The h Bar

tpg2114

05:00

@0celo7 Can you cite a source that says I would even need to do that?

Slereah

[40] Private communication overheard in the mens' room at a recent conference, but we didn't see who it was because we didn't want to get up.

0celo7

[34] Toilet stall conversation with unknown physicist at HEP conference

Slereah

[22] All our friends (until now), "Conversation of Energy: ∂φ/∂k," private gossip.

0celo7

[69] Thesis: unpublished

tpg2114

^ in Slovakian

0celo7

05:02

@tpg2114 what

Slereah

[38] (WRITE YOUR NAME HERE), My favorite paper, My Favor. J. 1 (1980) 1.

0celo7

suppose you need to know about geodesics on Lorentzian manifolds

just make some shit up and cite HE

people will believe you

ACuriousMind

Hmmm...is it acceptable to write "This is a straightforward calculation:" and then do three lines of turning symbols into other symbols to show it?

tpg2114

I actually reviewed a paper for an English language journal where more than half of their citations were for Slovakian theses (or thesii?) that were unpublished in any form

0celo7

@ACuriousMind The straightforward calculation is omitted.

This theorem is a trivial modification of the previous one.

Slereah

05:03

"the reader can show"

0celo7

Left as an exercise for the reader.

tpg2114

I sent the editor a message saying that without access to their sources, I couldn't verify anything they said. So then my inbox got blown up with copies of several different masters' theses in Slovakian

0celo7

THEOREM: Blah. PROOF: Clear.

ACuriousMind

@0celo7 Well, it's not so straightforward as to be obivous, but explaining the individual steps would be really boring.

@tpg2114 Did you then request a translator?

0celo7

@tpg2114 wait you're not a peer?

tpg2114

05:04

The worst is "The extension to 3D is trivial"

0celo7

yeah

try that for Gauss-Bonnet...

tpg2114

@ACuriousMind No, I just ripped apart their text for crappy english that made it impossible to understand the content of the paper.

Slereah

"The extension from 0 D to $\omega$ D is trivial"

0celo7

"The extension to complex vector bundles is trivial"

Slereah

The extension from integers to Calabi-Yau manifolds is trivial

tpg2114

05:06

@0celo7 I actually sent the editor an email that said "I'm happy to review this, but you do know I'm a grad student, right?" To which he replied "That probably means you are eager to not disappoint somebody who may hire you someday and so I know you'll not f**k it up."

0celo7

why would a lit person hire you

oh!

not an English journal

a jouirnal in English

was wondering why a fluids grad student was reviewing a lit journal ;)

Slereah

Many works of literature include the sea

Which is made of fluids

tpg2114

I thought you were making the really insulting leap that I must be below engineering and study literature...

At Georgia Tech no less. Where I'm fairly certain nobody, ever, has decided to go to grad school for literature

Slereah

[42] No, dummy, it's an exponent, not a footnote.

0celo7

probably not true

Slereah

05:10

heheh

tpg2114

@Slereah Wait, 42 is an exponent? How big are the numbers you work with?

Slereah

It's a joke paper

insti.physics.sunysb.edu/~siegel/parodies/stuperspace.html

0celo7

@tpg2114 you know what's a shitty journal

J. Math. Phys.

don't ever cite it

tpg2114

@0celo7 From AIP?

0celo7

yah

tpg2114

05:13

I don't associate them with crap journals usually

0celo7

@ACuriousMind can tell you all about it

poor @yuggib has a publication there...it's worthless now

ACuriousMind

@0celo7 Stop trolling.

tpg2114

But I only deal with Physics of Fluids from them -- and apparently the entire editorial board resigned recently to start a competing journal because of some BS that went on with AIP

They do the Gallery of Fluid Motion which has some pretty awesome pictures. I'm all about the pictures.

ACuriousMind

@0celo7 Jokes and hyperbole are not really funny when the other person has no way to tell to what extent you are joking.

0celo7

@ACuriousMind $\pi \Leftrightarrow$ quantum mechanics

Slereah

05:15

I think you mean $\pi + e$

You dolt

tpg2114

@ACuriousMind Yeah... so is it actually a bad journal? I haven't come across it before.

0celo7

@tpg2114 no

I've looked through a dozen of their papers tonight

but there is the paper of doom

DERIVATION OF PI FROM QUANTUM MECHANICS

DanielSank

@dmckee Yes. Yes, it does.

ACuriousMind

@tpg2114 Well...the journal as such is surely not bad, but there was a recent singular instance where a paper "OMG, we found $\pi$ in quantum mechanics!" was published there after the suspiciously low review time of...one day.

tpg2114

@0celo7 Good, cause now I'm pulling a bunch of papers applicable to my research

Slereah

05:16

was it reviewed on friday

"I want to go home, let's just approve this"

0celo7

I must sleep

tpg2114

@ACuriousMind Eh, that happens sometimes...

0celo7

good night everyone

ACuriousMind

@tpg2114 I'm just not sure how exactly that happens, especially when it's not really clear what kind of advancement in either maths or physics the result actually signifies.

Slereah

Was the result even non-trivial

You can find pi with random processes

ACuriousMind

05:19

Or is there some way to have the paper reviewed prior to officially submitting it?

tpg2114

@ACuriousMind I've seen several papers that intentionally troll people and usually the editor is in on it

Slereah

It has been known since the 17th century

0celo7

Throw sticks at a circle or something

Pi!

ACuriousMind

@Slereah imo, not really - they put in some gamma functions and used that those have powers of pi as special values to get it. Or something like that.

tpg2114

It happens more often in the pure sciences though. But I did see a paper that was criticizing how relaxed standards for numerical papers were

ACuriousMind

05:20

@tpg2114 Hmmm...that might be the most reasonable explanation.

tpg2114

The authors wrote this crazy complicated numerical scheme and then just said they neglected all terms higher than O(0) and their code just output the solution they wanted exactly

And so their error was 0 always

Slereah

0celo7

String theory

tpg2114

@Slereah There's another one that I can't seem to search for where the stickman says something about including second-order effects and why does that field need its own journal...

0celo7

What about...*hyper*symmetry

ACuriousMind

05:23

If you need some help with the math, let me know, but that should be enough to get you started! Huh? No, I don't need to read your thesis, I can imagine roughly what it says.

0celo7

ACM is too quick

tpg2114

Dang... I couldn't find the right search terms for that one

Good job

0celo7

He literally has all of them memorized

ACuriousMind

I, uh, searched "xkcd complicated simple", it was the top hit :D

tpg2114

I have the pictures memorized, but not the titles so I can search for them...

See -- I was searching for "xkcd secondary terms"

And... nothing

0celo7

05:25

Apparently the spin bundle proof of the positive mass theorem is a simplification of the original proof

Which used calculus of variations

Slereah

but wait, what if there is no spin bundle???

0celo7

Then the photon can't go round and round D:

Slereah

there's a few papers on non-orientable spin structures, but they are all done by math people

There's never a paper like "Here's a non-orientable spacetime, this is what happens to particles on it"

Sir Cumference - Pies

05:46

Oh my God....I just bountied the wrong question.

How do I undo it

I just lost 50 rep on a moronic question...

Please tell me there's a way to fix this

Slereah

"If there is no gravity on the moon why is the american flag waving?"

ahahah

Sir Cumference - Pies

No...no...

;-;

Welp...fml

user54412

06:01

@tpg2114 Just for you, I expanded all the definitions in the ADM equations, which are the Einstein equations put in "time derivative = stuff" form.

user54412

Note all $\Gamma$, $\bar{\Gamma}$, and $\pi$ terms have first derivatives, and all $R$ terms have first derivatives and products of two first derivatives.

user54412

Then the evolution equations are:

Sir Cumference - Pies

-17

Q: If there is no gravity on the moon why is the american flag waving?

If there is no gravity on the moon, how could this flag be flapping in the wind? (see link) http://www.stumbleupon.com/su/2wD6eg/hea-www.harvard.edu/~fine/images/desktops/Armstrong.jpg

user54412

Sir Cumference - Pies

06:02

So how do I get the bounty off this

Slereah

Ask on Meta

Sir Cumference - Pies

Er...ask who?

user54412

You can check that all the terms are either 1st derivatives, products of 2 1st derivatives, or second derivatives.

Slereah

The board

user54412

The mods can clear bounties. They probably can't give the rep back though.

Slereah

06:06

50 points lost isn't a tragedy, either

Sir Cumference - Pies

It was 25% of my total rep

That's a tragedy

Slereah

Well go earn more :V

Or ask mods

Sir Cumference - Pies

Th-thanks...

user54412

@tpg2114 I leave it as an exercise to the reader to verify that the above equations are indeed hyperbolic :p

user54412

Also, those are the vacuum equations. They are "trivially" extended to include dynamical matter (as one professor of mine actually once said).

Slereah

06:15

"Trivial" is basically meaningless

Not quite sure what the whole spacetime as a fluid would accomplish, tho

"Instead of solving GR, we just have to solve Navier Stokes!"

OH WAIT

Secret

06:31

If spacetime is a fluid, then what does it mean for the velocity of a fluid. Surely, spacetime cannot actually flow, right?

I actually found this approach kinda hard to wrap my head around

Ghost

07:56

hope you all enjoy your little exclusive club

found a place where I can collaborate with real physicists

Slereah

wat

Slereah

08:37

Mixmaster universe

The Mixmaster Universe is a solution to Einstein's field equations of general relativity studied by Charles Misner in an effort to better understand the dynamics of the early universe. He hoped to solve the horizon problem in a natural way by showing that the early universe underwent an oscillatory, chaotic epoch. == DiscussionEdit == The model is similar to the closed Friedmann-Lemaitre-Robertson-Walker universe, in that spatial slices are positively curved and are topologically three-spheres . However, in the FRW universe, the can only expand or contract: the only dynamical parameter is overall...

This sounds like a rap album

innisfree

12:29

@Slereah lol

I'm mostly just an idiot

@Ghost um wut

0celo7

^

I'm mostly just an idiot

in Academia, Nov 27 at 23:29, by Ghost

deleted my answer - I will not tolerate being lectured and talked down to by some one who seems to think that they are better than everyone else. (@ChrisWhite)

0celo7

He's just pissy. Nothing new.

But now I want to know what got deleted!

@ChrisWhite Any idea?

I'm mostly just an idiot

It was this I suspect: academia.stackexchange.com/posts/58979/revisions

The time stamps fit.

in Academia, 2 days ago, by Ghost

The irony is that another colleague mentioned their interest in joining Academia.SE (and a few other sites) - I echoed the advice of my other colleagues and told them not to (and mentioned that as they are not American, they are not likely to get a warm reception).

Better not mention that I am not american ;)

0celo7

12:42

This is a very cheerful post, and it no doubt gets upvotes from everyone who wants it to be true, but how does it answer the question? There are certainly fields (like mine, and more importantly the OP's) where no one in that part of the world is doing similar research. How does someone in country A convince someone in country B to actively collaborate, when the person in B can find a substitute collaborator in B with the same level of knowledge? The problem is that for people in B, finite resources really are best spent with the local collaborations, yielding more results per unit effort. — Chris White Nov 27 at 23:20

^offending comment

I'm mostly just an idiot

I don't even get how that would set him/her off so much.

Anyway I suspect we are breaking the be-nice policy

0celo7

@I'mmostlyjustanidiot Probably.

0celo7

13:07

@I'mmostlyjustanidiot Judging by his reaction to this site, he easily feels "persecuted." Not sure if that's the right word, but maybe you get my meaning.

ACuriousMind

13:25

@SirCumference-Pies I raised a flag for you informing the moderators. Next time something like this happens, just click the flag button on the post, and choose "needs moderator attention" in the dialogue that pops us.

ACuriousMind

13:37

@user36790: While your willingness to improve others posts is commendable, please try to exercise more care when using your edit priviledges. This is not the first case where your edits are partly superfluous - \begin{equation}...\end{equation} is fully equivalent to $$...$$in MathJaX (though not in LaTeX) - and partly just wrong - the correct English word is neither stillstand nor still-stand, but standstill.

0celo7

13:51

@ACuriousMind Did you "finish" Fallout 4?

ACuriousMind

@0celo7 Not yet

0celo7

@ACuriousMind : Why are you inconsistent with colons after @'ing people? This is just a general comment.

Both you and the Frenchman.

ACuriousMind

@0celo7 The colon is there when I type the address by hand - it is not there when I use the reply function or autocompletion

ACuriousMind

-1

Q: How fast would you have to shoot a fish through the air in order to cook it?

This came out of a debate in a pub - if you were to accelerate an uncooked fish through the air, how fast would you have to shoot it before the friction would cook it to an edible level?

friction

Sometimes, the "homework" close reason is unintentionally hilarious.

I wish cooking fish by shooting them had been homework at my school :D

user116211

14:09

@ACuriousMind: Yep! Actually the word didn't flashed that time; the word was marked by the red line as usual in Windows. So, I added the hyphen between those other distinguishable words which removed the line. I didn't think beyond that.

John Duffield

14:19

@DanielSank : point out what? I've had things pointed out carefully and politely to me, and I've learned from it. The difficulty seems to be when I then point these things out carefully and politely to people like you. It's something like the Avatar quote: "Moat: It is hard to fill a cup that is already full. Jake Sully: My cup is empty".

0celo7

14:31

Got a 100 on the last pre-final ODE test. I somehow managed to compute the convolution of two cosine functions by hand! Correctly!

John Duffield

@0celo7 : right. You don't travel along a worldline, you don't travel round a CTC, it would signify Mayfly Day instead of Groundhog Day. There is no way you can move such that everything else not only moved back to where it was, but never moved at all. Time travel is science fiction.

0celo7

14:47

Proof?

Can you prove that there are no CTCs in physical spacetime?

I'm mostly just an idiot

"

I will reiterate: there is no reason to assume a constant curvature. Because when you posit that the universe is homogeneous and isotropic, you also posit that there is no gravitational field and no spacetime curvature. There are no observations of any curvature, and for all we know some guy 46 billion light years away sees half the sky as black."

That is a really interesting thing to think about!

David Z

@0celo7 haven't you been down this road before? It might not be a good idea to go there again

I'm mostly just an idiot

What is a CTC?

David Z

Closed timelike curve

In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime that is "closed", returning to its starting point. This possibility was discovered by Kurt Gödel in 1949, who discovered a solution to the equations of general relativity (GR) allowing CTCs known as the Gödel metric; and since then other GR solutions containing CTCs have been found, such as the Tipler cylinder and traversable wormholes. If CTCs exist, their existence would seem to imply at least the theoretical possibility of time travel backwards in time, raising...

I'm mostly just an idiot

Not enough Analysis in my history apparently :D

0celo7

14:54

@DavidZ Is that a friendly reminder or a moderator reminder?

David Z

Friendly for now

The Man

@DavidZ Hey I just edited my question to better fit the "homework-like" question requirement. Do you mind reopening my question?

David Z

@TheMan can you post a link?

The Man

@DavidZ physics.stackexchange.com/questions/221623/…

David Z

Asking whether your work is correct isn't on topic either

The Man

15:00

So how can I make my question better?

I'm obviously just trying to make sure that I got my concepts right

David Z

I'll let you know soon (sorry, working on three things at once at the moment)

0celo7

15:15

@Danu I did $\int \mathrm{d}r/r^3$ correctly on a test. I'm very proud of myself.

Danu

Stellar progress

David Z

@TheMan OK, now that my other thing is done: the thing is, you don't really have a question for us. You encountered a problem, you came up with a solution to the problem and an explanation for the solution, and that's that. I understand wanting to check with someone to make sure you understand it, but that's not what we're here for.

0celo7

@Danu :)

next I'll be able to find the inverse of a 2x2 matrix!

Danu

Maybe start with a 1x1 matrix first ;)

Don't wanna go up to PhD level in one step

David Z

Now, if something seemed wrong about your solution, and you made a decent effort to figure out how to fix it but couldn't find out, then maybe you'd have a good question for this site. But that doesn't seem to be the case, from what you wrote.

0celo7

15:21

I think one can safely do matrix inverses in 5th year courses.

Triple integrals, on the other hand...

David Z

And make no mistake, that's a good thing. After all, your real goal is (or should be) to solve your problem, not to come up with a question to ask here.

I appreciate you asking about it, by the way. That is the mark of a great contributor. :-)

I'm mostly just an idiot

So the universe seems consistent in temperature and such?

The Man

@DavidZ Im just beginning to learn the concept of momentum. I saw a question on it. I tried my very best to come up with an answer to that problem to satisfy the homework-lie question requirement. However, I have no idea whether or not that explanation even makes sense, and since the last time I took physics was a few semesters ago, I have no real human being to check with. So I am asking a question to the community on how they can answer the problem.

0celo7

@DavidZ Can I ask a 10 part question about photons going round and round

David Z

@TheMan yep, I know, I see that's what you did. Like I said, that's not the sort of thing we're here for.

Secret

15:23

@ACuriousMind @0celo7 @DavidZ @I'mmostlyjustanidiot @TheMan @Danu A small newtonian question, that although I know how to do the prove mathematically, don't really understand how to phrase what is happening in words:

Why does the inability of a function U to obey the symmetry of mixed derivatives, which implies that dU cannot be a total differential, will mean that the force F derive from U must be non conservative?

That is, what differs between a total differential $dU$ and some non conservative "differential" $\delta U$ in a geometric sense?

David Z

@0celo7 no

ACuriousMind

@Danu What about...a 0.5x0.5 matrix?

@I'mmostlyjustanidiot huh?

0celo7

@ACuriousMind how is that even defined o.o

The Man

But isn't that a question? So should I just delete all of my explanation and have the community attempt to answer my question?

0celo7

@ACuriousMind CMB

I'm mostly just an idiot

15:24

@ACuriousMind I am just reading the Horizon problem, and the temperature of the CMB is roughly the same everywhere

0celo7

@DavidZ :(

ACuriousMind

@Secret 1. You don't need to ping everyone in the room. 2. What?

0celo7

@ACuriousMind if only @all worked...

David Z

@TheMan no, that would be going backwards. Remember, your goal is to get a solution to your problem, not to come up with a question to ask here. You already have a solution to your problem. So what do you need us for?

I'm mostly just an idiot

What is the solution to the horizon problem?

ACuriousMind

15:26

@I'mmostlyjustanidiot Wiki says "Two theories that attempt to solve the horizon problem are the theory of cosmic inflation and variable speed of light.". From that you should conclude we don't have a unique solution yet.

I'm mostly just an idiot

It said that and then went on to talk about assumptions of homogeneity and isotrophy

0celo7

variable speed of light

but isn't that just GR

ACuriousMind

It's Isotropy; isotrophy would be a state of "same nourishment", literally translated

0celo7

lemme find an Einstein quote...

I'm mostly just an idiot

@ACuriousMind :D

The Man

15:28

@DavidZ Because the solution is possibly incorrect, and I am trying to understand the concept of momentum, therefore trying to seek someone to answer the question listed with an explanation, which is what questions on StackExchange are. Plus, it isn't actually a homework question for me anyways. Just a random question that I came across and wanting to know more.

David Z

@TheMan Well then, let's try this: why do you think the solution might be incorrect?

Secret

We know that a conservative force $F$ can be described by a potential $U$. Thus dU is an exact differential, that is

$$dU=U_x dx+U_y dy$$

We also know that for mixed 2nd derivatives of U, $U_{xy}=U_{yx}$

If a function $S$ of which a force $A$ can be derived does not have equal mixed derivatives, then this S will not be able to satisfy the exact differential equation and thus $A$ cannot be conservartive

But why is that , what makes a total differential differ from a non total differential, what extra things (or terms) that the non total differential that give it path dependent properties…

ACuriousMind

@TheMan Questions on Physics.SE are a little different from what is par for the course on e.g. SO or math.SE. We don't want "homework-like" questions that are just about solving a specific problem or correct a user's solution to a problem.

David Z

@TheMan By the way, the fact that it's not actually homework doesn't matter. Even though the hold reason still talks about homework for historical reasons, it's more about requests for calculations these days. (We're in the process of figuring out new wording, as a long-term project)

Danu

When ACM doesn't know something, he asks in disguise :D

The Man

15:30

@DavidZ Because I learned the concept on my own, and I have nobody to check with.

David Z

@TheMan you mean, as a reason you think the solution might be incorrect?

ACuriousMind

@Secret WTF is a "non-total differential"? How can the mixed second derivatives not match, is your function $S$ not twice differentiable?

David Z

If your only reason to think you might be wrong is that you don't trust yourself, that doesn't cut it for a question on the main site. It'd be fine to ask here in chat though.

The Man

Alright then.

ACuriousMind

@Danu Damn, you caught me :P

Danu

15:33

@ACuriousMind Note that the match on membership times is pretty good ;)

I'm mostly just an idiot

"Maybe I can be talking a lot of bullshits, so I'm sorry.", " can them goes through every kind o materials" - Only a Curious Mind

Secret

ok, let me rephrase it

$U$ is a potential function if it satisfy the following equation
$$dU=U_x dx+U_y dy$$

Where $(U_x,U_y)$ is the force $F$

Because by integration, it can be shown that the change in U depends on the endpoints of the integration only, and not its path

If we have a non conservative force $G$, then it cannot be described by a potential. Any attempt to find this potential $K$ one will find $K_{xy}\neq K_{yx}$ thus a potential function does not exist for $G$

Therefore if we try to compute the total change of the quantity K by integrating

0celo7

@I'mmostlyjustanidiot that about sums up his GR knowledge

ACuriousMind

@Secret The quantity $K$ does not exist if the components of $G$ were continuously differentiable (because mixed derivatives are equal for twice continuously differentiable functions). It doesn't make sense to speak about its "total change" - it does not exist.

0celo7

@Secret what

$\mathrm{d}U$ is defined by that equation

$U$ is a potential function if $\nabla U=-F$

or $\mathrm{d}U=-F^\flat$

or is $\nabla U$ a covector...

Wow, @ACuriousMind VTC'd on his alter ego

Secret

15:46

Because we have this quantity, work $W$, which in most textbooks infinitesimal work is denoted by $\delta W$. If one integrate this along some curve from the positions a to b we then get $\Delta W$ which is path dependent

Therefore, my question is fundamentally speaking, why is $\delta W$ integrated vs $dU$ integrated give different properties for $\Delta W$ and $\Delta U$, how does the mixed derivatives give $\Delta U$ the property of path independence, other than it allows U to satisfy the total differential equation?

0celo7

how is infinitesimal work defined, @Secret

$(\sum F)\cdot \delta x$ or somthing like that?

ACuriousMind

@Secret "$\delta W$" is suggestive notation, it is not obtained from $W$. Quite the opposite! You define $\delta W$ (without having a quantity called $W$ at your disposal), and then you define $W[\gamma] = \int_\gamma \delta W$.

The notation is suggestive of the fact that if the integration of $\delta W$ is path independent, then $W$ is a path-independent function - a potential - and indeed $\delta W = \mathrm{d} W$.

0celo7

is it not a functional

OH GOD CHAT SESSION

ACuriousMind

But the $\delta$ is not an operation which is executed on $W$, $\delta W$ is one object, not $\delta$ applied to $W$.

Balarka Sen

what is this chat session about?

ACuriousMind

15:50

@0celo7 Not if it is path-independent, that's the point!

@BalarkaSen Chatting and sessioning

0celo7

@ACuriousMind I mean in general

ACuriousMind

@0celo7 Yes.

0celo7

@ACuriousMind good

brb class

David Z

@BalarkaSen nothing in particular. Sometimes we have a topic, but I don't think there's anything particular lined up for today

Balarka Sen

@ACuriousMind riiiight.

@DavidZ Ah, I see.

Secret

15:51

Ok, so basically the question boils down to how $\delta W$ obeying mixed derivatives (hence satisfying the total differential equation) result in it to be path independent, other than the fact that all the algebra works out and the outcomes just popped out from the computation

Is there some more fundamantal reason in terms of what underlying property that obeying mixed derivative changes $\delta W$ to make it path dependent?

ACuriousMind

Mar 24 at 16:04, by ACuriousMind

C'mon session harder

@Secret "Mixed derivatives" is just an integrability condition. $\delta W$ is a one-form. If it obeys mixed derivatives, then its exterior derivative (its "curl" if you wish) is zero, i.e. $\mathrm{d}(\delta W) = 0$. On topologically trivial spaces, this directly implies there is some 0-form $f$ such that $\mathrm{d} f = \delta W$, and since we want to use the suggestive notation, we call this $f$ then $W$.

Balarka Sen

more sessioners incoming.

Secret

Ah, makes sense, thanks

Balarka Sen

@0celo7 not sure what your morphisms are

I am not convinced you can make such a category.

Дау

s

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