I recently came across a new user who has asked several questions on the site but has not accepted any. Probably he is not aware of this feature. Is it okay if I inform him about it?

@TonyStark It's alright to comment once to tell them they can accept answers, but don't pressure them to do it.

i.stack.imgur.com/PcJid.png I feel certain someone has asked this question before. If so, does anyone know where to find the answer? If not, I will post that question, but it's a bit vague so if anyone has suggestions to improve it please let me know :)

@Allure Just like GR is the (geo)metric theory that reduces to Newton's law in a certain limit, Yang-Mills gauge theory is the geometric theory that reduces to Coulomb's law in certain limit. See physics.stackexchange.com/q/76354/50583, physics.stackexchange.com/q/111784/50583 and their linked questions

@ACuriousMind thanks!

In an Adiabatic Free expansion, the entropy increases, but in an Adiabatic Slow and Reversible Expansion, the entropy doesn't increase, why? What is the intuitive explanation for this phenomenon? What happens at the molecular level that the entropy increases in the first case but doesn't in the second?

@DevanshMittal Temperature is constant in the free expansion, so the volume determines the entropy. In the reversible case temperature changes. I am not sure what you mean by molecular level, entropy is not really a molecular property, but rather a property of the phase space

Is there any way in word to copy paste citations generated online? Instead of manually all the details that it asks - Name, Publisher, etc.

@Archer I'm not quite sure that you mean by "citations generated online" but most sites intended to provide citation offer BibTeX or similar formats to copy-paste.

scribbr.com/apa-citation-generator

Like this @ACuriousMind ^

I can copy it. But there is no way to paste it in word.

I have no idea what that is nor why you're writing your paper in Word :P

If you're writing a paper in physics or mathematics, people kind of expect you to use TeX

ugh

well, it's not that terrible a thing to do that you'd need to delete that :P

Possible silly question, but if we perform a transformation on a system in which first we apply the active version, and then we apply the passive version, do we expect to end up with the original pre-transformation system again, or the system with the transformation "applied twice" to it?

In other words are the active and passive transformation identical, or are they in some sense "inverses" of eachother

I don't know why I get tripped up on this so much, but based on what I've been attempting, applying the passive followed by the active (or vise versa) should land us back at the original, untransformed system

wait no now I think I'm wrong, hold up

I prefer not to think about this "active/passive" business all that much :P

there's a clear notion, mathematically, of what it means to apply e.g. a matrix to a vector, or a diffeomorphism to a manifold and its tangent spaces.

there's also a clear notion of what it means to change basis, or change coordinates

Talking about this in terms of "active" or "passive" transformations just seems to muddle the waters unnecessarily to me

hi everyone

good evening

if you think this really matters, I'd ask what problem you're trying to solve - in what situation do you have to first "apply the active version" then "apply the passive version"?

does here know about any online magazine on physics

I referred to some indigenous magzine

I'm afraid I don't really know what an "online magazine" is, nor why there would be a specifically indigenous magazine on physics or where you referred to it.

like a science newspaper which gives me update about the latest event

in science

it does sound funny indigenous refer to 'local update"

"indigenous" means usually "native to a certain region", and applying it to things means things produced by the indigenous population of that region

you haven't specified a region and you probably don't mean 'indigenous' anyway, but I don't know what you mean by 'local update' either

are you looking for science news from your local area?

no, I am seeking a platform to seek an update on the latest science event

what latest science event?

like new research, historical science events (like merging of blackhole)

...and what does that have to do with 'local update'?

the local update refers to the telegram or the social media platform for the physics enthusiast

@ACuriousMind

Sorry, I don't understand what you're trying to say

do you want something to send you a telegram message when something in science happens?!

no! all I am saying can you refer some link? like a website

@ACuriousMind

I have no idea if that's what you want because you haven't really managed to formulate a coherent English sentence about what you want. "can you refer some link" is not how 'refer' works as a word, and I still don't know what the 'indigenous' and 'telegram' business was all about.

@ACuriousMind Thanks for the link, sorry for using the word "indigenous" it is clearly out of the track. second about telegram all I was saying is I get some inputs from my friends using a group on this platform

@ACuriousMind Sorry didn't see this, I agree the wording is a bit loaded if that's what you're saying. It just bugged me because even if it is bad wording it still has to make sense somehow :P I drew some pictures and I was right the first time

So in more precise language we would refer to (on manifolds) active transformations as diffeomorphisms $F:M\rightarrow N$ and passive transformations as linear maps between all tangent spaces, which I suppose essentially is a map between tangent bundles $J:TM\rightarrow TN$

where $J$ is the Jacobian

Or rather, each individual map $J:T_pM\rightarrow T_{F(p)}N$ is a Jacobian, idk if there's name for the entire map

It's getting a bit elaborate now, but I think I get it

For correlation functions in QFT, when we have just two field operators (ie. $\langle0|\hat\phi(x)\hat\phi(y)|0\langle$ this is the propagator, but what is the purpose/interpretation of correlation functions with more than two field operators?

@Charlie you'll see :P

ask again when you know how to compute scattering amplitudes and it's still not clear to you

if I had to guess based on what I know about that, it looks like the amplitude for a field to spontaneously produce $n$ particles at the points $x,y,...$ and then for those particles to disappear, so we start and end in the vacuum state

QFT is not so simple

but it will turn out to be closely related to the probability of $k$ of those particles to scatter into the other $n-k$ particles

If you just learned about the propagator, you can't really see that. Wait for whatever you're reading to derive the LSZ formula and scattering amplitudes

ah ok then

without that it is not at all obvious that the n-point correlation functions describe scattering

Srednicki chapter 5 does LSZ in an easier way than Peskin, the jist is here, you can see the correlation functions arising as a by-product in working out $<f|i>$

It feels in qft that each new topic is quite a step up from everything that comes before it :p

QFT is usually taught in a fairly weird way

Mostly you're building in some kind of mathematical order

Rather than building it in a physically-relevant way

It takes a while to get to a measurable thing in QFT

arxiv.org/abs/2009.08903

Quite an impressive name

I'm prob stupid but does anyone understand why Mathematica is giving three surfaces?

looks cool

really doesn't

makes simple plots take way longer to load

Mathematica is the worst

GRAH