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12:36 AM
to the people who frequent here and help out, especially (but not only) @naturallyInconsistent and @ACuriousMind, thank you for your guidance! having a space to ask questions and learn
is so important to me as a student
as i begin my phd in computational chemistry next year i think you might be seeing me a lot more often
1:16 AM
mewth~
1:45 AM
off topic: I have so often heard fairly recently you can microwave metal bowls/ round smooth metal containers completely safely but I don't wanna try it out lest I become known as 'overeducated but brain-dead idiot microwaves metal and burns down building'
2:06 AM
you can do almost anything if you put water inside the microwave. Then the water will absorb the microwaves and reduce the chances of catastrophe significantly
2:23 AM
@qwerty i have definitely microwaved a container lined with aluminum before (unknowing that it was lined with aluminum lol). It was all good. I think I googled afterwards and there was something about sufficiently smooth surfaces of aluminum being okay.
2:36 AM
i wonder if this is related to the fact that the E field (in electrostatics) diverges at a sharp edge of a conductor
2:59 AM
it was something about you only get arcing between sharp edges. like the tines of a fork is bad for instance
@SillyGoose yes
3:30 AM
@SillyGoose how
this information would significantly improve my life
 
2 hours later…
5:19 AM
@SillyGoose Yes please tell us how !
 
1 hour later…
123
123
6:42 AM
Hello Everyone...
 
2 hours later…
8:38 AM
@ACuriousMind ACM, my plan was to learn some rudimentary group theory / Lie group concepts next; not for its own sake but it was more just for the concepts I had listed. is there anything you would suggest in particular that you think i should study? I don't really think I'm a super hardcore maths person, I just want to know enough for the physics to be solid.
you mentioned that physicists conflate DG and GR for instance so would reading a proper DG book be helpful or would it not directly address the issues in physics for instance
basically your comments last night threw a bit of a wrench in my study plans :p
@qwerty Do you just want to learn GR i.e. learn it well enough to understand basic calculations, or do you want a rigorous understanding of the theory?
@qwerty For Lie groups you need almost no differential geometry, but I would nevertheless suggest reading an actual math text on them - I deeply dislike the way most "for physicists" approaches do representation theory. A classic recommendation is the book by Hall.
@JohnRennie Hi. I'm not after learning GR per se - if you mean like what you'd see in a course, as I've already been and done that :). I am fine with basic calculations for sure. however, there are some particular concepts that I am exploring which I have found are not nicely covered in the textbooks i've read.
For GR vs DG I'm not really sure what I recommend. For me it all coalesced into a big picture after I'd learned both GR and a lot of differential geometry, but I don't think it is particularly helpful to work through all the formal geometry before starting with GR
@qwerty It sounds like yes you do need to read the sort of text ACM is recommending.
8:49 AM
maybe @Slereah has recommendations for more formal approaches to GR that aren't "read half a dozen books on differential geometry and pseudo-Riemannian geometry"
@ACuriousMind yeah basically my medium-term goal is to get to the bottom of the concepts I listed - more or less at a level such that I could read the answers you linked and understand them. my plan had been to attack it directly but from what you said yesterday it sounded like you maybe sorta would think I'd need to be less direct and do something like DG first
I mean I think differential geometry on its own is worth it but then again I just like pure math :P
I think I get intimidated. it's very dense
although I did have a bit of a book crush on Arnold's book
I think that was the only hardcore maths book that didnt completely spook me
well...yes, it's dense because it doesn't sweep all the details under the rug as the physicists do ;)
@ACuriousMind I wouldn't mind if they spent more time explaining in words as well! but they are efficient
@ACuriousMind I started on this one "Harriet Suzanne Katcher Pollatsek. Lie groups: a problem-oriented introduction via matrix groups 2009" because I saw it on a friend's bookshelf and it seemed gentle. but I'll look up Hall as well
9:02 AM
Are there guidelines on editing tag descriptions? I know that small edits to questions tend to be disfavoured because it unnecessarily promotes the question to the home page, but I didn't think it applied to tag descriptions.
I ask because I've just noticed this edit was rejected even though I thought it was pretty good (and approved it).
@ACuriousMind may I ask what physicists do re: representation theory?
@JohnRennie No, small edits to tags have no problems from my point of view - and even if it had been a question or answer edit, this particular edit should not have been rejected since it corrects an actual error (it's->its)
Thanks.
@qwerty They're usually just terribly confusing about it. For some reason they often refuse to properly conceive of a representation as a map $\rho : G \to \mathrm{GL}(V)$ and keep talking about vague things like "the value of the generators in the representation" or whatever
@qwerty Ted Shifrin has a link to his diff geo textbook on his profile page. math.stackexchange.com/users/71348/ted-shifrin I haven't read it, but it's quite popular, and it was "road-tested" by Ted's students.
9:11 AM
see e.g. this rant of mine about treatments of the Poincaré group
@ACuriousMind isn't it also a sense of knowing which details are necessary and which are superfluous? I mean I am a stickler for detail but I never found for example that rigorous mathematical definition for every little thing really seemed to make a difference? I think I actually did write fairly mathematical manuscripts but it was never mathematical physicist mathematical. I felt like the important thing was the concepts being honest somehow
@qwerty Well, I didn't claim that every detail was relevant for physics :P
as I said, I just like math for its own sake and I always found it more gratifying to have the mathematical picture and see for myself what parts of it actually applied to physics instead of having someone else make that decision for me
mhmm, fair
partly because I had made the experience that the average physicist made this decision differently than I would have
This doesn't necessarily mean that they are wrong and I am right, but I notice that many of my popular answers are about putting something vague or otherwise confusing on a more mathematically sound basis, so there is a bunch of people out there feeling the same
i suppose you learn extremely quickly, so it's actually practical for you. I think I learn at a much slower rate, so having someone else filtering is just what has to be done for me to progress
@PM2Ring thanks
Bml
Bml
9:26 AM
@ACuriousMind I re-submitted the edit. I hope it will not be rejected again :-)
@qwerty No worries. Ted used to be a regular in the main Math chat. He suddenly stopped inhabiting chat a few months ago, but he's still active on the main site.
@ACuriousMind sorry if you already said this in your answers and I missed it. are there any particular topics in DG books at any rate which I could look up to untangle the "general covariance"/gauge issues?
The relation of general covariance to topics in mathematics is an issue that is nowadays fairly niche
It is the topic of "geometric objects"
Which was a topic of discussion of differential geometry in like the 1920's
It is also related to equivariance in modern geometric parlance
@Slereah Hilbert?
No
There were other mathematicians back then
9:36 AM
lol
@Slereah thanks
> In 1926 O Veblen and J M Thomas [in Projective invariants of affine geometry of paths, Ann. of Math. 27 (1926), 278-296] defined an "invariant" in the following way:
"An invariant ... is an entity with definite determining components in any coordinate system, such that the transformations of the components from one coordinate system to another form a group, isomorphic with the group of analytic transformations of the coordinates".
In 1927 O Veblen [Invariants of quadratic differential forms, Cambridge Tracts, No. 24 (1927), 14] gave the definition:
@qwerty The question is really what your goal here is - what do you mean by "untangling" these issues? For instance, as in many other cases, my preferred way to deal with the problem of "general covariance" is to just not use the phrase :P
Yeah it is pretty rare that you need general covariance
getting a consistent picture of GR that does not require the use of the phrase is honestly a lot easier than trying to understand what the heck people mean by it
It's mostly interesting from a historical perspective
It does pop up in some very niche applications but it's not a particularly important topic to learn
9:41 AM
well, the problem is that every time I read it (or things like gauge) in a text or a paper my mind now grinds to a halt because of a) not really have any definition at all in my head besides the fuzzy one I gave and b) all the baggage it has given all these discussions and c) different authors sound contradictory and I can't make head or tail of it. so If I at least understand in what ways people are (mis)using it I can at least parse their work
I mean how often do they use it :p
pragmatically you can usually really just ignore it - it will occur in some general waffling about GR and what's so special about it or whatever but it won't be part of any actual derivation, precisely because its meaning is so vague :P
but I want to understand :(
There's plenty of more or less related things which fall under the term of "general covariance" but it is rarely something that will be useful to know except for the broad things - fields transform with the coordinates, the metric and connection transform in fancy ways, etc
I mean if you really want to you can check like
and yes, different authors sound contradictory probably because they are contradicting each other, you won't find a single definition that suddenly makes it clear what everyone is talking about
@Slereah yeah but ACM said the "right" way was to learn DG :P
It is part of DG
Part of "general covariance" is what the DG people call natural bundles
if you thirst to learn more about natural bundles, mat.univie.ac.at/~michor/kmsbookh.pdf
@qwerty What I meant is that differential geometry gives you the tools to clearly articulate what's going on (and propose a definition of "general covariance" that is at least consistent if you really want), but more importantly you will realize that "general covariance" is not at all a concept as central as physicists like to pretend; most of pseudo-Riemannian geometry in math is done without ever mentioning it
also absolute objects are part of the conversation : philsci-archive.pitt.edu/3005
sorry that was just a response to seeing the link to the norton paper and something under gr-qc
9:46 AM
Well if you want math :
and GR is just pseudo-Riemannian geometry done by non-mathematicians :P
You can do classical mechanics in a "general covariant" way, depending on what it means :p
oh, sure
@ACuriousMind who needs physicists anyway T_T
Unless you mean in the sense of absolute objects, in which case it isn't
9:48 AM
I love to cite Hennaux and Bunster on the vanishing Hamiltonian for "generally covariant systems" (where it essentially just means that the action is invariant under reparametrizations of the integration variable)
but that usage of "generally covariant" is certainly not equivalent to all the things people might mean in a GR context
@qwerty they need to use their strange powers of figuring stuff out so that then mathematicians can invade the theory and make it proper ;)
"General covariance" is an idea that was inspired by Ernst Mach's big book of rambling on mechanics and how fitting that it would be just as big of a mess
@ACuriousMind and there's just really no shortcuts I guess
it's a symbiosis, not an antithetical relationship
"Is general covariance too general? Or not general enough?"
Diff*********m inv******ce
4
9:52 AM
@qwerty the only shortcut is to not care :P
@Slereah people really hate Mach's bucket these days to the point of not wanting to mention it
Think of general covariance the same way that people use "absolute time" and "absolute space" in newtonian mechanics :p
@ACuriousMind I'm doomed, ACM. doomed!
@Mr.Feynman yes, it's that time again, I already remarked on it:
yesterday, by ACuriousMind
I must've done something really bad in a past life to be cursed to debate diffeomorphism invariance for all eternity :P
at this rate I will never progress to remedial QM/QFT lol
classical forever
9:54 AM
It is your curse for trying to learn the historical way :p
@ACuriousMind I mean, you must have done plenty of that, because it's not just diffeomorphism invariance. Passive/active, asymptotic states, Haag theorem to mention a couple
It's never a good idea to teach a topic the way it was discoverd
@qwerty I mean...sometimes it's the right decision to not care for now. I have repeatedly advocated that people should train their ability to say "I don't understand this" and then just file it for later investigation instead of becoming obsessed with understanding that particular detail :P
@Mr.Feynman At least asymptotic states and Haag's theorem are topics I in principle enjoy
but I don't even like GR! :P
Oh, I didn't expect that. I think I enjoy GR more than Haag-like discussion. Probably because I feel safer mathematically
@Slereah ...you're right. I was even worse younger. I was very briefly convinced to understand Newtonian mechanics I needed Principia for the ~historical context~. and then mach's principle when I started GR.
9:58 AM
But I'm afraid that the biggest mystery today is RR randomly pinging me with a question about ST, which I don't even know
And I wasn't even online
@ACuriousMind ooo noted.
The discoverers of an idea are typically the worst ones to explain it
I promise this will come up in the future. To give a simple example of this occurrence, I'm reviewing some non relativistic many body physics and Green's functions are the first thing you see. Guess who's back? Interacting vacua. So I occasionally read old conversations with ACM or old answers
@Slereah I was in the stage of life when everyone around me were fanboys of the OGs so obviously you gotta read them and get in their head right?
I don't know how reassuring it sounds but there are topics that will haunt you for the rest of your life, so don't worry about forgetting about your doubts, because they will get back in the future over and over
10:01 AM
General covariance is really just a reaction to that weird era from Descartes to the 20th century where everyone got really invested to go geometry entirely with analytic geometry and with a very heavy focus on coordinates
It was a practical method but also kind of obscuring what was going on
@Mr.Feynman insert some joke about asymptotic safety
My Professor hates asymptotic safety, so my jokes would be savage :P
@qwerty If you think Mach's bucket is weird, check out what he thought about atoms.
light-hearted physics-related question: does anyone here know subharmonics?
In singing, of course
@Mr.Feynman sure, it's what underlies (some forms of) throat singing. I like what The Hu do with it
10:08 AM
I expected you to know it. :P

I'm learning how to produce subharmonics and it's such a weird sensation to sing them
Because you don't have to push like you would to sing such a low note but you still get it (although you can still hear overtones)
I wonder if I can find some source describing the physics of vocal chords and this tecniques at a decent level
@qwerty Some historical context can be helpful, once you have some grounding in the topic. The historical info can help you understand why the topic is organised the way it is. Later refinements can create better organisation, and smooth out some of the weird stuff that was created by the founders of the theory, but there will always be residual traces of the early weirdness.
@PM2Ring well, that was part of my thinking.
The historical context of say Newton probably won't be super helpful because you will also need the historical context of Aristotelian mechanics :p
Like what are you gonna do, start reading with the presocratics to understand all of physics?
@Slereah this is what you said philosophers do in philosophy :P
Yeah and they still haven't solved their problems
Obviously not a good method
10:22 AM
@qwerty I don't think that people hate Mach's bucket, nor are avoiding to mention it. Instead, it would be helpful if people mentioned that GR does not agree with Mach's principle a bit more.
this whole discussion is xkcd.com/435 vibes lol
@naturallyInconsistent a lot of books barely mention it, if at all it's just a historical note. but I havent looked it up in years
@qwerty well, what good would it do to introduce students to a concept that, in the end, is a dead end?
@naturallyInconsistent Im not arguing for or against it I'm just responding to you saying that "people aren't avoiding to mention it".
that's kinda chicken and egg. can't exactly prove that they are avoiding...
@qwerty I don't know if it hurts more to see physicists doing math or mathematicians doing physics
10:28 AM
@naturallyInconsistent I guess
@Mr.Feynman are mathematicians bad at doing physics?
@naturallyInconsistent Of course it depends, just like not all physicists are sloppy :P
I mean, have you taken a look at gasp programmers pontificating about physics?
@naturallyInconsistent have you seen physicists programming??
:P
@naturallyInconsistent I don't even know what that is :P
10:31 AM
@qwerty yes; but those who are bad at it don't tend to make that much noise
But I've heard nasty stuff at conferences. Like mathematicians in the audience discussing with physicists (about physics)
@Mr.Feynman what do you mean?
I'm just discussing about personal experience and joking a bit, but I've heard mathematicians saying that supposing that the laws of physics hold (in a physical context) wouldn't be a safe assumption D:
The average physicist butchers math; the average mathematician thinks everything in physics is an unreasonable assumption. The best species is the one standing in the middle :P
then you please no one lol
I'm bitchy, if I may say so
10:40 AM
@Mr.Feynman death to the centrist!!!
11:35 AM
@naturallyInconsistent it's not as if physicists don't also love to pontificate about other fields :P (obligatory SMBC)
I think you mean
that's the obligatory xkcd on the topic :P
@ACuriousMind to be real tho the happiest part of the physicist lifecycle definitely seems to be emeritus. emeritus staff are just chill whereas everyone who's in their academic "peak" just seems stressed, angsty or territorial
plus who doesnt want to wander around the apartment talking about aliens or playing bridge lol
@qwerty sure, but that's less a function of age and more a function of the stability of their position, they don't have anything to prove anymore
yeah thats what i meant
well stability but also life wisdom helps too
11:43 AM
If they had life wisdom they wouldn't have gone into physics
eh it worked out for them
bit of a survivorship bias :p
also it can turn out very differently between people; there were two old math professors when I studied that still gave lectures; one was clearly starting to suffer from dementia and it was really sad to watch, the other was frail but was still sharp as a knife and once he started writing on the blackboard you could easily forget about his age
@Slereah i dont think life is really about planning a smooth trajectory as much as just taking the next best move according to you as it comes honestly
@ACuriousMind yeah that's sad to hear :( i guess i was more thinking about the generic wisdom being a function of age and changes in expectations around roles but you're right health can change too
12:26 PM
I think we should all take inspiration from Manfred Steiner who finally achieved his dream of getting a PhD in physics at the age of 89.
Don’t all scientific fields attempt to ultimately use a simple model to describe their class of phenomena
@SillyGoose careful, don't cut yourself with Occam's razor!
But since that was his third doctoral degree, I guess it's not that inspiring
@think_meaning_builds did he have 3? im looking at the news articles that came up and they only mentioned he had a phd in biochem before the physics one
Yup, he had an MD.
12:41 PM
that's not a phd though. for example in australia (medical) doctors lose the title Dr. after they stop practising
TIL, thanks 🙏
you can do phd's in medical science but theyre not the same as a medical degree. actually know someone who's doing that lol, so i guess they're a dr. dr.
Yeah, honorary degrees don't really count either, I guess.
actually im on wiki en.wikipedia.org/wiki/Doctor_of_Medicine and what it means seems to vary heavily according to country/decade and institute even
> There is nothing so absurd or incredible that it has not been asserted by one philosopher or another. - Descartes
1:04 PM
> You just keep pushing. You just keep pushing. I made every mistake that could be made. But I just kept pushing. René Descartes
1:24 PM
@think_meaning_builds That's a fake quote, see also the wikiquote talk page
@Slereah And this is just a rephrase of Cicero
He probably stole it from a famous caveman himself
The "made every mistake that could be made" part is beyond ridiculous.
at least it appears to be a genuine quote from Descartes' Discourse on the Method
1:33 PM
> When it was dicovered that the stamp honoring Descartes (No. 330) had the wrong title for his book, the French government quickly printed numerous copies of a corrected stamp (No. 331).
The mistaken ones must be worth a lot.
Probably
My memory is definitely not as good as it was 50 years ago, but I still remember trivial stuff like that. :)
:)
@qwerty any idea why?
cc: @PM2Ring
1 hour ago, by qwerty
that's not a phd though. for example in australia (medical) doctors lose the title Dr. after they stop practising
1:54 PM
anyone else having troubles with arXiv
seems to be down, yes
Here's a famous Aussie science communicator, who was formerly a MD.
Karl Sven Woytek Sas Konkovitch Matthew Kruszelnicki (born 1948), often referred to as "Dr Karl", is an Australian science communicator and populariser, who is known as an author and a science commentator on Australian radio, television, and podcasts. Kruszelnicki is the Julius Sumner Miller Fellow in the Science Foundation for Physics at the School of Physics, University of Sydney. == Early life == Kruszelnicki (Polish pronunciation: [kruʂɛlˈɲitskʲi]) was born in Helsingborg, Sweden, to Polish parents, Rina and Ludwik. Kruszelnicki's background was hidden from him for a long time, with his mother...
or, well, slow - arXiv pages now load for me again
arXiv was a bit flakey for me yesterday.
Internet Archives and now Arxiv
tough times
2:01 PM
And I've noticed a few sites lately with recently expired certificates.
2:26 PM
@qwerty smooth trajectories sounds like something you'd find in a diffeg book ;)
@nickbros123 @Arjun I am not sure what type of phone you both use, but if your browser has a bookmark feature then I just added the usual "start ChatJax" link to my bookmarks. Then, I open hbar in a new tab and go into my bookmarks and tap the start ChatJax bookmark. You will stay in the hbar tab, but mathjax will thereafter be rendered.
I guess I should not have said "ultimately". But it seems that in practice people often try to come up with a simple explanation first. Then, if that doesn't seem to work to build some more complicated thing
@SillyGoose chemists having laws that works in 2 situations:
2:43 PM
Of course there would be one
The real mystery is how you and ACM are so damn fast digging to find these
I just google it
Google can usually find smbc and xkcd with just vague hints
What do you google? "Chemists many laws xkcd"?
something like that yeah
2:46 PM
The trick of course is that when I can't find it, you won't notice :p
@qwerty what's funnier is that in Italy you could be called "doctor" with just a bachelor (although no one would take you seriously, they just do it in formal ceremonies :P)
@Slereah that reminds me of the anthropic principle
I'm not sure it's called that
2:59 PM
@ACuriousMind yay!
3:24 PM
They did it, they actually did it.
It's a good job I'm not a betting man as I would have lost money betting on the outcome.
@Mr.Feynman it is because u r learning string theory. I'm just wondering, do they call it bosonic string theory based on the spin or the statistics
@ACuriousMind it was never stated that this was a strict rule. u phrased it like a guideline.
3:46 PM
@Mr.Feynman literally actually a reason that i started to do physics xD
@SillyGoose were you a baby chemist?
@Mr.Feynman a fresh out the womb chemist (i only took up to 1st semester of orgo)
@Mr.Feynman hi
4:15 PM
@JohnRennie earlier, they used to require legs to land
I think Starship will still use legs to land, but the booster will be caught.
what is booster
The Starship "landed" in the ocean, but judging from the telemetry the landing was a little fast and would have resulted in a fireball if it had happened on land.
I don't know if that was deliberate or not. They successfully landed a Starship on land several years ago.
@RyderRude The booster is the first stage. That's what was caught by the tower.
oh. would they install these towers on Mars or the Moon?
The Starship is the second stage and it went on to achieve a stable orbit, then "landed" in the Indian ocean.
4:19 PM
@JohnRennie because of lack of fuel?
@RyderRude No, the booster is only needed to get from Earth's surface into orbit. The escape velocity on the Moon and Mars is low enough that Starship can take off without needing a booster.
@JohnRennie so now they can re-use boosters. Got it
it doesn't feel like a historical achievement... idk
@RyderRude No, it was deliberately landed in the ocean. Presumably for safety reasons i.e. they needed to show they could land it with enough precision that it wouldn't come down on an inhabited area.
@JohnRennie oh
@RyderRude that is explicitly strict. You just don't ever think that rules are a thing to be followed.
4:23 PM
maybe it is a revolutionizing technology for rockets @JohnRennie
@JohnRennie what is the purpose of starship. Do they need it in orbit for years
to study
ok so starship can later send other stuff to the planets/moons
including humans
That's a good question. As of right now it's hard to see what such a big rocket is needed for. SpaceX's existing Falcon Heavy is perfectly adequate for even interplanetary missions like the Europa Clipper.
Musk's stated reason for developing Starship is to enable manned missions to Mars, but it's hard to see it making back the billions it cost to develop taking a handful of astronauts to Mars.
I guess he's hoping that demand will appear once Starship starts routine flights. I believe Starlink plan to use it for launching the Starlink satellites. It can launch so many at a time that it makes the launch cost per satellite very small.
@JohnRennie also, NASA hires them for making many spacecrafts, so it may partially be government funded
@JohnRennie oh
yes, they could make money back with Starlink
4:56 PM
There's the possibility of a mission to Uranus in the mid 2030s, using a Jupiter gravity assist. space.stackexchange.com/q/67134/38535 But the probe will need a RTG, and there's a shortage of plutonium-238.
Allegedly, there were plans in the USA & Europe to ramp up Pu-238 production, but I guess that hasn't happened yet. space.stackexchange.com/q/55822/38535
 
1 hour later…
6:04 PM
@RyderRude hi (?)
@SillyGoose Is my memory failing me or did you mention biochemistry specifically?
I could check the transcript but ACM will do/has done it
6:51 PM
@Mr.Feynman i did not heh
7:02 PM
Damn it
7:54 PM
Physics puns are no joke. It’s a relatively dark matter.
 
1 hour later…
9:01 PM
the hbar during the 1950's
 
1 hour later…
10:04 PM
@ACuriousMind In case you have a bit of time, could you help me with something about finite Lorentz transformations? May I post a screen shot in german here?
From the lecture?
@think_meaning_builds because it's a professional title, not an academic qualification

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