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19:00
It's trying to keep this site interesting to physicists.
@barrycarter Stack Exchange went through a time when all the active high-rep users could run through their close votes in an hour every day, too.
If we allow through crap like this we just turn into Reddit.
It seems to be a stage in the life of growing sites.
@JohnRennie Do closed posts still take up space in the servers?
@Obliv Questions continue to "exist" even once they are deleted. But disk is cheap.
19:02
@Obliv Posts that are closed and meet some other criteria get automatically deleted.
But they still exist as high rep users can see them.
oh okay. So the primary reason of closing hw questions is so they don't mess with the SEO
The problem isn't the load on the servers, it's the load on the minds of the users trying to provide good answers about physics.
@Obliv In my opinion, the promary reason for closing question is as a signal to users about what kinds of question we want them to ask here.
2
@dmckee Are you a C++ Junkie?
Could someone approve the pending edit on this:
0
Q: Schrödinger equation in momentum space

AlexIn literature on an introduction to quantum mechanics which I am working through, there is a section which explains that a vector has different representations based on the basis you choose. It then makes a statement that The same is true for the state of a system in quantum mechanics. It is...

I want to correct the title but I can't do so until the pending edit has been OK'd or rejected.
Oh, it's just been corrected :-)
19:08
I think I killed the pending edit with my edit
@JohnRennie do you get a cool trophy/badge when you hit 200k rep? Also who is that in your image?
Why does no one ever think it's he himself?
@Obliv Not as far as I know. I didn't get anything for reaching 100K except a congratulations e-mail from the SE guys. Basicaly once you're at 20K that's as as regards extra stuff.
@ACuriousMind no-one believes I could be that handsome and distinguished? :-)
@Obliv that is me in the picture, though the picture is twenty years old and I have since lost the face fuzz.
@JohnRennie This.
@ACuriousMind The black and white makes me think it's some early 20th century physicist lol
19:13
I have a hard time believing you're not a brain in a jar.
oh wow didn't know.
@Obliv I used to do amateur operatics back then, and that was me in costume for Die Lustige Witwe by Franz Lehar. The operetta is set at around the beginning of the 20th century, so yes it was in the heyday of classical physics.
I do hope I get through chapter 3 of Peskin soon
All those Lorentz transforms are giving me hives
I love the cover of Peskin
Then I opened it with some knowledge...
@JohnRennie that sounds like fun. I always admired operatics even though I never did any myself :p
19:19
Now I like bits of chapter 9
@Slereah dude
@dmckee Just physics, or all subsites?
Most normal people don't buy into this "particles are not defined at the interaction level" nonsense
Do you mean THE TRUTH
I don't even buy this "a fermion field pervades all of space at all times" nonsense
19:20
@JohnRennie But what do you define as a "physicist"?
@bolbteppa The only thing I know for god-damn certain is that inhomogeneous spacetime causes pencils to fall down.
I wonder how many theorems actually apply to like
All of QFT
@Slereah Define "All of QFT" :P
Well without any assumptions beyond those of basic QFT
Is a pencil billions of excitations of one fermion field or just billions of fermion fields
19:22
Well there are several fermion fields
I recall some AQFT presentation with all the theorems proved in it
Let's see
I wonder how hard the proof of the mass gap will be once someone finds it
If it's gonna be something nobody proves a second time because it's so awful
Or if it will be obvious in retrospective
@bolbteppa It's one pencil field :)
20:00
I guess the first chapters of Weinberg is all generic theorems on QFT
By the way, what's a paper that actually talks about continuous spin states
I'm having a hard time finding one that actually discusses it
@ACuriousMind Agreed, the map $X\mapsto (X_p,\nabla X_p)$ has to be injective.
@ACuriousMind But how is this equivalent to $X_p=0=\nabla X_p\implies X=0$?
Lo and behold
He is risen
"The others are the “continuous spin representations” (CSR), which describe a massless object with an infinity number of helicities."
That is a lot of spins
@0celo7 Because a linear map is injective if and only if its kernel is trivial?
Sometimes I feel like a broken record :P
@ACuriousMind How do we know that map is linear?
@ACuriousMind No shit.
@Slereah I thought you wanted to learn QFT, and not "representations that never appear in QFT" :P
20:08
I want to learn everything
@0celo7 By...looking at it?
CSR intrigue me
@ACuriousMind Huh?
Because they are never talked about
Here's one
@0celo7 What exactly about verifying that that map is ($\mathbb{R}$)-linear looks difficult to you?
20:10
@ACuriousMind Wait, is it just evaluating $X$ and $\nabla X$ at $p$?
No tricks?
None that I could see
@ACuriousMind Then yes, it is trivial.
Apparently CSR are done by doing the usual massless representation method
BUT
there is an Assumption
That you can drop to get CSR
"They are characterized by a space-like Pauli-Lubanski vector, and describe a massless state with an infinite number of integer-spaced helicities"
that is a lot of them
@Slereah You already said that a few minutes ago :P
Well yes, but
An infinity certainly is a lot
20:15
@ACuriousMind That's a fun proof.
I want to prove more things like that
Easy proofs
@ACuriousMind What is the absolute slickest way to prove that an antisymmetric matrix has no real eigenvalues?
Is there like a "physicist proof" of that?
@0celo7 There is no such proof because the zero matrix is antisymmetric and has real eigenvalues :P
Sigh...
@ACuriousMind No real nonzero eigenvalues.
@ACuriousMind Would you happen to know of a non-simply connected even-dimensional manifold with everywhere positive curvature?
@0celo7 assume $v$ is a normalized eigenvector with real eigenvalue $\lambda$. We have $\lambda=v^t A v$. Therefore, $\lambda=\lambda^t=v^t A^t v=-\lambda$.
for a "physicist proof", we could try to think what would happen if $F^{\mu\nu}$ had non-zero eigenvalues
maybe something weird
Heh
I remember Lubos once gave a physicist proof of something with eigenvalues of $\mathfrak{so}(n)$-valued curvature forms.
Or something.
It was terrifying.
Errr... isn't de Sitter like that?
I mean the REAL de Sitter
$S^3 \times S$
20:26
@Slereah Riemannian.
Oh
Try doing it in Riemannian?
Just look at the maximally symmetric metric on $S^3 \times S$
@0celo7 Do you mean this thing?
@ACuriousMind Indeed.
I wish the clipboard would hold more than 1 copy/paste
like seriously how much ram would it take to get like 2 or 3 c/p
> like seriously how much ram would it take to get like 2 or 3 c/p
Phrasing, dude.
20:32
i hope you don't mean what I think you mean..
@Obliv Maybe, maybe not.
you go on 4chan too much man
@Obliv Haven't been on there today yet.
@Slereah Do you know the proof of why $g_{ij}(p)=\eta_{ij}$ in normal coordinates?
It's not commonly written down anywhere it seems.
Isn't it in HE
Is it? What page
20:39
you know how the prime factorization of $a$ and $b$ is $a = {p_1}^{\alpha_1}{p_2}^{\alpha_2}...{p_t}^{\alpha_t}$ and $b = {p_1}^{\beta_1}{p_2}^{\beta_2}...{p_t}^{\beta_s}$ and the G.C.D of them is $(a,b) = {p_1}^{min({\alpha_1},{\beta_1})}{p_2}^{min({\alpha_2},{\beta_2})}...{p_s}^{min(‌​{\alpha_s},{\beta_s})}$? How come it says minimum g.c.d?
@0celo7
I dunno m8
going to bed soon
@Obliv What
Minimum gcd?
I don't really know.. Just got this definition
i read it as minimum
@0celo7 lol
what else could min($\alpha$,$\beta$) mean
20:41
Where does it say minimum gcd?
@Obliv minimum of $\alpha$ and $\beta$
@Obliv It means "minimum of $\alpha$ and $\beta$"!
oh
stupid notation
????
It's literally the most obvious notation ever
Not like fucking Hawking-Ellis
Grrrr
lol
I don't think it's very obvious but then again I'm really new to it
20:42
he hasn't ever used min in the book before. I should have known though since thats how minimum is used more or less in java'
Who ever thought $$\frac{\mathrm D}{\partial t}\left(\frac{\partial}{\partial t}\right)_\lambda=0$$ is good notation!?
The reason it's not obvious is that in number theory $(a,b)$ is also used for lcm or gcd or something so it can be confusing as all of number theory is
@Obliv Oh wait
You're sure it's not $\min\{\alpha,\beta\}$?
That's how we did it in my course
no its parentheses
Min of the set $\{\}$
20:44
i guess he could have meant coordinate pair
$\langle , \rangle$
They have some $(a,b)$ lcm and $[a,b]$ gcd thing
@Obliv Ok, that's shitty. But you should have figured it out from the proof.
@bolbteppa I think [a,b] is always the lcm
@0celo7 it doesn't show a proof. I'm guessing I have to prove it unless it's really difficult
@Obliv The proof is not hard.
4
A: Prove that $\gcd(M, N)\times \mbox{lcm}(M, N) = M \times N$.

Patrick Da SilvaI don't know what you mean by ''algebraically''. I'll show you a proof. I write $(m,n)$ for $gcd$ and $[m,n]$ for $lcm$. If $(m,n) = 1$, then $m$ and $n$ both divide some integer $r$ if and only if $mn$ divides it (easy consequence of the Euclidean algorithm); it means that $[m,n] = mn$. Otherw...

20:46
You show it's a common divisor and then argue it's the greatest.
Use a contradiction proof for the second half.
@bolbteppa weird I actually searched that up yesterday
@Slereah WAT.
@NeuroFuzzy Note the file name: Evil.jpg :D
@Slereah Is that QFT
20:53
So much evil

@0celo7 group theory, actually
You know what is evil
Although I had to use it for QFT
@Slereah what's the difference?
@Slereah Uhhh...shouldn't the pentagram be upside down to be evil?
group theory isn't QFT
Are you a satanist
20:55
@Slereah Only on Tuesdays.
I don't know if he's kidding.
People with long hair can be unpredictable as hell.
heheh
That's me
It's funny because it's true
Except I can't do any of that math
I think people in my physics class think I'm retarded
I can't add double digit numbers in my head any more
20:58
Do math problems get harder as you learn more math? I was under the assumption that it did :o
@Obliv what
like taking up more working memory than simpler math
@Obliv Dunno, math has always been easy until lately
Math problems don't get so much "harder" as...different.
Until the toxoplasmosis
21:00
the self-diagnosed toxoplasmosis
sorry but I'm a bit skeptical :x I feel like it could be anything honestly
Mental degredation induced by excessive drinking
maybe you got into a horrific accident without knowing.. or you just didn't sleep for a year straight. I don't know many causes for brain damage admittedly
just ask @ACuriousMind
He can tell you I'm getting stupider
prime numbers are so interesting...
not really
21:09
^I concur :P
it's like describing a proton by its quarks (if quarks could also exist alone). It's still mass but one can be used to describe the other.
what
stop doing so much LSD dude
LOL i'm sober i think
Someone halp
C++ is killing me
bernard.help(yourself);
sry that was mean D:
21:12
@BernardMeurer What are you doing? Getting impaled by misdirected pointers?
@Obliv That's not even proper C++
Should be a parabola in one or both of those graphs :p
@ACuriousMind Getting a bunch of SIGSEGV errors when using GMP for arbitrary precision variables while calculating $A(4, 1)$
@BernardMeurer yeah I don't remember c++ syntax anymore. rip
And also having my IDE be very angry at me for not instantiating an unknown structure (which idk how to do)
21:15
@BernardMeurer Are you compiling with all warnings on?
@ACuriousMind well you also think Riemannian geometry is boring >:(
@barrycarter Always (if by that you mean -Wall flag)
@BernardMeurer "-Wall -O2"
back when I did C at work, one thing I did when I was bored was to set the compiler to maximum alert and try to close all the possible errors
Also try adding printf's or otherwise accessing the variable in question and the problem may disappear magically.
21:17
@barrycarter These numbers are too big, printf's will break everything :p
@BernardMeurer What kind of numbers do you serve in this place?
Alrighty, building wiht the -O2 flag enabled
@barrycarter Ackermann of 4,2
$A(4, 2)$
built with no errors
this is some bull
No errors, no warnings? Try accessing your large numbers somewhere. C++'s optimization can be excessive sometimes.
I've run into this problem myself and searching stackexchange will show others have too.
1
Q: Code crashes unless I put a printf statement in it

AlexThis is a snippet of code from an array library I'm using. This runs fine on windows, but when I compile with gcc on linux if crashes in this function. when trying to narrow down the problem, I added a printf statement to it, and the code stopped crashing. void _arrayCreateSize( void ***array, i...

9
Q: When program run correctly while debugging but otherwise not?

Jichao Possible Duplicate: Common reasons for bugs in release version not present in debug mode Sometimes I encouter such strange situations that the program run incorrectly while running normally and it will pop-up the termination dialog,but correctly while debugging.This do make me frustrat...

@BernardMeurer OK great, I actually have to LOOK at it? :P
21:22
Yeah I'm using valgrind on it right now
@barrycarter Only if you want to :p
Just a project I started yesterday
@BernardMeurer I prefer to offer vague meaningless and unhelpful advice in chat rooms.
@barrycarter That alright too I guess
@BernardMeurer Hmmm, fairly short code, not much to fail there
@BernardMeurer Try having ackermann() print its arguments each time to see how far it gets. Possible stack overflow?
@barrycarter Exactly! I was trying to keep it simple to build on a stable base and bam
@barrycarter I don't like SO, but I might
21:24
@BernardMeurer LOL! :) OTF! No, I meant literally... you're overflowing the stack by having too many recursions.
@BernardMeurer Can you compute A(4,2) outside the for loop? IE, just for a single pair of numbers?
@barrycarter Hahahahahaha yes stackoverflow
I checked on valgrind yesterday
@BernardMeurer Nice variable shadowing, btw :) Good use of private namespace.
Was about to do that, I'll do A(4, 1) because that also overflows
And I added the prints
@barrycarter Updated the code on git
@BernardMeurer Still failing? Any output at all?
Oh yes, it's outputting
SO MUCH stuff
21:31
Infinite loop?
Ha, think I caught a bug
You can always rewrite recursive functions as non-recursive functions.
@BernardMeurer That means you caught a cold in America, but ok.
"Recursion, like life, is only valuable because it terminates"
@barrycarter There, changed it again
It'll still overflow though I think
There is no think, only do.
Yes yes, I'm not that dumb :p
Hmm it's taking a while to break now
wonder if it worked
21:37
Any output?
From the cout's I mean?
I deleted the cout lines because they made it too slow
(after I checked that it worked for a while)
@BernardMeurer How many recursive calls are you expecting?
@BernardMeurer Yes I read that earlier. I'm sure your recursive stack size is limited. Consider a global variable that increments when you call a recursion and decrements on return. See how deeply you're nesting your stacks.
jesus christ A$[4,2]$ would have 19k+ digits
21:41
@BernardMeurer Try implementing the Fibonacci numbers recursively and watch things crash :)
@barrycarter Crap, sigsegv
I bet if you do a global count of stacks, it will die right around where that count is a power of 2.
I remember learning the fibonacci sequence in first year java in hs :D @barrycarter fun times..
Sigh, lemme valgrind this again
Yes, it can be done recursively, via loop, or closed form. Very versatile numbers.
21:43
@ACuriousMind Is the change of internal energy during a Carnot cycle 0?
--num-callers=50
that's a flag?
Yes, but it tops out at 50.
--num-callers=<number> [default: 12]
By default, Valgrind shows twelve levels of function call names
to help you identify program locations. You can change that
number with this option. This can help in determining the
program's location in deeply-nested call chains. Note that
errors are commoned up using only the top four function
locations (the place in the current function, and that of its
three immediate callers). So this doesn't affect the total
number of errors reported.

The maximum value for this is 50. Note that higher settings will
Also look at the --main-stacksize option and consider changing your ulimit.
@ACuriousMind Ignore that I'm a dummy
@0celo7 I don't care
21:46
@ACuriousMind Jesus
What? I just find that sort of thermodynamics so incredibly boring.
I do too, these signs are driving me insane.
They have an equation with a + in the lecture notes, but on the homework you flip a sign for some fucking reason
@barrycarter valgrind --num-callers=50 --main-stacksize=9999999 ./ackermann.o
@BernardMeurer And the exciting results?
So far nothing :D
21:48
Now somehow positive numbers are negative
I just don't get it
@0celo7 Did you overload your signed variables?
@0celo7 negentropy
I'll be dammed if I ever figure out if work is positive or negative
Work has to be positive, no?
21:48
@0celo7 Depends on the salary
If work isn't positive, you have an endothermic thingy, yes?
@BernardMeurer You're familiar with 'ulimit', right?
@barrycarter Yes, but that's not a code-based solution
changing the stack on Linux
although it would work
@BernardMeurer True, but if you can get your code to run once, at least you'll understand what's causing the problem.
You're still in the debugging stage.
The code solution would be to not compute this recursively. The recursion is far too deep.
espaco@Arch ~> ulimit
unlimited
@BernardMeurer Try "limit". You should get more than one word out of it.
limit
cputime unlimited
filesize unlimited
datasize unlimited
stacksize 10240 kbytes
coredumpsize 0 kbytes
memoryuse unlimited
vmemoryuse unlimited
descriptors 1024
memorylocked 64 kbytes
maxproc 1024
21:55
espaco@Arch ~> ulimit -s unlimited
espaco@Arch ~> ulimit -a
Maximum size of core files created (kB, -c) unlimited
Maximum size of a process’s data segment (kB, -d) unlimited
Maximum size of files created by the shell (kB, -f) unlimited
Maximum size that may be locked into memory (kB, -l) unlimited
Maximum resident set size (kB, -m) unlimited
Maximum number of open file descriptors (-n) 1024
There
OK OK.
Let's see what happens now
and hope it doesn't involve fire
I didn't start the fire.
@BernardMeurer Relax, it's not cooking :P
@ACuriousMind That trauma is starting to affect other parts of my life :p
Crap the makerspace is closing
I'll let it run overnight
and run it at home also
21:58
makerspace = computer lab?
I manage a makerspace @barrycarter, so I' stay here most of the day
oh, I don't know what that word means... computer lab?
SEE
Real people don't know what that is
Just wp'd it, but hadn't heard that before.

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