The h Bar

someone who raps

user218912

according to wikipedia khaled is a rapper.

he's not a good one then

he's a producer

where is John?

I bet the hemorrhoids finally got him...

@obe so, there is a book I might need

although Lee is still preferred

user218912

4:06 AM

do you listen to dubstep?

user218912

which one is cheaper?

when I was a teenager, yes

@obe not lee.

user218912

alright what book is it?

user218912

sigh... you know what I'll just buy it now...

user218912

link it

4:09 AM

well now wait a moment

I'm not sure

user218912

._.

do I need a book on measure theory first? that is the question.

user218912

idk

(they're the same price)

user218912

idk if I can afford that right now. let me see

4:11 AM

> 112 Days Loan

wtf is this

who can take a library book for a third of a year?

user218912

user218912

sad face

45

user218912

what?

user218912

lee is 70

4:12 AM

I'm so goddamn stupid

user218912

@SirCumference flagging

I forgot to study for a stats test tomorrow

I'm screwed

user218912

how can you be screwed?

No...I can do this...

@obe Don't threaten to flag, just flag and move on.

user218912

4:13 AM

if you went to the lectures and did the homework

user218912

it should be ok

@0celo7 Who did that offend?

user218912

@SirCumference you made fun of ducks.

Swearing is allowed

You said "godd*mn"

That is offensive to certain people.

user218912

4:14 AM

@0celo7 it's 45?

user218912

what book?

Jan 29 at 4:28, by 0celo7

@SirCumference No, I won't give you a goddamn source.

well

oh man

I was trolling you so hard there

...yeah

But yeah, I'll be up all night studying

Or maybe I should sleep instead

I have no idea what to do

user218912

then get off this chat

4:15 AM

@obe The question is: do I need a book on minimal surfaces, advanced analysis, or measure theory.

@obe Good idea

user218912

i'll flag you so you get banned?

user218912

let's try it.

Bl00beLord, thanks

Flag and move on.

4:16 AM

Wait what

Don't talk about flagging.

@0celo7 That's the first and second rule of flag club

user218912

hmm it's not working

I don't get your reference to Fight Club.

Could you explain?

user218912

@SirCumference make a ban worthy offense.

4:17 AM

call @obe a kanuck

@0celo7 Yes. First rule of fight club, don't talk about fight club. Second rule, tell everyone about fight club.

Third rule: ignore the first two rules. Fourth rule: the third rule is invalid

I'm so out of it...

user218912

@0celo7 whichever one is cheaper :D

I really should be studying

Later y'all

user218912

bye

user218912

@0celo7 do you want hatcher?

4:19 AM

...At least I hope it's y'all

Last time I misspelled it @0celo said

Feb 20 at 2:05, by 0celo7

We ain't gon have a yank use a word he can't spell

user218912

@0celo7 pick a book right now I feel like buying it for you.

user218912

you have 3 mins

amazon.com/Analysis-Graduate-Studies-Mathematics-Elliott/dp/…

it's the cheapest

user218912

and do you need it?

Yes

Sobolev spaces

Also distributions

user218912

4:22 AM

so you wouldn't rather wait a few weeks for lee?

user218912

because idk if I can buy another book after this for a while.

dude idk

@obe yes it's more important than Lee.

user218912

okay

user218912

go on skype for a sec

ok

user218912

4:35 AM

@0celo7 lmk when you receive it.

user228700

@JohnRennie:

user228700

in The Periodic Table, 10 hours ago, by M.A.R.

@Kaumudi ORTHO'S A SHE. ORTHO'S A SHE!

4:51 AM

@obe @0celo7 Ffs, what was flaggable about what I said?

Can we disable this chat bot? It seems to blindly ban people.

user116211

You got banned @SirC?

@MAFIA36790 Yeah

Lol. I didn't flag

For writing — and I'm quoting

"Fduck fduck fduck fduck fduck fduck fduck fduck"

Under stress

DavidZ banned me from flagging.

user116211

4:53 AM

._.

For fduck's sake, I didn't even swear

I can't flag

@0celo7 :P

You did abuse it, tho

user116211

ohh; you insulted duck, I see @SirC.

user116211

Morning @JohnRennie.

4:55 AM

@MAFIA36790 Darn ducks and geese.

user116211

:(

Morning all. What's an fduck?

How Pi was nearly changed to 3.2 - Numberphile

@JohnRennie Euphemism.

►

This video details what's wrong with the world

user116211

Yeh; two things are wrong: Commies and ISIL.

Are there any commies left? I thought they'd all given up and gone capitalist.

4:59 AM

For all the things the Soviets did wrong, they did do two things right:

Starting the space race, and having a nice anthem

Which I am forced to listen to daily

user116211

@JohnRennie Castro is not dead.

user116211

The fat Kim is alive.

@JohnRennie ACM

user116211

ACM is commie?

user116211

Proof?

user116211

5:02 AM

@SirCumference WTH ?!

user116211

Nazi Germany had also a nice anthem.

user116211

The tune is quite nice.

@MAFIA36790 People outside the campus are chanting and playing it every goddamn day

I don't even know why

I asked them about it and they mentioned something about trump, i don't even know

and they just kept marching

@MAFIA36790 To call North Korea communist is stretching the definition of communist rather badly. As for Cuba, they're headed inexorably along the road to capitalism.

@MAFIA36790 Yeah, North Korea has some unique economic system

user116211

5:17 AM

@SirCumference WTF ;/

user116211

Is your university Far-Left ?!

user116211

Hey @JohnRennie, could you help me in optics?

@MAFIA36790 Most people in the state are liberals

@MAFIA36790 I can try ...

At least as far as I can tell

user116211

5:18 AM

Why the Soviet Union anthem daily O.o

@MAFIA36790 Been trying to figure that out

It's like a wakeup call

It's the second weirdest thing I've dealt with recently

user116211

@SirCumference O.o

user116211

@SirCumference It's one of the weirdest things I ever heard.

@MAFIA36790 Hah. Get what happened a few days ago.

I meet a few people, they seem polite and all. Religion comes up, I mention I'm Jewish, and one of them kindly tells me Jews are evil political figures or some nonsense and the rest agree.

user116211

5:21 AM

@JohnRennie, in the above excerpt, I'm not getting how the authors write $\log \mu\,.$

The next few days they've been telling me that Egypt won the six day war. I've asked them why Egypt lost land, they can't answer.

user116211

@SirCumference okay.

The people here are seriously scaring me...and they're supposed to be smart.

user116211

You should now play with loudspeakers Nazi Germany anthem.

@MAFIA36790 "Jewish"

user116211

5:23 AM

Although it is not provocative, but it would make them embarrassed how a jew can listen to that.

@MAFIA36790 More or less risk offending tons of other jews

Or getting thrown out of the college

user116211

@sirC, always be alert; something is wrong with your place.

@MAFIA36790 It's supposed to be a really good college

Yet some of the people here are really, really strange

user116211

But the way you described gives me a weird picture of the people there.

@MAFIA36790 Most of the people here are great. Some are questionable, though.

user116211

5:25 AM

@JohnRennie, halp ?

user116211

@SirCumference okay.

user116211

But the SU anthem being played daily is really something I don't want to witness. It's really weird.

@MAFIA36790 I think they're trying to make some political statement

Or maybe they think it's funny

user116211

That's why I asked whether your university has a left leaning.

user116211

@SirCumference Listening to it on daily basis is surely not a funny act.

5:27 AM

@MAFIA36790 There's been worse. Yale had some screwed up kids chanting something I'd rather not explain

user116211

Holy crap ;/

user116211

You should carry a gun for your safety.

I don't think anyone's trying to make that much of a point...

user116211

hmm.

@MAFIA36790 I get $\nabla\left(\frac{1}{\mu}\right)$ rather than $\nabla(\log\mu)$

user116211

5:29 AM

yes.

Dammit! I was supposed to be studying!

It's 1:30am here

user116211

I concluded that too.

user116211

But they used the $\log \mu$ term from nowhere :(

user116211

@SirCumference Have some hot coffee and start studying.

Hmm, $d/d\mu(\log\mu) = 1/\mu$

user116211

5:32 AM

I know.

user116211

But the identity didn't say of anything.

user116211

It mentioned of $u\mathbf v\,.$ The gradient term, they wrote, was $\textrm{grad}~u\,.$

user116211

So, that would mean, the term in this context should have been $\textrm{grad}~\left(\frac{ 1}{\mu}\right)\,.$

Agreed

I can't see how they ended up with $\nabla(\log\mu)$

Is there a sneaky integration by parts going on?

user116211

@JohnRennie At least I am not seeing any.

user116211

5:36 AM

It's a direct substitution and use of the identity, at least at the first sight.

I must admit I can't see it either

user116211

okay, nevertheless thanks for sneaking into the query @johnR :)

user116211

Hmm, I should ask a question on this in Math.SE.

user116211

But I need to think of a good title.

user116211

I'm bored with dull titles like Problem in understanding.....

user116211

5:40 AM

Let me ponder over it a bit.

user228700

6:09 AM

Hi :-)

user228700

(Again :-P)

Morning :-)

user228700

I've got a quick question. Does charging by friction work if the two objects are smooth?

@Kaumudi You thinking about the triboelectric effect?

Q: Why did the authors write ${\rm grad}~\log \mu$ instead of ${\rm grad}~\mu^{-1}$?

@Kaumudi yes, both triboelectricity and friction are related to adhesion.

user228700

6:12 AM

That's a very fancy name for something my book describes as "charging by friction", but yes, that's what I'm talking about.

user116211

0

This snapshot is taken from Principles of Optics by Born and Wolf. I'm not able to comprehend the equation $(3)$ above. First they take the identity $$\textrm{curl}~{u\mathbf v} = u~\textrm{curl}~{ \mathbf v} + \left(\textrm{grad}~ u\right)~\boldsymbol\land~ \mathbf v\,.$$ Using it in $(2),$...

vector-analysis

@Kaumudi those fancypants physicists! :-)

user116211

^ @JohnRennie, done.

user228700

@JohnRennie Related to adhesion huh? OK, thanks :-)

user228700

@JohnRennie :-P

6:14 AM

@JohnRennie It's better than astronomers. Astrophysicists call all elements that aren't hydrogen or helium "metals", just because they don't care about differentiating them

@MAFIA36790 It might be worth noting that both u and v are fields - u is a scalar field and v a vector field

user228700

@SirCumference What? O.o

user116211

yes. so?

@MAFIA36790 I'm not sure that's clear in your question ...

user116211

ah!

6:15 AM

@Kaumudi Yep, it's true. Since most elements are metals, they just call anything besides hydrogen or helium "metals". So in astronomy, a star's metallicity means how much of it is composed of elements beyond helium.

user228700

@SirCumference Nice. Thanks for sharing :-)

@SirCumference they also call all non-dark matter baryonic even though it contains leptons as well :-)

@JohnRennie Oh yeah, and they also have dozens of names for apsides

user116211

@JohnRennie But I have used \mathbf; also grad is computed for scalar field, right? curl is taken for $u\mathbf v$, which means it is a vector field.

It gets ridiculous

user116211

6:16 AM

But I will explicitly write it there.

user228700

I read a little bit more about this effect and wait, if friction starts acting when the adhesive bonds are broken, then whenever we say "objects are smooth", we mean that there were no adhesive bonds to begin with, yeah? So, how come this effect works even without friction? ie. No adhesive bonds in the first place?

Variable star designation

@JohnRennie Worst though, is how they name variable stars

Variable stars are designated using a variation on the Bayer designation format of an identifying label (as described below) combined with the Latin genitive of the name of the constellation in which the star lies. See List of constellations for a list of constellations and the genitive forms of their names. The current naming system is: Stars with existing Greek letter Bayer designations are not given new designations. Otherwise, start with the letter R and go through Z. Continue with RR...RZ, then use SS...SZ, TT...TZ and so on until ZZ. Use AA...AZ, BB...BZ, CC...CZ and so on until reaching...

Just read that checklist

It's laughably complicated

> • Stars with existing Greek letter Bayer designations are not given new designations.
• Otherwise, start with the letter R and go through Z.
• Continue with RR...RZ, then use SS...SZ, TT...TZ and so on until ZZ.
• Use AA...AZ, BB...BZ, CC...CZ and so on until reaching QZ, omitting J in both the first and second positions.
• Abandon the Latin script after 334 combinations of letters and start naming stars with V335, V336, and so on.

A: How is frictional force dependent on normal reaction?

@Kaumudi for background have a look at:

5

You need to start by considering the microscopic origin of the frictional force. In most circumstances surfaces are rough so when to touch two surfaces together they actually only make contact at the highest points on the surfaces. We call these high points asperities, and in the diagram below I...

Smooth surfaces, smooth on the molecular scale that is, have relatively very high real area of contact and therefore enormous friction.

user228700

OK, I haven't read about yield strength yet. Gonna save that page and read it once I do. Thanks!

A: Orbital velocity of a planet - why is my calculation off by about 10%?

@JohnRennie Did you hear about what happened here?

7

Well done you. I double checked the calculations and couldn't fault what you had done. So I contacted the lead author of the paper about it and here is the response: "After checking the numbers in our paper, I found an error: we actually used a mass of 1.0 MSun for J1407 in our simulations, inst...

Really cool

6:23 AM

Yes :-) I saw that the first time you posted it.

@JohnRennie Say, is there any source that goes further in depth about how Sag A* and such aren't yet black holes?

It'd be useful to have when answering astronomy questions

user116211

@SirCumference Rob is a space-cop; he works in Enterprise.

@MAFIA36790 It's strange how he never goes on a chat

Yet is so active on SE

user116211

Lumo never came here.

He's #1 on Astronomy

user116211

6:27 AM

So, that's not something rare.

Yeah, but still. Rob J knows so much GR and physics overall that he'd be really helpful to speak with in a chat

I don't know of an article that specifically discusses this. It's one of those things that is obvious to all GR students because you learn it early on when you study the Schwarzschild metric.

Uh, maybe this?

nature.com/news/…

Welp, nvm, just looked at it

user116211

You want to study quantifiers @SirC?

@MAFIA36790 Uh, not in particular?

user116211

6:32 AM

I don't know; I wanted to reply; but I forgot earlier.

user116211

@SirCumference ohh, okay.

@JohnRennie Wait, you're only describing the Schwarzschild metric?

I thought you were applying this to finite black holes?

@SirCumference what you learn in studying the Schwarzschild metric is that it takes infinite coordinate time for an object to reach the event horizon. A simple extension of this tells you it takes an infinite coordinate time for a horizon to form ina real black hole.

The simplest model for a real black hole is the Oppenheimer-Snyder metric, which makes this quantitative.

I wonder if we need a canonical Q/A here to cover all this stuff. Then we can use it as a reference.

user116211

@JohnRennie Why not propose it at meta then?

@JohnRennie All right, but considering the OS metric, would an apparent horizon form?

6:40 AM

@MAFIA36790 my preferred route would be to write one first to satisfy myself that a good Q/A can be written. Then and only then would I seek approval in a meta post.

I have to go back to work for half an hour or so. see you all later.

user116211

@JohnRennie sure.

6:55 AM

@SirCumference the term apparent horizon has a strict definition, which I must confess I sometimes abuse. An apparent horizon is generally inside a true horizon. It's a kind of pseudo-horizon that appears in some reference frames e.g. for a freely falling observer.

@JohnRennie Yes, but while the absolute horizon is essentially theoretical, this should exist, right?

So the OS metric has no apparent horizons in the strict meaning of the term. However to outside observers it looks as if it has a horizon in the sense that light from the thing that looks like a horizon is taking a very long time to escape.

@SirCumference I don't think you can have an apparent horizon without a true horizon, though I wouldn't swear to this.

@JohnRennie Huh. I thought the OS metric describes absolute horizons as forming exterior to, and before, apparent horizons?

This is only from what I've read

Isn't that what I just said? Apparent horizons exist inside true horizons.

@JohnRennie Yes, but you said the OS metric makes no note of apparent horizons

Is that true?

@JohnRennie I'm just saying an absolute horizon should be theoretical, shouldn't it? We can't preclude that in reality, a black hole might accrete some more mass in the future, right? That should certainly make it impossible to determine an event horizon.

7:00 AM

Ah, OK. Yes, in the OS metric once a true horizon is formed there will be apparent horizons inside it. But for observers outside this takes an infinite time to happen. So neither true nor apparent horizons will ever be seen from outside.

Meanwhile, an apparent horizon can be defined at any point in time

I'm just going by what I've read in the past. I've never looked deeply into the maths.

I could easily be misinterpreting it.

@SirCumference you're quite correct that to identify an event horizon requires knowing the future evolution of the system out to infinite time. In our model metrics the metric is defined for all time so this is no problem. In real systems you're correct that the future evolution is unknown.

Ah, okay

But that brings us back to reality. In reality, would an apparent horizon ever form around something like Sag A*?

I'd have to check, but I don't think any apparent horizons can exist in the OS metric for external observers. They certainly exist for someone falling into the forming black hole.

@SirCumference Not for external observers. Yes for someone falling into it.

@JohnRennie But an event horizon would, for reasons we just discussed, take an infinite time to appear, right?

7:05 AM

Yes.

Because knowing the event horizon's radius would require knowing a priori when the apparent horizon would stop growing?

> when the apparent horizon would stop growing?

Eh?

Crap, I'm out of it, it's 3:06am

Say the black hole survived until the Black Hole Era of the universe. Surely there'd be nothing else for it to accrete, right?

At that point, if we ignored Hawking radiation, could we determine where the event horizon would be?

@SirCumference Have a read through this article. In a collapsing object apparent horizons can form only after the true horizon forms. And that takes infinite coordinate time.

@JohnRennie Yeah, that'd make sense. Absolute horizons form from the center of the black hole outwards, right? And if the apparent horizon must be interior to the absolute horizon, it must've formed after, right?

7:09 AM

@SirCumference I guess in the situation you describe the future evolution would be dictated by the fact there was nothing left to fall in, so yes the future position of the horizon could be calculated.

@SirCumference Yes.

Cool

Oye, I oughta head to sleep. 'Night

Q: How quickly do farts spread?

7:25 AM

Aha: $$\text{grad}\left(\frac{1}{\mu}\right) = -\frac{1}{\mu}(\text{grad}\log\mu)$$

I must admit that isn't obvious to me, though I guess if you work through it the proof should be straightforward ...

user116211

10:21 AM

@JohnRennie yeh, I also didn't see it initially.

user116211

but they are equal to each other.

user116211

A simple application of chain rule and definition of gradient operator.

user116211

\begin{align}\textrm{grad}~\log \mu & = \frac{\partial \log \mu}{\partial x} + \frac{\partial \log \mu}{\partial y }+ \frac{\partial \log \mu}{\partial z } \\ & = \frac{1}{\mu}\left(\frac{\partial\mu}{\partial x}+\frac{\partial\mu}{\partial y}+ \frac{\partial \mu}{\partial z}\right) \\&= \frac1\mu~\textrm{grad}~\mu\,.\end{align}

user116211

And $$\begin{align}\textrm{grad}~{\mu^{-1}} &= \left(-\frac1{\mu^2}\right)\left(\frac{\partial\mu}{\partial x}+\frac{\partial\mu}{\partial y}+ \frac{\partial \mu}{\partial z}\right) \\ &= -\frac{1}{\mu}~\textrm{grad}~\log\mu\,.\end{align}$$

user116211

It's that simple and I simply overlooked that while re-reading the excerpt again and again for God knows why.

user116211

10:52 AM

0

Basically I started thinking this question in regard to farts. I thought to myself, say Alice and Bob are in a room. If Alice farts, how long would it take for Bob to pick up on the smell? After a bit of thinking, I realised that this could apply to all gases. Is there a formula for gases sprea...

diffusion gas

user116211

...

user116211

QMech continues his links therein ritual without any feelings; he might be an AI.

Qmechanic

@MAFIA36790 : Luckily we still can't smell the posts... :)

user116211

See? I just wrote about him and here he comes! He is an AI!! And wow Qmech talks with feelings ;P

Aakash Tomar

11:17 AM

Wow, Mafia, that's so thoughtful.
Anyway, there is graham's law of diffusion to answer that. :P

user116211

Welcome to the h bar @AakashTomar.

Aakash Tomar

Thanks Mafia! Well, this is my first message here, or on any stack chat for that matter. Actually, second. :)

A: Put on hold and rephrasing the question

1:01 PM

Hey @MAFIA36790, what's with editing this?

4

Well, I tried to play the game and join in (hence my 10,100 points) but this forum is almost totally useless for anyone doing experimental physics. I particularly dislike the snobbish attitude of some moderators who cannot tell the difference between experimental physics and that stinking low cla...

The man is no longer here and cannot vet your edits. Why, oh, why, do you feel the need to nitpick his language?

user116211

I don't think I've edited something that would make him bother if he were there.

@EmilioPisanty Why do you mind if it's just language improvements?

@Danu It's not "improvements", it's de-railing specific stylistic choices he made.

This is the man's leaving note. It's a sign of respect to leave the language alone.

If it were an obvious typo, then sure. This isn't that.

user116211

The point of the post is still what it was prior to the edit.

user116211

I don't think it harmed his post.

user116211

1:07 PM

However, that's my opinion only.

@MAFIA36790 I disagree. It wasn't a lot of harm, but it was unwarranted messing with someone else's stylistic choices.

@EmilioPisanty I think you're taking this too symbolically. It's not a significant edit.

I understand if you don't like it, but it's not a big deal either way.

I don't want to raise a fuss.

Okay :)

I just want to say: please don't do edits like that.

anyways, next topic

ducks, maybe?

user116211

1:08 PM

I really don't have any idea still why it was wrong; but anyways, sorry.

scontent.xx.fbcdn.net/v/t1.0-9/…

user116211

@EmilioPisanty Someone insulted ducks today; and he got banned.

Filling out the "Significance" section of a grant proposal, the grad student gets a chance to exercise his creative writing skills.

Lel

oh, for crying out loud

user116211

1:10 PM

This time 0celo7 didn't flag ;P

1:26 PM

@MAFIA36790 I'm not allowed to flag any more.

user116211

:(

vzn

2:24 PM

@Secret nice table whered you get it?