The h Bar

0celouvsky

7:03 PM

The physicist proof is just Fourier transformation

Bernardo Meurer

@JohnRennie ::pokes::

Emilio Pisanty

@BenNiehoff not a genitive, a patronym

I can't point you to anything rigorous, though

0celouvsky

@EmilioPisanty sad side effect of being a physicist

Emilio Pisanty

@0celouvsky har har

have a go at this one if you're feeling uppity

3

Q: Does the $p^+p^-e^-$ system have bound states?

This is batted a bit off the wall, so bear with me. The hydrogen molecular ion, $\mathrm{H}_2^+$, is the simplest three-body system in molecular physics, and in the Born-Oppenheimer approximation its electronic Schrödinger equation is separable (though it's unclear whether it's actually exactly s...

=P

0celouvsky

@EmilioPisanty I don't do PDE with coefficients that are not $L^\infty$ :P

Emilio Pisanty

7:11 PM

@0celouvsky whazzat? You don't do physically relevant PDEs?

0celouvsky

@EmilioPisanty heat flow on a Riemannian manifold is physically relevant :o

Apparently it has to do with the melting of ice cubes...

Emilio Pisanty

@BenNiehoff goodness me, the first page of Google results is absolute bull

the untrustworthy Spanish-language internet has this distinctly 90s web-1.0 feeling

0celouvsky

Oh lord, the proof uses the Fourier-Laplace transform

Holy shit, any strongly elliptic PDE has a green's function

now THAT is not something your physicist magic could tell you

@EmilioPisanty what?

what the heck is a conjugate space

probably the dual...but why not say dual?

Emilio Pisanty

7:35 PM

@0celouvsky I was googling the -ez thing

Most search hits are people just making stuff up

But over really 90s web design

Ben Niehoff

7:56 PM

@EmilioPisanty I found the answer to the -ez thing

-ez

== Breton == === Suffix === -ez f -ess; suffix forming the feminine of a noun ==== Derived terms ==== == French == === Pronunciation === IPA(key): /e/ Homophones: -er, -é === Suffix === -ez suffix forming the formal second-person singular or second-person plural (vous) present indicative of an -er verb. suffix forming the second-person plural imperative. == Hungarian == === Etymology === -e- (“linking vowel”) +‎ -z (“verb suffix”) === Pronunciation === IPA(key): [ɛz] === Suffix === -ez (verb suffix) Added to a noun to form a verb. hely (“place”) → helyez (“to place”) ==...

the patronymic suffix comes from Latin -icus, genitive -icī

but I guess the dream of the 90's is alive on the Spanish internet ;)

Bernardo Meurer

Anyone here familiar with using memmove to do a circular shift on an array in C?

Ben Niehoff

@BernardoMeurer 1. no, 2. why?

do you mean a bitwise shift, or actually shifting one entry of the array to the next?

Bernardo Meurer

@BenNiehoff I have an array like [3, 1, 0, 0, 0], I want it to be [0, 0, 0, 3, 1]

Ben Niehoff

and you think temporary variables are too expensive?

Bernardo Meurer

I believe you can do that nicely with memmove() but not sure how

@BenNiehoff I just wanted to try doing it with memmove to be honest :P

Ben Niehoff

8:04 PM

I don't know, but I have a hunch memmove() can't make it wrap around

but I couldn't tell you for sure

the only low-level memory functions I've used have been malloc and free

Cows

Can someone walk me through why we ignore surface terms in integration by parts in qft most of the time?

Bernardo Meurer

That might just be right

Why malloc and not calloc?

Ben Niehoff

that's free as in empty, by the way...not freedom, or beer

Bernardo Meurer

I always do calloc :P

Cows

@BenNiehoff can you walk me through it?

Ben Niehoff

8:07 PM

@BernardoMeurer I forget the difference, what does calloc do?

Bernardo Meurer

@BenNiehoff your free function is actually FREE SOFTWARE if you're using GNU's libc!

@BenNiehoff Allocate and initialise to 0

Cows

@BernardoMeurer do you have a github account?

Bernardo Meurer

Generally safer than just doing a malloc

ACuriousMind

@Cows Nothing to do with QFT - we usually silently assume that physical fields fall off at infinity.

Bernardo Meurer

@Cows Yes

Ben Niehoff

8:07 PM

@BernardoMeurer Oh, I just assume every memory location is junk until I've initialized it myself

Cows

@ACuriousMind omg I had no idea. Is there a physical reason why?

Ben Niehoff

@Cows Surface terms are at infinity and we usually work in the infinite-volume limit. You do have to assume sufficiently-fast falloff of all of you fields

Cows

wow!

@BernardoMeurer what is your github handle?

ACuriousMind

@Cows Almost every field configuration you can write down that does not fall off at infinity will have infinite energy, which is a bit unphysical, don't you agree?

Bernardo Meurer

@BenNiehoff Fair, I just like to be double safe. Calloc is know to be slower than malloc though, which is something to keep in mind

Cows

8:09 PM

@ACuriousMind I agree totally

0celouvsky

@ACuriousMind ridiculous

Bernardo Meurer

IIRC there was once some issue with some linux syscall that made calloc significantly slower than malloc

@Cows bemeurer. My previous answer was a link to my profile

ACuriousMind

@0celouvsky ?

0celouvsky

spacetime is infinite, what's wrong with infinite energy?

Ben Niehoff

@BernardoMeurer Indeed. And the only reason to use plain C is for something where speed is more important than high-level code structure. So, malloc it is!

0celouvsky

8:10 PM

And besides, you'd have to justify why there's some magical point beyond which the field is <epsilon

I'm assuming there's no source or anything, just a magical field

Cows

@BernardoMeurer my github handle is kevintah add me. hehe

Bernardo Meurer

@BenNiehoff Note however that there are occasions where 0-initialization is needed, in fact the code I am working right now, and then calloc is often times better than malloc + zeroing

@Cows I'm not sure one can add people on GitHub

Cows

@BernardoMeurer I meant follow lol

Ben Niehoff

@Cows There is another reason to ignore surface terms, which I call "What if my action had surface terms to begin with, that I've been ignoring this whole time?"

0celouvsky

@BenNiehoff that's the "Why physics is unconvincing" Lemma

Emilio Pisanty

8:14 PM

@BenNiehoff Yeah, I saw that, but it's not particularly thick with references

Cows

wow

ACuriousMind

@0celouvsky That, in most cases, the "infinite" theory is really just for convenience and we're actually only interested in the theory inside a box.

Ben Niehoff

@EmilioPisanty True, although it's quite plausible. They love their Visigothic heritage in Spain

Emilio Pisanty

@BenNiehoff "plausible" is what it says on the top of the door to the rabbit hole of folk etymologies

Ben Niehoff

What do you call someone wearing all black in a dark room?

Invisigoth :D

0celouvsky

8:16 PM

@BenNiehoff an emo

ACuriousMind

You are right that the reasoning doesn't work if I have an infinite matter-filled universe I'm trying to describe but that's simply not the goal of most physical theories.

@BenNiehoff lol

0celouvsky

@ACuriousMind again...the universe is infinite and matter filled as far as we know

ACuriousMind

@0celouvsky So what? That's not what most physical theories are trying to describe!

We compartmentalize

Ben Niehoff

@EmilioPisanty Sure, but folk etymologies are usually folksier than that :P

ACuriousMind

When you're desribing two attacting opposite charges you don't care that there's a third charge in Andromeda

Emilio Pisanty

8:18 PM

@BenNiehoff this feels pretty folksy tbh

ACuriousMind

When you're describing collisions in the LHC you don't care that aliens on mars are doing a similar collision experiment right now

Emilio Pisanty

it has that distinct mark of Too Good To Be True that distinguishes all good folk etymologies

ACuriousMind

And so on

Emilio Pisanty

@ACuriousMind If there's aliens on mars doing precisely the same experiments as the LHC I would most certainly care

Ben Niehoff

@EmilioPisanty Well, it survives analogy with Italian patronymics

ACuriousMind

8:19 PM

@EmilioPisanty Sure, but for reasons entirely unrelated to your LHC calculations ;)

Emilio Pisanty

@ACuriousMind indeed

not that you'd catch me calculating anything relevant to an LHC experiment

unless, of course, they decide to turn it into an FEL in ten years, like they did with SLAC

Bernardo Meurer

@EmilioPisanty Are you versed in C low-level memory hax?

0celouvsky

@ACuriousMind you really should

That would be pretty neat

Emilio Pisanty

@BernardoMeurer nope, but I can help with Mathematica if that's your thing

Bernardo Meurer

@EmilioPisanty We talk in a couple years when I get that class

I'll bother you a lot :P

Emilio Pisanty

8:21 PM

@BernardoMeurer good

I suspect you'll quite like the universal-representation aspect of it

John Doe

How is 'spin' defined classicaly and not in quanum mechanics?

Emilio Pisanty

and you'll probably despise yourself for it, too

John Doe

spin of say an electron

Emilio Pisanty

@JohnDoe "spin" is a tricky term

but there is such a thing as intrinsic vs extrinsic angular momentum

0celouvsky

@JohnDoe it isn't

Inb4 Emilio or Bajoran say it is in some strange way

Ben Niehoff

8:24 PM

classical spinors are certainly well-defined

just not classical fermions

Emilio Pisanty

bam!

John Doe

@EmilioPisanty On page 4 of Sakurai he talks about the Stern-Gerlach experiment. He states that the magnetic moment $\mu$ is proportional to spin $\mathbf{S}$ but he doesn't mention what he defines as spin $\mathbf{S}$ and later refers to the $z$ component of spin i.e. $S_z$. This is before he introduces any QM, so I'm assuming he is relying on some classical notion of spin.

@BenNiehoff So there is no classical counterpart to quantum spin?

Ben Niehoff

@JohnDoe I'm not sure my answer is particularly helpful

I wouldn't read too much into it

Emilio Pisanty

@JohnDoe if you model electrons as little balls of charge, then yes, they have a magnetic dipole moment that's proportional to their intrinsic angular momentum

that's the portion of their angular momentum that's independent of the origin

0celouvsky

@EmilioPisanty what?

John Doe

8:28 PM

@EmilioPisanty So spin is classically just taken as orbital angular momentum in a sense?

0celouvsky

Has anyone actually done the calculation for the classical rotation speed of an electron?

Ben Niehoff

if you literally imagine an object spinning, that is classical spin, sure...but you can only get spin 1 that way, not spin 1/2

Emilio Pisanty

As opposed to the orbital/extrinsic angular momentum, which transforms as $\mathbf L \mapsto \mathbf L'=\mathbf L - \mathbf r_0 \times \mathbf p$ under a translation by $\mathbf r_0$, with $\mathbf p$ the total momentum.

ACuriousMind

@0celouvsky Uh...assuming what electron radius?

Emilio Pisanty

@JohnDoe no

Ben Niehoff

8:29 PM

@0celouvsky yes, it's a famous result! The electron magnetic moment is 2x what it "should" be

John Doe

Damnit

Emilio Pisanty

note the if

0celouvsky

@BenNiehoff spin 1 is still intrinsic, so not classical...

ACuriousMind

It's not hard. I think if you set a reasonably small radius you get faster-than-light rotation

0celouvsky

@ACuriousMind we'll there's the classic tale of Pauli or whoever doing it and concluding all is lost

So I wonder what radius to use

John Doe

8:30 PM

@EmilioPisanty Yes that's how he is modelling it I think.

Emilio Pisanty

@JohnDoe Ultimately, the spinning-ball-of-charge argument only serves to suggest that a hamiltonian of the form $H=g \mu_B \mathbf S\cdot\mathbf B$ might be an interesting thing to try

0celouvsky

@BenNiehoff that's QFT, I'm not talking about that.

Ben Niehoff

@0celouvsky no, QFT is for getting you the "anomolous" part; i.e., it's actually slightly more than 2x what it should be

Emilio Pisanty

but the intrinsic magnetic moment of the electron is just that, a fundamental property of the electron

Bernardo Meurer

@EmilioPisanty What do you mean by "universal-representation aspect of it"?

Ben Niehoff

8:31 PM

the 2x just comes from quantum mechanics

Cows

@BenNiehoff @0celouvsky do you guys mean something like this physics.stackexchange.com/questions/205580/…

Bernardo Meurer

TBH I'm just worried that it will be yet another class that forces me to use proprietary malware

0

Q: Circular shifting an array with `memmove`

Say I have an integer array like such #define MAX 5 int bar [MAX] = {0}; int foo [MAX] = {3,1,0,0,0}; Now I'd like to shift this array so that all null entries are to the left, i.e. foo = {0,0,0,3,1}. I thought I could do this by Finding the number of shifts I must perform Using memmove() t...

c arrays memory memmove

0celouvsky

@BenNiehoff the Dirac equation gives you the 2. It's the higher order corrections that give the little extra

Emilio Pisanty

@BernardoMeurer the fact that code and data and variables all come in on the same footing

Bernardo Meurer

O.o

That sounds like BASH

Emilio Pisanty

8:33 PM

@BernardoMeurer no, way better than that

John Doe

@EmilioPisanty I'm just trying to get an idea of what classical notion of spin Sakurai is referring to when discussing spin before ever mentioning any QM notions. And where the classical prediction of the SG experiment comes from. I think the spin in taken as orbital angular momentum of charge moving around a sphere.

Ben Niehoff

@0celouvsky I could have sworn there was a non-relativistic equation that gave you the 2...like just writing the Schrödinger equation in spin 1/2

but ok

0celouvsky

There is, but it's just a trick.

The Dirac equation gives it directly.

Emilio Pisanty

so you can take some big complex function/construct/graphics object/whatever, and operate on it with the same functions, say Part or Map or whatever, that you would use for some structured list of data

0celouvsky

Obviously the Dirac equation is more fundamental

Ben Niehoff

8:34 PM

@BernardoMeurer Mathematica borrows heavily from LISP for its basic structure

Emilio Pisanty

@JohnDoe So long as you understand that this is for motivation only, then yes, he's taking about a model that's just a spinning ball of charge with orbital angular momentum.

Bernardo Meurer

@BenNiehoff ::shakes::

John Doe

@EmilioPisanty Okay thanks.

Cows

Ok opening calculus book now lol.

DanielSank

@ACuriousMind I agree with @OBE. You have a habit of responding to questions by immediately telling the asker that their question is bad/unclear.

And "what" doesn't even tell the asker what to clarify.

0celouvsky

8:37 PM

Is it his fault that they frequently are? :P

DanielSank

@0celouvsky No.

I noticed just this morning a question on main where ACM responded with a comment basically telling OP "You have no idea what you're talking about". That's not particularly helpful.

0celouvsky

Without contex I have to say there's a chance OP has no clue what he was talking about.

Especially if it was one of those posts claiming to have discovered something...

ACuriousMind

@DanielSank I see the point about the "what?", but my comments on the main site usually specify what irks me about a post, at least I intend to leave comments like that. Which one are you talking about?

0celouvsky

@ACuriousMind Does this make sense to you?

He's using $||\cdot||$ for the $L^2$ norm in that section, so that equation doesn't make sense.

And I don't know what the "conjugate space" is supposed to be.

John Doe

Why does the magnetic field have to inhomogenous in the Stern-Gerlach experiment?

ACuriousMind

8:51 PM

@0celouvsky I suspect the author forgot to divide the exponent by half on the r.h.s. then, and I agree with your earlier guess that the conjugate space is probably the dual

0celouvsky

@ACuriousMind But if it's a Hilbert space, the dual should be itself unless I'm missing something.

Cows

what is the cheapest new car one can buy in 2017

ACuriousMind

@0celouvsky Oh, the dual is isomorphic to the original, sure, but I think the claim is that if you ask "What are the continuous linear functionals on $L^2_\epsilon(\mathbb{R}^n)$?" the answer is "integrating against a function from $L_{-\epsilon}^2(\mathbb{R}^n)$".

0celouvsky

@ACuriousMind So the dual pairing is the usual $L^2$ pairing?

ACuriousMind

Yeah

0celouvsky

8:56 PM

I'm not sure that makes sense, what is the inner product supposed to be?

↑ huh

Oh it's because there would be no force unless it was.

Emilio Pisanty

that's actually quite handy

ACuriousMind

But since I know neither your context or what this is supposed to be for, I don't know any more than you do

0celouvsky

Time to hunt down Hörmander's original paper unless @yuggib is around

GPhys

8:57 PM

rubs eyes

last midterm tomorrow: statistical physics

rob

@JohnDoe Yes, a magnetic dipole feels a torque in a uniform field, but a force in a nonuniform field.

John Doe

@rob Understood thanks.

0celouvsky

@rob I asked Handler this last semester but he didn't know. Suppose I charge a capacitor and just yank it out of the circuit. What happens?

Emilio Pisanty

@0celouvsky never do that

rob

@0celouvsky You have a charged capacitor in your hand. You can use it to zap people. When you compute $RC$, use a half-megaohm for the skin resistance of your target.

Emilio Pisanty

9:01 PM

an explosion at the very least

rob

@EmilioPisanty You and I are thinking about different-sized capacitors.

0celouvsky

@ACuriousMind

Let's see about this dual business now

It's just $L^2$ with a different measure, and such spaces are well-studied

Emilio Pisanty

@rob he did say a 1F capacitor under 100kV, right?

0celouvsky

I don't know why this is confusing me.

@EmilioPisanty jayzus

Emilio Pisanty

wait

@0celouvsky is that Zeus in bird form?

0celouvsky

9:03 PM

It's Kanye West in Ancient Greece

Er

Jay Z in Ancient Greece

Ok apparently it's just the dual

Time to prove that

rob

@EmilioPisanty $U = \frac12 C (\Delta V)^2 = \rm5\,GJ$, holy cow, why would such a thing exist?

0celouvsky

Why did he say "under" 100kV

DanielSank

@ACuriousMind I'll try to remember to notify you next time I see an example. This morning is had something to do with translations on manifolds.

0celouvsky

Why not make it more?

Emilio Pisanty

@rob so roughly 10ktons?

0celouvsky

9:06 PM

Why do you think a general curved spacetime has a "generator of translations"? What does "translation" mean to you on a general manifold? — ACuriousMind ♦ 4 hours ago

Emilio Pisanty

yeah, as I said, explosion, definitely

0celouvsky

@ACuriousMind That seems fairly condescending

DanielSank

@0celouvsky I was going to say this was on the "usefully Socratic" end of the spectrum.

0celouvsky

He probably just means a transitive isometry group or something...

DanielSank

There are others that are less so.

rob

9:08 PM

@EmilioPisanty Yeah, I was thinking millifarad, 10V, like you might extract from a wall-wart power supply. Not explosion.

Emilio Pisanty

@rob boring, though

DanielSank

@rob Oooooh we're blowing things up?

Emilio Pisanty

for extra fun, though, you could plug it back in the wrong way and then turn the circuit back on

rob

@0celouvsky, or whatever your name is today: don't zap people with a gigajoule capacitor. Strictly a low-voltage phenomenon.

0celouvsky

O'0celouvskyopolous

3

I like it

OBE

9:09 PM

@0celouvsky lol if you actually do that.

0celouvsky

@rob But, science!

ACuriousMind

@0celouvsky It does? I have genuinely no idea what a translation might be on a general manifold! Is it the quotes? How could I have asked less condescendingly for what OP means by "translation" in their question?

0celouvsky

@ACuriousMind It's the italics imo

OBE

@0celouvsky I see you figured out the infinite name change glitch.

Emilio Pisanty

speaking of kilotons, though

0celouvsky

9:10 PM

@OBE I did?

Emilio Pisanty

boy does googling anything remotely nuclear end up with some gruesome science

0celouvsky

I just changed my name, what's a glitch about it?

DanielSank

These guys are pretty good

Folsom Prison Blues

►

OBE

@0celouvsky you changed from 0celou7 to 0celouvsky in less than 30 days.

there's a glitch that allows you to do that which I revealed last year.

hasn't been fixed yet.

Run like hell

Hey guys, I always hear and read that the signature of spacetime is a matter of taste of the author, but I never read what kind of taste. I mean what a guy using $(+---)$ prefers compared to $(-+++)$ and viceversa?

0celouvsky

9:12 PM

@Runlikehell according to @ACuriousMind the signature matters for string theory

Emilio Pisanty

@Runlikehell $(+---)$ people tend to be your standard whisky on the rocks folks

$(-+++)$ are more G&Ts or rum and coke

DanielSank

^ What?

ACuriousMind

@0celouvsky Only possibly, and the existence of Majorana spinors is not something of purely string-theoretic interest

0celouvsky

@EmilioPisanty My favorite drink is bourbon on the rocks and I use -+++

DanielSank

Whisky on the rocks is more "standard" than gin and tonic?

OBE

9:13 PM

from my experience advanced books used mostly minus and beginner books use mostly plus.

dunno why

0celouvsky

@OBE no one asked you and you're wrong

DanielSank

@OBE Minus on what part!?

That is such a confusing comment.

Emilio Pisanty

@DanielSank I'm making this up as I go along, and in a hurry before all y'all post something serious, take it easy

OBE

@0celouvsky wow

rob

I find that whiskey by itself is neater.

Emilio Pisanty

9:15 PM

@rob har

har

0celouvsky

did you have to link wiki?

what is it with mods and their link fetish

Emilio Pisanty

link

0celouvsky

I'm not clicking.

Run like hell

@EmilioPisanty mmh what kind of guy (or alcohol) you think I should trust more?

Emilio Pisanty

link?

0celouvsky

9:16 PM

@EmilioPisanty I thought Zelda was the boy

Legend of Zelda, no?

Emilio Pisanty

link, I think

@0celouvsky wut?

0celouvsky

what?

Emilio Pisanty

@Runlikehell seriously though, I think it just comes down to what notation your college textbook used

@0celouvsky why would you think that?

0celouvsky

most people do

DanielSank

@EmilioPisanty ::take it not easy::

0celouvsky

9:18 PM

and it's a meme, never mind

I know who Zelda is

Emilio Pisanty

@0celouvsky he says, five minutes later

0celouvsky

She's the little fairy, right?

rob

@0celouvsky Linking is what the hypertext is for.

0celouvsky

Hey, listen!

OBE

omg

i remember that

Emilio Pisanty

9:19 PM

@0celouvsky I had that as my sms notification for a while

folks from my parents' generations got weirdly upset by it

0celouvsky

Run like hell

@EmilioPisanty got it, so one basically takes the signature used in the course he struggle more and has to use that signature for months eheh

0celouvsky

mm

bad choice since she isn't in that one :P

Run like hell

@0celouvsky We've got a pilot grip guy here. The best

0celouvsky

well, she's in a flashback

Emilio Pisanty

9:22 PM

@0celouvsky you're gonna pretend that flashbacks count now?

0celouvsky

@EmilioPisanty I have oot right there on the screen

And a link between worlds

You have to believe me

And a link to the past!

OBE

@0celouvsky what note book do you use?

0celouvsky

@ACuriousMind Here's the deal. By Riesz-Fisher, the dual of $L_\epsilon^2$ is indeed $L_\epsilon^2$, by the identification $T(f)=(g_T,f)=\int g_Tfe^{2\epsilon|x|}\,dx$. But if we absorb that exponential we find $\tilde g=g_Te^{2\epsilon |x|}\in L^2_{-\epsilon}$, so the dual can be viewed as $L_{-\epsilon}^2$ with the pairing given by regular integration $T(f)=\int \tilde gf\, dx$ without that factor.

@OBE a nice one

OBE

what's it called?

0celouvsky

Hans

OBE

9:35 PM

oh okay.

0celouvsky

the back flap says markings by CR Gibson

@Danu Lol, Drake OD'd on weed in Amsterdam