The h Bar

ooolb

12:00 AM

and then bring him to this universe

and replace him with the IT ACM

0celo7

12:12 AM

@dmckee ah, an American adult

I need help

DanielSank

Hi, everybody.

bolbteppa

It only took a few months to read 4 pages, but finally $d = 2 \ \mathrm{mod \ 8}$ are the dimensions in which spinors can be Weyl and Majorana :D

Mithrandir24601

@DanielSank 你好

DanielSank

@Mithrandir24601 Ho boy. So now we're playing the language game, eh?

0celo7

@bolbteppa @ACuriousMind disagrees.

DanielSank

12:25 AM

Дадно. Давай говорим по Русский.

bolbteppa

@0celo7 disagrees with what

Mithrandir24601

@DanielSank Well... I am going to China in a day or so :D

DanielSank

Хорошо!

В какой город?

Mithrandir24601

Remind me to bring my Russian-English and English-Russian dictionaries back with me next time I'm home :P

DanielSank

Да

ладно

Mithrandir24601

12:27 AM

@DanielSank 广州 (Guangzhou)

0celo7

@DanielSank Nein danke

DanielSank

@0celo7 Мне всё равно что ты хочешь.

@Mithrandir24601 Блядь! В Гуанжо ограбят тебя слепыми!

Mithrandir24601

@DanielSank I can at least count in a close enough approximation, so am willing to haggle... But yeah, they put the prices of everything up for foreigners :/

DanielSank

@Mithrandir24601 I just really wanted to use the word "блядь" in conversation.

Mithrandir24601

Although China is really cheap anyway :)

DanielSank

12:33 AM

It's a catch-all swear in Russian.

@Mithrandir24601 Yeah. When I was there some years ago, a baozi was 0.15USD.

Mithrandir24601

@DanielSank Ahh, fair enough :)

DanielSank

It's one of the most versatile words in existence.

Mithrandir24601

@DanielSank :D I can't wait to get me some delicious tea :) What was it like?

DanielSank

@Mithrandir24601 The baozi? Delicious. There were several kinds this one stand: peanut, carrot, pork, mushroom, etc.

Mithrandir24601

I've been told that the food stalls tend to be good

0celo7

12:38 AM

@DanielSank Что я сделал тебе?

David Z

@Mithrandir24601 Yeah, just don't ask about health inspections :-P

DanielSank

@Mithrandir24601 yessir

@0celo7 Нечего

bolbteppa

12:53 AM

Yes this is real:

sciencedirect.com/science/article/pii/0167278985901733

"Abstract:
We prove, once and for all, that people who don't use superspace are really out of it. This includes QCDers, who always either wave their hands or gamble with lettuce (Monte Zuma calculations). Besides, all nonsupersymmetric theories have divergences which lead to problems with things like renormalons, instantons, anomalons, and other phenomenons. Also, they can't hide from gravity forever."

0celo7

1:05 AM

@Semiclassical I have a question that's kind of related to my fluids final so maybe you don't want to answer

@Semiclassical I want to do a time-dependent contour plot. Is this possible?

Like create a gif

Semiclassical

1:19 AM

sure, that's not too bad @0celo7

Animate or ListAnimate

I'm not sure which is better tbh though

0celo7

cool

the actual problem is ridiculous

double perturbation theory for Navier-Stokes in (2+1)D

Semiclassical

sheesh

Mohammad Areeb Siddiqui

1:58 AM

hii

quick question of poisson brackets

how do i prove

${{F(p, q), p_i}} = \dfrac{\partial F(p,q)}{\partial q_i}$

the left hand side is inside the poisson brackets

0celo7

I don't see them

but the proof is a direct computation

Mohammad Areeb Siddiqui

i cant use the original definition of the brackets

thinking of the properties of the brackets as axioms

0celo7

how are we supposed to help then

Mohammad Areeb Siddiqui

and using them to prove this

for example

0celo7

then taylor expand

Mohammad Areeb Siddiqui

2:05 AM

the function can be anything

its a general function

0celo7

@MohammadAreebSiddiqui if you're not using the definition in terms of derivative you have to assume you can taylor expand

Mohammad Areeb Siddiqui

by taylor expand you mean the function is smooth and can be represented by a polynomial right?

so i can use the linearity

of the brackets to prove it

right?

0celo7

well it's not a polynomial

$e^{xp}$ can be represented by its taylor series

@MohammadAreebSiddiqui yes. You have to suspend disbelief that you can move the bracket past the infinite sum though

Mohammad Areeb Siddiqui

prove by induction will be best then

0celo7

@MohammadAreebSiddiqui the induction is too trivial to bother

if you're doing induction here you're missing the point

Mohammad Areeb Siddiqui

2:15 AM

i actually am not familiar with taylor expansion of a function with 2 variables xD

it will just be the use of the product rule tho i suppose

0celo7

google

ooolb

3:24 AM

@SirCumference howdy

ooolb

3:37 AM

or not...

enumaris

5:06 AM

weez

Abcd

6:42 AM

@JohnRennie Not possible to tag you in Problem Solving Strategies.

Because it's been a while...

John Rennie

Hang on, I'll rejoin the room ...

ooolb

@SirCumference howdy (the sequel)

or not...

Secret

7:08 AM

::infinite loop::

2

Slereah

8:19 AM

Mornin

Slereah

8:48 AM

Captain Bohemian

what a cv!

Abcd

9:03 AM

@JohnRennie How can a gravitational field possess momentum and energy as stated by my book?

John Rennie

Can a gravitational field possess momentum? A gravitational wave can certainly possess momentum just like a light wave has momentum, but we generally think of a gravitational field as a static object, like an electrostatic field.

Slereah

You can have momentum yes

just expand the gravitational field as a spin 2 field and take the momentum

John Rennie

I doubt Abcd's book is at that degree of sophistication ...

Captain Bohemian

but actually i never like to write my cv, just feeling having nothing to show.

Slereah

lie

Say you invented Facebook

Captain Bohemian

9:14 AM

well, i even hate facebook.

Slereah

I'm sure Zuckerberg does, too

"You have to understand that compared to other professions such as programming or engineering, ethical standards in academia are in the gutter. I have worked with many different kinds of people in my life, in the U.S. and in Japan. I have only encountered one group more corrupt than academic scientists: the mafia members who ran Las Vegas hotels where I used to install computer equipment. "

Balarka Sen

9:32 AM

ZUCC

New face filters on Instagram today

Maybe he prefers instagram over facebook

Slereah

I don't think he does a lot of coding these days

Mostly just swimming in his bin of money like a nerdy Scrooge McDuck

Secret

10:30 AM

mentalfloss.com/article/500940/…

reddit.com/r/AskScienceFiction/comments/1af2to/…

Why are downtimes sooooooo long

soooo bored

Anonymous

10:52 AM

@Slereah Zuckerberg was never a great coder. He is a good entrepreneur and team manager.

Kane

is this the right place to ask a simple physics question

John Rennie

@Kane yes, ask away!

Kane

so Ive got a small bottle that I filled up with salt. I put it on the scale and it's mass is 83g. I've also got a jup of water that has 500g of water. I put the bottle in the jug and it sank to the bottom. I have to figure out how much salt to take out of the bottle such that the weight force of the bottle equals the buoyancy force.

For the buoyancy do I: density of water * volume of water displaced * gravity acceleration?

so: mass of bottle * gravity = volume of water displaced * density of water * gravity?

Emilio Pisanty

Yes

Kane

sweeeet

John Rennie

11:00 AM

That was easy :-)

Kane

im taking physics as an elective class, never done it before haha

what will the bottle look like after I take out the correct amount of salt?

John Doe

Hi Emilio

Kane

will it float in the middle of the jug of water?

John Rennie

@Kane the bottle will have neutral bouyancy, so it will stay wherever you put it in the water.

Kane

wow cool, ill test it out

thanks for the help

Alan

11:18 AM

Is it okay to ask a (physics) question here?

John Rennie

@Alan yes,ask away!

John Doe

@EmilioPisanty The measurement operators than I suggested in the comments of the post are fine but I additionally would like to control the width of the Poisson Distribution (much like we can do for the normal distribution using variance). Do you know that this can be achieved while still maintaining the completeness condition $$\int A^{\dagger}_{C}A_CdC = 1$$?

Alan

Ah, thank you! Is classical mechanics (simple harmonic motion) okay? O:)

I'm still in high school (thought I'd mention it)

Secret

Maths chat is currently so boring. There is no one home except help vampires

Alan

Mr. @JohnRennie Could you have a quick look at that PDF? :-)

Secret

11:22 AM

I wish I can just ddos it to nonexistence in order to vent my boredom, but of course I am not allowed to do it :P

Alan

(I understand what's in the PDF, but I tried applying that logic to another question..and it didn't work out well enough :/)

(I'll post the question once you're done reading O:) )

John Rennie

OK ...

Alan

Fairly straightforward... but solving SHM questions via energy analysis is new to me. Now I'll upload a picture (it's on the variation of gravitational acceleration with depth, on earth)

Now the question I have combines SHM and gravitation..

(I'd like you to have a look at the proof, and see if the final relation in the picture is okay)

^ The picture I just posted O:)

John Rennie

OK. Is this going to be showing that an object falling through a tunnel through the Earth executes SHM?

Alan

Yep :D

I tried proving it...got something weird (I'll be uploading my steps in a second)

John Rennie

11:27 AM

The defining characteristic of SHM is that the equation of motion is: $$\frac{d^2x}{dt^2} = -kx$$

Alan

So yeah, Question: Find the period of oscillation, for a body executing SHM in that case

John Rennie

So all you have to do is show that the equation of motion is of this form, and you have shown that the motion is SHM.

Alan

@JohnRennie Yup, I used it ... but I still screwed up somewhere... (Gimme a second, finding the pictures) :)

(Hope the handwriting's not a turn off :3)

John Rennie

You're making life hard for yourself by measuring $x$ from the surface.

Alan

:O

The equation I got (for time period) varies with depth (x)... clearly not SHM :/

John Rennie

11:30 AM

Suppose instead you take $r$ to be the distance from the centre of the Earth, and write the equation using $r$.

Alan

^ Need to think about that... gimme a minute please. Thank you! :)

John Rennie

@Alan Well your equation $g(x) = g(1-x/R)$ becomes $g(r) = -gr/R$. The minus sign is because we measure $r$ outwards while the acceleration is inwards i.e. in the opposite direction.

Alan

Ah! Can you give me 5 minutes or so, I need to try that out O:)

Hopefully, you'll still be around by then?

John Rennie

I'm around for a while yet.

Alan

Thank you! :-)

Alan

11:50 AM

@JohnRennie Okay, so there's a significant improvement in the final result...but I still screwed up XD

The angular frequency (according to the answer key) ought to be the square root of (g/R)

But I've somehow managed to get sq.rt(-2g/R)

Hmm..

Anonymous

The formula for $g_d$ there is an approximate one.

Alan

@Blue Yup, but the the angular frequency I got is sq.rt{-2} times the frequency I'm supposed to get... something else must've gone wrong :/

Anonymous

@Alan Notice the word "approximate"

Anonymous

You shouldn't use that

John Rennie

@Alan the usual way of doing this is to guess $x = A\sin\omega t$

Alan

11:55 AM

But, Mr. Rennie said it was okay?

^ Blue

I'm sorry, I don't quite get the "A\sin\omega" part :(

<- Doesn't code

"A" is amplitude?

So that would be the erth's radius "R" (I guess)

John Rennie

$A$ is some constant that we'll worry about later.

Alan

Alright :-)

But..

John Rennie

If you start with $x = A\sin\omega t$ then differentiate it with respect to time you get:

$$ \frac{dx}{dt} = \omega A\cos\omega t $$

And if you differentiate it again you get:

$$ \frac{d^2x}{dt^2} = -\omega^2 A\sin\omega t = -\omega^2 x $$

Alan

Which is $$\frac{d^2x}{dt^2} = -kx$$

John Rennie

Correct.

Alan

11:59 AM

Gimme a minute (again) :-)

John Rennie

And if you compare it with your original equation $$ \frac{d^x}{dt^2} = -\frac{g}{R} x $$ you'll find that ... shall I do the last step for you?

Alan

I'll give it a try, thanks!

Wait, my original equation had -\frac{2g}{R}

^ There was a "2" there..

Or are we talking about different equations...

John Rennie

29 mins ago, by John Rennie

@Alan Well your equation $g(x) = g(1-x/R)$ becomes $g(r) = -gr/R$. The minus sign is because we measure $r$ outwards while the acceleration is inwards i.e. in the opposite direction.

Alan

Okay..I think I'm kinda lost now :3 I'll take you up on your offer then... I'd appreciate it if you could work out the last bit, thanks! :-)

<re-checking steps in the meanwhile>

John Rennie

@Alan You correctly state that the equation for $g(x)$ is $g(x) = g(1 - x/R)$. OK so far?

Alan

12:04 PM

<Nods>

John Rennie

And I pointed out we can rewrite this as $g(r) = -\frac{g}{R} r$. Still OK?

Alan

John Rennie

$g$ is the gravitational acceleration i.e. it is $\frac{d^r}{dt^2}$. Yes?

Alan

Yup! :-)

(Bear with me, it takes time to interpret the LaTex equations)

John Rennie

So substituting for $g(r)$ we get: $$ \frac{d^2r}{dt^2} = -\frac{g}{R} r $$

To get the latex to render see:

23

A: Any chance of MathJax in chat?

As a workaround while this request is pending, there exist several client-side workarounds that can be used to enable LaTeX rendering in chat, including: ChatJax, a set of bookmarklets by robjohn to enable dynamic MathJax support in chat. Commonly used in the Mathematics chat room. An altern...

Alan

12:09 PM

@JohnRennie Okay that's clear (and thanks for the link)

Ah, yes! I get it now! :O

That was brutally straightforward :O

Thank you very much Mr. @JohnRennie :D

John Rennie

You're welcome. If you go on to do a physics degree you'll encounter simple harmonic motion over and over again :-)

Alan

Ah, so you're a professor of physics? :-)

John Rennie

No, I'm a retired physicist

Alan

:O

Ah? How many years has it been? :D

John Rennie

Many too many :-)

Alan

12:14 PM

You're pretty well-versed in this stuff (I guess, you retired recently?)

@JohnRennie :O

John Rennie

Basic stuff like simple harmonic oscillators you never forget because you cover it so many times over the course of a physics career.

Alan

Can I put a bounty on one of your answers? O:)

John Rennie

@Alan no, don't do that. Maybe one day you'll ask a question on the main site that I can answer. That would be a better use for a bounty.

Alan

<- Was breaking his head try to get that problem done for the past hour or so

John Rennie

Trying to do it using energy wasn't going to work.

Alan

12:16 PM

@JohnRennie Hmm, okay, I'm looking forward to that :-)

@JohnRennie Before you showed me how to do it, I was under the impression that energy analysis was quickest way to get Things done XD

^ for classical mechanics

I'm still shocked it didn't work out as well/neatly as I hoped

@JohnRennie If I have questions like these again, will I still be able to ask you? (Judging from the chat transcript, everyone's bombarding you with questions, so it might be a bother) O:)

John Rennie

You're always welcome to ask. One of the reasons I hang around in the chat room is because I'm happy to answer this sort of question. Obviously I'm sometimes busy doing other stuff, but if I have the spare time I'm always happy to answer.

Though as it happens I have to go now - lunch time! :-)

Alan

I see, thanks! This really helped :-)

Enjoy your lunch Mr.@JohnRennie :-)

Anonymous

@JohnRennie It's possible to do it using the energy method. Just we need to carefully write down the potential function which is $U(r)=\frac{1}{2}\frac{mg}{R}r^2$ with zero point at the center of the earth.

Anonymous

Also I don't particularly like this SHM problem because it causes a lot of misconceptions. The motion is SHM only under particular conditions :P

user228700

1:01 PM

@JohnRennie Ooh, what'd you have?

ACuriousMind

1:22 PM

I see with concern the close queue has not shrunk considerably in the last week and is still at 73 items. This may be an effect of increased traffic but not increased reviewing or something else, I'm not sure

ooolb

2:04 PM

waddup it's a fine day

Sid

It's a horrible day..

user228700

@Sid :-/ Why?

Sid

@Kaumudi.H I just acquired a cold

user228700

Ah, dang, that sucks :-/

Sid

ANyway, How was your exam?

user228700

2:12 PM

It was alright, thank you :-)

user228700

Graphics on Thursday! Gosh, I am so screwed!

Sid

What is Graphics?

user228700

Engineering Drawing.

Sid

Isn't that supposed to be practical/lab work?

user228700

Unfortunately, both.

Balarka Sen

2:15 PM

@Blue Learnt about an important class of fractals today

Sid

ED is easy. AutoCAD makes things so much easier and time-saving...

Anonymous

@BalarkaSen Awesome. Let's discuss about it tomorrow evening. :) (Doing some work now)

user228700

@Sid Dude, I gotta use multiple pencils and everything! CAD, my foot :-/

Sid

@Kaumudi.H ..Hm? What? Why would you use pencils?

Balarka Sen

@Blue I don't know much about them yet but for sure

user228700

2:17 PM

@Sid To draw on paper!

Sid

@Kaumudi.H What on earth? Why? There is software to do exactly that.

user228700

Their argument is that Engineers must know the very basics of this stuff and must be able to do it with minimal tools.

Anonymous

We face the same torture @Kaumudi.H I sympathize :P

Anonymous

From second year we can use the CAD software though (I think)

Sid

@Kaumudi.H Yeah, well... these days, minimal tools=use of software

Anonymous

2:24 PM

@Sid I wish they realized that...lol

user228700

@Blue Eyyy, we don't have this crap next sem, or in any of the others!

user228700

@Sid x'D Ikr?

Sid

Do you have workshop?

Curio

What is wrong in bohr model?

Sir Cumference

Morning :)

ooolb

2:44 PM

@Sid you sound like a robot

Sid

@ooolb You watch Evangelion? :o

ooolb

yes

can't wait for 3.0+1.0

ooolb

3:13 PM

@Sid coincidence though

Sid

@ooolb Hm?

ooolb

i don't even remember the last time i watched it

did you mean my profile pic?

coincidence i'm not a big fan of evangelion right now.

Sid

Yeah

Balarka Sen

so your avatar is anime?

you know what they say about anime avatars

ooolb

i can't say i disagree

@BalarkaSen

urgggggg i have to go right now

@BalarkaSen i'm not done with you

hipster commie

John Rennie

3:38 PM

@Kaumudi.H Cool :-) So far the exams haven't been too bad then?

@ooolb hipster commie? Isn't that a contradiction in terms? :-)

Balarka Sen

It's a new form of communism

Sid

3:59 PM

@BalarkaSen over-modern communism. :P

Balarka Sen

No, postmodern communism

We believe in Derrida

Sid

Derrida was a genius

Balarka Sen

Well. He's a controversial figure.

Sid

Every Great person is controversial

Balarka Sen

Not sure about that, but the converse is certainly false :P

Derrida has received a lot of criticism from the experts on the fields he tried to comment on

I personally do not know much about postmodernist philosophy, so I shall not comment on it myself

I do have strong affirmative opinions on textual interpretation, made disjoint from authoritorial intent, however, which is a central part of Deconstruction theory. But I think that dates back to Heidegger.

I can see why a man of that generation would be leaned towards that idea. I do too.

0celo7

4:21 PM

@BalarkaSen explain

Balarka Sen

@0celo7 our motto is "Marx debunked communism yeaaars ago"

the fundamental premise is a contradiction

0celo7

@BalarkaSen can you be genuine for a second and say if you’re being ironic or not

Balarka Sen

lol

Narcissus jewel

@0celo7 Why would he be ironic? What he says is demonstrably true.

0celo7

@Narcissusjewel gulag please

Narcissus jewel

4:28 PM

@0celo7 I already ate.

Giskard42

Hey, quick question: does anyone know if the material that electrostatic deflection plates are made of is critical?

Secret

doing some kind of measure theory stuff in maths chat. Currently trying to prove the rationals are of Lebesgue measure zero by using an uncountable cover jut for the sake of completeness of proof

Giskard42

All of the commercial electron guns seem to be made from bent steel or nickle

But the literature doesn't seem to discuss material selection

I assume metals that rapidly form oxide layers (aluminum?) would be undesirable?

Anonymous

4:48 PM

@Giskard42 I suppose metals with high ionization energy would be better for such purposes. I don't know about the exact material they use though

JamalS

Can anyone remember that cool integral where the first 15 values are the same, and then suddenly the next value is many orders different? It's something like nested sine functions.

Balarka Sen

I think it's the Borwein integrals

JamalS

@BalarkaSen That's exactly it, thank you!

Balarka Sen

Strong law of small numbers in action

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