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user228700
2:30 PM
@JohnRennie Yayy! :-D
 
@Kaumudi.H Hi :-)
 
user228700
Hi! :-)
 
user228700
How goes it? How was the drive and all? Is ur house all dusty now?
 
Apart from a massive pile of junk mail inside the front door you wouldn't know I'd been away for two weeks :-)
But ... it's good to be home!
 
user228700
Ah :-)
 
user228700
2:33 PM
@JohnRennie Yay! :-)
 
Though I'm going to miss my Mum's cooking.
 
user228700
Oh, yes :-(
 
user228700
At least now it will be easier not to eat everything though.
 
I weighed myself when i got home and I was 69.9kg.
When I left I was 65k.
5kg in two weeks!!!
 
user228700
^ YES!! :-o
 
user228700
2:35 PM
Your mum must be one hell of a cook!
 
On Friday she made the best kedgeree I've ever tasted!
 
user228700
::Googles "kedegree"::
 
user228700
> Kedgeree is a dish consisting of cooked, flaked fish, boiled rice, parsley, hard-boiled eggs, curry powder, butter or cream and occasionally sultanas.
 
user228700
Oh, wow, that's a lot of stuff!
 
The name comes from kitchuri.
 
2:38 PM
@Kaumudi good luck on trip to darjeeling
 
user228700
@JohnRennie Ah, I see...
 
The dish originated in the days of the Raj as a variant of kitchuri.
 
user228700
@BalarkaSen :-( Are u mocking me? I'm broke at the moment.
 
not well-informed on the news i see
 
user228700
@BalarkaSen Not at all.
 
2:39 PM
google darjeeling news
 
user228700
Sigh. What's going on there? ::Googles::
 
user228700
MONTH LONG BANDH?!?!
 
What does bandh mean?
 
user228700
@BalarkaSen Wtf, man? It was a mistake trusting you -_-
 
2:43 PM
@Mithrandir24601 Hi, are you at all familiar with the concept of a 'typical sequence' in information theory?
 
Anonymous
Can someone suggest me a Classical Mechanics book suitable for self-study? I checked goldstein but it seem all the problems are unsolved in it.
 
@Kaumudi It's one of my favorite scenes from one of my favorite horror films.
 
Anonymous
@JohnRennie It means a strike during which shops and transport isn't functional
 
user228700
^ YES. Strike! That's the word I was looking for!
 
Aha, thanks.
Bandh is rooted in Sanskrit word 'Bandhana', meaning "a bond". Also used in a Urdu word meaning "closed". It is a form of protest used by political activists in South Asian countries such as India and Nepal. It is similar to a general strike. During a bandh, a political party or a community declares a general strike. A Bharatha bandh is a call for a bandh across India, and a bandh can also be called for an individual state or municipality. Often, it is going to be the best community or political party declaring a bandh expects the general public to stay in at home and not report to work. Mo...
 
2:45 PM
indeed
 
user228700
@BalarkaSen Damn.
 
Maybe I was lying. It's still a damn good movie.
 
user228700
@BalarkaSen I had the better fortune to watch another scene from the movie and nope, it's a bit too gruesome for my taste.
 
lol
It is body horror. A genre which isn't for everybody, but can communicate ideas no other genre of film can.
 
user228700
@BalarkaSen How so?
 
3:01 PM
Well, per se, I suppose not many horror directors actually exploited this genre - most of non-Cronenberg movies boiled down to slasher, which is like, action movie with a lot of blood.
But the point of the genre is to inflict horror from bodily transformations, and Cronenberg at least uses it symbolically.
 
user228700
> uses it symbolically
 
user228700
Guess I'd have to watch the film but I don't want to subject my brain to more of what I just watched >.<
 
haha. well, the scene you just watched (i don't know what the other one is) is basically a violent hallucination
His artistically best movie from this genre is unquestionably Dead Ringers, which is not very violent, but I am not sure if many people are comfortable with the subject matter
 
user228700
A violent hallucination? Oh, wow, I see.
 
(Videodrome is not artistically significant, but contextually awesome)
 
SBM
3:04 PM
hallucinations? scary
 
user228700
@BalarkaSen Oh, I see...
 
The popular joke is that Jeremy Irons won Oscar for Dead Ringers, just on the next movie he did :P
 
I want to make a movie named 'Canceling the d's in dy/dx'
 
I would watch that movie.
 
Thank you. I knew I could count on you
 
SBM
3:13 PM
@Avantgarde Fallacious title
 
user228700
I would too x'D
 
SBM
I'll also go watch it.
 
:')
Canceling the d's in dy/dx: The return of the genius
 
@Avantgarde I'm gonna pirate it
 
It's ok. It's an independent movie made just by me. I give you all them rights
 
3:18 PM
String theory debunked in 50 seconds:
 
lol the second comment goes "What's with all the drama?"
some youtube comments are precious
 
@Avantgarde It would have to be an 18 certificate. You couldn't risk impressionable young mathematicians being corrupted by such extreme material.
 
hahah
 
hehe
 
the new X-rated movie which divides by 0
 
3:27 PM
The sequel is widely believed to be a horror
 
exploitation film brah
exploiting the fact that Z isn't a field
 
going down in history for being a very bold, experimental film back in the day
 
Anonymous
@Avantgarde Any idea about a good self study book for classical mechanics (with solved problems) ?
 
Anonymous
I saw goldstein but all problems in it are unsolved
 
Anonymous
Is L&L series good?
 
3:39 PM
Yes, it's a hard book too. That's the one I used. And lecture notes.
It's supposed to be, yeah. Though I've never studied from them
 
Anonymous
@Avantgarde Do you recommend any other book?
 
Anonymous
And is the solution manual for goldstein available?
 
Unfortunately, I don't know about classical mechanics/field theory books other than the 2 above. Not sure about Goldstein solutions either; you should google and check
Maybe look for some pdf lecture notes on classical mechanics.
on the internet
 
@blue It must exist, and you can pirate it.
 
Anonymous
@Mostafa I found one....it has only solutions for 3 chapters (on libgen)
 
hahaha
 
SBM
@AccidentalFourierTransform haha
 
go away dude
-+++ is the right one
 
Hi @Accident
 
No way!
 
-+++ is the only correct one here
 
SBM
@AccidentalFourierTransform supremum of?
 
inf
 
NaN
 
@Avantgarde isn't that just += 2?
 
@Giskard42 eh?
 
-+++ is +=2
 
O_o
 
(sorry, C operators.)
 
@Giskard42 Yeah that's true
 
3:48 PM
oh god
lol, I don't remember my C anymore.
 
SBM
@Giskard42 not only C but almost anything
 
@SBM with the exception, for some damn reason, of python.
Every time I switch languages that trips me up
 
SBM
@Giskard42 And Pascal
I don't like switching languages unless I need to.
 
@SBM me neither, I'd rather be lovingly caressed by a Python ad infinitum, but it's a job requirement :)
 
i like programming with a pen and a paper
 
SBM
3:53 PM
@BalarkaSen that's the best
no syntax no rules just plain fun
 
@BalarkaSen I like programming with a steady hand and a magnetized needle.
 
SBM
but no complex stuff
 
you can also make... doodles!
 
I also like programming with rocks xkcd.com/505
@BalarkaSen Huh, since graphite's conductive, I wonder if you could actually program with pencil and paper
Given a suitably doped graphite pencil
 
Very nice xkcd
 
3:57 PM
Now I've got this image of Bob Ross making AND gates on a canvas "Now just use a little bit of p-doped paint here, use what comes natural...a happy little flip-flop there"
5
 
lol
 
@JaimeGallego That's pretty darn cool.
"Papers Please for people who want to be that much more nerdy."
 
SBM
@BalarkaSen huh
 
4:06 PM
@BalarkaSen this is gold
 
SBM
oh
 
@BalarkaSen I am infinitely disappointed that there's no We Are Number One in there
 
oh my god, this is even better youtu.be/ynslyx0G0Vs
 
I cracked at the Mario tune
 
4:12 PM
same
 
@BalarkaSen Not quite in the same spirit, but youtube.com/watch?v=93fqlzu9pqk
 
lol Thomas & Friends
 
hah
 
4:20 PM
@Avantgarde My favorite version of that: youtube.com/watch?v=VhqeNUFCyI0
 
in Mathematics, 33 secs ago, by Danu
Does anyone here happen to have access to the book "Global Riemannian Geometry: Proceedings of the symposium held at the University of Durham, Durham, July 1983. Edited by T. J. Willmore and N. J. Hitchin."? I am really interested in obtaining (scanned copies of) certain chapters.
 
Guys, why do they choose $R_2=-50$ to be negative? As far as I know, we have to determine whether the surface is convex or concave, and in the case of the planar-convex lens, I would say then that $R_2=+50$?
 
@Giskard42 I didn't get the reference very well, but this series is hilarious
 
SBM
oh
 
This is also very good, but not relevant to the meme-series
pretty much whatever from that channel is dank
 
4:29 PM
lol
 
@BalarkaSen I recommend we now rename this chat room "The Meme Bar".
 
ive never seen trump not in a suit
 
we never discuss physics in hbar aaaanyway
 
@Avantgarde He's been caught on camera getting out of a suit - though it was in court
:P
 
4:32 PM
lol hyperphysics
I was reading this site 15 years ago
 
I thought that was going to be a meme page for some reason.
 
I don't think it has changed at all since
 
I should have spent my day doing something productive. Oh well.
 
@Giskard42 lol
 
It's still in glorious HTML
 
4:33 PM
@Slereah It's basic in design and badass in content
 
it's quite nice though
 
Sometimes I wish more sites went that way
 
A lot of physics websites are like that
Because physicists, being academics, have used the internet since the olden days
 
Sometimes you go to a resource and it's a 10 Mb Bootstrap-enabled CSS-heavy HD-asset page for two paragraphs on Newton's Method
 
@John thanks
 
4:35 PM
So a lot of their websites were made in the 90's
 
It's refreshing to find a page you can actually wget
 
man, the people who made these memes are quite creative I must say
 
what do you mean? it's the aesthetics. this is postmodern art.
 
@Avantgarde Oh man, spend some time around me_irl on reddit and you'll be blown away
 
lol
 
I'll check that reddit thing out
 
Ok, I'm done. I should do real work now.
 
@Avantgarde
 
Lol! the first rule is "All posts must be surreal"
why have i never seen this before
 
4:46 PM
 
heh, very creative.
Thanks for this valuable link, @Giskard42
 
@Avantgarde One more excellent resource: reddit.com/r/hmmm/top
(I'm sorry.)
 
lol seriously?
 
Speaking of things in the oven, my vacuum chamber is almost golden brown now
 
0
Q: Can my question be reopened?

Drew WijayaI asked a question which had a problem that the Hamiltonian was not Hermitian. However, after thinking about it for some time I managed to make the Hamiltonian Hermitian. While it may still have problems I think it can be re-opened at least. Hamiltonian that can produce random primes?

 
4:55 PM
@Giskard42 Hmmm
 
The concept of having the baseplate to a vacuum chamber in my family's oven is infinitely amusing to me, though less so to my mother
The recipe's quite tricky, you have to make sure the Delrin doesn't caramelize.
 
5:15 PM
@JohnDoe Never heard of it...
 
 
1 hour later…
6:34 PM
@Sanya "I talk about topology in the hbar because ACM knows topology, not the other stuff I work on" - @0celouvskyopoulo7
 
6:55 PM
@ACuriousMind halp
sanity check please
the Killing metric of $\mathrm{SO}(1,n)$ is therefore unique, right?
cuz the only invariant subalgebra is $\mathrm{SO}(n)$
 
7:08 PM
@BernardoMeurer :D
 
@AccidentalFourierTransform I don't understand what "invariant subalgebra" has to do with it being simple - the full classification of simple Lie algebras does not involve the Lorentz algebra, so it's not simple, so the metric is not unique.
 
hmm but its semi-simple
 
so acc. to the text, there should be one arbitrary constant per simple factor
and $\mathrm{SO}(1,n)$ has only one simple factor, right?
or does it?
 
No, I don't think so
 
It being semi-simple means it being the sum of simple algebras, and it not being simple means there's more than one.
 
Has anyone here dabbled with numerical relativity?
 
@ACuriousMind argh
nothing makes sense
FML
I h8 you
(thank you anyway, sweety)
 
7:28 PM
@AccidentalFourierTransform It isn't so bad: Do you understand why simple Lie groups admit a unique bi-invariant metric up to scaling?
 
I mean, I know that. But I don't think I understand why.
 
@AccidentalFourierTransform OK. Do you want to go through it?
 
I probably don't know enough group theory to understand it :-P
im just a kid
 
You need zero group theory
It's geometry
 
then lets do it
 
7:32 PM
So do you know the definition of a bi-invariant metric? :P
 
So what is it?
 
well, $g(p,q)=g(Xp,Xq)$, right?
 
Where $X$ means what?
 
an element of the Lie Group, $X\in G$ and $p,q\in TG$
 
7:36 PM
Okay
That doesn't quite work
but you're headed in the right direction
Maybe I'll introduce a bit of notation. Say we have a Lie group $G$; all of its elements $g$ induce natural self-diffeomorphisms $L_g$ and $R_g$, left- and right-multiplication by $g$.
 
Then, as usual with diffeomorphisms, there are the induced maps on the tangent bundle $D_h L_g$, $D_h R_g$ and those can in turn be used to yield maps on tensors (if you don't have a metric, just on $(0,k)$-tensors I guess) by pulling back, i.e. defining $(L_g^* T)_h=T_{gh}\circ D_h L_g$ and similarly for $R_g$
So there is now an obvious notion of left- and right-invariance for tensors: $L_g^*T=T$ $\forall g\in G$ and similarly with $R_g$
Does this all make sense?
 
yep
so basically I missed the * before
 
@Lozansky I guess KyleKanos can help you.
 
So what is a bi-invariant metric? Well, a metric which is both left- and right-invariant.
 
7:43 PM
right?
oh right
 
Well, $X$ (my $g$) does not even act on tangent vectors
and you seemingly wanted to only mention left-invariance; that's not enough.
 
yes, I forgot about the right :-P
 
But it was in the right spirit anyways
Now, do you know how to produce left-invariant metrics?
 
let me think
:-(
I swear I used to know
 
Okay, so maybe some philosophy is in order
 
7:45 PM
I have forgotten most of my diff. geom.
darn
 
Why are Lie groups so nice?
Or topological groups in general?
 
they define a differentiable manifold?
there are plenty of reasons I guess
 
Because you can do mostly anything you like at a single point and "push it around" to get global statements. The pushing is guaranteed to preserve all structures (smooth, or topological, whatever category you are working in)
 
There are many incarnations of this philosophy
(i) representation theory can be essentially reduced to that of its Lie algebra
 
7:47 PM
good ol exp
 
(ii) For topological groups, Hausdorff is equivalent to $T_1$, which is equivalent to $\{e\}$ being a closed subset (oddly specific example that I happen to know)
(iii) the tangent bundle of a Lie group is trivial
All consequences of the homogeneity of groups
Anyways
Given this, can you find the application of this philosophy?
(to be clear, the exercise is to determine the left-invariant metrics on a Lie group)
Any guesses?
 
it has to be something about the algebra
 
A metric is an inner product on each tangent space
What does the homogeneity suggest?
 
I dont know what homogeneity means in this context :-S
you know what? I feel like I'm wasting your time lol
 
What it does is suggest that perhaps an inner product on a single tangent space will contain all the data you need
 
7:54 PM
how about we continue this conversation after the summer?
I'll have some free time
 
Don't worry---this is only taking so long because I choose it to :P But if you're not interested, then we can forget about it.
 
@Danu I don't know why when I see the word Hausdorff I suddenly think of @0celouvskyopoulo7 discussing some topology stuff with ACuriousMind....
(sorry for the interruption)
 
@Danu something about Killing vectors
and geodesics
 
I'll give you the main results you need:
1. Left-invariant metrics are in bijection with inner products on $T_eG=\mathfrak g$.

2. Same goes for right-invariant metrics.

3. Bi-invariant metrics are in bijection with inner products on $\mathfrak g$ which are *invariant under the adjoint action* of $G$ on $\mathfrak g$.
@AccidentalFourierTransform You're being way too fancy.
There is a priori no metric, so no Killing nothing, no geodesics
4. Simple Lie algebra is equivalent to (or the definition of, depending on who you ask) being irreducible under the adjoint representation.
5. Incarnation of another philosophy: Schur's lemma. This tells you that any two inner product invariant under an irreducible representation of a group must be proportional.
 
so to find a bi-invariant metric you only need to know its action at $e$, right?
 
7:58 PM
Conclusion: Simple Lie algebra/group means there is only one (up to scale) bi-invariant metric.
@AccidentalFourierTransform What does "its action" mean? You mean it is determined by an inner product on $T_eG$? Then yes, by the above, it is given by a special kind of inner product on $T_eG$.
 
ah yes, that's what I meant
and that's where the $\mathrm{tr}(\mathrm{ad}_X\mathrm{ad}_Y)$ comes from
 
It is not hard to check that the Killing form on $\mathfrak g$ is invariant under the adjoint action, indeed.
You have to be careful though
You may be confusing $\operatorname{Ad}$, the adjoint action $G\curvearrowright \mathfrak g$ with $\operatorname{ad}$, the adjoint action $\mathfrak g\curvearrowright \mathfrak g$.
It is the former that determines whether you get a bi-invariant metric.
(but the latter appears in the definition of the Killing form)
 
anyway, as I was saying, I'll have some free time this summer
what should I study?
my main options were differential geometry, CFT and string theory
what should I do?
 
Depends on your aims
 
I dont have aims
also, @Danu you are a mathematician
so you will probably tell me to study maths
ACM say sumfin
 
8:09 PM
@dmckee @JohnRennie Do y'all miss my spam of C questions yet?
 
@AccidentalFourierTransform Not string theory ;)
 
@BernardoMeurer That would imply that I want to think about pointers. Which I don't most of the time.
 
@Mithrandir24601 y tho
a lot of people seem to like it
it might be nice
idgaf about phenomenology
 
You should try to find concrete aims in order to decide what to study
 
8:25 PM
@AccidentalFourierTransform One reason: A lot of people want to do string theory because it's 'cool' or 'popular and therefore amazing' (i.e. they've heard of this thing called string theory that claims it solves a lot of problems - whether or not it actually does isn't my point) and so go off and study string theory without actually knowing what areas of theoretical physics they actually like or are good at. Meanwhile there are loads of areas of theoretical physics that are amazing and often ignored a lot more
 
well, to me it's more about recreational physics
so if it is actually fun - as a lot of people claim - then its fine for me
it's not that I think it solves problems, or that I could be good at it
it's just for fun
 
@AccidentalFourierTransform If you look properly at various areas of physics and go 'oh, this is interesting' and it ends up you looking at string theory, then fair enough, especially if you just want to have a look and see what it's like. One of my friends actually started going into string theory before luckily realising that he didn't like it (just before starting a PhD...), so changed track completely. So it is nice to know whether you like it or not
 
yeah, that's kind of my point: I want something to study this summer
so I wont be able to get into the details
I just want a taste of it
 
so... What do you enjoy?
 
otoh, if I studied diff.geom. then I would get into the details
diff.geom would be useful, ST would be for fun
@Mithrandir24601 pizza
@JaimeGallego hey how did your exams go?
 
8:30 PM
Still with them
 
selectividad right?
 
Nope
That's next year
 
@dmckee I only think of pointers
 
@BernardoMeurer I have an exam on pneumatic circuits tomorrow. It's basically logic gates with air, you would like it.
 
8:32 PM
I hate circuits
 
all my circuits
 
And on Tuesday, Biology. ::runs away screaming::
 
ah, the Calvin-Krebs cycle
good old times
 
@AccidentalFourierTransform damn, this whole discussion (people telling AFT not to go into string theory, and his answer I just want to learn its basics for fun) reminds me of von Jolly's advice to Planck against studying physics, and Planck's answer that he just wants to study the fundamentals for fun....
 
@AccidentalFourierTransform I think I'd go for CFT
 
8:35 PM
@Mostafa Holy shit
 
@ACuriousMind so, no pure maths?
 
AFT is bound to become an angry German then.
 
@AccidentalFourierTransform Well, if you wanna do pure math, then do pure math
 
idk
CFT could be fun - it's something new
 
But in the "math for fun" department, differential geometry does not rank highly on my list
 
8:37 PM
but pure maths could be useful
 
I think CFT is pretty fun even if you only learn it superficially, because it's so much "nicer" than general QFT
 
@ACuriousMind Really? Surprising
I guess it depends on what you call differential geometry
 
@ACuriousMind CFT = classical field theory?
 
I don't like the analytic parts too much
 
No, conformal field theory
@Danu My sentiment precisely
 
8:38 PM
Which do tend to be quite prominent in certain parts
 
And let's be honest - how much string theory are you going to be able to learn in ~3-4 months?
 
Some of the results are nice, but I don't like the general style of reasoning
 
But I'm generous and call all topology of manifolds differential geometry too ^^
That makes it into the best field ;)
 
@Mithrandir24601 Since AFT already knows QFT quite well, he should be able to make decent headway
...so many FTs
 
In any case, it really doesn't have to be all to computational
 
8:39 PM
@ACuriousMind Fair enough
 
Especially complex geometry---no functions or vector fields to work with :D
 
From my point of view - last year, it got to a point where I could have done a dissertation in error correction, or weak measurements. I was interested in both and thought that error correction would probably be more useful, but weak measurements more interesting to look at for a few months without PhD obligation. So, I did weak measurements and it's ended up being more useful than error correction would have been. I'm not sure if there's a point to that, or if it helps or not though
 
did I tell you guys? I can eat whatever I want: my pants are invariant under dilatations
 

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