12:20 AM
I would appreciate if someone is able to help me by answering this question
0

I was reading this article called "Joint Axis and Position Estimation from Inertial Measurement Data by Exploiting Kinematic Constraints", but I'm not understanding one part, which doesn't let me proceed in the reading of the same. From the physical point of view, we have the following situation...

5 hours later…
user228700
4:52 AM
@JohnR: Morning! :-) Here's my to-read list for the summer:

user228700

user228700
It's embarrassingly short, yes, but I don't want to overwork myself. Besides, I think it would be better for me to spend longer with good books than race through easy reads.

@Kaumudi.H I haven't read any of those.

user228700
Huh? Not even The Giver?

Who wrote The Giver?

user228700
4:56 AM
Lois Lowry

user228700
The Giver Quartet is a series of four young adult novels by Lois Lowry. The quartet consists of The Giver (1993), Gathering Blue (2000), Messenger (2004) and Son (2012). The first book won the 1994 Newbery Medal and has sold more than 10 million copies. The story takes place in the world of the Giver. Each book has a different protagonist, but is set in the same futuristic era. == Plot overview == === The Giver === The Giver is a 1993 American children's novel (generally Young Adult or older) by Lois Lowry. It is set in a society which is at first presented as a utopian society but gra...

By 1993 I was already too old for young adult books :-(

user228700
EEK, forgot to add All Systems Red. Just did. Anyhoo, what is that book series you'd recommended to me about a person living the same life over and over?

user228700
@JohnRennie :-/ Hmm, OK...

user228700
Mar 5 at 11:03, by John Rennie
There is a really good book you should read by Kate Atkinson and called Life After Life

user228700
5:02 AM
AH, there it is!

user228700
Wokay, added three more books to the list. That's it, no more!

All Systems Red is a novella really, and it's a very quick and easy read.

user228700
@JohnRennie I see. The Velveteen Rabbit is a children's book :-P An easy 15-minute read.

I wouldn't dismiss the quick and easy reads. Sometimes it's nice to have a book you can just pick up and sprint through without having to work at it.

user228700
Yes, of course. I'm not doing any dismissing :-) I said that I think it would be better for me to sit with some of the tougher reads this summer.

5:06 AM
If you like bittersweet endings then The Death House by Sarah Pinborough is an excellent read. Although I don't think it's specifically described as young adult it is really.

user228700
Well, tough relative only to the quicker reads.

user228700
@JohnRennie I don't restrict myself to the YA genre, you know...

I think Life After Life is a very, very good book though it can be hard going at times. It is most definitely not young adult!

user228700
But I will be sure to check it out, thank you :-)

½ a day to go now? :-)

user228700
5:09 AM
@JohnRennie I see. I've added it to the list so I will let you know how it goes! I might not find it at the library so I might just actually buy it. After having read some on a screen, I'm finding that I'd much rather read the physical copies; I find it hard to finish softcopies...

user228700
@JohnRennie Yeah! :-)

My niece prefers real books as well.

user228700
I see :-)

I'm genuinely not fussed. I do still read physical books, usually when someone has given me a book, but I'm just as happy reading on my tablet.

user228700
Like I said, I find it more difficult to engage with the text on screen as actively as I would on paper, which subsequently at least partially results in this inability to actually finish the book.

user228700
5:15 AM
I read both Fangirl and Eleanor & Park on screen.

user228700
The latter was a delightful book, which is why it wasn't a pain to read it on my laptop but the former, well, that book sort of sucked, which added to the misery of having to read it on my (now dead) tablet.

user228700
Anyhoo :-) I'd better get back to revision. I'll see you tomorrow, then!

See you later.
@BernardoMeurer this is pretty much how Windows does it if you're writing Windows apps in C/C++. The downside is the overhead of processing definitions you don't need for every file, but with modern compilers that overhead is pretty small.

5:54 AM
Hello all! Does anyone have any experience in PPN formalism for the field equations of general relativity? If so, would you mind checking out my recent question. There is a cold hard +50 bounty awaiting whomever puts forth a comprehensible derivation.

1 hour later…
6:56 AM
hey hey

@BernardoMeurer In my experience, that is the transpose of how code should be organized.

Just put all your code in a gigantic single C file

7:14 AM
In differential geometry and theoretical physics, the classification of electromagnetic fields is a pointwise classification of bivectors at each point of a Lorentzian manifold. It is used in the study of solutions of Maxwell's equations and has applications in Einstein's theory of relativity. == The classification theorem == The electromagnetic field at a point p (i.e. an event) of a Lorentzian spacetime is represented by a real bivector F = Fab defined over the tangent space at p. The tangent space at p is isometric as a real inner product space to E1,3. That is, it has the same notion of vector...
Why have I never heard of this

7:47 AM
Sulla teoria e sulla classificazione delle omografie in uno spazio lineare ad uno numero qualunque di dimensioni
Good thing Italian just sounds like bad French

Let philosophy hour begin

If one subscribes to "existentialism", does this mean "morality" is now a choice?

I'm a physicist tho
I want the opinion of physicists
they're more logical

8:01 AM
@Slereah At least italian was not ruined by the influence of anglo-saxon languages (at least up until the late twentieth century).

please, it's the other way around
French corrupted English
Have you ever read pre-french corruption english

they clearly corrupted each other

Yore folk, such as the British builders of Stonehenge, mealfully wondered about the heavens. The first folkdoms, such as those of Sumer, Midrike, Egypt and the Mayas, sometimes built big losseners, where they would keep watch on the heavens.
These folkhoods knew much about the shrithing and setting of the stars and wanderers, and often drew bilds of them. The Olden Greeks cleaved the night sky into starbilds, still ongot by roomlorers and starcrafters to this day. However, they were also unaware of the true workings of the heavens; Aristotle believed the Sun whirled about the Earth

ye aulde english?

The coming of Einstein was one of the biggest leaps in tunglecraft. His Beholding of onlay showed that sweerdom is a forthmake of the bendings of roomtide made by bulk, and also showed how bulk and dodrive were the same. It also unraveled the wonder of the speed of light; showing how as things shrithed almost at that speed, it would both slow its tide and gain bulk. Edwin Hubble later found out that stars were not huddled in one cluster, but set out in shedded clusters known as starswirls.
Good old Einstein and his Tunglecraft

8:06 AM
Hie dygel lond
warigeað, wulfhleoþu, windige næssas,

under næssa genipu niþer gewiteð,
flod under foldan. Nis þæt feor heonon
milgemearces þæt se mere standeð;
ofer þæm hongiað hrinde bearwas,
wudu wyrtum fæst wæter oferhelmað.

þær mæg nihta gehwæm niðwundor seon,
fyr on flode. No þæs frod leofað
gumena bearna, þæt þone grund wite;
ðeah þe hæðstapa hundum geswenced,
heorot hornum trum, holtwudu sece,

feorran geflymed, ær he feorh seleð,

Beowulf?

yep

Old french is fairly unreadable too although a bit less than old english

Mama mia

8:08 AM
that is even wrongly spelled

That is a spicy meatball
Que maint conteres vos aconte,
Conment Paris ravi Helayne,
Les maux qu'il en ot et la paine,
De Tristram qui La Chievre fist,
Qui assez belement en dist
Et fables et chançons de geste,
Romanz de lui et de sa geste,
Maint autre conte par la terre.
Mes onques n'oïstes la guerre,
Qui mout fu dure de grant fin,
Entre Renart et Ysengrin,
Qui mout dura et mout fu dure.
ye olde french

This I can understand pretty well (compared to old english)

yeah it's mostly a few words and verbs that are weird

Even middle English is hard. Try reading Chaucer.

Is that the Canterbury Tales guy

8:12 AM
@JohnRennie can you read old english?

It is at least obviously English, unlike Old English, but it's still hard to understand.
@yuggib No.
@Slereah yes

@JohnRennie Is it studied in schools anymore (or ever)?

I think english books start at Shakespeare in schools

@yuggib it was in 1977 when I did O Level English :-)

@JohnRennie old english I was meaning

8:14 AM
I mostly remember what we read in school
They loved making us read Zola and Hugo
the most boring books
Never make school children read authors that were published in newspapers
ie paid per page

@yuggib I didn't do it in school (yeah, started at Shakespeare), but they might have looked at it in A-level

@yuggib no, or at least not at O Level (age 16). You might study Old English if you do A Level English (age 18) and I'm sure you would if you do an English degree.

Beware, Chaucer is nothing but FILTH

We'd still be speaking Old English if it wasn't for the bloody Normans :-)

Normans get out

8:17 AM
@JohnRennie I see. We still study latin during all high school

Mamus mius!

@yuggib We did two years of Latin but gave it up unless pupils studiedfor the O levels.
It was actually quite fun to do a bit of Latin, though I'm not sure it's been much use to me over the years.

@yuggib I think the school I went to just started latin classes up a few years ago - not sure if it was a success or not

Like Ubu Roi
Which I'm sure was hilarious when it came out 120 years ago

8:20 AM
We cannot choose...it's five years of latin, five hours per week (compared to two/three of mathematics and two of physics (only in the last three years)) ;-P

OK, that's a bit much :/

And that is the "scientific" program; I'll let you imagine the "classical studies" program

@Mithrandir24601 no-one reads the Principia unless they're studying the history of science or they're a masochist :-)

@JohnRennie which Principia ? the interesting or the boring ones?

@yuggib :o Oh dear. Oh dear

8:22 AM
The Principia Discordia
Not all of it, but I redid by hand the first 14 chapters or so

^ those are the interesting ones ;-P

I was talking about Newton's Principia Mathematica, but whatever works :P

@Mithrandir24601 to be fair, russell's ones are not in latin

Where's the fun in that?

There's a 2008 math paper in latin

8:25 AM
I think that, apart from the pope and very other few exceptions, nobody in the twentieth century wrote in latin

2006, actually
Well the Vatican has its own science paper

@Slereah latin is still official language of the vatican iirc

it is
The modern publications are in english

1 hour later…
9:33 AM
Oh god, Lagrangians require jet bundles
Kill me
"Let $Z$ be a manifold"

@JohnRennie Halp

@BernardoMeurer morning. What's up?

@JohnRennie Good morning John :)
So, nice advancements to my final project, I have my parsers almost done :^)
I have decided to store the CSV entries in a doubly linked list
And I've written all the functions for it, push, reverse, remove, print, etc
@JohnRennie Can you test-run it?

9:49 AM
OK ... ?

I'm afraid one of the parsers isn't 100%

@BernardoMeurer yes of course. Is it on github?

That link gives me a 404 error

Try this one, it's project02
(Make sure you're logged in)

9:55 AM
Ah it was failing because I wasn't logged in. Give me a mo while I remember the syntax for git clone ...
@BernardoMeurer: huh, why doesn't git clone git://github.com/bemeurer/prog.git prog work?
Oh, because it's a private repository. Hang on, I'll clone using the key pair.

Hm
Over here I just do git clone URL and it asks me for username and password

Ah, OK, that's working.

"A system of $k$-order partial differential equations on a fiber bundle $Y\to X$ is defined as a closed subbundle $\mathfrak E$ of a jet bundle $J^k Y \to X$."
Help

Oh, dangit, it may not work for you because of strsep()

@BernardoMeurer: I can't build it I'm afraid because it uses SDL which isn't available in Windows

10:01 AM
@JohnRennie Ah, I'm not using any of those, just take them off
They're just there 'cause I'll need them later

utils.c(52): error C2059: syntax error: '}'

wat
Ah
Are you using C99?

MSVC 2015

ID newID = {};
It's saying that this initializer for structs is illegal

parser.obj : error LNK2019: unresolved external symbol _strsep referenced in fun
ction _parseStations
utils.obj : error LNK2019: unresolved external symbol _strptime referenced in fu
nction _mkDatetime
utils.obj : error LNK2019: unresolved external symbol _asprintf referenced in fu
nction _IDToStr
main.exe : fatal error LNK1120: 3 unresolved externals

10:04 AM
Dangit
It's all POSIX stuff
:P
You should get cygwin John
It's black magic, but it works
Anywho
The problem is at parseTrips over in parser.c
It should work just as well as parseStations but it seems to not include much of the file
Ah
I think it was an issue in my IDE's terminal :^)

@0celouvskyopoulo7 So it turns out the presentation I did on differential geometry in school, jet bundles were indeed necessary to prove that geodesics are the shortest curves
$$\delta L = \sum_{0 \leq | \Lambda |} (-1)^{|\Lambda|} d_\Lambda (\partial_i^\Lambda \mathcal L) \theta^i \wedge \omega$$

10:19 AM
@JohnRennie Can I have a struct in foo.h with a function pointer to a function in foo.c? I would think no

Yes, I don't see why not. You just need to know the function type when writing the struct declaration. You can't initialise the pointer in the header file.

Ah, right
Hm
I guess my function newStationNode in utils will be the initializer :)
To work!

A function pointer is just a variable. You need to set it yo the function, but of course you can't do that at compile time because the address of the function isn't known.
Hi @nbro. Did you see my comment about proper acceleration?

@JohnRennie Hi, I guess I missed it...

yesterday, by John Rennie
@nbro: GR gives us an expression for the four-acceleration, and this includes both terms due to non-inertial motion and due to spacetime curvature: $$a^{\mu}= \frac{du^{\mu}}{d\tau}+\Gamma^{\mu}_{\alpha \beta}u^{\alpha}u^{\beta}$$

11:06 AM
@dmckee Is it okay if each element of my linked list has a pointer to head and tail?
This way I can avoid O(n) brouhaha whenever I want to do something in reverse

Guys, say we have a uniformly charged sphere. I see it makes sense to assume the charge density is finite. However, what would go wrong if we assume a finite charge density in the interior of the sphere, and a charge density described by the Dirac delta function at the surface? To me it seems that we would still have a finite charge, so I'm guessing the distribution wouldn't be uniform. How to make this a bit more rigourous, tho?

@ShaVuklia I'm not sure I see the problem. You can have any charge density function you want as long as it integrates to a finite value.

@JohnRennie Well, my question is if we still have a uniform charge distribution, if we choose our charge density to be finite in the interior, and described by the Dirac delta function on the surface. Because as far as I know, in the case of uniform charge distribution in a solid sphere, we have zero charge on the surface, because our charge density if finite. However, if we described the charge density by the Dirac delta function, we would not havr zero charge on the surface.

@ShaVuklia No, that's not uniform (and that charge density does not really make sense- you have a charged sphere but a part of the charge is confined exactly to the surface?

If you have a uniform charge distribution then $\rho(r)$ would be a step function and it's value isn't defined at $r = R$ (R = radius of sphere).
But that doesn't matter because the edge of the sphere has zero volume so the charge contained within it is necessarily zero.

11:20 AM
Yea I see it now
I donno why I was confused

Why does LaTeX first load as really small text, then as bigger text?

If you just decrease your volume, the charge will decrease, so it can never be non-zero on the surface
(volume of a Gaussian surface that encloses part of the sphere surface)

@JohnRennie These doubly linked lists with head and tail pointers are mushing my brain

Doubly linked lists shouldn't present any problems.

@JohnRennie It's just that I'm trying to embed the head and tail pointers into the list
typedef struct tripnode {
trip data;

struct tripnode *prev;
struct tripnode *next;

struct tripnode *tail;

void (*headInsert)(struct tripnode *self, trip data);
} trip_n;
And it's a bit confusing at first

11:35 AM
The prev pointer is null and the head and the next pointer is null at the tail.

You store the head and tail pointers in every list element?

Uhum
It makes things easier after you've implemented the functions

Well it means if the tail changes because a new element is added you have to go all the way through the list and update every element with the new tail pointer.

Because then I can do all my operations on the whole list just passing any element of the list :^)
Shit
I had not thought of that
Goddamit

11:37 AM
I must admit it isn't obvious to me why a linked list is the best solution in this case. I'd have used an array to store the data.

Because I want to get fancy

If necessary use a second array of the same dimension as an index for things like sorting.

i.e. harvest points by using what we learned in class
Goddamn
I thought I was super smart
With that those head and tail pointers
nope, lel
THEY CAN BE A DOUBLE POINTER
AHA!
Bingo
AH!
Can't I just have two bools
Ah, that doesn't help me
Dammit
Balls

The head is uniquely identified by prev being null. Likewise the tail by next = null. Why would you need extra bools?

Yeah, exactly, why I said it didn't help me :P
I can make the head and tail things be double pointers to an external variable
This way I can still call operations by just passing any node

11:44 AM
I've never seen that done in an implementation of a linked list.

Do you understand why I want it though?

The head is normally stored ina variable because it never changes, and the tail is found by navigating the list.

Yeah, I will store it in a variable, but all the elements will have a pointer to that variable
Well, the variable is a pointer to head and everyone points to that pointer
POINTERS WOOHOO

Presumably you want a quick way to find the head or tail when given any member of the list?

Yes, as well as being able to pass any node to it's function pointers and have them work
i.e. inserting at head passing a middle node

11:47 AM
I guess storing a pointer to a pointer to the head/tail in every list element is a way to do this, though obviously at the expense of extra storage space.

Meh, a pointer is what, 8 bytes?
Good lord, this is going to be a mess :D

Are you allowing adding at the head to you can build a sorted list?
i.e. you sort when adding the new element

my CSV's are already sorted though, so as long as I invert the list after reading it's sorted

You normally wouldn't add at the head, though if you're sorting as you build the list I can see why you would want to.

Maybe I should just read the file backwards le

11:53 AM
That is a bit bizzare

Yes
Which is why now that I have a tail pointer I can just add at the tail

Most of us would just keep the address of the current tail in a separate variable.

Yep
@JohnRennie How do people do those C functions with variable arguments? Like, say, printf can take any number of arguments depending on the string

@BernardoMeurer In general it's a good idea to code in the obvious way because other people reading your code will expect you to code that way. If you work on a collaborative project not using the obvious solution will not make friends and influence people.

So, not make everyone point to head and tail?

11:57 AM
@JohnRennie I must be honest I'm not familiar with general relativity...at the same, I'm wondering why some people define proper acceleration as the acceleration measured from an inertial frame of reference... Is this last version a special relativity version?

@nbro Proper acceleration is not the acceleration measured from an inertial frame of reference, it is the acceleration measured in the rest frame of the observer.
The rest frame of the observer isn't an inertial frame if the proper acceletration is non-zero.

Guys, do we know what the electric field is on the surface of a charged hollow sphere? I know that we have a discontinuous electric field (the difference between the upper and lower perpendicular electric components is $1/\epsilon_0\sigma$, if you are close enough to the surface), however, what about at the point of discontinuity? Is it "infinite" there?

The discontinuity doesn't exist in reality - it's an idealisation. In real life charge distributions always change smoothly. So handling charge localised in a 2D surface is always an unphysical approximation.

Gauss theorem

I understand it's not realistic, but I was wondering what the theoretical/ideal value then would have been

12:07 PM
@ShaVuklia It's best to think about it as "undefined". I can't imagine an actual situation where that value would matter - if you are close enough to the sphere that it matters what the value there is, it's likely that the approximation of the sphere as being infinitely thin breaks down anyway.

Ok, sounds good @ACurious

The theoretical value of the field at the discontinuity is gonna be the value outside

there are two values outside, so I'd go with undefined too

and then drop to $0$
you can define it as a distribution

@JohnRennie Well, maybe I'm misinterpreting this Wiki article en.wikipedia.org/wiki/Proper_acceleration, but it says "It is thus acceleration relative to a free-fall, or inertial, observer who is momentarily at rest relative to the object being measured.", even though here it also says that the observer is at rest relative to the object being measured, which I suppose is equivalent to your statements, but, at the same time, the sentence also says that the observer is inertial...

12:17 PM
Yo dawgs

Another point from en.wikipedia.org/wiki/Inertial_frame_of_reference: "an accelerometer moving with any of them (i.e. inertial frame of reference) would detect zero acceleration.", where here acceleration is supposed to be proper acceleration, since an accelerometer measures theoretically proper acceleration

If i accelerate beyond the speed of light, do I go back in time?

@Slereah you do not need fucking jet bundles
and geodesics are not the shortest curves; the shortest curves are geodesics

Well no, not for a general connection

who cares about a general connection?
send me this thing pelase

12:25 PM
Ehresmann

Fucking physicists
You know they are the worst
They either do it completely wrong
Or with so much overkill it't stupid

^ you can't live with them or without them :P

how would u solve a differential equation without the jet bundle, mister funny man

@Slereah I do it all the time
just use that

Local mom solves PDEs without the jet bundle
Geometers hate her!

12:30 PM
Seriously though, the jet bundle is not necessary.

Yeah I am wondering what it's for

On what page is it claimed they are necessary for whatever

Well they define the Lagrangian via bundle wizardry
And in my proof of geodesics during that presentation I did use the length function
which is roughly a lagrangian

Serious question
Have you actually read O'Neill's material on geodesics

I think so?

12:34 PM
You do NOT need the jet bundle, you do it via an old-fashioned variational technique, or the Gauss lemma or whatever

I don't really read O'neill in order so it's hard to say
I know
I am just sayin'
you could
And that professor back then mentionned it
that the method I used was to be done properly via jet bundles

"properly"
That's like saying the correct definition of group is a "groupoid with one object"
It's an overcomplication

well, is it not

@Slereah Yeah and Picardy-Lindyhop is really just elliptic regularity theory
Don't bother with ""proper"" proofs

what would u say the proper definition of the Lagrangian on a manifold is

12:38 PM
What Lagrangian?
He's not making sense

The lagrangian in general

I don't know about Lagrangians in general. In Arnold there are no jet bundles. Some people have jet bundles. For what reason, I cannot say.

I am unsure
Maybe it's easier for like gauge theory

I flip a coin 99 times and it is heads
what is the chance of heads comnig up on the next flip?

1/100

12:48 PM
realli?

1/2?

100%

the book says there's two people
Dr. John the man of science, he says 50%

doesn't it depend on how you ask the question?

and Fat Tony, he says it is a loaded coin

12:50 PM
or maybe the coin has heads on both sides

And @0celouvskyopoulo7 is in between
yes good point @ShaVuklia
now ur thinking like fat tony

Fat Tony sounds like a sicilian
I trust him with a bet

I wouldn't trust if he was flipping the coin tho

@0celouvskyopoulo7 It's the name of a mafia boss in The Simpsons...

12:58 PM
@ACuriousMind Why would I know that?
And how come you respond to that and not Sam's ramblings

Hi @JohnDuffield

I'm sure most Americans know more characters from the Simpsons than from the Bible
Homer Simpson is of course the equivalent of biblical Esau
Selling his birthright for a bowl of lentils

1:34 PM
lentils are good
except if they are cursed

who cursed them lentils

they contain potassium benzoate

yes
you may go now

[random question] We know that string theory often give the impression of a physical theory that look so much like it should be in mathematics, but are there examples where the opposite happens?

1:40 PM
where should I go?
Lentils are good

@JohnRennie Still around?

oh simpsons. I never watched/liked that

1:55 PM
Reposting this question here with the hope somebody qualified can help me.
0

I was reading this article called "Joint Axis and Position Estimation from Inertial Measurement Data by Exploiting Kinematic Constraints", but I'm not understanding one part, which doesn't let me proceed in the reading of the same. From the physical point of view, we have the following situation...

@dmckee @JohnRennie Someone save me, I have angered the pointer gods