The h Bar

John Rennie

17:01

@0celo7 that's a really interesting question. There must be a simple explanation.

You realise I'm going to lie awake worrying about it now ...

0celo7

Good.

John Rennie

Sadist!

0celo7

I didn't sleep for weeks because of a damn normal vector.

TheGhostOfPerdition

thanks to @JohnRennie and @dmckee for giving me good advices and suggestions :)

0celo7

@JohnRennie It's a sign that I like you.

Hari Prasad

17:03

@TheGhostOfPerdition they do have really good experience with physics

John Rennie

:: John looks around nervously ::

3

TheGhostOfPerdition

@HariPrasad experience in the sense I finished whatever my colleges in the past had to offer me.. you know the usual ones

David Z

OK, well, chat session time is over, and I should get off to sleep. See everyone again in two weeks for the next one!

0celo7

@JohnRennie I read 50 Shades, I know what to do.

vzn

@0celo7 lol see the movie too then :P

TheGhostOfPerdition

17:05

@HariPrasad are u planning to pursue physics in ur degree?

Hari Prasad

@TheGhostOfPerdition yes of course.

TheGhostOfPerdition

@HariPrasad where ? IIT?

John Rennie

@HariPrasad if you find out why the value of $k$ in the FLRW metric is constant come back and tell us

0celo7

Constant in time!!!

Constant in space is not hard

If you know a little geometry

John Rennie

Well it would have to be constant in space wouldn't it - the FLRW metric assumes homogeneity

0celo7

17:08

Yes...but you have to prove that.

Hari Prasad

@TheGhostOfPerdition where did you do your UG?

John Rennie

Why would I have to prove that if I assume the metric is homogeneous it turns out to be homogeneous?

TheGhostOfPerdition

@HariPrasad PESIT.. you woudnt probably know.. I had a seat in NIT surathkal But I didnt want to quit my music class so I stayed

0celo7

@JohnRennie The proper statement is that FLRW spacetime is a foliation and the leaves are Riemannian space forms.

@JohnRennie because our definitions of homogeneity are probably different

Hari Prasad

@JohnRennie Well a lecture note by Sean Carroll on GR says that "k is some constant"

ACuriousMind

17:11

@0celo7 Do you realize that you are doing exactly what I am doing when I say things like "no, the wave function doesn't fall off toward infinity"?

John Rennie

@HariPrasad yes, and any introductory GR text will say the same. The question is why can't k be a function of time?

0celo7

@JohnRennie a space is homogenous if the isometry group acts transitively

John Rennie

@0celo7 I love it when you talk dirty

2

@HariPrasad there's probably a simple explanation, and I'll probably think "oh that's obvious" when I see it. But for now I can't see the reason.

@0celo7 ... in comoving coordinates. That wouldn't be true in other coordinate systems.

TheGhostOfPerdition

Happy Womens day btw.. to @0celo7 ?

0celo7

@ACuriousMind huh?

@TheGhostOfPerdition what?

@JohnRennie huh?

ACuriousMind

17:13

@0celo7 You're demanding that people who neither know nor care for mathematical rigor conform to it.

John Rennie

In comoving coordinates the FLRW geometry foliates nicely into spacelike slices with constant comoving time

Hari Prasad

Deriving Friedmann Robertson Walker metric and Hubble's law from gravitational collapse formalism

►

@JohnRennie check this

John Rennie

This looks a fun question:

1

Q: Effect of relativity on Newton's second law

Let's say I have some object, like a long pole, that is 30 km long with mass $m$. One end is resting on the Earth's surface and it is sticking straight up, perpendicular to the surface. According to the gravitational time dilation equations, a clock at the far end should gain about 49 microsecon...

0celo7

@ACuriousMind I don't know what you're talking about

ACuriousMind

6 mins ago, by 0celo7

@JohnRennie The proper statement is that FLRW spacetime is a foliation and the leaves are Riemannian space forms.

I guarantee you many people who work with GR would not agree that that is the "proper" statement

John Rennie

17:17

I risk exposing how little I really know now, but isn't my statement:

In comoving coordinates the FLRW geometry foliates nicely into spacelike slices with constant comoving time

saying this?

ACuriousMind

Honestly, I have no idea because I don't know what a "Riemannian space form" is :D

John Rennie

Space form

In mathematics, a space form is a complete Riemannian manifold M of constant sectional curvature K. The three obvious examples are Euclidean n-space, the n-dimensional sphere, and hyperbolic space, although a space form need not be simply connected. == Reduction to generalized crystallography == The Killing–Hopf theorem of Riemannian geometry states that the universal cover of an n-dimensional space form with curvature is isometric to , hyperbolic space, with curvature is isometric to , Euclidean n-space, and with curvature is isometric to , the n-dimensional sphere of points distance 1 from...

0celo7

@ACuriousMind you were serious when you said you don't know Riemannian geometry...

John Rennie

So presumably the submanifold with constant proper time is a space form.

ACuriousMind

@0celo7 And it only took you about a year to catch on to that!

0celo7

17:19

Sorry guys, typing here and following my analysis lecture at the same time isn't working.

@ACuriousMind next you'll tell me you don't know PDEs

@JohnRennie yes. And I prove that in a PSE post.

Actually you need a theorem in Jost to complete the proof.

But it's mostly there

@ACuriousMind I'm pretty sure I'm just a mathematician.

@JohnRennie so the question is why the type of space form we have is constant in time

John Rennie

Well, why it's curvature is constant in time ...

DanielSank

@JohnRennie Awww, that's not a nice outlook.

I also think it's not true.

0celo7

@JohnRennie proof?

John Rennie

@DanielSank I didn't mean it to sound depressing. It's like being a senior academic - you achieve stuff by managing highly motivated young scientists willing to work stupid hours.

0celo7

Ok the space forms are completely determined by curvature in 3 dimensions

That's a theorem in do Carmo

DanielSank

17:25

@ACuriousMind Google is atypical, but even so I'm not sure @JohnRennie's statement is correct, even as a broad generalization.

John Rennie

@0celo7 I knew that :-)

DanielSank

@JohnRennie Ah yes, in that sense.

The thing about the glorified lab tech. though... I dunno about that.

John Rennie

I think as you get older your horizons broaden and you want to achieve more than you ever could on your own. Heading a team allows you to do that.

DanielSank

@0celo7 Maybe you need more RAM.

@JohnRennie Well yeah, my manager is such a person. Intestingly, he still does some of the heavy lifting in the group.

I think you get out of your career what you put in.

John Rennie

@DanielSank My experience was in the 80s and 90s so it may well no longer be relevant.

dmckee

17:28

@DanielSank It takes a strong cultural commitment on the part of the company to allow that. Dad worked as SwRI, which has such a commitment.

John Rennie

But back then there were skilled and ambitious young scientists queueing up to work

dmckee

Other wise the powerpoint load and meetings with clients suck down every available hour.

John Rennie

Unilever would take them on with the intention that they'd become team leaders or even more senior managers

After a few years if you hadn't made the grade, now you are older and less motivated and there are fresh faced youngsters (who are paid less) available.

DanielSank

@JohnRennie Yeah, that's rough.

John Rennie

Some of the older guys became specialists (I did - sort of) but others didn't

0celo7

17:32

@DanielSank perhaps

DanielSank

@0celo7 Maybe you should stop using snapchat.

John Rennie

But this is exactly the same in academia. We've all met people on their tenth 2 year postdoc.

0celo7

@DanielSank :o

DanielSank

@JohnRennie Exactly.

0celo7

@DanielSank do you have a snapchat

DanielSank

17:33

@0celo7 I don't even know what that means.

vzn

@DanielSank are you talking about Martinis or someone else...?

0celo7

@DanielSank do you use snapchat on your phone?

DanielSank

@vzn Yeah. He manages a pretty big group, but he also does a considerable amount of research.

@0celo7 No.

@0celo7 I didn't even have a smart phone until four days ago.

0celo7

@DanielSank how do you send random pictures to your friends?

DanielSank

@0celo7 See previous comment.

0celo7

17:35

@DanielSank oh, now get snapchat

John Rennie

Bite my balls, a Synology has just failed at one of our big customers - this could be messy :-(

DanielSank

@0celo7 No, it sounds stupid.

dmckee

@0celo7 Rarely and by text.

DanielSank

@JohnRennie They have off site backup, right?

Right?!?!

0celo7

@DanielSank take that back

vzn

17:36

@DanielSank re ACMs earlier query, is there an age spread among workers in your lab? or mostly younger?

John Rennie

Yes, there's no danger of data loss.

0celo7

@dmckee but what if you don't want the picture to last forever

DanielSank

@0celo7 What? No, that's an observation. I can't change it. You should never rewrite observations in your lab notebook. It's dishonest.

@JohnRennie ::breathes easily::

John Rennie

Actually I started worrying too soon, one of our engineers is already on the way :-)

0celo7

@DanielSank I'm colorblind, I always had to edit the colors in chemistry

pink? nope, that's purple

dmckee

17:37

@0celo7 Then I don't send it. There are no take backs in Digital Land.

0celo7

or maybe it was blue!

who knows!?

@dmckee not really

What if you just want to send a pic of your face with a caption

TheGhostOfPerdition

@0celo7 I'm color blind too.. Had to get fake reports when I joined my company :D

0celo7

That would be awk to text

Or what about videos

And the snapchat story

@DanielSank you really need a snapchat

dmckee

I'm still waiting for a use-case I care about...

0celo7

I'm surprised some of the young guys in your lab haven't asked you about it before

@dmckee one girl in high school put pics of her butt on her snapchat

John Rennie

17:41

@DanielSank we always have backups - it would cost someone their job if we didn't. Still, it's always stressful when something fails big time. In this case I overreacted. A disk failed in an array, but the array was fine and the disk is now replaced.

ACuriousMind

@0celo7 Like...why would you want to do that?

dmckee

The young have often been foolish. If it can be displayed it an be captured. There really are not take backs in Digital Land.

DanielSank

@0celo7 Give me a reason.

John Rennie

@0celo7 if you're under 16 that's a criminal offence in the UK

dmckee

^ For your own good, of course.

The failure of well intentsioned laws are an endless source of horror combined with amusement

DanielSank

17:43

@0celo7 Wow. So if I get snapchat I can see a butt? Amazing! If only there were some other medium for accessing various pieces of media on demand...

What a world that would be...

A grand network of information... interwoven across the globe.

"Networkglobe"

no...

"Intwined-network"

"Internetwork"

"Internet"

My god...

0celo7

Lol that reaction

DanielSank

@JohnRennie Nice.

@JohnRennie Making public an image of your own butt?

John Rennie

Yes

DanielSank

Self-generated underage pornography is illegal? That's a little funny.

dmckee

@DanielSank Yes. The equivalent laws in the US apply up to 18 years of age. Welcome to the lingering effects of our puritan past.

John Rennie

17:46

Transmitting a nude picture of a minor is an offence no matter who does it. In practice no-one is going to jail a minor for posting a photo of their own arse - they'd just get a stern talking to.

dmckee

@DanielSank And horrible. Because kids have been put in jail to protect them from themselves...

0celo7

@DanielSank A girl at my high school got charged for sexting pictures of herself.

(Different girl)

vzn

"sexting". a massive cultural shift in a single generation. psychologists are studying it now.

@0celo7 criminally charged? by who? who "pressed charges"? werent charges dropped?

DanielSank

@0celo7 Why do I feel like I'm at the neighborhood pool listening to the kids vie for the tallest tale from last school year?

John Rennie

@0celo7 I would guess this was intended as a wake up call. It's hard to see what would be achieved by giving a young girl a criminal record and (in the UK) putting them on the sex offenders register.

0celo7

17:52

@DanielSank last year? That was freshman year

vzn

@JohnRennie its gotten a bit out of control in US. think/ iirc there was a case a young male got put on sex offender list for running naked across football field (as prank).

John Rennie

The UK has always been fairly lenient of that sort of thing. It's generally regarded as funny rather than dangerous.

Stephen Gough

Stephen Gough (born c. 1959), popularly known as the "Naked Rambler", is a British activist and former Royal Marine. In 2003–04, he walked the length of Great Britain naked. He did it again in 2005-06, but was arrested in England and in Scotland. Since then he has spent most of the intervening years in prison, having been repeatedly rearrested for contempt of court for public nudity, each time within a short period after release. He spent most of his sentences in Saughton Prison and Perth Prison in Scotland. == Activism == === Naked rambling === Gough, an ex-lorry driver, is from Eastle...

DanielSank

@JohnRennie That's because y'all persecuted the Puritans so hard that they had to cross the goddamned ocean and come over here.

Thanks.

vzn

@JohnRennie yes, in my "youth" same but there seems to be a (questionable/ strange?) generational shift there also.

DanielSank

You guys literally exported your prudes.

John Rennie

17:56

Interestingly Stephen Gough got imprisoned in Scotland, but in England everyone just laughed.

vzn

@DanielSank (lol) uh, they seemed to "self-vacate"....

DanielSank

@vzn Listen, buddy, you don't get on a wooden boat and cross the Atlantic mother------g ocean because you felt like it.

John Rennie

If you promise not to tell anyone ...

vzn

@DanielSank they were said to be "persecuted" but not for their prudishness.

John Rennie

As a student I had to go to a college sherry party and was told I had to wear a tie

So I did

DanielSank

17:58

@vzn I know I know. I 'm joking around.

@JohnRennie o_O

John Rennie

A tie and a sock

DanielSank

"College sherry party"...

vzn

@DanielSank it could have been regarded as a case of religious extremism... bordering on fanaticism!

John Rennie

@DanielSank Cambridge is like that

"was like that" I should say, but I bet it still is.

DanielSank

@JohnRennie Is a sherry party what it sounds like?

vzn

18:01

if the guys wore socks what did the women wear? :P

John Rennie

@DanielSank you stand around in stuffy room with a glass of sherry (usually really good sherry to be fair) making small talk and trying not the attract the attention of the really boring dons.

@vzn Single sex college

Peterhouse, Cambridge

Peterhouse is a constituent college of the University of Cambridge, England. It is the oldest college of the university, having been founded in 1284 by Hugo de Balsham, Bishop of Ely and granted its charter by King Edward I. Today, Peterhouse has 226 undergraduates, 86 full-time graduate students and 45 fellows. The modern name of Peterhouse does not include the word "college". Despite being one of the smallest colleges, it has one of the highest overall endowments, more than £250 million, including property in central London such as the Albany apartment complex in Piccadilly. Peterhouse is one...

vzn

@JohnRennie ewww

DanielSank

@JohnRennie Surely for parties you imported members of the other sex?

import fun

self.converse_with(fun.member_other_sex_factory(4))

Right?

John Rennie

Some of the dons wives would be there. The students wouldn't bring guests.

DanielSank

@JohnRennie What's a don?

John Rennie

18:04

University don

A don is a fellow or tutor of a college or university, especially traditional collegiate universities such as Oxford and Cambridge in England, Trinity College, Dublin, in Ireland, and Delhi University in India. Like the term dom used for Roman Catholic priests, the term don derives from the Latin dominus meaning "lord" and is a historical remnant of Oxford and Cambridge having started as ecclesiastical institutions in the Middle Ages. The term don meaning crime boss also derives from dominus. == Canada == In some universities in Canada, a don is a resident assistant, typically an upper-year student...

DanielSank

@JohnRennie Are you allowed to walk on the grass?

John Rennie

I feel like we live in different countries :-)

@DanielSank No! Big time no! That's a disciplinary offence! Only the dons were allowed to walk on the grass.

DanielSank

@JohnRennie Can't tell if is joke.

John Rennie

Well that's not really true. The colleges usually had a formal and well maintained lawn out front and you couldn't walk on that. But on the other courtyards no-one cared.

DanielSank

@JohnRennie I see.

0celo7

18:07

@ACuriousMind because girls like it

hubot

Hi there!

John Rennie

Last sentence in the Scholarships and Prizes section:

Trinity College, Cambridge

Trinity College is a constituent college of the University of Cambridge in England. With around 600 undergraduates, 300 graduates, and over 180 fellows, it is the largest college in either of the Oxbridge universities by number of undergraduates. By combined student numbers, it is second to Homerton College, Cambridge. Members of Trinity have won 32 Nobel Prizes out of the 91 won by members of Cambridge University, the highest number of any college. Five Fields Medals in mathematics were won by members of the college (of the six awarded to members of British universities) and one Abel Prize was...

DanielSank

Hey, quantum mechanics people, I haz question:

2

Q: How does one compute the state of a quantum system following imperfect measurement?

Suppose I have a quantum system $S$ ("system") with Hamiltonian $H_S$ and initial density matrix $\rho_S(0)$. I allow $S$ to interact with another system $P$ ("probe"), which has Hamiltonian $H_P$ and initial state $\rho_P(0)$, via an interaction Hamiltonian $H_I$. Then I measure $P$ in the basis...

quantum-mechanics quantum-information measurement measurement-problem density-operator

John Rennie

That was Trinity not Peterhouse, but the same rules applied.

hubot

How to check is my answer correct on Stack Exchange?

I've question related to collimated atomic beams.

0celo7

18:10

@old people If young people are doing some stupid shit it's probably because girls do it for some reason.

hubot

Landau & Lifshitz are good for me?

You said that Landau & Lifshitz are on top.

0celo7

@vzn I don't know. I just know that she had to show up in court, I didn't ask more about it

She was kind of a slut

vzn

lol. ancient/ slow moving legal system still trying to catch up to cyberspace & sometimes its not pretty (or worse, a mess...) eg re recent apple crypto case (blogging on that)

ACuriousMind

@0celo7 Girls like it when you send them non-permanent pictures of your face with a caption on it? Your girls are strange.

vzn

@ACuriousMind think of it as a personal internet meme (and how many of those have been posted in this chat room?). all about the heading/ msg. eg LUV. gfs like that kind of thing

ACuriousMind

18:23

@DanielSank Already upvoted that ages ago, can't do more

0celo7

@ACuriousMind like a reaction to something they send you, yeah

ACuriousMind

You make me feel old

0celo7

I thought it was stupid too

But, as you said, students want a "hands on" experience

So you gotta play the game

vzn

Huh? cyber pictures are "hands on"?

ACuriousMind

Oh, I'm not confused about the idea of doing something because the other sex likes it. I'm confused about the fact that anyone likes sending/receiving those pictures.

(Talking about the face+caption pictures, not the naked butt ones :P)

@vzn No. He's saying sending snapchat pictures gets him laid :P

...aaaaaand this chat is at a weird place again

0celo7

18:28

Lol.

From space forms to the female form.

vzn

@ACuriousMind dont think you want to speak for 0celo7 :P

John Rennie

@vzn we badly need an opposite to starring comments

ACuriousMind

That's called "flagging", I think.

0celo7

@vzn why

vzn

@0celo7 uh, hes explaining/ clarifying your sex life/ modus operandi?

← feels older

0celo7

18:44

@vzn being nice and doing stuff she likes is generally a good MO

Do you disagree?

vzn

@0celo7 "stuff she likes" = "snapchat (sexting?) memes + hands on"? sounds good to me...! thought you said earlier you were still "closing the deal" but maybe misunderstood :P

user54412

@dmckee Something something action-angle variables maybe?

user54412

@JohnRennie While this might be common, I'd probably have quit grad school if my adviser was like that. I want to work with a researcher, not for a manager. Or at least, if the latter, I want better than minimum wage.

0celo7

@vzn oh god I'm not sexting

You'd have to be retarded to sext

Memes?

user54412

^ there's literally no other reason to snapchat

0celo7

18:50

What are you on about

vzn

@0celo7 havent you seen all the internet "memes" posted in here over the ages? maybe it was KK who enjoyed that. some were really funny/ hilarious. formula is basically/ roughly silly picture + silly caption. (so figure you qualify.) :P

0celo7

I know what a meme is

@ChrisWhite what's your snapchat

user54412

@0celo7 I'm old enough that if I had one I'd probably be arrested on suspicion of sexting minors

0celo7

@ACuriousMind Actually in high school my snapchat was an Einstein picture delivery device

vzn

@ChrisWhite good luck finding a place to work without a mgr (or "mgr-like entity") :|

0celo7

18:54

@ChrisWhite I'm not a minor.

Oh if you had one

Bernard Meurer

Yo

user54412

@ACuriousMind Since we all must ask you questions directly... Do you know about massive gravity? I just attended a talk about it, given by a field theorist to a bunch of astrophysicists, so we were a bit in over our heads. What's up with the DVZ discontinuity? In particular, is Vainshtein known or just merely suspected to solve it?

Bernard Meurer

@DanielSank Are you around?

@ChrisWhite Are you a python ninja?

user54412

@Danu might weigh in on this? ^^^

0celo7

@BernardMeurer yo

@ACuriousMind et al. don't understand snapchat

user54412

19:01

@BernardMeurer I'm a python... something. Definitely not a ninja.

0celo7

they don't get why we would snap our face with a reaction

Bernard Meurer

@ChrisWhite Are you familiar with sockets, non-blocking interfaces and pure networking magic?

Networking should be called Notworking

0celo7

@ChrisWhite ok can you please explain why $k$ is constant in time?

Is the reason topological?

user54412

@BernardMeurer wat

0celo7

@JohnRennie See:

user54412

19:06

is this a python question or a network-theory-(in-python) question?

0celo7

The proof is not fun.

Bernard Meurer

@ChrisWhite stackoverflow.com/questions/35875369/…

It's a question

that involves all those spells

DanielSank

@ACuriousMind is my comment here correct?

5

Q: Why is the divergence of a magnetic field equal to zero?

We know due to Maxwell's equations that: $$\vec{\nabla} \cdot \vec{B}=0$$ But if we get far from the magnetic field, shouldn't it be weaker? Shouldn't the divergence of the field be positive? If we define the vector field as a function of distance, then if the distance increases then the magn...

electromagnetism magnetic-fields maxwell-equations

John Rennie

@ChrisWhite my supervisor was Steven Elliott. He was very well known, on the board of several journals, good contacts and a nice chap

Also very easy to talk to and gave good advice

And I consider my self fortunate that he was able to supervise me

But I don't think he had been first author on many papers for quite a while. His research was done by guiding us. And that was just fine by me.

I think this is him, though he had a full beard in the 1980s ...

ch.cam.ac.uk/person/sre1

user54412

@JohnRennie I just found a brochure advertising my department back in the 80s. My professors were indeed more bearded then.

ACuriousMind

19:17

@ChrisWhite I have no clue about massive gravity

@DanielSank I'd say it's wrong. $\nabla \cdot B = 0$ and $\nabla\times E = 0$ are the necessary equations for electromagnetism to be a proper gauge theory. If the divergence of the magnetic field doesn't vanish, you have magnetic monopoles, and cannot define the four-potential at the points where they sit. You can try to have the monopoles sit at points cut out from your actual spacetimes on which the gauge theory lives, but in standard EM, it's not possible.

user54412

9

A: Does magnetic monopole violate $U(1)$ gauge symmetry?

No, a magnetic monopole a la the Dirac string does not "violate" gauge symmetry. Rather, the statement "we have a magnetic monopole" means only that we are forced to consider the gauge theory not on the whole spacetime, but on the spacetime with the location of the magnetic monopole removed. Why?...

Bernard Meurer

@ACuriousMind What race of student are you?

ACuriousMind

@ChrisWhite Yeah, there we have $\nabla\cdot B = 0$ everywhere where $B$ is defined, so all is well

user54412

When I eventually learned this, I was annoyed at how often in my earlier studies I would be told "and look how symmetric Maxwell would be if only there were magnetic monopoles -- they're so natural."

ACuriousMind

@BernardMeurer I don't understand the question

Bernard Meurer

19:24

@ACuriousMind undergrad, grad, PhD, omniscient entity, ...

user54412

German

ACuriousMind

@BernardMeurer I'm a master student

I.e. grad but not (yet) PhD

Bernard Meurer

@ACuriousMind I was betting on the latter of the options :p

John Duffield

@0celo7 : because it's circular mathematical handwaving, and it's wrong because you confuse space and spacetime. The FLRW metric starts off with the assumption of the homogeneity of space. As the PhysicsFAq says, the distinction is crucial.

Besides, why are you complaining? I got 9 downvotes for my correct answer.

0celo7

No, you got 9 downvotes on an incorrect answer.

2

It probably doesn't even answer the question

ACuriousMind

19:33

@ChrisWhite Well...the unanswerable question here is is it more "natural" if Maxwell's equations are symmetric than if the Lagrangian is gauge symmetric. One can make a "duality" Lagrangian with two field strengths which then get identified as electric-magnetic duals of each other on-shell, but from the Lagrangian perspective, that doesn't seem at all "natural".

John Duffield

@0celo7 : it does. Two out of three "shapes of the universe" were always going to be wrong, WMAP found the universe was flat, and there's no overall gravitational field in the universe because it didn't collapse under its own gravity when it was small and dense. There's no reason to assume a constant curvature term k, and there's every reason to assume no curvature.

hubot

I've to go

DanielSank

19:50

@ACuriousMind I could've sworn Jackson explains that you can get ride of divergenceless magnetic field requirement if you instead make some assertion about the ratio of electric and magnetic charge.

ACuriousMind

Ah, well, if you have magnetic charge, then yes, the field won't be divergenceless. But electromagnetism doesn't have magnetic charges.

DanielSank

@JohnDuffield I have a curiosity: when you find yourself repeatedly telling people they're wrong about something, and you have a rather strong prior reason to believe they are probably reasonably intelligent, how do you avoid the obvious conclusion that it's more likely that one person (i.e. you) is wrong than it is that everyone else is wrong?

ACuriousMind

But there is something to the "ratio" thing. If you put in a magnetic charge and do the usual gauge description of EM away from it, you find these coordinate artifacts - Dirac strings. If you insist that they really shouldn't be there, you must require that the Aharonov-Bohm effect incurred from the "pseudo-flux" of these lines vanishes, and that makes the magnetic charge be $\frac{2\pi}{e}$ where $e$ is the electric charge.

That's variously known as "Dirac quantization condition", "flux quantization" or other names.

DanielSank

Obviously the situation would be different if you were talking to a group of kindergarteners. In that case there's a good reason to believe they may not know the thing in question.

@ACuriousMind Well allright then.

In that case I think my comment is vague enough to not delete it, yes?

ACuriousMind

Since you're not specific about what exactly you mean by "electromagnetic theory", it's not really wrong, I guess

0celo7

19:56

@DanielSank From his PoV we're saying Einstein himself is wrong.

Which I'm happy to do

John Duffield

@DanielSank : it's because I've read the Einstein digital papers and lots more. When I tell some 17-year-old kid that he's wrong, I'm not doing it because I'm making stuff up. I give robust references to support what I'm saying. Then when the kid dismisses those references and declares that Einstein woz wrong, it's crystal clear that he isn't reasonably intelligent at all.

0celo7

Einstein waz totes wrong tho

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