The h Bar

ZeroTheHero

2:28 PM

@JohnRennie any relation to one of en.wikipedia.org/wiki/John_Rennie ?

ACuriousMind

He's clearly the MI6 director who faked his death and now answers physics questions on the internet.

0celo7

@ACuriousMind but why would he still use the same name ?

ACuriousMind

Vanity

0celo7

he is British after all

ACuriousMind

No one will believe he's a retired spy anyway, so why hide?

Emilio Pisanty

2:37 PM

@ACuriousMind have you seen Secret's search starred above?

ACuriousMind

@EmilioPisanty Yes

SE is working on a method to remove these spammers in bulk, it's infeasible for individual mods to try and combat them

Emilio Pisanty

@ACuriousMind a'ight

0celo7

Russian bots?

ACuriousMind

For instance, the poor SU mods would have to remove so many users that it would take them more than a day at 1 deletion every 5 seconds :P

Emilio Pisanty

@ACuriousMind oooof

I guess that, so long as those users aren't posting, they're not really that harmful, and their removal can wait until SE has a solid automated solution

ACuriousMind

2:42 PM

@EmilioPisanty yup

Emilio Pisanty

physics.stackexchange.com/posts/373142/revisions <- that was fast

like, faster than @ACuriousMind fast

and purely community action too

ACuriousMind

Nope, it was my spam flag that deleted it :)

Emilio Pisanty

huh

then why doesn't it attribute it to you?

ACuriousMind

@EmilioPisanty Because I didn't delete by "mod deletion" but by casting a spam flag (which is binding when coming from a mod)

Emilio Pisanty

or were there more flags before yours?

ACuriousMind

2:47 PM

@EmilioPisanty You can usually assume that Smokey will raise flags on obviously spammy posts before even I get to it ;)

Emilio Pisanty

@ACuriousMind ah, fair enough

-1 no hand drawn circles on the graphs, I don't know where to look. — Bhargav Rao Feb 20 at 15:29

lolz

ACuriousMind

Ah, meta.

I'll bet there's a "-1 not enough JQuery" somewhere there, too :P

John Rennie

@ZeroTheHero Not that I know of.

Sid

@JohnRennie Your namesake was an MI-6 officer? :o

John Rennie

One of them ...

0celo7

2:56 PM

JR is a killer

John Rennie

There were lots of engineers called Rennie

It's a Gaelic name

Sid

@0celo7 he is a spy.

ACuriousMind

@JohnRennie That's exactly what the MI6 director would say!

3

John Rennie

@ACuriousMind this is one of those no win situations isn't it :-)

ACuriousMind

yup :)

0celo7

2:59 PM

this is how I concluded ACM is Alicia Keys

ACuriousMind

I wonder if being the top rep user on physics.SE fulfills Wikipedia's criteria of notability...

@0celo7 No, you concluded that because I was on fire ;)

0celo7

@ACuriousMind well, you saying very suspsicious things didn't help your case

John Rennie

The most closely related famous Rennie is Michael Rennie who's a second cousin several times removed or something like that. But then he's not really very famous ...

ACuriousMind

Ah, you hail from a "space visitor"

0celo7

John Rennie is an alien.

ACuriousMind

3:04 PM

Sure, try to throw us off the spy trail with an alien conspiracy theory

John Rennie

Explains a lot really :-)

Anonymous

@ACuriousMind Well, he's the only soap scientist in the world who is also an expert in GR and Javascript! :P

ACuriousMind

[citation needed]

John Rennie

I know all about bubbles in spacetime.

Master Yushi

I can derive the distance traveled by a point on the circumference of a pure rolling circle in one revolution, it makes a cycloid and the arc length comes out to be 8R but all this is considering constant angular velocity. What if the circle starts from rest being accelerated by a constant tangential force?

Anonymous

3:07 PM

@ACuriousMind lol

Master Yushi

Will the distance traveled by the point in one revolution change?

Anonymous

@MasterYushi Yeah, in the case of sliding on surface, that distance traveled should increase. Suppose you apply a very large force it will travel a large distance before it completes one full rotation.

Master Yushi

Assuming that I know the force and other info about the circle, how can I find that distance?

John Rennie

Who would have thought car insurers were so interested in physics :-)

Anonymous

Looks like the same person made all those accounts XD

Anonymous

3:16 PM

@MasterYushi I'm thinking

Anonymous

Will let you know if I come up with something

Anonymous

Meanwhile try searching the net

John Rennie

@Blue I imagine it's part of the big spam accounts attack on the SE

What surprises me is that it's worth the time taken to do it.

Master Yushi

@Blue I did, couldn't find anything for accelerated motion

ACuriousMind

@JohnRennie I'd wager they just have a botnet doing that, no? But I don't really know how spammers work

John Rennie

3:19 PM

@ACuriousMind Yes I'm sure the attack was automated, but will anyone actually get any extra business as a result of all this?

Master Yushi

It's Q.83

ACuriousMind

@JohnRennie Does anyone get extra business from all the other spam I find in my mailbox daily? If yes, then why wouldn't they get some from this, if no, why do any of this in the first place? :P

Anonymous

@JohnRennie That's not at all surprising. Spammers and trollers find spamming and trolling to be a refreshment or a "nice" method for procrastination. Also, I don't think that's any real company. Probably a troll who got bored and decided to play pranks on PSE.

ZeroTheHero

@JohnRennie well... was worth asking.

John Rennie

@ZeroTheHero Rennie was originally a clan name, so in principle all Rennie's are related though you'd probably have to back a long way. I think the name originated in about the tenth century in the Kingdom of the Gaels.

Anonymous

3:26 PM

@MasterYushi Do you know the derivation of the general cycloid equation?

Balarka Sen

@JohnRennie Oh, that guy

ACuriousMind

@JohnRennie All Europeans are related to basically everyone who lived in Europe around 1000 CE :P

Master Yushi

@Blue
x = r(t-sint)
y = r(1-cost)
This?

John Rennie

@ACuriousMind well humans will keep shagging each other. Disgraceful really - no better than rabbits.

ACuriousMind

@JohnRennie Eh, we build nicer abodes than rabbits :P

Balarka Sen

3:32 PM

That's what the rabbits say when they see us

I need to keep up with British slangs...

Anonymous

@MasterYushi Yeah, now try to find the $x$ coordinate when there is a force applied at the top

Anonymous

Any guesses from this picture? @MasterYushi

Anonymous

@BalarkaSen Tell me about the fractal stuff you learnt!

Anonymous

We could shift to the other room if you wish :P

Master Yushi

3:41 PM

@Blue the x coordinate would be the same in terms of t, it's just the angle(t) that will change according to angular acc.

wait

x = r(t-sint) + (aT^2)/2?

Balarka Sen

@Blue Well there's not much on my end to say so I'll just say it here

Anonymous

Wait a bit. The question asks only for the distance travelled by the "sphere". Not a point on it's surface. That is a way simpler problem

Balarka Sen

Say $f : \Bbb C \to \Bbb C$ is a rational function of $z$

Anonymous

@BalarkaSen Okaies

Balarka Sen

Fact: There is an open dense subset (not necessarily connected) $U$ of $\Bbb C$ such that $f(U) = U$. So under successive iterations of $f$, $U$ is left invariant

Anonymous

3:48 PM

What does iteration mean in this context?

Balarka Sen

Oh, like, $\{f, f\circ f, f \circ f \circ f, \cdots\}$

It's a discrete dynamical system

Master Yushi

I thought you were gonna talk about the fun part of fractals

@Blue Oh right, that's easy

Balarka Sen

where each time interval is taken to be 1 and what happens after each interval of time is an extra iteration of $f$

Anonymous

Alright, gotcha

Balarka Sen

$\Bbb C \setminus U$ is usually the fractal-like subset that comes out of this system

This is called the Julia set, I think

Anonymous

3:51 PM

Dense set just means that all the limit points are contained within it, right?

Balarka Sen

Oh, no, dense means every point of $\Bbb C$ is a limit point of $U$

Rationals are dense on the reals for example

Master Yushi

Anonymous

@BalarkaSen But $5$ is not a limit point of the rationals, no?

Anonymous

It is a rational!

Balarka Sen

sure it is

it's limit of the sequence $5, 5, 5, \cdots$ :P

Anonymous

3:53 PM

@BalarkaSen Oh, lol

Anonymous

Got it

Balarka Sen

So yeah I have been given an example I don't really understand

And that's $f(z) = z^2 + c$

For large enough $c$, apparently it's Julia set is supposed to be a Cantor set

Sid

Only today did I find out why my college leaves were not getting sanctioned..

Anonymous

"The Cantor ternary set ${\displaystyle {\mathcal {C}}}$ is created by iteratively deleting the open middle third from a set of line segments." and "Julia set consists of values such that an arbitrarily small perturbation can cause drastic changes in the sequence of iterated function values"

Balarka Sen

I just told you what a Julia set is

Anonymous

3:59 PM

@BalarkaSen Uh, where?

Anonymous

Did I miss something?

Balarka Sen

Yes. :P

Anonymous

12 mins ago, by Balarka Sen

Fact: There is an open dense subset (not necessarily connected) $U$ of $\Bbb C$ such that $f(U) = U$. So under successive iterations of $f$, $U$ is left invariant

Anonymous

This?

Anonymous

If we change values of that open dense subset...

Anonymous

4:01 PM

The successive iterations will no longer be invariant

Balarka Sen

No, read carefully

10 mins ago, by Balarka Sen

This is called the Julia set, I think

Anonymous

Oh, damn. I dunno how I missed that message

Anonymous

Let's see..hmm

Anonymous

I don't know the notation C/U

Balarka Sen

C \ U. Setminus

Anonymous

4:03 PM

C-U. Oh, alright, got it

Anonymous

Interesting. This makes more sense

Anonymous

The complement of an open set is closed

Master Yushi

What fractals are you talking about?

Anonymous

@MasterYushi Fractals in mathematics...

Master Yushi

These are not the fractals I've worked with lol

Anonymous

4:08 PM

This is indeed related to the fractal geometry you worked with, I guess. Unless you are talking of fractals in art

Anonymous

@BalarkaSen Silly question. C-U would be the complement of U, or no?

Balarka Sen

Yes it is

Anonymous

Phew. It's making sense. Go on

Balarka Sen

I don't have anything else to say, really

Master Yushi

I am talking about procedurally generated graphics, I've coded a few. L-systems and stuff

Balarka Sen

4:09 PM

I'd like to understand the picture for $z^2 + c$, like I said

Anonymous

@MasterYushi If you are talking of these fractal images, the yes...it is related to mathematics

Anonymous

In fact it IS the mathematics we are talking about

Master Yushi

I know this one, I've written code to generate it

Anonymous

@BalarkaSen I see. Interesting. Will think about it

Anonymous

4:15 PM

In fact the Mandelbrot set is the set of complex numbers for which the function $f_c(z)=z^2+c$ does not diverge when iterated from $0$ (as wiki says)

Anonymous

hmm

Sid

@blue ping me whenever you are free.

Abcd

@JohnRennie Got an exam tomorrow. Confused about some concepts. Are you busy?

Anonymous

@BalarkaSen Did you check the Wiki page on Mandelbrot set? That function looks exactly similar. But I think there is a more generalized version

Balarka Sen

I did

Anonymous

4:16 PM

I'm reading it

Anonymous

@Sid Ping!

Sid

@Blue What all things do I have to revise for that ML Course that you had convinced me to sign up for?

Master Yushi

Anonymous

@Sid I think one of those two courses are already online (as one of them is a re-run of a previous course). Try to go through the course material once. Also complete the Python (Algo and Data Structures) lecture series by them

Anonymous

I think you can do that by December

Master Yushi

4:20 PM

One buggy line and you end up with spiderman instead of mandelbrot

Anonymous

I'll start as soon as my exams end

Anonymous

@MasterYushi That's characteristic of Julia sets it seems. I'm going through the Wiki page

0celo7

@BalarkaSen Is the scalar curvature of $H^n$ like $-n$ or is it just $-1$

Abcd

@Blue Could you help? Or are you busy?

Anonymous

@Abcd A bit busy. But you could ask...someone might be able to answer...

Balarka Sen

4:23 PM

@0celo7 i have forgotten scalar curvature

can't help

Master Yushi

@Blue youtube.com/watch?v=fAsaSkmbF5s

Abcd

In $y = y_m \sin (kx-\omega t+\phi)$, could anyone tell me the significance of $k, \omega$ and $\phi$?

Master Yushi

phi is the initial phase

Anonymous

@Abcd $k$ is the wave vector. $\omega$ is angular frequency. $\phi$ is the initial phase.

Anonymous

Check those terms on Wiki

Master Yushi

4:25 PM

tells you about the particle at x=0 at t=0

John Rennie

@Abcd I've just finished lunch so I'm happy to chat now. What's up?

Master Yushi

k is wave number, it is the number of waves in unit length

Anonymous

@MasterYushi "trying to get into a good university to pursue cse without having to burn my nerves on pcm". Story of every kid nowadays :) Good luck :P

Abcd

@JohnRennie what is wave vector, and phi?

Anonymous

@MasterYushi Nice. Will watch

Master Yushi

4:27 PM

@Blue Haha, thanks!

John Rennie

@Abcd Let's take $\phi$ first because that's quite easy.

Abcd

Ok

John Rennie

Suppose you look at the point $x=0$ and $t=0$ and ask what is $y$ at the time and place.

Abcd

@JohnRennie Is this the correct use of phi : desmos.com/calculator/ojbbwm5tjw

John Rennie

We know the values of $y$ range from $-y_m$ to $+y_m$, but there's nothing in our equation to say what the value of $y$ is.

Master Yushi

4:30 PM

@JohnRennie I've seen k as number of waves in unit length so that should be 1/wavelength but somewhere I've seen it written 2pi/wavelength also. What's up with that?

John Rennie

The value of $y$ is what we call an initial condition for our equation.

Anonymous

@BalarkaSen Got the PDE book by Farlow's, today. It looks quite thorough and readable! So all book-buying done for the next one year! XD Hirsch-Smale, Farlow, Shakarchi-Stein, Gallian/Artin, Nash-Sen and Arfken. I really hope I can complete all of them. Lot of interesting stuff to learn. Just waiting for 14th Dec :p

Balarka Sen

Nice

John Rennie

@MasterYushi in these sorts of situations you sometimes see angular frequency $\omega$ used, and you sometimes see frequency $f$ used. They're basically the same thing with just a factor of $2\pi$ difference.

user228700

Hello, everyone :-)

John Rennie

4:33 PM

Well we get the same thing with the wave vector. Sometimes we include the factor of $2\pi$ and sometimes we don't. The difference is that people tend to be sloppy and use the same symbol for both.

ACuriousMind

@Kaumudi.H Hey. Getting enough food? ;)

Abcd

@JohnRennie I can't understand.

Emilio Pisanty

fun fact: the usage of the term 'gauge field theory' seems to be in steep decline

user228700

@ACuriousMind Hi :-) Yes, today was a good day, in terms of the amount of food I was able to eat :-P How's it going?

John Rennie

@Abcd that was an answer to @MasterYushi ...

@Abcd suppose we take your equation $y = y_m \sin (kx-\omega t+\phi)$ and set $x=t=0$ then we get $$\frac{y}{y_m} = \sin\phi $$

Abcd

4:35 PM

Yes

Emilio Pisanty

Also, @ACuriousMind: boom, double rep-cap =D

ACuriousMind

@Kaumudi.H Going well, just watching the chat while listening to music and playing Dungeon Crawl

@EmilioPisanty grmbl

John Rennie

@Abcd so if $\phi=0$ that means $y(0,0)=0$, or if if $\phi=\pi/2$ that means $y(0,0)=y_m$

Emilio Pisanty

@ACuriousMind ::grin::

ACuriousMind

You're much more profilic in answering than I am, currently

user228700

4:37 PM

@ACuriousMind Sounds fun :-)

user228700

I am slowly dying.

ACuriousMind

@Kaumudi.H We all are, it's called living.

John Rennie

@Abcd The parameter $\phi$ just determines our initial value for $y$.

user228700

:-P Right, of course.

Emilio Pisanty

it might yet be rolled away by the system's anti-systematic-voting defenses, I suspect

yesterday I got one vote each on my top-scoring answers

I appreciate the gesture from whoever it was, but the system does tend to not like that

user228700

4:38 PM

I meant to emphasize the fact that I feel like I am dying :-P

ACuriousMind

@Kaumudi.H Well, that sucks. What's up?

user228700

Engineering Drawing exam tomorrow :-/

ACuriousMind

@EmilioPisanty The reversal script runs each day at 3 am UTC, so yesterday's serial votes should already be reversed if it wanted to reverse them

Emilio Pisanty

@Kaumudi.H if everything else fails, bring some crayons and a crayon sharpener

Master Yushi

@JohnRennie So we use omega in the wave equation if we use k = 2pi/wavelength, and if we use k=1/wavelength, it should be Asin(2pi(kx-ft)) ?

Emilio Pisanty

4:40 PM

::grins::

aaaaand

::ducks::

user228700

:-)

user228700

@EmilioPisanty It's OK, I lack the energy to come at you :-P

Balarka Sen

I rate my day as NOT GOOD/10

Emilio Pisanty

which one was it?

this one?

Tim Minchin - Americans and evolution

►

user228700

@BalarkaSen :-/ Damn, what happened to you?

Abcd

4:42 PM

@JohnRennie Understood. Wait a minute. Let me note that.

Balarka Sen

Allergy

General idleness

user228700

Ah :-/ Dang, I hope you feel better soon.

Balarka Sen

I don't

ACuriousMind

@Kaumudi.H Well, that's tomorrow-you's problem ;)

Balarka Sen

I like to live in perpetual misery

user228700

4:43 PM

Ah, right, I'd forgotten.

John Rennie

@MasterYushi Yes, I suppose so, but no-one will applaud you for using $f$ in a wave equation, or for using $k$ that doesn't include the factor of $2\pi$.

Emilio Pisanty

no, that's not it

ACuriousMind

@EmilioPisanty Whatcha looking for?

Abcd

@JohnRennie Ok, what next?

user228700

May 15 at 9:41, by Balarka Sen

@Kenshin I don't want to feel good! That's the point!

ACuriousMind

4:44 PM

I think I know most Minchin clips by heart :P

Emilio Pisanty

@ACuriousMind Tim Minchin going "aaaaaaand, duck"

user228700

I remember this well enough :-P

John Rennie

@Abcd can't remember, were you asking about $k$?

Emilio Pisanty

then says he's just practicing for his American tour

Abcd

@JohnRennie $k$ and $\omega$

ACuriousMind

4:45 PM

@EmilioPisanty Ahhh

Hm

Emilio Pisanty

@ACuriousMind exactly

it's one of those, innit

the question is which one

John Rennie

@Abcd shall we go to the problem solving room? It's getting noisy in here ...

Master Yushi

@JohnRennie Well I'm confused now, according to the definition of k, shouldn't the proper way of writing the equation would be using ft?

Abcd

@JohnRennie Yes, sure. Even I was getting confused by the parallel discussion on the same topic with Yushi.

John Rennie

@MasterYushi I would define k as $2\pi/\lambda$ ...

Master Yushi

4:47 PM

@JohnRennie But it is defined to be the number of waves in unit length

John Rennie

Wave vector

In physics, a wave vector (also spelled wavevector) is a vector which helps describe a wave. Like any vector, it has a magnitude and direction, both of which are important: Its magnitude is either the wavenumber or angular wavenumber of the wave (inversely proportional to the wavelength), and its direction is ordinarily the direction of wave propagation (but not always, see below). In the context of special relativity the wave vector can also be defined as a four-vector. == Definitions == There are two common definitions of wave vector, which differ by a factor of 2π in their magnitudes....

> Its magnitude is either the wavenumber or angular wavenumber of the wave

ACuriousMind

@EmilioPisanty Well, unfortunately there are a lot of spots in his routines where he might want to duck in front of certain audiences ;P

Balarka Sen

@Kaumudi It's a good ontological exercise to defend my position about misery against the sword of "enjoyment of misery is also enjoyment of a sadistic sort". I leave this as an exercise to the reader.

ACuriousMind

So I don't recall either where exactly that was

Emilio Pisanty

@ACuriousMind I'm pretty sure it was religion-related

which narrows it down by maybe 10 to 15%

user228700

4:49 PM

@BalarkaSen -.- I'll pass, thanks.

Master Yushi

@JohnRennie So technically it is both, but angular wave number is usually used in physics

ACuriousMind

@BalarkaSen Enjoyment of your own misery would be masochistic, not sadistic.

John Rennie

@MasterYushi usually, yes

Balarka Sen

@ACuriousMind Well, depending of course, upon which end of the action that inflicts the pain you are in. In this case, uh, both?

Master Yushi

@JohnRennie thanks

Secret

4:51 PM

If I see anymore "Please help me", the other chat room will be dead :P

Emilio Pisanty

@MasterYushi yes. In signal processing, as I understand it, they do use the FT normalized as $\int f(x) e^{2\pi i k x}dx$, but that would just get you some very puzzled looks if you used it anywhere I've been in physics

Anonymous

@Secret There's an user who well known for using that phrase. Hehe

0celo7

@ACuriousMind Coordinates are $\{x^i\}$ or $(x^i)$?

Probably the latter because we use parentheses for vectors

Secret

@Blue not that I knew for h bar. h bar recently is relatively free of help vampire infestation

Balarka Sen

hbar is full of trolls though

Anonymous

4:54 PM

@Secret I was talking about Math SE chat

Anonymous

hbar did have some vampires previously

Anonymous

Now it's just trolls

Balarka Sen

not sure that's any better than help vampires

Anonymous

Trolls are at least funny

Anonymous

Vampires...are well....just annoying

Balarka Sen

4:55 PM

and help vampires at least ask productive questions :P

even though, well, in a vampirish manner

Anonymous

Well, lol

Secret

I have been observing Maneesh Narayanan for more than 2 months already, and he is not showing signs of improving. Mary Star is only a bit better than him. It seems he will become the next koolman

This is one reason I hate the downtimes in maths chat, because a chat inhabited with vampires is so boringand annoying I just want to spam it to nonexistence

Anonymous

@Secret Maybe someone should point that out to Maneesh directly. Apparently he is preparing for some entrance exam and needs help every now and then.

Balarka Sen

@Blue But, as the pejorative against the vampire culture goes, "Still a better love story than Twilight"

So I prefer trolls anyway

Secret

@Blue I know. but for other people (recall the numerous JEE people a year ago), at least they are showing effort

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