The h Bar

12:51 AM

0

Q: Does 'Semiconductor Physics' have a home in Physics SE?

I have seen a number of questions on PN-Junctions, Avalanche Breakdown, Biasing and MOS...etc, that have gone largely untouched; which is understandable, do these types of questions belong here or should they be relegated to Electrical Engineering SE?

antimony

1:13 AM

well biasing perhaps belong in EE, but others maybe they want the real physics answers?

Charlie

That's a bot posting that

antimony

oh oops

hehe

thanks

123

2:47 AM

Hello World..

123

6:12 AM

Hi @JohnRennie Sir

John Rennie

@123 hi :-)

123

I am confused with magnetic field and magnetic force phenomenon. Many thing are clear but not fully.

In ampere law experiment compasses align in circule to show the magnetic field of wire. Compass also has magnetic field and made up of charges electron & proton.

But in beam of electron it apply force as Lorentz phenomenon perpendicular to magnetic field.

I don't understand how magnetic field behaves differently in some situations it behave as perpendicular force like in beam of electron. In other situation behave linear attraction/repulsion like in two magnets.

John Rennie

6:29 AM

A compass needle is a magnetic dipole, so in the first case we have an interaction between a magnetic field and a magnetic dipole.

In the second case we have an interaction between a magnetic field and an electric charge. So they are completely different interactions.

123

6:43 AM

Superb @JohnRennie sir. But magnetic field has always dipole. Pls clear this in both cases.

In wire how circular magnetic field can be seen as dipole.

John Rennie

The magnetic field of the compass needle is a dipole. The magnetic field of the wire is not a dipole.

RewCie

Hey there!

Does @PM2Ring comes here?

i think he left the SE Network permanently...

Ah, last active 16 hours ago!

I sent him an invitation to h bar

@JMac Do you know a good book that covers Robot Kinematics and Mechanical stuff?

Need one for engineering tho....

Slereah

8:42 AM

I do love the questions that aren't questions at all on PSE

you can spot them pretty quick

"How come this part of the theory makes no sense and also it smells and here is a paper by a crazy man proving it is wrong"

John Rennie

Pentcho Valev again?

Slereah

not just him

it is a whole genre of feigned surprise

except they are not good at pretending

John Rennie

Oh you young people!

Admittedly it used to be a pain distributing preprints before Gutenberg invented the printing press.

It used to cost me a fortune paying those scribes.

ACuriousMind

It's pretty amazing that there are now people alive that cannot imagine a world that is not instantaneously connected

Nihar Karve

omg, like, you guys didn't have snapchat?

John Rennie

8:48 AM

We used semaphore or at night we lit fires on hilltops.

Slereah

I assume @JohnRennie used a ham radio

chatting up truckers

I remember the pre-phone era

I wasn't more social, I just read a book while walking instead

ACuriousMind

I think you mean the pre-smartphone era?

unless you're over a 100 years old

in which case good for you

Slereah

I was born in a log cabin

there were phones, but it wasn't the phone era

123

9:09 AM

@JohnRennie Thanks. But I learnt magnetic field is never observed a monopole. What is the magnetic field of current carrying wire?

John Rennie

I'm not sure to be honest.

I don't know how you would express it using a multipole expansion.

123

Thanks a lot @JohnRennie Sir, this is problem with the magnetic field phenomenon. It is not well cleared on book also in internet. Once I understand the idea and experimental results. Then it is always easier to understand mathematics used to explain the phenomenon.

I have problem to understand the magnetic field behavior in current carrying wire. Which attract/repel another current carrying with linearly. But in case of magnetic field attract/repel electron beam perpendicular.

ACuriousMind

@123 There isn't only "monopole" and "dipole". There is a whole series of $2^n$-pole moments you can expand a function in, see Wiki. Many fields are not purely one of these poles.

123

Also in Ampere law experiment magnetic field it attract/repel magnetic field of compass.

@ACuriousMind thanks I see the link

I always assume in magnetic field phenomenon by magnetic field due to one object and magnetic field due to second object. Which has force effect on each other. Which should be same and completely explainable. But unfortunately it is not.

ACuriousMind

A magnetic field always exerts the same force - $qv\times B$ - on charges in it. The differences you're seeing between e.g. a wire and a free electron beam is because the electrons in a wire are not free to change their direction of movement since they cannot just leave the wire to describe a curved path like the free electrons in a beam do

There's nothing "unexplainable" here, it may just involve a bit more careful arguments than what you're used to so far

123

9:25 AM

@ACuriousMind yes I need experts discussion like you and @JohnRennie and this community to understand these phenomenon. Which seems very difficult to me.

@ACuriousMind pls explain what happened in two magnets case. Two magnets attract/repel each other linearly in straight line. As per Lorentz force it should pe perpendicular attraction/repulsion but it is not. What happens in this case?

ACuriousMind

@123 Think about this: Two magnets attract each other even when they both start at rest. But the Lorentz force only acts on charges that are already moving - why is there a force between magnets at all?

You seem to have tried to apply the Lorentz force directly to the magnets as "charges", but that just doesn't work at all - that's what I mean when I say you have to use more careful arguments

123

@ACuriousMind in magnet magnetic field due to spin of electron. It means Lorentz force only work when electron in motion and spin is not considered as motion of electron?

It means Lorentz force does not work in two magnets case. They do not behave as perpendicular force

ACuriousMind

@123 Ah well, that's now a bit too detailed - you need proper quantum theory to talk about the interactions between the magnetic field and quantum spin. Classically, you should just think of the electron as a tiny current loop (the spin corresponding to the orientation of the loop).

But even in this model (called the Ampere model) it is not easy to see how the force on the magnet arises, since a uniform magnetic field exerts only torque but no net force on a single current loop. The truth is that explaining permanent magnets and their interaction is pretty complicated, you'll have to read a proper treatment of the Ampere model to see how we usually understand the arising force classically

you have to think about the field between the magnets not being uniform, so when the torque rotates the current loops so that one part of the loop is closer to the other magnet ("higher up" in the magnetic field), there is then a net force on the loop towards the direction in which magnetic field strength increases (if the magnets attract)

123

9:51 AM

@ACuriousMind thanks let me read this quantum phenomenon. Also next step with theory of relativity.. Thanks

@ACuriousMind pls explain it completely so I can understand the both phenomenon.

@ACuriousMind I have an idea of QFT QED and theory of relativity. Pls explain so I can understand the phenomenon.

ACuriousMind

@123 I don't think that would be useful - I think the Ampere model is the best way to explain permanent magnets.

There aren't really "forces" in quantum theory, everything is a lot more complicated. The most reductionist explanation isn't always the best or even feasible

123

@ACuriousMind whatever approach you think better to explain you can if you have time. I am keenly wanted to understand this behavior. You can use multiple approach.

Thanks for your participation.

ACuriousMind

10:07 AM

@123 I've already given you the name of the model and a rough explanation. I don't have the time or inclination to go through the entire derivation with you, you'll have to find a text that does that.

user 85795

do you have any textbook recommendations?

123

@ACuriousMind pls share the good link. So I can read exact article. Then I will search myself for further. Thanks

ACuriousMind

@123 I don't have a link, it's been years since I've done this

123

@ACuriousMind any Google link

I know spin behavior in quantum theory. Quantum entanglement etc..

You can suggest any good book.

Slereah

10:46 AM

Alice in Wonderland?

satan 29

11:10 AM

lmao

user 85795

11:24 AM

@ACuriousMind I left a message for the ELL mod in their chat room about this.

dunno what else to do?

there's more chat history here

ACuriousMind

@user85795 Flag a post (on the main site, not chat) by the account with a custom mod flag. Chat is not the right venue to request mod actions.

user 85795

ok, thnx

-_-

user 85795

1:21 PM

@ACuriousMind I don't have enough rep to do that.

Thanks for your participation.

ACuriousMind

@user85795 ...and you are certain that you don't have an account on ELL that does have enough rep?

user 85795

ok

Daniel Underwood

Is it just me or are superthings comical?

Somehow I got to the wikipedia article for Lie superalgebra and it's pretty much just "super" before every mathematical term

ACuriousMind

they were supercreative with that naming, yes

I'd much prefer we'd all stop using it and just say $\mathbb{Z}_2$-graded like proper mathematicians :P

user 726941

1:38 PM

linguistics uses "super" a lot also

but, it's not at all as comical

mithusengupta123

Is quantum Hall effect some kind of phase transition?

Daniel Underwood

But who wants to study "$\mathbb{Z}_2$-graded gravity" when you can be a superphysicist? :D

2

Nihar Karve

1:55 PM

@user85795 what an incredible interaction

user 726941

2:07 PM

@NiharKarve yeah, I got her suspended chat

Nihar Karve

@user726941 where'd the profile picture go?

user 726941

@NiharKarve who knows, she wanted a meme for a picture; I think

I am 10 years old.

Nihar Karve

Ah well

A for enthusiasm

user 726941

A++ for making my skin crawl...

> This user has been temporarily suspended by a moderator and cannot chat for 41 days.

ACuriousMind

@DanielUnderwood we should probably just be glad this naming convention did not continue otherwise there'd be courses on megahyperultrasymmetry EXTREME.

user 726941

2:21 PM

just like the UFC

though they do have super heavyweight wrestling and weightlifting also

>265 lbs or 120 kg

Charlie

The year is 2150, the search for supersymmetry has hit a dead end and the search for megahyperultrasymetry EXTREME has begun at the colossaluberhadroncollider+++

3

ACuriousMind

@Charlie I, for one, welcome your new hyperphysicist overlords

Charlie

may they be merciful

user586228

Always start with

In - Out + Generation = Accumulation

Out = Area*h*(T-Tair)*t
Gen = V*density*Hfusion

Somebody told me this

Even then I am not getting the answer

Any help in the direction is welcome

Do I post it in problem solving strategies?

user 726941

2:36 PM

yes, please

ACuriousMind

@user586228 You already posted it there, so I'm not sure why you're asking this.

user586228

o

It is a very quiet room

As of now..

Anyone who intend to help me there..

Can join in

Nihar Karve

physics.meta.stackexchange.com/u/283253: in less than a month, this user has upvoted 641 questions on physics meta, placing them in the top 20 voters of all time

unbelievable

Charlie

doing god's work

user 726941

creating a rock he can't lift?

Charlie

2:51 PM

surely the opposite of god's work is constructing his own fallacies

Nihar Karve

I don't think I've even read 641 Q&A's on the main site

(like, properly)

but 600 question upvotes/month on a site that gets around 20 questions per month is a little excessive, don't you think?

user 726941

yup

ACuriousMind

Some users do a lot just to get shiny badges (there's a gold badge for 600 votes)

user 726941

badges? we don't need no badges

Nihar Karve

well

you'll be glad to find out that (from a quick survey), this user has obtained this badge on 89 SE's

user 726941

3:03 PM

:O

Nihar Karve

holy Nebuchadnezzar

Yashas

any classical intuitive example of gauge symmetry vs global symmetry?

Nihar Karve

he is insane

he's upvoted 60,000 times this month

user 726941

pandemic fever

Nihar Karve

...or a well-crafted bot

user 726941

3:06 PM

mhmm

ACuriousMind

@Yashas You will not find "intuitive" examples of gauge symmetries because gauge symmetries are a property of the mathematical description of a system, not of a system itself

but the classical example is of course the 4-potential of electromagnetism, maybe you could explain what's dissatisfying about that example to you?

Charlie

speaking of gauge symmetry, I had the reparameterization invariance of worldsheets in string theory described as a gauge invariance, however, I've also read in a number of places that GR isn't a true gauge theory of the general linear group, is this just a similar slightly imprecise use of terminology?

ACuriousMind

oh dear here we go again

Charlie

:(

Nihar Karve

::*grabs popcorn*::

ACuriousMind

3:09 PM

@Charlie My view on this is for example here, there is more discussion at physics.stackexchange.com/q/46324/50583 and its linked question

Charlie

oh I was ok with the idea that gr isn't a gauge theory, i just wondered if the same argument basically applied to worldsheet reparameterisation invariance

the phrase "I was ok with" here should be interpreted as "I have skimmed"

ACuriousMind

there is also the question of what a "true" gauge theory is, since even when you formulate GR in a "gauged language", you still have that things that are not invariant under the gauge group (like the stress-energy tensor, which isn't a scalar) are nevertheless observable (the gauge symmetry is not "internal")

Charlie

ok that seems reasonable

Yashas

@ACuriousMind I am hearing 4-potential for the first time :)

Nihar Karve

@Yashas you should probably take a look at this first then

Yashas

3:15 PM

I am a CS student. My background in physics is non-existent but I am doing stunts by taking electives like "particle physics". It's been qualitative so far and the math is yet to start but the prof mentioned that gauge symmetries/local symmetries lead to force and global symmetries lead to a conserved quantity. The only symmetries I have ever seen were in an into classical mechanics course which led to conserved quantities like angular momentum, energy, etc.

Charlie

What topics can you cover in particle physics as a CS student? Most physics courses don't cover actual particle physics until final year

Yashas

So I was just looking if there was a simple example I could understand. I am quite intrigued that symmetries lead to new particles. Maybe I haven't understood properly or yet to understand (the prof did say it's all in the math) but I don't know why the idea of symmetry -> particle is a bit fascinating (as of now).

ACuriousMind

Well, the simplest useful example is really classical electromagnetism.

But understanding classical electromagnetism is usually a full course on its own so I'm not sure how helpful that is to you :P

Yashas

@Charlie Quantum Mechanics, Electromagnetism (no relativity), Classical Mechanics (had a bit of SR, Hamiltonian mech, Lagrangian mech), Spectroscopy & Symmetry (more of chemistry) and now doing Intro 2 Particle Physics.

Charlie

I'm not sure if symmetry $\rightarrow$ particle is true, but I guess you could argue that the mathematics of Lorentz symmetry is tied very closely to particles

ACuriousMind

3:19 PM

@Yashas Ah, okay, if you know EM just not the covariant formulation, then you know the scalar electric potential $\phi$ and the magnetic vector potential $\vec A$, yes?

Charlie

as in, they are co/invariant under the lorentz group

@Yashas oh ok that seems reasonable then

Yashas

@ACuriousMind Yes.

Nihar Karve

@Charlie is this the politically correct way of writing Lorentz "invariance"

ACuriousMind

In that case gauge symmetry is quickly explained: For any function $f$ you can do $\phi\mapsto \phi +\partial_t f$ and $\vec A \mapsto \vec A + \vec \nabla f$ without changing the electric and magnetic fields.

This is a gauge symmetry - a transformation of the physical variables $\phi, \vec A$ that leaves all observable quantities $\vec E,\vec B$ untouched.

Charlie

@NiharKarve if you use the phrase invariance wrong three times in a mirror you can summon acm like in Betelgeuse

Nihar Karve

3:23 PM

ah yes, Beetlejuice

ACuriousMind

As for "gauge symmetries lead to forces", I personally think this is the wrong way to think about things, see e.g. physics.stackexchange.com/a/370783/50583, physics.stackexchange.com/a/257100/50583

Nihar Karve

I said Beetlejuice ironically but turns out that's an alternative spelling of the movie

I somehow used to read it as "Beltugese", pronounced "Belch-you-geeze"

Yashas

@Charlie I want to clarify that I have to take more science courses than an average CS student because I am also doing my masters in computational natural sciences (it's in bioinformatics) along with bachelors. My set of courses is not representative of what an average CS student would take at my uni but the intro 2 particle physics course is an elective that is available to just CS students too

Charlie

oh ok

Yashas

@NiharKarve 84600 * 30 / 60000 ~ upvote every 40 seconds... I don't think this is humanly possible.

makes me wonder if there are people selling SE accounts (won't be very surprising given the amount of scams I see in resumes of engineering graduates/peers)

Nihar Karve

3:34 PM

how he's been evading anti-bot measures I have no idea

I've seen the user talk on the English chat before, so there was definitely a human behind the account at some point

I guess he was later consumed by Skynet

RewCie

Does anyone know how to build 3D Models from Engineering Drawing

(Don't suggest 3D Printer)

I'm lookin' cool ideas only!

Phutrixtik ideas belcom

Sed Noises

123

4:25 PM

I don't understand the phenomenon of nature.

Charlie

get in line

ACuriousMind

welcome to the human condition

123

You study whole life to understand physics, and to understand physics you to understand mathematics. The whole life you life. And when you understand you become older to die.

The whole you did this, what is the benefit.

Hello @Charlie @ACuriousMind

ACuriousMind

I believe you're ready for the advanced class

123

@ACuriousMind to whom you gave the above msg.

ACuriousMind

4:30 PM

To you, but I'm partly jesting

123

Aye.. I have read the link. :D you are right I am ready for advance classes

ACuriousMind

No one can tell you what the benefit of studying physics for you is. Some study it for practical purposes, to build new machines or more accurately predict the future. Some because understanding of reality in itself, even if always incomplete and incremental, is satisfying to them. Some because they believe in some sort of innate purpose or truth about the universe they might be able to glimpse if only they understand it enough (I'm looking at you, platonists).

123

I am studying physics to understand, how the nature work.

Slereah

The secret is that there is about two meanings to "a gauge theory"

One is "There is an extra degree of freedom that can't be solved by the EoM" and the other is "There is some connection form of a principal bundle"

The second kind is also the first kind, but not vice versa

ACuriousMind

@Slereah that's true but that's not the issue in the GR case because there you do have the spin/Levi-Civita connection

it just doesn't work quite like the other gauge theories because it's soldered to the tangent bundle and not acting in some "internal" space

123

4:46 PM

@ACuriousMind what topic I need to study to understand the behavior of magnetic field in amperes law and between 2 objects.

Slereah

@ACuriousMind but people also call diffeomorphism invariance a gauge!

which is... kind of related?

Slereah

5:10 PM

Boy AI libraries are way unstable

I'm sure they're complicated but it's amazing how something that badly behaved is industry standard

That's the real AI catastrophe we should be worried about

Nihar Karve

As a lazy weed dealer would say, why not roll your own?

2

Slereah

Because I'm not insane

It's a lot of work

Nihar Karve

What's the project?

Slereah

playing around with GPT

Nihar Karve

5:26 PM

Welp

I thought it was a custom GR NN or something

What's happening with GPT?

Emilio Pisanty

0

A: Opt-in alpha test for a new Stacks editor

Disclaimer: Let me start off by apologizing for the blunt tone in this post. I very much appreciate the spirit of openness and hard work that underlies the development side, and I want to thank you for both the hard work in improving the UI and for the time and willingness dedicated to soliciting...

↑ important!

I'll add a pointer on our own meta

Slereah

@NiharKarve You can make it say rude words

Yashas

@Charlie Oops, I was going through the messages and I realized that you asked for what topics are covered in PP. I read it as what topics have I covered as a CS student.

It's not very rigorous (CS students :P) but it's fun and interesting. The only per-requisite was introductory quantum mechanics.

ACuriousMind

@EmilioPisanty oh dear

Nihar Karve

Speaking of GPT, play.aidungeon.io is reasonably interesting

ACuriousMind

5:40 PM

it's "We have other sites than SO?" time again, it seems

4

Emilio Pisanty

@ACuriousMind indeed

ACuriousMind

and you're completely right that accomodating math formatting needs to be a feature considered from the beginning, since there is essentially no way to do it right without a separate preview

@NiharKarve I lost at least one day to that one when it first appeared :P

Slereah

I tried using GPT-3 but its bulky ass will not fit in my GPU's cold heart

So back to GPT-2

Emilio Pisanty

0

Q: The proposed changes to the text editor would break this site

You have probably seen already a link, on the Featured on Meta sidebar, to an announcement of an upcoming upgrade to the text editor on Stack Exchange, Opt-in alpha test for a new Stacks editor. As a whole, the project looks extremely useful, and it looks like it will do a lot of good for general...

discussion

ACuriousMind

no patience for the poor meta bot :/

Emilio Pisanty

5:55 PM

@ACuriousMind no indeed

Physics Meta

The proposed changes to the text editor would break this site

4

Emilio Pisanty

@ACuriousMind maybe I needed to invoke it?

ACuriousMind

it's pretty erratic with its delay

(edited for better readability on the starboard)

Emilio Pisanty

@ACuriousMind oh, you can do that?

nice

ACuriousMind

mod powers are infinite :P

Charlie

6:08 PM

@Yashas that is an enormous range of topics for an "introduction" class lol

tpg2114

@ACuriousMind Given the smart-quote-breaks-all-code issue, it seems it's really more "We have other sites besides Teams?" now

ACuriousMind

true, true

tpg2114

I've given up on posting something many times now with the new editor because hitting Enter never seems to give me the line breaks and spacing I actually wanted

And it drives me crazy because it does it in both Markdown and non-Markdown modes

Nihar Karve

6:25 PM

@ACuriousMind ok, I'll bite. How should I report the 60,000 votes/month guy? Is he even within the jurisdiction of the mods of a single site, seeing as he hasn't violated the rules of any single community per se (even though his account shows obvious bot like behaviour SE-wide)?

ACuriousMind

@NiharKarve given that meta.stackexchange.com/q/326199/263383 is the last discussion on what perhaps should be done about automated voting it seems SE currently has no policy that says you shouldn't do this

Nihar Karve

Hmm, one of the main points there is that it's usually difficult to distinguish, so any automated counter-checks/"authoritarian" mod intervention is likely to hinder real humans in some cases. This proof for this guy using a bot is a lot more concrete though. Regardless, if there's no official policy, I'll leave it aside for now.

tpg2114

It reminds me of when Twitter started taking down bots a few years ago, and an old lady had her account removed. But it turns out she really was sitting down for 14 hours a day, every day, from waking up until bed, tweeting and retweeting things.

ACuriousMind

@NiharKarve you could ask on mother meta (without linking to the specific user) to ask what to do

Nihar Karve

@ACuriousMind I'll consider it, thanks.

schn

7:01 PM

What does it mean when light has a high degree of polarization? What is the brilliance of light?

Charlie

7:30 PM

@schn Is this relevant, about partially polarised light?.

I have a brief question. When we choose worldsheet coordinates, we label one of them as $\tau$, does this imply that we always embed our strings in spacetime in the same "orientation"?

As in, I assume the $\tau$ coordinate on the worldsheet has to be related in some way to the time coordinate in the target spacetime

schn

7:45 PM

@Charlie Very relevant I think, thanks. Do you know what is meant by "streams" in "Unpolarized light can be described as a mixture of two independent oppositely polarized streams..."? Is it just waves?

Charlie

I'm not certain, but it sounds like streams~beams here

As in, a "polarised stream of light" would just be a beam of polarised light

schn

And would you say that a beam is a wave? :)

Charlie

Well, a photon is (in classical EM) a vacuum solution of Maxwell's equations, which is a wave, so I guess so

I'm not sure what other information you're attaching to that statement though so I'd be hesitant to say it :P

schn

Fair enough

G. Bergeron

9:13 PM

So what's going on the time-sinking blackhole I just landed back on?

@Charlie Well, more like: local symmetries made global $\implies$ new fields $\implies$ new particles...

Charlie

I don't really follow, is this something to do with gauge theory?

G. Bergeron

@Charlie yes

Slereah

Beware, though

Not all symmetries lead to gauge fields

G. Bergeron

Indeed

But close enough if you consider only local symmetries that leads to non-trivial connection when you make them global

Slereah

I'm not quite sure what's the actual condition for bumping up a mere action symmetry to a principal bundle

I guess the symmetry obeying some Lie algebra condition maybe?

probably in Henneaux

G. Bergeron

9:31 PM

You could always do it trivially. I suppose some structure beyond Lie algebras could be used, but then it is not really a gauge theory anymore, is it?

Slereah

I'm sure nlab has a 5000 word article on what that would be

G. Bergeron

LOL

I'm sure that article would look super deep and interesting from afar but I won't be able to tell from up close ;)

Slereah

the $(1, \infty)$ category of topoi of moduli stack of vector bundles in a supercartesian site

G. Bergeron

XD

Slereah

heck I'm remaking a new text Neural Network

Maybe I should make it generate fake ncatlab articles

G. Bergeron

9:33 PM

deep n' cat

So you're doing AI now?

Slereah

I have for a while, ye

G. Bergeron

Most of my friends have jumped ship by now. It makes me wonder...

Slereah

I mean, I kinda hate it

as a job it is boring and drab

but it pays the bills

G. Bergeron

lol, yeah but exactly

Slereah

Computer science is basically what physics would be if before proving anything, you had to arrange all your physics books on the shelf in a specific order

Lest nothing works

G. Bergeron

9:36 PM

Still in academia now, but you can easily get stuck in a post-doc rut running after imaginary positions, you know

and I'm at a point I need to decide what to do regarding that

And as the MILA is in my city, you can imagine the pull that community has on other disciplines

Transcript for