12:04 AM
@Charlie I think Feynman diagrams are just a representation of these propagators, not the definition thereof.

The German u's are always watching :E

@Bohemianrelativist what about e.g. Wikipedia is unclear to you?
@Charlie there are also äs and ös
it's an umlaut trifecta

Are there any words spelled entire with letters with umlauts?
@Bohemianrelativist Also, the section just below what ACM linked on Wikipedia talks about their use in Feynman diagrams if you're interested

@Charlie not really, they're all vowels but real words need some consonants

aww

12:14 AM
the only German word without any consonants I can think of is Ei (=egg), but it doesn't have any umlauts

@ACuriousMind I have not read the Wikipedia; I am reading a lecture note, in which the lecturer is trying to derive the propagator.

if they're deriving a propagator but you don't know what a propagator is it's likely you're missing some prerequisites they're assuming

5 hours later…
5:33 AM
@Charlie Not unless you're a heavy metal band.

6:24 AM
Is the scri-plus and scri-minus symbol of Penrose diagrams (the future and past null infinities) from some language? What's it's origin?
Talking of these J looking thing...Got it from Carroll's book on General relativity...appendix H on penrose diagrams

6:47 AM
@ManasDogra It's an "I" not a "J".
"I" for "Infinity"
Why the curly form of "I" was chosen I don't know. I guess it just looked good :-)

@JohnRennie That makes sense
But why call "scri"..is it in some language's alphabet?

I doubt there is any deep significance to this. I suspect it's just short for "script I", where the term "script" is often used to describe fonts that look like stylised handwriting.

7:04 AM
@JohnRennie This turns out to be right!

:-)

@JohnRennie Have you read Chandrasekhar's book on black holes?

Yes...It's incredibly hard for me :( Do you have any recommendation on some introductory tetrad formalism?

To be honest I'm just an interested amateur in GR and I don't know it in the sort of mathematical depth that Chandrasekhar discusses.
I don't know the tetrad formalism at all.
Have a look at the book recommendations question and see if there are any suggestions there.
0

What is the current scope of doing research in tetrad formalism to understand gravity? What is the material, book I should refer for doing research on it?

2

I'm still self-learning general relativity. I have been a huge fan of Andrew Hamilton's amazing lecture notes on GR, black holes and cosmology. He goes through GR in pretty much full tetrad formalism. The reason he wants to do that is because some of the physics becomes much easier to understand ...

7:15 AM
Thanks...will check it out!

4 hours later…
10:54 AM
@Charlie You should try Finnish
something like hyvää pääsiäistä

11:41 AM
Is there a way to augment Wigner's classification so that it can distinguish between, say, gluons and photons (i.e. with an internal symmetry group)?

@NiharKarve I wouldn't call it augmenting Wigner's classification, but sure, gluons transform in the adjoint of SU(3), photons in the trivial

11:59 AM
So it's reasonably straightforward then (I say augment because I've only ever seen it with just the Poincaré reps)

12:46 PM
https://physics.stackexchange.com/questions/618841/measurement-in-quantum-mechanics
anyone who answers get a bounty

2 hours later…
2:36 PM
2020: Weird Wigner friend experiment and implications on superdeterminism
2021: We have just found a way to physically measure (somewhat) whether complex numbers or subquantum physics (postulated extensions to quantum mechanics) are physical entities

2 hours later…
4:34 PM
@NiharKarve It's fairly straightforward
The "big" classification wrt gauge groups is usually done for effective field theories for mesons and hadrons
Although that's more the flavor group usually
then mesons transform under one of the $1 \times \bar{1}$ rep, baryons under $1 \times 1 \times 1$, etc
And you can make it more complex by throwing in spin and whatever else
I think you can also throw in color?
things get a little wild if you throw everything in

5:32 PM
@Slereah very cool
course, my knowledge of real QCD is woeful

that kind of stuff

I mean, I'm familiar with the basics
But this old physicist dude radicalised me into doing proper field theory just when I was about to start practical QCD

QCD is proper field theory

His words, not mine

sounds like he doesn't know how to do QCD properly!

5:37 PM
I assume he meant getting real comfortable with the functional formalism of field theories
Nair's book is good in that regard, it felt like a QFT rebirth
The amniotic fluid being the sheer abundance of functional derivatives