« first day (2521 days earlier)      last day (1016 days later) » 
00:00 - 16:0016:00 - 00:00

4:00 PM
if not, though, it tends to be chewy and that's not so great.
My bitterness taste buds are deficient.
roasted broccoli is really good
"A number of important properties of the scattering amplitude can be established by considering it as a function of the energy $E$ of the particle undergoing scattering, the energy being formally regarded as a complex varriable."
@Semiclassical I like all the brassicas.
There's a giant football game tomorrow so the town will be gridlocked
Not fun
4:01 PM
@bolbteppa sure. you'd expect to have poles at the bound state energies, if memory serves.
@0ßelö7 and if you're doing scattering in a periodic potential, then you'll have bands of allowed/ forbidden energies. I think in that case the scattering amplitude has square root branch points at the band edges? I'm not certain that's right, though.
@Semiclassical what?
Q: Why am I still alive?

morgan freemanIve tried everything, but im still breathing! WHY? does anybody have an idea, ive tried trains, nooses guns, and even burying myself alive.

Beats me :-)
... D:
Someone upvoted that!
Alright not so strange now
4:09 PM
ugh, brain work now
4:22 PM
@JohnRennie that's...
@EmilioPisanty Jon Ericson has just deleted it
@JohnRennie curious
the roomba was going to get it anyways, no?
this is curious, though
> This question was marked as spam or rude or abusive and is therefore not shown - you can see the revision history for details.
has it, really?
and if so, how?
were there rude-or-abusive flags on it?
I doubt there's any significance to that. Presumably spam/abusive was the closest of the available reasons for deletion.
(CC @ACuriousMind)
@JohnRennie you don't need a reason, no?
It is a bit odd Jon Ericson got involved. I wonder if someone notified the SE mods that it was a suicide post.
4:29 PM
@JohnRennie maybe there's an automated alarm system hooked up to mentions of suicide?
yeah the head mod shouldn't meddle himself with such petty affairs
@EmilioPisanty entirely possible ...
I'm not sure why this merited any more attention than just closure and five downvotes tbh
@EmilioPisanty interesting
@EmilioPisanty could we have an automated system to warn of posts about career suicide e.g. questions about the interpretation of quantum mechanics :-)
4:32 PM
hiya @JonEricson
Ah, here's the man.
@JohnRennie We ask the mods to contact us if they see them.
just wondering why this caught your attention
in Tavern on the Meta on Meta Stack Exchange Chat, 59 secs ago, by E.P.
is @JonEricson around? if so, we've got a couple questions for ya over at hbar
@JonEricson aha, thanks. That makes sense in case the post is a genuine plea for help.
I sent a personal email and deleted the post.
4:33 PM
@JonEricson was there a flag of some description? or why hide the content even when it was already deleted?
@EmilioPisanty I think there was a flag. We delete these posts quickly because there's a documented copycat phenomena.
@JonEricson ok, fair enough
We can never know if someone is in real danger or just pulling our leg. But it never hurts to treat it as real.
so, if i post an answer to a homework question on the main site expression frustration towards OP's zero attempt and say "i have level 4 cancer; i want to die now", do i get to have a personal email?
i am a depressed fella
the homework spams depress me
4:35 PM
I'm not sure I would really read it as actual suicide talk (looks more like taking the mickey than anything else), but yeah, there's no harm in treating it in full seriousness
@JonEricson that.
beat me to the punch
thanks for clarigying =)
but then you let the "leg pullers" win
@0ßelö7 well, no, the post gets deleted anyway.
@JohnRennie getting a personal email from an internet celebrity is neat
@BalarkaSen Maybe. Sometimes mods jump the gun and take things the wrong way. But then the user has a chance to talk about it with a CM.
@BalarkaSen stage 4
4:37 PM
@0ßelö7 Attention from one person isn't nearly as satisfying as attention from an entire site. So delete, delete, delete. ;-)
Or flag.
@JonEricson yeah, that's an interesting point. My instinct was just to let it lie (closed + deep negative score) until the roomba got it.
but I guess if someone is just out for attention then deleting does help
@JonEricson I was being silly, but yeah, I actually applaud the email policy.
Cool. I imagine someone wants to talk Physics now? ;-)
@JonEricson I was going to bring up that woman, but yeah, physics sounds like a better idea.
@0ßelö7 Let's talk about topological quantum field theories
which are allegedly the very mathematical model of physics
4:40 PM
@BalarkaSen I was just looking at Folland's QFT for Math People book
Is it good?
I think my advisor wants to read it, he randomly talked about it the other day
@BalarkaSen I'm not a physicist, I dunno
@JonEricson How about a nice game of physics
you might need to be fairly familiar with QM first
4:42 PM
well, this is QM we're talking about
he's in a superposition of being and not being a physicist until he observes himself working on a physics problem
I should have seen this coming
@JohnRennie there's also this, fwiw
@EmilioPisanty just a dickhead then, not a suicidal dickhead
@Semiclassical "There's no way to win. The game itself is pointless!" (The second law of thermodynamics is so depressing.)
4:45 PM
And gravity sucks!
@JohnRennie well, we are all nerds here, mostly
but still
@JonEricson well, that's why we invented quantum thermodynamics
and quantum quantum thermodynamics
Aug 5 at 9:52, by John Rennie
@BalarkaSen nonsense, you can't have too many nerds. It is a noble and honourable calling :-)
Q: What is a multiple characteristic of a hyperbolic PDE?

0ßelö7In section 3.1 of The Wave Equation on a Curved Spacetime, Friedman defines what a characteristic hypersurface for the wave equation $$\Box u+a\cdot\nabla u+bu=f$$ is, and then shows that these are precisely the null hypersurfaces of the Lorentzian manifold. He also talks about tensor wave equati...

@EmilioPisanty what?
hello @JohnRennie
I have doubt in array
int input[i],i;
    for(int i = 0; i < 2; i++)
        printf("Please enter an integer: ");

        scanf("%d", &input[i]);

Can I use it like this
Does it compile?
4:55 PM
also can I take the input value of 'i' from user ?
actually it is working on terminal but not on hacker rank
@JohnRennie Seriously?
Does anyone know the derivation of the Arrhenius equation? I couldn't find it on the internet. Or, are we simply supposed to learn it without deriving it?
I saw one here: ww2.d155.org/clc/tdirectory/JSayles/Shared%20Documents/… ..but it's really vague/informal.
when I try to compile it on MSVC I get:
error C2065: 'i': undeclared identifier
rat.c(6): error C2057: expected constant expression
rat.c(6): error C2466: cannot allocate an array of constant size 0
rat.c(6): error C2133: 'input': unknown size
@JohnRennie Is it possible to reply without tagging/pinging the person just as you did?
when I searched on internet , I found that 'i' is uninitialized
but I could not understand how to initialize it
@Abcd it was just a normal reply, but with multiline posts it hides the name of the person you're replying to
4:59 PM
Hmm..well if you add the iterator of Reinman function and couple it with dimensional instability of Wittenian manifolds, it gives you a globally stable field.
Please let me know (by pinging me) if there's a derivation of arrhenius equation. Bye.
@Koolman in standard C or C++ you cannot declare an array with a variable length. You need to allocate the memory for the array using malloc or for C++ new
I believe GCC has extensions that do allow for variable length array declarations
okay , then I should learn malloc
int* input = malloc(i*sizeof(int));
5:04 PM
Oh and you also need to integrate the charge over the 5 dimensional manfold
Or in C++ int* input = new int[i];
It then produces a symmetric locally invariant gauge, doesn't it ?
@JohnRennie No , I will first learn C
is it fine to learn from here
I will learn from there
@JohnRennie thank you
@JohnRennie Just wanted to ask you whether I can ask my question on Chemistry.SE.
5:07 PM
@JohnRennie Is it good for begineers
@Abcd Derivation of the Arrhenius equation?
@Koolman Yes
@JohnRennie yes
okay, thanks
Q: Arrhenius equation derivation

Lamichhane88I couldn't find the derivation of Arrhenius equation. So, How can we derive the Arrhenius equation: $k = A \mathrm e^{-E_\mathrm a/(R T)}$?

Only one upvote? Wow
@BalarkaSen has my back
5:09 PM
@Koolman it's bound to be downloadable from the Internet somewhere. The book is 50 years old!
@JohnRennie Bad treatment of that question by Chem.SE.
They shouldn't have migrated it to hsm.SE.
Derivations are science and not history IMO.
You also need to make sure the function is locally integrable, though.
@JohnRennie : I did inform the SE team, in case it was real.
Anyway, I understood that it doesn't have any derivation.
Oh, they didn't migrate it. I misread :P.
@Abcd I think it can be derived if you make various assumptions. The problem is that those assumptions may not be true of real eqautions.
5:12 PM
But there could be a better answer..
@JohnRennie Yes, understood.
As I recall it ultimately comes from the Boltzmann distribution
No well often time the stuff is topologiacally inconstant, and applying Thue-Witten-Morse theory, we often see that mesons keeps inhalting in and out.
@JohnDvorak Hmm...still not :P
@Qmechanic Jon was saying they have a policy of responding to all posts that look related to suicide, and it makes sense to assume they're real. If they aren't there's no great harm done.
@JohnRennie : Agree.
5:30 PM
@ACuriousMind Where would I learn about the classical theory of constrained Hamiltonian systems with applications and PDE in mind?
To what email do the overlords CMs send the personal email? The one the users registered with?
I emphasize classical. If I see BRST I will not be happy.
@0ßelö7 Just about to head out the door. The only thing I've really read on constrained systems is QoGS
@0ßelö7 The BRST-BV formalism is not necessarily quantum
@Sid Yes
@ACuriousMind Is QoGS of independent mathematical interest?
I don't really care about constrained quantization.
@JohnRennie I love the tag on that question. "half-life". lol
5:43 PM
@JohnRennie thanks I have downloaded it
(Sorry for spamming the chat with nonsense theories. It was part of a inter-chatroom experiment)
@AlexKChen Ah, the old XKCD experiment
@DavidZ XKCD has everything relevant to life. :-)
Randall Munroe for President, anyone?
This may be an instance of anyone qualified to be president being smart enough not to take on the job
1 hour later…
7:15 PM
@Koolman I would say that Kernighan and Ritchie is good for newcomers to C that already have at least a little of the programmer mindset. It often overwhelms complete newbies.
In any case you have to read the book carefully as there are many places where each word is chosen for a reason (as is each word left out).
Also do the exercises—all of them.
@dmckee that sounds long
@0ßelö7 It is a bit long, but c is full of traps for the unwary and the exercises expose many of them to clear view.
7:32 PM
@Abcd I think it depends on what you count as a 'derivation.' One can certainly obtain it by starting from certain assumptions/approximations, e.g. Arrhenius's logic about activation energy. But if you don't include those approximations then you probably won't get such a simple result. (And you can find systems that will violate it; google 'non-arrhenius behavior' for examples.)
It's a bit like Ohm's law in that regard, I suppose.
see for instance the notes/slides here: www1.udel.edu/pchem/C444/Lectures/Lecture5.pdf
8:00 PM
what's the term for diagonalising a hermitian / breaking it down into it's vector images
I think it was Jeff Atwood that said "there's three truly hard problems in programming: garbage collection, and off-by-one errors"
I have a bad case of the latter =/
@Phase you mean "diagonalising"?
if not, what do you mean by "a hermitian"? (i.e. a hermitian what?), and what do you mean by "images"?
preimages and images
$f:a \to b$
nevermind, it's probably just another poorly formed question
8:19 PM
Q: "Repeated Deletion of Answered Questions"

PhaseWhat exactly does this mean? Could I be banned from asking questions just for clearing out trash / malformed questions that received answers that are just of the form "Your premise is wrong" or Duplicates? I could maybe understand the virtue of leaving the former up so anyone else caught in the s...

Hello! I have a professor that described one stream of light traveling through a birefringent medium as going "slower" than the other, but this is making no sense to me. Is there another word besides slower that would better describe this?
The tricky word in here is really velocity and what that means for a ray of light
citing WP for convenience: "The refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 299792458 m/s, and the phase velocity v of light in the medium, i.e. n=c/v. The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves."
I was not aware that there was a phase and group velocity... this is probably why I was lost. Would one of these velocities stay constant? For example: the phase velocity of light is always the same, but the group velocity can differ when passing through different materials.
well, if you look at that definition, you'll note that the index of refraction is c divided by the phase velocity
@KristinaL the phase-velocity argument is a bit of a red herring. What you care about is the group velocity and that can also be slower.
Light in a medium need not travel at the speed of light in vacuum.
This is true for both phase and group velocity.
8:27 PM
the phase velocity bit is really a further subtlety
@KristinaL To answer your core question: no.
Here the word "slower" describes something that gets from A to B in more time than something else.
@PhysicsMeta @Phase This is unanswerable without examples.
the point, I suppose, is that while light doesn't have a medium, it does interact with its environment. hence light propagating in vacuum need not behave the same as light propagating through glass, for instance.
mercifully, in the case of glass it mostly behaves the same, but with a lower velocity than you'd get in vacuum
there are other media which are weirder in that regard.
@Semiclassical I just started in a mineralogy course, so I might see these weird ones soon. Do you know of any examples off the top of your head?
And thank you, @EmilioPisanty and @Semiclassical for the explanations!
the fashionable example nowadays are metamaterials, but that would definitely be outside the context of mineralogy
ah, actually, your birefringent query originally is an example:
"Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light."
setting aside the polarization part, the propagation direction bit is clear enough: if you send in the light horizontally, it can take longer to pass through the medium than if you did so vertically.
@KristinaL we were discussing this in here relatively recently, there is a material known as BBO (Beta Barium Borate) used in QM bell experiments for its birefringence properties. en.wikipedia.org/wiki/Birefringence en.wikipedia.org/wiki/Barium_borate
8:40 PM
There's a macroscopic vs. microscopic question here, though. Macroscopically it's enough to say that, for whatever reason, a birefringent material exhibits a difference index of refraction for light entering vertically vs. horizontally.
@Semiclassical Colloquium about nonlinear nonlocal elasticity was...complicated
@0ßelö7 What a surprise :P
Pretty good though. Lots of jokes, some at the expense of engineers
microscopically, though, you'd want someone to explain why light interacts with the medium in that way
oh those silly engineers
too busy making sure things don't collapse to worry about proving theorems
@Semiclassical With two different rays entering the object, the propagation direction difference makes sense. From what I understood from the lecture, one ray of light was entering at the same position, separating into two different rays, and then recombining to form an interfering wave that was different from the original. Would this be where the microscopic explanation would be of use?
8:50 PM
not really. that's still macroscopic
hey, simple question i think, are mass and velocity commuting variables?
but this goes to a word that I put aside earlier: polarization
one usually talks about either linear or circular polarization of light; the latter is more annoying, so I'll only address the former.
the basic point is that what constitutes light, at the macroscopic scale which matters for crystals, are electric and magnetic fields which vary in space and time
@shaihorowitz yes, though the (rest) mass isn't a variable
see for instance this picture:
the propogation direction of the light would be to the right, and it's constituted by the electric and magnetic fields.
that's an example of polarized light: the electric field always point up/down, and the magnetic field points left/right
@Avantgarde why cant rest mass be a variable? for refrence i'm considering the equation $E^2=m^2c^4+((1/sqrt(1-v^2/c^2)mv)^2c^2$ which seams to accept m as a variable
8:56 PM
if you're just doing special relativity then commutativity is irrelevant. you don't have operators
i realize this. i'm doing a no no and playing in both realms
(continuing from above): but nothing is stopping us from having the electric field pointing in a different direction; we could swap the blue and red vectors and that'd still be a valid light wave, just with a different polarization
@Semiclassical I've got such an explanation sitting in a book just a few metres away...
Which book?
@Semiclassical Boyd Nonlinear Optics
9:00 PM
ah, that figures
Kind of hinted at by the title, yeah :P
(continuing) Additionally, we could take two waves with different polarizations and take their superposition. Then we'd get a light wave which is a mixture of the two polarizations
side really bizzare question. do entropy and temperature commute i.e. a ST diagram.
if it's an equal mixture of all possible polarizations, the light in that case would be unpolarized.
@shaihorowitz I think you mean the 'relativistic mass', which is different from the rest mass. Some people don't like talking in terms of relativistic mass because it can provide an incorrect picture of what happens to a particle's energy content at different velocities. In any case, you're right, this is classical mechanics (with special relativity) and there are no operators here, so commutability follows.
9:03 PM
Anybody know why \tableofcontents takes like 2 builds before the actual toc updates?
now suppose we shine unpolarized light on an optical medium. if the material isn't birefringent, then nothing terribly weird happens: each polarization of light has the same velocity in the medium, and so they all refract in the same way.
but if it's birefringent, then that's not the case and each polarization can have a different index of refraction.
in the end, i'm trying to consider a mass-less particle gaining mass. so i have to worry about quantum commutabillity only in the sense that i need to make sure i'm not leaving out a part of the system. in that sense i care about the variables (m,v) and (S,T)
and therefore the different polarizations will refract with slightly different angles at the interface of the medium
now, why different polarizations of light behave differently in that that question is another question entirely. but if it can happen, then you get various experimental consequences
ew bell...
just characterizing these different ways the medium can respond can be a hard enough task. (oh god, flashbacks of stokes parameters)
9:09 PM
but really does anyone know if (S,T) temperature and entropy commute in quantum mechanics
you're acting as though entropy and temperature are quantum mechanical operators.
with entropy, at least, you can write it in terms of sums over probabilities. so that's fine
same with Temp as average kinetic energies
That's not the definition of temp, though. Temp is just $T=\frac{\partial U}{\partial S}$
its a definition of temp
you can relate the temp to the average kinetic energy if you have a boltzmann distribution, but that's not it's definition.
the questions you're asking sound to me like the equivalent of asking what the color blue tastes like.
or what color the letter e has.
9:15 PM
@Semiclassical You guessed correctly what my next question would be... but considering that this topic is now delving into the realm of "oh god" flashbacks, I won't harass you any further. You already helped me understand polarization of light much better. Are there any good books I could pick up to understand the details better?
I don't know any off the top of my head.
what i know is mostly dim recollections of undergrad physics and grad-level E&M.
i'm saying you can define temp that way. and entropy as a measure of macro states. in this sense they become quantum operators. i'm asking if these specifically commute
this would be a good question for your prof @kristina
@shaihorowitz and I'm saying that I doubt that such a definition is meaningful.
can you give me a situation in which its not meaningful?
@Semiclassical I think I'll do that. Thanks!
9:21 PM
@KristinaL I've missed most of the conversation, so what is it that you're looking to know/find out?
Ah, I've found the start, I'll just have a reread :P
in the end. i'm writing a paper on the mass of information by the way
@Mithrandir24601 Why different polarizations of light refract differently on a birefringent medium
@shaihorowitz As far as von Neumann entropy is concerned, then yes, because it's just a number (as the trace of an operator)
@shaihorowitz First thing is that the Boltzmann distribution starts by assuming a gas, so... Would it work in a liquid or solid?
in my paper i'm assuming a gas but i believe it has been recreated in other mediums
yeah i looked that up, it has the thing that changes is the degrees of freedom of the molecules in a solid
10:14 PM
@Semiclassical oh god I have a month to prepare three talks
Can I fake my death and transfer universities?
> Do not multiply entities without necessity
ah, wikipedia, I see your grammar is ship-shape today
I guess it makes sense, just
@EmilioPisanty not without contex
What's it supposed to say
Occam's razor
In philosophy, a razor is a principle or rule of thumb that allows one to eliminate ("shave off") unlikely explanations for a phenomenon. Razors include: Occam's razor: When faced with competing hypotheses, select the one that makes the fewest assumptions and is thus most open to being tested. Do not multiply entities without necessity. Grice's razor: As a principle of parsimony, conversational implications are to be preferred over semantic context for linguistic explanations. Hanlon's razor: Never attribute to malice that which can be adequately explained by stupidity. Hume's razor: "If the cause...
I initially read
> Do not multiple entities without necessity.
but it's not like correcting it makes it that much better
That seems like an old-English phrasing.
@0ßelö7 ooof
Is there anything the three talks have in common?
10:22 PM
@Semiclassical Two are about different aspects of the same problem (geometric/analytic) and one is related to the geometric part
I could roll two up into a coherent 1.5-hour talk, in other words
Hmm. Split it in two, abd spend some time in the second talk reminding people about the first one?
@Semiclassical Different audiences.
Board talk or slide talk?
10:25 PM
The analytic one will be for my PDE class and the geometric one at the geometric analysis seminar
@Semiclassical Now that I don't know. I need to ask.
@Semiclassical I told the organizer for the seminar that I'd have a somewhat outline by Monday. These talks aren't out of the blue and it's well within my field.
It's just daunting now.
@0ßelö7 Enjoy your weekend!
@Mithrandir24601 not possible
10:29 PM
@Semiclassical my advisor said I should get two talks for the PDE class. That would be four talks.
are these talks being graded?
@Semiclassical how does a board talk differ from a slide talk
in the former people don't fall asleep.
@IcEmybReaD no
@IcEmybReaD I've fallen asleep in board talks
maybe I should rephrase that, you also won't fall asleep in board talks.
10:31 PM
No offense, but mathematicians should be worth less points. — AbleArcher 7 hours ago
0celo will like this mathematical rigor:
Q: Physics terminology about smeared and unsmeared fields

user1620696Let $M$ be a smooth manifold and denote $C^\infty_0(M)$ the space of smooth functions with compact support. In Mathematics a distribution is defined to be a continuous linear functional $\phi : C^\infty_0(M)\to \mathbb{R}$. The space of distributions is usually denoted $\mathfrak{D}'(M)$. So a d...

> Quantum (unsmeared) fields are often referred to as operator-valued distributions in mathematical physics.

They surely are linear maps from some symplectic linear space of test functions to the self-adjoint operators affiliated to a W*-algebra.
Hmm, so that would meam: $\phi \to \hat{O}$
W* algebra seems quantum
10:47 PM
Wikipedia said it is von neumann algebra, a subset of $C^*$ involving only bounded hermitian operators
10:58 PM
Is there like a children's book for systems of equations in infinite dimensions or something, like some old crazy book on them anybody knows about?
I don't think there exists something like infinite dimensions for dummies, because infinite sets and functional spaces requires quite high level maths to understand
Yeah, I'd still say there's some good stuff from people around the 1920's or so before they used words like operator on this stuff, just can't find it
Unrelated, but I also suspect had we humans need to deal with infinite sets on a daily basis, then finite number mathematics will still be easier to us. If we had already being able to count to infinity, then finite is a very very small number and possibly all finite maths will have been done in an instant
00:00 - 16:0016:00 - 00:00

« first day (2521 days earlier)      last day (1016 days later) »