreds of very significant papers with no wikipedia entry

4:45 PM
@JohnDuffield When did I say electron diffraction or spin 1/2 is not mainstream? And of course the Dirac equation is mainstream...

@0celo7 : it's called gamma-gamma pair production, and it works becuase displacement current does what it says on the can. See how Wikipedia says pair production occurs because pair production occurs spontaeously? Like worms from mud? That's cargo-cult trash, and that turbine hum you can hear is Feynman turning in his grave.

@yuggib Life is unfair sometimes :P

@Danu In physics, it is pretty average...

haha cargo cult.pdf

considering the experience

4:46 PM
@yuggib Being a "pretty average" theoretical physicist---professionally---is nothing to sneer at.
Do you have any citations?

@Danu still not worth a wikipedia entry IMHO
@Danu yep

@yuggib That's awesome :D I'm really looking forward to getting even a single one.

I have an h-index of 3; in math it is not so bad for a start

@yuggib Then flag it for not being noteworthy enough (that's a thing right, on Wikipedia?)

@Danu Scroll to the bottom of this page. Look for Lumidek.

4:47 PM
@Danu The flagging is even less worth :-D

@JohnDuffield You don't understand QED, the theory Feynman got so famous for. Yet you have the audacity to use him in an appeal to authority to make a point that goes against the principles of QED. Good job.

@Danu They will come...just give it time.

@yuggib Not without a paper...

@Danu You are waiting the right time to contribute in a meaningful way... ;-)

@0celo7 : so think it through. What happened to the +511keV photon? Did it magically disappear? Did the electron magically pop into existence courtesy of some "God did it" creation operator? What happens in annihilation? Let's see now, we started with a wave going thataway, we ended up with a wave going thisaway, and in between we had... a point particle made of cheese!

4:50 PM
@yuggib Waiting for the ability to do so haha
@JohnDuffield Maybe you should read Feynman's little layman-level book on QED to get acquainted with the basics.
It doesn't go too heavy on the equations---just as you like it :)

We lost him :(
@Danu Oops. Sorry for the confusion. As mentioned I use this website rarely, am not familiar with many of its features, and it's been a long day. Anyway, I think I've said enough. It's back to the salt mine for me. – Michael A. Gottlieb 1 min ago

@Danu You probably will have that ability soon

@yuggib I think so. I hope to start working on my first bits of research this academic year.
@Rigor I saw that. Thanks.

I tried.

@Danu : I understand it, and I've read his little books. And nothing that I've said goes against the principles of QED. Nothing. What goes against the principles of QED is cargo-cult nonsense peddled by popscience quacks.

4:54 PM
@Danu And the most difficult part would probably not be the research by itself, but redacting the paper afterwards...

@yuggib Hahaha. I fancy myself to be a rather good writer (at least when compared to most physicists I know) so I'm not too worried about that.
My undergraduate degree was more interdisciplinary than most, so I had a bunch of writing courses etc. besides the usual physics courses.

@Danu Writing a paper is another type of "writing"
:-D

@yuggib I mean writing in an academic context; I'm guessing papers won't be too different.

@Danu Well, probably not; but they have rules that are not written. There is a lot of probability that your first referee will dub the paper as obscure and involved
even if in your opinion was pure source water clear
at least, it was like that for me ;-)

@yuggib Hahaha, right. I hope to write it together with a more senior researcher who can hopefully help out with that part.
@yuggib Well you are interested in obscure and involved things... (no offense).

4:58 PM
@Danu Of course I am...and I take pride on it ;-) but I do not write papers on that...
that's just for fun

@yuggib I'm not talking about your foundations of mathematics/physics stuff. The QM/functional analysis is... also obscure to me ;)

@Danu o.O
it is easier than it seems
and yes...I have to admit that I write papers on that

may I ask what language you write your papers in?

editing language? or language as in "english language"?

english

5:03 PM
in english

@Rigor There isn't really a lot of serious research in other languages than English nowadays

apart from english, maybe just french (and maybe only in the math community) is accepted as a language for scientific papers

...though there are some notable exceptions (EGA,SGA).
@yuggib Right :P But even that is a thing of the past, I think.

at least in internationally renowned journals
@Danu Annals of math still publishes papers in french
last one I saw was by Joseph Ayoub

@yuggib Interesting point.

5:05 PM
but you know...france is one of the cradles of math

a math PhD still requires a second language

in physics, I think there is no non-english-only journal

@Rigor requires, in what sense?
(I don't think that's true in any sense)

mandatory
at least a reading knowledge

in my home country, you need english to do a math/phys PhD, but because of the fact that research is almost exclusively in english

5:08 PM
@Rigor Really? Can you show me where you found this information?
@yuggib But besides English, I don't think you need anything, right?

in france, in mathematics, obviously there is not that restriction
@Danu not in the countries I am aware of

some uni's are accepting some computer languages

i.e. european ones (France, Italy, Germany, Spain, UK,...)

@yuggib Same for me.

check any US university PhD requirement
in math
1 min ago, by Rigor
some uni's are accepting some computer languages

5:10 PM
@Rigor Bizarre...it's easier for foreigners

yep

@Rigor Interesting!

@Rigor Nevertheless, if you stay in the academics you would probably have to travel a lot; and that additional language may come to use

math remains true to its language of science roots :P

@Rigor :-D
or maybe american PhD students are lazy, and they have to be motivated (read obliged) to learn additional stuff
in europe, you have everywhere to study english in mandatory education; and maybe another additional foreign language
and if you are spanish/french/italian/(at a lesser extent)portuguese you will learn the other ones in the group with little or no effort

5:21 PM
@yuggib And as Dutchies we all know our language is useless, so we just learn everything else :D

5:34 PM
I don't see a need to learn a 2nd language if you know English
unless you are interesting in learning languages
then all the power to you
but I hate being forced to learn a 2nd language

@FenderLesPaul To most Europeans, that comes across as pretty narrow-minded.
3

user54412
I can understand forcing a second language in high school, just like we force students to learn chemistry and history and maybe even a bit of music, for the purpose of breadth.

user54412
But the whole "learn something other than English so you can communicate" was one of the most ridiculous lies I was told as a child.

@Danu why is it narrow minded though?
like why would I ever need to use my knowledge of French?

@FenderLesPaul In a PhD in mathematics, I can see how it'd be useful. I don't know what you're doing in life, so I can't go into the specifics for you particularly.

user54412
5:39 PM
@Danu I think that says more about the stubborn old-fashionedness of mathematicians than about the the usefulness of French ;)

@ChrisWhite Yeah, that's not true. Communicating in English is fine most of the time---but just in order to realize what else is out there and maybe even read something in a different langauge is pretty nice.
@ChrisWhite Sure, but that won't change ;)
It implies the usefulness of French

It's nice and all if you care for it sure

The most important books on algebraic geometry, for instance, have never been translated.

but language classes in college are pretty hard
there's a lot of HW and lectures
so it puts a lot of pressure when I'd rather be focusing on physics for example

@FenderLesPaul I've taken some. They were very easy compared to the physics :P

5:41 PM
Well at my university anyways

They may have been different from yours, of course

like spanish 1 here had 1 lecture and 4 discussions a week and HW everyday
it was insane
and we are required to take 3 semesters of language

user54412
Keep in mind that in the US, it's not like you can go and practice most languages outside of class. It's pretty hard to master a language when your teacher is the only person you know who speaks it.

@ChrisWhite All the more reason to study it in uni: You won't encounter it anywhere else :D (semi-joking)

I should note though that my main complaint of workload is not really restricted to language so I don't want to come off as criticizing mandatory language classes specifically

user54412
5:44 PM
@Danu When choosing what college to go to I almost went somewhere else that had a much broader language program. I actually wanted to learn some dead languages for fun.

a med student who is forced to take physics would have the same complaints

@ChrisWhite Yeah; Latin has been well worth the 6 years I spent learning [the basics of] it.

@ChrisWhite dead languages can be fun I'll admit

what age did you start?

I'm taking Sanskrit now and it's pretty trippy

5:46 PM
@Rigor 12
The highest level of high school education (it is split in several disconnected programs in Holland; kids get grouped based on test scores from before etc.) requires either Latin (6 years) or Ancient Greek (5 years).

user54412
both of which are apparently more useful than Dutch :P

@ChrisWhite Hmm... Probably true for people who don't live in Holland.
Both provide a deep insight into the structure of most Western languages

That deep insight into the structure of a language is useful in mathematics.

Is it? How?
linguistics is way more complicated than math

Translating words into symbols.

5:57 PM
oh

user54412
Linguists are also more hardcore than mathematicians

user54412

woah how long have we had xkcd previews on chat?

user54412
a long time I think

user54412
even the hover text works :)

5:59 PM
yep

@FenderLesPaul forever

Warning: this comic occasionally contains strong language (which may be unsuitable for children), unusual humor (which may be unsuitable for adults), and advanced mathematics (which may be unsuitable for liberal-arts majors).

damn
that's fancy as fuck

6:16 PM
So let's go with a classic late latin/early romance riddle:

Se pareba boves, alba pratalia araba
et albo versorio teneba, et negro semen seminaba

@yuggib No idea what that'd describe.
Those verbs don't seem to follow the usual rules of the Latin that I learned :P
the t's are missing, maybe?

the most complicated thing to understand is "se pareba boves"; so the translation for that: had the oxes in front of him

or a-> o?
oh yeah that wouldn't work with "se"

is the t missing
latin: fun for all family since 600 BC

I can't solve the riddle, and had to look up some of the words

6:25 PM
you came up with the translation?
Mine (in a not so good english) would be:

He had oxen in front, ploughed white fields
and held a white plough, and seeded black seed
(it definitely looses elegance in english)

7:06 PM
@JohnDuffield So the most technical and complicated books on the market are popsci? What do you think popsci actually is?
popsci = stuff you disagree with?
3
@yuggib lol, what on Earth is that supposed to mean

@0celo7 :
The Veronese Riddle is a riddle written in late Vulgar Latin written on the margin of a parchment, on the Verona Orational, probably in the 8th or early 9th century, by a Christian monk from Verona, in northern Italy. It was a very popular riddle in the Middle Ages and has survived into dialects to date. Discovered by Luigi Schiaparelli in 1924, it is considered the first document ever written in the Italian language along with the Placiti Cassinesi. == Text == Original Text: Se pareba boves alba pratalia araba albo versorio teneba negro semen seminaba Rough Translation: In front of him (he) led...

@Loong nice lurk

a Looong lurker :P

@Rigor you should watch the Oklahoma game, I'll tell you where to find me
@Loong I see

@0celo7 I thought you're too sick

7:15 PM
@0celo7 That in late latin the riddle is much much better

@Rigor I actually feel much better
and I'll be good by Saturday

good, good
now will the sooners cover the 3 points :-/

what

they are the favorites to win by 4 or more points
which is huge considering the home field advantage...
...in a hostile enviroment :P

7:41 PM
@ACuriousMind Got it, I think, thanks.

In the spirit of answering unanswered questions:
0

We consider the metric $$\mathrm{d}s^2=-\mathrm{d}t^2+a^2(t)\mathrm{d}\vec x^2$$ where $a(t):= a_0e^{Ht}$. To show that these coordinates do not cover the entire spacetime manifold, we consider the trajectory of a freely falling observer, which of course extremizes the proper time \tau=\int...

@DanielSank You expressed interest in this answering-unanswered-questions type of thing, right? Who else starred my comment (there were a lot of stars!)?
Sep 5 at 21:50, by Danu
Guys, over at TeX - LaTeX they have this (weekly?) session called "answer the unanswered" which means a bunch of (serious) users get together in chat and try to answer a whole bunch of questions that have either been answered in the comments, are easy to answer, etc; sometimes they also vote to delete an old unanswered question that is deemed too bad. What do you guys think about potentially doing this here on PSE?

@Danu That seems way too complicated.
Hawking-Ellis shows that that form of the metric is obtained from the embedding equation in $\mathbb{R}^5$ and that the coordinates are simply not defined everywhere.
Ah, it seems OP is insisting on doing it using the geodesic equation.
> partially to prevent this answer from being abused by lazy students