The h Bar

@heather MIT OCW has a lecture series which follows the book by Hecht an the one by Goodman (including readings and assignments from those two books). I'm using it at present and am finding it quite useful. The best part is that in the OCW they specify the exact amount of time you need to learn each topic.

0celoñe7

@EmilioPisanty Well known or well-known?

heather

@Blue oh, nice, i might go check that out

Emilio Pisanty

@0celoñe7 depends on context

Anonymous

2:17 PM

@heather Yeah, and also it will be helpful if you learn a bit of differential equations and take a basic electromagnetism course before starting that. Good luck with your studies!

0celoñe7

@EmilioPisanty "It is well known to be true for a proper compact extension of the given manifold."

Emilio Pisanty

@0celoñe7 space instead of hyphen

Physics Meta

1

Q: Can we get rid of the general-physics tag?

This has bothered me for some time, and since bad tags are on the table, I think this is a good time to do it. There are currently 60 questions tagged general-physics of which roughly fifteen have that as their only tag (and of those, five are closed). The tag is absolutely terrible and it is ab...

0celoñe7

@EmilioPisanty thx

heather

@EmilioPisanty fwiw, I already asked this question (A generally terrible tag) which Qmechanic marked as a duplicate of an old "list tags to burninate" post. i do agree though, with your meta post.

Emilio Pisanty

2:19 PM

@heather ah, right you are

9

A: Tag burninate requests for voting

Please burninate general-physics. It is no more descriptive of a question than the fact that it was posted in this site. If absolutely no other tags apply to the question then it is very, very unlikely to be on-topic here. Unless anyone objects I'll start to slowly de-tag the current 54 mistagg...

heather

which, i'm sorry, but i don't really agree with posting in that old thread.

it's hardly visible, even with duplicate links, and nothing's gotten acted on there.

think food, general-physics (or whatever it is now), etc.

Feeds

Qmechanic has made a change to the feeds posted into this room

0celoñe7

@ACuriousMind Maybe as in I have reason to believe.

heather

@Qmechanic did you add the "new please-delete-this-tag questions" feed?

Emilio Pisanty

@Feeds @Qmechanic Just to clarify, that was adding the please-delete-this-tag feed into this room?

Also, here's a thought

migrate everything tagged physics-careers off to academia

heather

2:28 PM

^ooh, that sounds like a good idea

Emilio Pisanty

a total of 39 questions

heather

one would have to work through the tag manually and delete the junk first, though

Emilio Pisanty

mostly too old to migrate as standard

user685252

please get rid of the drop-down thing @Qmechanic

Emilio Pisanty

eight of them still open but frankly they shouldn't be, they're all in the close queue now

and then blacklist physics-careers

Qmechanic

2:29 PM

@heather & @EmilioPisanty: Yes. I want to see if my half-baked idea works.

Emilio Pisanty

@Qmechanic so what are the criteria for success?

i.e. what are we looking for?

Or, alternatively: is it possible to put a historical lock on the existing questions, and then blacklist the tag?

Qmechanic

@EmilioPisanty : Concretely feeds of posts with please-delete-this-tag.

Emilio Pisanty

@Qmechanic I mean, yes

but what would X and Y be in "if X happens, then the plan worked" and "if Y happens, then it's not working"?

heather

i assume what's being looked for is less junk tagged mathematical-physics or mistagged, or that sort of thing

user685252

Success = X + Y + Z

heather

2:32 PM

if there's no real difference, then it's not working

@user685252 what drop down thing?

user685252

where X = work

@heather notification of new questions

heather

ah.

i find that kind of useful, actually.

user685252

Y = play

heather

keep an eye out for weird titles.

user685252

can you guess what Z = ?

Fawad

2:36 PM

laZiness

user685252

close

seriously close

Emilio Pisanty

No, seriously, we're mucking with some pretty important tags here

what are the criteria for success?

user685252

Z = keep your mouth shut

Sid

Yeah, right tags. You guys need to write tag-wikis for the site.

Qmechanic

2:53 PM

@user685252 : This is my first feed creation, so I don't know if I did it correctly. However, if it works, it will not be in form of a drop-down, but as a comment.

user685252

ok

thanks for replying

heather

how do you cross out a mathematical expression in mathjax?

user685252

--- before and after

~~cross~~

chat.stackexchange.com/faq#formatting

heather

no, this is within mathjax. nevermind, i can just explain what is meant.

Loong

@heather \require{cancel}

heather

3:00 PM

$\require{cancel} \cancel{x^2}$

okay, that's awesome

thanks!

Emilio Pisanty

@heather mind your partial derivatives

user685252

LaTeX

Emilio Pisanty

$\frac{\partial^2}{\partial x}$ is inconsistent

user685252

note 2

Emilio Pisanty

but frankly if you think this one can be rescued then I disagree with you ;-)

heather

3:02 PM

@EmilioPisanty ack, yes.

@EmilioPisanty never hurts to try =)

Emilio Pisanty

Also, the laplacian is $\nabla^2$, not $\nabla$

heather

okay

there we go.

Emilio Pisanty

And also, I would transcribe the LHS of the bottom row as $\nabla \times \mathbf E$, not $\mathbf J + \mathbf E$

heather

oh, okay.

does the question at least make a little more sense now?

Emilio Pisanty

@heather not particularly

starting with "deriving the Maxwell equation"

presumably OP means deriving the wave equation from the Maxwell equations

but frankly it's such a low-effort question that any external help will just encourage OP to ask more terrible questions

so, e.g., OP is clearly referencing a derivation that works in reduced dimensionality, as is usually done in intro texts, but which is a very simplified version of the full derivation you can do. That's OK if OP references the text where they got this from.

which is not the case here.

heather

3:11 PM

they say they got it from a video.

it'd be nice if they linked to the video.

i really wish you were able to vote from the close vote review queue.

Emilio Pisanty

@heather it would be the absolute minimum they could do

Abcd

My attempts:Found T (tension in string). For mass 2M, T = 2Ma, For mass M, T - Mg = -Ma, therefore a = g/3

But the answer given is 2g/3

(a is acceleration of system which will be same for both blocks)

(T is tension i string which will be same throughout the string. )

Sid

@Abcd Eh, that would be 2T instead of T for 2M mass.

(constraint(I think, is how you get it) relation)

Abcd

(Question 31)

Sid

3:26 PM

Yep, still 2T for pulley B.

Abcd

@Sid Still I can't obtain the desired answer ($2g/3$)

@mbq Now it's quite active :)

@JohnRennie Can you tell me where I am going wrong?

heather

can you migrate to beta sites?

Emilio Pisanty

@heather mods can

who the hell flagged that?

Sid

Can you see flagged things @EmilioPisanty ?

I thought only mods and ROs can.

heather

@BenI. hello!

Emilio Pisanty

3:34 PM

@Sid 10k+ users can see chat flags

Ben I.

Hello! I chased a flag here.

Some other mod already removed the flag. I assume it was flagged by accident.

Emilio Pisanty

aaaaaand, that's what happens when people flag things that don't need to be flagged

we get this huge influx of people from other rooms who see the flag flock in and we just have a nice long discussion about flagging

4

heather

yup, happened to me once - lead me to the Nineteenth Byte

Ben I.

^^^^^^ What Emilio said :)

HDE 226868

@heather Yeah, just mods. But normal users can raise a custom flag and ask a mod to migrate, if they're pretty certain of it.

Sid

3:37 PM

maybe that flag was just to bring mods attention to that question? :P

heather

i think the question was deleted, so it doesn't much matter.

@HDE226868 okay =) I thought I saw one for HSM

HDE 226868

@heather Oh, yeah, I saw that one too.

Abcd

@HDE226868 and @BenI. must be moderators, therefore there names are in blue. Am I right?

Ben I.

Yup

Sid

mods in a different site, but yeah

HDE 226868

3:42 PM

@Abcd Yeah, on a couple different SE sites. I've been in here for a while today, just following along with the conversation. Don't mind me.

Abcd

Okay.

Ben I.

We were summoned by the flag on Heather's comment, which is why Heather has been banned for the next 6 years.

heather

heheh

the first thing that came to mind: "curse you, Perry the Platypus..."

Sid

only 6 years? I thought the punishment for calling mods unnecessarily in their time of work ought to be far harsher. :P

Ben I.

Cursing! That's another 2 years!

So, @heather, this is your main hangout?

HDE 226868

3:44 PM

All right, back to physics. I suspect the previous discussion's been derailed long enough.

user685252

^

Ben I.

Seems like a nice enough place.

Was there a previous discussion?

user685252

yes

heather

@BenI. yup

@HDE226868 I was looking at some of your blog posts - they seem quite interesting.

Ben I.

I'll leave then, and let you all get back to it. Enjoy your physics! Remember folks, F=MA. Ciao!

heather

3:46 PM

and amusing, I might add.

user685252

newtonian @BenI.

btw welome to the hbar

:-)

Avantgarde

(removed)

HDE 226868

@Abcd Have you tried drawing a free-body diagram?

@heather I'm flattered and surprised that anyone reads them. :-) Which one(s)?

Abcd

@HDE226868 Yes

Bye...

heather

@HDE226868 all the ones i saw - the introductory post, the one on analogy in physics teaching, one that was a part of worldbuilding's blog

you've actually made me consider writing some sort of post for worldbuilding's blog =)

HDE 226868

3:54 PM

@heather Oh, that's great! You might want to check out our blog chat room, then. It's a bit busier during the week. Monica Cellio's probably the go-to person if you want to submit a story.

heather

okay. I've popped in, I'll see when she responds =)

Anonymous

4:16 PM

Any idea how to typeset a small part of a code snippet, for example "System.out.println" as they do in the Stack Overflow comments ?

Abcd

Where $a_r$ is acceleration of blocks wrt pulley P

I just want to understand how is net acceleration $a_r-a$ and $a_r+ a$ for the two blocks.

Alyosha

Does anyone know (or know a link to) the precise statement of Schrodinger's equation in an electromagnetic field, using connections? In http://www.ihes.fr/~celliott/Dirac_quantisation_talk.pdf they use
$$i \frac{\partial \psi}{\partial t}= (\nabla+iA)^2 \psi$$
but this is a type error since $A$ as a $1$-form here ( they later talk about $A+d\chi$).

Abcd

Anyone?

0celoñe7

@Alyosha On $\Bbb R^3$, there is no difference between vector fields and 1-forms.

Anonymous

@Abcd Imagine P is at rest. What would the motion of the 2kg and 3kg block look like?

Abcd

4:19 PM

@Blue WIth accelerations $a_r$ and $-a_r$ . I don't understand the need for addition of a.

Alyosha

I'm not necessarily working in $\mathbb{R}^3$.

Anonymous

@Abcd Yes. Now if the pulley P begins free fall (with acceleration due to gravity g), what would the motion be like?

0celoñe7

@Alyosha Then interpret $\nabla$ as a covariant derivative.

It's a map $C^\infty(M)\to \Omega^1(M)$, so no issue.

Abcd

@Blue It ain't intuitive to me :( though I think it'd still be ar and -ar

Alyosha

OK, but how does one view $A$ as an operator $C^\infty(M)\to \Omega^1(M)$ in this case? You'd have to do that to avoid a type error.

0celoñe7

4:23 PM

@Alyosha $\Omega^1(M)$ is naturally a $C^\infty(M)$-module. Multiplication of $\psi\in C^\infty(M)$ by a 1-form is well-defined.

Hmm, the other side doesn't make much sense in that case.

Anonymous

@Abcd How can the accelerations be still $a_r$ and $-a_r$ ? Won't they become $g+a_r$ and $g-a_r$ ? That is because now P is falling down with accn $g$. So the net acceleration of the blocks can be found by superposition of P's accn and their accn w.r.t pulley P.

Alyosha

OK, so to be concrete, the right side should be
$$\nabla^2(psi)+iA \wedge \nabla(\psi)+i\nabla(\psi A)-A^2 \psi$$

And yes, I'm not sure about that side.

Abcd

@Blue SO basically we are adding to g to all bodies under consideration.

0celoñe7

@Alyosha That's one possibility, and then contract with the metric... But usually one sees $(\nabla+\mathrm i A)^2$ in the contex of Yang-Mills, where you get the curvature. So that's a 2-form. That equation does not make sense as written, I see your point now.

Anonymous

@Abcd Right

0celoñe7

4:28 PM

@Abcd I would say just contract that with the metric, because without a potential you would expect to have $\mathrm i\psi_t=\Delta \psi$.

Abcd

@0celoñe7 @Alyosha *

Alyosha

OK. I'm not really happy to assume Yang Mills stuff because I'm trying to find out from foundations how much of it is physical assumptions and how much is maths. Thanks for the input!

Metrics don't really appear in Maxwell's equations, though, so I'm not sure if it's good to use one.

0celoñe7

@Alyosha As a mathematician, most of these equations seem to be well-motivated in a classical setting or whatever, and then by some version of the Equivalence Principle are greatly generalized.

@Alyosha They do.

Curvature appears.

Maxwell's equations are $\mathrm dF=0$ and $\delta F=0$. Defining $\delta$ requires a metric.

Emilio Pisanty

0

Q: Why doesn't dupehammer work when it's the fifth vote?

I just voted to close this question over on physics and I'm confused with the result. When I was shown the question in the review queue, it had four close votes as off-topic, but I disagreed with that assessment - the way I see it, the question is a clear duplicate of this other question, and th...

support bug vote-to-close dupehammer

of possible relevance

Alyosha

Oops, sorry. You're right.

It might be annoying to check that the differential forms definition of that equation matches up with the original functions one.

But thank you, this was helpful.

0celoñe7

4:42 PM

@Alyosha It's still inconsistent because $A^2$ there should be $A\wedge A$, which is $0$ for a $\mathrm U(1)$ gauge theory. In QM you have an $\mathbf A^2$ though, I think.

Emilio Pisanty

5:05 PM

1

Q: Publication Authorship Credits

Many physics papers now have dozens of authors per paper. Experimental physics may have multi-organizational and multi-country contributing staffs, but I'd guess that most of the names don't contribute a word or equation to a paper, yet they get individual authorship credit. My question is who de...

soft-question general-physics+

anybody care to plop that on the close queue?

I'm outta votes, it seems

also, how come this one managed to stay open?

2

Q: Does physics address the topic of consciousness?

Does physics address the topic of consciousness? For instance, does physics say anything about how it might arise or what might be its qualitative properties? I'm wondering because it's interesting how certain combinations of particles are conscious (at least apparently) while others are not.

soft-question general-physics+ biology

heather

@EmilioPisanty in queue

0celoñe7

@EmilioPisanty it's 6 years old

seriously?

Emilio Pisanty

@0celoñe7 so?

it's still off topic by current standards

heather

@EmilioPisanty too broad? not mainstream?

I'm leaning toward too broad on this one.

Emilio Pisanty

@heather just off topic, it's not physics, period

I would say

heather

5:09 PM

mmm....not so sure.

i think the answer illustrates that it is related to physics.

0celoñe7

@EmilioPisanty you're using up internet

Emilio Pisanty

@0celoñe7 no, I'm just in a grumpy mood is all

heather

i think i'll not cv that second one.

i think it's on-topic, if a little broad.

Emilio Pisanty

I've been clearing out the bottom row of general-physics and there's a lot of bad stuff there

@heather that's perfectly reasonable

heather

-5

Q: Are there any expressions in physics which always equal 1?

Are there are any mathematical expressions in physics that will always be equal one or to a number which is almost equal to one. If there are any I would highly appreciate if you could show me and tell me the name of it/them.

general-physics+

^can someone vote to delete this?

it adds zero value to the site.

Mithrandir24601

5:11 PM

@EmilioPisanty I'm going to mildly disagree with that - 1. There are physics-y people that talk about consciousness and 2. Asking 'Is there a known classical [non-quantum], relativistic law that allows massive particles to travel faster than light' isn't off topic, it's just that an answer that isn't no isn't 'mainstream'

(i.e. is total nonsense)

Emilio Pisanty

@Mithrandir24601 I... have no idea what your point is, but sure

Mithrandir24601

@EmilioPisanty If you believe that the answer to the question is no, then the answer is no. It doesn't make the question off topic

heather

and can someone vote to delete this as well:

1

Q: What are the most important unsolved problems in physics?

What problem/s in physics would have the most far reaching implications if solved?

general-physics+ big-list

no content really, much too broad.

Emilio Pisanty

@Mithrandir24601 I think the question is off topic. You apparently think it is on topic. You have a perfectly legitimate right to think it is on topic. I have a perfectly legitimate right to think it is off topic. I don't see what else there is to discuss.

@heather yeah, I agree. I voted to delete but it won't do much, community deletion only works for negatively-scored posts.

heather

the number of things tagged with general-physics with more specific tags is astounding

@EmilioPisanty in that case, it just needs one more downvote to get it negatively scored.

Mithrandir24601

5:16 PM

@EmilioPisanty Because I wanted a short discussion/explanation of why you think it's off topic and thought that the easiest way to get that started was to explain why I felt that it was on topic :P

Emilio Pisanty

@heather ... and two more user delete votes, or a mod deletion, the former of which is a rare thing around here.

Mithrandir24601

@heather Done

Emilio Pisanty

@Mithrandir24601 in which case you might start with an explanation of why you think it's on topic that's intelligible.

Mithrandir24601

Although I don't have anywhere near the required rep to cast delete votes

@EmilioPisanty Fair point

Emilio Pisanty

"There are physics-y people that talk about consciousness" doesn't make a question about physics. There are physics-y people that overstep the boundaries of what they can intelligibly talk about and talk all sorts of nonsense about philosophical topics that they should really stay out of.

Mithrandir24601

5:19 PM

@EmilioPisanty Yeah, that's true - hence I didn't want to use the word 'physicists'

Also, I do get that that may put it firmly in the 'not mainstream' area

Emilio Pisanty

@Mithrandir24601 oh, no, there are plenty of physicists who fall under that description, but that still doesn't make what they talk about physics.

There's plenty of physicists who talk about, say, politics, but that still doesn't make politics physics.

Mithrandir24601

Generally, when I say physicists I suppose I tend to mean someone with a job that involves publishing papers in mainstream physics, but it's not something I've thought much about

Emilio Pisanty

@Mithrandir24601 unless you have something specific to discuss, I would rather not go into rambling-rant territory just for the sake of rambling rants.

Mithrandir24601

Erm... OK. My main point is that (if this makes any more sense, which it probably won't) asking something along the lines of 'is x mainstream physics?' isn't a non-mainstream physics question (although I don't actually know if such a question is on or off-topic here) regardless of whether or not x is mainstream physics

Emilio Pisanty

@Mithrandir24601 that's entirely irrelevant; at no point did I claim the question was off-topic as non-mainstream. You're attacking a strawman.

Mithrandir24601

5:29 PM

So why is it off-topic

?

Emilio Pisanty

because I don't think it's about physics.

Mithrandir24601

And now I'm the one that's confused :/

Slereah

5:55 PM

Emilio Pisanty

@Slereah wut?

heather

probably a), mercury, because all others are solids?

Slereah

But what of question 2

Emilio Pisanty

hoo carez

Sanya

iv

without any doubt

Emilio Pisanty

6:06 PM

it's plenty easy to pose nonsense questions

Anonymous

@Sanya Well, it's more funky than the other options obviously :P

heather

wait @Slereah, i just noticed something -

question 1: a, b, c, d are options

question 2: i, ii, iii, iv are options

::shivers::

Anonymous

Don't know why but heather's comment reminded me of this meme :D

Avantgarde

6:25 PM

What do you prefer calling $\hbar$? h cross or bar

Anonymous

From where did the cross come?

ACuriousMind

@Avantgarde Look at how this chat room is named, then take a guess ;P (I've never heard "h cross")

Avantgarde

haha, I realize the chat room name is bar. I also realize I've probably been pronouncing it incorrectly all this while. Oh well

I've heard both being used. It will be hard to shift to bar after so long

Well this is fun pronouncekiwi.com/Reduced%20Planck%20Constant

Danu

6:43 PM

o/

AccidentalFourierTransform

what happened to SRS?

Mithrandir24601

@heather Another reason for mercury is it's the only non four-letter word :P

AccidentalFourierTransform

I said hey, what's going on?

Ooh, ooh ooooooooooooooooh

0celoñe7

@ACuriousMind Consider the following... $\Omega\subset\Bbb R^n$ with $\partial\Omega\in C^1$ so we have a geometric outer normal field $\nu$. Suppose $\bar\Omega$ has a Riemannian metric $g$ and a Riemannian outer normal $n$. For $w\in T_p\partial\Omega$, we have $g_{ij}w^i n^j=0$. But the geometric normal gives $w^i\nu^i=0$. Since $w$ is arbitrary we are left to conclude $n^j=g^{ji}\nu^i$, no?

AccidentalFourierTransform

I wasnt all crazy

you do need a connection over the space of fields, in a sense

0celoñe7

6:53 PM

You're crazy for considering a configuration space that isn't a vector space :P

vzn

@Kaumudi.H you might find this interesting, it answers a huge question that was on my mind lately after reading other movie reviews of Dunkirk thedailybeast.com/… ps my uncle was here en.wikipedia.org/wiki/Omaha_Beach

0celoñe7

@AccidentalFourierTransform Do I even want to know what $m$ and $n$ are supposed to be

AccidentalFourierTransform

DeWitt indices :-P

an index running over $\mathcal M\times\mathcal A$, where $\mathcal M$ is a manifold, and $\mathcal A$ is a discrete index set

0celoñe7

mathcal is a bit pretentious

manifolds aren't that special

AccidentalFourierTransform

\mathcal{0celo7}