The h Bar

5:00 PM

@JohnRennie I got pretty good at searching paper textbooks anyways. If you were a bit familiar with the textbook already, you can find most things pretty easily with the index.

Johan Liebert

@AaronStevens here the markets are only open from 7:00 am to 12:00 pm. When LockDown started some people bought about 20 kg potatoes.

ACuriousMind

@JohnRennie I've found that "get the book and work through it start to finish" is a much rarer usecase (you'll do that at most once per book!) than "look up this one thing". PDFs are much better for the latter especially once the number of books expands beyond physical shelf size

John Rennie

@ACuriousMind that's certainly true.

skullpatrol

but a lot easier said than done for a novice

ACuriousMind

@JohanLiebert Meanwhile, we have 700% increased sales of toilet paper. How's that for priorities ;P

(I find very little unusual about buying 20kg of potatoes - these things keep for months!)

John Rennie

5:06 PM

@ACuriousMind let's put this behind us.

ACuriousMind

that seems to be the plan, yes

@skullpatrol ...what? A novice is able to press Ctrl+F as well as any expert!

Johan Liebert

@JohnRennie 😂 😂

John Rennie

@ACuriousMind The supermarkets in Chester have also sold out of high fibre cereal

Semiclassical

knowing what to search for, is the trick

skullpatrol

^

@ACuriousMind

JMac

5:09 PM

@Semiclassical That's why I liked using the indices so much. If you didn't know the specific word you were looking for, you could look through index topics until something stood out as a good lead.

Semiclassical

yeah. same with tables of contents

ACuriousMind

all of these are also better if the PDFs are properly hyperlinked (sadly a rare art)

Semiclassical

though that's one reason I hate a certain math book i've got---the index is totally borked.

it's not an easy book to understand regardless, but the index just does not reflect the actual text

Johan Liebert

USA now surpassed Italy in the no of reported cases.

ACuriousMind

@JohnRennie I'm beginning to suspect there are many literalists when it comes to idioms like 'shit's about to get real'

Semiclassical

5:11 PM

that was inevitable, but still yikes

JMac

I found table of contents was good for finding higher level things that are covered by chapters or sections. Index was good for topics that are only mentioned in passing or just don't have their own section.

Semiclassical

yeah

oof:

washingtonpost.com/sports/2020/03/25/…

JMac

Knowing the ToC was definitely helpful for using the index too, since if you know approximate what pages are what topics, you have a better idea which page in the index will actually be on the topic you want.

Since sometimes the index has multiple pages listed if it comes up several times

PM 2Ring

@ACuriousMind Well, PDF started life as a file format for stuff that's intended to be printed. And not edited. But it's kinda mutated a bit since then...

Semiclassical

back in February, there was a soccer game in Italy with about 40k visitors

and now that's an epicenter of covid in Italy

ACuriousMind

5:14 PM

@PM2Ring Don't get me wrong, the actual file format is, like many others, a historically grown abomination of compatibility nightmares, but with stuff like hyperref in TeX it's not hard to at least produce properly hyperlinked technical texts.

Semiclassical

so that's a pretty good candidate for a super-spreading event

PM 2Ring

PDF is closely related to PostScript, except that PDF is a data description language, so similar in principle to HTML

bolbteppa

'Weeks later, Bergamo has earned the tragic distinction of being the hardest-hit province in the hardest-hit region, Lombardy, of Europe’s hardest-hit country. ' yikes

skullpatrol

True @Semiclassical

Semiclassical

yeah

JMac

5:16 PM

@Semiclassical Man that timing sucked for them. It was 2 days before the first death in Italy, and they were one of the first to get hit hard. You can't even blame them for not distancing, because people weren't fully aware yet.

PM 2Ring

Whereas PostScript is a full programming language, with the advantage & disadvantages that entails.

Johan Liebert

I think if Italy hadn't become the epicenter then we could have stopped the spread of covid-19 .

ACuriousMind

I once looked into the PDF spec to figure out whether it would be feasible/interesting to implement a "print to PDF" option on my own for something. I decided to leave that for more desperate souls rather quickly :P

bolbteppa

Was the world paying the same attention until what happened in Italy

Semiclassical

yeah. this does illustrate something I was suggesting: it having such an impact in Italy was as much a matter of bad luck and timing as anything

JMac

5:17 PM

@JohanLiebert I have my doubts. Italy was one place it spread; but there were others. Based on how the US went, I kinda doubt we would have stopped that from happening, Italy or not.

PM 2Ring

@ACuriousMind Assuming people are using such tools... and not creating their docs in Word. :)

Semiclassical

until it became clear how bad it's going to be, people were going to keep going to big eventns

ACuriousMind

@PM2Ring Even Word can produce passably linked PDFs, as I know much to my own regret

Semiclassical

so it's unfortunately rather plausible that such mass-transmission events were inevitable

"Valencia said last week that more than a third of their players and coaching staff had tested positive for the virus, implying in a statement that the club’s participation in the Champions League first-leg tie against Atalanta is linked to the high number of positive tests."

skullpatrol

@PM2Ring how do they do it so well in Wikipedia?

JMac

5:20 PM

@Semiclassical That's kinda how I see it. There's a tough balance to strike between "everything functioning the way people expect and are used to" and "prevent the spreading of a possible pandemic we know little about". I'm not sure how we could have known enough before it was too late, unless we had gotten lucky.

PM 2Ring

I don't have much need to create PDFs. Mostly, I just feed PostScript files to GhostScript. And those PostScript files are either hand-written, or created by a simple Python script.

bolbteppa

Biden is going to steer the ship back to sanity luckily (oh yes, this is sarcasm)

Semiclassical

though the mayor also noteos:

“The football match is one factor, but the hospital is the most credible explanation,” said Gori. “We don’t know exactly when, but on a certain day a patient turned up with pneumonia, but the symptoms weren’t recognised. That patient was together with other patients who became infected, as well as doctors and nurses.”

PM 2Ring

@ACuriousMind Oh, ok.

Semiclassical

@bolbteppa it's remarkable how much of a non-entity Biden has been since COVID became the news

bolbteppa

5:23 PM

He was literally blaming his ceiling height as a reason for not appearing on camera nearly all of last week as Trump did daily briefings and raised his approval to near/above 50%

ACuriousMind

Unfortunately, the people responsible for corporate style guides do not usually understand requests for a "LaTeX template" :P

Semiclassical

@ACuriousMind "what good is a template made of rubber"

ACuriousMind

But they'll be happy to provide you with a dozen Word templates and cheerfully ask you if you're sure your document shouldn't be a ::shudder:: Powerpoint

those monsters

They love power.

5:26 PM

In mathematics, the Iverson bracket, named after Kenneth E. Iverson, is a notation that generalises the Kronecker delta. It converts any logical proposition into a number that is 1 if the proposition is satisfied, and 0 otherwise, and is generally written by putting the proposition inside square brackets: [ P ] = { 1 if P...

PM 2Ring

^ I read his APL book. I never got the chance to do much coding in APL though.

John Rennie

APL. Happy memories. A "write-only" programming language as I recall.

PM 2Ring

It is very dense. And the use of Greek letters as well as Latin makes it even denser. Sort of.

But to be fair, RAM was very expensive back then, so coders did all sorts of stuff to keep the RAM footprint as small as possible.

OTOH, APL doesn't just blindly mix Greek & Latin. The Greek letters are for the built-in operators, the Latin for user variables & constants. I don't remember much about it, since I haven't touched it since the late 70s.

Johan Liebert

5:46 PM

‘People Are Dying’: Battling Coronavirus Inside a N.Y.C. Hospital | NYT News

►

Patients with no fever but just abdominal pain were later found out to have chest X-rays consistent with covid-19. Is that just a coincidence?

Yuvraj Singh...

@JohnRennie my brother installed for me, for the first time he was so patient full and polite with me, after two year we talked for more than an hour.

Yes we talked about our child hood memories, let us all share our some memorable event that had happened in our life.

@PM2Ring start with you sir! Something memorable and joyful you haven, t forgot yet?

Though I am just 18, so my memories are not yet much old, but yeh topic refer here is moment with joy:-)

Johan Liebert

@YuvrajSingh... When I used to to live in West Bengal, in the rainy season it floods so much that the nearby play ground gets filled with water. In those days we used to go on fishing (and fun thing was that we used to catch fishes with polythene bags). Once the local people found a red fish which sort of looked like a pirahna. Those days were amazing!

Coronavirus Pandemic: Will N.Y.C.'s Homeless Students Be Left Behind? | NYT News

►

I feel sad for that little girl.

skullpatrol

6:11 PM

2 days ago, by skullpatrol

"newest updates on corona" bar

or

♦ Coronavirus Chat Zone

RPG.SE's offtopic room for COVID-19 related discussion. May be...

2 days ago, by ACuriousMind

I don't really mind people discussing how they're doing, but I do mind random news posts that do little more than push it back into my consciousness when I've just managed to escape it for a few minutes. If I'm the only one that feels that way, that's fine and I'll suck it up, but I'd appreciate a bit of restraint.

2 hours ago, by skullpatrol

::last virus update::

Johan Liebert

Oh sorry! I just forgot about that.

skullpatrol

np, pal

:-)

bolbteppa

6:53 PM

Is there a simple fast way to derive the non-relativistic Coulomb scattering matrix :(

Regge theory

In quantum physics, Regge theory () is the study of the analytic properties of scattering as a function of angular momentum, where the angular momentum is not restricted to be an integer multiple of ħ but is allowed to take any complex value. The nonrelativistic theory was developed by Tullio Regge in 1959. == Details == The simplest example of Regge poles is provided by the quantum mechanical treatment of the Coulomb potential V ( r ) = − e 2 /...

Semiclassical

@bolbteppa brb, will give pilot wave derivation :P. (very much sarcastic)

unless I succeed

bolbteppa

Bohmian Regge Trajectories, my god...

Bohmian complex angular momentum related to complex energies on Riemann surfaces, it all makes sense now...

Semiclassical

lol

to the extent that i know about pilot-wave treatments of scattering, it's via this paper: arxiv.org/pdf/1210.7265.pdf

which is for 1D

oh lol

@bolbteppa bohm-c705.uibk.ac.at/about_daten/theses/mmeisinger-dipl.pdf

bolbteppa

7:09 PM

Those notes look great wow

Section 2.2.2 bit of a chore but need to do it that way anyway

Semiclassical

the bohm stuff doesn't start until chapter 3, so you might appreciate that much at least

(my own sympathies are towards the "Bohmian velocity field" setup more than the historical setup. analogies to Hamilton-Jacobi theory don't do as much for me as "hey, the probability distribution generates integral curves")

bolbteppa

(I don't see why you'd even need probability if particles followed paths in any sense of the word)

Semiclassical

(because what you're able to prepare is the initial wavepacket, not the particle's position within said wavepacket.)

(knowing the initial wavepacket and the potential is enough to determine what paths are possible. but it doesn't determine which path is actually taken. that is still a matter of statistics.)

bolbteppa

(It doesn't matter if we can't prepare it's position it'll still follow a path which is solved by some equation which becomes the new F = ma even if we're prevented from seeing the path, it's still there, ahhh :p)

Semiclassical

(pilot wave dynamics are first-order, not second order. so it's dx/dt = stuff not d^2x/dt^2=stuff)

bolbteppa

7:19 PM

Quantum scattering is so hard, right here we have to know asymptotic approximations to confluent hypergeometric solutions to a radial wave function after isolating off it's $r \to 0$ and $r \to \infty$ behavior just to even get started

Semiclassical

(if you could prepare the initial wavepacket -and- know where the particle is within that wave packet, then you would be able to predict the particle's trajectory. but that's not an option even in Bohm: in order to know where the particle is, you have to measure its position. and once you do that, you no longer have the initial wavepacket. if you prepare the wavepacket again, then you again no longer know where the particle is.)

(an additional subtlety (one which I'm not confident is helpful to the Bohm position) is that one can't really "measure" the position of a particle to infinite precision. at best, one localizes the particle to a very volume of space. but in that case one has really just traded one initial wavepacket for another, highly localized packet)

Student404Mus

Is it right to change the argument of this integral to $R_0R$ holding the same result since it spans over all the rotations?

$\int D_{m_{1}, m_{1^-}}^{(j_1)}(R) D_{m_2, m_{2^-}}^{(j_2)}(R) D_{M M^{-}}^{(J) *}(R) d R$

D are the transformations induced by arbitrary rotation R

Semiclassical

oh hey, Wigner-D coefficients

Student404Mus

yesss

good guess

Semiclassical

i had to learn about them a bit last year

mostly the little-d coefficients

Student404Mus

7:30 PM

The book I snipped this from De Shalit which treats Shell Model

bolbteppa

It looks like the usual group rep argument that comes up e.g. in proving Schur orthogonality or defining an inner product

Semiclassical

so we've got $$\int \langle j_1 m_1|R|j_1 m_{1^-}\rangle \langle j_2 m_2|R|j_2 m_{2^-}\rangle \langle J M|R|J M^-\rangle~dR$$

and we want to replace $R\mapsto R_0 R$

more formally, we'd say that $R=R_0 R'$

Student404Mus

Actually the full expression is,

Semiclassical

ugh, normalization

Student404Mus

where a's the Clebsch-Gordon coefficients

He used the fact that ,

to obtain,

that's it.

Semiclassical

7:37 PM

that looks right

we definitely have: $$\langle j m |R_0 R|j m_-\rangle = \sum_{m'} \langle j m |R_0|j m'\rangle \langle j m'|R |j m^-\rangle$$

Student404Mus

@Semiclassical Normally it would be as you mentioned

Semiclassical

yeah. i'm fine with doing $R=R_0 R'$ and then relabeling $R'\to R$, though

or vice versa

Student404Mus

so it's matter of notation (definition)

Semiclassical

yeah

anyways, what i just wrote amounts to what they have. though I'm a bit perplexed by the ordering

Student404Mus

alright

Semiclassical

7:40 PM

on the left-hand side, they've got $RR_0$

but they get products in the order $R_0,R$

Student404Mus

What if someone goes to use substitution

the expression above doesn't account for $R_0 dR$

yes here where I get confused.

Semiclassical

i think that the measure is invariant under rotations, so $d(R_0 R)=dR$

Student404Mus

as usual

:)

mathematically acceptable? or we say, if it is in physics it would be okay?!

or..

Semiclassical

i think the mathematical statement is that you're dealing with a so-called Haar measure

Student404Mus

the a's coefficients are equal up to a phase?

Semiclassical

7:48 PM

see for instance: en.wikipedia.org/wiki/Haar_measure#Haar_integral

Student404Mus

I didn't hear about this before. Let me check it out

Semiclassical

in 3D, i think it comes down to saying that $d\Omega = r^2 \sin\theta ~d\phi\,d\theta$ is invariant under rotations

e.g., if you were to rotate your coordinate system and pick $r,\theta,\phi$ according to that system, you'd still have $d\Omega=r^2 \sin\theta \,d\phi\,d\theta$

Student404Mus

indeed.

this invariance applies to rotations only. Right?

Semiclassical

hmmm

should, yes.

if you do reflections, then there's probably the possibility of minus signs etc

Student404Mus

i.e., cannot be applied to Cartesian coords.

Semiclassical

7:54 PM

right. for cartesian coordinates, $dxdydz$ is invariant under translations

Student404Mus

Good.

Semiclassical

i'm sure i've proven this at one point or another

(proven that $d\Omega \to d\Omega$ if $\vec{r}\to R\vec{r}$, i mean. the invariance of $dx\,dy\,dz$ under translations is trivial)

Student404Mus

Besides the proof. Is it true that $d(R_0R) = R_0d(R)$? If so, how can we reconcile it with the invariance of element of integration under constant rotations?

Semiclassical

no, i don't think it is

that would be true if $R$ were just a variable

but $dR$ is really shorthand

the correct version is more like: $d(R_0 R)=\det\left(\frac{\partial(R_0 R)}{\partial R}\right)dR$

(jacobian)

Student404Mus

Ah!

the determinant

Semiclassical

8:04 PM

and so (I hope) the jacobian is just $\partial (R_0 R)/\partial R = R_0$

Student404Mus

the determinant should equal to unity

Semiclassical

right.

one has to be a bit careful with this

i forget how matrix derivatives work

ah, here: math.stackexchange.com/questions/19518/…

Student404Mus

According to the first answer of that post, he didn't argue why it equals 1

I think this is a property of groups of rotation

Semiclassical

the point in my book is that (proper) rotation matrices always have determinant 1

Student404Mus

yes.

If I'm not mistaken we saw that in group theory class.

Semiclassical

8:15 PM

right

Student404Mus

Can we say the same thing about the invariance of infinitesimal rotations?

I mean does the same trick work on infinitesimal rotations?

Semiclassical

probably, yes? I think that goes from being a statement about determinants to a statement about traces

e.g. $\det(I+\epsilon A) = 1+\epsilon (\text{tr} A)+O(\epsilon^2)$

so this probably goes to the fact that the generators of rotation matrices are all traceless

Student404Mus

I see.

Semiclassical

8:31 PM

(as in: for an infinitesimal rotation, we have $R=e^{\epsilon A} = 1+\epsilon A+O(\epsilon^2)$. Hence $\det R = 1+\epsilon (\text{tr} A)+O(\epsilon^2)$, and therefore $\det R=1$ for all $\epsilon$ requires $\text{tr} A=0$. So the generator $A$ must have zero trace.)

Student404Mus

you mean, $det R = \exp (\epsilon tr(A))$

Semiclassical

yeah, that works

(that's the exact version.)

Student404Mus

cool

Semiclassical

it's amazing how useful $\ln \det = \tr \ln $ is

Student404Mus

indeed.

Semiclassical

8:37 PM

I will never understand why \tr isn’t a default latex command like \det is

Student404Mus

maybe it's not defined

Aaron Stevens

9:31 PM

0

A: How does the lost mechanical energy transforms to a heat?

First of all, you need to know that any external force can change the mechanical energy of a system. The proof of : $$\Delta E_m=W=-Q$$ Maybe you can use the first law of thermodynamics, it can be helpful !! It's given by the following relation : $$\Delta U=W+Q$$ Note that : $\Delta U= U_{fi...

Maybe I am missing something, but I am confused by the use of $\Delta U=0$ here

Aaron Stevens

10:11 PM

Their comments to me make even less sense

insomniac

10:30 PM

@AaronStevens : what is missing is the (physically reasonable) assumption that the body is initially and finally in equilibrium with the surroundings. ie, the initial T,P = final T,P, hence, initial U = final U. For such a system, we may indeed conclude that the net work done by friction is equal to the total heat dissipated, But I agree, the comments don't make sense

JMac

Yeah that answer is confusing. It doesn't explain terms or really explain why it's doing what it is with them. It seems like the type of answer that only makes sense if you already know exactly what they are talking about.

insomniac

@JMac : well, its "high school physics". We all at least know how physical arguments are packaged to us when we are in high school. Totally incoherently ;) (personal experience)

JMac

@insomniac I actually had an okay experience with HS physics. Practically all the math and science teachers at my school were actually pretty passionate about the subjects, thankfully. I actually felt like I came out with a pretty good grasp on the basics.

My grade 11 advanced math teacher didn't understand rounding though, so I guess it all evened out.

insomniac

10:50 PM

Lol. I had a few good ones too, now that I think about it. for some reason, more good math teachers than physics teachers. I think chemistry was the worst for me personally. All the chemistry teachers I had in HS seemed to encourage mindless cramming. No wonder I ran as far away from chemistry as soon as I had the chance.

Its all very subjective though. A friend who is now a chemist credits a chemistry teacher we both had, so..

JMac

I had one really bad math teacher, one really good one, and two pretty good ones. Practically all the science teachers I had were into their subjects. My biggest gripe with a science teacher was that my grade 11 chemistry teacher was more hardcore than I would have wanted. We only realized when we graduated and checked our transcripts that it actually counted as an IB course.

Aaron Stevens

@insomniac Yeah, maybe that is what they meant

@JMac Yeah they just say $\Delta U=0$, and acted confused when I asked about why that assumption was made

Sir Cumference

11:22 PM

anyone else play piano and notice that they aren't making much progress after practicing throughout a day, but when they come back the next day they're suddenly much better at the part?

super weird how that works

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