The h Bar

6:00 PM

@KyleKanos I agree, but that doesn't mean there aren't viable exceptions to the rule. Perhaps flagging it to make it into an answer is good. I didn't read it, is the answer mostly just an opinion, or is it based on science and facts?

Danu

@KyleKanos The answer is a very good 'artefact' as Spolsky calls it in his public talks

ACuriousMind

Well, I'd reopen the question if the "Are they right?" bit is removed. It'd still show no research effort, but that's no ground for closure, imo.

Danu

@JimdalftheGrey I think it's an above-average quality answer

@ACuriousMind I'll edit the question

ACuriousMind

I actually dislike the fact that it got two upvotes more than the question itself

DanielSank

Can one of you guys explain to me what the "non mainstream" thing is supposed to mean?

@ACuriousMind Sour grapes.

ACuriousMind

6:02 PM

11

Q: Is non mainstream physics appropriate for this site?

Is non mainstream physics allowed here? What defines non mainstream physics? What sort of questions and answers are disallowed by this policy? What should I do if I see a question or answer containing non mainstream physics?

It's probably another policy you'll find giant holes and inconsistencies in, @DanielSank ;)

Danu

1

Q: What is a tachyon?

What is a tachyon? Is it true that tachyons represent faster-than-light particles?

special-relativity faster-than-light tachyon

Improved

Kyle Kanos

@JimdalftheGrey It's based on science

But even still, it's an insufficient effort problem

Jimdalf the Grey

@Danu But is it an opiniony answer? If not then it's evidence that the question shouldn't have been closed as primarily-opinion in the first place. If it is, then I agree with Kyle that we shouldn't allow this to circumvent the disability to post answers.

DanielSank

@ACuriousMind Indeed yes, but as usual it appears to me that what is written in the meta does not dictate how people actually reason about VTC, which is what I'm asking about.

Danu

@JimdalftheGrey Not really

Kyle Kanos

6:03 PM

OP did not even bother checking Wikipedia or doing any search

DanielSank

I'm asking for your actual thought process.

Jimdalf the Grey

If not opiniony, I say make it an answer and re-open the question

Danu

@KyleKanos But that's a different VTC reason (you can start the process again if you want, if it gets reopened)

@JimdalftheGrey I think it should be an answer

Kyle Kanos

@Danu (a) as a close-voter I cannot, (b) reopening just to close it again is a wasted effort

ACuriousMind

@DanielSank Well, the "Most physicists think... . Are they right?" bit is essentially the question "Is this piece of mainstream physics correct?", which is off-topic by the policy, I think.

Danu

6:04 PM

@KyleKanos Also, if you'd actually checked the wikipedia page you would see that it has some seemingly contradictory statements

ACuriousMind

We have not unanimously decided that insufficient effort questions should be closed, right?

DanielSank

@ACuriousMind That's off topic by the mainstream policy?

I actually don't see that.

ACuriousMind

@DanielSank Mhhh, you're right

Danu

@KyleKanos First sentence: "A tachyon or tachyonic particle is a hypothetical particle that always moves faster than light. " while much later on in the article we find "Because the group velocity for such a field is superluminal, naively it appears that its excitations propagate faster than light. However, it was quickly understood that the superluminal group velocity does not correspond to the speed of propagation of any localized excitation (like a particle)."

It is not really obvious (at least not to the layman) what the deal is here

ACuriousMind

Okay, it seems we have no close reason corresponding to "You can't just come here and ask us if this or that bit of established physics is correct!"

Kyle Kanos

6:06 PM

Then the question should be about clarifying that statement, not the idiotic "what is X" question it is currently

DanielSank

@ACuriousMind Ok. As usual I bring this up mostly because when question get closed for reasons that don't make sense to OP, they aren't getting the information they need to make it better.

ACuriousMind

@DanielSank Yes, I see the issue here.

Danu

@KyleKanos Meh, I think you're overreacting

ACuriousMind

@Danu I think Kyle is completely correct in saying the question is bad, and his proposals are a good step to make it better.

It's just that we are not decided on whether or not to close insufficient effort questions

Danu

@ACuriousMind I think it's not completely clear that this is a case of insufficient effort

I'd imagine that it'd be quite difficult for a layman to find any clear info on tachyons

Kyle Kanos

6:09 PM

I think any question that is only "What is X" is absolutely insufficient effort

Doing a Google search for X should be the first thing

Danu

It's not just that

DanielSank

@KyleKanos Ok, here's the thing:

ACuriousMind

@Danu Yes, but then the question should incorporate the contradictory statements. As it stands, it shows no effort.

Kyle Kanos

And if they did that, then they ought to include that

DanielSank

For a lot of people, doing a Google search on $X$ simply is not helpful.

Danu

6:10 PM

@ACuriousMind Ugh, whatever, I can include it for you if you wanna be nitpicky. I really don't think you're making a good point.

Kyle Kanos

@DanielSank Then they should say, "I read <this> on <site> and did not understand what they meant."

Jimdalf the Grey

a lot of people don't know what to type into google to find what they want

DanielSank

@JimdalftheGrey Yes!

A thousand times yes.

0celo7

Are we arguing about in general or this specific case?

Kyle Kanos

I'm arguing the general case.

Jimdalf the Grey

6:11 PM

I'm only in favour of vtc as no effort when the exact title of the question can be typed into google and the first hit is the wikipedia page that explains it entirely

ACuriousMind

Great, now were arguing about what were arguing about.

DanielSank

@KyleKanos Imagine you Google some topic and find a page with a thousand words on it. You don't' understand it. What parts should you paraphrase in your answer?

Danu

I'm saying this case is very difficult since tachyons are not easy to google

0celo7

@ACuriousMind Sorry, I came late to party.

DanielSank

My point is just that it's really hard if you don't already know what you want to ask.

0celo7

6:12 PM

@JimdalftheGrey Agreed.

Kyle Kanos

@DanielSank None of it should be paraphrased. It should be quoted directly.

Danu

^the entire wiki page?

DanielSank

@KyleKanos All 1000 words?

Danu

For a non-expert, it is not clear what contains useful info and what doesn't

Kyle Kanos

1000 words isn't that much. But I'd say whatever is key to not understanding.

Jimdalf the Grey

6:12 PM

Only 1000? That's a short wiki page

DanielSank

Ok whatever, 10,000 words.

Danu

@KyleKanos lol, would you answer a 1000 word question?

I doubt that many of those are answered

ACuriousMind

@JimdalftheGrey I'm also not in favor of VTC because of this, but I still think the question shows no effort. That's a downvote reason, not a close reason.

DanielSank

You want me to copy-paste that into my question?

Kyle Kanos

@Danu If I know the answer, why not?

Danu

6:13 PM

Because you'll have to wade through an entire wiki page's worth of info that is mostly useless

Kyle Kanos

Show me a 10,000 word wiki page that you don't understand all 10,000 words

DanielSank

@KyleKanos I feel like you may not be arguing in good faith here...

Danu

...if the OP follows your advice, that is.

ACuriousMind

I'd be happy with a short statement like "The Wikipedia articles is not helpful because, for example, X"

Danu

@ACuriousMind Yeah yeah

I'm editing it in

Also, all of you could have done that too

but okay

DanielSank

6:13 PM

@KyleKanos Ok: en.wikipedia.org/wiki/Cohomology

Danu

I guess I'm the only one who thinks this answer is worth fighting for

Kyle Kanos

@DanielSank VTC: too broad

:D

Danu

@KyleKanos yeah, that's a lame response though

DanielSank

You asked for a page I don't understand. I gave you one.

Kyle Kanos

Why is it lame?

Danu

6:14 PM

You just said we should just quote the entire thing

Kyle Kanos

@DanielSank And I gave you the appropriate response

0celo7

Functor

In mathematics, a functor is a type of mapping between categories, which is applied in category theory. Functors can be thought of as homomorphisms between categories. In the category of small categories, functors can be thought of more generally as morphisms. Functors were first considered in algebraic topology, where algebraic objects (like the fundamental group) are associated to topological spaces, and algebraic homomorphisms are associated to continuous maps. Nowadays, functors are used throughout modern mathematics to relate various categories. Thus, functors are generally applicable in areas...

Danu

it's obvious that your approach will not lead to anything useful

Kyle Kanos

Did I say that?

Jimdalf the Grey

So every right now go and type "what is a tachyon" into google. Then come back and tell me if the question shows an unallowable amount of insufficient effort

Kyle Kanos

6:15 PM

2 mins ago, by Kyle Kanos

1000 words isn't that much. But I'd say whatever is key to not understanding.

Whatever is key to not understanding = all 10,000 words? I think not

DanielSank

@KyleKanos Huh? You're voting to close my example of a wiki page that I don't understand? I don't even know what that means.

0celo7

Kyle Kanos

@DanielSank A copy-pasta of a wiki page with a question, "Can you explain this to me" should be VTC as too broad.

0celo7

seems pretty non-effort

Danu

0

Q: What is a tachyon?

What is a tachyon? The linked Wikipedia page shows some seemingly contradictory statements, and they are confusing me (note that I am, personally, not a physicist). For instance, the first sentence states that tachyons "always travel faster than the speed of light" whereas, in a later section, ...

special-relativity faster-than-light tachyon

DanielSank

6:16 PM

Anyway, I do think there have been a nontrivial number of questions I've seen where I could tell what information and logical link OP was missing despite the fact that the question was poorly worded.

Danu

now it's a good question

@0celo7 Except it's wrong :D

Kyle Kanos

It could be, but that's not OPs words there. This is what I consider a drastic change:

15

A: Correcting incorrect answers

I think that edits of another user's post should be reserved for typographical errors (e.g., grammar, LaTeX formatting) adding/modifying relevant links (e.g., updating broken links) word choices (e.g., using liquids as a generalized replacement for fluids) These edits are minor, changing the...

Danu

@0celo7 ...or at least brushing aside a lot of important info

DanielSank

@KyleKanos You asked for an example of a wiki page where I didn't understand all the words. I was giving you that, not an example question.

Kyle Kanos

@DanielSank I know.

Jimdalf the Grey

6:17 PM

now it's a good question

Danu

@KyleKanos oh come on

Kyle Kanos

It's Danu's question, not The Smart Guys

Danu

It's not my question: I know the answer already

0celo7

@Danu Meh.

Danu

I just reworded his and expanded it to be much more explicit

@0celo7 ? I don't see how this link is relevant

Kyle Kanos

6:18 PM

But you did it without knowing that's what OP wanted. He/She never clarified what their issue was

You invented that

DanielSank

@Danu: Am I taking crazy pills?

Danu

I'm not going to argue this any further

Jimdalf the Grey

Yeah, this is a radical change

0celo7

@Danu It's a simplistic answer that leaves out info.

Danu

@0celo7 Yeah, so it's no good.

ACuriousMind

6:19 PM

@Danu Don't add info to questions that is irrelevant to the question. Nevertheless, the question is now neither downvote- nor closeworthy. It'd be interesting to see if your edit would have been approved in the review queue, though.

Kyle Kanos

I actually have half a mind to roll it back to the 1st edit (removing the personal opinion question)

0celo7

@Danu The title should be "Can tachyons be slower than the speed of light" or something

Danu

@ACuriousMind Good first point. I don't really care about the second point. I'm doing this to salvage that valuable answer.

ACuriousMind

So, what do we do? Open a meta discussion on whether or not this was allowable aditing?

DanielSank

I actually don't quite understand why the original was closed.

ACuriousMind

6:20 PM

From this chat it seems consensus is not in sight

DanielSank

It's a simple yes/no question.

Kyle Kanos

@ACuriousMind I'd argue it's been done before, given my answer above

Jimdalf the Grey

@Danu Your edits now partially answer the OP's question within the question. It's clearly conflicting with their intent

Danu

@ACuriousMind It's not worth pursuing

DanielSank

oh wait

Arg, I fail at reading version history.

user54412

6:21 PM

I don't understand why there are any objections to editing a closed question to bring it in line with our expectations of good questions.

4

Danu

@ChrisWhite This 100 times

user54412

Closing is meant to encourage good content, not to punish users.

Kyle Kanos

@ChrisWhite Because of the drastic change. It's changing OPs question to being something he/she did not ask

user54412

@KyleKanos Which is only bad if what they did ask were to remain on the site.

Danu

@KyleKanos I think it was implied in his question; the original was just terribly worded

ACuriousMind

6:22 PM

@KyleKanos Then again, the SE model doesn't really care about the OP.

user54412

If they are unhappy with the rephrasing, they can rollback or delete.

DanielSank

@ChrisWhite I agree with this, except that one then has to campaign on the chat channel to get others to reopen.

Danu

@ChrisWhite Also 100 times this

@DanielSank In this case that's 100% worth it

DanielSank

@Danu I don't disagree with that :)

Kyle Kanos

@Danu I disagree with that 100 times that. OP wanted to know what a tachyon was and if scientists were right/wrong in believing it was real. OP did not want to clarify a statement made on a wikipedia page

Danu

6:23 PM

I usually couldn't care less, but this answer was good

DanielSank

@Danu 100 times this.

Danu

@KyleKanos OP wanted to know whether tachyons are faster than light or not. The main question is still exactly this

"Is it true that tachyons represent faster-than-light particles, or not?"

ACuriousMind

@Danu You're no fun. Pointless arguing is fun.

DanielSank

The question was so bad that I don't see any reason to try to preserve its original form. OP obviously just wants to grok tachyons to some degree. I see nothing wrong here with proposing a rework of the question which allows a good answer to stick.

Danu

@ACuriousMind I'm getting really annoyed at the pedantry I'm encountering here is all

DanielSank

6:24 PM

@Danu You may now join my special exclusive club for grumpy people on this chat :D

ACuriousMind

@Danu Living in Germany must be hell for you :D

Danu

@ACuriousMind YES

I really don't like it nearly as much here :(

Kyle Kanos

@Danu I don't think we're looking at the same question. It was originally Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. Are they right?

0celo7

So what is the resolution? Does Wiki have the tachyon defined in two different ways?

Kyle Kanos

With the Title saying, "What is a tachyon"

Danu

6:25 PM

@KyleKanos Lol, did you not even look at my edits?

@0celo7 The resolution is outlined in Leonardo's answer

(which is in the comments)

Kyle Kanos

@Danu The linked Wikipedia page shows some seemingly contradictory statements, and they are confusing me (note that I am, personally, not a physicist).

When did OP say that?

Danu

basically, the only type of tachyons anyone is pursuing are the perfectly-fine ones from field theory

Kyle Kanos

Where?

DanielSank

@Danu: I think you should probably keep the edited version such that a non-physicist would actually understand the question.

"Field theory" is probably no bueno.

Danu

@DanielSank Just paraphrasing wikipedia

that part is under the section header "field theory"

@KyleKanos See @ChrisWhite's response

DanielSank

6:27 PM

@Danu Right, but that kind of came from nowhere. To me this question is basically "What is teh tachyonz (I am not a physicist)?"

Danu

@DanielSank Yeah, agreed. Edit it!

DanielSank

@Danu The problem is that I have absolutely no idea what a tachyon is.

ACuriousMind

@DanielSank I don't think that "What is X?" questions with no context where "X" is mentioned aren't good questions, either.

Kyle Kanos

@Danu IMO: That's a drastic change. You're literally changing the entire aspect of OPs question

ACuriousMind

"What is X?" could be a nice featured series on a possible blog, though

DanielSank

6:28 PM

@ACuriousMind I agree.

Kyle Kanos

There is nothing left of OPs question left in that box

user54412

I do feel like pointing out, though, that this poor person has been lurking for a month on the site, probably refraining from saying anything before understanding the rules, and now we've bombarded him with edits, closures, opens, comments, comments on comments, etc.

DanielSank

@KyleKanos So what?

The original question was awful.

user54412

Perhaps further debate should be confined to meta

Danu

@KyleKanos See @ChrisWhite's response

Kyle Kanos

6:29 PM

@DanielSank So it's not OPs question

DanielSank

@KyleKanos So what?

user54412

I'll also point out that this is what happens when too many high rep users have simultaneous free time ;)

Danu

Ugh, you're being really atypically pedantic @KyleKanos. I'm going to leave now.

Kyle Kanos

5

A: Correct way to deal with minor detail in answer

You did the right thing. When an answer is wrong, there are three tools you can use. Each tool apply to different circumstances. If the answer is fundamentally flawed, downvote it. Preferably leave a comment to explain both to the poster and to people who will read the answer what is wrong wit...

Danu

Hah, true @ChrisWhite

ACuriousMind

6:30 PM

@Danu Not atypic, really :P

Jiminion

So Corsika is even more annoying than me.

DanielSank

@ACuriousMind agreed.

user54412

@Jiminion I've seen that name around the network, mostly in comments.

DanielSank

@Danu: Plz see edit.

ACuriousMind

@Jiminion You're not talking about an island, are you?

Jimdalf the Grey

6:39 PM

So I've been going through mother meta, because why have a debate if it's been had before. And I found a couple related posts, let me dole them out

4

I just came across this suggested edit. It does qualify for a reject as "Radical edit" in my opinion: This edit changes too much in the original post; the original meaning or intent of the post would be lost. The edit does indeed completely change the question. But, it's also transforms a...

discussion suggested-edits

10

A: Users should not be able to edit closed questions in order to make them more palatable!

Closing does not prevent editing by the OP, it merely prevents new answers from being added. Currently on SO, a question can be closed and then edited by its original author. Potentially, this editing can change the context and even spur a reopening of the question. Yes, that is the intenti...

ACuriousMind

@JimdalftheGrey That one seems to say: "If there are no answers present that would be invalidated by a change, it's okay"

Jimdalf the Grey

@ACuriousMind or answer-like comments

ACuriousMind

Well, in this case, the edit rather tried to validate the answer-like comment, right?

Which is again an unusual case

Jimdalf the Grey

hang on

my computer is being dumb and not letting my copy things to the clipboard at the moment

Kyle Kanos

@ACuriousMind To some extent. I think it was more in relation to my comment about it should be asking something else, not "what is X"

Jimdalf the Grey

6:46 PM

1

Q: Closed question is edited to ask a completely different question -- approved edit?

A new user asks a question: Which database server should be preferred for developing mobile application with IBM Worklight? This question is closed by the moderators (an agreed action indeed). The user then comes back some months later and edits the question completely, to ask something else en...

discussion

no answer on that though

Kyle Kanos

@JimdalftheGrey No answer on that because OP did it

We've got the case that someone else edited

Jimdalf the Grey

12

A: Should we disallow vast (self-)editing of a question, completely changing it's meaning?

The general rule is that you should not edit the question in a way that invalidates the existing answers. Doing that causes a lot of problems and is rather unfair to the early answerers. There are some exceptions, e.g. if the question would otherwise be closed it can be justified to perform some...

general thought seems to be that this particular case is okay, but everyone would be much happier if we could avoid doing these kinds of okay things in the future

Jiminion

@ACuriousMind No, I might be a minion but I can SPELL! He/she/it is a user.

ACuriousMind

@JimdalftheGrey Everyone! Don't do okay things! sounds exactly like the kind of policy @DanielSank loves so much

Kyle Kanos

@JimdalftheGrey I'm not too keen on it. It is not at all A Smart Guy's question in any way and I'm not sure that LeonardoM's comment-answer is valid (I'll have to re-read both Q&A). I'm obviously outnumbered, so I'll concede that I can do nothing else.

I don't think the precedent should be set on such a change either.

Jimdalf the Grey

6:55 PM

@KyleKanos If this questions weren't close-worthy in the first place, this wouldn't have been okay. But as the OP's question didn't belong here, the opinion seems to be it's technically okay to change it the way we did. I don't like it either, but as it isn't the same at all, the OP can still ask another question that more appropriately addresses their concerns

Kyle Kanos

I mean, think about it: allowing this means that any question that's closed as off topic can be radically changed by anyone (with sufficient rep) into something entirely different.

I can take a blatant zero-effort HW question and turn it into a complex question about cosmic ray production.

And that's bullshit, even to SE's standards.

ACuriousMind

@KyleKanos Ordinarily, people aren't gonna do that because they could just ask the different question themselves

Jimdalf the Grey

@KyleKanos I know, I know. But the majority opinion seems to be "You're one hair more on the side of 'this is okay', but try not to walk so far into the gray zone, just in case the line shifts"

Kyle Kanos

@JimdalftheGrey I think that it is a slippery-slope that I don't think we should tread

Jimdalf the Grey

The question was at least close-worthy as too broad originally

Kyle Kanos

6:57 PM

@ACuriousMind Agreed, but that the fact that it happened today shows that it can happen.

Jimdalf the Grey

Why don't we do this then, undo all the radical changes. Get the question closed under the proper close reason. Allow the comment-answer to become a full answer, then make a meta post concerning handling of stuff like this and let everyone decide on the correct policy. If they decide we should be able to do it, we can rollback to the current changed state

Kyle Kanos

Well I cannot VTC because I used mine initially

Jimdalf the Grey

I think you can vtc again if it gets reopened

Kyle Kanos

Nope

You get 1 vote per question

So if it's reopened or fails to close, you can't do it again

Jimdalf the Grey

I'm sure we have enough people to make sure it gets closed (and reopened if necessary) properly

Kyle Kanos

7:04 PM

Yep.

Which is why I conceded that I couldn't do anything about it

Jimdalf the Grey

The question is who should write the meta post and which meta should we use? I think someone who doesn't have a strong opinion on one side or the other should write the post so as not to pre-bias anyone not already involved in the debate

Kyle Kanos

I could write something up without much of a bias.

Jimdalf the Grey

@KyleKanos I'm waiting to do anything until more people agree we should revert to the original until policy gets firmly decided

Kyle Kanos

It's simply asking for the state of the community's opinion on the matter

@JimdalftheGrey I think we could ask the Meta question without having anything happen to the post

Jimdalf the Grey

Okay

Mother meta?

Kyle Kanos

7:08 PM

I think we can do it on ours

Jimdalf the Grey

might be helpful to get a mother mod's point of view. Problem is the mother meta posts often get ignored

DanielSank

@ACuriousMind Huh?

Kyle Kanos

@JimdalftheGrey: meta.stackexchange.com/questions/218183/…

DanielSank

@ACuriousMind: I'm reasonably sure the only policy issue I've ever really argued here is that the overwhelmingly theory oriented high rep user base should be consistent about what it considers physics and what it rejects as not physics. I stand by that and I have no idea how that could be construed as saying not to do ok things.

Jimdalf the Grey

@KyleKanos Fair enough. If you're still writing our meta post, I'd include that along with a few other links. At best this tips the scales toward "don't do it, but there's a bit of a debate surrounding this"

Kyle Kanos

7:21 PM

Will do

Jimdalf the Grey

Unrelated question: Are questions about the culture of physics or things culturally significant to physicists specifically on topic here? I know they aren't about physics material, but they are about physics as a concept; a community. I suppose it's akin to asking if you have to speak klingon to be considered a trekkie on scifi&fantasy.SE

not sure if that would be on-topic either

Kyle Kanos

I'm inclined to say they'd be off-topic, but it might be a case-by-case basis.

Have an example in mind?

Jimdalf the Grey

Not a good one

every example I can think of right now would be something that would probably only be related to physics culture by sheer luck. But I want to be prepared if I see anything in the future

I started thinking about it because of this edit to a post:

physics.stackexchange.com/revisions/166128/2

Kyle Kanos

The arrow bit certainly is on-topic, but the bit about the attire really isn't physics (at least I don't see it)

Jiminion

I was looking into some weird quasi-physics stuff and actually found something "real". It's conventional physics, but new (to me) but I hesitated to post a message because it even references some fringe stuff.

Jimdalf the Grey

7:35 PM

I thought the bolded question is off-topic, but say that outfit was culturally significant to physicists or say all physicists wore that kind of dress on formal occasions, would it be on-topic to ask about it?

Jiminion

@JimdalftheGrey I think the stamp is a valid physics question.

Jimdalf the Grey

yes, but not quite what I'm asking here

ACuriousMind

@DanielSank I meant that it is a policy that's inconsistent - saying It's alright, but don't do it is not really a clear cut policy

(and you like pointing out inconsistent (-ly applied) policies)

Kyle Kanos

@JimdalftheGrey Yes, the bolded question should not be allowed

Maybe as a comment

(as it was originally)

Jimdalf the Grey

@KyleKanos no doubt. But what if physicists did always wear that silly outfit. In that case, would asking why they have that cultural peculiarity be on-topic here? Basically, is asking about topics that are specific to physicists but not necessarily about physics material off-topic?

Kyle Kanos

7:40 PM

That might be worth a Meta post, I suppose

ACuriousMind

@JimdalftheGrey We have a culture?

Jimdalf the Grey

Might be, but I can't think of any realistic examples that might come up. I don't want to bother meta with something that may never become relevant

@ACuriousMind Yes, but only the one

0celo7

@ACuriousMind How do the Borel sets (on $\mathbb{R}^n$) generate a $\sigma$-algebra? Are the open sets in the Borel sets clopen in $\mathbb{R}^n$? I thought this was impossible because $\mathbb{R}^n$ is connected.

In the definition of a $\sigma$-algebra we have that it is closed under complementation.

ACuriousMind

@0celo7 Why would they need to be clopen? The Borel sets are kinda defined as "the $\sigma$-algebra made out of open sets".

0celo7

How do you make a $\sigma$-algebra out of open sets though?

Don't you need the closed complements too?

ACuriousMind

7:45 PM

@0celo7 You start with the open sets (the topology), and add their complements und countable intersections until what you have is a $\sigma$-algebra

The technical procedure uses transfinite induction and whatnot, but that's what happens

0celo7

Ok. So when we say that the Borel set is the open sets, we also mean includes the complements (and intersections) so that it forms a $\sigma$-algebra?

ACuriousMind

@0celo7 A Borel set is any set that you can get from the open sets by taking the complement, unions and intersections as often as you want

0celo7

Got it. Thanks.

user54412

Ok, h bar, I have a challenge.

innisfree

that stamp of feynman is strange ("physicist, adventurer, raconteur") - feynmann is surrounded by hagiography and myth

Jimdalf the Grey

7:50 PM

@ChrisWhite Challenge Accepted!

user54412

Regarding this question, I've seen that picture before

user54412

Jimdalf the Grey

@innisfree he's also wearing clothing that clearly predates his time

user54412

I need a better version

user54412

it's a painting on the wall on the right-hand side (for the audience), toward the front, of the Bridge Lecture Hall at Caltech

ACuriousMind

7:51 PM

@innisfree Worshipping idols seems to be quite intrinsic to human nature.

user54412

(I grabbed that shot from 1:07:56 of this video)

ACuriousMind

@ChrisWhite Was that taken with a potato? What kind of quality is that?

user54412

I want to know if the arrow in the question is present in that picture

user54412

This could confirm or refute @KyleKanos 's hunch that the arrow was clipped for the stamp

ACuriousMind

So, we just need someone at Caltech to go there and look?

Jimdalf the Grey

7:54 PM

@ACuriousMind Ahh yes, the SpudEye 800. I remember when that came out

user54412

would that be easier than finding a better youtube video?

0celo7

@ACuriousMind Soft question: Why do we call the characteristic function measure $\delta_y(A)$ (1 if $y\in A$ and 0 if not) a Dirac delta measure and not a Kronecker delta measure?

innisfree

am i the only person who found feynman's books to be amusing but ultimately vacuous sets of anecdotes? no meaningful reflections or introspection about his life, his influences or mistakes.

0celo7

Is associating Dirac with an infinite spike naive?

innisfree

funny, yes, but not profound or memorable, or worthy of mythologizing.

ACuriousMind

7:57 PM

@0celo7 I won't say yes or no to that. The Dirac and Kronecker delta aren't really all that different to me.

Jiminion

@ChrisWhite That's kind of freaky, but a bit blurry. I think that's Bigfoot behind his right shoulder.

Jimdalf the Grey

@ChrisWhite I'm extremely impressed that you saw the stamp in the question and then managed to find that tiny, unnoticeable image at the very end of a random hour-long lecture on youtube. What are you?

ACuriousMind

@JimdalftheGrey He was at Caltech and just searched for any lecture in that lecture hall

Not that superhuman :P

user54412

@ACuriousMind Shhhh. Let's keep up the ruse

Jimdalf the Grey

He still found the only 5 second window where the camera zoomed out far enough to see the picture

and he remembered exactly what lecture hall had a visually similar picture

user54412

8:02 PM

and to be fair, it was the BICEP2 announcement at one of the home institutions for the project

user54412

@JimdalftheGrey Caltech has only 2 lecture halls of note, one of which I had half my physics classes in.

Jimdalf the Grey

@ChrisWhite, are you Jeebus?

user54412

Also, I may have spent my time looking around and not paying attention in lecture...

Jiminion

youtube.com/watch?v=QKFYKzJytwk (Heston throwback....)

0celo7

Does the h bar like cats or dogs or both?

Jimdalf the Grey

8:10 PM

the h bar is not sentient or conscious. I don't believe it has a preference, therefore

0celo7

Does the master of Jims have a preference? (He's the only one that matters, anyway.)

Jimdalf the Grey

The question is Jimmaterial to him

user54412

^ A new name suggestion!

0celo7

Jimpossible! One must have a preference!

Jimdalf the Grey

@ChrisWhite If you like that, you should read the comment I wrote under the "Jimperator" suggestion

@ChrisWhite Also, I don't think the original painting is of feynman. I think they took that painting and modified it so that the guy is holding a feynman diagram for the stamp. So, this search is going to be a lot harder

user54412

8:15 PM

perhaps this is a task for a Philately Stackexchange

Jiminion

Isn't there some Caltech resource that could get us a better quality picture? Ask a student or whatever?

Jimdalf the Grey

why is feynman holding a lock in his other hand in the stamp?

Jiminion

From his safe busting days during the manhattan project?

user54412

@JimdalftheGrey a reference to his safe-cracking bar trick?

Jiminion

jinx buy me a coke.....

Jimdalf the Grey

8:20 PM

can he only crack safes in bars? Or is a more useful skill that can be applied elsewhere?

user1667423

What is the physical interpretation of the dot product between the electric field and the magnetic field (E . B) and of E^2 - B^2? These are Lorentz invariant quantities associated with the electromagnetic field tensor.

user54412

@JimdalftheGrey I mean that it's the kind of ability one can imagine bragging about over a beer, not the kind of thing that gets one Nobel prizes

Physics Meta

0

Q: Radically editing closed questions

This post is in regards to events occurring with the question Do tachyons move faster than light (see also chat starting around here), but the question is to be applied to a site-wide policy. The aforementioned post contained a bad question that was quickly closed as primarily opinion based. A s...

discussion editing policy

Jimdalf the Grey

@ChrisWhite Right, but can he actually use it to crack safes all over? Because that's a useful skill to have

user54412

I think that's the idea

user54412

8:23 PM

he was pointing out that safes have far fewer effective combinations than they advertise

user54412

@user1667423 you can think about E^2-B^2 as the difference between energy per unit volume stored in the electric field vs. the magnetic field

ACuriousMind

@0celo7 Cats.

user54412

I suppose at some level the larger that number the less the velocities of the sources matter in determining the dynamics (compared to the mere fact that they are charged)

Jimdalf the Grey

@ChrisWhite But how long would it take him to crack a safe? Because I've done it in an hour by listening to the pins. If it's not faster than that, the trick isn't impressive

0celo7

@ACuriousMind Measure theory is incredibly boring. Transfinite induction is bleh. Monotone class theorem is bleh. Sets are bleh.

ACuriousMind

8:32 PM

@0celo7 Hahaha, I never said anything else

0celo7

Either you or Danu warned me of this.

I think he said functional analysis is even worse.

TIL Zeidler considers his four volume set of applied funtional analysis prerequisite and supplemental material.

ACuriousMind

@0celo7 It's essentially analysis, but in infinite dimensions, what do you expect? :P

alarge

@0celo7 You could do it in the context of probability theory. Makes it a bit more interesting.

ACuriousMind

I find "algebraic" stuff much prettier, generally. But the analysis stuff is better if you want to apply it to things. Probability and measures and functions arise more directly than rings and sheafs and groups, often.

Jiminion

Most of his tricks for cracking safes during the war was knowing were to look for combinations, glancing at people when they were opening their safes, etc. He used that info plus the fewer-real-combinations trick to open safes quickly and impress people.

0celo7

8:36 PM

@alarge I need some analysis for both Zeidler's and Hall's books. I just want the basics, but even the 40 page introduction to measure theory and integration in Lieb & Loss is excruciating.

I'm doing the "read a page twice, realize I forgot all the definitions, go back 5 pages, read everything again, continue reading, get confused by proof, forget definitions" cycle ad infinitum.

Danu

@KyleKanos It's up to OP to rrrrrrollback

Kyle Kanos

@Danu No it's not. I can certainly do it.

Then flag it for content dispute

Danu

OP can rollback

Kyle Kanos

Of course, but it's not "up to OP" to do so

0celo7

@ACuriousMind What does the monotone class theorem even say? That I can construct a $\sigma$-algebra using open sets? But isn't that the definition of a $\sigma$-algebra?

Kyle Kanos

8:40 PM

Anyone with sufficient rep and enough will can roll it back

Leandro's answer doesn't quite answer your question now

ACuriousMind

@0celo7 Mhh. To be a monotone class is a bit weaker than to be a $\sigma$-algebra, and the theorem says that if you take the smallest monotone class containing some algebra, you've already got the $\sigma$-algebra generated by that algebra

0celo7

@ACuriousMind I kinda figured that was the case from skimming the proof.

ACuriousMind

It's a technical statement you need for some proofs, I think, because its easier to show that something is a monotone class than that it is a $\sigma$-algebra

0celo7

Seems really arbitrary though.

ACuriousMind

But I'm not very sure

0celo7

8:47 PM

@ACuriousMind "Note, however, that measurability does not require a measure." I'm crying on the inside.

What does that even mean!?

Do we use the level set to measure functions?

I thought we used the $\mu$ belonging to $\Sigma$! Or is that for sets? What is happening? Who is John Galt?

ACuriousMind

@0celo7 lol, yeah, this is confusing. You take any $\sigma$-algebra for which at least one measure exists and call the elements of it measureable, but this does not require you to fix any specific measure.

0celo7

@ACuriousMind I'm still not sure how one tells if something is measurable.

The author defines the Lebesgue measure for $S^n_r$, but what about other sets?

ACuriousMind

@0celo7 The whole shebang with the $\sigma$-algebras and so means that you need only define the measure on the open sets to define it on the Borel sets.

0celo7

Aha! Upon careful reading, I see that this is left to the reader. As an exercise. :: Sigh ::

ACuriousMind

It "extends naturally"

0celo7

8:55 PM

How? Suppose I take the interior of unit cube. That's open. How do I measure it?

ACuriousMind

@0celo7 The open balls are a basis for the topology, and a measure on the basis extends to the whole topology :D

0celo7

@ACuriousMind WTH does that even mean...

Do I try to fit a bunch of tiny spheres in there XD or what

ACuriousMind

@0celo7 Essentially, yes (at least, that's how I think about it)

I'm not sure I've even seen the explicit construction once

0celo7

@ACuriousMind I thought the whole point of freaking measure theory is that we don't have to cram a bajillion tiny things into bigger things to do integrals!

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