The h Bar

Bah, I can take that for granted.

1:04 AM

I get what you mean - despite the fact that it's obviously not true.

@NeuroFuzzy Actually, no.

Do you see Lebesgue integrals mentioned?

Hm? In the fischer-riesz theorem linked?

No, in your physics books.

No, but I also see people differentiating functions of integers $f(n)$ with respect to $n$ in my physics textbooks

(I'm not arguing FOR it, by the way)

I think that analysis has uses for people working in rigorous QM and QFT, but beyond that I don't see important uses.

1:11 AM

Hey @0celo7, have you ever heard of this: en.wikipedia.org/wiki/POVM ?

By analysis I primarily mean Lebesgue integrals. Various inequalities have uses of course.

@NeuroFuzzy I don't do rigorous QM, so no.

Rigorous QM is way down on my reading list.

I probably won't get to that until I retire.

@NeuroFuzzy Why do you ask?

1:30 AM

@0celo7 oh just curiosity because I heard the acronym from some (99.9% crank) person on the internet.

No real reason.

@NeuroFuzzy My reading list in order: strings, more GR, cosmology, QFT in curved spacetime, advanced QFT, that kind of QM

I'm sure I'm missing stuff.

I was mostly just curious if it was a common acronym I missed by chance

@NeuroFuzzy So what do you want to do anyway?

Quantum computing ;P

@NeuroFuzzy You need to use the GR tag more, it's pretty much the best.

Hm! Yeah school kind of is eating up my time to do anything new though, so GR is off my reading list for quite a bit.

@NeuroFuzzy Are you a junior or senior?

1:40 AM

@0celo7 Trying to whittle down the choices. Theoretical or mathematical physics research if possible; this year and the next are the times to work my ass off and do all side projects I can manage. But I have a decent background in programming.

@0celo7 junior

@NeuroFuzzy When are you taking GR?

@0celo7 it isn't scheduled, and the undergrad GR classes are pretty bad

but I'm planning to work through L&L classical theory of fields this summer so

@NeuroFuzzy Bad? What book do they use?

let me look

I should probably read that, but I think it'd be a step down from my current level of GR.

1:42 AM

@0celo7 Ok, so there's two courses: "Cosmology" and "black holes"

and cosmology is pretty general and doesn't do any GR

"Black holes" is an undergraduate course?

What's the prereq?

physics.ucsd.edu/students/courses/spring2014/physics161

Hartle. That explains it.

prereqs are just the general undergrad courses I guess.

He outsources a lot of derivations.

1:44 AM

general lower div*

And there's a grad level course on it buuuuut

@NeuroFuzzy I'm surprised UCSD's undergrad GR is so paltry.

@NeuroFuzzy You can do it.

I would probably rather try to do the grad level QFT or QM

What's the prereq for that?

eww

Quantum

I don't see any wavefunctions around, do you? Ha, disproved quantum.

user54412

@0celo7 I don't know of any university that has good undergrad GR, and very few have good grad level courses (in the US)

Lol

1:45 AM

@ChrisWhite I talked to a guy from Caltech who used Wald in undergrad.

(He might have been in a grad course though.)

Anyways I have to keep working through my representation theory book!

@NeuroFuzzy Can you link me to the grad GR?

user54412

@0celo7 Me? :P

@ChrisWhite No, a Junior.

user54412

Caltech's only GR course is grad-level, but a number of undergrads take it anyway.

1:48 AM

@ChrisWhite I hear Wald teaches the GR, QFT in curved spacetime and black hole courses at Chicago.

@0celo7 physics.ucsd.edu/students/courses/winter2014/physics225b/… physics.ucsd.edu/~mcgreevy/f13/225A-lectures.pdf physics.ucsd.edu/students/courses/winter2015/physics225b (hit notes)

@NeuroFuzzy Zee? Holy crap!

That's the graduate course?

Zee says in the intro that the book is for motivated high school students, lol.

oh lord

well it's not too late to go to germany next year!

@NeuroFuzzy He says diffeomorphism all of thrice in the book.

@NeuroFuzzy The notes are good, but that's sad if it's really the only GR course.

the pdf notes on ~mcgreevy is the 1st 1/2 of the course

so ten weeks of study

1:53 AM

I figured. I compared the ToC with the course description.

@NeuroFuzzy You could totally take that next year. Zee is a fun book.

Hmm!

well cool, I'll think about it.

@NeuroFuzzy The reserved books here range from high school (Zee) to scary hard (Hawking).

Hmm, upon further investigation, that might be Zee's other book where he makes the high school comment.

@NeuroFuzzy Ahh, I read that in Lubos Motl's review of the book.

Q: How much magnetic field is required to magnetize a 10x10 cm mumetal shield?

2:15 AM

@NeuroFuzzy I'm really confused by that course. The recommended text is definitely undergraduate, and the notes aren't particularly advanced either. However, the reserved books span all difficulty ranges. You have the introductory Zee and Hartle, the intermediate Carroll and Weinberg, the advanced MTW and Wald and then the really advanced Straumann and Hawking & Ellis.

David Z

6:47 AM

-1

My main goal is to magnetize and demagnetize a mumetal shield. Till now I am using a Helmholtz coils setup and I can generate 10 mT applying 2 A DC. I am using a DVR425 fluxgate and a hall probe sensor to measure the magnetic fields. To demostrate that my mumetal shield is being magnetized I am c...

would be good to get the community to pitch in on this one: close it or not? It might be "engineering", or something else

Emilio Pisanty

10:30 AM

@DavidZ Looks like a valid experiment question to me.

Physics Meta

11:19 AM

0

Q: Problems trying to post a second answer to question (second from same user)

So I was trying to update this question with a link to another well asked question here - I tried twice, but each time my answer did not appear- I am wondering if it is being checked because I asked the original question and already posted one answer... ...the idea was to post good examples of h...

support bug

Physics Meta

12:17 PM

0

Q: What should I do with long exhaustive answers in lack of a question and in general?

I'm in the situation of having a long answer but no question. regarding this question I can discard the answer so nobody has any benefit from it and the time I spent writing it is deemed wasted. Or I could post that answer somehow. I did the later, asking for verification of my answer. It got ...

discussion answers

1:54 PM

I wonder who starred my message about that one girl and what it means they think about it

@Danu Apparently many people liked the story or the discussion about her

Yeah, fair enough

2:21 PM

I'm in an annoying linear algebra-pickle.

Do any matrices, except multiples of the identity, commute with all anti-symm. matrices?

Partial results: Take $A$ any $n\times n$ matrix, $X$ any anti-symm. $n\times n$ matrix. Then, if $[A,X]=0$, $[A^T,X]=0$

Furthermore, $A=A_\text{symm}+A_\text{anti-symm}$ and $[A_\text{symm},X]=[A_\text{anti-symm},X]=0$

Did you check Math.SE?

@Danu Heavy artillery solution: That's equivalent to asking whether the representation of the $n\times n$-antisym. matrices upon $\mathbb{F}^n$ is irreducible (Schur's lemma). Since the antisymmetric matrices are $\mathfrak{o}(n)$, this in turn is the same as asking whether the rep of $\mathrm{O}(n)$ is irreducible, which is true (the fundamental representation is irreducible), so all matrices that commute with all antisymm. matrices are multiples of the identity

@ACuriousMind ..... Well duh. Stop patronizing us :P

@ACuriousMind Okay, thanks, that makes sense

@ACuriousJim Ooooh, someone's cranky :P

2:33 PM

@ACuriousJim Hey, I got that and I only took about 2 weeks of a course on Lie algebras at this point ;)

@ACuriousMind Honestly, it's as if you mathematicians think the layman doesn't even know what a lemma is.

Meanwhile, I'm stuck at showing that $\frac{\lvert x -y \rvert}{1 + \lvert x-y \rvert}$ fulfills the triangle inequality. It's supposed to be obvious, but apparently I suck at inequalities :D

@ACuriousJim Hey! I'm not a mathematician!

@ACuriousMind $|x-y|\le1+|x-y|$ is obvious

actually, take out the equal

@ACuriousJim Yeeees. And how do you get $\frac{\lvert x-y\rvert}{1+\lvert x-y \rvert} \leq \frac{\lvert x-z \rvert}{1+\lvert x-z \rvert} + \frac{\lvert z-y\rvert}{1+\lvert z-y \rvert} $ from that?

That looks horrible without TeX rendering.

2:41 PM

@ACuriousMind How about multiplying by the product of the denominators?

then factoring stuff out

maybe

@ACuriousMind It's obvious and the solution is really simple, but I can't show it to you I have to go

The chat box is too small for your proof, eh?

@0celo7 Hehe

@Danu I tried that, I didn't get it (which doesn't mean that it doesn't work, I must be overlooking something)

Stupid French [Canadians]

2:43 PM

@ACuriousJim :)

@ACuriousMind How does one prove it's irreducible though?

Qmechanic

2:58 PM

@ACuriousMind : [Spoiler alert] math.stackexchange.com/q/698869/11127

@Qmechanic You got any spoilers for me? ;)

@Danu Ehhh...that's surprisingly non-obvious, the only way I can find is taking the character of the representation and showing that its product with itself is $1$, which is ugly for high $n$...

@ACuriousMind Right... it should be for all $n$ too, so unless there is some inductive process I'm not happy :P

@Qmechanic Thanks!

That really wasn't that hard...

Is it possible to incorporate friction in the lagrangian without regarding the normal force?

The Dark Side

3:12 PM

Hey guys, please clear something up for me.

This question.

OP's asking how to get the EoMs, given the action S.

How is this not HW (i.e. off-topic HW)?

Yeah, I agree

The Dark Side

Asking because of this:

@TheDarkSide Three close reviewers voted to leave open, hence the flag was declined.

@ACuriousMind I pushed it back into the queue

FWIW, I fully agree it is off-topic as HW-like.

The Dark Side

3:17 PM

@ACuriousMind Okaaa...y

@Danu Very good.

See you guys :)

@Danu Very good. Now I'm unhappy too because I can't prove that the standard rep is irreducible :(

Qmechanic

@Icosahedron : For a friction force proportional to speed, see e.g. physics.stackexchange.com/q/147341/2451 and links therein.

3:44 PM

@ACuriousMind Just as planned...

Qmechanic

@Danu @ACuriousMind : So I guess what needs to be shown, is that $SO(n)$ acts transitively on $S^{n-1}$.

@Qmechanic Oh, hmm I hadn't thought about it like that. I don't think that's very hard, right?

How do you show that that's equivalent to the defining representation being irreducible though?

4:12 PM

@Danu Reducibility means there is an invariant subspace, and transitive action on the unit sphere means that every 1D subspace is transformed into every other 1D subspace by some element of the group, so there cannot be an invariant subspace that's not the whole space.

Emilio Pisanty

4:58 PM

@TheDarkSide I don't think it's a great question, but I feel it shows a misconception on how Euler-Lagrange equations relate to the action more than anything else. (To the limited extent that I can discern anything in there, that is.) Hence the Leave Open vote, which doesn't have particularly grave consequences in any case.

7:01 PM

@ACuriousMind I meant more like, why does this correspond to the fundamental representation. I know the idea of transitivity $\Leftrightarrow$ irreducibility

@EmilioPisanty I think the guy is probably rushing into "cool" topics without doing the necessary "pedestrian" stuff first

@Danu Uhhh...because the fundamental rep is the one on $\mathbb{R}^n$, where $S^{n-1}$ then is the unit sphere? I think I'm misunderstanding your question^^

Emilio Pisanty

7:56 PM

@Danu In which case he needs someone to set him straight on the concepts, not to be told to keep fumbling at over-advanced exercises.

Q: How can I explain to my friend that running a "hackintosh" is a violation of the law?

lol...I can get bonus points on an assignment if I can provide a less laughable term than "snake-able diagram" for a diagram to which the snake lemma can be applied. Suggestions? ;)

Kyle Kanos

8:14 PM

I got nothing, sorry

Kyle Kanos

8:36 PM

Interesting:

6

I have a friend who has never owned a Mac. He has just built a "hackintosh" by installing VMWare on his PC running Windows, and downloading a pirated, hacked copy of Mac OS X Yosemite in a portable VMWare virtual machine volume from somewhere online. He claims to be astonished and unbelieving wh...

osx hacking

Note to self: don't run Hackintosh

@KyleKanos "He actually said to me "It wouldn't be on the Internet if it was illegal"."...wat.

How...just how can anyone actually believe that? :D

@ACuriousMind What does it mean when people say that something is a theory of everything? Let's be concrete and assume superstring theory is a ToE. Does a ToE have to be able to derive everything within its framework or does it get some previously known facts? i.e. does one have to be able to derive Lorentz symmetry or Born rule from the ToE, or is one allowed such facts to count as a part of the ToE itself?

For instance, as far as I can tell string theory needs basic quantum mechanics and analytical mechanics to be true for basic results to be obtained. Must we demand from superstring theory that we can derive e.g. the canonical equations?

8:52 PM

@0celo7 I believe there is no "one" universal meaning of ToE, but I think the minimum requirement is: Reproduce all (tested) predictions of QM and GR, and make predictions for all energy scales. Ideally should require minimal or even no experimental input to be able to make predictions (where the exact definition of what constitutes experimental input is probably also not universal).

@ACuriousMind I think the ideal situation would be one input, such as the string tension, right?

physics.stackexchange.com/questions/178917/… move to math?

@0celo7 Probably

@ACuriousMind Holy shit this question is painful physics.stackexchange.com/questions/178817/…

@ACuriousMind It's not even a homework question...

@0celo7 Your links are not correct.

@Icosahedron They are.

8:59 PM

It's not hyperlinked.

Your PC is broken.

@Icosahedron Uh...it is.

@ACuriousMind My current calc teacher has gotten tired of the subject and doesn't want to teach it next year, so I'm training her replacement.

it's not

I can teach him all the proofs.

Where the heck did I post that?

@ACuriousMind He's back

9:03 PM

@0celo7 That's how it supposed to be, I guess

@0celo7 <3

@ACuriousMind I have no idea what you're saying here.

@0celo7 what do you mean by that, aren't all ap calc proofs trivial?

@KyleKanos Your plugin is broken!! The links don't work!!

@0celo7 Then I'll stay cryptic ;P

@Icosahedron The proof of $\mathrm{d}\ln(x)/\mathrm{d}x=1/x$ is nontrivial, for instance.

The chain rule, proven properly, is nontrivial.

9:05 PM

uhhh

but it's still just high school level

it's in almost every book

Nope. The full proof is found in Rudin. (For the first one.)

Things don't become trivial by being in many books :P

The second one is given in the appendix to my college calc book, which is more advanced than the AP book at school.

@ACuriousMind The first one is better.

The proofs of the series crap are also nontrivial, but I don't know them.

9:08 PM

if you don't get a 5 on the exam. then

I got a 5 on the practice by a wide margin.

Come July and I don't come on here again, know that I didn't get a 5 and an hero'd.

do they also tell you your percentage?

Who? Collegeboard? Nope.

i don't like exams.

Have you not taken any AP exams yet?

You should have taken two history exams and chem.

9:11 PM

chem only. 5

what two history exams?

Euro and World

why should I have?

Those are sophomore classes.

i'm not even in any ap classes at school.

they are for seniors only.

i'm just taking the exam myself.

What kind of fucked up school system are you in?

9:14 PM

canada.

Are they making you pay for your exams?

yes, well the centre i signed up at did.

@Icosahedron $90 a pop?

y

That's fucked up. Have you been practicing at all?

9:15 PM

no.

i don't need to.

Which ones are you taking this year?

english, phys c m/em, calc bc, stats

perhaps for english

i can't write 3 essays that fast.

So you know how to do a $\chi^2$ test?

....

What does that mean?

9:18 PM

you don't do a chi squared test.

I took Stat and got a 5. Of course you need to know that.

I had a two-page chart of inference tests. You have that all memorized without doing any problems?

i don't know what you're taking about.

You're telling me you don't know what an inference test is?

i don't know

:D

Pray to Jesus, boy. You're screwed.

What do you think is on the AP test for Stat?

9:23 PM

......

Can you at least do a $z$-score?

Do you know what blind, double blind is? At least one experiment question is guaranteed on the FR.

i was not acting serious.

yes ofc i know all of that, i read the ap stats book a month ago.

You...read the book, didn't do any problems and hope to recall it all at the drop of a hat?

worked for chem.

I suggest you do a practice exam.

9:27 PM

i'm fine...

i only need to figure out how to write 3 essays in 45 mins.

or is it 1 essay in 45 minutes, 3 times?

@0celo7 why are you not writing the english exam?

@ACuriousMind OP has me worried I said something incorrect here. You mind reading it?

@Icosahedron AP English 12 is demonic.

@ACuriousMind Is it wrong to provide a source for physics.stackexchange.com/questions/178903/… 's desired calculation as a comment?

9:43 PM

@0celo7 too bad for me.

@0celo7 Grey area, see here

@ACuriousMind Jim's comment no longer makes sense.

@0celo7 Looks alright to me.

@0celo7 Heh^^

dmckee

11:33 PM

@0celo7 I sometimes get the feeling that user##### whoever (s)he might be, is trying to launch a Hilberteque attempt to set all of relativity on a solid and rigorous foundation. Other times that (s)he is just suffering from a bad case of OCD.

And I don't know enough relativity to sort the two possibilities out.

Anyone knowledgeable what to chime in with an opinion?

vzn

@NeuroFuzzy nice/ impressive work! any interest in 3d solitons? =D

@dmckee hilbert was visionary :)

dmckee

And Gödel was a grumpy curmudgeon. What of it?

Not that Gödel's theorem takes anything away from what Hilbert accomplished in laying firm foundations for mathematics, but it sure derailed the Program (tm).

alemi

11:49 PM

Why don't the user##### accounts always correspond to their userid?

vzn

@dmckee agreed actually godel sort of sabotaged one of hilberts main aims/ goals, the systematization of math. by proving it impossible. but there were different aspects of the Program so to speak. my favorite is hilberts 10th problem that took ~¾ of the 20th century to solve. (again proved undecidable...)

dmckee

:21359314They can change their name to have that form. We might, also have merged accounts for them, but I would have thought the dominate account set both number and name.

alemi

I've certainly seen many that do correspond, or if they have another account on another SE site, the number corresponds to that userid, but for some it doesn't correspond to any of the linked accounts

just seems odd, another peculiarity of the site