Ok, because the pullback acts on functions too.

Got it.

FYI: While doing some cleaning around here, the mods found some socks behind the dryer. Please brace yourself for possible lost reputation points. Although it should be much less than previous clean-up.

Let's hope they stay gone, I could do without another round of Return of the Sockpuppet.

so what do people on this room talk about?

@Mikhail Prety much anything.

@0celo7 You wish.

@ACuriousMind Huh?

@0celo7 Pfffft.

@ACuriousMind I'm still curious what that meant.

Editing just makes the transcript look confusing :P

@0celo7 It was meant to imply that you wished other topics were bannable. Not a good joke, apparently :P

@ACuriousMind Deleting too...

@ACuriousMind I thought the last chapter of the book "Bosonic String Theory" would be a bit more physical than the rest of the book, but it's just 30 pages of Riemann surface geometry.

@ACuriousMind I was being sarcastic. (Perhaps you knew that.)

Anybody tried doing (optical) coherence theory in Dirac notation? I'm having trouble finding a good reference :-)

@0celo7 Perhaps I knew :)

@0celo7 "Assume that no one can tell sarcasm on the internet." --0celo7

Shit.

Got me there.

@Danu Consider then a hypothetical system of 'justice' that, when a man is convicted of kidnapping and murdering a child, puts the parents in jail and gives their belongings to the kidnapper thus punishing the innocent.

Deleting my account now. This is pretty much the biggest embarrassment a man can endure.

@AlfredCentauri I think the point is that "crime" is not universal to most people. By extension, "innocence" is hard to universally define as well.

@Mikhail Not an optics person, sorry

(Although I wear optics :P)

@0celo7 This isn't difficult to understand and I'm frankly surprised at how such intelligent people here seem to be tripping over this. On the other hand, this a physics site, not a philosophy site so maybe I shouldn't be surprised.

@AlfredCentauri I don't have a problem with your stance. I think I understand where they are coming from though.

@0celo7 Will you come back as 0celo8 or rather as something like pan7her?

@ACuriousMind It's a Metal Gear Solid reference.

Revolver Ocelot is pretty much the coolest guy in gaming.

Feels weird spelling that correctly for a change.

@ACuriousMind I am considering becoming "Ryan Unger" on SE.

Ahhh, it's not because you like the cats? :D Never played Metal Gear.

@AlfredCentauri This due to the fact that unlike physical laws, judicial laws don't always hold.

@0celo7 I've got a feeling one has to have played the game to appreciate these

@ACuriousMind Yeah, I didn't actually watch it before posting it...bad vid for introductory purposes.

It's pretty much "143 random dialogue quotes from Revolver Ocelot"

@Icosahedron I'm afraid you've missed the point too. Honestly, I did not anticipate that it would be this difficult to make. In the example I give, the system explicitly and objectively punishes the innocent parents by design. Whether or not some consider the kidnapping a crime is irrelevant. Whether or not judicial laws don't always hold is irrelevant.

By the way, has anyone here read Nozick's Anarchy, State, and Utopia?

@AlfredCentauri I didn't read the comments prior to that.

@AlfredCentauri But to say that such a system is unjust is a value judgement - you proclaim that it is unjust to punish "innocents". Others might proclaim it just to punish the parents since they failed to protect their kid. If there are possible personal beliefs that might lead to a situation be called just and unjust by people holding different beliefs, then it follows that just and unjust are not objective criteria.

@ACuriousMind 17 pages until I can move on to properly learning SUSY!!

@0celo7 The ending...

Tell us what happens after that.

@0celo7 ...which you will learn from which text?

@ACuriousMind Lecture notes provided by Micheal Jordan Jr.

@Icosahedron Don't remember.

@ACuriousMind The parents in my hypothetical are not being punished because they failed to protect their kids. This is important and crucial. The parents are being punished instead of the kidnapper for the actions of the kidnapper.

@0celo7 Stop referencing under a pseudonym.

@Icosahedron Nah.

I feel left out. ;(

@Icosahedron Oh you don't know it?

lol

Not the Jr. at least.

We only know two dutchmen

I didn't know that there were two.

Gerard and Danu

Clearly one is junior to the other

@AlfredCentauri But it is not even universal (hence not objective) that that should matter. For example, consequentialists do not acknowledge that reasons matter at all.

@0celo7 What about horseradish?

@Icosahedron Maybe

So I'm not far from mayonnaise^^

$\infty$, easy. Holy crap

D:

@0celo7 Yeah, I did not find an upper bound

@ACuriousMind This functional measure is ridiculously complicated.

today is a boring day.

@Icosahedron No.

:(

@ACuriousMind I do not wish to take a tangent here with competing ethical theories (though that would be fun if I had the time). To get back to the subject, I initially wrote that "punishing the innocent is unjust". Two responses were that innocence is subjective. I've given a counter-example. Further, I hold that any theory of justice that finds the hypothetical situation given to be just is necessarily false.

@ACuriousMind "Now the total measure is written as $J\sqrt{\operatorname{det}(\Phi_r,\Phi_s)}\mathrm{d}^nt \mathcal{D} \delta\phi\mathcal{D}\delta\tilde v \mathrm{d}^k\delta a\mathcal{D} \delta\tilde X$"

What is that.

@Icosahedron (14.70) in Nakahara. It's (part of) the integration measure of the Polyakov string path integral.

"summing over all string configurations" is actually a very difficult thing to do.

The latter part looks like random letters.

Are you retaining that for the future?

No, it's a step in a larger calculation.

@AlfredCentauri Ah, then I think the "issue" might be that there are different levels of "objective/subjective" floating around. You hold an ethical theory within which there is an objective conception of justice. I'd interpret that the objections to you were more in the vein of pointing out that yours is not the only ethical theory, and perhaps not the only one to have objective justice, and that one thus cannot consider "justice" to be an objective concept (on a meta-ethical level).

I'm doing electrodynamics currently.

Almost done griffiths text, thus I think it's better if I complete it.

@Icosahedron The problem is that schematically, when calculating the string amplitude, we have to sum over topologies and metrics. The topologies part is easy. The metrics part is harder, because we have to worry about double counting and orientations in spacetime.

Then we have to try to dodge anomalies too.

I understood that.

@Icosahedron I'm not sure you can say this without knowing (at least) QED path integrals.

Forget it...

@Icosahedron The traditional phrase to use would be "Hey, I understood some of these words!" ;)

It's a start.

@Icosahedron nice profile pic!

@ACuriousMind Commentary: even nicer profile pic meatbag!

That guy is awesome

@ACuriousMind Holy crap, done. (I skimmed the computationally intensive parts of the last chapter.)

Dude, just saw a cool trick in a World Building answer!

That "Click to Show Math" thing is pretty nifty. Could be useful around here sometimes if there's long stretches of equations

@tpg2114 It seems to be a feature by Mathjax / Latex.

$$\require{action}
\toggle{
\begin{array}{cl}
& \bbox[2pt,color:black;border-radius:3px;box-shadow:4px 4px 8px black]{
\verb/Click to show math/}
\end{array}
}

Source of post: worldbuilding.stackexchange.com/revisions/…

@AlfredCentauri As @ACuriousMind already pointed out, I think we are/were simply talking past each other. Even if there exists an ethical system that one may objectively call unjust (which I'm not saying), this would not mean that all ethical systems (or notions of innocence) can be classified as either just or unjust in an objective way. No number of examples can prove such a "for all"-type statement, as you surely know.

@ACuriousMind between you and me (and everyone else who reads this :-P) I suspect we will see more of this in the future. Such is the pattern so far.

@ACuriousMind lolmythesis.com

In other news, a post on Physics Forums about askers who don't thank answerers and whether this constitutes rudeness. Something for us to always have in the back of the mind as well.

I don't know if it's such a big problem here.

@DavidZ I think the accept is like a thank you right? I mean, i thought we were supposed to dispense with all salutations, thanks, etc. and the "thanks" comes from upvotes and acceptance, ie recognition and confirmation that an answerer's answer is valuable, which is a sort of implicit thanks

Hey, guys I want to apply for International Physics Olympiad. But I don't get what resources should I have to make it. Any help?

@ChrisWhite Kant's "Categorical imperative" is actually a subjective principle based on community shared acceptance.

@DavidZ lol

"also note that the physicists working at the particle collider are able to see the collision real time in 3d on their screen."...that isn't true, is it?

@DavidZ Yeah, I'm thinking the same way but I suppose it's a 'cost of doing business' type of thing.

@StanShunpike yeah

@ACuriousMind I dunno... they certainly can't see the collision, but there might be some kind of realtime status display of detector activity... the OP seems to have added in a link

@DavidZ I think the image that is visible there is not a real-time display, but an image of the jets generated from the recorded data afterwards, but...well, I don't know

@ACuriousMind What would it be a real-time display of? The jets aren't visible in the traditional sense, right?

@0celo7 Yeah. It might be a "real-time" display of the traced back trajectories, but I don't think we can calculate that fast enough to be real-time in any sense.

@ACuriousMind $\{,\}=0$ does not imply $[,]=0$, right? I'm not going crazy?

@0celo7 I won't pronounce on your mental state, but yes, stuff that anticommutes does not commute.

@ACuriousMind I think I've had two SUSY sources proclaim it does.

Time to crack open Weinberg for a 10 page explanation.

Oh, in the context of superalgebras you'lll often find the notation $[,]$ for the superbracket, which is the anticommutator for two odd objects

So there you need to be careful whether $[,]$ is the commutator or the superbracket

@ACuriousMind No one told me this...I think it's the commutator.

@ACuriousMind Essentially, I'm trying to figure out why the superderivative works.

@ChrisWhite What, don't you usually watch movies that are only nanoseconds long?

@ACuriousMind Ahhh, I think I found it. When we write $\delta\Phi=[\epsilon^*Q+\epsilon Q^\dagger,\Phi]$, $\epsilon$ is fermionic, so it produces a minus when moved past the fermionic superderivative. Maybe.

@ACuriousMind Weinberg confirms :D

::turns to OP, puts on Jack Nicholson voice:: You can't handle the answer!

@ACuriousMind Er ... for many experiments, the "online" reconstruction and event display can show you the events a few seconds or minutes later. Details vary from experiment to experiment, but this is considered a high priority tool as it helps shifters detect problems fairly early.

@dmckee Finally not just a guess, thank you!

@ACuriousMind Why does a(n) (anti)self-dual antisymmetric tensor form a $3$? I know that an antisymmetric tensor is a $6$, how does the duality condition half this?

I guess the duality condition matches up a component of the dual to a component of the tensor, right? How do we know that this exactly halves the $6$?

@0celo7 Do you mean by "form a $n$" that it has $n$ independent components?

@ACuriousMind Yeah, sorry. I'm reviewing my representation theory for SUSY.

@0celo7 How do we know? By inspection - write down what the dual is, and observe that you get half the d.o.f. as independent equations for a tensor that must be self-dual

@ACuriousMind Oh I figured there was some slick way.

Not that I'd know right now

@ACuriousMind What is a physical situation in which the identities $(\frac{1}{2},0)\otimes(\frac{1}{2},0)=(0,0)\oplus(1,0)$ and $(\frac{1}{2},0)\otimes(0,\frac{1}{2})=(\frac{1}{2},\frac{1}{2})$ appear? I know how to get them, but I don't have a feel for what they mean.

@0celo7 First one: The combined system of two (non-relativistic) spin-1/2 particles decomposes into a singlet and three triplet states. Second one: A Dirac spinor consists of two Weyl spinors.

And the latter should have $\oplus$ instead of $\otimes$.

@ACuriousMind You sure? Rules of addition of angular momentum works for $\otimes$.

@0celo7 No, I'm not sure, nvm

$\otimes$ is correct

:D

I can correct @ACuriousMind

(sometimes)

Then my interpretation for the second one is wrong, btw

I was wondering about that.

$\oplus$ would stack them like a Dirac spinor.

It then means that the combined system of a left- and a right-handed Weyl-spinor behaves as a four-vector.

When would this arise in QFT?

I don't think it does. At least I don't think I've ever seen it

@ACuriousMind Ah, Wald uses that identity to motivate writing two spinor indices on vectors I think. (I only skimmed his spinor chapter, so I'm not completely sure.)

Hey I was wondering how to derive the formula $\Phi = BAcos \Theta$ from $\Phi = \int B ◦ n dA$

@Howcan Take $B=\text{const}.$ and do the integral. Remember the dot product angle rule.

@Howcan $|\hat n|=1$. So $\mathbf{B}\cdot\hat n=|\mathbf{B}||\hat n|\cos\theta=B\cos\theta$

Yes.

Thanks, this was embarrassingly simple.

hi guys can I can inviti somebody to the chat?

@havarka Yes. Click on the name of the target user and select "start a new room with this user".

but hi is not yet in chat, :)

he is in forum

just

Oh, to this chat?

There is a way, let me think.

to any

Can't you just copy and paste the link to the chat? Worked for a friend of mine.

@Howcan Probably. @havarka I can't remember how to ping a user who isn't in here already.

@0celo7 You cannot, if they haven't been in here for the last few days.

@ACuriousMind I was invited here somehow.

@0celo7 I bet someone just left you a comment on one of your posts.

@ACuriousMind That might be the case, but I thought it sounded like an official "user X has invited you to join chatroom Y".