Conversation started Dec 29, 2015 at 16:11.
Dec 29, 2015 16:11
Anyway. We have a topic to discuss today!
@DavidZ Go on then ...
@DavidZ My dad had some during his stay in Taiwan (military) and he said it's not very good, at least for his western tastes
vzn
vzn
@0celo7 "not very good" how?
@DavidZ Yes, my attempt to prove the Lebesgue number lemma
I think it's about time we start thinking about broadening the homework-like close reason. A lot of the questions we close as homework-like these days are not homework, or anything close to homework, and aren't even meant to have educational value.
Dec 29, 2015 16:12
or that
@vzn anything that eats only meat isn't good eating. The protein breakdown products make the meat rank.
Instead it seems like we tend to close things that are pure calculation requests, where the asker is trying to get us to do some math for them, rather than asking about some concept so they can do the math themselves.
@JohnRennie BTW, Kahlers are according the the answers (quick read) a strict subset of Riemannian manifolds?
@vzn Let me go ask him
@TerryBollinger Yes.
@JohnRennie Yep, tensors can be rank too...
Dec 29, 2015 16:14
All manifolds can be made Riemannian, but only some manifolds can be Kahler.
So it would make sense to introduce a new close reason that talks about calculation requests being off topic. This would replace the current homework-like close reason, and to go along with it we would replace the current homework policy with a new policy prohibiting calculation requests.
@TerryBollinger Boom boom
This implies they are a subset of Riemannian ones.
At least smooth paracompact manifolds can be made Riemannian.
At least, that's my thought. My first thought. What about everyone else?
@JohnRennie Rank?
Dec 29, 2015 16:15
@DavidZ I think everyone refusing to do math for others would help... this site should be concepts and good pointers...
What's the most general topology you can have for GR, though
Conifolds with boundaries?
@Slereah Hausdorff, paracompact
I've seen non-Hausdorff manifolds used
@TerryBollinger Yeah, I suspect people mostly agree on that. We need to work out the details of what exactly would be on or off topic under this new policy.
vzn
vzn
@0celo7 why are you interested in Kahler manifolds?
Dec 29, 2015 16:16
Hausdorff is not very physical, also like all of Riem Geo breaks
Huy
Huy
where does one use non-Hausdorff in GR
@0celo7 sort of sour tasting. It's hard to describe tastes.
Rarely.
The Taub-NUT spacetime isn't hausdorff in parts
@DavidZ Warning folks that calculation requests are subject to quick downvotes, and encouraging such downvotes, might help even without an official rule.
Items of particular interest: (1) are there a significant number of questions which are off topic under our current homework policy which would be on topic under the new policy? If so, should they be on topic? (2) exactly what sorts of questions would become off topic under the new policy which are not covered by current close reasons?
Dec 29, 2015 16:17
@DavidZ I feel as though I've tried many times to come up with a good alternative and failed
@JohnRennie @vzn All he remembers is that it tastes funny somehow.
@TerryBollinger right, but we can do that already. (I certainly favor continuing to do so) The reason I'm proposing this is that I actually see the homework-like close reason being used, in "the wild", for calculation requests that are not, strictly speaking, homework-like by our current policy.
And they always put so much sauce/stuff on it that you didn't really taste it
@JohnRennie yeah, that's why it takes a whole community ;-)
@JohnRennie I think the American phrase, somewhat dated and localized, is "roof rabbit."
Dec 29, 2015 16:18
Roof rabbit?
This is the start of a process, FWIW. Don't expect that we'll be dumping the HW policy any time in the next month or two, at least.
@0celo7 Yup!
As the official American, I've never heard of that before.
Complex conifold with boundaries and a general connection
@0celo7 me neither
Dec 29, 2015 16:20
Probably not a good idea
@DavidZ I like the goal. To me, any concept question is OK, even if the understanding behind it is very weak. A good answer to a weak concept question can have high value. Do a calculation, though, and all you did is give away a fish, not teach how to fish.
I think the "unclear" reason also gets used to mean "this is a s**t question" rather than that it's actually unclear.
Oh and the metric can have a dynamic signature
So what is the most general type of manifold applicable to GR??
@David Z : maybe "calculation request" should be replaced by "mathematical assistance".
Dec 29, 2015 16:21
There is an extent to which we (well I) see questions that I think are rubbish and cast around for a reason to close them.
@JohnRennie One thing I've noticed about some very naive but sincere questions is that they can uncover surprisingly common misconceptions that otherwise can be overlooked.
Well pure GR is a hausdorff second countable connected manifold with a Levi Civitta connection and a Lorentzian metric
Closing the question seems more important than specifying the precise reason for the closure.
@TerryBollinger Well, I mostly agree, but that's another story... taking that as a premise, though, I think that sidesteps the issue a bit. Under our current policy, homework-like questions which are asking for a conceptual explanation of something are still supposed to be off topic, but it's not based on whether the asker's understanding is weak.
@Slereah what does that even mean
Dec 29, 2015 16:22
You can probably get rid of connected but beyond that I wouldn't call it GR
The main reason I want to close questions is that they're lazy
@Slereah there's no point there
@0celo7 ditto?
That is the OP couldn't be bothered to think through what they are asking
@JohnRennie indeed, and that happens on all sites. It's kind of part of the design of the system. The idea is that, when people find themselves closing a lot of questions for a particular reason, then you make that reason one of the standard reasons in the list.
Dec 29, 2015 16:23
@0celo7 The signature can vary from point to point
@Slereah what does that even mean
Let alone take the trouble to Google it before posting
It can be (+---) at one point and (++--) at another?
@JohnRennie which brings me to my next thought: we could go so far as to define an "insufficient effort" close reason, which would include calculation requests and other questions that are lazy
@Slereah what does that even mean
Dec 29, 2015 16:24
@JohnRennie That's certainly a point: Anyone too lazy to Google a question likely deserves quick, quick closure.
5
Actually that's not such a bad idea ...
I'm not sure how to break it down further
After all, asking us to do homework is lazy
Asking us to do a routine calculation is lazy
Asking us to Google is super lazy!
Not bothering to use correct grammar and punctuation is lazy
Dec 29, 2015 16:25
It'd be a bit different, though. I think there are some questions of the form "I want to calculate [thing]. I tried [X], [Y], and [Z] but I don't know how. Can you just do it for me?" which I think should be off topic, but an insufficient effort close reason wouldn't include those.
And so on
@DavidZ e.g. this question physics.stackexchange.com/q/153715/46304 I answered a while back was just calculation. Should it have been closed?
@Slereah what is the physical meaning
vzn
vzn
@Slereah funny, but ??? ... did enjoy the youtube video on LHC as opening up "portals" etc
@JohnRennie Sometimes grammar is language barrier though...
Dec 29, 2015 16:25
what you're saying is equivalent to "the cohomology is the dual of the homology"
sure the signature varies, but that has no meaning
unless you point it out to me...
@TerryBollinger sometimes yes, but when the text isn't even properly capitalised I doubt it
@alarge well, that's the thing we have to figure out. I suppose that one is close to the boundary of what would be considered conceptual.
the dimensionality of space and time would vary
@JohnRennie Agree, just making the point. I've seen a few questions, hmm mostly years ago, that were actually good but very hard to read in the English rendition.
what does that even mean
stop making vacuous statements
Dec 29, 2015 16:27
Well you asked for a more general version of the spacetime
no?
You can do the work to check the physical consequences
when did I do that
@JohnRennie, can you give an example of something from GR that is "interesting" in the way you were talking about earlier, phenomenologically?
Eh whatever
Dec 29, 2015 16:29
For me the main sin is laziness. I'm actually not that fussed about a calculation type question is the user has put ina lot of effort already.
I asked what happens if we restrict our attention to Kahler manifolds
@JohnRennie what is the main cos
Yes, if it's gratuitously homework I'd still close it
But the example that alarge gives would be fine by me
@0celo7 Are the Kahler manifolds sufficient? Is GR over-generalized at present?
@TerryBollinger I don't know. Probably not.
And sometimes I want to see how the calculation is done - even if it's a bit routine
Dec 29, 2015 16:30
@alarge What I have in mind when I say "calculation request" is not so much something like that, but the questions that ask us to do some basic physics. Like no-effort homework questions, for example, but also something like this
Well, actually GR is not really Riemannian geometry. It's Lorentzian, which is in some ways a generalization of Riem. geom. and in some ways a restriction.
@0celo7 Boom boom!
Generalization in the sense that we have the distinction timelike/spacelike/null and the consequences of that and a restriction in the sense that there are some Riem. manifolds that are not Lorentzian.
@DavidZ Right, but then I don't see why the current homework-like policy is not enough to cover these cases. Can you give an example of something that you think does not fall under the current policy but is something that should be closed?
@0celo7 That's interesting! So, the Venn diagram has an intersection of Riemannian manifolds and Lorentzian? What is that intersection called? Does it even have a name, other than "GR"?
Dec 29, 2015 16:32
@alarge the one I linked, in fact
It doesn't appear to be a homework question, nor for educational value
@TerryBollinger Riemannian metrics are positive definite
So I wonder if a Lorentzian Kahler manifold makes any sense. If it doesn't, then my question doesn't make sense, either.
While GR uses a -+++ signature
@TerryBollinger Well...any Lorentzian manifold is Riemannian.
But not the other way around.
@0celo7 That's not true!
Riemannian manifolds are positive definite
Dec 29, 2015 16:33
@JohnRennie We need to be more precise here.
He means topologically
@David Z : if I were you I'd be wary of making any changes that would make "serious" users unhappy. Instead of downvoting/closevoting a physics-maths question, other users could simply point out that this website isn't for that sort of thing. IMHO you should instead focus your efforts on the naïve/lazy questions from kids.
@DavidZ Granted, it is probably a problem the person formed themselved, but it looks like something that might appear in a book as well.
A manifold is a set $M$ and a metric $g$.
every lorentzian manifold can be equipped with a riemannian metric instead
Dec 29, 2015 16:34
If $M$ admits a Lorentzian $g$ there is also a Riemannian $\tilde g$
The converse is not true.
Ah OK, yes, I withdraw my objection
@0537 nicely symmetric icon there...
@alarge I think the issue is less that the current policy doesn't cover it but that "homework" is a bad name and the talk about "questions of primarily educational value" in the faq post does not reflect the kind of questions we mean and close with that reason.
@ACuriousMind hmm, you've been helmeted also...
@TerryBollinger Thanks I drew it myself.
Dec 29, 2015 16:36
@0537 Is it a particular object, or just an abstraction?
@TerryBollinger basically, to get a Lorentzian manifold you have to get a line element, which is basically a vector field which vanishes nowhere, and this is not possible on all manifolds
Because of the hairy balls theorem
(heheheh)
@Slereah probably knows the proper defintion of a line element in this sense
@TerryBollinger It's a special icosahedron I guess.
How about making the close reason: an lazy, idle and slapdash attempt at a boring question that could be answered by googling in half the time it took to write it
Dec 29, 2015 16:37
Line element is basically a directionless vector field
@TerryBollinger Of course! Winter time is hat time!
@JohnRennie I like that one...
IIRC it's a doublet of $(V, -V)$
@TerryBollinger BUT once you have a Lorentzian manifold we can do cool stuff like Lorentzian hypersurfaces and causality and all sorts of cool stuffs
@JohnRennie Hm...but those are closed with "insufficient effort", not "homework-like".
Dec 29, 2015 16:38
@ACuriousMind yeah, that alludes to something that @DanielSank suggested at some point: the wording of the homework-like close reason no longer reflects the kinds of questions we actually want to close
That way you don't have to worry about which way it's pointing
which is sort of the whole reason we're having this discussion
@ACuriousMind yes, but @JohnRennie got hatted mysteriously.
(You have to use a line element instead of a vector field in case of orientability shenanigans))
@ACuriousMind but the thing is, there technically is no "insufficient effort" close reason
Dec 29, 2015 16:39
Now that everyone is here, how is this off-topic?
3
Q: Cauchy stress tensor for a spherically symmetric problem

CaosGiven a sperically symmetric problem, I am asked to show that its Cauchy stress tensor, in spherical coordinates will assume the form: $${\overline{\overline{\sigma}}}=\sigma_{RR}(r)\overline{e_r}\otimes\overline{e_r}+\sigma_{\theta\theta}(r)(\overline{e_\theta}\otimes\overline{e_\theta}+\overli...

Currently, a question which merely shows insufficient effort is supposed to be downvoted, but our policies don't offer any justification for closing it
Well how is this off-topic:
0
Q: Can there be acceleration without velocity?

AmphibioIf I was in a spaceship continually accelerating at $9.81m/s^2 =1g$ in a straight line, I would reach near light speed within a year. On Earth, we are in a gravitational field of $1g$, which according to the equivalence principle means we are effectively accelerating at $9.81m/s^2$. What does ...

@DavidZ Well, I see it often enough as a close reason these days that it effectively is. We just don't have another custom slot for it, have we?
@0537 I dunno, it looks okay to me...
@ACuriousMind you mean, as a custom close reason? (which shows up in a comment)
@DavidZ Yep
Dec 29, 2015 16:40
@0537 do you know how to solve it yet
I've seen that too, and that's one of the big reasons I think it'd be a good idea to replace the HW reason with something that covers insufficient effort questions
@ACuriousMind I might have used it as a custom reason once or twice
@0celo7 No :(
@0celo7 Cool... I have questions I don't know how to ask, related to my tendency to view the universe as being "a lot like" a computer simulation, even if it is not. That would put... odd constraints on the manifolds in play?
@JohnRennie I think you might have started it, and others caught on, but of course I don't have a timeline
Dec 29, 2015 16:41
@ACuriousMind we don't have another slot, but if we really needed one we could ask for the SE team to enable one. The system allows up to 5. (But I don't think we really need an extra one that badly.)
@0537 do you not want to do the Lie derivative thing
@ACuriousMind You might say that, but I couldn't possibly comment :-)
@0celo7 Can you show me please, because I don't know how to do it.
@JohnDuffield downvoting and closing is exactly how we point out that this website isn't for that sort of thing
@JohnRennie Argh, is that a variant of "why don't electrons at rest in a gravity field give off EM?"
Dec 29, 2015 16:44
@0celo7 I figured it out.
I'll answer it.
@TerryBollinger No it isn't it's a really subtle question, though with a surprisingly simple answer
I would provide that answer, but the question got closed
Can you guys mark that question as on-topic please?
or...
VOTE TO REOPEN NOW
@JohnRennie Hm. I see your point, but I also see the point of the close voters - there's a very obvious and boring reason we're not accelerating, and that's because the force of gravity is counteracted by the normal force. The equivalence principle doesn't say that acceleration is equivalent to standing on a surface in a gravitational field.
@ACuriousMind Wrong, we are acceerataing
We are accelerating relative to a freely falling observer
Dec 29, 2015 16:46
@0537 don't worry, it's fine to ask ;-) (i.e. if the community actually thinks a question should be reopened, it's fine to nudge people toward actually voting for it - and similarly for voting to close)
and as a result of the acceleration our speed is increasing relative to a freely falling observer
Just like being in an accelerating rocket in fact
@JohnRennie that to me is a question that would have been worth answering, because I am very sure it bugs a lot of folks who are first told about gravity/acceleration equivalence.
@JohnRennie that one I gotta say looks a lot better now than when it was put on hold
@DavidZ some good editing?
Yeah
Dec 29, 2015 16:48
@DavidZ True, but it was always clear what the OP was getting at
@JohnRennie done.
@TerryBollinger Too late ;)
@ACuriousMind Tried!
@ACuriousMind And it's open :-) Now to sharpen my pencil ...
@JohnRennie it wasn't clear to me
not at first
Dec 29, 2015 16:49
@DavidZ: have we made progress on the close reason?
@JohnRennie you do good answers on that kind of question, I look forward to it.
@DavidZ : some people take umbrage at downvoting and closing, and some of them might be the sort of people PSE wants to attract and retain. A polite explanation might serve better.
@JohnRennie changing from homework-like, you mean? Well, you're here, you can see what sort of progress we've made ;-)
@JohnDuffield no they're not (the sort of people PSE wants to retain)
@DavidZ The trouble is that in these chat sessions we talk a lot without ever reaching a conclusion
Just saying.
Dec 29, 2015 16:50
@JohnRennie I like "lazy not allowed" a lot, with specifics about doing a bit of homework first. But as @JohnDuffield says, polite is always better, especially when a question is first-time.
yeah, that's okay. This is just starting a discussion, at this point. The goal here is not to reach a conclusion, it's to draw attention and generate ideas.
For the record:
I don't think homework is always bad
I often see homework questions at a fairly advanced level that I can't answer and for which I'd really like to see an answer
(removed)
Dec 29, 2015 16:53
Whoa, what's happening?
lol...
mod intervention was imminent
On the subject of deleting, was it you @DavidZ who deleted the question about the physics of erections?
Now how will I ever find out?
What exactly was the question
@JohnRennie r/askscience
@JohnRennie that's true, but let's also consider the case where someone posts a really interesting homework question that is, nevertheless, directly from a homework assignment. Do we want to provide an answer to that?
My inclination is "no" but I could see it going either way
Dec 29, 2015 16:55
@DavidZ Yes
@JohnRennie nah, I don't think I had anything to do with that
@DavidZ I don't think we should care at all about whether things are actual homework or not.
Well, I'd probably try to broaden my answer to be more conceptual
@JohnRennie Context dependent, I think some homework questions are legit, especially if presented honestly.
@ACuriousMind it certainly does make things a lot easier if we don't have to care.
Dec 29, 2015 16:56
But lazy homework questions will still be nuked - yes?
@JohnRennie again, not saying I necessarily disagree, but if I may play devil's advocate, this position is (apparently) that it's okay for us to help people cheat on their homework if the problem is interesting enough.
@JohnRennie presumably so. But this is what we have to decide.
Later gents, and ladies if any at present? Nice question discussion, and nice thread about manifolds. "It is your destiny... your manifold destiny." (attempt at Star Wars humor...)
@JohnRennie Questions about "advanced" calculations I think can and do stay open - if they make plausible that this is not a straightforward calculation. The question should demonstrate (or state) that the standard methods in the field do not work, and indicate the reason for that. All too often these questions lack that kind of context, and I don't think there is anything worthwhile in a question whose answer is two pages of lengthy, but standard manipulations.
Advanced calculation: QCD to 15th order
Wonder how many pages the result would be
Or Post-Newtonian stuff to 15th order
@DavidZ homework is intended to be part of the learning process. If I can answer a homework question in a way thatI think leaves the OP knowing more than they started with then I'm happy with that.
Dec 29, 2015 16:59
15 order would be p. long
There's a certain class of homework questions which I suppose everyone agrees should be off topic, but the question is why they should be considered off topic. Should it be because they are homework? Because they are educational? (as current) Because they don't show effort? Because they are just asking for a full answer instead of help on the step they're stuck on?
@JohnRennie that's a good point
A 14-fold integral at the end
I had to do 4th order things once and it was already a nightmare
Gosh, 5 already.
@0celo7 I am lost now... can you help?
I have to slope off I'm afraid
Dec 29, 2015 17:01
@Slereah $$\int\int\int\int\int\int\int\int\int\int\int\int\int\int F(a,b,c,d,e,f,g,h,i,j,k,l,m,n)\,\mathrm{d}a\mathrm{d}b\mathrm{d}c\mathrm{d}d\mat‌​hrm{d}e\mathrm{d}f\mathrm{d}g\mathrm{d}h\mathrm{d}i\mathrm{d}j\mathrm{d}k\mathrm{‌​d}l\mathrm{d}m\mathrm{d}n$$
Nooo
I've got to be off too, but I'll get a meta post up about this close-reason thing sometime soon. Thanks for the thoughts everyone
 
Conversation ended Dec 29, 2015 at 17:01.