@Semiclassical yo why would I ever write $-\frac{1}{4}F^2$ as my action

it gives the right field equations and it's $T-V$ if you break it up, but without breaking Lorentz invariance, it's kind of random

Simplest Lorentz invariant scalar quantity you can write down using F?

@Semiclassical Ok, plan for tomorrow: Noether's theorem for general invariant functionals and an application to semilinear wave equations. Then physics: stationary action, point particle in a potential, conservation of energy, simple harmonic motion, particle in polar coordinates (shows the power of general canonical coordinates)

@Semiclassical proof by Occam's razor?

Well, it’s also what works in describing reality

You could also ask why the simplest possible Lorentz-invariant Lagrangian is the one which describes reality

And there I dunno

Probably one could invoke QFT to some extent, namely that the QED lagrangian is renormalizable etc

@Semiclassical GR isn't renormalizable but it works

Point

@Semiclassical do you know how to delete pages from a PDF? Acrobat wants me to pay for the ability...

No idea

found an online tool. what a pita

now they're gonna steal my math

To follow up on the earlier point, it’s worth noting that you get the same kind of field^2 term in the QCD lagrangian

so that’s what you’d use to talk about qcd in the absence of color charge (I think that’s the language)

@Semiclassical All Yang-Mills actions are F^2

with a trace over the group index

@0celo7 convert pdf to latex, then delete the page and compile?

@Mithrandir24601 Well it's my thesis, I don't want to do that

I just found an online tool that let me select pages to delete

@0celo7 Then don't you have the latex file anyway?..

Why would I delete pages from my thesis...

and If I delete random pages the numbering will get screwed up

I have to edit the PDF itself

@0celo7 "how to delete pages from a PDF?" As in, can't you create a copy (or use one of the backups that you've obviously created as you're a sensible person who's prepared for all eventualities :P ) and edit that to get what you want?

@Mithrandir24601 I just said I can't edit the TeX of any copy because that breaks equation, theorem, and section numbering.

(obviously, I'm totally missing the context, but just felt like interrupting the conversation, as it's been a long day :P)

@0celo7 then there you go :p

nice

@0celo7 If you're that desperate, there's bound to be a way to fix the numbering to be what you want where you want?

make extra sure that it's positive infinity

@Mithrandir24601 IT'S DONE

Actually. A thought. If you print a Word document, you can choose to print only some pages. Doing print tk PDF should do the same thing

@Semiclassical I'm emailing it to my advisor, not printing it

@0celo7 Aw, I was looking forward to the epic use of some crazy Latex skills :P

No, I mean

When you print a Word doc to PDF, you can pick what pages to print and you’ll get a saved pdf with just those pages

some italian dude seems to have the proof I need

thank god

time to eat and then do complex analysis homework...

@Semiclassical I did not know that

let me try

That’s for word docs, but there may be a similar workaround for PDFs directly

I'm opening the PDF in Word

"Word will now convert your PDF to an editable Word document"

...hmm

I wonder what it will do

:(

@0celo7 Doesn't it work if you open it in adobe acrobat, ctrl+p and select the 'print to pdf' option?

Yeah, that’s what I had in mind

And that’s what this YouTube vid suggests: youtu.be/zbmBtdcxbIY

wow, genius

gg peeps

Huzzah for simple solutions

@Semiclassical it's still a hack

@Semiclassical so you think there's anything more I should mention tomorrow?

It’s definitely not obvious

i don't have much time but I just wanted to talk about physics for a bit in that class

Hmm

Could possibly say something about hydrodynamics since that pretty much hinges on conservation laws

Hydrodynamics is not a Lagrangian theory though

Yeah

Unless you’re doing relativistic hydrodynamics maybe