The h Bar - 2016-08-15 (page 2 of 5)

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1:00 PM

@Danu Thanks a lot :(

I loved you all this time...

@BernardMeurer I use an old pair of Sennheisers that are older than you are. They aren't the very best headphones but they are comfortable and I love them dearly. We geriatrics get sentimental about such things.

@BernardMeurer Oh, don't worry---the topics you talk about just don't interest me that much but it doesn't mean I don't want you hanging out here. You are a nice guy ;)

@JohnRennie Lol, You must have a lot of nice things older than myself, it's not exactly hard :p

@Danu Awwwwww :*

@Danu You have a weird idea of fun :P

@Danu Interesting content: can you explain to me exactly what it is that a creation operator creates? ACM explained it but I didn't understand a word.

1:01 PM

@Danu I don't get how blowing up fruits is not interesting to you

@BernardMeurer lol

@ACuriousMind Oh, that's not fun :P It's just something that needs to be done.

@BernardMeurer don't worry, I look forward to you telling us you set your kitchen on fire again :D

Like when I retagged 350 questions in 2 days

@BernardMeurer my speakers are Heybrook HB2s bought in 1980. I've tried many other speakers since and I like the sound of the Heybrooks more than any other speakers I've tried, including Linn exotica.

1:03 PM

@JohnRennie In what context? In the Fock space context I think it makes some sense. In more general QFT, I couldn't really tell you.

user image

I just learned to work with it without understanding what I'm doing in a deep way ^^

@BernardMeurer: when are you in Germany?

@Secret You don't need K-theory :P

Caption: My learning progress in terms of preparing for string theory (red out items)

1:04 PM

@ACuriousMind Hopefully I will live a long and kitchen-fire full life :)

I don't even know what K theory is nor it existed until now

but lol thanks for telling

@Danu I'm still thinking about that what is a photon question. I realised that I didn't know what any particle is.

@JohnRennie I don't know yet. Will depend on how well I can manage my money in Portugal. If not late this year early next year

@Danu Why not?

In my simple-minded way I've been thinking that a particle is what a creation operator creates (in the free field limit)

I thought - aha - a creation operator creates a plane wave, because that seemed to be what the free scalar field beginners articles were saying.

1:05 PM

@0celo7 The burden of proof is on you to show me why one does need it :P

@JohnRennie I don't currently own speakers, although sometimes I wish I did. I'm currently looking to get a better DAC than the portable one I have

ACM agress but says it isn't actually a plane wave in the sense of a de Broglie wave.

@JohnRennie Why do you ask?

@Danu AS index theorem?

Whenever I head "de Broglie" I go back to trying to read the QM book by Bohm my grandma gave me. Traumatizing

1:07 PM

Sakurai smells funny

I wonder why some books smell good and some don't.

@0celo7 I don't think you strictly need K theory for it---it was also the thing that popped into my head.

@BernardMeurer Well you were talking about getting a laptop and I have some broken ones here that would be easy to fix. I was going to say you can have one if you want, then i saw how much it would cost to ship one to Brazil.

But I think that literally no single professor at LMU except Mayr and perhaps Brunner knows some K theory.

@JohnRennie Well, maybe I should qualify that statement a little: When you go to the "QM limit" where wavefunctions make sense, it would be a plane wave. But in full-blown QFT, states are not given by wavefunctions, so "plane wave" as such doesn't make sense

They get away with using Atiyah-Singer just fine.

1:08 PM

@BernardMeurer But it would be much cheaper to ship one to e.g. ACM in Germany if he doesn't mind holding it for when you visit.

@SirCumference : both. We have no evidence for an infinite universe. We have evidence for a "flat" universe, but that isn't actually evidence for an infinite universe. There's a non-sequitur in there. See this answer for more information.

@Danu Depends on your background. Analysts like my advisor like the heat kernel proof (see Gilkey), but if you're an algebraic toplogist/geometer you might like the K theory approach better.

That's not to say that that's a good state of affairs (I personally dislike this thing, that people in physics often do)

@JohnRennie Oh wow, that's so nice of you!

@BernardMeurer what are you getting?

1:09 PM

Let me see if I have space on my beer list... I'll need a new notebook soon

@0celo7 Your mom

TSSSSSSS

Well I get broken kit from the place I work, and I can usually cobble together working stuff from the broken ones.

You want a 60 yo woman? Ok

@BernardMeurer lel

If that's all you can manage to get...

@0celo7 SIGH

1:10 PM

And to be honest the third gen i5 laptops are dirt cheap anyway.

@0celo7 Tell that to all them german ladies

For example look at:

ebay.co.uk/itm/-/291843264699

@JohnRennie If I get my hands on the ICC this world is over

The only reason the human race still exists is the fact that I have an AMD processor

3

I've got beef with our reviewing system. We seem to be closing questions as unclear all too often lately. Especially long questions. Are people too lazy to take the time to properly review long questions so they auto-vtc for whatever reasons are already there?

I'd guess that will sell for about £150, and thats with a 2.9GHz i7!!

1:11 PM

@ACuriousMind @Danu Sakurai postulates that the eigenkets of an operator span the Hilbert space it acts on, but can't one derive that?

@0celo7 Spectral theorem.

and half the unclear-closed questions I come across aren't unclear at all

@JohnRennie What the hell

@ACuriousMind Not a Hermitian operator.

@0celo7 wat

1:12 PM

^

Then it's false

not-self-adjoint operators aren't guaranteed to have eigenbases at all

So anyhow, what I'm thinking is that if I can put together a laptop from bits that work or mostly work would be feasible to ship it to ACM and have him hold it for when you deliver the beer you owe him?

Also I don't know what the spectral theorem is

He probably postulates that the eigenkets of an observable span

Oh, he uses $A$ for an observable, and he slyly switched to $A$ in that section

1:13 PM

@JohnRennie I'd be extremely grateful for that, as long as ACM is okay with it too. You know he doesn't like immigrants... :p

This is why physics books need clearly delineated results

@JohnRennie This sounds hilarious

@Jim I do think that unclear what you're asking is often used in places where a custom reason would be more appropriate (and I'm probably guilty of that as well)

@BernardMeurer ???

OK, when you firm up your plans give me a few weeks warning to sort something out. If ACM isn't around or for whatever reason that isn't possible we could use some other address in Germany.

e.g. the illegal immigrants detention centre? :-)

Sure I'll hold the laptop ransom till I get my beer

3

1:15 PM

I am also in Germany

I also am interested in beer

Do I qualify?

We should do an h-bar meet-up in Germany

@ACuriousMind Well now wait a moment, why doesn't a general operator have a complete eigenbasis

You guys can chat about physics and I can be confused and drink until I pass out

@0celo7 Because it may not have any eigenvectors at all

Think rotations

Sounds like Christmas

1:16 PM

@Danu The vector space it acts on is complex.

@ACuriousMind I don't use unclear as a reason unless I'm familiar with the subject and the question simply makes no sense at all. If I can figure out a hidden question, maybe it's too broad, maybe it's off-topic, or maybe it's just too tedious for me to want to answer. But that last one isn't grounds for closing

@Jim : I share your sentiment. I think some people are too keen to close questions. I see intelligent questions from educated posters getting closed.

@JohnDuffield as unclear?

@0celo7 Consider the shift operator in $\ell^2(\mathbb{C})$ that sends $(1,0,0,0,\dots)$ to $(0,1,0,0,\dots)$, $(0,1,0,0,\dots)$ to $(0,0,1,0,\dots)$ and so on

You know I don't know what that means

1:17 PM

^that's the standard example

@Jim : I'll have to check.

@0celo7 You can google---this is really not hard to look up.

@0celo7 $\ell^2(\mathbb{C})$ is the space of square-summable complex sequences, the things I wrote there are sequences (the sequences that have 1 at the i-th spot and 0 else form a Hilbert basis of the space). Now you know what it means, and it should be clear that this thing doesn't have eigenvectors (try writing one down).

@Danu: see, this is the way the chat should work. Constructive conversations about physics rather than bitching about each other. It can be done!

@ACuriousMind sure it doesn't have eigenvectors, but what about the finite-dimensional case

1:20 PM

@JohnRennie But there is some serious background noise :P

@ACuriousMind You dirty downvoter!

@JohnRennie You old lettuce!

@Danu Pesky mathematician!

@JohnRennie There, ruined it :p

@Jim Well, I tend to use "unclear what you're asking " also when there's details missing that would be needed to unambiguously decide what kind of answer is needed there

the fuck, I'm not important enough to be insulted

(I hope lettuce is an actual cursing. If not it is now)

@0celo7 Consider any projector.

1:21 PM

In the days when the Commodore Pet was the peak of personal computing power I wrote a program that prompted the used for an adjective, then a noun, then randomly selected from the ones that had been entered and flashed up the message "you <adjective> <noun>"

3

It used to produce things like "you old lettuce"

@JohnRennie It is me father

Until people worked out what it did and erm livened up the database with some gynecological terms.

@ACuriousMind why does the characteristic equation fail

@JohnRennie Great

@BernardMeurer lol

@0celo7 wat

1:23 PM

don't you get $n$ eigenvectors from that

It gives eigenvalues, at best

@ACuriousMind as long as unclear is being reserved for questions with a non-positive amount of clarity, that's fine with me

For some reason I hate the name eigenwhatever

^me too

hipsters

1:24 PM

@BernardMeurer Yeah, why did Mr Eigen have to find all this stuff, right?

@0celo7 there's no way anyone ever told you that, because that would be an insult, which you aren't important enough for

Dr. Eigen.

@ACuriousMind He should've just been a baker

Isn't there a mountain called the Eigen?

And let someone with a better name discover this crap

1:25 PM

@BernardMeurer Baking the eigenbagel?

@ACuriousMind IT NEVER ENDS

@JohnRennie I think that's Eiger

Oops, oh yes :-)

@JohnRennie I'm afraid only Mrs.Eigen could climb that mountain

@ACuriousMind does one have an eigenbagel with eigenbutter?

1:26 PM

WINK

my statistics book is written by Marx

@0celo7 Watch out, you may be calling mainstream physicists opressors before noon

@Jim Probably. Although I do not wish to know where eigenbutter comes from

@Jim : I had a look at the recent questions, and I struggled to find any intelligent questions by educated posters that were unfairly closed as being unclear. Is it my imagination or is the close queue much shorter these days? As if fewer questions are being nominated for closure?

@BernardMeurer I'm generally unconcerned with physicists

pretty sure they make up whatever

1:27 PM

@ACuriousMind Ah! This reminds me! I need your help with some notation

Let me get a link, 1 sec

@ACuriousMind Here Page 16, Rice's theorem

ACM will not know

He doesn't even know what a flow is

I have no clue what he's telling me in Definition 8

pages are not numbered

@0celo7 He will definitely be more helpful than you though

@0celo7 They are if you have a proper PDF reader

what part exactly confuses you

1:30 PM

@JohnDuffield mostly it's the really long or low-level questions that are unfairly closed as unclear. They could be fairly closed as something else, but using unclear as a default is what I find despicable

@0celo7 What's that C with an underscore?

@Jim : can you provide a couple of examples?

x ∈ L ⇔ y ∈ L

@Jim to be fair I think unclear is often used to mean you haven't explained your question clearly i.e. haven't given enough detail for it to be answerable.

Also this

1:31 PM

@ACuriousMind What do I do in my topology course when AoC is invoked? Can I refuse, like conscientious objectors in war

C with an underscore

⊆

@BernardMeurer $x\in L$ if and only if $y\in L$.

...

subset or equal to

Then what's just the C?

I thought that was just C

depends on their notation

Oh god

1:32 PM

I use subset for subset or equal to

and then specify when it's not equal

Also I don't really get how they define triviality

they should tell you, but very likely will not tell you

@JohnRennie let's be fair. I've been reviewing long enough to know that constitutes unclarity. I'm talking about the questions that are worded outside the normal terminologies but still completely clear, the questions that have less than ideal prior research, the really long ones that take a while to as a clear question but you'd have to read the whole post.

@0celo7 Why do people do this

@JohnDuffield I can search for em

1:33 PM

@0celo7 You specify it every time? :P

@BernardMeurer I'm no comp sci expert, but I think it's saying that the language is "closed" with respect to Turing machines in the sense that if one element produces a Turing machine with a certain set of accepted inputs, then all other descriptions producing a Turning machine with the same accepted input are also elements of the language

@Danu It's usually a minority of the times when $\subseteq$ should really be strict.

@JohnRennie go to the mod tools and sort the recently closed questions by close reason. There's a disproportionate number under unclear

So writing $\subsetneq$ every now and then doesn't hurt.

@0celo7 When talking about chain complexes and their subcomplexes it's... very often :P

1:35 PM

I'm not an algebraic topologist

I do agree that it's a big annoying shitty thing that people don't say they use $\subset$ to mean $\subset$ or $\subseteq$ at convenience

I remember when I started self-studying math and this confused the fuck out of me.

Asshole Spivak :D

If I wrote a book I'd probably use $\subseteq$

I would use $\subset$ because no confusion arises in practice, plus I value keeping notation simple

@Jim : yes please.

But I can respect using $\subseteq$ if you really wanna ;P

1:37 PM

@Danu No confusion after you just said it really confused you?

@ACuriousMind I should've just gone for visual arts

@Jim I haven't counted them but the number closed as unclear seems about the same as the number closed as duplicates or off-topic.

@0celo7 Of course I'd mention it in a notation section (at the start of the book!)

@Danu You wouldn't happen to know about the Euler class of a sphere bundle?

Neh

Ask again next semester, I'll know.

I'm giving a class talk on Euler classes of bundles in december or so

1:38 PM

Next semester I'll be way beyond that

But I think it's a trivial issue anyway

hahaha

good

And unless you take the double complex approach, you won't know

Let the triviality flow through you

@Danu Bott & Tu use double complexes and it's really freaking hard to understand

There's three cohomologies floating around at any given time

@JohnRennie but dupes are expected to be higher and off-topic covers more reasons. Unclear shouldn't be equal to them.

1:39 PM

de Rham, Čech-de Rham, Čech

I'll be basing my talk on Milnor & Stasheff

What's Cech-de Rham cohomology? I know the two others.

[Rambles] Next time when I ask question and someone (Let's say X) got confused, I should invite them to join and debug exactly where and how X.exe stop working

@Danu Consider Mayer-Vietoris in de Rham cohomology with 2 open sets

@Jim I'm not sure I see how you can justify that argument. I think there is an issue in how charitable we want to be in accepting questions that are not well thought through.

[Joke] You will never know, perhaps I will become an expert human computer nerd in debugging people one day

1:42 PM

I tend to err on the side of charity (when a sore back isn't making me grumpy) but not everyone takes the same view.

so you have a long exact sequence and a map $\delta:H^q(U)\oplus H^q(V)\to H^q(U\cap V) $ in that long exact sequence

@Jim: but at the end of the day this is a site for physicists and we expect a minimum level of effort to be put in to writing a question. We aren't reddit and don't try to be.

@JohnRennie I'm looking through the list. I think I should clarify my problem now. Most of these questions closed as unclear should be closed. Just not as unclear. My issue is with the usage of unclear more than with the questions that are closed

You now make a double differential complex $K=\bigoplus_{p,q\ge0}K^{p,q}$ where $K^{0,q}=\Omega^q(U)\oplus \Omega^q(V),K^{1,q}=\Omega^q(U\cap V),K^{p,q}=0$ for $p\ge2$

^you wanna edit that last part @JohnRennie

@0celo7 Sure

1:44 PM

@Danu Oops :-)

using the exterior derivative you can turn this into a cochain complex itself by writing $D=\delta+\mathrm(-1)^pd$

Since we seem to have a reasonable number of people in the room at the moment, I have a question for the panel.

and have this act on the "diagonals" $K^n=\bigoplus_{p+q=n}K^{p,q}$

is this a test? I didn't study!

@0celo7 Sure

1:45 PM

Suppose we could get Sabine Hossenfelder to do an AMA by paying her usual rate of $50/20 mins. Would that make for a good AMA?

So the $D$ cohomology of $\bigoplus K^n$ is the Čech-de Rham cohomology

It's isomorphic to the de Rham cohomology

@JohnRennie who do you propose would be paying her?

@0celo7 OK

The real power is that it can be generalized to a countable number of open sets, not just $U,V$

@Jim Leave that aside for now, just assume it could be taken care of.

1:46 PM

@0celo7 ok, sounds like fun!

@JohnDuffield Hence why I said "If our universe is infinite"

then by some tricky Riemannian geometry, you can cover your manifold with a countable collection of open sets, which are contractible, and all finite intersections are contractible

Not "Because our universe is infinite"

My concern is that the AMAs have really just been the usual bunch you see in the chat.

well then, I've never heard of her, so I couldn't say if she'd make for a good AMA

1:47 PM

because contractible sets have good cohomology, this makes like very good

they said they have probem inviting the high profle guests

@0celo7 That doesn't work with just (disk-shaped) chart domains?

And I'm not sure what would happen if we got someone who is actually (slightly) famous.

probably the press will be dragged in afterwards

I am pretty sure that works with just chart domains, at first glance.

1:48 PM

@Danu If you can make it work rigorously, good job

@BernardMeurer No, you shouldn't!

The proof is mostly folklorish

yes, what was the point of the AMAs again? People can already ask me anything in chat, why do I need to sign up for a specific timeslot for them to do that?

I constructed the full proof here:

3

Q: Why are geodesically convex sets diffeomorphic to $\Bbb R^n$?

In the construction of a good cover for a manifold $M^n$, Bott & Tu use the fact that each point in $M$ is contained in a geodesically convex set (after picking a Riemannian metric). They then claim that the intersection of geodesically convex sets is a geodesically convex set, and that makes sen...

differential-geometry algebraic-topology differential-topology riemannian-geometry

@Danu the intersections give you trouble, I think

1:48 PM

@ACuriousMind An intersection of finitely many disks will not be nice?

I read that as geodesically convex sex

3

The right way to read it

@JohnRennie the best kind of sex

great curves

That word salad does not make sense. I mean, how will you describe it, hmm...? (lol)

@JohnRennie Well, if you look at DanielSank's AMA, there were several non-regulars there, and they seemed rather interested (one even explicitly requested to keep the thing on-topic because they were interested in what Daniel had to say). I think the non-regulars come if they get someone interesting. A random physics.SE user isn't, a quantum engineer at Google is, and an actual quantum gravity researcher would probably also be

1:51 PM

@Danu You use Čech-de Rham to prove that de Rham $\cong$ Čech, but looking at Čech-de Rham on its own is interesting.

@ACuriousMind Dude. I could easily draw the sistine chapel with only my mouth

@Danu Well, you have no guarantee that the disk-homeomorphic regions intersect nicely, e.g. that the intersections are connected

What do you guys think will happen if T Hoot is a guest of AMA?

Hoot?

Is he an owl

@0celo7 Hmm, sounds very different from the proof i saw

1:52 PM

't Hooft :-)

@ACuriousMind I guess the problem is making the disks round ^^

sorry, cannot spell

@Danu Duh! :D

@JohnRennie Yes! Correctly spelled! 10/10!

@ACuriousMind And you think it cannot be done?

@Danu Oh, I think it can, but I'm far from convinced that's easy or straightforward

1:53 PM

@ACuriousMind Quoting the great thinker from our time, Justin Bieber, Never say never

Knowing where the apostrophe goes in 't Hooft's name is the ultimate proof you are a nerd (... or Dutch I suppose).

@Danu Gromov wasn't able to do it.

user image

And what's wrong with my proof, anyway

@JohnRennie what's the difference? ;)

1:54 PM

@ACuriousMind INCOMING :-)

@Slereah no chinese?

@0celo7 It's not conceptually clear to me.

The Chinese got it right @Secret

@Danu Do you know what a geodesically convex set is

Nope

Ayy

1:55 PM

Is it a set where all points can be joined by a geodesic lying in the set

ie

I thought you took a Riemannian geometry course

a convex set

it is the roundest of sets

@0celo7 As I told you many times, we just did Chern-Weil theory there

what about the disk

1:56 PM

Riemannian geometry was 2 weeks as a "trivial application"

Disk is pretty round

@Slereah the disk is geodesically convex!

but not all geodesically convex shapes are disks

ie : the square

@Danu The vast majority of Riem geo has nothing to do with connections in the Chern-Weil sense.

Anyway, I have to go shopping if I want to eat today. I'll see you all later.

1:57 PM

@0celo7 I know that

@Danu Ok, a geodesic convex set is a set in which any two points may be jointed by a unique minimizing geodesic lying entirely within the set.

@0celo7 I did already see that in your post

Ok, see Lemma 6

that's why it works

Anyways, the proof you outline is just what I suggested (take "disks" and make sure they're round)

they have well-behaved intersections

1:59 PM

For me personally, it's more important to have a one sentence set-up like that at the beginning than the gory details---I lose interest when you get bogged down in the details.

But it's good that you worked it out! +1

@Danu The proof is sketched in BT, my goal was to have the details worked out

Btw, @Slereah, are you currently free to check my scifi time travel setting with me to see if it is logically sensible?

@0celo7 Well, good job then

@Danu And Ted thanked me for it because he never did it ;)

Good for you :P

2:01 PM

Off to the shower

only took me 3 hours to get out of bed today

I'm getting back into the school schedule

@JohnRennie Look at the back of the freezer first ;)

2:19 PM

@ACuriousMind How does vacation/holidays schedule work in european unis

@BernardMeurer I'm not sure how universal that is. Even within Germany, the exact dates vary by as much as a month

Sigh... Why can't we just have consistency

@ACuriousMind Since you keep good track of the review queues: How is the proposal regarding deleting low quality content going?

Is more stuff getting deleted/flagged for deletion?

@Danu If not for the meta post, I would not have noticed anything is supposed to be different at all

I had the same feeling, haha

2:24 PM

This close to getting Linux on my Kindle again

Or rooting the Linux on my Kindle rather

Nice weather today

@BernardMeurer You didn't answer my question

@0celo7 Which one?

Why do you think Germans have any authority over shoes?

@ACuriousMind how can I track what happens in the review queues to a post I flagged?

@Danu Either go to the timeline of that post, or search for its reviews in the review history

You go to the timeline by going to the revisions page and then replacing revisions by timeline in the URL

2:27 PM

@0celo7 Because she was reasonable enough to not like those dreadful Jordans of yours

If you flagged it recently, going to the review history is probably easier

Neat.

Ehrmaged

I think I got it

Linux on the Kindle

@Numrok I am wondering why you thought this post was an answer to the posed question.

@BernardMeurer Wait wait, which ones did you even show her

2:29 PM

@0celo7 The ones you just bought and was snapping me

Jordan 5s or some bullshit?

Seeing as how the poster literally said "I don't know" to the main questions.

Dude these are fucking space boots

They're awesome

Not wearing them today

I've got black Jordan Eclipses on

you should like those, actually

@ACuriousMind How do I find the revisions page if there are no revisions?

@Danu Then you need to compose the URL manually, i.e. find out the postID by some other means. The easiest way is to click on "share" - the number behind the /a/ is the postID. Then you go to physics.stackexchange.com/posts/<postID>/timeline.

2:33 PM

Thanks.

Why is all of this so obscure?!

@0celo7 Those look OK at best

@Danu I suspect because it is a strain on the servers to generate the timeline and they don't want people to generate a timeline for a given post unless they really need it

But I haven't got a really good reason

@heather Can you explain why you thought this post addressed the posed question (despite containing no mathematics at all!)?

@ACuriousMind Fair enough.

@BernardMeurer what's wrong with them

@0celo7 They don't look good

2:39 PM

Why not

It's subjective, I don't know why they look good or not, I just know they don't please me

which doesn't mean your or someone else will have to agree with that

There is no truth in opinion

Of course there is

3:00 PM

@Danu @Danu when I first read it I thought it did address the question but was just wrong. and votes are there to point out that an answer is wrong. reading it again you are probably right it does not address the question at all, my bad... sometimes hard to tell the difference between "wrong answer" and "not an answer" :P

@Numrok I suggest you flag it again (to push it back into the queue).

user image

Ha!

@BernardMeurer lol

And I've now added a provision against their silent updates that kill my jailbreak

Suck it²

@BernardMeurer Bob doesn't like hackers

thinks they're scum tbh

3:18 PM

@0celo7 Good for him

3:51 PM

@JohnDuffield liked the hossenfelder ref to hydrodynamics and drops "pinching off" as a natural model of singularities, did you see that? think shes much closer to reality than she realizes.

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