The h Bar - 2015-08-28 (page 1 of 2)

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12:51 AM

*Who's

:(

1:02 AM

@Qmechanic @DavidZ: I'm confused. One my flags for a res. rec. post was declined (by David, I think) with the reason "we don't make these CW anymore". But Qmechanic converted this res. rec. question into CW just now. What's going on?

Could be miscommunications twixt the mods. My thoughts were similar to the message that was issued on your flag.

^

Perhaps everybody didn't read the memo :P

Is it discussed on meta?

@dmckee I also looked on meta, but I couldn't find a clear post whether the policy is to make them CW or not, just personal opinions with various votes.

CW?

Community Wiki

1:08 AM

I think there was something on the mother meta that mostly disowned the idea of imposing CW on posts as a policy decision. That is post should be either in or out, not in some wishy-washy middle ground where they are tolerated when CW.

Well, there's this blog.SE post.

Within that outline of what Community Wiki is for, I think it's clear we shouldn't make res. rec. questions CW.

What is the purpose of the community wiki? I don't want to read fairy tales of meta :)

But then, I have a hard time seeing how we shouldn't close them for being primarily opinion-based or too broad.

@FreeMind here

the only thing which might remain in my memory after leaving SE would be the name of the damn moderator who closed my question :\

1:22 AM

the community-wiki tag has no usage guidance

:/

:\

:/

:\

(removed)

re(removed)

you're a bunch of children

god where are the adults

1:34 AM

busy adulterating

5

^

@0celo7 You're old then?

Jul 30 at 21:27, by 0celo7

I'm not even an adult.

@ACuriousMind so

I never claimed I was one

these kids here are out of line

Aug 2 at 20:24, by HDE 226868

This room stars things waaaaaaaaay to much.

1:38 AM

*too^

@ACuriousMind so?

what is the baseline

and why do we star "too much"

maybe we're not uptight f*cksticks and we say lots of interesting things

@ACuriousMind "The spaces of states on Σ is given by some gauge invariant wavefunctionals Ψ[A]."

I thought you didn't know anything about wavefunctionals

@0celo7 do you see me actually doing something with them? I mention them and then immediately switch to $L^2(\mathfrak{G})$ as the space instead.

@ACuriousMind ok, just skimming

Skimming is good.

1 hour later…

2:58 AM

ello

2 hours later…

4:28 AM

ello^2

user image

4:40 AM

can someone help me with my question

2 hours later…

6:44 AM

@Danu Jaffe and Thruston....very intersting indeed ;-)

@Slereah In euclidean time you have a good number of rigorous results on path integrals (stochastic integrals); at least QM and low dimension QFT (interacting, for the free one there is not much problem)

for recent works (but the language is not so easy) see the fields medal Martin Hairer: arxiv.org/abs/1508.05261

3 hours later…

9:20 AM

Heyyy

Not cool paper

You don't put a theorem in the intro

When you're a Fields medalist you can put a theorem anywhere you want :P

9:59 AM

So rude

10:13 AM

I meant it as joke, albeit a sarcastic one. I apologize pal @Slereah :(

I meant him, not you :p

There should be a list of cardinal sins

@FreeMind I don't think there is much of a purpose anymore. At best, it's an indicator that a post is meant to be collaboratively edited - but since anyone can suggest edits anyway, it's just an indicator, it doesn't really enable a whole lot.

2

You shall not put useful informations in the intro

You shall not reference unpublished papers and private correspondance

You shall not let the reader show

You shall not make up notations for the purpose of this paper

@DavidZ there should be some "usage guidelines," no?

Perhaps I'll ask on meta.

10:29 AM

Is it legal for me to ask "Who runs Bartertown" on the SF Stack Exchange

SF?

science fiction

who run barter town

SAY LOUD

Why would there be a legal issue?

Because it would just be for the purpose of fun :(

user image

Looks like one of those post apocalyptic movies?

WW IV will be fought with sticks and stones.

10:44 AM

@Rigor for community wiki? I'd say the usage guideline is "don't use community wiki" :-P

:D

@Rigor : What about WWIII

I wouldn't worry too much about civilization collapse really

Civilization collapsed a bunch of times

We got out okay

That's an Einstein quote @Slereah

Remember when civilization totally collapsed in Europe and writing and governments disappeared for 300 years?

I know

Nukes are scary.

10:50 AM

Are they that much, though

Unless people go way overboard with nukes, they would just blast the big sites

Which is bad news for me since I live in the capital

But otherwise most areas should be fine unless they just throw out everything they got

Nukes aren't really that popular anymore, really

They turn football fields of dessert sand into glass :(

a football field is p. small

The effective radius of an atomic blast is only about 20 km

Beyond that you're pretty safe

(unless radioactive fallout, 'course)

^

I'm not saying it's gonna be fun

But I don't think a nuclear war would wipe out civilization

Would it send us back to the Stone Age?

10:54 AM

Why would it

The only people who actually went back to the stone age were people who were already in the stone age

After all the mutations

It's pretty hard to regress that much

Some pacific islander lost some technologies after getting on some really shitty islands

But they were already stone age people

Wait, maybe some mesoamerican people did go back to the stone age, too?

I forget

What happened during the Maya collapse

Mayas were p. close to the stone age too, though

Let's say the radiation was so bad we all had to live in caves for 100 years?

Or 500 years.

Dunno about food though.

Game Over

If the radiations are that bad we are all dead

Back to evolution

Return to the planet of the apes.

11:03 AM

If the radiation is too bad for people, odds are pretty good apes are dead too

They're already endangered

Sorry for being a downer.

I meant when all life on the planet lived in the sea.

1 hour later…

12:09 PM

@Rigor Fallout 4 will probably be training for the real deal...

at least the music will be good~

Plus if you're a ghoul you get to be immortal

12:25 PM

Some of us, cf. e.g. ~~Alexander~~ Bajoran, are already immortal.

1:23 PM

@ACuriousMind Why did you not direct me to your thesis when I asked about connections on bundles?

(I am compiling a list of typos :P)

(Not really, but I agree that there are some.)

@JohnDuffield How goes it today?

@ACuriousMind The most egregious error I see thus far is you missed a \mathrm on page 9 D:

you dare write a constant as a variable? tsk.

reads on ...is QoGS out of my reach until I can understand this?

I get the geometry but not this group theory

1:40 PM

What is QoGS

amazon.com/Quantization-Gauge-Systems-Marc-Henneaux/dp/…

@Slereah you should get it and give me a full list of stuff I need to read it

Hans Reichenbach has proposed in 1924 [...] but this would result in an unnecessarily complicated formulation of special relativity.

Damn, Gourgoulhon cooking the Germans up!

2:06 PM

@0celo7 I...didn't think it would be all that helpful

@0celo7 There is no group theory in QoGS, since they consider general constrained systems which need not have a gauge group in the usual sense.

@ACuriousMind it's a short and sweet overview, I will read it more carefully later!

I skimmed your paper in LA, and I also read some in Gourgoulhon

Mr. LATA was not impressed when I proposed tensor product as an operation on vectors ;(

now I get to do constant acceleration kinematics...woo

@ACuriousMind OK so once I've got some algebraic topology under by belt I'm set?

what on earth is homological algebra

@0celo7 The general conception of (co)chain complexes and their (co)homologies in arbitrary categories.

@ACuriousMind do I need to read a whole book for that, is it self contained, ...?

@0celo7 You should read Alice in Wonderland

@0celo7 The topology aspect will be rather useless, but that you've seen the idea of chain homotopies and homology will be useful.

2:10 PM

@Slereah I should read some fiction again...what are some good LQG books?

@0celo7 I have no idea if there are pure books on that, we did it during an algebra course, and also a bit in algebraic topology

Rovelli

@ACuriousMind do you think Hatcher will give me enough algebra to get to QoGS?

I don't even really know why I want to read QoGS

@0celo7 Probably yes

Also derivations and graded modules and such would also be useful :P

@0celo7 That I don't know either :P

@ACuriousMind is my comment here OK physics.stackexchange.com/questions/203067/…?

I mean, you'd need to have such a vertex, right?

and I can't come up with one, but then again I couldn't find page 16 on that powerpoint yesterday....

2:14 PM

@0celo7 If the process is to be at tree level, then yes

However, there is e.g. that $2\gamma \to 2\gamma$ box diagram, so I'm not sure you can't make a $\gamma\to 2\gamma$ at higher levels.

@ACuriousMind ahhh

@ACuriousMind interesting point...

Annav's answer is correct though - even if the process would be in principle allowed, it's kinematically forbidden by the conservation laws.

I remember an anecdote about Norbert Wiener

He was with some big mathematician in a classroom

And they were both staring intently at a wavy line on the blackboard

@ACuriousMind call me stupid, but I don't get that

Like it was

THE MOST COMPLICATED PROBLEM

To this day nobody knows what the line was

2:17 PM

if it emits each with 1/2 the energy, what is violated?

@Slereah you're like /b/ and /sci/'s love child on drugs

random!

scientific!

high as a kite!

I wish

@0celo7 Hmmm...perhaps the argument doesn't work for splitting into photons

It forbids $\gamma\to e^+ e^-$, but perhaps not $\gamma\to 2\gamma$...

lol did you remove your +1?

I don't think $\gamma \rightarrow 2\gamma$ is allowed

I READ THAT

lol

2:20 PM

You son of a motherless goat

@ACuriousMind this is going to sound stupid again, why

@0celo7 The $e^+ e^-$ system has a center of mass frame with non-zero mass: The square of its total four-momentum cannot be zero.

But the square of the four-momentum is zero for a photon.

OOOOOOOHHHHHHH

like 2 years of confusion gone

@ACuriousMind I left a comment on annav's answer

You can totally have $e^+ e^- \rightarrow 2 \gamma$ though

Although I think there are restrictions on the polarization

Since positronium only has two polarization states

@Slereah Is being a son of a motherless goat worse than being the son of a goat or being the son of a fatherless goat?

2:24 PM

ortho and para

Can you get spin 1 from two photons?

Or is it gonna be 0 and 2

Let's check Clebsch Gordan

Hm, you can apparently

Tho you couldn't get angular momentum of 2 then

@Slereah are you a GR or QFT person

you seem to go back and forth

@ACuriousMind physics.stackexchange.com/questions/203067/…

@0celo7 True in all but the degenerate case where the two photons are collinear.

@ACuriousMind are you gonna comment that?

Yes

@ACuriousMind because the sum of two null vectors need not be a null vector?

2:39 PM

@0celo7 Exactly, and when the spatial momenta are not collinear, you can explicitly show that the sum won't be a zero vector.

cool beans

@0celo7 I prefer GR

both are fine, though

What I hate is thermodynamics :V

And fluid mechanics

I take a dozen courses of those

why do you hate them

@ACuriousMind you should fite Floris

he's trying to rek you

@0celo7 That comment thread is going to be glorious :P

oh shit Crazy Check Man jumping in!!!

HNQ TIME

2:49 PM

Haha, he's on my side (I think)

I THINK HE REKT YOU

maybe

(that misspelling was intentional)

I'm wary of bots looking for the correct spelling

He just told me that indeed the collinear splitting is not forbidden by conservation laws, but by the gauge symmetry. I'm not sure he's thought about it deeper than "there's no 3-photon vertex for Abelian gauge theories".

...did he agree with me that gauge invariance is the real reason?

@ACuriousMind I misread that at first

I want to jump in for no good reason

@0celo7 "real reason" - for the collinear case, yes, but in all other cases, the conservation argument also forbids the splitting of non-Abelian massless gauge bosons.

Lumo is the Donald Trump of physics

@ACuriousMind of course

2:53 PM

@0celo7 Although they have a three-gluon vertex!

@ACuriousMind of course

I'm agreeing with what you're saying

The amplitude of that for non-Abelian gauge bosons is the structure constant of the group.

I vaguely know that....

will relearn when I get to page 700 in Weinberg

@ACuriousMind Let's do that colinear bit. Two null vectors which are spatially colinear are proportional in 4-space, right?

@0celo7 Yes

oh well that's trivial then

now how to show "only if"...

@0celo7 Yes

@ACuriousMind is "only if" trivial?

3:05 PM

@0celo7 It's pretty easy. :P

Just take the sum of two null vectors, set it do zero, and derive a condition on the angle between the spatial parts from that.

back soon. Going swimming the only costless pleasure left in Greece. — anna v 13 mins ago

@ACuriousMind I'm waiting in line for free stuff

It's not easy in my head :/

Also damn my backpack is heavy

Oh no I got a downvote

It must be an EVIL CONSPIRACY TO BRING DOWN REAL PHYSICS

You're welcome

@ACuriousMind I really want Lumo to go on a Greek politics rant

3:24 PM

@0celo7 I think there are enough of that on his blog :P

Hi. I have to participate in a quiz this September on "Inventions and Discoveries in the field of Physics in the past 100 years". What topics should I focus on? Since it is a quiz, I think the Historical facts will be the basis.

3:43 PM

hi

@FenderLesPaul yes... could you help me with a few tips?

@ambigran_maker sorry I don't think I'll be of much use there

I know very little about physics history

okay... does someone else know?

Well we don't know what the quizz will be about

Although usually, pop culture of physics in this era is like

heavy on QM and such

quantum mechanics, relativity

Those are the crowd pleasing topics for some reason

Though if inventions are included, that makes it a bit wider

Semiconductors are probably gonna be there

black holes

lots of black holes and multiple universes

3:49 PM

oh... they said they'd focus on Einstein

Einstein was a punk

he didn't even know geometry

@0celo7 I'm gonna pretend I didn't see what I clearly just saw.

?

hi

o/

3:52 PM

@0celo7 No... seriously. Quizes are on facts right?

sometimes

in principle, yes

@Danu hates me

@0celo7 You really overestimate the amount of feelings I have about you :P

@Danu oh god you're ambivalent

Will years, names of scientists and theories suffice?

that's worse!

@ambigram_maker what do you know about

we can see if we need to add anything

3:56 PM

@0celo7 ... who is a programmer?

@FenderLesPaul wassup?

sigh...

@ambigram_maker maybe that Einstein got a nobel proize

if you're taking a quiz on developments in modern physics, you're not average

at least in my experience

@0celo7 slightly above. ;-)

will talk later

This is a nice book on the history of modern physics.

I read it last year and it was very boring so beware.

4:00 PM

@FenderLesPaul ::replaces Skyrim soundtrack with She Twerkin :: ready for a stream?

Is it the weekend already?

it is Friday

@0B3 who are you

@0celo7 obe.

dammit not this again...

prove it

eww I have to take statistics to get a double major in math

@0B3 are you doing QM? if yes, you're a damn imposter, if no, stop procrastinating

ok why does my roommate leave his desk light on

@FenderLesPaul I just noticed your avatar is not a spaceman holding a laser rifle

4:21 PM

Found this

@0celo7 I think that book ought to suffice, eh? What do you think? :-)

@ambigram_maker sure

why is Hawking the scientist of the decade for the 80s

he did his good work in the late 60s and 70s

Perhaps because of his book/movie

4:36 PM

his only good book is The large scale structure of space-time

@Rigor lol

@0celo7 It was Hawking's Brief History of Time that compelled me to take up Science in 11th grade.

(along with some others)

you should read his real book

It's about high school senior level

The movie is good.

lol Hawking left his wife for his nurse?

@0celo7 The large scale structure of space-time?

4:41 PM

and the nurse's husband is the one who made his speech thing?

@ambigram_maker si senor

ok

I have my copy right here

I assume you know some algebra and calculus (@0B3 ;D)

@ambigram_maker (Don't let the comic sans like font fool you. It's an extremely hard book to read. Unless you have experience with differential geometry, point set topology and PDE theory, you won't be able to enjoy it.)

@0celo7 right the basic type, enough to read his book.

I think the quadratic equation is used at some point in the book...

4:58 PM

@0celo7 Hawking did a lot in the 80s

@0celo7 Good thanks, though seeing as it's Friday night, I have to go. PS: don't stay in your room too much. Get out there.

Have you read any of Hawking's work @JohnDuffield?

user54412

5:27 PM

@0celo7 You have to thoroughly know statistics to be considered an educated ~~scientist or engineer~~ person in the 21st century.

@ChrisWhite I get stats through my regular courses...I'd have to take an additional stat class

@ACuriousMind I guess he should read algebraic topology.

my book is still not here

maybe they lost it in transit?

Track it

@0celo7 Czech, lol

@0celo7 Use the explicit expression for the inner product of two vectors

5:33 PM

@Danu I forgot about that, I'm sure I can get it

I typed that while standing in line for BBQ

@0celo7 His GR work is not as important as his QFT in curved spacetime work, if you judge by current "hot"ness

@Danu that was '75, no?

3 hours ago, by 0celo7

(that misspelling was intentional)

my mother is Czech...

@0celo7 That would logically be referring to the last made spelling error (REKT instead of WRECKED) (#rekt)

May I read your essay that started with The Lament? @Danu

@Rigor Nah, sorry.

5:35 PM

:(

It's about mathematical beauty, not about the fine points of Hardy's argument anyways.

Thurston's essay that I linked yesterday in a message to yuggib nicely complies with my conclusions :)

@Danu I see your point, but rest assured I know how to spell Czech

But what about this? :P

I've heard of it

user54412

Is that the compactification guy?

5:40 PM

Yes

Stone–Čech compactification

In the mathematical discipline of general topology, Stone–Čech compactification is a technique for constructing a universal map from a topological space X to a compact Hausdorff space βX. The Stone–Čech compactification βX of a topological space X is the largest compact Hausdorff space "generated" by X, in the sense that any map from X to a compact Hausdorff space factors through βX (in a unique way). If X is a Tychonoff space then the map from X to its image in βX is a homeomorphism, so X can be thought of as a (dense) subspace of βX. For general topological spaces X, the map from X to βX need...

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