The h Bar

naturallyInconsistent

00:19

@ACuriousMind if he aint in the reeds, then he is in the weeds

naturallyInconsistent

01:26

@Relativisticcucumber Can you check if the "answer" is just missing a square? It really looks like a typo.

Relativisticcucumber

02:32

@naturallyInconsistent wdym check if the answer?

so u also think its a typo 0.o? bc i tried to do free electron case to see if i could get a concrete case to work and indeed the issue persists even tho i feel like i did that "test" calculation right

user430580

@weeab00 Hello, welcome to the chat

naturallyInconsistent

02:51

@Relativisticcucumber Check if the $\int\mathrm dS|v(k)|$ is not just a typo of $\int\mathrm dS|v(k)|^2$

weeab00

@user430580 Hello, what's up?

user430580

03:11

@weeab00 Not much, just happy to see new people join in. Do you like physics?

naturallyInconsistent

More than just liking physics, which is almost a certainty, miao miao is more interested in whether weeab00 is actually a weeaboo~

user430580

Hey is there any interesting story or background lore why does miao miao refer to miao miao as miao miao and why does miao miao usually finish miao miao's messages with "~~" characters?

naturallyInconsistent

03:26

Yes, miao miao iz a kitten ~~

user430580

ok thanks!

qwerty

"pats cat*

naturallyInconsistent

purr purr

Watching the election battleground swinging is very much a cardiac arrested development

user430580

🐱❤️🐱❤️

naturallyInconsistent

mew mew~

qwerty

03:47

@naturallyInconsistent surely the outcome won't be known for a little while

weeab00

@user430580 Yes, I love anything related to relativity and classical mechanics. Also, yes I am a weeb.

How about you?

user430580

I like chemistry more than physics, but the chemistry's site chat seems inactive nowadays. And physics is cool too, so I ended up hanging around here and in the other physics chat room. Mostly interested in physical chemistry and electromagnetism.

naturallyInconsistent

@qwerty yes, but le heart iz naat rational

@weeab00 weee~~

Relativisticcucumber

@naturallyInconsistent er sorry im blanking -- where do u propose i check this information? when i worked it out i indeed got that its squared but i do not trust myself

@naturallyInconsistent its making me v sad

@qwerty we'll prob know tn

yay for cali

Mr. Feynman

04:04

@Relativisticcucumber lol

qwerty

04:14

@Relativisticcucumber really?! I thought it would take a couple of days

naturallyInconsistent

@Relativisticcucumber I'm not sure. There aint many systems that are cubic in the nice textbook sense and so it is not easy to compare with experiments to know which is correct. I'd suggest you just chuck this problem in a drawer and forget about its existence

qwerty

@Mr.Feynman you're up early!

naturallyInconsistent

@qwerty usually the results are good within 24h

qwerty

D: impending doom

naturallyInconsistent

@qwerty miao miao hath only played doom and puked. Never got the chance to try quake at all

and no castle wolfenstein either.

Life is weird

qwerty

06:41

eep the numbers

naturallyInconsistent

07:13

eep

qwerty

07:44

welp i guess that's it

Nairit Sahoo

The Maurer Cartan form $g^{-1} dg$ looks so much like the Lie algebra generator $g^{-1} \partial_\mu g$ since $g=e^{i \theta T}. Is there any actual connection? If there is: how to put that in words?

naturallyInconsistent

@qwerty let's distract ourselves with this. There are many places in physics where results from basic complex methods are being used. I'm not sure how you managed to not see it everywhere?

qwerty

@naturallyInconsistent avoiding anything quantum maybe? it just didnt seem as important as everyone told me it would be - i'm sure there's loads of applications, but at a more elementary level it didnt seem like a real prerequisite. In hindsight I would have told young me to take discrete maths, diff geo, and a more advanced linear algebra course.

well, maybe not the diff geo. but no required stats, and only taking standard lin alg and not discrete was probably a much bigger hindrance than complex

i guess this tparker agreed somewhat physics.stackexchange.com/questions/509868/…

naturallyInconsistent

08:26

@qwerty All good choices, but surely you cannot be expecting that a recommendation that clearly is suggested for use in quantum theory, would be useful recommendation when you avoided its intended targeted use...

Like, complex analysis is also applied to electrodynamics if you want to deal with the details of electrical engineering, say, but it seems like you are far from applications?

Diff Geo is a great organiser of concepts but it is also surprisingly how little is required knowledge in physics, despite the obvious application range.

Lin Alg is just, more is better

Mr. Feynman

@qwerty It was a virtual me

qwerty

@naturallyInconsistent I guess even now it's not clear to me that it's important at a foundational level, like as a prereq, rather than in terms of particular tricks and applications? it's not currently anywhere on my list of things I want to review or (re)learn

Sir Cumference

ugh

maybe i should just read a textbook to distract myself from today

think_meaning_builds

08:42

yup, looks like a landslide victory

Sir Cumference

@Mr.Feynman well if nothing else, at least i got a good laugh from this

Ryder Rude

CU-Prime's 1st Annual Physics Debate: Many-worlds (Dr. Graeme Smith) vs QBism (Dr. Chris Fuchs)

►

naturallyInconsistent

@qwerty The issue is more that if you didn't know about Cauchy's integral formula, then the moment it gets used for the first time, you will be utterly thrown into the next universe, whereas if you dont know a specific detail in diff geo, or lin alg, at least you can make do with an explicit calculation.

@SirCumference yes

Mr. Feynman

08:58

@naturallyInconsistent I hate when it's used in calculation :P

I prefer to get away with residue theorem (which you prove using it)

naturallyInconsistent

I'm really referring to all contour integrals, really. Like, including residue theorem, and how to convert real number line integrals into them too

and especially using complex shenanigans to define integrals rigorously.

Lucky Chouhan

11:43

@naturallyInconsistent Finally US has got its 47th president :')

@user430580 if you can bring people who like gossips in the chemistry's room then that room won't be inactive or frozen anymore.

user430580

12:03

@LuckyChouhan I think you are 100% correct, but I tend to prefer scenarios where I come to a room that is already active. And if I bring people who like gossips together, why not just start my own room (wink wink) :P

Lucky Chouhan

@user430580 yeah, that's why we have "Thinking Out Loud" :P

user430580

Exactly! =D

naturallyInconsistent

13:55

@LuckyChouhan crycry

think_meaning_builds

14:10

A twice impeached convicted felon gets re-elected with 20 million less people voting compared to 2020.

Slereah

Oh America

naturallyInconsistent

14:25

literally can't see

Silly Goose

Today's a good day to talk about Haag

naturallyInconsistent

Im sure there are worse topics than Haag

Silly Goose

@NairitSahoo maybe the nLab page will help you here: ncatlab.org/nlab/show/….

Slereah

Can we at least talk about another Haag theorem for once

Like

Haag–Łopuszański–Sohnius theorem

In theoretical physics, the Haag–Łopuszański–Sohnius theorem states that if both commutating and anticommutating generators are considered, then the only way to nontrivially mix spacetime and internal symmetries is through supersymmetry. The anticommutating generators must be spin-1/2 spinors which can additionally admit their own internal symmetry known as R-symmetry. The theorem is a generalization of the Coleman–Mandula theorem to Lie superalgebras. It was proved in 1975 by Rudolf Haag, Jan Łopuszański, and Martin Sohnius as a response to the development of the first supersymmetric field theories...

Mr. Feynman

14:54

motion: failed

Slereah

15:14

For your crimes, I sentence you to the Haag tribunal

Mr. Feynman

I beg you, give me a death sentence instead

Slereah

15:54

To the Haag with you

Slereah

16:58

> In the populous land of Egypt there is a crowd of bookish scribblers who get fed as they argue away interminably in the chicken coop of the Muses

Apparently not everyone approved of Ptolemy funding the Science

Silly Goose

19:37

how do I write the minkowski metric in a coordinate independent way?

Slereah

It's the flat metric of signature n, 1

ACuriousMind

20:49

@SillyGoose $\eta$.

@Slereah I talk about that one, too, but I say Coleman-Mandula unless the context explicitly includes supersymmetry :P

user430580

Hello, could someone please familiarize me with yet another piece of this chat's background lore? What is "Haag", and why was it on the very bottom of than funny image with Mr Incredible getting progressively more uncanny? Thanks.

BTW Yes, I tried Google and Wikipedia, but I cannot find this chat's background lore on Wikipedia.

ACuriousMind

@user430580 Haag's theorem is what I weaponize when people start asking unresolvable questions about the usual presentation of QFT in terms of the interaction picture

user430580

thank you! <3

ACuriousMind

Mr. Feynman is specifically scarred by this because I wrote this answer when he was expecting a proper explanation instead :P

Mr. Feynman

You monster.

Up to that moment, Haag theorem was just a cool flex. It hadn't ruined my life yet

I could use it during breaks "hey, do you know that interaction picture is a lie?"

I even remember a day, during a break, chatting here with ACM about it

Silly Goose

20:57

is the proper time $\tau = \int_{s1}^{s2} \frac{ds}{c}$ valid for arbitrary worldlines $x^\mu(\alpha)$?

user430580

I am far too uneducated in physics to understand the details, but thanks for trying :P

Silly Goose

When I first learned about proper time, it was only in the context of uniformly moving clocks. however the presentation of proper time by L&L seemingly allows for arbitrary motion of clocks by the above formula

ACuriousMind

@SillyGoose what do you mean by "valid"?

i.e. in what context is that not just the definition of the proper time of an observer?

Silly Goose

i think due to my unfamiliarity, I am more asking if this formula is generally true in special relativity

it is unclear to me if it only applies to particular types of motion of objects

ACuriousMind

it's the definition of proper time

it's "true" in the sense that that is what we mean by "proper time"

it's a bit like asking if $a+b = b+a$ is always valid in a commutative group - yes, it is, but merely by definition, there is no content to this

Silly Goose

21:04

hm i see

Mr. Feynman

I can't believe that I was checking the log in search of Haag stuff and I was about to fall in the rabbit hole

Silly Goose

so why do presentations of SR not include discussion on accelerating objects

or some even say SR does not describe accelerating objects

ACuriousMind

because they don't understand SR

it's the same problem as when people call basic differential geometry "GR methods"

Mr. Feynman

I should have added "SR can't handle accelerations" to the meme

Okay, I clearly need to make an entire meme about relativity

ACuriousMind

SR is just physics in Minkowski spacetime; but if you've been tricked into thinking SR is mostly talking about weird thought experiments with light clocks, you might not realize what that means

qwerty

21:11

as long as we're talking about objects and not like photons. probably worth an asterisk.

ACuriousMind

oh no, "photons" are another can of worms

Mr. Feynman

Where did "photons" come from now?

I swear if someone says "vacuum", I quit

Oh ok, I'd misread qwerty's message

ACuriousMind

I should vacuum more

Mr. Feynman

You dare defy entropy?

qwerty

@SillyGoose interesting discussion here physics.stackexchange.com/questions/6742/…

@ACuriousMind what's wrong with photons?

Mr. Feynman

21:18

I think that the geodesic path is a simplifying assumption

qwerty

@ACuriousMind i just say I'm in my dust dominated era

4

well tbh I don't, I need to wage war on clothes moths

ACuriousMind

dustpilled, unbothered, in my lane

Mr. Feynman

There are layers of dust on my Weinbergs

ACuriousMind

@qwerty they suck

particles that aren't localizable, infinitely many soft photons, etc

Lamb was famously "anti-photon"

qwerty

D:

ACuriousMind

21:20

see e.g. this take of mine

Mr. Feynman

A friend of mine just replied to a message after 2 months because he was busy "understanding what was happening around" 💀

I think we also discussed how much you hate photons here in the chat. I remember being on a bus and chatting with you and luca

qwerty

21:37

@ACuriousMind I am feeling rather disturbed right now. I was always told photons were a perfectly sensible/rigorous idea

in fact to make classical stuff easy i.e. talking about null geodesics --you say stuff like "ah yes, the photon path", and talk about things like intensity as number of photons

and then palm off the idea of a photon to quantum physics >.>

ACuriousMind

I mean, "the photon" as the state created by the c/a operators of the EM gauge field is a perfectly fine idea

but it doesn't really behave like you would expect a naive particle to behave

no wavefunction because no position operators, no non-relativistic limit

the world is not as simple as you thought, it's lies-to-children all the way down :P

qwerty

>=[ the lies should at least be consistent

fiction is fine, but no plot holes please :P

> More specifically for the photon, note that a classical electromagnetic wave - a state with definite electric and magnetic fields - is a coherent state where the number of photons is indeterminate. Hence it is not particularly useful to conceive of any electromagnetic radiation as being "composed of photons"

this is the part which I was reacting to a bit more specifically, in terms of "it is not particularly useful..."

Mr. Feynman

@ACuriousMind well, no other particle in relativistic settings has position :P

qwerty

(and fiction is ok only if the book is marked "fiction" and not "the true Gospel of LL" or something)

ACuriousMind

the whole purpose of a lie is to make the world simpler than it is; if the lie was consistent with all other facts it would be the truth

Mr. Feynman

21:47

Then, it all boils down to the fact that the EM field is intrinsically relativistic

qwerty

@ACuriousMind but your point was kinda it's not, for what we hand-waving say in classical physics

ACuriousMind

@qwerty there are no photons in classical physics

qwerty

@ACuriousMind that's why I said "handwavingly" ... I mean like what i was saying a couple of comments ago

Mr. Feynman

By classical do you mean non-quantum or non-relativistic?

ACuriousMind

no one would have ever considered such a strange idea before the photoelectric effect

qwerty

21:52

14 mins ago, by qwerty

in fact to make classical stuff easy i.e. talking about null geodesics --you say stuff like "ah yes, the photon path", and talk about things like intensity as number of photons

ACuriousMind

(cue @Slereah citing some 17th century discourse about corpuscular theories of light)

Mr. Feynman

That's more an assertion about electromagnetism being a relativistic theory at its core than a statement about just photons, imho

qwerty

@ACuriousMind (sorry, was distracted) I'm aware! but you see it all the time, for the sake of convenience. the wording tends to be more of a mouthful when you can't say "photon".

ACuriousMind

sure; and if everyone understands all the subtleties of the notion of a relativistic particle that's fine

qwerty

@Mr.Feynman you mean the one I quoted from ACM's answer?

ACuriousMind

22:02

@qwerty You have to pay attention to my careful wording here - I stop before I call the notion of the EM wave being composed of photons wrong, because you can always technically argue it's true, for some values of "photon" and "composed"

but it doesn't really mean what anyone with less than a complete understanding of the subtleties of quantum field theory would assume it means

qwerty

@ACuriousMind and do you mean that those "values" (definitions?) are somewhat incompatible with the understanding of a photon as "the state created by the c/a operators of the EM gauge field"?

ACuriousMind

also I'm not the average physicist - someone once called me (as a compliment) "extremely nitpicky about what's right"

@qwerty no, but I mean that they are incompatible with the "little localized blob of light" that almost everyone associates with the word "photon"

an electron is, in many useful approximations, a little localized blob of charge

a photon is not localized, nor a blob of anything, to any useful approxmation

Mr. Feynman

@qwerty everything that has been said so far

@ACuriousMind to some extent I would consider "special" the particles that are, instead :P

ACuriousMind

@Mr.Feynman by which you mean every particle except the photon and the weird things where we don't even understand why they don't occur freely (gluons)? :P

qwerty

@ACuriousMind how would you rigorously refer to null geodesics / a "photon path", in the classical limit, but knowing QFT?

ACuriousMind

22:10

@qwerty I'm not sure what you mean

A null geodesic is just a path through spacetime, it has nothing in particular to do with photons

qwerty

Hum. So clearly the idea of a "photon path" is not "correct" in the view that photons aren't localisable?

ACuriousMind

that's another complicated question :P

I mean, no particle has a "path"

qwerty

Please go on!

ACuriousMind

but if you do Mott's bubble chamber argument you can reconstruct the classical path from the quantum position operator

Mr. Feynman

@ACuriousMind I mean, your prejudice against photons seems to be based on the fact that they are not intuitive in a sense that corresponds to the everyday idea of a particle. The electrons in a relativistic QFT calculations are as unlocalized as photons, as you said

ACuriousMind

22:12

this doesn't work in relativistic physics because you can't access a position operator

Mr. Feynman

I will defend photons because they are the only gauge boson that deserve my unconditional love and respect (and exist)

ACuriousMind

but instead you can have a "surface crossing function" for photons where you compute how likely a detector that's a 2d surface is to detect a particular photon crossing it

qwerty

@ACuriousMind in relativistic physics you can't access a position operator? what does that mean?

ACuriousMind

so sure, if you stack a bunch of such detectors you can imagine a variant of the bubble chamber argument, with the small problem that every known detector for photons destroys the original photon :P

Mr. Feynman

What I mean is that ultimately nature is relativistic, so I don't like considering the non-relativistic behaviour of some (most) particles as the standard for a particle :P

ACuriousMind

22:15

@qwerty There are no operators $x^\mu$ with $[x^\mu,p_\nu] = \delta^\mu_\nu$ with the properties we'd want from position operators $x^\mu$ and momentum operators $p_\mu$

Mr. Feynman

@qwerty there is no such thing as a position operator in relativistic QFT, unfortunately

ACuriousMind

there is extensive literature on this topic, the relevant buzzword is "Newton-Wigner operators"

Mr. Feynman

The propagator as a propagation amplitude is about as true as my cousin owning a PS6

qwerty

I need to go write SQL now D: but this is suuuper interesting

much to think about!

Mr. Feynman

You'll end up either hating QFT or hating it and pretending not to :P

qwerty

22:18

tchuss/ciao friends

@Mr.Feynman ive already been scarred for life by my undergrad soviet lecturer :P

3

ACuriousMind

@Mr.Feynman It's not bad to hate QFT - it's important to hate it for the right reasons.

qwerty

I also retained/learned nothing in the process :p

ACuriousMind

"Genuine anger was one of the world's greatest creative forces. But you had to learn how to control it. That didn't mean you let it trickle away. It meant you dammed it, carefully, let it develop a working head, let it drown whole valleys of the mind and then, just when the whole structure was about to collapse, opened a tiny pipeline at the base and let the iron-hard stream of wrath power the turbines of revenge." - Terry Pratchett, Wyrd Sisters

Mr. Feynman

@ACuriousMind like the unmentionable.

More seriously, I do not hate QFT at all. I hate the hypocrisy of writing texts at the physics level of rigor, using it as an excuse to fill them with just vacuous formal manipulations, because "ultimately the calculations are the same for everybody". If you only care about calculations, be coherent and write a book featuring the perturbation expansion in the first page and the Feynman rules in the next pages

You can't just write a book with vacuous proofs and then say that

Or at least stress the level of formality. You should tell the reader you are giving just an idea, not a proof and possibly not even working with existing objects. Should I believe that most authors don't know that? This is something so worth a mention, even just a paragraph

ACuriousMind

@Mr.Feynman just watch <insert any standard QFT book here>

Mr. Feynman

22:28

To whom do you think my dissing is addressed to? :P

Sidney Coleman used to say "they gulp down without digesting and then throw it up on the pages of a book" or something like that

Incidentally, if anyone has read/heard the original quote and can find a source, it would be appreciated

ACuriousMind

22:43

@Mr.Feynman I cannot even find any place that even attributes a quote like this to Coleman, where did you get this from?

Mr. Feynman

From a lecture, I've been looking for it too :P

Please tell me also your professors had a thing for quotes that no one ever actually said

ACuriousMind

no, not my professors

Mr. Feynman

Yeah, I can see Germans never saying a word out of place (memes have heavily influenced this vision)

ACuriousMind

but this question might have been inspired by something I found in promotional material by a big corporation :P

Mr. Feynman

Ah, another representation of ACM

Well, you had some places to link to

I had professors telling the same story twice a week and I couldn't never find a source. When I did, the story was heavily different

Like that time a Professor told us about Newton destroying Hooke's career

Planning to become secretary of the Royal Society to banish all his works because he had contradicted him in a debate about the nature of light :P

Mr. Feynman

23:12

*couldn't ever

When I write here from my phone I never check what I've written D:

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