12:43 AM
@ACuriousMind what are you doing here? Aren't you supposed to be asleep at this hour?

@Sofia I'm not supposed to be anything at the moment, the semester is over.

1:12 AM
@ACuriousMind aren't you bored? You are alone in the room.
@ACuriousMind I would go to sleep too, but I have a problem with Emilio that instead of being in bed at this hour, made an editing of an answer of mine, and I don't know what to do, because there is no field there that allows me to approve its editing.

@Sofia The edit was automatically applied to your post already - edits from users with more than 2.000 reputation are always applied without waiting for approval

@ACuriousMind what is I# ?

I pressed Enter too soon

@ACuriousMind don't you have exams?

I wanted to say that I'm not bored because I'm also doing a few other thing except watching this chat
@Sofia The next exam is more than a week away

1:20 AM
@ACuriousMind : don't spend nights being awake, you will regret later on. I am having terrible problems because I got used to be awake by nights. That destroys my mornings.
@ACuriousMind well whenever you'll decide to go to sleep, sweet dreams!

@Sofia Thanks, you too!

You three :D

1:36 AM
←dreaming of sof...'s av

1 hour later…
2:53 AM
While trying to find something in my browser history, I somehow, after highlighting "Last Month", managed to make the browser open all of the web pages I visited the last 30 days.

1 hour later…
4:15 AM
@KyleKanos Ouch. This is why having a lot of RAM is important.

I couldn't tell the lag because I just ended up opening a new window and closing the old one before it managed to do too much

5 hours later…
9:07 AM
@ACuriousMind How'd your exams go?

3 hours later…
12:28 PM
@KyleKanos This question and the answers make me want to yell at everybody:
7

I've been using Ruby to write scripts for research, but I want to get into some heavier stuff that Ruby is just too slow for. I noticed there are a few things written in C and C++, but there is an oddly large proportion of software used in computational chemistry that is written in FORTRAN (which...

1:16 PM
@tpg2114 I've been a member of chemistry.se for a month or so now, and I find the frequency of questions asked very dismal in comparison to Physics. I know it's still in beta, but it's been so how long? About six months or so?
Surprisingly, sites like bicycle.se ( yes, there is such a site) have already progressed from beta before chem.se has. Any idea what the reason may be?

@tpg2114 That Geoff guy is big on Python over at Computational Science too, IIRC

@KyleKanos Yeah, the answers regarding Fortran seem to be the same in both places
And they seem to be out of touch with reality in both places

1:33 PM
As a matter of pure opinion, as a developer who has developed in perhaps a dozen languages, there is no reason I would recommend FORTRAN for any developer for any task unless there is a legacy FORTRAN codebase to interface with. People are simply not learning the language anymore, meaning you will have to maintain any code you write forever. It will be hard to hand off to anybody. — Cort Ammon 8 hours ago
I declare shenanigans

Agreed

@Danu Oh, quite well. String theory is still to come, but I passed algebraic geometry.

Seems to me someone needs to learn about stellar lives: cad-comic.com/cad
Well SabreTooth seems to have left this site (though I see his ever-watchful eye in the corner)

2:03 PM
Now even his watchful eye is gone :O

Well, someone ought to apologise to him. Dissecting someone's personal life (which none of us have a clue about) that agressively is not a good thing.

In unrelated news, I hate it when people write stuff like sneutrinos. I always think it is a typo until I recall these stupid supersymmetry names.
2

@KyleKanos I noticed that as well. I put together some puzzle pieces that I won't elaborate on in open chat. But I don't think that was the first account associated with that person. He asked for his previous account to be deleted because of some (arguably) mean/disrespectful things that went down on Meta before.
So out of respect for privacy, I won't explain any more. But it's regrettable that he left again. Although this time I don't know that things were actually as offensive as last time
He seemed pretty upset about my rubber duck physics comment in chat. I don't know why it was thought to be demeaning, it's a legitimate thing to do.
In other news, GT messed up yet again and sent my W-2 to the wrong address. Right about the time that happened, somebody used my SSN to change my mailing address with my bank and ordered a new debit card that was activated this morning
Never got any notifications about changes. Only found out because my card was declined and I called to find out why.

@tpg2114 It's probably not your fault. I think what put him off is the unneccessary verbal abuse by another member.

2:18 PM
@Gaurav I wouldn't attribute it to any single source. There are multiple reasons evident from the discussions here and on Meta. None of it was helpful.

Can you use animations in answers on SE?

-2

Sometimes, specifically with sorting/clustering algorithms, it would be nice to be able to provide a gif to illustrate a point. Does anyone else think this could be a good feature?

Thanks.

Gif is awesome, so long as you don't need a ton of colors

I suppose SVG would be even more awesome (well, it has limited uses but for illustrating concepts and plotting you probably want vector graphics, anyway)

2:23 PM
Even static SVG doesn't work here
But the reason GIF is great -- it's been around forever so it's going to work everywhere
We started using GIF in our PPT presentations rather than dealing with trying to embed movies and crap. It was just so much simpler
Back before GIF was cool again

GIF isn't very efficient at compression, though, and as you mentioned does have a restricted colorspace. I mean, depends on the contents of course, but I guess that much of the new GIF use could well be replaced by webm (and some GIF sites allow both formats to be uploaded).

Size is the real problem, the animations can get gigantic
But the colorspace is actually a good thing. I try to explain to people that most scientific visualization shouldn't have more than 16-32 colors anyway
Otherwise you lose a lot of information about the fields

Well, depends on what you want to show. If you want to show a cool movie-like animation as motivation, you would probably appreciate the full color.
But as you said, the size is the big problem. I don't think GIFs are buffered either, so you can't play back like a one gig GIF file.

Yeah... it depends on the audience. To demonstrate technical data, limited color spaces are generally much clearer. For showy presentations to non-technical project managers, the Colorful Fluid Dynamics approach can be better
Side note, I'm super picky about data visualization and am trying hard not to get on a soapbox :)

At one point I kind of developed a fetish for vector graphics, meaning that I wanted all my figures (in publications as well as presentations) to be scalable.

2:36 PM
Vector is pretty cool, but my fetish is actually making as many things as I can into Tikz/pgfplot data so it's magic

Right, that's what I'm into as well.

It ends up scalable because when I tell it the size in the presentation or publication, it redraws it
So my catalog of images is just a bunch of tex files

Well, that and .js files for me as I've done a lot of presentations in HTML5.
So I could throw in some cool animations and stuff, where appropriate.

I've never had an opportunity to play with that

I used D3.js if you're interested.

2:38 PM
I can't even get my advisor to let me use Beamer for presentations most of the time. He only wants PPT

It feels like playing around with TikZ/pgfplots. Except now you have to learn it all again.

So HTML5 would be impossible
And since he's on Mac, all of the equations have to use the old style editor and not the new style one that works more like Latex

Wow, ok. Is there a reason? I'd imagine that PDF is more portable.

He doesn't want to learn Beamer
And he thinks that you can't put enough stuff on a single slide in Beamer
So yeah... I'm forced to make that kind of PPT

Right, but does he micromanage all of the details in your presentation? Or does he want to recycle the material into his own presentations as well?

2:40 PM
Recycle
I'll make like 50 slides for a presentation because he asked for it. Then find out he only used 2 or 3 and cobbled together things from other projects
Heh
I haven't seen D3, but I did see impress.js and it looked pretty cool: bartaz.github.io/impress.js/#/bored

I did several posters with beamer as well. It works surprisingly well. Although from what I remember, I did have to manually throw in the coordinates which was a bit of a pain (well, I did do one with automatic boxes, but that results in a bit of a boring presentation)
Yeah, I've used impress.js. D3.js is more for the visuals, plots and stuff.

I haven't tried digging into it yet, but I've always liked the idea that Beamer can have a single document and compile as a report or a presentation

I used impress.js, or rather jmpress.js (with jQuery) once. I wouldn't recommend it though: The Prezi-style presentation might seem cool, but I think a traditional one is more efficient.

I couldn't imagine giving a presentation at a thesis defense, seminar or conference using the examples in impress.js
To me, it just seemed like a more modern PPT slide transition
I might as well windowshade each slide
I could see in business domains or non-technical presentations that being somewhat appealing

I made a poster with it. But used that as a talk, so the slides would be like different parts of the poster, kind of. I think it wasn't bad per se, but wouldn't do it again.
At least I had fun making it.
Here's someone's more or less technical presentation with impress.js
The good thing about that is that if the presentation is as nice as that one, it can sort of make an interesting website.

2:50 PM
See, for me, all the transitions just get in the way of what they are trying to say.
I don't think it adds anything to it, aside from a "Look what I can do!" appeal

But of course PDFs are easier to store etc.

I dunno about that -- the js are just text files, they can be version controlled
It's easier to send somebody a PDF rather than a bunch of js files

I agree, but I also think that if you go to a conference, say, where there are dozens or hundreds of presentations, that'll help your stand out.

@alarge Since you were asking whether animations can be used in answers on se, a recent high-voted answer had a pretty neat animation: physics.stackexchange.com/questions/161406/…

Well, I meant for the end user. As in if you want to download the presentation there, there in principle is no single file for you to download, you'd have to download several. Furthermore, you might also have problems running the presentation locally (so you might have to install a dummy webserver).

3:10 PM
0

Back when we sorted out the book recommendation and resource recommendation requests, we made it so there was a permanent banner instructing those answering to abide by our policies regarding the questions. Since we get so many homework questions, can (or should) we have a similar banner for th...

@tpg2114 Yeah, I don't know what to think of your suggestion
Well-posed HW questions wouldn't need the banner. Poorly posed questions shouldn't be answered at all and should be closed
4

@Jim Well-posed questions would still need the banner for people answering it (reminding them not to provide a solution)
Which is what we do on book questions, the banner reminds the answerer that they should base it on experience and explain why it's a good reference

Well-posed questions wouldn't be looking for the solution, so it shouldn't be something that we would have to worry about

So in some ways, the banner is more to alert people answering a question to the policies, rather than OP (because if the question was against policy, it would be closed). But we can use it to gently remind OP also

Yeah
Like I said, I don't know what to think

3:18 PM
I disagree. Somebody could show a problem, explain why they are stuck and ask for the physical explanation. That's well posed. But somebody could come along and answer it by saying "Here's the concept where you got stuck. And by the way, the answer is 10"

You're right, they really shouldn't be giving away the answer like that (especially when the answer is clearly 11.2)

So I think you could get policy-violating answers to well-posed questions
And the banner just serves to remind everybody what to do
As well as provide a more visible and lasting way for people to have links to the policies

Hmmm... this might be a status-not-worth-the-effort thing
But, as I don't know what to think, I'm not going to agree or disagree until I hear a second and third opinion
What does @Qmechanic think?

@EmilioPisanty : Yeah, only proves one should read question carefully before answering.

@Qmechanic I'm really curious about what you'd written.

3 hours later…
6:09 PM
@ACuriousMind what's your opinion to change "some day" that vampire-style picture? You frighten people. There are all sort of eccentric picture but your one is on top.

@tpg2114 He has denied those claims (of being here before).

@ACuriousMind You current avatar is freaky enough to approach awesome.
@tpg2114 Being bothered by a suggestion that is widely applied in all kinds of detail-oriented creative enterprises is not something that I can easily understand. Telling it to the duck is a tool for bringing to the surface things you already know.

@Sofia I'm not sure if having the most eccentric picture is a bad thing :)
At least it is recognizable ;)
And I don't think anyone is really frightened by it. They might think I'm a freak, but I don't care about that

Hello there

@ACuriousMind I personally choose to believe you have a certain affection towards Clowns.

6:20 PM
@KyleKanos I might have been banned from visiting the local circus...

Can someone help me on a formula related to the Young-Laplace equation on a pdf ? I'm new to those, and there's one coefficient which's formula I don't get.

@ACuriousMind I'm frightened by it

@alarge PDF is way more portable and reliable. To the point that many particle physics types will actually grumble aloud if a presenter uses powerpoint, because we don't feel confident we can display it right on our machines.

@Jim Ooooh, have you lost your security blanket?

@dmckee I much prefer the PDF presentation too for that very reason.
It's sad when someone has animations (either videos or equations/text popping in) in their presentation then does simple Export to PDF and ruins it :(

6:28 PM
I had two classmates give a presentation in a seminar recently. They used Powerpoint. Font: Comic sans. Half the room audibly groaned when the first slide came up :D

Basically, on this image below, how does one get the $2\kappa^*$ coefficient ? All I know about the Young-Laplace equation is from Wikipedia.

Every Fermi (the Space Telescope) presentation I've seen uses Comic Sans

@KyleKanos Must be an inside joke.
Has to be.

I asked someone about it once, they said "Why not?"
I didn't want to tell them how bad it looks, so I moved on

It's not actually atrocious, it's just not that good and way overused.

6:30 PM
@dmckee It could be. It could be that they've got a template file & everyone utilizes it. But I'm curious as to the reference

@ACuriousMind No, but it doesn't help this time

Do you guys think I could ask my question above on PSE ? I fear it might be a bit too "homework-like"

@Hippalectryon I think it is homework-like and doesn't ask a conceptual question, so I'd advise against asking it. I know nothing about what you are doing there, though, so I don't know the answer, either.

@ACuriousMind and here i thought yours was a nice face-painting :D dont know why it is frightening

@Hippalectryon At a glance that just looks like the definition of curvature.

6:41 PM
@Gowtham It is just a nice face-painting. 'Twas for Halloween, though :D

@ACuriousMind was it supposed to be some-sort of cat ?

Haha...Kyle wasn't off the mark, it was supposed to be some kind of clown. There was a red nose, too.

@Hippalectryon If you look at Wikipedia, I think the equation there is the same (except the sign).

@alarge It indeed seems similar, but I don't see where the ^3 exponent comes from in the last term of the numerator (in the paper)

@Hippalectryon By multiplying the latter term (on Wikipedia) by (1+(dS/dr)^2) to match the denominators
That, for clarity, is the equation I'm talking about.

6:52 PM
Oh I was not looking at this one
@alarge Thanks a lot !

No problem. I'll leave it to you to derive the Young-Laplace equation (from what I remember the derivation for the general curved surface is painful; sphere is obvious though, so you might convince yourself using that as an example and just "generalize").

@ACuriousMind : what means the word "freak"?

@Sofia Someone who is weird or strange, someone who deviates from the norm. Often, but not always meant to be derogatory.

@ACuriousMind : Oooo! now I understand (I have no idea what is derogatory).

A degoratory word is one that is meant to be insulting in some way.

7:04 PM
superfreak :D

@EmilioPisanty Did I miss something while I was gone?
@KyleKanos I would deny them too if I left because I was (in my mind) being bullied etc.. There's just way too many coincidences for it to be somebody different
@dmckee I didn't understand being upset either, nor do I feel bad about it now. I'm just saying it seemed to be upsetting. No idea why
Okay. Now I think I'm caught up

7:19 PM
@tpg2114 I suppose I could understand the rationale there.

Which is why I refrained from bringing it up before, and I will not discuss details in public now
Oh man, so in extra fun bonus day -- not only was my debit card jacked up, but GA rejected my income tax return as one of the batch of returns from TurboTax that is fraudulent!

Ouch
My family was part of the ~1 million people whose SSNs were stolen a couple years ago from SC

8:11 PM
@ManishEarth Hey, there's a new user's answer that isn't an answer but really deserves to be made a comment as it's a referenced correction to the question. Flag for mod attention or do the usual not an answer thing anyway?

@Jim flag is fine
either, really
but if you want it to become a comment flagging is the only way

hmmm... question is 9 months old. Probably isn't worth the effort
On a completely unrelated note, I love the sound bug zappers make. That bzzt lets me know how much it's working. Now, I know it's not possible, but wouldn't it be great to have that kind of thing for protected questions? A little counter or something that indicated each time it prevented a new answer to an old question from a random google searcher. For instance, don't you wonder how many answers the protection on this question has prevented?
I'd just feel so soothed to hear that little bzzt everytime some young kid is prevented from posting his own theory of what gravity is. Alas, too bad it will never be possible

@ACuriousMind s'amazing! s'wonderful! s'neutrinos!

8:49 PM
@Jim use the standard "not an answer" flag for that sort of thing
@Hippalectryon I actually think that would have been a fine question. To my mind, "Where does [factor] in [formula] come from?" is unlikely to be the bad kind of homework-like, and is the sort of thing I see this site as being for.

@DavidZ Ok.
I'm having another problem now :c
According to the article we have the formula above for $H$ created by a uniformly magnetized spherical magnet

@Hippalectryon don't take my word for it though ;-) It seems like we might have to hash this out as a community. Anyway, you were saying...

I went to their reference (25), and here is the relevant page :
I hardly see how one goes from the book's formula to the paper's formula

I suppose the paper is talking about the field outside of the magnet, in which case you'd have to look up the formula for the magnetic field of a dipole of magnitude $\mathbf{m}$ and compare that to the paper's formula.
($\mathbf{m}$ is the magnetic dipole moment, right?)

Let me check... that's right
@DavidZ Like that, from Wikipedia ?

9:06 PM
Yeah, that'd be the one

@DavidZ That gives me $H=\dfrac{r(m\cdot r)M}{r^2}-\dfrac{M}3$, which doesn't look like the expression in the paper (I know that we're considering a centered sphere here while it's not centered in the article, but the formulas do not even seem similar)

@Hippalectryon Write r (and m) in cylindrical coordinates.

@DavidZ Something is wrong in my formula above, but you get the idea
@alarge Isn't $r$ already in cylindrical coordinates ? The wiki is unclear to me, are they using spherical coordinates ?

@Hippalectryon In cylindrical coords r is the projection to the plane. In the equation you posted above (wiki?), you're indeed dealing with spherical coords

In the wikipedia article, $\mathbf{r}$ is the position vector, which means that its magnitude $\lVert\mathbf{r}\rVert$ is a spherical coordinate

9:21 PM
So, e.g. $r^5 = (R^2 + z^2)^{5/2}$, where R is now the cylindrical radius

Yep

Is there a standard method of estimating the probability of a stochastic path? Suppose I have some stochastic process that I sample at regular intervals; What's the density kernel? I suppose I could use e.g. Parzen windows (or just discretize the state space) assuming that the process is Markovian, but that's not particularly elegant.

@alarge In the wiki's formula (image above), $m$ and $r$ are vector right ? If so, how do I turn $r$ in spherical coords to $r$ in cylindrical coords ? $\sqrt{r^2+z^2}$ is not a vector.

@Hippalectryon Yes, $\mathbf{r}$ and $\mathbf{m}$ are vectors. Do you remember how to change basis? Basically you want to figure out how to write out the spherical unit vector $\hat{\mathbf{r}}$ in terms of the cylindrical $\hat{\mathbf{R}}$ and $\hat{\mathbf{z}}$.

I can change from cartesian to spherical/cylindrical, but spherical to cylindrical is more troublesome

9:36 PM
It is. I think Wikipedia probably has the formulas somewhere, but it's useful enough that you should try to figure it out at least once if you have time.

Ok let me try :-)

Hey @DavidZ is there a way to cancel an edit to a post? I edited the title of the "Why do most formulas in physics have rational exponents?" but it was actually perfect the way it was

@alarge Isn't it just $\hat{r'}=\cos\theta\hat{z}+\sin\theta\hat{r}$ ? $(\hat{r},\hat{\theta},\hat{z})$ is cylindrical, $(\hat{r'},\hat{\theta'},\hat{\phi'})$ is spherical
@pentane Can't you re-edit it ?

@Hippalectryon That looks about right. Throw that into the equation and see if your final result agrees with the one from the book.

9:58 PM
@alarge I get $\hat{H}=\dfrac{R^3}{r^5}C\sin\theta\hat{r}+\dfrac{R^3}{r^5}C\cos\theta\hat{z}-\‌​dfrac{R^3}{3r^3}\hat{M},C=\hat{M}\cdot(\hat{r}\sin\theta+\hat{z}\cos\theta)$

Ok, let's see...

The system is axisymmetric, so I thought I could choose any $\theta$ I want... but we see above that it leads us, for $\theta=0$, to no $\hat{H}$ towards $\hat{r}$...

@pentane you can revert an edit, which gets recorded as a new edit that undoes the previous one (though there is no way to erase the evidence that the edit ever happened)

You have like $r = r'\cos\theta$, $z = r'\sin\theta$, though, right?
So, the first term, for example, should be $\frac{\mathbf{r}'(\mathbf{M}\cdot \mathbf{r}')}{r'^5} = \frac{(r\hat{\mathbf{r}} + z\hat{\mathbf{z}})(M\hat{\mathbf{z}}\cdot (r\hat{\mathbf{r}} + z\hat{\mathbf{z}}))}{(r^2+z^2)^{5/2}}$

@alarge Don't you mean $\theta'$ ?
I'll post the pics of the angles I've chosen
And for spherical :
Notice the swap between $\theta$ and $\theta'$ (..' are for spherical coordinates)

10:08 PM
You won't need the angles, I think.

Well I need $\theta'$ to express $r'$ with $r$ and $z$, don't I ?
That being said, I just noticed that above in my formula is should be $\theta'$ and not $\theta$ everywhere

Well, in principle, but you can see that because $r$ and $z$ (in cylindrical) are orthogonal, things will drop out

Oh I see what you did above, that's clever indeed. Let me correct my formula.

So if you multiply what I have above by $R^3$ (the radius of the actual sphere), you'll get the $\hat{\mathbf{r}}$ component (the other term is in z-direction, so there's no contribution)

@DavidZ : Why some questions are restricted only to the pleasure of people who have > 10K? I saw a question to which I am forbidden to answer because I have less, and though, I have an answer.

10:11 PM
And that r-component agrees what with you have in the book.

@Sofia because those questions are deleted. You (or anyone else) shouldn't be answering them. (and I'm not sure what you think pleasure has to do with it)

@alarge How can you replace $\hat{m}$ by $M\hat{z}$ ? How do we know it has this direction ?

@DavidZ : I see sometimes questions to which I would have simply pleasure to answer. But I don't have 10K, and I won't have soon.

@Hippalectryon By definition, see your figures. Anyway, we have to pick some direction for the magnetic moment, and we might as well align our coordinates with it (as we don't expect it to be moving).
And m should really be replaced by 4/3 pi R^3 M z, right.

@DavidZ sometimes there are questions for which the answer is ""written on the sky".

10:16 PM
@Sofia yes, well, it's the nature of the community here that you don't get to simply answer whatever you want. 99% of the time, sure, you can, but there are some cases where people are prohibited from answering questions; always for a particular reason.

@alarge Yep, that's what I'm doing :)
@alarge Ok I'm still missing one last step. I got $\hat{H}=\dfrac{rzR^3M}{(r^2+z^2)^{5/2}}\hat{r}+\left(\dfrac{z^2R^3M}{(r^2+z^2)^‌​{5/2}}-\dfrac{R^3M}{3r^3}\right)\hat{z}$
The first factor (before $\hat{r}$) is good
But the coefficients of the last one do not match the paper's formula

@tpg2114 you win this round

The paper's formula would be $$\hat{H}=\dfrac{rzR^3M}{(r^2+z^2)^{5/2}}\hat{r}+\dfrac{1}3\left(\dfrac{2z^2R^3M‌​}{(r^2+z^2)^‌​{5/2}}-\dfrac{R^3M}{(r^2+z^2)^‌​{5/2}}\right)\hat{z}$$

@DavidZ : why is the Copenhagen interpretation the most accepted? And I, that work exactly in the domain of fundaments of QM for long, long years, and I have a chance to answer well, because I know the other interpretations - I can't answer. I don't want to argue with you, of course not. But it's very bad. At least I should be able to see such questions.

@Jim Heh, for which one?

10:24 PM
@Sofia no idea

@tpg2114 the moon is the sun at night

Yeah... forget photons and peer review. It's really Black Sabbath FTW
I do like how we got 3 almost identical answers within 30 seconds

me too. Great minds do think alike

@Sofia I believe it is more fair to say that "Shut up and calculate" is the most accepted interpretation. Most physicists I know emit a pained groan as soon as someone wants to talk to them about "interpretations". Also, >10k people can only see these questions - they cannot answer them, either.
2

@Hippalectryon You have a mistake in your denominator in the formula you derived. Fixing that and making the denominator the same, you get... almost the same formula as in the book.

10:27 PM
Plus some comedy in the meantime

@ACuriousMind yes, a good point, even 10k users cannot answer deleted questions

@alarge I don't see where the mistake comes from; the last term comes from $\hat{m}/(4\pi r^3)=\dfrac{4\pi R^3/3M\hat{z}}{4\pi r^3}$, doesn't it ?

You should have r'^3, not r^3. So that translates as (r^2 + z^2)^(3/2)

Oh I forgot that indeed :D

Other than that, I don't see any mistakes in the derivation, so it is possible that the book has a typo and is missing a "3".
Or it could be that I missed something.

10:30 PM
Well if it were a typo, there wouldn't be an additional "2" either I guess
But I have no idea where it comes from

Well the 2 is correct

When does it appear ?

If you normalize the denominators
i.e. multiply your latter formula by (r^2+z^2) / (r^2+z^2)

But wouldn't that give us $bladiba^{3/2}$ on the denominator ? They have $bladibla^{5/2}$ too. Or am I missing something ?

Umm... That would give you bladibla^5/2 throughout.

10:33 PM
I don't get what multiplying by $(r^2+z^2) / (r^2+z^2)$ does then

$\frac{MR^3\hat{z}}{3(r^2+z^2)^{3/2}} = \frac{MR^3\hat{z}(r^2+z^2)}{3(r^2+z^2)^{5/2}}$
Multiply that out, add the other term, boom, stuff cancel out.

Oh I had made a mistake on my sheet of paper
Ok thanks a lot for the help. I just have one last general question on notations.
Where I come from (France), I have only seen the $\vec{e_r}$ etc .. notation for vectors. What's the meaning of the notation $\hat{h},H^*$ ? Do we use them in different cases ? The paper seems to use $^*$ all the time for non units vectors.

Hat usually denotes unit vectors.
I've got no idea why your book uses * all the time, but it doesn't seem to mean vecotr, but rather some other coordinate system (maybe?)

And they also write $r^*$ for $r$, so yeah indeed it's not only for vectors
Like in

@jim You keep getting into philosophical discussions on that question

10:40 PM
Yeah, so I think the * probably refer to some coordinate transform. As in, the non* ones for h might have a different definition for 0, say, so h* = c - h (for example).

Ok, thanks a lot for helping me :-)

No problem, that's that SE is for.

copenhagen interpretation: nothing to see here folks, move along!

325

This just came up again: homework is not acceptable if the asker has made no effort whatsoever I'm developing a bit of an involuntary twitch that manifests itself when I encounter the word "effort" on Stack Overflow these days. Don't get me wrong — effort is good. At least, productive eff...

interesting...

11:40 PM
@DavidZ That is an interesting thread, and certainly a long one. It's always interesting to see how other sites deal with the same kinds of things that crop up here -- our problems really are not all that unique.