12:00 AM
and then bring him to this universe
and replace him with the IT ACM

12:12 AM
I need help

Hi, everybody.

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

@DanielSank 你好

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

@bolbteppa @ACuriousMind disagrees.

12:25 AM
Дадно. Давай говорим по Русский.

@0celo7 disagrees with what

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

Хорошо!
В какой город?

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

Да
ладно

12:27 AM
@DanielSank 广州 (Guangzhou)

@DanielSank Nein danke

@0celo7 Мне всё равно что ты хочешь.
@Mithrandir24601 Блядь! В Гуанжо ограбят тебя слепыми!

@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 :/

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

Although China is really cheap anyway :)

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.

@DanielSank Ahh, fair enough :)

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

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

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

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

12:38 AM
@DanielSank Что я сделал тебе?

@Mithrandir24601 yessir
@0celo7 Нечего

12:53 AM
Yes this is real:
"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."

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

1:19 AM
sure, that's not too bad @0celo7
Animate or ListAnimate
I'm not sure which is better tbh though

cool
the actual problem is ridiculous
double perturbation theory for Navier-Stokes in (2+1)D

sheesh

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

I don't see them
but the proof is a direct computation

i cant use the original definition of the brackets
thinking of the properties of the brackets as axioms

how are we supposed to help then

and using them to prove this
for example

then taylor expand

2:05 AM
the function can be anything
its a general function

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

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?

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

prove by induction will be best then

@MohammadAreebSiddiqui the induction is too trivial to bother
if you're doing induction here you're missing the point

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

1 hour later…
3:24 AM
@SirCumference howdy

3:37 AM
or not...

1 hour later…
5:06 AM
weez

2 hours later…
6:42 AM
@JohnRennie Not possible to tag you in Problem Solving Strategies.
Because it's been a while...

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

@SirCumference howdy (the sequel)
or not...

7:08 AM
::infinite loop::
2

1 hour later…
8:19 AM
Mornin

8:48 AM

what a cv!

9:03 AM
@JohnRennie How can a gravitational field possess momentum and energy as stated by my book?

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.

You can have momentum yes
just expand the gravitational field as a spin 2 field and take the momentum

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

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

lie

9:14 AM

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. "

9:32 AM
ZUCC
New face filters on Instagram today
Maybe he prefers instagram over facebook

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

10:30 AM
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.

is this the right place to ask a simple physics question

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?

sweeeet

11:00 AM
That was easy :-)

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?

Hi Emilio

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

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

wow cool, ill test it out
thanks for the help

11:18 AM
Is it okay to ask a (physics) question here?

@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 $$\int A^{\dagger}_{C}A_CdC = 1$$?

Ah, thank you! Is classical mechanics (simple harmonic motion) okay? O:)
I'm still in high school (thought I'd mention it)

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

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

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

(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:) )

OK ...

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:)

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

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

11:27 AM
The defining characteristic of SHM is that the equation of motion is: $$\frac{d^2x}{dt^2} = -kx$$

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

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.

@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)

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

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

11:30 AM
Suppose instead you take $r$ to be the distance from the centre of the Earth, and write the equation using $r$.

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

@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.

Ah! Can you give me 5 minutes or so, I need to try that out O:)
Hopefully, you'll still be around by then?

I'm around for a while yet.

Thank you! :-)
<scoots>

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.

<checks calculations again>
@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

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

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)

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

Alright :-)
But..

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

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

Correct.

11:59 AM
Gimme a minute (again) :-)

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?

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...

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.

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>

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

12:04 PM
<Nods>

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

<Nods vigorously>

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

Yup! :-)
(Bear with me, it takes time to interpret the LaTex equations)

So substituting for $g(r)$ we get: $$\frac{d^2r}{dt^2} = -\frac{g}{R} r$$
To get the latex to render see:
23

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...

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
<rubs hands in glee>

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

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

No, I'm a retired physicist

:O
Ah? How many years has it been? :D

Many too many :-)

12:14 PM
You're pretty well-versed in this stuff (I guess, you retired recently?)
@JohnRennie :O

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

@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.

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

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

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:)

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! :-)

I see, thanks! This really helped :-)

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?

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

2:04 PM

It's a horrible day..

user228700
@Sid :-/ Why?

@Kaumudi.H I just acquired a cold

user228700
Ah, dang, that sucks :-/

user228700
2:12 PM
It was alright, thank you :-)

user228700
Graphics on Thursday! Gosh, I am so screwed!

What is Graphics?

user228700
Engineering Drawing.

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

user228700
Unfortunately, both.

2:15 PM
@Blue Learnt about an important class of fractals today

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 :-/

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

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

user228700
2:17 PM
@Sid To draw on paper!

@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)

@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?

Do you have workshop?

What is wrong in bohr model?

Morning :)

2:44 PM
@Sid you sound like a robot

@ooolb You watch Evangelion? :o

yes
can't wait for 3.0+1.0

3:13 PM
@Sid coincidence though

@ooolb Hm?

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.

Yeah

you know what they say about anime avatars

i can't say i disagree
@BalarkaSen
urgggggg i have to go right now
@BalarkaSen i'm not done with you
hipster commie

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? :-)

It's a new form of communism

3:59 PM
@BalarkaSen over-modern communism. :P

No, postmodern communism
We believe in Derrida

Derrida was a genius

Well. He's a controversial figure.

Every Great person is controversial

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.

4:21 PM
@BalarkaSen explain

@0celo7 our motto is "Marx debunked communism yeaaars ago"
the fundamental premise is a contradiction

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

lol

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

4:28 PM

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

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

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

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.

I think it's the Borwein integrals

@BalarkaSen That's exactly it, thank you!

Strong law of small numbers in action