The h Bar

8:55 AM

I try to understand if the "particle on the ring" is the same as the QM problem of a particle in 2D space with a say (negative) delta-type potential ($V(\vec{r}) = -\delta(r - R_0)$ that forces the particle on a ring of radius $R_0$. Not sure if the question is clear?!

One can read that "particle on the ring" problem as if one would simply use the particle in the box with its solutions and just "bend" the 1D linear space to become a ring. Of course it also could be (thats what I guess) that the two things are equivalent, in this particular case. But I cannot see immediately why that should be the case.

maybe you could eplain it, @EmilioPisanty ?

Archer

9:14 AM

Could anyone please explain the difference? '

Emilio Pisanty

9:27 AM

@Rudi_Birnbaum no, the problem is not equivalent

if the angular energy becomes too large, the particle can leak out of the radial barrier

it's only equivalent in the limit where the delta function is much deeper than the angular energies you're interested in.

AnOrAn

@JohnRennie ok, I copy and paste: So when we measure the 3-curvature of the universe we are measuring something that is valid only for a certain set of coordinates, comoving coordinates, then do you know what's the point of doing that? I mean a closed universe for us may be open for a non comoving observer right?

John Rennie

@AnOrAn hi :-)

When we derive the Freidmann equations there is a natural choice of coordinates i.e. the comoving coordinates and we almost always work in these coordinates. They are a natural choice because then every comoving observer agrees on the time.

And given this choice there is an obvious meaning the spatial curvature. It's certainly true that non comoving observers will differ about the choice of time and space axes, though I'm not sure a non-comoving observer would every observe a spatially closed universe to be open.

Rudi_Birnbaum

@EmilioPisanty Thank you! Yes that makes sense :-)!

AnOrAn

9:42 AM

@JohnRennie About this, I think it could, for example a deSitter spacetime can be described both by flat (the usual textbook exaple), closed and open space. I just read this in 1.3.6 of Mukhanov where he use the 3 different 3-metric to describe the same spacetime in different coordinates

John Rennie

I've got a copy of Mukhanov somewhere, though I won't have time to dig it out today.

Rudi_Birnbaum

@EmilioPisanty another question (maybe for you its easy): When I add a constant homogeneous mag field with potential like $A=(1/2) R_0 B (-y,x,0)$. What will be the solutions of the particle on the ring?

AnOrAn

But anyway when we say something is a solution of the Friedmann equation, it's automatically in comoving coordinates right? We've already made the choice of coordinates before deriving the equation I think

John Rennie

True ...

AnOrAn

Ok then, if you'll remember maybe you can let me know what you think after you read that chapter of Mukhanov. In the last paragraph of it, he derives that the results I told you about as solutions of the Friedmann equations, therefore I think all 3 equivalent results are in comoving coordinates. But the 3 results have different spacial curvature, therefore disagree on that despite being all comoving. Maybe I'm missing something, I'm pretty slow these days :)

It all started from the new paper about the "evidence" from Planck data that the Universe is closed..

John Rennie

9:54 AM

@AnOrAn well the CMB is as close to a comoving system that we can get ...

I've seen the media reports about the latest findings, though I haven't read the original publications. I'm not ready to conclude the universe is closed just yet since it isn't clear the data is good enough for a deginitive statement.

AnOrAn

Unless you have free access to Nature the ArXive paper is here if you want to read it: https://arxiv.org/abs/1911.02087 . I didn't cause I don't get it so :)
But from my point of view any conclusion about the 3-curvature of the Universe, being it flat open or closed, can be criticized with the above arguments. At least until I get a better grasp of the subject :)

John Rennie

Any paper that has the word crisis in its title needs to be taken with some skepticism :-)

2

AnOrAn

@JohnRennie I agree, but that skepticism gets mitigated a bit when you read that Silk is one of the authors :)

Slereah

11:48 AM

@JohnRennie Is Einstein under threat again

Emilio Pisanty

11

Q: The position-representation matrix elements of the propagator for a particle in a ring

I have a question about obtaining matrix elements of time evolution operator. I have the following Hamiltonian for a particle in a ring with magnetic field $$H=\dfrac {\hbar ^{2}} {2mR^{2}}\left[ -i\dfrac {\partial } {\partial \theta}+\dfrac {\phi e} {h}\right] ^{2}$$ and since $\left[ H,P\righ...

has magnetic fields

John Rennie

@Slereah no :-) No-one is arguing about the FLRW metric. People are just arguing about the contents of the stress-energy tensor.

Slereah

I have the stress energy of the universe in a textfile somewhere

it's $10^{60}$ GB

Physics Meta

1:13 PM

0

Q: Would a site on the interdisciplinary topic of climate change in StackExchange be supported?

The topic of climate change is an exemplary case of an interdisciplinary topic. There is not one single scientific discipline of science working on it, but a wide spectrum. It primarily includes meteorologists, physicists, sociologists, psychologists, statisticians and researchers of media studie...

discussion scope

yuvraj singh

1:54 PM

Do anyone suggest me which book should I refer for classical mechanics, and analytical mechanics at ug level.

Ezze

Goldstein, maybe?

Aaron Stevens

2:11 PM

Is there something I am not aware of in different parts of the world where the English word for a car tire is spelled "tyre"? I feel like I see (and edit) a question or an answer with this spelling every once and a while one here.

Ezze

@AaronStevens I mean isn't tyre the correct English spelling?

Oh, it definitely is: dictionary.cambridge.org/dictionary/english/tyre

Aaron Stevens

@Ezze My computer always underlines it in red. Is it a UK thing?

Ah yes, it is

Ezze

Its the same as color vs colour

Aaron Stevens

Odd that I have never heard of that

Abhas Kumar Sinha

2:31 PM

we use tyre here...

Aaron Stevens

@AbhasKumarSinha Thanks, got it figured out

Abhas Kumar Sinha

@AaronStevens :)

yuvraj singh

3:36 PM

@AaronStevens can you help me.

Aaron Stevens

@yuvrajsingh Depends on what you need help with

yuvraj singh

Can you refer me book on the given topics

Like quantum optics and photnics for ug level

@AaronStevens

Aaron Stevens

@yuvrajsingh I am not familiar with books for those topics. Sorry

yuvraj singh

On analytical mechanics @AaronStevens

Ezze

@yuvrajsingh you already decided you don't like Goldstein?

yuvraj singh

3:45 PM

Sorry @Ezze I haven, t seen your comment, actually I am not familiar with book.

Is it good for ug level

Ezze

Well, ug level can mean a lot of things

Also Goldstein is pretty old, however, classical mechanics doesnt change that fast nowadays

I quite like the book to be honest

yuvraj singh

Thank I will mark your opinion.

Aaron Stevens

@yuvrajsingh For undergrad I used Taylor for classical mechanics

yuvraj singh

OK have to you heard about David j Griffith book on electrodynamics @Ezze @AaronStevens

Ezze

Griffiths electrodynamics is pretty good IMO

Aaron Stevens

3:50 PM

@yuvrajsingh It is pretty much the main book everyone uses haha

Ezze

Our university had a book compiled to our class and we didnt use a textbook for electro

Actually we didnt use textbooks for a lot of classes

yuvraj singh

OK both of you can refer me one more book on inertial and non inertial frames, transformation of velocity and displacement in different frames.

@Ezze @AaronStevens

Ezze

@yuvrajsingh that should be in any ug mechanics book

yuvraj singh

Here we have so much non sense authors who just published the book irrelevant, can you refer one.

ZeroTheHero

@AaronStevens I want to take a few moments to discuss your HNQ post but I don't want to do this as an answer.

Ezze

3:54 PM

@yuvrajsingh you already got two suggestions, Taylor and Goldstein, choose any of those, it should contain a discussion about inertial frames

Aaron Stevens

@ZeroTheHero Aren't answers intended to be part of a discussion? I don't think it necessarily has to be an actual answer on meta for you to post as an answer

ZeroTheHero

Overall I think they are useful but what is bad is the way they are selected. I realize this is too much to ask for but my bone is with the way they make it there.

it is automated.

yuvraj singh

@Ezze do you know Ajoy ghatak modern physics.

ZeroTheHero

so my non-constructive criticism is that this selection process ought to be changed.

Aaron Stevens

@ZeroTheHero Ahok. Yes that makes sense

ZeroTheHero

3:59 PM

Ok 'have to go.

Be well.

Aaron Stevens

@ZeroTheHero Thanks for the input

Abhas Kumar Sinha

@yuvrajsingh If you need a rigorous (mathematical) book, then no book comes even close to - Landau and Lifshitz. (both Quantum Mechanics - 3rd and Classical Mechanics (lagrangian and hamiltonian, no Newton's ). If you need some easy book then go to Griffiths. I believe Goldstein is good for QM and good for beginners but not good as L&L.

Ezze

4:17 PM

L&L is definitely not a first book

Abhas Kumar Sinha

@Ezze I started with it when I was of 11th grade, and yes it was by first book.

Aaron Stevens

@Ezze I'll second that

yuvraj singh

Why.

Abhas Kumar Sinha

I completed the classical volume of L&L vol - 1 and I'm heading to QM vol - 3

@yuvrajsingh usually harder language and very advanced mathematical manupulation

Ezze

also I'd like to add that the English translation of L&L is horrible

the book is hard to understand as it is, but it is not well translated either

yuvraj singh

4:20 PM

Ooo poops.

Abhas Kumar Sinha

you'll have a lot, a lot of problems when starting with L&L. I remember the first time, it took me 4 days to understand the first chapter, deriving the Lagrangian equations of motion. But, thanks for the patience of @bolbteppa @ACuriousMind @JohnRennie I've completed that.. :) It'll take time, but you'll get along with it easily after some time...

bolbteppa

@AbhasKumarSinha did you read the whole book

Abhas Kumar Sinha

@bolbteppa Yes sir and completed too... :)

@bolbteppa What you are learning these days?

bolbteppa

Some very hard parts of that book

yuvraj singh

So should I go on with the book.

Abhas Kumar Sinha

4:23 PM

@bolbteppa I used V Arnold too as reference

@yuvrajsingh I can't say anything for sure, depends on what you want, mean. :( You can try, if you have some doubt, you can tag me here, I think I've remembered most of the book and i'll help... :)

yuvraj singh

Great. Actually soon we will get the book.

bolbteppa

I am trying to understand AdS/CFT

Abhas Kumar Sinha

@yuvrajsingh archive.org/details/Mechanics_541

yuvraj singh

Thnx.

Let me bring @JohnRennie too.

Abhas Kumar Sinha

@bolbteppa I'll learn GR, Tensor Analysis (to master that area), statistical mechanics (until I master it), then QM and QFT (QFT is really hard!! :( )

@yuvrajsingh JR can help you with GR... He likes that topic

bolbteppa

4:27 PM

@AbhasKumarSinha vol. 2 to 5 ;)

Abhas Kumar Sinha

@bolbteppa ah ;)

bolbteppa

Tensors are explained in vol. 2

yuvraj singh

@AbhasKumarSinha I amaze some time I think you are preparing for jee, and you have boards +jee main+jee advanced.

@AbhasKumarSinha I am amazed some time I think you are preparing for jee, and you have boards +jee main+jee advanced.

Abhas Kumar Sinha

@yuvrajsingh I'm preparing for JEE (it's hard actually), my weak part is chemistry and for JEE you've to balance all three subjects. I dun like Chem most of the time, but I still have to waste my time swallowing a lot of things, I dun understand :(

John Rennie

@yuvrajsingh hi

yuvraj singh

4:32 PM

@JohnRennie as abhas mentioned, please be with us during the period we read GR. What you think about L and l

John Rennie

Landau & Lifshitz is a hard book. It's useful when you already know the basics and want to hone your skills, but it would be a tough way to learn from scratch.

Abhas Kumar Sinha

@bolbteppa Do you have some good book (rigorous mathematically) for celestial mechanics?

John Rennie

I'm not sure learning GR at this stage is an ideal move. If you're really interested and want to learn it for that reason then great, but it will take a lot of time and I get the impression the college doesn't leave you with that much free time.

yuvraj singh

, that, s correct, but I am just 18,and when I see the physics it just attracting me more, and make me more curious. @JohnRennie

Abhas Kumar Sinha

@yuvrajsingh I understand that feeling much better

Sometimes I rage my emotions to such an extent that I'll master theoretical physics better than any other individual of the world, will study 17+ hours a day, will do this, that, fantasies and I start it doing just after that :)

yuvraj singh

4:41 PM

@AbhasKumarSinha why you are wasting for iit, buddy go for iisc.

Abhas Kumar Sinha

@yuvrajsingh I'd have to score decent (in JEE Mains) for IISC, I think..?

I'll be back after dinner...

yuvraj singh

Where are in India only naa, dinner at 10 surprising

bolbteppa

Not sure about celestial

Abhas Kumar Sinha

@yuvrajsingh lol... I live with my family :) I've to take care of time,

btw I go to sleep at 2 AM

yuvraj singh

Ooo ooo.

Abhas Kumar Sinha

4:52 PM

@bolbteppa was thinking to start with Raybov...

barlop

5:19 PM

If a car goes at 100mph and doesn't accelerate, then F=0. So what do we use measuring force for?

And also, if a car goes at 100mph having not accelerated for a whlie, and hits me, was that force = 0 ?

JMac

@barlop What's the cars speed after it's done hitting you? Probably not 100 mph, right?

barlop

5:35 PM

@JMac good point

@JMac so what is more relevant in terms of impact when hit by a car. The force, or the momentum? And what effect does each have?

JMac

@barlop Depends on what you want to figure out. The two are very related. As per Newton's second law, Force is the rate change of momentum with respect to time. Second law. If you want final velocities, momentum probably makes more sense. If you're trying to figure out if things will break, forces are a start; but then you also have to look at impact mechanics.

barlop

@JMac so putting aside iif things will break.. which makes more sense in a sport like tenns or badmiinton.. when hitting the object?

what is the difference in effect between the force of the racket, and the momentum of the racket, on the shuttle or on a tennis ball?

schn

Was the double slit experiment ever carried out with 'classical' particles? These are images from some lecture notes, and quantum particle are defined as electrons, neutrons, photons, etc., but it doesn't say what 'classical' particles are. Could it simply be point masses?

JMac

@barlop Biggest difference would be how you set up the math. If you set it up and solve properly, either one would get you the correct answer. Off the top of my head, I don't think you would want to use momentum. Momentum conservation is really complicated in that case, because momentum is transferred from your body, which is actually coupled with the ground, so figuring out conservation of momentum is over-complicated.

barlop

5:51 PM

A) Would it be correct to say that force from the racket is imparted to the ball?

and B) Would it be correct to say momentum from the racket is imparted to the ball?

Emilio Pisanty

@schn that's an innacurate picture of the classical behaviour

You will only see interference if the individual classical lumps (R3 and R4) have substantial overlap, giving a classical distribution that looks like D0

> Could it simply be point masses?

have you got real-life point masses in your lab? then sure.

JMac

@barlop I would be fine saying momentum was imparted, because momentum is conserved in an isolated system, so you can look at it as the racket giving some of it's momentum to the ball. I wouldn't say the force is imparted. I don't like the word in that context. The force of the racket acts on the ball, but the force doesn't really go from one to the other, there's an equal and opposite force acting on the racket (which is basically how it imparts the momentum).

schn

@EmilioPisanty What is the definition of a 'classical particle'?

JMac

@EmilioPisanty Do you know any good places to buy point masses? I heard all about them when I started physics, but I haven't seen them sold anywhere.

barlop

6:08 PM

@JMac Thanks.. Is it possible that a racket moving at high acceleration and low speed, would hit a ball faster, than a racket moving at no acceleration and low speed?

assuming the racket is the same speed when it hits the ball

schn

@EmilioPisanty With what are R03 and R04 produced?

JMac

@barlop The accelerating racket's momentum still increases during impact, because it's velocity is increasing. The constant velocity one should have constant momentum. If the impact started with the same velocity, then the accelerating racket would impart more momentum, because it had extra momentum added to it while the ball was in contact. I'm not sure how much more that would be in practice though.

Aaron Stevens

6:37 PM

Up votes on questions are going to be work 10 rep points now instead of 5

ACuriousMind

Thanks, I hate it.

4

Aaron Stevens

@ACuriousMind The 10 points for question up votes?

ACuriousMind

@AaronStevens Yup. I think good questions are much rarer than the amount of positively scored questions suggests. The reasoning in the blog post doesn't work because many of the people voting don't know how to write a good question either.

And while there are questions that require skill to ask, many popular questions don't, really - they just require the OP to be the first one to formulate it in a somewhat intellegible manner. They're popular because many users have the very same question, after all!

In essence, I still support the reasoning that led to the original split and I don't think anything has changed (except leadership at SE, apparently :P)

JMac

I'm sure my 368:0 Q:A ratio has me pretty biased; but I'm definitely not a fan of the change. Answers being worth more than questions always made sense to me.

ACuriousMind

@JMac Don't you mean A:Q ration? :P

JMac

6:47 PM

@ACuriousMind Yes. I wrote it opposite the first time too, and in correcting myself I over-compensated, double-negative style.

ACuriousMind

Guess you shouldn't not have done that

Aaron Stevens

I just don't see how they are going to reward people based on the change. How are they going to take into account if you would haven hit the max rep for the day based on question votes?

JMac

@AaronStevens ... That's a good question. Are they back-tracking all that, or will people essentially skirt the cap that way?

ACuriousMind

@AaronStevens Since the timelines record the amount of votes at the end of every day it changed, the data is technically there to recalculate that faithfully

Aaron Stevens

@ACuriousMind Ah ok. Hopefully that is taken into account

Well I will get 15 extra rep points. Yay for me

ACuriousMind

6:50 PM

But of course I have no idea if they're going to do that - might be too expensive to calculate in a reasonable time, I dunno

Aaron Stevens

Why not just make it different from now on and not compensate for the past? Certainly when they made the change before they didn't lower people's reputation points?

JMac

It might potentially offend someone speaking on behalf of an oppressed group that they don't even belong to. We wouldn't want that.

Sorry I'm just salty because IDK what SE has been thinking lately.

Aaron Stevens

Its cool to be offended. Everyone is doing it

ACuriousMind

@AaronStevens They did, actually: stackoverflow.blog/2010/03/19/important-reputation-rule-changes says the change also applied retroactively to all users back then

Aaron Stevens

@ACuriousMind Ah ok. Interesting

ACuriousMind

6:56 PM

Also, Jon says the recalc handles the rep cap correctly retroactively

Aaron Stevens

@ACuriousMind Ah ok. Well then it seems like everything after the decision is being handled fine

But yeah... I can see why questions shouldn't be worth as much as answers.

ACuriousMind

Yeah, I disagree with the decision itself but, for once, I have no concerns over its implementation.

What miffs me in particular is that this means that where 1 downvote needed 2.5 upvotes to offset before, it now needs 5. That means lots of low-net-scored contested questions suddenly give more rep, too.

6

And I already thought that downvotes should be more balanced with upvotes before.

What next, reintroducing the 1-rep cost for downvotes on questions?

4

DanielSank

7:25 PM

I came here to complain, but I can see that @ACuriousMind is already handling it.

3

Thanks!

ACuriousMind

You're...welcome?

DanielSank

:-D

How's life?

ACuriousMind

Interesting. My team at work got, uh, disbanded due to shifting priorities and I'll have to get used to work in a different one.

DanielSank

So what does that mean in terms your daily work?

Are you now on the pizza-making team?

ACuriousMind

@DanielSank No, but that would be awesome!

DanielSank

7:30 PM

Hmmmm

So what team are you on?

ACuriousMind

I'll actually do pretty much the same as before, but branded under a more general "quality tools" umbrella than the more specific "static code analysis".

DanielSank

Interesting. You work on an analysis tool?

ACuriousMind

Yes, a driver framework for all sorts of analyses on development artifacts in SAP's ABAP environment as well as some specific linters for ABAP code itself.

DanielSank

@ACuriousMind ABAP?

ACuriousMind

@DanielSank SAP's proprietary programming language and the origin of a lot of COBOL jokes in this room (search the transcript for COBOL)

DanielSank

7:40 PM

uh oh

Yes I've heard a lot of not good about COBOL.

ACuriousMind

It's not COBOL, it's what you get when you continually mutate something like COBOL in a backward-compatible fashion ;P

DanielSank

egad

ACuriousMind

Yeah, "pretty" is not the first word that comes to mind. But it does its job.

And how's life in octopus land?

Loong

heh

61

A: Should the weight of question upvotes be reduced?

I'm fine with that. Something that's bugged me for a while is that while I don't mind helping out by editing poorly-asked questions, it seems wrong that some users come to rely on this, coasting through asking one lousy question after another, occasionally garnering significant amounts of reputat...

DanielSank

@ACuriousMind Good enough.

Emilio Pisanty

7:56 PM

@ACuriousMind 'ration'?

ACuriousMind

@EmilioPisanty I like me some delicious answers!

Emilio Pisanty

@DanielSank COBOL

Despite occasional attempts in this chatroom to deny that equality

Huh

On the plus side of the change, I'm now 10k+ on the Mother Meta

Now I can go and ogle at the trainwreck that was the most downvoted question in MSE history =P

ACuriousMind

What, meta.SE didn't already have enough trainwrecks for you to see?

Aaron Stevens

@ACuriousMind No, you get +2 rep for down votes because hey, at least you tried, right?

ACuriousMind

8:13 PM

@AaronStevens That would increase my rep by more than 50%, but I'd still hate it!

Aaron Stevens

@ACuriousMind Ah I was meaning if someone downvotes your post you get +2 rep

ACuriousMind

Oh :(

Aaron Stevens

@ACuriousMind I actually would prefer + rep for giving down votes though.

Maybe it would encourage people to give them when they should

ACuriousMind

Nah, I think the "questions free, answers cost a little" is just right. Otherwise you start incentivizing people to downvote all other answers to a question they have answered by default.

Aaron Stevens

@ACuriousMind Yeah I know. (Mostly) Kidding :)

I only down vote all other answers on a question when mine is obviously better

:p

Jk I don't do that

When is the new policy going to be put in place?

ACuriousMind

8:18 PM

Dunno, I guess you'll know when you see that fat +15 in your inbox ;)

Aaron Stevens

@ACuriousMind I am going to view it as 15 free downvotes

ACuriousMind

That's the spirit!

Aaron Stevens

In line with your vote ratio analysis earlier, if they are going to double up votes they should double down votes as well

ACuriousMind

@AaronStevens That'd then be: Upvotes: +10 on questions/answers, Downvotes: -4 on questions, -2 on answers. I don't really want downvotes to be more effective on questions than on answers, either.

Aaron Stevens

@ACuriousMind Yeah I suppose that's true.... Oh well

@BenCrowell Someone is fudging your identity. — Bill N 2 hours ago

lol

ACuriousMind

8:35 PM

Oh, come on, Cren?!

Novalium Company

8:47 PM

Since businesses buy resources to produce goods and services from the resource market, doesn't that make other businesses part of the resource market? So when I buy plastic from another business to produce toys, I'm actually purchasing from the resource market (i.e the business that produces plastic)?

I know, I know, this chatroom is the perfect place to ask this question.

Semiclassical

@ACuriousMind I can’t figure out which pun is better for that: cobbled (together) or COBOLD

TanMath

Hello all!

I don't know if any of you remember me, but I used to be really active on SE like 3 years ago.

Have any of you read "Road to Reality" by Penrose?

Emilio Pisanty

@ACuriousMind apparently not. But it did provide meta.stackexchange.com/questions/337757/… and the mighty puzzle of meta.stackexchange.com/questions/337604/redacted-private-info lately

Was the original announcement on Teams really that bad?

I mean, are the quotes accurate?

Because boy are they a bucketload of stupid

ACuriousMind

9:04 PM

@EmilioPisanty ...I do not have 10k on meta and have no idea what you're talking about!

Emilio Pisanty

> (population) ask about 18% more questions on Stack Overflow than (other population)

Aaron Stevens

@ACuriousMind I prefer Crentist

TanMath

I wonder if you guys have any resources for better understanding one-forms and p-forms? I guess I have some understanding of it, but I would like to improve my geometrical intuition of it.

Also, any resources on Clifford and Grasmann algebras? Those sections in Road to Reality were a little confusing

ACuriousMind

@EmilioPisanty I'm still guessing (I haven't really engaged with non-physics.SE related SE issues at all in the last few weeks), but if you're asking about something from the Team for moderators, I won't comment on that because it's supposed to be private.

Emilio Pisanty

9:34 PM

@ACuriousMind ah, our mood team is too good at resisting temptation

Transcript for