The h Bar

Silly Goose

00:07

who likes penguins

qwerty

@ACuriousMind that was partly just me backing off and excusing my ignorance :p. given people in cosmology (in the context of this discussion, glibly?) refer to the energy density and that we can have energy conditions means that I feel like there is something more for me to unpack for my own understanding here: I'm aware you don't have a conserved energy, but to say energy is ill-defined in GR seems like something I don't deeply understand

@SillyGoose NOOT NOOT

Silly Goose

omg

@ACuriousMind hm but shouldn't we be able to prove a statement by just using the algebra structure

qwerty

@qwerty versus well defined in other cases

User1865345

01:49

@imbAF what is your level?

For beginners: Complex Variables and Applications by Brown and Churchill.

An exceptionally well-written book covering analyticity, Cauchy Integral formulas, Taylor, Laurent series, various singularities and singularity at infinity and their properties, Bromwich integral -- all the stuffs that one should definitely know.

There are ample examples.

Once a certain proficiency is achieved, one can switch to Functions of one complex variable by Conway.

Just to get a glimpse, Conway uses homotopy to explain analyticity. There are other concepts that he discussed like analytic continuation, Hadamard factorization theorem for entire functions etc that are missing in Brown and Churchill. Good book but bit dry and sometimes bit condensed.

@Relativisticcucumber any professor flaunting their cats while giving lecture is approved.

@User1865345 banned at Maths and MO. Has started posting at Physics. Flagged already.

Silly Goose

02:27

@ACuriousMind Actually, what is the model part necessary for anyways? Isn't the wanted result achieved immediately upon specifying the Lie bracket?

Semiclassical

@User1865345 i'm fond of Stewart and Tall as the book i learned from but i'm not familiar with the second edition

Silly Goose

Here is what I mean by it seems it is unnecessary to consider a specific model. But I might misunderstand what is meant by model.

Also, formally to actually write down that expression as an exponential, must I project onto the second slot of the direct sum?

User1865345

@Semiclassical indeed seems to be a good book as I had a glance over the contents of the book.

Relativisticcucumber

03:20

U know I have actually noticed on average physicists seem to be strong cat supporters. Most people I know in physics either have a cat or very proudly show their support for felines

I wonder what the root of this correlation is

User1865345

Schrodinger also chose cat for his thought experiment.

qwerty

Oct 15 at 8:11, by John Rennie

Now I'm wondering if there is a correlation between preferred pet and area of research ...

User1865345

Newton did have a dog, wasn't it?

Diamond (dog)

Diamond was, according to legend, Sir Isaac Newton's favourite dog, who, by upsetting a candle, set fire to manuscripts containing his notes on experiments conducted over the course of twenty years. According to one account, Newton is said to have exclaimed: "O Diamond, Diamond, thou little knowest the mischief thou hast done." The story is largely apocryphal: according to another account, Newton simply left a window open when he went to church, and the candle was knocked over by a gust of wind. Some historians claim that Newton never owned pets. The story of "Newton's Mischief" has been reproduced...

Apocryphal it seems though.

BBC: Cats understand physics, according to a group of scientists in Japan

qwerty

@User1865345 "thing inside box makes noise" = understanding physics seems like a stretch xD

User1865345

Yeh.

Typical news title gimmick

But I can accept cats outsmarting dogs in intelligence.

Wait. Newton invented the cat doors?!

Hubble had a cat named after Copernicus.

Feynman, Boltzmann, Maxwell had dogs.

And Tyco Brahe had an elk. Lol.

Source: Great Physicists and the Pets Who Inspired Them

2

qwerty

03:42

@User1865345 eh let's not get into ranking-by-intelligence

User1865345

😺

We can't. True. We love all.

although cats....

qwerty

I'm a minority dog person on the hbar xD

User1865345

@qwerty you are in the league of Feynman, Boltzmann, Maxwell though.

I need another elk loving physicist. Otherwise Brahe would be an outlier.

qwerty

I find it strange that that listicle mentioned Tesla's cat instead of his pigeon love affair britannica.com/story/nikola-teslas-weird-obsession-with-pigeons

User1865345

He died with only pigeon having his companion. Those pigeons had secret to keep.

Allie

03:54

does anyone know where i can find the solutions to griffiths electrodynamics 2e? I can only find 3e and it seems some of the problems have changed :(

User1865345

Do 3e then, @Allie. Would that be a problem?

Allie

the textbook i have is 2e :(

the physical one!

User1865345

Ah. I see.

Is the book well established?

Allie

yes quite

User1865345

I mean is it taught globally?

Okay.

Chances are solutions have been discussed in forums, SE etc.

Or you can ask your own showing your attempts.

Allie

03:58

thats a pain in the ass

User1865345

😅

Tobias Fünke

06:25

morning

qwerty

@TobiasFünke :)

User1865345

Hey.

Allie

can anyone help me outtttt

im trying to figure out the vector field whose curl is $\vec{b}\delta^3(\vec{r})$

the equation im given is $\nabla\times \frac{1}{4\pi} \int_{\textrm{all space}} \frac{\vec{b}\delta^3(\vec{r'})}{|\vec{r} - \vec{r'}|} d\vec{r'}$

when i do that integral i get $\nabla \times (\vec{b}/(4\pi |\vec{r}|))$

have i made a mistake yet

User1865345

06:59

Wish I could help, but why do you think there would be mistake?

Do you have any particular relation in mind @Allie?

Allie

07:14

because the curl of that vector field is not bd^3(r)

User1865345

Ohh

John Rennie

07:28

You're looking for a field that has a curl of zero everywhere except at the origin?

Allie

well thats what the book says

John Rennie

v = (-y, x, 0) ?

Allie

doesnt that have a curl everywhere

John Rennie

I just wrote out the curl to check and I get it zero everywhere. There is some trick to computing it at the origin that I forget ...

Allie

what

is it not (0,0,2)???? am i crazy??

naturallyInconsistent

07:36

@Allie when you want to seek the solution to a DE whose RHS is a Dirac delta distribution, it is always the case that you should transform to the equivalent integral form of the DE.

Allie

meow

John Rennie

Oops, sign error :-)

naturallyInconsistent

And it tends to be the case that your curl is a Dirac delta along an entire line

@Allie M E O W ~

Allie

what???

im so fucking confused

John Rennie

$$ \mathbf{v} = \left( - \frac{y}{x^2 + y^2},\frac{x}{x^2 + y^2},0 \right) $$

naturallyInconsistent

07:39

@JohnRennie this is what I meant: its curl is Dirac delta, but along entire z axis

Allie

naturallyInconsistent

its not just origin

John Rennie

@Allie My mistake, I managed to get 1-1 for the z component instead of 1+1. In my defence it is the day after Boxing Day and I overdid it yesterday :-)

Allie

but the question says its the 3d dirac delta

if its a typo i will be so angry

d is divergence, c is curl

naturallyInconsistent

@Allie well, there can be a curl that gives that, but we'll gonna hatha think hard

Allie

07:41

this is the first chapter in griffiths T_T

naturallyInconsistent

I dont think I want to work so hard on that question. Because in the end, when we really want to use the theorem, it is for the purposes of Maxwell's equations (as is the main topic of Griffiths), and so I don't want to have to deal with a Dirac delta curl source that is so unphysical and difficult to connect with the Ampèrian loop concept.

Allie

ok :( but why woudl it be hard?????

this is literally 30 pages into the book

User1865345

Allie can go through the book! Have faith in yourself! :-)

Allie

im feeling stupid

oops

User1865345

Why 😬

naturallyInconsistent

07:46

@Allie again, I'm trying to say that the curl source that makes sense is the one that is a Dirac delta along a line, i.e. 2D Dirac delta, not 3D

Allie

oh im sorry

User1865345

For not getting one relation? You have help here if you don't understand.

Allie

wait im sorry

do you mean that the field whose curl is the 3d dirac is the 2d dirac?

or do you mean that the problem should be a 2d dirac

naturallyInconsistent

@Allie this

@Allie not this

Allie

UGH SO ITS A FUCKIKNG TYPO

AASDKLASJDKLASDJALKSJD

John Rennie

07:49

@Allie Griffith's Electrodynamics?

Allie

i spent the past 2 hours questioning my entire existence because of this stupid question

yes

naturallyInconsistent

I mean, maybe the question is asking you to consider a Dirac magnetic monopole, which is ridiculous and doesn't belong in the first chapter.

User1865345

@Allie wtf

Allie

@naturallyInconsistent yeah and in the 3e solution manual this problem is gone

i have 2e)

thats annoyng as fug

whatever

User1865345

@Allie did I say something like this somewhere? 🤔

2

A: Is this Hellinger Distance expression correct?

From $[\rm I], $ So, it's a typo. Reference: $\rm [I]$ Encyclopedia of Distances, Michel Marie Deza, Elena Deza, Springer-Verlag Berlin Heidelberg, $2016, $ p. $268.$

One thing I would suggest would be to follow the latest edition of any literature. — User1865345 Oct 7, 2022 at 22:04

😅

Allie

07:52

OJK WELL THIGNS COST MONEY AND I DONT LIEK TO SPEND A LOT OF IT

User1865345

@Allie 🤧🤧

You have a physical 2e copy, right?

Allie

yes....

i like physical copies a LOT more

NO

my research is all programming i want SOME TIME WHERE I DONT HAE TO LOOK AT A SCREEN

User1865345

@Allie I mean all who loves reading loves their physical copies more than their other material possessions.

Allie

sorry ive been up for long and this problem drove me insane

lol

User1865345

@Allie I can feel your pain. I know.

@Allie ahh. I see. You don't have to read everything from 3e.

Allie

07:54

i should go to bed but for some reason i want to do more

qwerty

@User1865345 remove this before mods do

Allie

im crazy

snitch...

User1865345

Just have a psf copy. Read as usual from 2e. Whenever you feel suspicious, try to look for that thing in 3e.

qwerty

I'm quite sure we can't tell ppl to go there on here

Allie

true

i suppose

User1865345

07:54

@qwerty 👍🏻

@qwerty good point.

Also don't forget to check whether authors maintain an errata list.

Books that are used this much do tend to have an errata list.

This would ease your study a whole lot.

@Allie that would be better. Fresh mind would be better for terse study.

qwerty

@User1865345 I was about to hint but ppl have gotten banned over this

altho that was for doing it repeatedly

User1865345

@qwerty I support that. At this point, we don't have to direct anyone to get anything. They know how to do that.

Allie

believe me

i do

meow

User1865345

😺

Allie

i wish typing with gloves was possible

my hands are like

frozen

its honestly painful

User1865345

07:58

😭

Allie

its probably my anxiety

okay i should go to bed

User1865345

Have a hot chocolate or something. Then take a 5 hours sleep.

qwerty

@User1865345 yup. all your points are great. i wasnt sure whether to chime in at the start of the convo when you were saying to use 3rd ed lol

User1865345

@Allie yes. 👏🏻

Allie

no ill take an 8 hour sleep

User1865345

07:59

Better

Allie

when i wake up grades should be posted for this semester

User1865345

@qwerty I think we all faced such conundrum at any one point of our careers. It's better to have a new edition close to you even though you may feel comfortable reading earlier editions.

@Allie all the best.

Probability God will be with you.

I will sacrifice some dice.

qwerty

I'm on a walk. I know I typed up some notes last year about energy in GR but Im literally re indexing my computer to search 100s of tex files to find it

Allie

well i think there is a 99.9% probability i got a 4.0

User1865345

😅

Tobias Fünke

08:01

is 4.0 the best?

Allie

all my grades were high enough to be in the A range the only thing that would stop it is if idk some random thing i didnt account for

yes 4.0 is all A's

User1865345

That's something significant. Anyway 😴 without getting tensed again.

Tobias Fünke

nice

Allie

it would be my 3rd semester in a row

User1865345

👏🏻👏🏻👏🏻

@qwerty wait you forgot where you kept those?

Allie

08:02

okay good ngiht besties

User1865345

Bye!

Allie

you are a good and pleasant bunch

naturallyInconsistent

@Allie it is most likely that they finally realised that the problem and its solution makes no sense, and thus removed it for 3rd ed

User1865345

@Allie H bar and a representative of Ten Fold. Good combo.

qwerty

bye allie

User1865345

08:04

@naturallyInconsistent exactly.

Allie

bye qwertyyy

User1865345

That's the point of having new edition.

Tobias Fünke

bye

qwerty

@User1865345 overleaf doesn't let you search all files so I downloaded them all to search on my computer, but it didn't automatically index to search tex files as text

User1865345

Ahh. I see.

qwerty

08:05

I actually think I wrote about it for a job application lol. how embarassing to forget all about it

User1865345

😅

naturallyInconsistent

@Allie miao miao bought an e-ink reader so as to have the good parts of both worlds

qwerty

@naturallyInconsistent ooo what kind? is it the remarkable? most e-readers don't handle pdfs well

naturallyInconsistent

@User1865345 thank you for your service~

@qwerty The remarkable 2 is a horrible thing to have bought.

User1865345

😺👍🏻

naturallyInconsistent

08:09

Although it has a linux interface, it is hidden; I'd have to do a lot of stupid shit just to activate it. Usually, ereaders have an android interface, so that apps for one will work for another.

qwerty

@naturallyInconsistent I tried it out in a shop and wasn't impressed but my partner saw a lot of academics using it at conferences

naturallyInconsistent

I actually prefer the usual ebook reader's PDF handlers. I just need them to look just like in the book form. Anything else is annoying.

User1865345

Tbf, seeing e-ink reader reminds me of remarkable devices too. Never used them. So no idea.

qwerty

:O that's surprising to me. is it a4 size? I've heard that the re-scaling or zooming gets difficult with pdfs

naturallyInconsistent

The go-to ebook reader is the onyx-boox, because you can get them at A4 sizes and bigger. This means that you can connect it to your computer and use it as a screen. The use-case is that you can put computer code on it and edit with your computer as-is, and that is much less hurtful on the eyes.

User1865345

08:11

Wow. Sounds nice.

qwerty

oooh

naturallyInconsistent

@qwerty What miao miao kept doing with half-page sized ebook readers, which is the common and pretty much the correct size, is to ask them to cut the page into halves, and read the portrait pages in landscape instead. Very very convenient.

qwerty

anything that has nI star of approval has my interest

User1865345

Good criteria 👍🏻

naturallyInconsistent

@qwerty It is NOT a good ebook reader screen, though, because while Onyx-boox has monopolised the big eink screen market, it is mostly glass, and easily cracks when you accidentally sit on it

And its China-based company actually does its manufacturing in Russia. As you can see with the war in Ukraine, the delivery times are now effectively infinitely long.

It is currently not a good time to buy any ebook reader; As a principle, I don't support Amazon, and so kindles, which are nice, are out of the picture. And with Onyx-Boox out of the picture because of not supporting the Russian aggression, I'm left with things like olde Barnes&Noble Nook (discontinued) and the likes

Maybe Kobo would do.

qwerty

08:18

actually I bought a very cheap /budget Lenovo tab last year but I ended up not using it because I couldn't hook it up to my monitor and keyboard setup for a continuous workflow. I just ended up selling it on. I think whatever I buy will need to be something that can be docked

John Rennie

Tablet style Chromebook?

naturallyInconsistent

There is also a nice spot for sizes. Half a page is quite the nice size, because it is lighter. When it drops on your face in bed, it will hurt much less than a full page size.

User1865345

@JohnRennie those student laptops? Not seen Chromebook tablets though.

John Rennie

I use an Acer Spin 513.

User1865345

I have some reservations re Chrome OS.

naturallyInconsistent

08:21

The youtuber Technology Connections covered some options for using eink screens for like the coding thing that I was mentioning. I am not sure where precisely the video is, maybe in his connextras alternate channel

User1865345

@JohnRennie what is your experience so far?

John Rennie

It's a Chromebook but the screen folds all the way round so you can use it as a tablet, though I mostly just use it as a laptop.

User1865345

I see.

John Rennie

@User1865345 I love it :-)

User1865345

Okay.

qwerty

08:22

@JohnRennie ooh. noted.

John Rennie

I do all my reading on it.

User1865345

That could be a good option.

John Rennie

Though note it's 1.2kg so much heavier than a tablet.

User1865345

Hmm

John Rennie

Chromebooks run Android apps, and indeed I use an Android app (Readera) for reading ebooks.

naturallyInconsistent

08:23

miao miao uses a HP Envy x360 at work but never used it as anything else but a standard laptop (plus external screen and keyboard most of the time)

qwerty

I think I looked into them but I thought maybe in like 6-7years I'd just consider a surface hybrid thing

to replace my laptop

John Rennie

@naturallyInconsistent Yes, I hardly ever use it in tablet mode, though it's occasionally useful.

In the UK a Spin 513 can be bought for £100 to £150 on eBay. I wouldn't pay full price for one.

naturallyInconsistent

I kinda should buy my own electronics. Its the motherland that makes all these electronics here

i.e. cheap like hell

@Relativisticcucumber toxoplasmosis

@qwerty too numerous to be a minority

Herr Feinmann

@TobiasFünke On a personal level, going through days-long draining fights, on a Japanese level I have to resume because I took a couple of days off and on a physics level, when I told I was just beginning to study for the SC exam from scratch, I hope to take it in January. I haven't done much though, since as I said I took a couple of days off. During those days I only focused on that evil magnetic work thing haha. What about you? :)

qwerty

08:38

@naturallyInconsistent how many people going WOOF do you see versus MEOW :p

on hbar

Herr Feinmann

Do you think that 2025 is a good timeframe for a hbar meeting? :D

2

Tobias Fünke

@HerrFeinmann oh, I am sorry to hear that. I hope you feel better soon. Re the exam: Ah, so you can take the exam whenever you want? On a personal level I am fine so far, a little bit stressed. For work I have to hurry up with a publication, but I'm stuck with several small things :d but OK, I hope to proceed next year

User1865345

@naturallyInconsistent who likes Amazon anyways? It's turned to a monopoly. But anyways, I digress. Not the right place to rant your frustration for a billionaire.

naturallyInconsistent

@qwerty that's how we identify. Miao miao hath no problem playing with doggos. In fact, a cute pupper is staying over... miehehehehe

@HerrFeinmann hugs

User1865345

True that. 😺🐶

qwerty

08:49

@naturallyInconsistent :) ok but I believe that ACM, herr F and cucumber are cat ppl too whereas u and Allie are cats

naturallyInconsistent

@User1865345 everywhere is a good place to rant on billionaires. They are unavoidable...

3

@qwerty Look, there is nothing preventing being simultaneously cat and dog ppl

User1865345

Absolutely.

@naturallyInconsistent lol. True. Few years ago, I was a proud doggo and meow.

naturallyInconsistent

yay~

User1865345

@naturallyInconsistent Normally we folks at Ten Fold have decided to not express any political thoughts. I maintain that in other rooms.

But there are dedicated rooms for that too.

This Is Fine

The news discussion offshoot of the Bridge. Stars reserved for...

6

2

(Mostly US and Canada's news and related rant)

naturallyInconsistent

mewth

User1865345

08:53

in This Is Fine, 2 days ago, by User1865345

I appreciate @think_meaning_buildß for letting me know about this room few months ago.

Oh. And the above one is due to Prof. Robert Reich.

User1865345

Enough digression. You folks can continue.

naturallyInconsistent

@User1865345 naise~

John Rennie

Corporations are regulated by the government, and the citizens elect the government. The corporations are only doing what corporations do, and they do it because the electorate believe letting them get away with it is the correct way to vote. Some of us may find this incredible (in the literal meaning of the word) but that's the way it is.

User1865345

09:08

John, US is a mess. UK is way better. There are regulations.

At least under Biden Lina Khan, boss of FTC went nuclear against corporate malpractices and monopolies. All will end once the stable genius re enters White House.

qwerty

I'm sitting at the park and a dog just came up to me and dropped his ball for me to throw :3

User1865345

@qwerty ohhh 🐶🐶🐶

John Rennie

Europe in general takes what seems to me a more rational view, though in the UK we are showing a worrying tendency to wander down the US path.

User1865345

🙂

Herr Feinmann

@naturallyInconsistent hugs

John Rennie

09:12

I have to say I find it incredible (again in the literal sense) that an electorate would elect a leader who gives tax cuts to corporations and millionaires while the ordinary citizens struggle with the basics. Maybe there is something in the water.

qwerty

@JohnRennie must be the fluoride /s

User1865345

@JohnRennie this is called misinformation.

Far right propaganda and fear mongering. Worked always efficiently.

in This Is Fine, Nov 28 at 15:53, by User1865345

To sell such a pathetic, incompetent figure, you need us vs them rhetoric and tsunami of misinformation. Bro podcasters, fox did just that.

John Rennie

Possibly, but ultimately the responsibility to elect a sane leader falls on the electorate.

User1865345

in This Is Fine, Nov 11 at 15:14, by User1865345

> As Angelo Carusone of Media Matters told MSNBC, "We have a country that is pickled in right-wing misinformation and rage." Political organizers and pundits don't want to face that, because it is such a massive, hard-to-wrangle problem.

@JohnRennie exactly. And they are misinformed.

Herr Feinmann

@TobiasFünke Publication stress, the spirit of Christmas, ain't it? :P So, this is how it works in Italy. After you've followed a course which is in your curriculum, you have some dates every year (you don't have to take it the same year) chosen by the professor to take the exam.

The number of dates depends on... I don't know what but they are about 4-5 per year. Then again, since the department is small and most master courses have like 10 people or less, there is a lot of flexibility regarding the date of the exam and you can agree a different one

User1865345

09:15

To the point that they fail to see they are electing more crises.

John Rennie

@User1865345 No-one has a right to be informed. It's up to everyone to make sure they know what's going on, and they are the ones who have to shoulder the blame when the demagogue they elected starts behaving like demagogues behave.

User1865345

True. Agree. But the propaganda was so large that it eclipsed any form of sanity.

in This Is Fine, Dec 17 at 6:09, by User1865345

Farmer who voted for Trump hopes he won't do what he promised

►

I have no idea what this is.

naturallyInconsistent

Dividing line between metals and nonmetals

The dividing line between metals and nonmetals can be found, in varying configurations, on some representations of the periodic table of the elements (see mini-example, right). Elements to the lower left of the line generally display increasing metallic behaviour; elements to the upper right display increasing nonmetallic behaviour. When presented as a regular stair-step, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line. The location and therefore usefulness of the line is debated. It cuts through the metalloids, elements that share...

qwerty

@JohnRennie "No-one has a right to be informed." i think i see it less as a question of rights and more about questions about trust

naturallyInconsistent

The metalloid staircase is a pretty fiction; the actual behaviour is much more complicated, as wiki correctly notes.

@qwerty lob it far~

qwerty

09:22

@naturallyInconsistent <3 ! I tried! it was covered in slobber haha

User1865345

😅

naturallyInconsistent

@qwerty slimy as it might be, it is definitely full of happiness.

User1865345

True 😇

naturallyInconsistent

09:38

@Relativisticcucumber I think you are nitpicking the paragraph a bit too much. yes, of course, a charged defect can be neutralised by minor manipulations of the electron sea, and then it does not have to be as strictly worded as the paragraph is wording it. But what A&M is really trying to get across, is only that the Coulomb interaction is too long ranged and too strong to be left unneutralised in a condensed matter scenario. It aint saying that much.

@Allie You can make it either. Seriously, this is why I really hate the many different standard presentations of Fourier transform. Yes, I hate them all. It is so much easier to just start with the discrete-discrete Fourier transform, and then take the different limits so as to obtain all 4 cases. There will be absolutely no ambiguity and cause for confusion leftover when one sees the full details.

@Allie It happens to be true, but we are not exactly interested in this factoid. Instead, we are interested in Bloch's theorem's guarantees. That is much more physically meaningful and interesting than your narrow conception of the idea.

naturallyInconsistent

09:57

@TobiasFünke Not practically. The first book to start discussing molecules without assuming Born-Oppenheimer approximation came out in 2006, noted as being first of its kind. It is incredibly involved just to begin dealing with those basic systems. I'd be willing to bet that if one goes about crystalline physics with the same tack, one might well NOT end up with the usual phonons as treated in cond mat QFT.

In particular, I would think that it would be unfair to subject students of Ed Witten or Feynman's calibre, to an exposition of cond mat of the kind that @SillyGoose is attempting to describe. There is a reason why, when dealing with phonons in QFT the way that cond mat wants to discuss, it is always almost at the context of superconductivity: the subject matter is just absurdly difficult and nobody should be expected to survive a direct treatment of the subject without baby steps.

Tobias Fünke

@naturallyInconsistent yes, I'd agree here.

of course solid state physics as it is "survived" because it introduced useful notions and "visualizations", which are the starting point of a good understanding and possible approximations (which is what I've said a few messages later for a specific scenario)

naturallyInconsistent

Or in more common language, while a mathematical physicist may start directly with postulates and proper mathematical derivations, the usual definition, theorem, proof, style, such treatments are confined to low complexity. No theoretical physicist can ever grow out of a stifling treatment of that kind. Everybody who is anybody in physics, starts approaching a problem by dealing with some appropriately simplified version, and then adds complexity bit by bit.

Tobias Fünke

yes. To add: Nothing against "rigor" and a clear, precise definition of concepts etc. But to motivate the concepts one wants to introduce is also very much important.

@HerrFeinmann I see. Seems nice

naturallyInconsistent

@TobiasFünke exactly; though in the case being treated, phonons, students do not need extra motivation. They can clearly see the importance without us spoonfeeding them.

Tobias Fünke

hehe

naturallyInconsistent

10:11

@Relativisticcucumber I am aware of the discussions that you had, later on, about rates v.s. final state, that obviously is good and correct and matters a lot. However, the whole final destination-esque exploding oxygen tanks thingy is just a TV trope: it doesn't actually work IRL. It wont explode the way that movies depict. However, there is another kind of nightmare that can happen.

Now, because of trigger warning, I wanna know how to spoiler. ||Test|| |test|

errm...

>! Test $${}$$

Oh, there is no spoiler feature for chat. I'd rather not mention it, then.

Anyway, the concentration and temperature of oxygen can drastically change the kind of burning that happens, with traumatic consequences that I wanted to mention but am going to self-censor.

John Rennie

Ah, no it isn't ...

naturallyInconsistent

I just found out that there is no spoiler feature for chat.

John Rennie

I imagine there is a MathJax way to do it, though that would only work for people using a MathJax renderer.

User1865345

4

A: Why doesn't chat allow spoilers?

This seems to make even less sense in Chat than it does on the normal site. The rationale for having it on Q&A is that readers who aren't asking or answering the question might stumble across it and have their lives ruined by exposure to surprise events in the topic of discussion. In chat, th...

> Ignore your messages (using the "ignore" feature).
Leave the room.
Ask you to take your discussion into a separate room.

Alternatives 🙂

Herr Feinmann

@JohnRennie I think it works but only for posts on the site if I recall correctly

Unless you were specifically talking about the chat

John Rennie

10:22

We were specifically talking about the chat

Herr Feinmann

But it seems it was the case

naturallyInconsistent

@Allie Anyway, yes, there will always be the Maxwellian tail, so that there are some chance that the fire starts from those quantum tunnelled particles (actually, basically all spontaneous ignition starts that way). The crucial point is that the heat liberated from those reactions need to be concentrated enough to be the ignition starter; if you have just one molecule reacting, then the generated heat is usually going to diffuse away. Not enough to start the fire going.

It is really all just stat therm @ACuriousMind @Relativisticcucumber

@ACuriousMind This is just straight-up wrong. Instead, what is happening is much more likely a density issue: What you should have brought up to the lorentzian vegetable is that, 1) definitely had seen Bunsen burners at school, and the blue flame is obviously much less luminiscent than yellow flames. 2) The blue flame is the hot molecules in the gaseous form being blackbodies. 3) The soot is brighter because, although the overall temperature is cooler (yellow is less hot than blue),

the soot is denser, so that there is a conversion of the fewer number of photons in the blue region, to greater numbers of photons in the yellow region.

The combination of these effects lead to the luminosity difference.

This is definitely also compounded by the fact that carbon has a bright yellow emission line.

@Relativisticcucumber Activation energy is not an energy transfer. There are, underlying it, a lot of complicated reaction kinetics happening. It just so happens that a good summary that fits well with experimental observations is the single activation energy approximation. Again, there is just a tonne of stat therm and quantum tunnelling involved, if you really want to derive all these things from scratch.

imbAF

11:18

@User1865345 I have done basic complex analysis, with residue etc, but that's about it. I want to know more as it helps in solving integrals which are otherwise hard to solve

thanks for the suggestion

User1865345

@imbAF you want problem books then?

imbAF

I primarily want theory books, which examples, because sometimes theory can be confusing

with examples*

User1865345

@imbAF Conway is a masterclass albeit with less examples but good problems.

imbAF

Yes I searched for both books

User1865345

B&C has lots of examples but as you said you know said, you know some aspects. There are good problems though.

@imbAF wait.

imbAF

11:23

yes?

User1865345

You can try Ponnusamy.

Foundations of Complex Analysis.

Or Ponnusamy & Silverman

Complex Variables with applications.

Detailed, with examples, have good problems. I prefer the former as it is more detailed.

imbAF

Foundations of Complex Analysis ?

User1865345

Yes

imbAF

ok

User1865345

Once you have somewhat good grasp, I can refer problem books.

imbAF

11:29

It will be sometime until I have the time to read the book but I will do it at some point

I have like a mountain of books to read atm

User1865345

@imbAF Yeh. Sure. Take your time.

ACuriousMind

12:17

@naturallyInconsistent so, uh, are you saying that I would see the same colour as a flame if I just heated a bunch of air to the same temperature? That seems wrong to me (hence my insistence on soot particles as blackbodies) - air is transparent to most radiation and hence an extremely poor blackbody, no?

Also I'm pretty sure the blue of flames isn't BB radiation - blue BBs (like stars) would be at ~10k K, but blue flames are around 2k - 3k K

Herr Feinmann

12:49

Oh god, I need to learn how to write one single gothic letters and it feels more difficult than kanji

ACuriousMind

@SillyGoose The explicit affine model is just an argument for why this multiplication/bracket definition is the correct definition. If you can pull it out off your hat some other way, that's fine, too.

Herr Feinmann

(Please nI do not tell me off for talking about magnetic fields)
I think that in the "QFT approach" in which you have $-MdH$ work i.e. $F$ is a function of $H$ and not of $M$ as it would be in other cases is that what they really mean is not $H$, which depends on $M$, but $\mathfrak{H}$, which is the external field that is established neglecting the field produced by the sample itself (like when you consider a particle moving in an external fixed field)

Yet, $\mathfrak{H}$ is such an ugly letter...

Up to now I think that the most reasonable description is in L&L volume 8, sections §31 and §32

More specifically, I'm saying that the "field theory" Helmholtz energy is the one appearing in $(32,5)$

You can ignore this vent, it's just here for future reference in case someone ever needs to search the transcript

Aditya Krishna Panickar

13:23

Can anyone help me with the following:

Entropy of a system, composed of subsystems that are adiabatically separated. Each subsystem is in equilibrium within itself. However, need not be in equilibrium with its neighbourhs.

In such a case, how can one even define, say change in entropy. The composite systems cannot be ascribed any particular temperature... s

ACuriousMind

@AdityaKrishnaPanickar (Gibbs) entropy is defined for any macrostate/probability distribution, not just those in equilibrium

Aditya Krishna Panickar

I proved that internal energy has to be additive in the above case using Carnot engines and 2nd Law. But can't prove the same additivity for Entropy.

@ACuriousMind I talking strictly under the terms of Classical Thermodynamic. How can one conclude without invoking stat mech. In one of the paragraphs of AB Pippard, he insists the reader to prove this additivity.

I tried, but seems to have no progress. One idea was to take a system where I raise the internal energy is raised using heat only. Now, I claim that that this heat, is a state function(cuz = dU). Now, I can say, that this heat will be additive(dQ1+dQ2). So, the entropy of the subsytems will increase by integral dQ/T.

Now, for physical reasons, I am forced to have additivity, because, say I put all my heat into one of my subsytem. No heat transfer takes place between the subsystem

So, the entropy of the subsystem I provide heat, will increase. The other subsystem will be intact, as they are adiabatically separated. So any change has to come up due to first subsystem only. The only sensible way of then defining the total entropy will be using sum of components. However, does this even constitute a "proof"?

A second question. Suppose I have a composite system C, which has two subsystem A and B, adiabatically separated. Now, say Tb<Ta. Can I ever have a reversing heat transfer between the two in any manner? Obviously, opening the adiabatic shaft is irreversible. I came up with the answer that it simply isn't. Here is my thought process, please help me find my holes in understanding

Say I run a Carnot engine between two temp Ta and Tb. And it does work. Essentially taking heat from A and depositing some of it to B, the rest used in work. Why is work non zero here? Because at any stage, I will want to reverse my process, so that my engine can function as a refrigerator.

Now, here is the problem. All my work can be converted to heat, but all my heat cannot to converted to work, due to 2nd Law. Say, I run the carnot cycle, extract heat from A, then deposit it back to B, and in the process doing non-zero work, quasi statically. But, consider it to be one composite system. So, I am, extracting heat from C and completely converting it into work.

Ok, maybe you say that it's illegal, cuz a Carnot engine has to operate between two cycles. And one cannot consider the two reservoirs to be a composite system, or else, it will lead to problems

So, what else could be the reason

Aditya Krishna Panickar

14:06

Can someone please help. My 2nd question might be crass, so don't bother reading much into it. But still, my 1st question...

Tobias Fünke

14:34

@AdityaKrishnaPanickar but after the new equilibrium is established you can. I don't know why you say that it is possible for the internal energy but not the entropy. I'd say it is exactly the same situation

additivity of internal energy, entropy etc. is a non-trivial matter

you more or less assume it, and sometimes the underlying assumptions which make things work in most cases fail

see Giorgio's answer here, for example

2

Q: Entropy function additivity

I am reading Herbert Callen's book Thermodynamics, which proposes a postulatory treatment to the subject. The postulate number 3 states the properties of the "entropy" function, one of which is the additive property: the entropy of a system composed of multiple subsystems separated via internal ...

I hope I did not misunderstood your question. If not, then this should be explained in good textbooks.

Herr Feinmann

14:56

I remember reading on Huang that it depends on the ensemble. It's always true in the thermodynamic limit but for example if you work with the microcanonical ensemble, combining a system with $N_1$ and a system with $N_2$ particles, $N=N_1+N_2$, then the total entropy is $\propto N$ only up to $\mathcal{O}(\log N)$

So, I guess it is relevant to put this too into the mix

Tobias Fünke

@HerrFeinmann no, it's not always true. you need the thermodynamic limit, which might fail to exist

put differently, thermodynamics more or less assumes all of this, i.e. puts it as axioms. stat mech however recovers thermodynamics usually only in the thermodynamic limit

This Is Fine

Transcript for