The h Bar

12:22 AM

@JMac I hit the wrong button on at least one of your flags... sorry about that. Thanks for flagging and I did take care of it though!

JMac

Attention everyone. I'm starting a petition to remove the tyrannical moderator tpg2114, who has abused his power to maliciously decline my flags.

@tpg2114 Thanks.

tpg2114

I <3 censorship

JMac

That's why we're all here, right?

tpg2114

Well, that and the physics. But mostly that.

</jokes> lest anybody later be confused.

JMac

I think we all know this platform exists to uphold the mainstream dictatorship™. /s

I don't know why but ever since I saw someone call the site that it's stuck with me as like the funniest thing.

tpg2114

12:27 AM

It has always struck me as funny how upset people get about a place they choose to go, moderated by people who chose to volunteer. Like... we're not an essential service, or a fundamental human right

I've used the analogy before, but I can't figure out what motivates some people to come into somebody else's sandbox and then angrily tell them they're playing in it all wrong. It's an interesting problem to tackle though

JMac

And like they don't even try to understand the platform and why it works like it does. When it doesn't work how they expect, you often get a lot of crazy assumptions about why, or treated as if you personally attacked them.

tpg2114

I was thinking about this when I was out riding today... I wonder how we can address the issue people have with "one rule for all" or "one approach for all". Ideally, we need an "N rule for M", where N << M, otherwise it's just not practical

And I wonder sometimes if people who are really unhappy about things work have suggestions on what rule(s) would optimally satisfy that constraint. The problem is, when people are already really unhappy, it's hard to engage in a "Please tell me what you think would fix this, knowing we probably won't do it" conversation

It's like doing an exit interview right after somebody is fired.

JMac

Yeah it's hard to say "you're free to suggest something else, but here are a million times it's been talked about already on this site" without discouraging people too. I'm sure it's a bit overwhelming if that's part of your first experiences.

But at the same time the site can't just contort to allow new users to get around established rules. It's tough all around.

tpg2114

I feel like there's probably a study out there that looks at learning curve vs. time for various things. When you're a late comer, there's so much to learn and so much history to know before you can meaningfully participate or affect change

And the effort to learn all of that is probably too high for a lot of people. To make things worse, when the effort to learn all of that exceeds the effort to learn whatever you're having issues learning, it's like extra insulting.

JMac

Yeah. I can emphasize with people for that. For me I felt I integrated fine, because I basically came to the site out of physics interest just to read some questions and answers. I didn't get directed here because I had a question; so I had time to get a bit of a grasp on the rules and culture before I started posting.

tpg2114

12:38 AM

I can sympathize for sure. If you're beating your head on the wall over a homework problem and you're desperate for any help at all, and you end up here and ask it... then you're told you need to go read long posts, and learn a bunch of reasons why what you did (i.e. asking) is wrong

It's like kicking them when they are down, and they're still no closer to figuring out why they can't learn the answer they need to learn

I actually don't remember how I ended up on this site

JMac

I first actually read some stuff on engineering SE and that's how I stumbled into the network.

tpg2114

It looks like I joined 2 days before I answered my first question (wow, terrible answer too) back in Dec 2011

And only a few days after I joined SO

Must have been poking through the list of sites

Yuvraj Singh...

1:40 AM

Is our solar system is stable?

I mean does a star get affected due to it, s own gravity?

In a solar system.

perilousGourd

2:12 AM

How do you all figure out when it's appropriate to give up trying to find the answer to a question independently and post it here? This says to do 'anything' else I can think of first: I could think about the question for a year, read every relevant textbook, and probably find the answer myself eventually, but this might defeat the purpose of a Q&A site?

Charlie

I'd say at an absolute least make sure your question isn't answerable with a google search

perilousGourd

Any rules of thumb for how much time should be invested looking independently, or level of frustration reached, etc, before asking is acceptable?

How long spent on google

Charlie

Most people don't appreciate having to recite the first paragraph of wikipedia for instance

If you have a conceptual question that you can't find an answer to you might as well just ask it

Try and make sure it's not a duplicate of something easily findable on the site too

perilousGourd

?* (accidental early enter, sorry.) If I googled for days, I'd probably eventually figure out my misunderstandings. Would that be too little time to search before asking?

Definitely makes sense, re Wikipedia

Charlie

what is the topic?

perilousGourd

2:17 AM

I don't have a specific question at the moment; I've just been wondering this since I joined, basically, years ago.

Charlie

You don't need to have a timer next to you recording how much time you've spend researching, if you can't find a satisfying answer online and you've made sure you're not submitting a duplicate it's probably fine to just go ahead and ask it

Worst case it gets closed, multiple questions are closed each day because they are just someone posting a picture of their textbook asking how to do a question

perilousGourd

Alright. I think the italicized 'anything' in the FAQ and the... spiciness with which I've seen some questions closed has made me over-anxious

Thanks ':)

Yuvraj Singh...

2:53 AM

@PM2Ring hi

ZeroTheHero

@perilousGourd I think I must have carefully typeset about 15 questions. I put them in, then look at the suggested links or closely related questions, and then I realize that there is already quite a bit on thetopicpiI never post the question.

so I don’t think measuring in length of time is a good metric.

PM 2Ring

@perilousGourd Seriously, if you spend 15 minutes to half an hour googling relevant key phrases, and then spend some time trying to understand the Wikipedia articles (& other stuff) that Google throws up, you're doing better than 90% of the askers whose questions get closed.

And if we do end up close voting your question it doesn't mean we think you're bad or stupid, it's just that we don't think your question is appropriate for this site. And if you do do that research, and mention what you've found and done so far, people are likely to give you good feedback in comments that will help you make your question a better fit for the site, and make it more likely for it to get appropriate answers.

Hi, @Yuvraj.

@YuvrajSingh... Sort of. The planet orbits are fairly stable. But over a very long time span, chaotic things are possible, making it hard to predict exactly what the orbits will be like many millions of years in the future, or what they were like many millions of years ago. And because we don't know the positions & momenta of all the planets & moons to unlimited precision, the errors in our predictions grow over time.

We have incredibly good data for our own Moon, down to a few centimetres, through decades of lunar laser retroreflector measurements. So we know how that orbit will evolve for many millions of years. However, we cannot say exactly where on that orbit the Moon will be located at a given time in 26 million years. Actually, that may have improved a little in recent years, it's been a while since I studied the long-term stability of the solar system.

This topic has a long & rich history. One European king even offered a substantial cash prize for a mathetical proof of the stability of the solar system. Not surprisingly, there is a large Wikipedia article on this topic: en.wikipedia.org/wiki/Stability_of_the_Solar_System

@YuvrajSingh... As for the gravitational stability of the Sun itself, that's a separate question. ;) A star is in a dynamic balance between the force of gravity, which is trying to compress it, and the fact that it's very hot, which wants to make it expand.

But it's more complicated than what you might expect from the basic physics of a hot gas because it's (mostly) plasma, with powerful magnetic fields. The study of such plasma is called magnetohydrodynamics. Also see:

Magnetohydrodynamics combines the intuitive nature of Maxwell's equations with the easy solvability of the Navier-Stokes equations. It's so straightforward physicists add "relativistic" or "quantum" just to keep it from getting boring.

:)

JMac

3:32 AM

I love the hover text for that one

PM 2Ring

Hover text: Magnetohydrodynamics combines the intuitive nature of Maxwell's equations with the easy solvability of the Navier-Stokes equations. It's so straightforward physicists add "relativistic" or "quantum" just to keep it from getting boring.

When people propose the idea of detailed computer simulations of the universe, à la the Simulation Hypothesis, my first thought is: "Have you even heard of magnetohydrodynamics?"

perilousGourd

4:03 AM

@ZeroTheHero I've done this a bunch of times as well! It's kind of interesting how the words I type trying to explain my question are so much more useful for the automatic search function than the seemingly most relevant keywords I pick to search beforehand by.

@ZeroTheHero If you're perfectionistic or anxious in mindset, though, having an upper bound to time/effort that should be spent prior to asking could be a useful sanity-check

@PM2Ring Reassuring! Thank you very much!

Yuvraj Singh...

@PM2Ring wow! Thanks sir.

You are always awesome.

PM 2Ring

@YuvrajSingh... No worries.

Yuvraj Singh...

But sir we got the answers from the Newton's formulas can we get these same result by einstein theory of general relativity? @PM2Ring

I have not read anything which says about stability of planet orbits.

PM 2Ring

4:20 AM

@YuvrajSingh... Yes. We get better results from general relativity, but the problem is that Einstein's equations are usually much harder to solve than Newton's. Fortunately, most of the time Newton's equations are good enough, and if we need a little more accuracy, we can apply a small relativistic correction to them.

The only time we need the full power of GR is when the gravity is really intense, like with black holes or neutron stars. Or when we are doing cosmology, where we need to model structures that cover a huge amount of space and time.

Yuvraj Singh...

@PM2Ring oh, sometimes I got confuses that who was correct Einstein or Newton about the gravity?

user434058

@AaronStevens That's right....FakeMod likes "fake analysis"

PM 2Ring

In Newton's gravity we can give exact analytical solutions (that us, we can write a straightforward equation) to any problem of 2 spherical bodies, like 2 stars orbiting each other, or a planet orbiting a star, or a moon orbiting a planet. But if there are 3 bodies, there is no general analytical solution. There are 3 body solutions for various simple combinations, but in general a 3 body system is chaotic, and we must use numerical merhods that give approximations.

user434058

@PM2Ring I put my focus on the need of intuitive explanation because I don't know QM :P

user434058

@Loong Thanks a lot for your edit :)

Yuvraj Singh...

4:27 AM

@PM2Ring OK sir.

PM 2Ring

However, in GR, there isn't a general solution for the 2 body problem. And over a long time span, a 2 body system that's perfectly stable in Newtonian physics can be chaotic in GR. This makes it very hard to analyze stuff like the orbits of 2 black holes around each other.

user434058

@AaronStevens Of course, no point of editing there, but I was just asking for my answers and I got the answer. Thanks!

PM 2Ring

@FakeMod That's ok, but the OP did ask for a QM explanation. ;) OTOH, a future reader may be perfectly satisfied with your answer.

But the problem with answering questions that are a little ambiguous is that you end up with a bunch of answers that are each answering slightly different things. That makes it hard for the OP & other readers to decide on how to vote for the various answers.

Yuvraj Singh...

@PM2Ring you know sir, question I had asked you today was asked by an interviewer to Stephen's hawking.

"that if Newton was alive what one question you would ask to him."

And he said that question.

PM 2Ring

Well, Newton didn't believe that the laws of gravitation & mechanics which he discovered / developed were sufficient to guarantee the stability of the solar system. He believed that it was probably necessary for God to intervene from time to time to prevent cosmic calamities from occurring.

Yuvraj Singh...

4:38 AM

Oh. If I ask you, to ask a question to Newton, what would be your question?

@PM2Ring

PM 2Ring

I'm not sure if I'd have a question for him. But I'd try to give him some advice. ;) Eg, don't sit on your results for years, publish them ASAP. Don't get into a pointless dispute with the Continental mathematicians about who invented calculus. Don't waste so much time on alchemy & mysticism.

Yuvraj Singh...

Lol!

PM 2Ring

Or maybe I'd just chat with him about designing sundials. :)

Yuvraj Singh...

I have a question for Newton, that what are his views on on Einstein theory of general relativity!

I would happy to see what Newton think about it!

PM 2Ring

He mostly grew up in his grandmother's house. He had a mirror on the windowsill of his bedroom, which reflected a spot of sunlight onto the ceiling. And he drew a giant sundial on the ceiling, so he could quite accurately tell the time from that spot of light.

Yuvraj Singh...

4:49 AM

Oh! Do you know some more stories about Newton? And his experiment?

PM 2Ring

@YuvrajSingh... That wouldn't be fair. He'd have to catch up on a few centuries of mathematics first, so that he could appreciate GR.

@YuvrajSingh... I know a few anecdotes, and various bits & pieces. But I'm getting old, and some of my old memories are a bit vague these days. ;)

Newton was a loner. He'd work for days at a time without leaving his house. He had a housekeeper who would bring him meals. Allegedly, when he died and people were going through the vast piles of paper in his room, they found a neglected meal or two buried under the paper. :)

Because he was a famous mathematician, the king made Newton the Master of the Royal Mint, a post he held for several years. He did make a few improvements, but what a waste of talent for one of the greatest mathematicians that ever lived!

David Z

@perilousGourd For what it's worth, you're right that was probably bad phrasing. It certainly doesn't mean you have to do everything else that is humanly possible to do to find the answer before asking here. What it's supposed to mean is that you should make an honest effort to check resources that are readily available to you, and you should do what someone who actually wants an answer to their question would do: check whatever you have, don't just look one place and then come ask here.

What we're trying to discourage is an attitude like, for example, "they need me to do prior research, so I'll look at a Wikipedia page, and then I will have done prior research and can ask a question"

Yuvraj Singh...

5:08 AM

@PM2Ring :-

PM 2Ring

But so many times, the OP doesn't mention anything about their research, and when a comment suggests a relevant Wikipedia page or an existing Physics.SE question, they say "Oh, I looked at that page, but it doesn't answer my question". And then you have to interrogate them to try & find out exactly what on that page they didn't understand...

David Z

Yeah, for sure. In those cases we can put it on hold as unclear or insufficient effort (or a duplicate, if it's another question here).

Anyway to finish what I was writing to @perilousGourd: the idea is you should make an honest effort to think of other places you might find the answer. Definitely try a couple web searches, think about relevant Wikipedia pages or pages on HyperPhysics, and if you have access to possibly relevant textbooks or papers, or people to ask, you can give those a try if you think there's a decent chance of finding your answer. But you don't actually have to do everything.

For some questions, 5 minutes of searching the web is all the prior research you need.

perilousGourd

@PM2Ring Maybe on the topic of wasted talent and vast piles of paper: Something I recently read (in The Western Intellectual Tradition) that really surprised me was that Leonardo da Vinci hardly 'produced' anything to completion in his life: '[he] left fewer than 20 paintings, not a whole statue, machine, or book, 5000 notes pages of notes and sketches which lay unread for 250 years'.

user434058

BTW if anyone was wondering about getting text in math mode with least effort, then $\rm d x$ works perfectly fine.

perilousGourd

Other interesting excerpts: he was commissioned to work on a painting by Pope Leo X, immediately began to create the varnish, and Leo X said 'This man will never do anything, for he begins to think of the end before the beginning'.

'And on page after page, [in later notebooks,] one phrase is scribbled: "Tell me if anything at all was done... Tell me if anything at all was done..."'

user434058

5:19 AM

\rm d x works perfectly fine.

user434058

Learnt that from QMechanic's answer.

Yuvraj Singh...

@PM2Ring do you know about Stephen weinberg?

perilousGourd

I'd had the impression from hearing about him in passing that he had been very prolific in output besides notes. Anyway, I found this kind of arresting!

Yuvraj Singh...

A physicist from Texas!

Sir Cumference

anyone else instinctively read "$\varphi$" aloud as "var phi" instead of "phi"?

PM 2Ring

5:26 AM

@YuvrajSingh... Not really. The name is familiar... Ah, Wikipedia says that Steven Weinberg is a Nobel laureate in Physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interaction between elementary particles.

user434058

@SirCumference Wait, that is called var phi, isn't it? ;P

Yuvraj Singh...

Yup. He shared some papers from Newton work

perilousGourd

@PM2Ring :53976098 , That seems reasonable and easily achievable, and is helping me develop an intuition for what is considered appropriate, so thank you. Is it appropriate to ask a question that has been caused by a readily accessible source (e.g. Wikipedia) being difficult to understand, probably due to lack of familiarity or background, when googling the question/s and keywords etc doesn't return anything that answers it?

Yuvraj Singh...

Which were not published.

perilousGourd

In this case, the source is readily accessible and probably easy to understand for people who have more background, but, as a newcomer who doesn't know what they don't know, figuring out the answer independently can be pretty difficult.

Yuvraj Singh...

5:35 AM

@PM2Ring

It was interesting to read them.

perilousGourd

that answers it for the person with the question, but would probably answer it for others*

user434058

Should I downvote a tag synomym suggestion if I want the proposed synonym to not be a synonym and instead be a umique separate tag?

PM 2Ring

@perilousGourd Sure! Wikipedia is reaching a stage where a lot of its science articles are becoming rather difficult to understand if you aren't already an expert in the topic. But if you ask such a question you need to give the readers enough info so they know what your level of understanding is so that they can frame their answer appropriately.

@FakeMod Yes. But maybe discuss it here first, to clarify potential misunderstandings.

And of course, when in doubt, don't vote!

perilousGourd

Okay, yay! Thank you for the clarification :D

I'll do that (describe level of understanding possessed and level of answer sought) in future

PM 2Ring

OTOH, Stack Exchange answers are supposed to benefit future readers as well as the OP, so a good answer may include material that's aimed at a higher (or lower) level, as well as material intended specifically for the OP.

Sir Cumference

5:46 AM

@FakeMod Yeah but I mean outside of latex context

user434058

@SirCumference I meant outside LaTeX context.

PM 2Ring

And of course, some topics may simply not be easily answerable at a certain level, so the answerer may need to educate you a little so you can grasp their answer. ;) Or they may not bother doing that if you ask an interesting question that's way out of your depth, but which the answerer happens to be an expert in.

Vivek

6:06 AM

the first orbit of hydrogen will have n=1, l=0, as the principle and orbital quantum nos.?

Yuvraj Singh...

@JohnRennie hi.

PM 2Ring

@Vivek Correct. And what does l=0 tell you about that orbital?

Vivek

well, it's ground state, so, l=0 indicates the orbital shape, i.e., it is round, having no ellipticity.

?

btw, what could be a better explanation to that - alkaline atoms don't have orbital (l) degeneracy?

PM 2Ring

@Vivek Yes. But it also tells you that the orbital angular momentum of that electron is zero. Which is very different to a classical orbit!

Vivek

6:22 AM

yup

well, if I write the Lagrangian for the central force, in the above case, the angular momentum remains conserved... even for elliptical orbits

how can it be zero? what does it signify? ...

Slereah

There's the fancy theorem for the holonomy formula

Although first you need the whole Chen paper showing that iterated integrals converge!

John Rennie

6:40 AM

@Vivek the electron in a hydrogen atom is not a little ball orbiting the nucleus like a planet orbits a star. It is delocalised over the whole atom i.e. it is more like a cloud spread out over the atom.

Vivek

@JohnRennie well

thanks for clearing it up, i clearified it more

much accurately, the meaning can be interpreted from the hydrogen wave function... 1,0,0 , meaning the wave, and the probability of finding the electron is irrespective of the the angular momentum

Novalium Company

0

Q: Can I use polymer clay to make a mold and then melt HDPE plastic in it in the oven?

Can I use polymer clay to make a mold and then melt HDPE plastic in it in the oven? My goal is to be able to dig a fairly uncomplicated shape in a polymer clay block and then fill that mold with small HDPE plastic filaments. I'm hoping that the melted plastic would fill the mold with the help of ...

Vivek

is there any android application for the chat room specific, well, I've downloaded stack app, and it redirects to site which I'm using in web browser

user434058

Should I leave comments like this 👇for the answers which I find great? Or would you delete such stuff.....

user434058

Great answer! +1 — FakeMod 58 secs ago

ACuriousMind

6:59 AM

@FakeMod It's noise (although well-meaning noise). Unless you have something substantial to add, you shouldn't leave such comments and they'll be deleted sooner or later.

@Vivek there's no app for chat afaik (and the normal stack app is also not really maintained anymore, using the site in your web browser might work better anyway)

Slereah

@ACuriousMind How are you supposed to compute the holonomy, in practice

Just compute the first few terms?

ACuriousMind

What would you need it for "in practice"?

It's a parallel transport operator, how often do you need to explicitly compute parallel transport? :P

Slereah

@ACuriousMind In my case, I want to see if I can relate spacetime measurements to the holonomy

and therefore the curvature

Since every GR measurement is just a loop at its core

ACuriousMind

Ah. Well, I'd suggest using non-Abelian Stokes

The path-ordered exponential is tricky to compute but a curvature surface integral seems doable

Slereah

Isn't non-Abelian stokes just Gauss-Bonnet for GR

ACuriousMind

7:07 AM

???

Slereah

I dunno, trying to find details!

ACuriousMind

No non-Abelian Stokes is literally non-Abelian Stokes, i.e. $\oint_{\partial S} A = \int_S F$, but there's some subtlety about the integral on the r.h.s. I think just like there is on the l.h.s with the path-ordering

Slereah

Not a lot of general sources on non-Abelian Stokes theorem

Even nlab is pretty glib! ncatlab.org/nlab/show/nonabelian+Stokes+theorem

ACuriousMind

ah, forget it, there's a similar surface ordering involved

arxiv.org/abs/math-ph/0012035

Slereah

I do know that there is some work relating GR measurements to holonomies of a connection

The sagnac connection

only for static spacetimes, though

ACuriousMind

7:15 AM

I'm not sure what exactly a "GR measurement" is but shouldn't the connection just be the Levi-Civita connection for standard GR?

Slereah

apparently so

The standard GR measurement is the photon bounce

send a photon, get it back

ACuriousMind

GR is a gauge theory, just not a Yang-Mills gauge theory

user434058

@ACuriousMind Ok, got it!

Slereah

The two photon curves + the timelike curve form the holonomies

user434058

1

Q: Doubt regarding application of Gauss Law

An infinitely long uniform line charge (charge per unit length = $\lambda$) lies parallel to the $y$-axis in the $y$-$z$ plane at $z=\frac{\sqrt{3}a}{2}$. Let $ABCD$ be a rectangle in the $x$-$y$ plane with origin as centre and with sides parallel to the $x$ and $y$ axes (Length of the side al...

homework-and-exercises electrostatics gauss-law

Slereah

7:16 AM

cf. arxiv.org/pdf/gr-qc/0204063.pdf

user434058

@ACuriousMind Should I answer this question partially or completely as it is a HW question but a thoughtful one?

Slereah

although since he's doing specifically static spacetimes, his holonomy is just done on the projection to the Cauchy surface

ACuriousMind

@FakeMod You should answer a HW-like question if and only if you believe it is not off-topic.

Slereah

Easier I suppose, since then every loop is an actual possible measurement!

ACuriousMind

(goes for non-HW-like questions, too)

user434058

7:18 AM

@ACuriousMind So is that off topic?

ACuriousMind

@FakeMod I'm not the sole authority on what's on-topic here. Use your judgement.

user434058

I feel it's on topic but that's maybe because it's a past year JEE Advanced question and I am biased :P

ACuriousMind

My personal opinion is that it's fine - it's not asking for the solution to the exercise, it's asking about a specific thing the solution does.

user434058

@ACuriousMind BTW it is satisfactorily(imo) answered

ACuriousMind

'BTW'?

user434058

7:24 AM

By the way?

user434058

There's no need for it but still...

user434058

0

Q: Ball motion in water

I am analyzing a video of a ball falling in water with tracker. The ball falls in a free fall for few seconds before it reaches a glass of water. The ball goes down till 3/4 of the glass, then stops and goes up again. I am trying to understand acceleration and velocity graphs. In the first one...

kinematics water

user434058

I reckon that there's a problem with the OP's tracker.

ACuriousMind

@FakeMod Yes, but that phrase usually introduces a new topic (perhaps in passing related to something just mentioned) - and we were talking about that question already anyway - so I'm a bit confused about its meaning there

user434058

@ACuriousMind Nvm it's not worth contemplating

user434058

7:32 AM

@ACuriousMind BTW Are Germans Christians or do you have any other religion?

user434058

0

Q: Why atmospheric pressure on a liquid and not on a solid?

An open cylindrical container (radius = 0.5 m) has 100 kg of water. a) Find the water pressure at the bottom of the container. b) Find the pressure exerted by the container on the floor (ignore the mass of the container) What confuses me, in a) We simply use p = p0 + ρgh then in b) p = F/A...

homework-and-exercises forces pressure fluid-statics

user434058

How did the OP make the font so big without using anything? Is this a bug?

user434058

I am sorry for posting every second question ;P will not do that in the future.....

ACuriousMind

@FakeMod Plenty of non-religious people around, but the majority of religious people are Christians, yes, mostly Roman Catholics and Protestants.

@FakeMod The ------------ marks what is above it as a heading.

Slereah

Is there a

Forbidden Stokes Theorem

that man was not meant to know

Hm

question is

What is the holonomy equals to for a photon bounce!

We don't really have the full parallel transport of a single vector

Complicated

Vivek

7:51 AM

why alkaline atoms have no orbital degeneracy?

user434058

@ACuriousMind Ohh!!

user434058

@ACuriousMind So were Germans Christians from the start? I mean when did you people become Christians. Was Christianity popular even during the Nazi regime?

user434058

Is it useful to edit a question to add the homework tag when itis anyways going to be closed?

ACuriousMind

@FakeMod Like most of Europe the inhabitants of the area have been Christian for well over a millenium - the late Roman Empire spread it pretty thoroughly before it collapsed.

There's plenty of material e.g. on Wikipedia if you interested in the historical spread of Christianity

@FakeMod If you're just adding the tag and editing nothing else I'd suggest to simply leave that to someone with editing privileges.

Slereah

8:07 AM

Germany is not Christian, it is Protestant

user434058

@ACuriousMind hmm... Makes sense

ACuriousMind

@Slereah There's about as many Protestants as Catholics when you look at the total population, but you'll find almost wholly Catholic and almost wholly Protestant areas

Slereah

Yeah I guess the old saying is true

user434058

@ACuriousMind You guys and gals were Aryans as well, right?

Slereah

Nul n'est prophète en son pays

no man is a prophet in his own country

John Rennie

8:10 AM

@FakeMod Aryans is just a name for the Indo-Europeans

user434058

@JohnRennie Wasn't it an ancient empire?

John Rennie

No-one knows who the Indo-Europeans were. We only have linguistic evidence for their existence.

user434058

BTW, we still use Swastika quite often, don't think Germans would be using it after Hitler using it.

ACuriousMind

@FakeMod Uh. No. That term isn't really currently used by any reputable scholars (what was once called "Aryan" is now mostly "(Proto-)Indo-European") and has an ugly history with the Nazis' beliefs about racial superiority.

user434058

@JohnRennie Hmmm... Didn't know that?

John Rennie

8:13 AM

@FakeMod yes, though note this is dangerous ground upon which you tread. NB the swastika used by the Nazis was parity reversed wrt the swastika used by Indians.

user434058

@ACuriousMind In fact Aryans are also not that famous when it comes to Indian history. They are not too ancient neither too modern. They are maybe somewhere in the middle and that's why they are left out and nobody teaches us about them.

ACuriousMind

If another German used that term in all seriousness I'd probably assume they're a believer in the racist theories associated with it

user434058

When I was young, I always messed up drawing that, I didn't know when and where to bend lines :)

user434058

@ACuriousMind We consider that symbol next to holy.

ACuriousMind

@FakeMod I'm not sure what exactly you're saying but there's considerable variance in what exactly "Aryan" is supposed to mean. See, again, Wikipedia - you might be talking about the Indo-Aryans

user434058

8:15 AM

Like it isn't completely holy.

user434058

2 mins ago, by FakeMod

@ACuriousMind In fact Aryans are also not that famous when it comes to Indian history. They are not too ancient neither too modern. They are maybe somewhere in the middle and that's why they are left out and nobody teaches us about them.

Korra

Anyone interested in answering a nuclear physics question?

user434058

I think i am mistaken

user434058

About my comment

ACuriousMind

@FakeMod With 'term' I meant still "Aryan", not the swastika. I'm well aware that the swastika is a symbol used in many contexts and cultures histoically.

John Rennie

8:18 AM

@Korra what's the question?

Korra

2

Q: Alpha emission of U-238

Alpha emission paradox says that an alpha particle with energy ~4 MeV is able to "come out" of the nucleus of U-238 but an alpha particle of energy ~9 MeV is unable to penetrate the Coulomb barrier of the nucleus. My question is, if the charge on the nucleus is positive then the barrier which the...

quantum-mechanics nuclear-physics coulombs-law

John Rennie

@Korra JEB's answer looks fine to me. What's the problem with it?

user434058

@ACuriousMind Yeah, i know. But I suddenly felt that I was wrong about that comment. Most probably Aryans were the oldest ppl our indian literature has talked about.

user434058

Bye

Korra

I don't understand why we use the Coulomb potential barrier when we are considering the inner alpha

Because a Coulomb potential would just aid the alpha to come out

On the other of we consider the nuclear force, then that would explain why the inner alpha cannot come out easily

@JohnRennie

ACuriousMind

8:22 AM

Well, if you choose to call these people "Aryan", okay, but since Sanskrit/Old-Indian is what linguists today call a indo-European language these people are just Proto-Indo-Europeans to everyone else (and no one knows where exactly they lived though there are different theories)

John Rennie

@Korra We don't consider a Coulomb barrier for the internal $\alpha$. The barrier is due to the rearrangement of the other nucleons that is required to spit out the $\alpha$. The 25MeV barrier that JEB refers to is due to the strong nuclear force.

Novalium Company

If I melt plastic in a microwave or an oven, would it need cleaning after that? I mean, would it toxify it or the fumes can get out and no problems?

Korra

Ohhhhkay. But for the outer alpha, we have to take Coulomb potential barrier, right?

@JohnRennie

John Rennie

@Korra there is a coulomb force on the incoming alpha, but this only dominates at distances greater than a few femtometres. To get the incoming alpha into the nucleus there would be the same strong force barrier that the outgoing alpha experiences.

That is, to integrate the alpha into the nucleus you have to rearrange the nucleons and pass through a transition stage with an increased energy.

Korra

@ACuriousMind sorry to jump in the conversation but I always wondered that the Aryans that is used in context of Nazis and the Aryans user in context of Indians are same or not

@JohnRennie thank you! I don't understand why my notes says Coulomb potential barrier. They even used that in the calculation!

By they I mean the reference book

@JohnRennie can you post the above message as an answer to my question so I can accept it?

As an answer

Novalium Company

8:31 AM

@JohnRennie Daaamn, how in the world do you have 358k rep. I can barely get to 200 with my inept posts.

John Rennie

@Korra I think I understand what your book means. Suppose we graph the strong force and the EM repulsion on the same graph. We'd get something like this (diagram incoming):

Korra

@JohnRennie the potential the book has used to get the probability is the Coulomb potential.

John Rennie

The total is the sum of these:

Korra

@JohnRennie yes, got it! But then why have they used the Coulomb potential for getting the probability of the inner nucleus coming out?

John Rennie

That's an approximation. For the alpha the difference in energy between the inside and outside is 4MeV. We know this because the alpha ends up with a KE of 4MeV. And 4MeV is small compared to the barrier height of 25MeV.

But I agree with you that the barrier should be 4MeV higher on the way out than it is on the way in.

Although note I've simplified the argument a lot because the strong nuclear force is complicated not a simple 1/r potential like the EM force. It depends on spin of the nucleons as well.

Whether you could really indicate the strong nuclear force by a simple curve like my blue line is debatable.

Korra

8:43 AM

@JohnRennie thank you so much!

ACuriousMind

@Korra That's difficult to answer because the Nazis (and some Indians, I guess) believe(d) things that current science does not support, especially about racial superiority.

What I'm talking about is mostly based on linguistics - we know that Sanskrit is related to other Indo-European languages like Greek and Latin but that doesn't tell us whether the relationship is because of contact between distinct civilizations or because there was a single ancient culture that spoke PIE that spread to all the places where the descendants now live

See e.g. en.wikipedia.org/wiki/Proto-Indo-Europeans#History_of_research and links therein for why modern researchers are reluctant to use the term "Aryan"

Korra

@ACuriousMind history is dope! And I already read that article

ACuriousMind

The existence of PIE speakers is a guess - other than linguistics we don't have any hard evidence of their existence

Korra

@JohnRennie that is very much like the book's barrier (the green one)

ACuriousMind

It's pretty annoying that early humans were too busy to survive to invent writing and history books on the spot :P

Novalium Company

9:00 AM

If my microwave watts knob can be turned to maximum 800W, how hot can the object get? I mean, would it be enough to melt HDPE plastic (which melts at ~130C)?

ACuriousMind

it's generally a bad idea to put anything but food in your microwave unless you know exactly what you're doing

Novalium Company

@ACuriousMind Why is it bad specifically for recyclable plastic?

ACuriousMind

I'm not saying it's bad for plastic - I don't know what happens when you put it in there

John Rennie

@NovaliumCompany the danger is that you don't know in advance how the material will interact with the microwaves.

ACuriousMind

But since the microwave doesn't really heat things but instead bombards them with EM radiation it's not reasonable to expect any given substance to just heat up evenly

(see: what happens when you put metal in there)

Novalium Company

9:04 AM

Doesn't that bombarded EM radiation translate to induced kinetic energy?

John Rennie

@NovaliumCompany it depends on exactly how the material interacts with the microwaves

I suspect that plastic interacts only weakly with the microwaves so it won't heat up much.

Loong

130 °C with 800 W? Perhaps if the part is not larger than about 5000 cm².

John Rennie

You can get unexpected effects in microwave ovens e.g. have you ever tried the "microwaving a grape" experiment?

Novalium Company

@JohnRennie I haven't. (I'm checking it out now)

John Rennie

You should try it. It's ... spectacular :-)

I have done it and my microwave is still working so you should be fine.

66

Q: Why do grapes in a microwave oven produce plasma?

Some of you may know this experiment (Grape + Microwave oven = Plasma video link): take a grape that you almost split in two parts, letting just a tiny piece of skin making a link between each half-part. put that in a microwave oven, and few seconds later, a light ball which seems to be a plasm...

electricity electrostatics physical-chemistry home-experiment

Novalium Company

9:11 AM

Well, I've seen plasma before

In terms of plastic, would a mini-oven do the job? (which doesn't use EM radiation)

John Rennie

I think you'll find forming plastic hard because it tends to oxidise and degrade before it melts.

I think industrial injection molding machines keep the molten plastic away from air to stop it degrading, though if I'm honest I don't really know how they work.

Novalium Company

Yes, unfortunately, I don't have an injection molding machine so I must find a way to do it at home (on the balcony) so that the final result is at least acceptable. I can then sand away any unnecessaries.

John Rennie

I would make your prototype from a material you can form more easily e.g. cardboard or metal sheet.

The aim of the prototype is just to check the whole thing works. It doesn't have to look or feel good.

Novalium Company

What am I gonna do with the prototype? I need to make it sellable so I can get funds for the next one.

John Rennie

This is why small companies need capital to get started.

Your company is going to soak up money to pay for all the development costs before you can make something you can actually sell.

Novalium Company

9:22 AM

Or I can make something a bit crappy looking, sell it, with the funds, make it look a bit better, with those funds, better... I need to find a way to do it without capital.

I have capital, but not enough to buy million-dollar injection molding machines.

John Rennie

Many have tried that approach, few have succeeded.

Novalium Company

I'll give my all to be one of them.

John Rennie

You don't buy your own injection moulding kit. You pay an injection moulding company to make the products for you.

That reduces the initial outlay, though it still costs a lot of money.

Novalium Company

What the injection molding machine's job is to fit melted plastic in a shape. I must find a way to do something close to that even if it means suffering with a mini oven on my balcony.

There's at least one person in the world who would be ready to buy the first crappy version of the device.

Alright, I guess I'll be buying a cheap mini oven with polymer clay and testing the whole thing on my balcony with HDPE plastic. Thank you so much for the help everyone. I hope it works.

John Rennie

@NovaliumCompany maybe check this out

user434058

9:34 AM

0

A: Is heat $\delta Q$ an exact differential for an isochoric process (ideal gases)?

No you don't. When volume $V$ is kept constant heat is indeed path independent. At least if we assume there aren't any external magentic or electric fields or other weird interactions. These would result in more work coordinates than just $V$.

user434058

Do electric or magnetic fields affect thermodynamic processes?

John Rennie

@NovaliumCompany Or this. This suggests melting the plastic by heating it in oil. That would keep the air out and help stop it degrading.

Actually that looks interesting ...

I might have a go myself.

Novalium Company

@JohnRennie Thank you! <3

I'll probably have to go through a few failures before I find a method that works

@JohnRennie Last thing, do you know if the melted plastic would stick to the baked polymer clay? If so, is there something I can spray over the baked polymer clay so the melted plastic doesn't stick?

John Rennie

9:50 AM

I have no idea I'm afraid.

I think the melted plastic will be far too viscous to spray.

Novalium Company

@JohnRennie No problems. As I said, I'll probably have to go through a few failed attempts.

user434058

10:34 AM

Hi

user434058

10:47 AM

user434058

What does @DavidZ mean when he says that the oost is temporarily deleted? Will it be reopened/undeleted in the future?

John Rennie

11:02 AM

@FakeMod If you give it a couple of weeks then post a custom flag asking for your answer to be undeleted one of the mods will undelete it.

user434058

11:18 AM

@JohnRennie So, why did they delete it in the first place? What was the motivation behind deleting and then undeleting after some time? (it's not as if I am angry or upset, it's just that I am curious)

John Rennie

The thing about homework questions is that if you answer them you encourage that user to ask more homework questions - because they know that if they ask they'll get an answer. And the fact their question is eventually downvoted and closed is irrelevant.

user434058

@JohnRennie Hmm.. I get it!

John Rennie

So if the mods see an answer to a homework question, or if some user flags the answer for the mods attention, they will delete it for a couple of weeks. Hopefully by then the homework will have to have been handed in and undeleting the answer no longer helps the vile criminal :-)

user434058

BTW, it wasn't really a HW question, but still it's okay.... Link : physics.stackexchange.com/questions/540971/…

user434058

@JohnRennie Haha! I expected that to be the motive but wasn't sure enough!

John Rennie

11:22 AM

That's blatantly a homework question.

It's been retyped straight from a problem sheet.

user434058

Who does this kind of homework?

John Rennie

People who post their homework here!

user434058

I don't expect physics teachers to give terminology questions as homework to their students :P

user434058

I got fooled!

tpg2114

High school physics and/or physics-for-non-scientists classes give stuff like that all the time

user434058

11:23 AM

I thought that the doubt was genuine :)

tpg2114

Heck, even my grad school turbulence class would have questions like "Explain the difference between homogeneous and isotropic turbulence"

user434058

@tpg2114 Didn't know that!

user434058

@tpg2114 That is still a bit technical, however that HW question was disguised as a naive and innocent doubt which the OP would have faced while solving a HW question.

user434058

But anyways, lessons learnt.

tpg2114

Yeah, the turbulence example is more technical because it's graduate level, but it's still a terminology question. But hey, great time to remember: fool me once, shame on you. Fool me twice, shame on me!

user434058

11:28 AM

@tpg2114 Exactly! I love this saying/quote.

user434058

0

Q: How to bring the two arms of the drawing together?

I have a basic design of two arms which need to be brought together. But I tried the design using a test setup and it did not work successfully. Here is a diagram of my design - What I observed is that arm 2 moves up before arm 1. I want both of them to move towards each other to cause a comp...

homework-and-exercises forces

user434058

Is this a HW question?

tpg2114

Maybe, maybe not -- but it's definitely engineering

user434058

Yeah I flagged it for that. However the most recent edit added the homework-and-exercises tag, which I think isn't appropriate....

tpg2114

11:41 AM

I mean, homework-and-exercises is not only for assignments. It's also used for problems where the method of arriving at the solution is more important than the solution -- which could apply here too

Rather than "Here's how to make your arms move correctly," an answer could explain how to set up the problem to work for any system or something. That would be more like a HW&E answer

So... not an issue to get worked up about, even if it's a marginal case.

user434058

@tpg2114 Thanks👍 I am slowly getting a better feel for this tag.

tpg2114

It's... the hardest one to figure out

But our homework policies have been hashed out over years with a ton of input and people, and the result is a compromise between all of the different views. Anything that's a compromise with a diverse range of opinions will end up complicated or murky...

user434058

@tpg2114 That's bound to happen when younare serving the whole internet...

tpg2114

One of the easiest ways to identify "absolutely 100% homework" is listed in our homework policy: "Don't just copy the exact problem from your homework assignment or textbook. In particular, when you are asking for help, writing in imperative mode ("Show that...", "Compute...", or "Prove or find a counterexample: ...") is at the very least impolite..."

Which is the easiest red flag to watch out for if trying to figure out whether something is an assignment... if it uses the imperative, like your early example that you answered did ("Also provide examples."), it's probably a good bet that it's homework

user434058

@tpg2114 Definitely!

user434058

11:50 AM

@tpg2114 It's definitely HW

user434058

@tpg2114 most of the times, the OP is so lazy that he doesn't even format the question properly. Bad grammar, a lot of typos, no spaces, no punctuation, no/weird capitalisation $\Rightarrow$ HW QUESTION!

tpg2114

Yeah, there's lots of warning signs... since you said that one fooled you, I thought I'd point out the warning sign that jumped out to me

Bad grammar, typos, and poor formatting are also calling cards of crackpots and cranks

user434058

Yeah sure!

user434058

@tpg2114 I don't usually get into crackpot stuff because it is mostly above the level of physics I know. So I am not able to judge it's "crackpot index". Though I do figure out the crackpottery by the comments and the downvotes and thus I assume that the question must be a crackpot one. So I flag it and downgote it.

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