The h Bar

5:00 PM

(I got him straight about misrepresenting what tpg says, but he's correct, the hw policy is murky when it comes to mathematical physics)

@Sklivvz yay. WFM

In the future, use current in place of the userid for a personalized-per-click link :)

tpg2114

So the rest of my transcript there for those who don't want to read it is whether "Help me in my derivation" is on topic or not

ManishEarth

...you probably know that

For example, look at these

Arc676

@DavidZ That's definitely very unlikely, but how would I represent the oscillation? And the W boson exchanged by the two neutrinos will have to carry a charge right? And wouldn't this violate the law of mass conservation? How can two neutrinos (very light) turn into an electron pair (heavier)?

Crazy Buddy

Well, I'm off to dinner (though I didn't message much, I did watch all of your speech people) :D

C'yall later ;-)

Arc676

@CrazyBuddy lol...

Bye @CrazyBuddy

ManishEarth

5:03 PM

Basically, what happens to questions in the more mathematical branches of physics where a student, after putting in work, is stuck on a step.

Here, the concepts may be physical but superficially mathematical

(i.e., math.SE won't take it)

Sklivvz

Isn't the "show no effort" rule enough to close all bad homework questions and leave the one you link above open?

ManishEarth

@Sklivvz ...that's changing

Sklivvz

what is changing?

David Z

@Arc676 Hm, I'm actually not sure how to represent neutrino oscillation in a Feynman diagram. Also, there's no law of mass conservation. You have to remember that the intermediate particles in a Feynman diagram aren't really particles, they're disturbances in the underlying quantum fields. So they don't have to obey the usual rules where higher-mass particles only turn into lower-mass particles.

ManishEarth

19

Q: Why don't we just ban homework altogether?

This site gets a lot of low-quality homework questions. Since I ignored the homework tag a few months ago, my experience of the site has been much more pleasant. The current homework policy is nice, but the questions that actually obey it are few. I think this is because if you've put the effort...

le new policy

Sklivvz

5:07 PM

yeah i tend to agree with david's answer there

tpg2114

@ManishEarth I think even in fluid dynamics you see questions like that a lot. Like "why did this term go away" when the answer is "Because Re \to \infty means it's smaller than other terms and can be neglected." The answer is purely math but it's based on purely physical insights

ManishEarth

@Sklivvz Reading list of all major recent meta posts: Let's have a journal club, book policy, ban homework, improve participation, improve own experience

@tpg2114 Ok, so how do we identify these?

tpg2114

My point in the conversation before was that just because it's newtonian mechanics asking for the same kind of physical insight, we can't just close it because it's "basic" to us but not to the OP

No more than we can keep open a ST question just because it's "advanced"

ManishEarth

True

The same policy should apply

David Z

I think the threshold for being too low-effort should scale somewhat with how advanced the topic is.

ManishEarth

5:09 PM

So...how do we make insightful steps more conceptual?

Or do we allow these as is?

@DavidZ yes

tpg2114

Yes, that's my only concern and the concern I was trying to bring up in chat before

David Z

BTW: book policy is edited, see what you think

Arc676

@DavidZ I didn't mean the intermediate particles, but the pre-final state electron pair. If we assign the number 1 to the mass of a neutrino, we have a mass of 2 units at that point in time. If these two neutrinos turn into an electron pair via a W boson, won't we have an incredible increase in mass? Or do the electrons also count as intermediate particles? But even if they do, won't the final state photon violate the law since it has zero mass?

tpg2114

@DavidZ Does it though? Isn't that way too subjective? ST to one may seem trivial but fluid dynamics to another seems virtually incomprehensible. Isn't that really in the eye of the asker (so not the one who votes to close)?

David Z

@Arc676 You can't consider mass at all in the intermediate "steps" of a Feynman diagram. All that matters is whether the final particles have a lower mass than the initial particles.

tpg2114

5:11 PM

I remember in high school being totally baffled by things that seem totally obvious now

David Z

@tpg2114 Even though different people understand different things, there's still a general agreement that string theory and fluid dynamics are more advanced than Newtonian mechanics.

So I personally tend to be a little more forgiving of homework questions on advanced topics. But then again, that's probably because such questions tend to be better explained anyway.

tpg2114

My only point is that if we want to welcome and encourage people to ask good questions and learn from them, the level of the topic shouldn't matter, just the quality of the question

Arc676

@DavidZ I'm drawing the diagram. Can you have a photon in a final state even though it has zero mass?

tpg2114

People learning (and writing good questions) about Newtonian mechanics today are the ones learning and asking good questions about ST, QM, etc later

David Z

Hm, yeah. It's not really that much of a difference. Questions that should be answered by a Google search or by reading a standard textbook get closed no matter what level they're at.

ManishEarth

5:14 PM

If anyone wants to join in, Dilaton has some points to make here

David Z

@Arc676 Yes, of course. How else do you think things like Compton scattering or e+ e- annihilation could be represented?

Arc676

@DavidZ I always thought in e- e+ annihilation you needed the photon to pair-produce the original electron-pair

David Z

@Arc676 Nope. Any e+ and e- can annihilate.

Sklivvz

@DavidZ Reading a standard textbook is ...a tricky guideline. Not all textbooks are clear or available to all students. Wikipedia is somewhat complete but often it's too complicated for non-specialists

tpg2114

@Sklivvz I agree with that too.

@DavidZ But easily Google'd is off topic regardless

Arc676

5:17 PM

@DavidZ So I can actually draw an electron-pair annihilate into a photon, leave the photon by itself in the final state, give it to a physicist, and say that the mass of the leptons is stored in the energy of the photon?

David Z

@Sklivvz The material covered in standard textbooks on basic subjects is, well, standard, though.

user54412

wayyyyyyyyyy too much transcript

user54412

Flavin

@Sklivvz If a question says "I was reading such-and-such book and didn't understand this concept..." wouldn't that be a fine question?

ManishEarth

@ChrisWhite your fault for coming late

Arc676

5:18 PM

@ManishEarth LOL...

David Z

@Arc676 No, you need two photons. If you're literally doing just e+ e- -> photons, you need two of them in the final state. If it's part of a larger Feynman diagram, then you can potentially have e+ e- to a single photon.

Sklivvz

@Flavin I totally think it would, iif the user can actually explain what the problem is

ManishEarth

@ChrisWhite We have 7 meta-issues to discuss, and @ShuklaSannidhya and @Arc676 have physics problems. So...long transcript.

@Flavin yes

conceptual :)

Sklivvz

"I was reading chapter x and I didn't understand topic y" is way too lazy :-)

tpg2114

@ChrisWhite Book policy good with minor tweaks; Manish will think about posting whether we should revise the list policy or not; Physics questions+multiple conversations abound

@ChrisWhite Discussing homework policy now

Arc676

5:20 PM

@DavidZ So a simple "e- e+ > photons" diagram would be like a big X in which the "legs" are an electron, a positron, and the other two photons?

user54412

so did I read that right - the consensus is to not CW book q's???

tpg2114

Yes

user54412

Can I add my own plea for if this goes through? Please make any list question CW, no exceptions. All too often, both on this site and others across the network, users get enormous boosts of quite frankly undeserved rep from asking/answering these questions. Now the reputation system isn't perfect, but these people completely break its functionality in discriminating between trusted answerers/insightful askers vs. random posters. — Chris White Jul 15 at 18:58

David Z

user54412

I'm gonna repeat that ^ ^

David Z

5:21 PM

@Arc676 ok I think I finally got it ;-)

ManishEarth

@ChrisWhite great, and we thought we had consensus

user54412

if we host quora-style questions, we're gonna get votes from everyone and anyone who used a book in college

2

David Z

I'm still on not allowing list questions at all...

ManishEarth

you and schizo-@tpg, always breaking the nice-nice unanimity of the site

user54412

I always thought books \subset lists

Arc676

5:22 PM

@DavidZ The line in the middle is not labeled because it can be either e- or e+ right?

David Z

@ChrisWhite No, with the new book policy, that shouldn't be the case anymore.

@Arc676 Uh, yeah, you can think of it like that.

tpg2114

@ChrisWhite I think the thought process is there won't be a single-book-per-answer type answers

And that answers will have the book(s) but also insight into why they are good or bad

ManishEarth

/me thirsty

tpg2114

Which also means that OP must specify what they need in a book

Arc676

@DavidZ You said that if it was part of a bigger diagram it was OK to just draw one photon. What is the exact criteria to draw a single one?

ManishEarth

5:23 PM

@tpg2114, with your hallowed fluid dynamics powers, please make clean water flow from the nearest tap into my mouth!

Sklivvz

@ChrisWhite if we get huge list questions of books and they suck we'll disallow them

Arc676

@ManishEarth Wouldn't it have been simpler to say "telekinesis"?

ManishEarth

If you throw enough upside-down deltas at the problem it should solve itself

tpg2114

@ManishEarth I had a joke but it might be kind of offensive (cultural insensitivity) and I don't want to upset anybody

David Z

@Arc676 The rule is simply that the total mass of the final particles need to be less than (or equal to) the total energy of the initial particles. And that itself is just a consequence of energy conservation.

user54412

5:25 PM

and what if we get small list questions of books, and whoever posts "Griffiths" first gets +100 from every college freshman who wanders by the site?

tpg2114

@ChrisWhite and others -- I've dug through most of the SE sites looking for comparable policies to what I think the books should look like

ManishEarth

@tpg2114 ...about the purity of water in India? :P I'm usually not sensitive to thes things.

tpg2114

The most similar I could find is:

ManishEarth

@ChrisWhite yes

tpg2114

34

Q: Are game recommendation questions on topic?

Many Stack Exchange sites, like the similar gaming.SE, do not allow game recommendation questions (aka shopping questions) because they pose an intractable quality problem. And this is after long, contentious discussion and learning on the topic. Should RPG.SE similarly reject game recommendatio...

discussion faq on-topic game-recommendation

@ManishEarth I wasn't worried about you, but you know the joke anyway

ManishEarth

5:26 PM

hah

Arc676

@DavidZ Then how can an electron-pair annihilate into a Z boson which then decays into a muon-pair if muons are heavier than electrons? Or is this impossible?

David Z

@Arc676 If the electron and positron have enough kinetic energy such that their total energy is more than that of a muon and antimuon, then that can happen.

Arc676

@tpg2114 I thought one of the rules for posting questions was not to post questions that were too opinion-oriented

@DavidZ Great! Thanks :D

David Z

@Arc676 One of the rules is that most SE sites don't follow the rules ;-) (well, that's somewhat of an exaggeration)

tpg2114

@Arc676 That is, but things like references and books tend to be opinionated. There's millions of books on fluids, so what I say is the best for understanding boundary layers may not be what others say

Which is why OP has to be specific in what they need and not just "Where can I learn about X"

Arc676

5:28 PM

@DavidZ Well, rules are "made to be broken" which is very true in some cases with some people I know

@tpg2114 OP? What does that stand for? I might have missed it

ManishEarth

Original Poster

the asker

Sklivvz

@ChrisWhite it depends. It's ok if there are popular or easy questions. every site has them. It's not ok if the questions become an easy way to get rep on a grand scale and take over the site.

Arc676

Oh

ManishEarth

@Sklivvz TAKE OVER

tpg2114

@Sklivvz I thought the way to take over was get a diamond and then squash all dissent?

Arc676

5:30 PM

@DavidZ If you need the final state to weigh less than the initial state, how does vector fusion work? Does the produced Higgs need to decay almost instantly?

David Z

@Arc676 Yes, exactly

Arc676

@DavidZ Same with associated production right?

ManishEarth

@tpg2114 you need rep to get diamonds

Sklivvz

@tpg2114 LOL or, get hired as a dev, you get a super heavy diamond that beats mods and community managers (and you don't have to get elected either).

David Z

@Arc676 Yeah, that sounds about right

Arc676

5:32 PM

@DavidZ OK

tpg2114

@Sklivvz Can we just call that the Sklivvz method? :)

David Z

So where do we stand on homework right now? I've kind of lost track of the discussion

ManishEarth

@Sklivvz do you really want SE to start running f97?

tpg2114

@DavidZ We all lost track of it... It got sidetracked by the CW issue

ManishEarth

@DavidZ we stand on a tpg position ... undecided

@tpg2114 CW's CW issue

No wonder he likes CW so much.

tpg2114

5:33 PM

@ManishEarth Touche

user54412

@ManishEarth we're standing on tpg? Does that hurt @tpg2114 ?

ManishEarth

hahaha

David Z

OK, well... let it be known that I think any question should either be acceptable as a non-wiki question, or not acceptable at all.

ManishEarth

@DavidZ but why?

user54412

@DavidZ I agree on an individual level, but having multiple people making a value judgement together, even if those people only make binary judgements, opens the door for a third option

ManishEarth

5:35 PM

> Community wiki is like a cheese knife: it is a specialized tool to be used sparingly.

user54412

there are things I think are unacceptable, but if the majority accepts them, i may prefer them to be in a gray "acceptable but CW" state, and the majority may be ok with that

David Z

@ManishEarth Because the kind of questions that prompts one to say "this should be CW" are lists and polls and such things that the SE model was never intended to take.

ManishEarth

@ChrisWhite And that was exactly the problem with CW

David Z

"questions rarely, if ever, need community wiki" from the same post

ManishEarth

> Community wiki should never be used as a get out of jail free pass

out of context, yes

tpg2114

5:37 PM

Like I said before, I think the guys at RPG.SE have a decent model with their system recommendation tag. They don't CW the questions and the asker has to be very specific in what they are looking for and why, and the answerer has to be very clear in how their recommendation addresses the issues

ManishEarth

But I resonate with that. We shouldn't use CW as an "Okay, I will allow this, but only if CW, because CW makes things OK"

tpg2114

So for books it's the same. "What should I read to learn X" is off topic, whether it's Newtonian Mechanics or String Theory. But "Which books have a mathematically rigorous approach to String Theory" is sufficiently on point to be answerable

Without being a rep-machine

user54412

i'm still very shaky about this - wonder what @dmckee has to say?

David Z

@tpg2114 I think something more specific like "What are the standard introductory books for string theory?" might be okay.

ManishEarth

@tpg2114 wait, maybe if the question doesn't explicitly state which type, it's still OK

dmckee

5:43 PM

I have been and still am strongly against allowing any kind of make-a-list.

user54412

also, i'm very much inclined to downvote any and all recommendations for Griffiths, Jackson, Halliday/Resnick, Weinberg, Landau/Lifshitz on principle

ManishEarth

As long as the banner states that the type must be mentioned

tpg2114

@DavidZ Only because "Introductory" is sufficiently on point

ManishEarth

@ChrisWhite Goldstein? That one is nice

dmckee

I feel that with any policy you write you are going to have people rules-lawyering at you about why their marginal questions ought to be OK.

user54412

5:44 PM

@ManishEarth oh that too - forgot about that

ManishEarth

Neumaier's book on Classical is actually pretty good though.

David Z

Yeah... as long as the question is written in such a way as to encourage each individual answerer to provide a complete overview of the books that are out there, it's probably okay with me. And that should be the intent of our new book policy.

dmckee

And it becomes and endless game of whack-a-mole governed by which users and mods are on the site just now...

Flavin

@ChrisWhite What if the answer cites specific chapter/section/derivation from one of the famous books and why it applies to the question?

dmckee

Then come the allegations of favoritism.

ManishEarth

5:45 PM

@dmckee which is why I tried to make the policy very objective with no wishywashyness

tpg2114

@dmckee Or accusations of self-promotion

user54412

@Flavin I just think those are universally terrible books to learn from, for differing reasons, and I can't in good conscience recommend students use them

David Z

@ChrisWhite well if someone asks for a general QM/EM/Newtonian/QFT/everything book (just kidding about that last one) then those would have to be on any self-respecting list. But if someone just posts a link to Griffiths' book's page on Amazon, then sure, that gets deleted (not just downvoted).

user54412

allowing book questions opens the door to opinions like mine, and i'm not sure that's what we want

ManishEarth

If you had hired the spiritualist who was spamming Stack Overflow last week, you might not need the divorce lawyer spamming today.

Tweeted by lizardbill on November 5, 2013 at 5:05 PM

everyone, remember the black magic spammer?

dmckee

5:47 PM

@ManishEarth The way I see it "objective" is much much harder than you think. People are going to be coming at this list of rules with different experiences, different expectations and above all slightly different understanding of the denotation of words than you are using.

tpg2114

@ChrisWhite So long as you can defend why you feel they are terrible then it's a useful answer no? I wouldn't recommend somebody new to boundary layer theory read Schlicting; but somebody looking for advanced books about BLT, that's the defacto standard

@dmckee Which is why real lawyers have to write 10 pages to say "Don't steal."

dmckee

...ans some won't even be intellectually honest in the way they interpret them or argue in favor of their own question.

ManishEarth

@dmckee Except that the proposed policy doesn't restrict questions much. It restricts answers! And it identifies what elements need to exist there.

dmckee

@tpg2114 Exactly.

ManishEarth

@ChrisWhite controlling voting here can be a pain, yes

CW makes sense from that POV

dmckee

5:49 PM

Just be aware that I'm going to say "I told you so" if it turns out I was right all along...

2

ManishEarth

even if the answers are substantiated the voting will probably go down the "my favorite!" alley

tpg2114

@dmckee That's the God-given right of the minority isn't it? Otherwise being the minority is never fun

David Z

@ChrisWhite well, the whole point of the new book policy is that it doesn't leave the door open for opinions. Look at this answer of mine for example. Even if you wouldn't recommend one of the books posted there, you should recognize that it's a good overview of the common options out there. (I think.) The point is, it's not the kind of answer that encourages people to downvote just because they disagree with one of the recommendations.

ManishEarth

@dmckee haha fine

@DavidZ now CW will downvote it to prove you wrong, I bet :P

David Z

lol

Well anyway, I have to go for now, but I'll hang around the chat room as much as I can later today

ManishEarth

5:53 PM

k

Alright, there's a lull. Lemme see what my meta-to-do-list is:

- Post something about downvoting and VTCing homework on the [improve participation post](http://meta.physics.stackexchange.com/questions/5139/). Also something about VTCing in general.
- Have a good look at lists. Explore the network, post findings later.
- Continue discussion on homework. Try to figure out objective criteria for mathematical-physical-insight
- Topic of the week? Anything to be done about that?
- Journal club! Post thoughts there

(Feel free to chip in for these :D)

Also, which posts (if any) do you think should be featured?

crickets

Flavin

I had hoped to talk about journal club here, but I guess we should keep it in meta.physics.stackexchange.com/questions/5144/…

ManishEarth

@Flavin no, we can discuss it here!

and then distil any useful thoughts and post as a comment or answer there

What are your thoughts?

Flavin

Mostly that I support the idea, but I don't have any paper recommendations.

Part of why I want to be involved is to help keep up with the literature, which is something I'm terrible at on my own.

ManishEarth

@Flavin yeah!

What would be nice are some papers that are understandable to a wider audience

Flavin

How wide? Experts outside the nominal field of the paper? Undergrads? Interested laypeople?

ManishEarth

6:08 PM

@Flavin not laypeople. as many from the other two as possible

(I myself am an undergrad, though I try to be able to understand higher level things :P)

tpg2114

I would say outside the normal field is good. Undergrads maybe not so much

ManishEarth

yeah, maybe not so much

tpg2114

Advanced undergrads perhaps -- ie. ones who go out of their way to learn beyond undergrad curricula

ManishEarth

be back in a few mins

@tpg2114 were you around when we discussed contacting the author?

Maybe for the first few, we should contact the author after the journal week is over

David Z

6:30 PM

We could discuss articles that have been featured in blog posts, APS viewpoints, that sort of thing, so there is a more generally accessible description out there

ManishEarth

yep

Arc676

Is there any way to have a Higgs boson in the final state of a feynman diagram?

ManishEarth

@Arc676 Why not? What do you think the folks at CERN did?

Arc676

@ManishEarth Didn't the Higgs decay into other stuff immediately after?

ManishEarth

sure, but not immediately

And feynman diagrams need not represent the reaction all the way through

Arc676

6:46 PM

@ManishEarth That instantly solves this question and a lot of related ones

ManishEarth

woo

Arc676

If Feynman diagrams don't need to represent the whole thing, then lots of diagrams that seem to violate the law of mass conservation could be drawn

ManishEarth

@Arc676 no not that way

Arc676

Why not?

ManishEarth

wait, maybe

Arc676

6:47 PM

Obviously you can't have ANY diagram

ManishEarth

I was saying that a diagram that doesn't put stuff into its final decay products but does conserve mass and all the other things is ok

Arc676

but diagrams showing processes like vector fusion can be drawn without showing the decay of the higgs boson at the very end

user54412

the last time someone asked about what could be freely propagating out of a feynman diagram, the result was general disagreement

ManishEarth

for example, even though muons decay, you can have a muon exiting the diagram

@ChrisWhite hmm

Arc676

@ChrisWhite What do you mean "freely propagating"

And about the question linked, aren't W and Z bosons detected via their ENERGY since mass/matter and energy are interchangeable?

user54412

6:49 PM

@Arc676 meh - i'm being loose with terminology - something like "asymptotically approaching the mass shell" perhaps

Arc676

Mass shell?

ManishEarth

On shell and off shell

In physics, particularly in quantum field theory, configurations of a physical system that satisfy classical equations of motion are called on shell, and those that do not are called off shell. For instance, in classical mechanics in the action formulation, extremal solutions to the variational principle are on shell and the Euler-Lagrange equations are on shell equations (i.e., they do not hold off shell). Noether's theorem is also another on shell theorem. Mass shell The term comes from the phrase mass shell, which is a synonym for mass hyperboloid, meaning the hyperboloid in energy-m...

Arc676

Oh

ManishEarth

The "classical" (non-quantum) curve

Quantum mechanics lets you leave the mass hyperboloid in E-p space

To get back to the classical state, you must approach the shell

Arc676

If you were to make a Feynman diagram to represent a full reaction, are there any particles that cannot be alone in the initial state?

Like photons for example, cannot be alone in the initial state without some other particle, right?

ManishEarth

6:52 PM

why not?

can'y you have pair production with a photon?

Arc676

First of all the tutorial said so, and second of all, I find it counter-logical to have a photon (mass zero) become anything else without some other particle, because the final state has to have equal or less mass than the initial one

So to have pair production don't you need some other particle? (Note that I did specify a FULL reaction)

user54412

certainly you can't have a "complete" reaction that has a single massless particle ingoing and some massive stuff outgoing

user54412

@Arc676 indeed - in scintillators and such it's usually the nuclei that are around (I think), whereas in the region around a neutron star it's the magnetic field itself that deals with the momentum conservation

ManishEarth

@Arc676 mass zero? but energy!

mais non! energy!

@ChrisWhite wait, why not?

Oh, momentum and energy don't hold up

Arc676

@ManishEarth So you CAN draw a diagram that starts with a photon (mass zero) and end with mass by assuming the energy of the mass come from the kinetic energy of the photon?

ManishEarth

6:56 PM

I'm probably confusing you here, listen to Chris

Arc676

Um, OK?

user54412

@ManishEarth slip into the center-of-momentum frame of the electron/positron - in this frame the photon couldn't have had momentum, so it couldn't have existed

user54412

@ManishEarth i'm probably confusing myself

Arc676

Center of momentum?

ManishEarth

@ChrisWhite Why not? Why can't the e and e+ move with an angle to each other?

Arc676

6:58 PM

What exactly ARE e and p?

Wait, isn't p momentum?

ManishEarth

electron positron