The h Bar

Sofia

12:09 AM

@ACuriousMind ah, no! You mentioned symmetry breaking and I thought that it was one with some Norton's Dome.

ACuriousMind

@Sofia Then you mean this one

0celo7

@ACuriousMind No school.

I have lost all faith in my school system.

Freaking rain

ACuriousMind

You...get off because of rain?! Should you think about building an ark or what?

0celo7

@ACuriousMind Slush

People die in that!!11 /s

ACuriousMind

@0celo7 Well, I, for one, prefer not to risk my life getting to school ;)

0celo7

12:16 AM

@ACuriousMind What does Tong mean by "dual of the Riemann tensor" on p. 27/87 of damtp.cam.ac.uk/user/tong/string/four.pdf ?

@ACuriousMind I found a Physics Forum post which shows it as a Riemann tensor contracted with some Levi-Civita tensors.

ACuriousMind

@0celo7 It's the Hodge dual.

But probably not the Hodge dual as you'd naively take it

0celo7

@ACuriousMind Hodge dual of a 0-form is a 4-form.

ACuriousMind

The Riemann tensor is not a 4-form on the space, because it is not antisymmetric in all indices

It is a 2-form as the curvature of the tangent bundle, and it is in this sense that the Hodge dual is taken

0celo7

Oh, I see.

The curvature form from the structure equations?

ACuriousMind

The Christoffel symbols are actually a connection on the tangent bundle, i.e. 1-forms, and the Riemann tensor is then just a 2-form

HDE 226868

12:29 AM

@ACuriousMind @ACuriousMind Yes; I've seen some of his/her posts.

ACuriousMind

The nLab has a good exposition of how to think of the Christoffels, and the Riemann tensor is then just their curvature

0celo7

@ACuriousMind So we take components of the dual of the curvature form?

ACuriousMind

@0celo7 Exactly

0celo7

Makes sense, there is a similar prescription for the Faraday form.

ACuriousMind

the intro to the nLab article speaks right out of my mind, and is part of the reason why I never got to real grips with GR

HDE 226868

12:33 AM

@ACuriousMind Why did you ask (about the other HDE)?

0celo7

@ACuriousMind I seriously recommend Straumann. Page 1 defines manifolds as topological Hausdorff spaces. (Page 1 in the sense that the last 150 pages are diffgeo review, and he tells you to read the last 150 pages before the first 600)

ACuriousMind

@HDE226868 Oh, I just stumbled across them and wondered if you knew

No particular reason, really

Except that I can now segue nicely into asking what the meaning of HDE is, if you want to tell me ;)

HDE 226868

@ACuriousMind Everyone's heard of Cygnus X-1, but I doubt many people know the name of its companion star. . .

Cygnus X-1

Cygnus X-1 (abbreviated Cyg X-1) is a well-known galactic X-ray source and black hole candidate in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3×10−23 W m−2 Hz−1 (2.3×103 Jansky). Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate and it remains among the most studied astronomical objects in its class. It is now estimated to have a mass about 14.8 times the mass of the Sun and has been shown to be too compact to be any known kind of normal...

0celo7

@ACuriousMind Straumann embraces coordinate free notation in GR...

HDE 226868

HDE itself is

Henry Draper Catalogue

The Henry Draper Catalogue (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the Henry Draper Extension (HDE), published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the Henry Draper Extension Charts (HDEC), published from 1937 to 1949 in the form of charts, which gave classifications for 86,933 more stars. In all, 359,083 stars were classified. The original HD catalogue covers the entire sky almost completely down to an apparent photographic magnitude of about...

ACuriousMind

12:36 AM

@0celo7 <3

0celo7

@ACuriousMind 30 page derivation of the Kerr metric is beautiful.

25 of them have no indices!

ACuriousMind

I can happily live without ever doing ugly proofs with Christoffels again. I might actually take a look at Straumann, then

0celo7

@ACuriousMind Stay away from chapters 5 and 6.

(G waves and Post Newtonian Approx.) (i.e., a whole lotta indices)

ACuriousMind

@0celo7 Yeah, I can do without that

@HDE226868 Well, I couldn't have told you what Cygnus X-1 was, either :P

But thanks

HDE 226868

@ACuriousMind Aw, astrophysics just had its feelings hurt. . . :-)

0celo7

12:39 AM

@ACuriousMind Of course, he writes indices on tensors when appropriate. But Christoffel symbols are not used unless he actually does a calculation with them, i.e. solves the geodesic equation.

ACuriousMind

@0celo7 Yeah, I'm not against writing indices. I'm against defining stuff by them, mainly

0celo7

@ACuriousMind I saw in a comment that you read Schutz.

Have you read any other GR?

@ACuriousMind $\Gamma^\lambda{}_{\mu\nu}:=\frac{1}{2}g^{\lambda\sigma}(g_{\sigma\mu,\nu}+g_{ \sigma \nu , \mu}-g_{\mu\nu,\sigma})$ is the way to go.

ACuriousMind

I didn't really read Schutz, I looked into it because our lecturer was following it closely, and I didn't understand much of what he said. I didn't understand more after reading some of the passages corresponding to the lectures, so I gave up

0celo7

MathJax in chat sucks.

ACuriousMind

But, no I haven't read any other GR

Heh

0celo7

12:43 AM

It was probably trying to prevent me from committing evil.

ACuriousMind

Straumann would be this one, yes?

0celo7

I'd imagine Hawking would give you quite the challenge, but it's more of a Cosmology rather than GR text.

Ja

I like Straumann, anyway, for whatever that's worth.

@ACuriousMind He also manages to call the geodesic deviation equation a Jacobi equation.

ACuriousMind

Well, I'm reading Quantization of gauge systems and Quantum physics: A functional integral point of view currently, but whenever I'm done with that, I'll consider actually learning GR ;)

0celo7

@ACuriousMind I'm sure someone has recommended Wald to you, right?

ACuriousMind

@0celo7 Yes

@0celo7 This guy sounds fun

0celo7

12:46 AM

@ACuriousMind Have you considered it? He does hit topology hard.

@ACuriousMind How so?

ACuriousMind

@0celo7 Because it is a Jacobi equation :P I didn't mean that ironic

@0celo7 Considered, yes, but I didn't decide anything except not right now

0celo7

@ACuriousMind Fun fact: the fact that geodesic deviation is a Jacobi equation is critical to the proof of the singularity theorems

Conjugate points (points where the Jacobi field vanishes) describe points where geodesics are not maximal.

@ACuriousMind I know you're not reading any GR now, but I don't get how you half-read one introductory book and then decided to throw away the subject.

ACuriousMind

@0celo7 It was rather having a lecture on it and it was horrible. Meanwhile, quantum physics and QFT were awesome, so I gravitated towards them rather than GR

I'm not a devourer of books about all subjects like you are ;)

0celo7

@ACuriousMind String theory is the best of both worlds! Maybe.

@ACuriousMind I'm just trying to figure out my interests.

ACuriousMind

@0celo7 Nothing wrong with that :)

ACuriousMind

1:10 AM

What is it with people bountying questions after I've cast a close vote on them lately?!

0celo7

@ACuriousMind Which one(s)?

ACuriousMind

4

Q: What concepts could I teach to children in a game about quantum-mechanics?

I think that games can also be used to teach things to young people or childs, different than for example a book and could be more "direct" than teaching them all the math background, I would like to hear about different concepts and way to teach them inside a game, the game could be a 3D game, o...

It's too broad because it's asking for a list

0celo7

@ACuriousMind IMO too broad is a poor reason for closure.

(In this case at least. I'm sure there are times when it is applicable.)

ACuriousMind

Additionally, it surfs along the border of primarily opinion based and off-topic, because, while asking about educational techniques is not a bad question as such, it is not really about physics

Stan Shunpike

2:00 AM

So, anyone got any suggests for books with interesting applications of ODEs

?

Stan Shunpike

2:16 AM

@0celo7 what are you reading these days?

0celo7

@StanShunpike I've been reading string theory, SUSY, and GUT lecture notes.

Although, I just started Dexter, so nobody knows how much reading I'll be doing.

Stan Shunpike

LOL via Netflix?

0celo7

Yup.

Stan Shunpike

My bro just introduced me to key and peele. They're very funny.

@ACuriousMind @DavidZ I think that's off topic

The post about the game I mean

0celo7

3:05 AM

Dexter's acting is really autistic. I mean, I know he's supposed to be a sociopath, but damn...this is pretty painful.

Also that girlfriend of his is pretty painful too.

And the sister...

Danu

8:05 AM

@StanShunpike They're actually in the show 'Fargo' (inspired by the movie)

infinitesimal

8:41 AM

Hi pal @Danu

Danu

Heyyo

infinitesimal

Have you heard the returning explorer puzzle?

I found it very interesting

Danu

I think I have, but I don't remember what it is

infinitesimal

Oops :(

Did you see it?

Danu

No

infinitesimal

8:50 AM

A person travels exactly one mile South, one mile east, and one mile north And returns back to exactly where he started. Where did he start?

my linen closet

Nope

Danu's linen closet

Nope

Santa's nuclear bunker at the north pole

infinitesimal

8:53 AM

The answer is the North pole. But the interesting question is there any other place on earth this can happen?

Sabre Tooth

South pole nuclear bunker

infinitesimal

How can you go south from the South Pole?

Sabre Tooth

jump

infinitesimal

Step 1. Go one mile south.

So start on the line of latitude one mile from the South Pole

Sabre Tooth

How to go south at the South Pole: jump

infinitesimal

8:57 AM

Step 2. Go one mile east.

Stan Shunpike

I was reading A Zee and he has this picture on page 346. It looks like Beethoven and Einstein. And suddenly a thought occured to me: imagine if the two families could somehow cross genealogically, they would yield the most awesome hair ever.

infinitesimal

:D))

Stan Shunpike

Lmao

It would elevate hair almost to the level of @JimdalftheGrey from Lord of the Rings

Danu

10:04 AM

Huh, the Wikipedia definition of a ring includes unity and associativity, while the book that I'm reading does not. Does anyone know what's 'standard'?

...Lang also includes unity and associativity. Is Vinberg's book on Algebra an exception?

Nick

10:47 AM

@Danu Go to the math room. They'll help you out.

Danu

11:05 AM

@Nick I know that, but I'm lazy. There are enough good mathematicians here too, if I wait a bit longer ;)

Kyle Kanos

2:33 PM

Is it wrong to say that $\mathbf a:\mathbf b+c\mathbb I$ is a dyad product plus something times the identity matrix?

Specifically the whole dyad products are really tensors and not really matrices

tpg2114

@KyleKanos I wouldn't think there is anything wrong with it, but you could be more precise and say that $\mathbf I$ is the identity tensor

Because you can do things like $\mathbf a : \mathbf I = tr(\mathbf a)$

Kyle Kanos

I hadn't considered calling that a tensor, I would have gone with the Kronecker delta

tpg2114

So you can treat I as a tensor

The Kronecker delta is just the indexed version of an identity tensor

Right? You could do $\delta_{i,j,k,l,...}$ I suppose. But I never do anything that needs more than 2 indexes

Kyle Kanos

I only need 2 indices here as well

tpg2114

I wouldn't mix notation then and just stick with $\mathbf a : \mathbf b + c \mathbf I$ or $a_{ij} b_{ij} + c \delta_{ij}$

And call \mathbf I the identity tensor

Kyle Kanos

2:39 PM

Yeah, I'm changing it now

tpg2114

I really dislike vector notation

Index notation is so much better

Kyle Kanos

I do prefer the index notation

Also, Latex is really cool:

12

Q: inserting sentences between subequations

My apologies for asking so many questions here. I want to introduce a sentence in between 2 subequations which is not in math mode. Here is the code that I am using: \documentclass[12pt,oneside,reqno]{amsart} \usepackage{amssymb} \usepackage[T1]{fontenc} \usepackage[utf8]{inputenc}...

equations amsmath align

Had no idea that it was possible to do that

ACuriousMind

2:54 PM

@Danu I think requiring associativity and unity is standard in the sense that almost everyone will assume it if you simply say "ring".

tpg2114

Latex is awesome, but then it's also frustratingly stupid sometimes. Like how you can't have your open and closing ( / { / [ separated when using align

So many times I want to line break equations and line up terms and stuff but I want my grouping marker to be sized automatically. But you can't do that

Kyle Kanos

Doesn't \right. give you the blank closing?

I'm pretty sure I've done stuff like that as hacks because the "right" way to do it doesn't work

tpg2114

I haven't seen that one before

Maybe that would work

Kyle Kanos

I also think that \bigg( doesn't require closing either

\begin{align}y&=\bigg(a+b \\ & -c - d\bigg)\end{align}

tpg2114

Yeah, the \bigg etc don't

But you have to manually pick how many g's you want

Kyle Kanos

3:00 PM

Isn't it 1 or 2?

tpg2114

artsci.kyushu-u.ac.jp/~ssaito/eng/tex/tips.html

Nope, you can keep getting larger and larger with more g's

Hang on, I must have those commands in a module somewhere

In my system

Hah. Go figure

I forgot we installed some custom packages designed in our lab on all our workstations

Kyle Kanos

o.O

That's a lot of g's

tpg2114

\begin{equation}
\begin{aligned}
f &= \left[ A \right. \\
  &\qquad \left. + B \right]
\end{aligned}
\end{equation}

That works

So I learned something new today. I guess I can just go home and go to sleep now! Mission accomplished.

CeceXX

Hey everyone!

May you please tell me if I formulated a good question here? physics.stackexchange.com/questions/168577/…

Kyle Kanos

@tpg2114 A GT faculty's (Dr Ballantyne) giving the physics colloquium today at Clemson

ACuriousMind

3:12 PM

@CeceXX I think it is off-topic per our homework policy, but it's not its formulation that is bad, it's simply that we don't want questions that ask for the solution to a problem.

Kyle Kanos

Also: whoever voted to migrate this question helped me get a silver badge (specifically, the 'yearling' badge)

Though I also had 25 rep migrated from here to there, but whatever

tpg2114

@KyleKanos What's the topic? I've only taken 1 class in the physics department, my E&M undergrad class, so it's unlikely I will know who it is

ACuriousMind

@KyleKanos Then you can "thank": ACuriousMind, Sofia, Neuneck, Martin, Chris Mueller ;)

Kyle Kanos

@tpg2114 X-ray observations of Active Galactic Nuclei

tpg2114

Definitely out of my area

Kyle Kanos

3:19 PM

Quick question on duplicates. For a case such as this one where it's pretty much an exact duplicate, should these be merged? Or is that not something we do here?

ACuriousMind

@KyleKanos Manishearth says exact duplicates are merged if you raise a flag asking for it.

Kyle Kanos

Aha, and my namesake asked it too!

ACuriousMind

Qmechanic seems to say that merging is perhaps more trouble than it's worth, though

Kyle Kanos

And reading his comment, I seem to concur that it's probably best to just not do it.

CeceXX

@ACuriousMind Thank you for your feedback! So are you recommending against posting numbers in a question if it is homework-related? Should I just ask for an explaination and possibly a formula? Thank you!

ACuriousMind

3:29 PM

@CeceXX No, it has got nothing to do with the specific numbers. Asking for a formula on how to solve a generic problem is off-topic for the same reason - it is not a "conceptual question", although I admit that the exact definition of conceptual is difficult to give.

Kyle Kanos

I suspect a good break-down between conceptual & numeric would be the "Why is" versus "What is"

Also: Transformers (the first one with Shia Lebouf) is available for free on Google Play, for those interested

Qmechanic

@KyleKanos : Speaking in general rather than the specific post: Merging is time consuming for the moderators because there can be slight variations in question formulations so the answers may need some adjusting, etc. Merging has happened in the past, but we (the moderators) don't have time to do it often. In general, answerers are encouraged to answer the oldest entry. Also answerers are urged not to post duplicate answers.

See also this meta question.

Jiminion

4:05 PM

OK, how come the search never finds the duplicate question, but once you post, someone IMMEDIATELY finds the EXACT question from somewhere else? (It sounds like something a stack exchange standup comic would say....)

@Qmechanic I'd only merge if there really is a union of questions. I'd say 95% of the time, the single, older question could suffice. OTOH, I do feel that multiple answers have value, even if only one can be the best. So maybe moving interesting answers to the old question would be good, but I don't know if that's possible.

ACuriousMind

@Jiminion The search is crap, but the users who have enough rep to call duplicates have probably simply seen the question before because they were around when it was first posted.

Using google with site:physics.stackexchange.com is actually often better for finding stuff here than the actual search, btw

Jiminion

@ACuriousMind I've heard that about the search (maybe from you....) so how annoying is it to see duplicates posted, even if the poster deletes them? (I am guilty of this, but I do try to delete them.)

ACuriousMind

@Jiminion Duplicates are the least annoying kind of closable questions - because they often are, at least good questions.

3

Kyle Kanos

@Jiminion The trick I've found with SE's search engine is to just use less words and then scroll

But it does also help to know the words you need

Jiminion

Why don't they fix the search? It seems it would also help locate unanswered questions that become lost in the void.

ACuriousMind

4:18 PM

7

Q: Why does (insert favorite search engine here) get better search results than SO, SF, or SU's own search function?

I've lost count of the number of times I've either asked a question or been answering a question only to have it closed as an exact duplicate. But searching for the question turns up a list of seemly unrelated results using the sites builtin search. Then if I switch to a general web search engin...

support search

Kyle Kanos

152

Q: Link up the excellent search engine that gives "Questions that may already have an answer" with the search box

The search engine that works behind the scenes to generate the "Questions that may already have your answer" is staggeringly amazing; even this first draft at a not very good title brings up exactly the question I was going to ask as the first result: On the other hand the search bar doesn't...

feature-request search

Sadly the 100x upvoted question has no answer

Jiminion

4:45 PM

@KyleKanos Ugh, another branch of SE to join.....

Kyle Kanos

You don't have to join there

Sofia

4:55 PM

@ACuriousMind you have big credibility. Can you give a hand of help. Please look at this question and at the exchange of comments. Can you?

@ACuriousMind are you here, at all?

ACuriousMind

@Sofia Yes, I am reading

Sofia

@KyleKanos I have some young fellow that insists very much with a question about pulleys. I feel pity, and I'd incline to copy to his post the answer that I gave to an old and approx. similar question. But, please decide you, because I don't have the heart to decide. I would have pity.

Jiminion

@Sofia I think this is more than one level to answer that question. Sort of like the "lying to kids" issue with explaining things.

ACuriousMind

@Sofia The issue is that "has a photon a well-defined wavelength?" is an ill-posed question. Photons in momentum eigenstates (or approximate momentum eigenstates, i.e. those with very narrow wavepackets in momentum space) do have a wavelength, and it is a definite property for them. Photons that are broader wavepackets do not actually have a well-defined wavelength. Since atomic transitions are very sharp though, most photons you'll find have a wavelength.

Nevertheless, it is true that you cannot, from a single measurement, tell whether the photon was in a momentum eigenstate or not, so a single measurement indeed tells you nothing about whether the photon you measured had a well-defined wavelength before you measured it

More generally, all questions of the form "Does X actually have well-defined property Y without measurement?" are ill-posed in the quantum world, and the question simply presupposes it can ask that question.

Sofia

@ACuriousMind no, no, he didn't say that his photons come from atom transitions, he said that the wavelength is established at the contact with the measurement. Well, how shall I redirect your answer to Jiminion?

@ACuriousMind you don't have to convince me, as we think the same.

Danu

5:09 PM

I just watched the Turing movie. Mediocre at best.

Sofia

@ACuriousMind won't you leave a comment at that site? Only that would stop the polemic about what have single particles. Pleeease! Otherwise there would be more and more arguing.

ACuriousMind

@Sofia I'm trying to fit all I want to say about that into a few words, but I think it might grow into an answer of my own.

@Danu I must be living under some sort of stone. Which Turing movie?

Well, it seems I'll answer the original, then.

Jiminion

@Danu I think it wasn't so bad. A lot of people didn't like it very much. I think I must be more comfortable with the creative license they took to cram a bunch of stuff into the one story (which would be somewhat dry if presented with strict accuracy in a video medium).

@ACuriousMind She started it!!! :)

@Sofia I'm here. I'm listening and reading.

Sofia

@ACuriousMind @Jiminion on what do we argue now? Look at what the Curious Mind said. It's a few comments above my one. I'd want to go to eat, until now I didn't take my lunch. Pleeease, look at what he said.

Danu

@ACuriousMind Recent Hollywood one - The Imitation Game

@Jiminion It was mostly just uninteresting, and the romantic component apparently misrepresents his real life

Jiminion

5:27 PM

@Danu I liked 'Enigma' (Kate Winslet, Saffron Burrows) better. It was still drama-ridden, but at least got a bit more into what they needed to do to crack a code, esp. finding letter 'loops'.

Danu

@Jiminion It did turn me onto the biography by Hodges, which is a good thing.

I attended a talk by Hodges recently, he's doing quite interesting mathematical research (as well as having written this extensive biography on Turing) on spinors and twistor geometry

His work provides the fundaments for the recently highly-hyped amplituhedron approach to QFT

CeceXX

I just reworded a question that was put on hold. If anyone could kindly reopen it I would appreciate it. If it can't be reopened, I would like to know why. Thank you very much! Link: physics.stackexchange.com/questions/168577/…

Jiminion

@Sofia I read it. I don't want to argue. Life is too short. I understand you can never know the wavelength of a single photon, because if you measure it, the wave has collapsed. I was trying to answer at a 'softer' level, which I thought was appropriate for that OP.

Danu

@CeceXX "the angle it forms of a body" is not correct grammatically

CeceXX

@Danu Fixing it right now. Thank you very much.

Danu

5:33 PM

@CeceXX I left a comment, but then noticed the accepted answer is saying pretty much the same thing. The key to simple questions such as this is geometry: Draw the picture (with the forces)!

Sofia

@Jiminion yes, but just, I hold that it's not a good thing to leave the OP thing that single photons have some feature. Indeed life is short, and my stomach is very empty. So, I jump to eat smth.

Jiminion

@Sofia OK, go eat. I am eating a salad right now too.

CeceXX

@Danu Thank you very much for your point and for editing my question. I would like to draw the picture online, could you please recommend an online tool that does the job?

Danu

@CeceXX No idea, sorry.

Jiminion

5:50 PM

@CeceXX google draw is good, I've heard.

CeceXX

I modified new question I posted recently, too. I would really appreciate comments on my recent changes and how I can improve it further. physics.stackexchange.com/questions/168589/…

@Jiminion Thanks! I'll check it out.

Wrong link, sorry: physics.stackexchange.com/questions/168589/…

Sean

Can someone explain to me why downvoting an answer makes you lose a rep point, but downvoting a question does not?

infinitesimal

Why not?

Sean

What do you mean why not?

ACuriousMind

102

Q: Should downvotes on questions be "free"?

As I'm sure you all know, downvotes "cost" 1 reputation. That is, every time you downvote: -2 to post owner -1 to you This is done to make sure downvotes are cast only when you feel strongly that something is incorrect / wrong / dangerous / of low quality. We've been tweaking a few thing...

discussion status-completed questions voting

infinitesimal

6:01 PM

@Sean^

:-)

Sean

Because I'm gonna be honest, I've ignored some bad answers because I don't want to lose the rep point haha

infinitesimal

Sorry pal, "why not" is just a reflex answer I have for "why" questions :-)

ACuriousMind

What is this weird habit of writing $F = ai + bj + ck$ for vectors and just assuming everyone knows that this is supposed to be the column vector containing $a,b,c$ in the standard Euclidean basis? I've seen it several times now.

Sean

@ACuriousMind They don't put hats on the unit vectors? Haha

ACuriousMind

Even with hats, I rather denote bases as $\hat{e}_i$ instead of $\hat{i}$

But, yeah, there aren't even hats

infinitesimal

6:07 PM

Everybody knows their abc's :-)

ACuriousMind

For all I know, that F is a quaternion.

(with zero real part)

infinitesimal

Well, all numerals could be viewed that way, no?

ACuriousMind

What's your definition of a numeral?

infinitesimal

A numerical expression.

ACuriousMind

And...you want to say that every numeral can be viewed as a quaternion?

Sean

6:11 PM

@ACuriousMind were you referring to the force/work question

I just added vector hats, just for you haha

and also because i wanted +2 rep

infinitesimal

with, as you said, zero components :-)

ACuriousMind

@Sean It was the latest example, but I've seen it before

I just never saw anyone writing it like that when I had to deal with these things

infinitesimal

They're just being lazy.

ACuriousMind

Nah, I'm more about using $i,j,k$ as the basis than leaving out the hats

In my world, unit vectors always were $\hat{e}_1,\hat{e}_2,\hat{e}_3$

I somehow was under the impression that was the standard

Sean

@ACuriousMind I've NEVER seen it that way

infinitesimal

6:14 PM

The Internet has created a whole new standard

Sean

in my mechanics class it almost always $\hat i, \hat j, \hat k$

infinitesimal

People can create their own notation

Sean

But especially in E and M, if we were working in other coordinates, we might do $\hat r, \hat \theta, \hat \phi$ or \hat r, \hat \theta, \hat z$ or what have you

ACuriousMind

Always $\hat{e}_\phi$ or $\hat{e}_r$ or somesuch with us

Because there is a difference between the coordinate and the vector associated to it, and we should never confuse the two

Sean

sure, that's why you add the hat

haha

ACuriousMind

6:17 PM

That's what my mechanics/linear algebra teachers told me, at least

Okay, I mean, it's not that $\hat{r}$ is not understandable

Sean

But I've learned that notation is very much at the whim of the person writing the textbook

infinitesimal

Exactly^

ACuriousMind

Weeeeeeinberg! ::shakes fist::

Stan Shunpike

@ACuriousMind Amen

:: shakes both fists ::

infinitesimal

-_-

Stan Shunpike

6:20 PM

MTW make up notation

It's really annoying

They like to use bold letters and write an entire word Riemann for the Riemann tensor

I have no idea why

They could just use R like everyone else

Sean

Am i the only physics major on the planet who never had to take an entire class on GR as an undergrad?

ACuriousMind

@Sean It's not obligatory here.

Sean

Oh. I went to a small school, so I didn't know if it was more common elsewhere

Sofia

@Jiminion Enjoy your salad!

infinitesimal

Einstein himself believed GR should be taught in high school.

As shown in his book for high schoolers Relativity: the special and general theory

Sean

6:35 PM

I mean, I took a Modern Physics class

there was GR in it

but it was more conceptual

I haven't done much mathematically with GR

Stan Shunpike

@infinitesimal I was asking about GR in high school and my teacher didn't want to get into it.

He new all about it.

He had done supergravity research. I was mad lol

I was like "Why won't you teach me!"

Kyle Kanos

@Sofia My suggestion: stop feeling sympathy for questioners. If the question is off-topic, just downvote (if you want), vote to close, and then leave it alone.

infinitesimal

@Sean @StanShunpike somethings are just better left as independent study for interested students. GR is one of them. Try reading the creator's own explanation for high schoolers is what I would tell those who show any interested.

ACuriousMind

@infinitesimal Actually, reading Einstein is probably a bad idea if you want to do GR

infinitesimal

Why so?

Sofia

6:47 PM

@KyleKanos what in fact is the purpose of our site? What we are doing here? For what is it good? I'll tell you why I ask. Because sending people to home exercise is sending them to all the winds. I tried to get into that home-work site, but it sent me here and sent me there, in short, good for nothing. This is what I saw.

ACuriousMind

@infinitesimal Because modern GR has a very different understanding from what it was at its inception. Recall that Einstein himself said something like "Since the mathematicians have invaded my theory, I do not understand it myself"

Kyle Kanos

We are trying to build a repository of good conceptual questions about physics.

Doing homework is not part of our goal. If you want to answer those types of questions, there are other places online for that

It is slightly more of a problem for self-studiers because they have no one to turn to, but actual students (i.e., ones in some school) have classmates and instructors to turn to for homework issues.

infinitesimal

@acu he said that about Lorentz and SR too

ACuriousMind

@infinitesimal Which is why I don't advise learning SR from Einstein, either :P

All these thought experiments and contractions and dilations obscure the simple idea of Lorentz transformations and the invariant interval

Sofia

@KyleKanos I see smth. strange. This question was put on hold only by 4 people. There should be 5, shouldn't there.

ACuriousMind

6:52 PM

@Sofia A moderator close vote closes a question immediately

Sofia

@ACuriousMind hmmm! What you say! Well, if this is the rule, O.K.

Kyle Kanos

Moderators can unilaterally close a question.

They don't need any 3k+ members to do so

infinitesimal

don't we learn Newtonian mechanics the way Newton discovered it? @acu

Kyle Kanos

You can see the moderators here: physics.stackexchange.com/users?tab=moderators

ACuriousMind

@infinitesimal Lol...try reading the principia. It's literally impenetrable for modern minds

infinitesimal

6:55 PM

I have.

ACuriousMind

No rigorous notion of calculus and emulation of Aristotle's style, among other things, make it very hard to follow

Sofia

@KyleKanos was it necessary to remind me that my reputation is small? You that sometimes tell me that I am rude? "they don't need ANY 3K+"

Kyle Kanos

@Sofia Uh, no, I was not trying to remind you that your reputation is small (mostly because I don't think it's small). Nor was I calling you rude.

ACuriousMind

@infinitesimal I think we probably do learn Newtonian mechanics rather like Newton, just modernized, but we do not learn electromagnetism like they discovered it, and we do not learn GR as Einstein conceived of it

And, unfortunately, we do learn quantum mechanics as it was invented

Sofia

@KyleKanos "they don't need ANY 3K+"

Kyle Kanos

6:58 PM

@ACuriousMind ...because we should be learning Bohm :D

@Sofia Yes, emphasis on any 3k+ members along with the word unilateral in the line above it.

ACuriousMind

@KyleKanos If I have to never hear "wave-particle duality" again, I'd consider paying that price ;)

Kyle Kanos

@Sofia, What I said was that the moderators don't need any votes from any member with >3000 rep to close a question, they have been given oversight of this website

It wasn't specific to you, it was the general population of people who are eligible to close a question

infinitesimal

Nobody is calling you "rude" @sof

Sean

@ACuriousMind why don't you like the wave/particle duality speak?

Kyle Kanos

Question on notation: We typically say that Leibniz "won" the battle over Newton because we tend to use $df/dt$ to denote the derivative instead of $\dot f$. Perhaps this is true in mathematics, but is that really true in physics?

Sean

7:03 PM

@KyleKanos I never saw dot notation until I was a junior in college, and then it was all we ever used after that

infinitesimal

In physics, yes.

ACuriousMind

@Sean Because it is wrong, and leads to all sorts of even wronger ideas in people's minds. How often have I heard someone say something like "The particle is a wave until it is measured". It's not. Quantum objects are states in a Hilbert space, and they are neither waves nor classical particles, though they exhibit properties of both.

Sean

But like I told ACuriousMind earlier about unit vectors, I suspect it really just depends on your textbook author

infinitesimal

"Wavicle" :-)

Sean

@ACuriousMind I suspect that's bad pedagogy. But I'm guilty at times, because when you're trying to explain youngs double slit to a high schooler, you have to simplify

"Lying to children" or what have you

infinitesimal

7:07 PM

Bad idea^

dmckee

I say "Quantum objects can exhibit particle-like and wave-like properties at under the right conditions."

infinitesimal

The Feynman lectures never lied :P

ACuriousMind

@dmckee Yep. I don't see how that is more difficult to understand than "Sometimes it's a wave, sometimes it's a particle", but it has the advantage of being correct

Sean

Wait, I never use that phrase

It's not like a particle picks which it wants to be

ACuriousMind

Okay, if you actually only say something like what dmckee said and call that "wave-particle duality", it's fine

That's not what most people understand under "wave-particle duality" though, in my experience.

dmckee

7:13 PM

The problem is I'm not sure that the students really appreciate the distinction between what I say and what they read in popular treatments.

infinitesimal

en.m.wiktionary.org/wiki/wavicle

ACuriousMind

@infinitesimal Did you just create that entry?^^

dmckee

@ACuriousMind It says "Last modified on 22 May 2014, at 23:51" ...

ACuriousMind

Oh, well you didn't. Who says wavicle, though?

infinitesimal

Feynman

Sofia

7:15 PM

@KyleKanos @KyleKanos if you say so, I believe you. It's O.K.

dmckee

@ACuriousMind Students at my undergrad school bounced the term around for a while (in the early 1990s), but it didn't stick because it wasn't much use beyond blowing peoples minds.

Sean

@dmckee what level do you teach?

dmckee

A undergrad-only department at a "directional state" school.

That means "[state] [direction] university" which is to say second-tier if we're being generous.

Sofia

@infinitesimal I wondered, what can mean the two dogs looking in different directions. Do you mean that there is a small difference between the position of their heads? It's a nice and delicate idea. (In general I like pictures with animals).

infinitesimal

thanks for noticing @Sofia :-)

They? are known as "Orthus" in Greek mythology.

@acu "particle-wave" could be used also, not to be confused with a "wave of particles" in the same sense that a "picture-show" is not to be confused with a "show of pictures." :P

That is to say, an entity in and of itself.

Stan Shunpike

7:38 PM

@infinitesimal GR isn't also really needed for HS students since most people either won't understand it or won't use it, do that would be another reason not to teach it

Jiminion

@infinitesimal or confused with Cerberus

infinitesimal

I'm not advocating what Einstein wrote in his HS book @StanShunpike just stating it :-)

@Jiminion that name is taken

user54412

@tpg2114 Hate to burst your bubble, but that will fail if you have different size things in the two lines. The delimiters only scale to the thing in their own line, so you could end up having paired delimiters of different sizes :/

user54412

(yes, this is very much a shortcoming of tex)

infinitesimal