@Charlie I now realize I have missed the chance there to plug one of the few books I actually possess physically: If you're interested in BRST quantization, the canonical source for that is *Quantization of Gauge Systems" (QoGS) by Henneaux/Teitelboim

Looking a bit at magnetic field history, it is amusing

bc in the 70's people used like superconducting magnets for fields of a few teslas

And by modern days, people use neodymium magnets, which are the magnets I use on my fridge

they pack quite a punch

@Charlie BRST is part of the quantization package

I don't know what is the current contender for Best Quantization

I hear geometric quantization is good?

Doesn't work for everything, but then again what does in the field of quantization

@Slereah Oh what does it not work for?

@MoreAnonymous There's always some ambiguity wrt quantization

because of operator ordering

also some theories can't be quantized IIRC

@ACuriousMind Ah I have seen that book referenced a few times in here, will have a look one day

Could someone please help me in a chemistry proof for no of electrons in a compound = summation of atomic number * no of moles * $N_A$ .

@Charlie

People are generally asked not to randomly @ people with their questions

But what is not clear about the formula you've give, in a neutral atom the number of electrons is the same as the number of protons. The number of moles multiplied by Avagadro's constant gives you the number of molecules

So the number of electrons in each molecule times the number of molecules is going to give you the total number of electrons in a sample

Ok. Then for Ca Co3 , why do we say that no of electrons = 50. It should be 50 * 1 mole * 6.022 * $10^{22}$

@Charlie I am sorry for the @.

You'd be right if you have a mole of $CaCO_3$, not just one molecule of it

Ohk. So here 1 molecule of CaCO3 has 50 electrons .

Yes

Thank you

np

Does anyone know what people mean when they describe a particular functional as an "H-function for the dynamics", something to do with convergence to equilibrium.

@DanielAdams very likely it's the "H function" from the H theorem

Thanks :)

it is

Anyone have a few minutes to discuss Loschmidt's paradox ?

My interpretation is that Newtons Laws determine the molecular collisions, these laws are time reversible. After the collisions are determined in this way, Boltzman showed that the entropy of the system increases over time, however since the dynamics were built from the assumption of time reversibility how does it make sense for a property of the system to definitely decrease over time? i.e we should be able to swap $t$ with $-t$ and everything still hold.

This is really bare bones interpretation^ wonder if anyone can make it clearer, or point out some other vital details.

@vzn No I am currently pursuing my Master's. But I will start a project on Cosmology that will require ML / Neural Networks. And my supervisor also doesn't know anything about it. So we plan to collaborate with someone who is an ML expert. Nevertheless I want to learn it by myself.

@DanielAdams It's less about "making sense" and more about a basic property of Newton's laws (or time-symmetric dynamics in general) - any solution is still a solution if you run it backwards, so for every solution where entropy increases we should get another solution where entropy decreases.

This just shows that the 2nd law is not derived purely from Newton's laws, though. I'm not sure why we call it a "paradox", really.

The modern understanding (as far as I know) is that the second law is statistical in nature anyway, so there isn't any modern claim that there are no evolutions of systems that temporarily decrease entropy to begin with.

@DarkVader I've heard Andrew Ng's course is pretty good for learning about (general) ML right from the basics

@NiharKarve Yes. I have done it as well from Coursera .It is great.

@ACuriousMind I agree, i dont see it as a paradox just maybe a simplified modelling. However I do know there are a lot of people (e.g Oliver Penrose) who worry about these things, or is it something different they are concerned with. Sorry I cant be more specific

Is there something wrong with the following sequence of steps? $$\frac{1}{\sqrt{-X^\mu X_\mu}}X^\nu X_\nu=\frac{1}{\sqrt{-X^\mu X_\mu}}\sqrt{(X^\nu X_\nu)^2}=\sqrt{\frac{(X^\nu X_\nu)^2}{-X^\mu X_\mu}}=\sqrt{-X^\nu X_\nu}$$

This gives me a bad answer, I think there is something subtle about how the minus sign works here that I'm doing wrong

@Charlie minus signs under roots don't work like that - consider: $\sqrt{-2}\sqrt{-2} = -2$, but $\sqrt{(-2)(-2)} = \sqrt{4} = 2$, so $\sqrt{-a}\sqrt{b} \neq \sqrt{-ab}$ in general.

oh that is sneaky

@ACuriousMind your neighbours on this query might interest you :-P

@ZeroTheHero will also get there in a few months

though I guess the User Who Must Not Be Named stopped participating in this site long enough ago that neither of you had overlap with them?

@EmilioPisanty yeah, I never had any direct interaction

@EmilioPisanty neither did I.

Interesting query.

@EmilioPisanty an interesting variant of your query would be reputation/posts (either answers or questions)

Can I just make sure I've got something right, I think it makes sense but it just sounds a bit strange to say it to myself. In classical field theory a field like $\phi(x,t)$ is just a scalar function $\phi:\Bbb R^{1,3}\rightarrow \Bbb R$ and when we think of the field "oscillating" (or doing whatever) we think of the output of this scalar function as changing. However in string theory a similar role is played by the embedding coordinates of the worldsheet. (1/2)

So rather than thinking of the field as oscillating, we think about the string wiggling around and as it does so the embedding coordinates change and this is analogous to the scalar function in classical field theory "oscillating"?

Maybe this is obvious, what's slightly strange to me is that manifold coordinates are being treated as "scalar fields" on the worldsheet, which is a slightly odd concept at first

@Charlie sure

Ok that's good ty

@ZeroTheHero for Posts, just change PostTypeId=2 to PostTypeId IN (1,2)

that's true, although I hadn't even encountered the point particle action until I started string theory either, but I guess it's the same idea yeah

if you want the ranking over total reputation, that's already ranked in the main site, both on Users and on the rep leagues (shortest link is via the "top X% in Y time period" under your reputation in your profile)

stackexchange.com/leagues/151/alltime/physics

if you want the rep-to-posts ratio... sure, can be done

let me know exactly what data you want and how to sort it, and I can SQL it for you if needed

@EmilioPisanty yes the rep-to-post ratio but this is just a random curiosity. I was puzzled by the ranking of your previous query: JR is much more efficient than I at accumulating rep.

unrelated (completely) to physics but such a fantastic picture: flic.kr/p/2k1yRfj