The h Bar

1:32 AM

@SirCumference yeah that charge is a complete scam.

1:52 AM

I want to appeal against the scam of the dormitory administration - they count my rent from October 1st while I didn't move in until October 20th by asserting the person who made the reservation for me from October 1. They cannot confirm me a reservation even less than one day before the flight on September 30th took off and have never had an agreement with me on when the reservation started.

Captain Bohemian

2:02 AM

So I have to book the flight on October 15th.

Nihar Karve

Did the mobile website change?

Now all the titles are in bold and visited links don't turn purple :(

ZeroTheHero

2:19 AM

a related situation happened in the 16th century because of drift of the vernal equinox: Thursday 4 October 1582 was followed by Friday 15 October 1582, and this caused quite a conflict for rent, as landlords wanted the full month rent while tenants argued they should get reduced rent.

fqq

2:35 AM

@CaptainBohemian ask the authors, or (illegal) sci-hub.se

Ryan Unger

4:00 AM

@ACuriousMind reports indicate it’s plagued with bugs :p

123

6:24 AM

Hi All...

RewCie

7:55 AM

JUST IN: The second person to receive the #Pfizer coronavirus vaccine is named William Shakespeare

source: dailymail.co.uk/news/article-9029435/…

satan 29

8:44 AM

hey everyone

Is it fair to say that currently, the entirety of magnetostatics is based on the assumption div(B)=0 ?

John Rennie

Are you asking if div(B)=0 is an assumption, or are you asking if magnetostatics is based on that assumption?

satan 29

both, actually

John Rennie

div(B)=0 is indeed an assumption, and if we ever observe a magnetic monopole we'll know the assumption is wrong. For now the assumption has proved correct.

satan 29

"proved correct" ?

John Rennie

I mean every time it has been tested experiment has shown it to be correct.

Yes, using the word proved there is misleading.

satan 29

8:50 AM

ah I see.

wait, what exactly do experiments measure?

Its hard to think of "measuring" divergence

or are you talking about biot savart law, (which I believe is something based on this assumption)?

John Rennie

The statement div(B)=0 is equivalent to the statement that magnetic monopoles do not exist. So the experiment is to search for magnetic monopoles, and they have never been found.

satan 29

@JohnRennie ah ok.

@JohnRennie so, can you then derive the biot savart law using div(B)=0?

John Rennie

No. EM is described by Maxwell's equations and you need all four of them. div(B)=0 is just one of the four equations and cannot describe the behaviour of magnetic fields on its own.

satan 29

okay, so coupled with curl(B)=uJ we can derive it?

what i am struggling to come to terms with is that:

1)div(B)=0 is an assumption.

2)biot savart experimentally produced his formula. And it turns out that if you apply div to it, the result is indeed zero.

John Rennie

I don't recall how the Biot-Savart equation is derived. In fact I'm not sure I have ever known. I suspect I was never interested enough to find out. But I'm sure a Google would find derivations.

satan 29

9:03 AM

wikipedia: "biot-savart "discovered" this relationship in 18xx"

John Rennie

I'm not sure I see the problem with your points 1 and 2. The experimental verification of Biot-Savart is also an experimental verification that divB=0. But no experiment can ever prove an assumption.

The experiment just shows that in the conditions used in the experiment the assumption is not untrue.

satan 29

suppose we do discover that there are monopoles.

RewCie

JUST IN: The height of Mount Everest has been remeasured as 8848.86 meters

satan 29

now, how will this explain the correct results that we obtained earlier?

since now the formulae have to be changed

John Rennie

It would show only that in the experiments done so far no monopoles were present.

satan 29

9:07 AM

:0

John Rennie

That makes sense since theory suggests that if monopoles exist they are probably exceedingly massive and wouldn't be created except in the most high energy processes.

satan 29

that actually makes sense

@JohnRennie oh wow

hmmm i am satisfied for now, thanks :)

John Rennie

There is probably a Wikipedia page on magnetic monopoles ...

satan 29

wait I had another question

why are 1/r terms called monopoles and 1/r^2 terms dipoles?

John Rennie

The field generated by a point charge (of any sort) has a potential that falls as 1/r. This follows from Gauss's law.

satan 29

9:12 AM

ah, so its based on potential.

John Rennie

Yes

satan 29

I thought why is it called monopole when the field drops as 1/r^2..

John Rennie

The potential of a dipole falls as 1/r², a quadrupole falls as 1/r³ and so on.

satan 29

\right

John Rennie

Do you know about multipole analysis?

satan 29

9:14 AM

no...

but I recall this being in griffiths, I will study it.

I had skipped the chapter on "potentials"

John Rennie

Any field can be expressed as a sum of multipoles i.e. a monopole component, a dipole component and so on.

In effect the multipoles form a basis for expanding the field.

satan 29

i see

John Rennie

Given any field you can calculate the amplitude of each multipole with an integral that ... erm ... I've forgotten.

satan 29

F

John Rennie

Multipole expansion

A multipole expansion is a mathematical series representing a function that depends on angles—usually the two angles used in the spherical coordinate system for R 3 {\displaystyle \mathbb {R} ^{3}} (the polar and azimuthal angles). Similarly to Taylor series, multipole expansions are useful because oftentimes only the first few terms are needed to provide a good approximation of the original function. The function being expanded may be real or complex-valued and is defined either...

satan 29

9:19 AM

@JohnRennie ah yes, now I remember whi I decided to skip that chapter

John Rennie

I struggled to find classical electrodynamics exciting.

It was always just something I needed to know.

satan 29

9:35 AM

what were/are your interests aprt from GR and QM @JohnRennie?

John Rennie

At university I got interested in quantum chemistry i.e. using quantum mechanics to predict properties of molecules. Back in 1983 this was a pretty new field as computers were only just getting powerful enough to do the calculations to a reasonable degree of accuracy.

In my final year I did all quantum chemistry courses.

Then I did a PhD on solid state photochemistry that I thought would be related to this, but in fact went in a different direction.

But when I started work I was asked to work on colloid science, and since Unilever was paying me to do this I thought it only polite to do as they wanted :-)

I only got back into physics when I switched to part time work and had the free time to start studying it again.

Nihar Karve

On the topic of quantum chemistry, I just finished a baby HFSCF program for helium

satan 29

9:56 AM

@JohnRennie oh!!

John Rennie

@satan29 I had always wondered how GR worked because it was so notorious for being complicated. So when I had the free time I started to learn it.

I was also curious about how QFT worked, but I have found that a lot harder than GR and I haven't made any significant progress in trying to learn it. Mainly because I'm unwilling to do the hard work required :-)

ACuriousMind

@NiharKarve HFSCF = Hartree-Fock...?

Nihar Karve

10:12 AM

@ACuriousMind Self-consistent field (method)

Although that addition might be considered redundant by some authors

My program wasn't all that special, it mostly mirrored Szabo and Ostlund. But I wrote all the contracted GTO code independently, so that's a plus

ACuriousMind

writing the same thing someone else already wrote is usually a good way to make sure you've understood how it's supposed to work, yeah

Nihar Karve

...I picked up but the slightest hint of sarcasm

ACuriousMind

no, I meant that!

Nihar Karve

Sorry :D

ACuriousMind

doesn't work if you're just copying code, obviously, but I really do think that works

Nihar Karve

10:26 AM

I forgot that S&O even had code at the back - I wrote my NumPy code based on their description of the HF method

I haven't ever touched Fortran, I think Julia is gonna be the next up and coming language in that niche

Physics Meta

10:43 AM

0

Q: Tags for calibrating answers to the level of question

A problem I find with the site is that often answerer's don't know how to calibrate an answer to the level of the questioner. This was both good and bad for me personally as I'd get insight into topics I'd never have known about if I had kept my narrow view on topics. But, some people wish for an...

RewCie

12:53 PM

Just In: Aliens exist but in hiding, until mankind is ready, says ex-Israeli space head Via WION

JMac

1:38 PM

@RewCie Yeah that sounds totally legit... The super pixelated Trump image they used really sold me on their journalism.

Nihar Karve

1:51 PM

@JMac I dunno if you've seen those, but I think that's the image that uber Trump-replacement conspiracy theorists use as the "replacement clone for the real Trump" after his COVID hospitalisation

RewCie

Alt. Ref. : Aliens in hiding until mankind is ready, says ex-Israeli space head

Alt. Ref. 2: Mankind has made contact with an alien 'Galactic Federation' but it has been kept secret because 'humanity isn't ready', former head of Israel's space security program claims

2

via : News AU, DailyMail UK

@NiharKarve Honestly new Trump speaks less than the old one.

JMac

2:44 PM

@RewCie Uhhh... He was talking quite a bit when the polls stopped being in his favour?

Physics Meta

3:24 PM

-1

Q: How can I improve my question from October 16, 2019?

One of my questions from October 16, 2019, Why is the ångström not a metric unit? And why is the ångström spelled with the Swedish/Finnish letters "å" and "ö"? has got useful answers. However, my question is downvoted to -5. How can I improve my question?

discussion support specific-question down-votes

Charlie

4:00 PM

Quick question, when we study symmetries in Lagrangian mechanics we change the Lagrangian function itself, but when we look at "canonical transformations" on phase space that preserve Hamilton's equations and the Poisson brackets we are effectively just talking about using another coordinate chart on $T^*M$, in the usual manifold sense, right?

Ratman

4:18 PM

I think you are right and I think even in the lagrangian case it can be expressed in such terms, as you can consider the lagrangian as functtion on the fiber bundle of the manifold. But it is better if you wait for someone more expert than me, I still have to find some time to study Arnold's book

Charlie

Oh you think you could express symmetries of the lagrangian in the same way?

After thinking about it for a moment I don't see how, but perhaps there is something I'm missing

Ratman

perhaps it's just me saying nonsense ahahah

ACuriousMind

6:00 PM

@Charlie it's an active/passive distinction again :P

you can think about a isomorphism $M\to M$ of any kind of manifold either as a map of the manifold to itself (active) or as a change of coordinates (passive), just like you can think of the invertible linear maps (=matrices) on vector space either as moving all the vectors in the space around or as changing the basis

neither is the "correct" view, but for some reasons in my impression mathematicians seem a lot better about sticking to one (the active view) while physicists seem to decide semi-randomly which things should be seen as active and which as passive :P

Charlie

7:09 PM

ahh ok I'm happy with the distinction in the case of charts and maps between manifolds, just wanted to make sure that's what we're actually doing

Ryan Unger

9:26 PM

@ACuriousMind can you download cyberpunk yet

ACuriousMind

@RyanUnger it's already sitting on my hard drive ;)

but I got it on GOG, not on Steam

Ryan Unger

I’m downloading on steam now

But there will probably be a 50 gig day 1 patch