The h Bar

Stan Shunpike

12:01 AM

@0celo7 what do those acronyms stand for?

0celo7

@StanShunpike Look up Briggs-Meyers personality test.

Stan Shunpike

Ah, that thing. I know about that. I was like an uber extrovert

Is this ur AP psych class? My bro is a psych major.

0celo7

Yeah

Sean

12:32 AM

@0celo7 Myers-Briggs is total BS

that's the physicist in me scoffing at the social "sciences"

0celo7

@Sean True, but Minnesota costs money.

Kyle Kanos

1:03 AM

@0celo7 I've taken it a few times & got ENTJ twice and INTJ once. So I'm not really sure how intro/extro-verted I am, but I do seem to be Intuitive, Thinking & Judging

NeuroFuzzy

@KyleKanos Really? I took you for something counterintuitive

infinitesimal simplicio

Psychology is a fun subject :-)

A great mix of applied biology and philosophy.

No need to scoff @Sean ;-(

Kyle Kanos

1:28 AM

@NeuroFuzzy Hmm, well the internet can do that, I think.

Stan Shunpike

How can we do integrals on a manifold? I am reading Bernard Schutz's book on GR and he talks about doing this.

I just don't get how we know we can do integrals on a manifold. Given all the complexities with differentiating on a manifold, why can we integrate like this along one of the coordinate lines?

user54412

1:48 AM

@Sean Which they sometimes deserve no doubt, but I thought even psychology understood that Myers-Briggs with BS.

user54412

It's just something AP psych teachers love, because it involves a worldview that's simple and easy to grasp, yet just complicated enough to make it seem worth studying.

0celo7

@ChrisWhite My teacher admits that Myers-Briggs is too simplistic. We learn about the Minnesota test, which has ~500 questions and is constantly tweaked, but it costs money so we can't feasibly take it.

infinitesimal simplicio

Free is good :-)

user54412

@StanShunpike Well, the simple answer is forget there's a manifold at all, since your coordinate chart means you can consider yourself to be on $\mathbb{R}^n$

0celo7

^

user54412

1:52 AM

The better answer -- Carroll covers this -- is to really understand what each symbol means

0celo7

Stole the words from my fingertips

user54412

for example, the d is the exterior derivative, the final x^1 is a scalar function on the manifold, dx^1 is therefore a 1-form, etc.

0celo7

How do I make a landscape Word doc?

user54412

@0celo7 Use latex :P

0celo7

@ChrisWhite I need pictures tho

Also I figured it out

user54412

1:55 AM

Honestly, I have no idea. Ever since Office changed from menu-based to picture-based navigation, I've never been able to find any features.

Kyle Kanos

@0celo7 Click the orientation button

0celo7

@KyleKanos I looked everywhere but that place

Then I asked and found it immediately

Kyle Kanos

@0celo7 You didn't check with MS Office support? Why?

user54412

@0celo7 What I really want to take some day is one of the psych tests administered for people in sensitive positions -- the kind that are supposed to stress you out and see if you hold up.

infinitesimal simplicio

Why?

user54412

1:56 AM

I know someone who interviewed for the NSA once, and they gave something like that clearly designed to catch you in a lie.

0celo7

@ChrisWhite Coming from a family rooted in government service, all clearances work that way.

In the military they ask you if you have performed or have received oral sex.

It's against the UCMJ to do so.

infinitesimal simplicio

Wtf

0celo7

@infinitesimalsimplicio Sodomy is also illegal in the military, obviously. Although that might be changing.

infinitesimal simplicio

Talk about out of context >8(

You go in the military to kill people. Nothing else.

0celo7

I hope you're kidding.

Stan Shunpike

2:01 AM

Have you seen Stripes?

infinitesimal simplicio

During war.

No @StanShunpike

Kyle Kanos

@infinitesimalsimplicio Yeah, I'm gonna have to go ahead and, uh, disagree with you here.

infinitesimal simplicio

I meant during war only

Kyle Kanos

Even during war, that's not true

You're talking about one specific subset of "military"

infinitesimal simplicio

Exactly

Kyle Kanos

2:04 AM

There's a bigger superset that is involved in not killing people

infinitesimal simplicio

True

Kyle Kanos

And not everyone joins the military to legally kill people

They do it for other reason (money, for one)

infinitesimal simplicio

But they just provide support

Kyle Kanos

That depends on your definition of "support"

infinitesimal simplicio

Bullets

Kyle Kanos

2:05 AM

That...does not compute

infinitesimal simplicio

Do people die in war?

Kyle Kanos

Usually, yes.

infinitesimal simplicio

How?

Kyle Kanos

I know that bullets kill people, but I asked for your definition of "support." You responded with bullets

infinitesimal simplicio

Providing bullets

Kyle Kanos

2:07 AM

The clarification of that word was in response to your statement that people provide support by joining the military

Do you even know how the military functions?

Like any siblings or close relatives in the military?

infinitesimal simplicio

Yes.

Most died in action.

Kyle Kanos

Perhaps you should ask the survivors how the military functions

Because your comments do not suggest any knowledge of military function

0celo7

@KyleKanos You a vet?

infinitesimal simplicio

They don't care about "function " only the loss

Kyle Kanos

@0celo7 No, but I've got 2 brothers in the military, one who served for 6 years, and several cousins both active and served

Plus a few uncles, most of whom went to Vietnam

0celo7

2:10 AM

@KyleKanos My father served in Vietnam.

My parents and godparents served. By extension, virtually all close family friends are military or former.

Kyle Kanos

My father was in the military during Nam, but his duties involved training helicopter pilots State-side, so he did not go over with his brother (who flew helicopters over there)

0celo7

My dad flew C-130s.

infinitesimal simplicio

I didn't mean to offend your military background. @KyleKanos

I still feel the loss.

Stan Shunpike

4:33 AM

Are flags reviewed just by moderators or by any community members with enough points?

infinitesimal simplicio

Yep, +10ker's

for chat flags

iirc :-)

David Z

@StanShunpike some flags are only reviewed by mods, but I forget which ones

Certainly at least custom flags for moderator attention

infinitesimal simplicio

Are there custom chat flags for mods only @DavidZ?

David Z

I think so, but again I'm not sure. Actually I never handle chat flags.

Y'all are too well behaved :-P

infinitesimal simplicio

:D

David Z

4:46 AM

^^amusing

from my friend in Penn State's SPS

infinitesimal simplicio

But is it really always a wise thing to say?

David Z

I guess "clever" is not necessarily the same thing as "wise"

but I'm sure there's some proverb about that

infinitesimal simplicio

I'm not so sure about the "clever" part either.

So according to that, during our interactions with people we should spend most of our time doing nothing, right?

David Z

Yeah, but I bet a lot of people would benefit from moving closer to saying nothing than moving away from it

infinitesimal simplicio

True.

If you factor in body language it makes more sense, I think.

Stan Shunpike

5:12 AM

Is there a way to use tensor products to describe the Riemann curvature tensor? Just curious.

David Z

6:12 AM

You can definitely express it as a linear combination of tensor products of lower-rank tensors, but that's true of all tensors

If you're asking whether the curvature tensor can itself be expressed as a tensor product of other tensors (or other meaningful tensors), that I'm not sure about. But I'd guess no.

Stan Shunpike

7:17 AM

@DavidZ I think I was asking about the latter. I wonder if @ChrisWhite has any thoughts about this.

infinitesimal simplicio

11:33 AM

Danu

Lol, an old question of mine just randomly got downvoted (?)

That has never happened to me before. Late upvotes are more common :P

@infinitesimalsimplicio ?? lol

@0celo7 Yes, a little bit! My course on diffgeo actually focused a bit on this---using Frobenius' theorem

@0celo7 Take the differential at the identity, I'm pretty sure.

@StanShunpike Its the point of the notation that is packs a lot into a small space

@StanShunpike Any law in tensor form is true in all coordinate systems---that's one of the most important motivations

0celo7

12:14 PM

@Danu I came up with the same program myself, so I think that is correct and makes sense to me.

0celo7

12:31 PM

@Danu Do you know what the standard basis for sl(2,C) is? I first thought the Pauli matrices, but the algebra is 6-dimensional.

@Danu Perhaps the Pauli matrices and then i times the Pauli matrices...although they are not linearly independent.

Ah, I recall from previous studies of the Lorentz group that this is correct, with a factor of 1/2 in front of everything.

Danu

12:54 PM

relaunch.hcm.uni-bonn.de/fileadmin/perrin/chap10.pdf? lol

0celo7

@Danu What's funny?

Danu

That I found it by googling "basis of sl 2 c"

0celo7

@Danu I'm sure you know that sl(2,R) != sl(2,C).

infohost.nmt.edu/~iavramid/notes/sl2c.pdf would have made your point better.

Danu

I just clicked the first link on google :)

Didn't look at the contents

ACuriousMind

1:24 PM

@0celo7 $L_+^\uparrow$ is some restricted Lorentz group? But, anyway, you obtain the algebra homomorphism as the derivative of the group homomorphism. In general, the adjoint group action $BAB^\dagger$ is $ba+ab^\dagger$ (for $a,b$ generators of $A,B$) infinitesimally, it has to do with some version of the BCH formula, I think.

@0celo7 Complexifying the algebra often makes things easier without significantly changing the rep theory, by the way.

tpg2114

I'm always amazed by questions that I figured nobody would look at and might get closed as off-topic and then they get me a ton of rep when I answer them

0celo7

1:42 PM

@ACuriousMind Algebra homomorphism? Is it not guaranteed to be an isomorphism?

ACuriousMind

@0celo7 For such covering maps, it should be an isomorphism, yes. But that doesn't change that the algebra homomorphism is the derivative (or, more precise, "induced map on tangent spaces") of the group homomorphism in general, regardless of whether one of them is an isomorphism or not.

0celo7

Cool, thanks.

user54412

2:25 PM

"This is a brief rebuke to arXiv:1502.03821, which falls in the category 'not even wrong.'"

user54412

arXiv papers titled "Comments on..." or "Response to..." are always a fun read.

alarge

@ChrisWhite I asked a question last year about a long chain of arguments. At least one or two papers have come out this year (which I have not edited into the question). One of the authors of one of the papers answered the question.

I also have read parts of this debate which has been going on for years.

And then there are the more "direct" fights such as this and this

David Z

5:05 PM

0

Q: How can I add a sound wave on top of an existing one, without harming the original's loudness?

I'm working on a program that has to do with adding sound waves on top of sound waves. Currently what I'm doing is simply an average of the two waves. So if I have wave A, and I want to write wave B 'on top' of it, every sample in the new A is equal to the average of two matching samples in the o...

off topic?

Kyle Kanos

@DavidZ Looks like it could be asking about superposition of sound waves

Jimnosperm

I'm leaning towards off-topic because, while it is kind of physicsy with adding waves, it's definitely in the realm of audio engineering and there's probably scores of people on another SE that deal with exactly this problem

user54412

I feel like ultimately the person wants to do audio engineering, but since the question is phrased in general, with no reference to file formats or software, I'm inclined to classify it as physics.

Jimnosperm

@DavidZ Argh you deleted that answer just before I managed to post my usual "-1, don't feed the bears!" comment

Spoil sport

David Z

0

Q: Where can I learn about Motors and Generators?

` Where can I learn about the functionality of motors and generators, so I can answer questions such as these? I need a good textbook that starts from the basics (eg faradays) then moves up with lots of questions. Can I also know if this is hard to pick up?

classical-mechanics general-physics

also off topic, I think

@ChrisWhite well the first sentence is "I'm working on a program..."

Jimnosperm

5:16 PM

@ChrisWhite in physics, we can simply play the two sounds simultaneously. That will add them together and won't decrease the volume of any individual sound. This question has to be taken in context of combining sounds in a software program, otherwise it is so trivial that it doesn't make sense

@DavidZ EE.SE?

David Z

I don't think EE deals with motors

but maybe I'm wrong?

ACuriousMind

@Jimnosperm The question the user wants to solve doesn't look like EE at all to me

Jimnosperm

Looks like an electric motor. electrical engineering works with them

Stan Shunpike

It's a resource request correct?

Jimnosperm

@ACuriousMind true. Fair point

David Z

5:18 PM

It doesn't matter, anyway. The question is whether it's on topic here or not. We don't need to think about whether it's on topic on some other site.

(until there appears to be a consensus that it is off topic here)

Jimnosperm

This is a not even wrong question

you don't need to know about motors and generators to answer the questions

ACuriousMind

The question asks where to find resources to solve the question. Seems on-topic in a sense to me, although I think anything about Newtonian mechanic will suffice, so I'd be a duplicate of the book recommendations

David Z

FWIW I think that if a question asking for books showing how to do X is on topic, it also implies the question that directly asks how to do X is also on topic. And vice versa.

Jimnosperm

@DavidZ I disagree. One asks "What can I use to learn/find out how to do [X]?" The other asks "Do [X] for me"

Stan Shunpike

@Jimnosperm ^ I agree with that

ACuriousMind

5:27 PM

^I agree with agreeing with that

Jimnosperm

^ I agree with chaining agreement in this manner

Stan Shunpike

Lololol

Jimnosperm

Stan! You should have written "^ I agree with that" and started an infinite loop

Stan Shunpike

Someone had to break the monotony :D

Are resource requests off topic?

I have found chat responds better than the actual SE.

user54412

@StanShunpike No, though I and a couple others put up the good fight to make them off topic.

Stan Shunpike

5:37 PM

Yeah some ppl clearly don't like them

dmckee

@StanShunpike I'm also in the "No" camp, but you win some and you lose some.

The"Yes" people effectively promised to keep them curated and maintained to maximize their value and minimize the basic incompatibility with the SE system. As far as I can tell they're mostly keeping up with the work.

Jimnosperm

I'm firmly in the "Undecided" category. And given the amount that this problem affects me, that's not likely to change soon

Kyle Kanos

^ Seconded

Danu

Oh man

I just got flagged over at History of Science and Mathematics for "welcoming" a user in a way that is standard on PSE. The flag says that I was being too rude :\

What do you guys think? hsm.stackexchange.com/questions/2100/…

Jimnosperm

@Danu Yes, we are a bit rude here

tpg2114

5:53 PM

@Jimnosperm I dunno if I'd say rude... more just bitter and jaded :)

Jimnosperm

@tpg2114 To a non-bitter and non-jaded person, they sort of appear the same

@Danu Your comment wouldn't have been rude if it were properly addressing the question. But given that it didn't, I'd say that added a "rush to judge" quality that increased rudeness above normal levels

Danu

@Jimnosperm It was a genuine misunderstanding :\

I somehow failed to read it properly

Jimnosperm

I know, but other people interpreted it differently

Danu

I improved my second comment to reflect it more clearly

innisfree

6:21 PM

why do the histories of maths and physics get lumped in together in one SE? they are sometimes related, but generally quite distinct, aren't they?

oops, i mean history of science and mathematics. now surely they are distinct? or not?

Kyle Kanos

Well Newton invented calculus to do physics

So there's some intermingling of science & mathematics

Jimnosperm

They aren't as distinct as you'd think

Math may not be a natural science, but it's still very scientific. And where else would you put it?

History of Math and Culture?

innisfree

I'd separate History of Math and History of Science - these subject have occasionally intertwined, but are generally detached

Jimnosperm

They aren't broad enough independently to be their own SE site

Okay, science may be, but math isn't

innisfree

That's my naive feeling. I'm quite interested in the history of science and of maths, but I haven't read much about the latter

Kyle Kanos

6:30 PM

-12

Q: Generalize the site

Proposal: History of Science and Mathematics Can we just go ahead and make this site a site for the history of academics, it is much more general and would attract more people to this site.

science

There was a request to make it more general, but it was firmly rejected

Jimnosperm

was there a request to make it more specific?

Kyle Kanos

Not that I can see

It seems like it was always about Science & Math

Jimnosperm

They do go nicely together

Kyle Kanos

@Jimnosperm: Uh, that was spam: hayneedle.com/patio-umbrella-buying-guide.cfm

Physics Meta

0

Q: Homework questions as a knowledge resource

I'd like to point on the idea of question as a knowledge resource. It has come to my knowledge that understanding a subject, especially Physics, requires experience with as many questions as possible. Reading many solutions of expert members of the Physics site may indeed help many students t...

discussion homework

Jimnosperm

6:34 PM

@KyleKanos what was spam?

Kyle Kanos

physics.stackexchange.com/a/172706/25301

Jimnosperm

Honestly, I didn't even look at the link. Either way it'd be deleted

Kyle Kanos

Yep

Jimnosperm

@KyleKanos If we're critiquing each other's LQ reviews, might I call on this one

The answer explains waveguides but the question is about how electrons carry signals through a cable

Kyle Kanos

7

A: Is it appropriate to use the low quality review system to get rid of 'bad' answers?

The options for Low Quality posts are No comment needed Commentary on another post A "Thank you" comment "I'm having this problem too" comment Different question than posted Link-only answer (and not spam) "This is a bad answer" does not appear on the list, so voting to delete such an answer ...

Jimnosperm

6:42 PM

You are saying that it doesn't fit with any of the choices?

Kyle Kanos

I do not think so, no

Jimnosperm

It does fit with No Comment Needed

Kyle Kanos

That, in my opinion, is to be used when someone else has already put the canned-comment in

Jimnosperm

It's not an answer to the question. It's an answer to a totally different question

@KyleKanos If someone else already put the canned comment in, then choosing the appropriate reason only upvotes the comment, it doesn't repeat canned responses

Kyle Kanos

@Jimnosperm I know. But the act of it is saying Someone else already told them why it's being deleted, I don't need to do so again

Jimnosperm

6:45 PM

I use No Comment when I add my own comment in. If I can explain why I think it's inappropriate as a posted answer to the question, then I write that comment and select no comment needed, because I provided one already

Kyle Kanos

I think if you have to write a comment that is explaining yourself, "Looks okay" is the button you need to press

Jimnosperm

2

A: Low quality questions - No Comment Deleted

Depends. A downvote should be the solution in the case the post is not useful. As in: It is technically correct, but isn't clear. It is technically correct, but is only a link or is of some other value. It is technically incorrect, but is of some value to the original question. Deletion is r...

Kyle Kanos

Sure, be selective

Skip out on the +10 accepted answer

9

A: Low quality questions - No Comment Deleted

If a question is flat-out wrong on top of being exceedingly brief, in poor English, or whatever, by all means vote to recommend deletion. If it looks beautiful except for being wrong, I'd stick with downvotes and comments.

> If it looks beautiful except for being wrong, I'd stick with downvotes and comments.

Jimnosperm

But it's not even wrong

Kyle Kanos

If it does not answer the question, then I do not see how it is anything but wrong

Jimnosperm

6:48 PM

Q: What's the square root of 2? A: Bananas are yellow.

Kyle Kanos

Downvote, comment that it doesn't answer the question, move on with life

Jimnosperm

That's not even wrong, it's just not really an answer

Not answering the question is grounds to flag it as not an answer

Kyle Kanos

And I somewhat agree

Danu

@innisfree There is a big overlap (also in terms of expertise of users) and the audiences for each separately are probably too small to justify a site.

Kyle Kanos

If an answer is (a) asking a new question, (b) meant to be a comment on another post, (c) a thank you comment, (d) a "me too!" post or (e) a link only; then by all means flag it as not an answer

Danu

6:54 PM

@innisfree There's a lot for you to discover, then!

Kyle Kanos

If the answer is just flat out wrong, then downvotes & comments as to why it's wrong are the necessary action

Jimnosperm

20

A: When should the "Not an Answer" flag be used?

A bad or a wrong answer is still technically an answer. Moderators aren't here to judge the correctness of answers. That's what the voting system is for, so the right way to handle those is to downvote, edit, or leave a comment. The "not an answer" flag is for posts that are either completely un...

Kyle Kanos

6

Q: Which flag do I use for an inappropriate post?

Other than off topic flags, there are several different types of flags that one can cast on a question, answer, or comment. What are each of these flags for? Question and answer flags: spam offensive/abusive/hate speech not an answer very low quality other Comment flags: rude or offensive ...

support flagging faq-proposed

David's answer there says,

> Do not use this flag for anything that answers the question or represents a good-faith attempt to answer the question, such as wrong answers

(where this flag is the "Not an Answer" flag)

Jimnosperm

It also says use it for answers to completely different questions

The first bullet point of when to use the flag

Kyle Kanos

With the caveat answers which just barely fail to address the question due to an honest misunderstanding

Danu

6:57 PM

@innisfree I think you're wrong about this. There really is a ton of overlap (of course especially between physics and mathematics)

Kyle Kanos

How do you know the poster of that answer didn't misunderstand?

Jimnosperm

Talking about how EM waves travel through a waveguide when asked about how electrons carry signals through a cable is sufficiently unrelated

Kyle Kanos

That's obviously debatable.

innisfree

@danu there's certainly overlap between maths and physics - as @kyle noted, occasionally revolutionary developments in maths were motivated by physics, and vice versa - but the overlap between the histories maths and e.g. biology is tenuous.

Kyle Kanos

I've answered a few questions (and commented on more) based on my misreading the question

innisfree

7:00 PM

and even with maths and physics, most developments in maths have nothing whatsoever to do with physics.

Jimnosperm

@KyleKanos What? Other than them both carrying information along something and being elements of the EM force, they're not related

And misreading is one thing but turning electrons into EM waves and cables into waveguides is something else

Danu

@innisfree I don't really agree on this point

The other point is fair enough... It just turns out that most people interested in either discipline turn out to have at least a mild interest in the other as well.

innisfree

for a start, maths is arguably much older than physics

Kyle Kanos

@Jimnosperm Yes, and if you gloss over the fact that it (clearly) says electrons, and read the "transmit " and "cable", you can think of waveguides

Jimnosperm

You'd also have to gloss over "electrostatics" and "twelve years old". Plus glossing over "electrons" is a pretty big oversight

Kyle Kanos

7:04 PM

I concur

It's a wrong answer

Jimnosperm

But that's neither here nor there. The answer is clearly only suited for a totally unrelated question

Kyle Kanos

But I don't think it's not an answer

Jimnosperm

Is it an answer to the question. Even remotely?

Kyle Kanos

It's a wrong answer

The correct action is downvote + comments why it's wrong

Jimnosperm

No it's not even wrong

It's correct but doesn't addres the question

Kyle Kanos

7:06 PM

Right, so downvote and comment why it's the wrong answer for the question

Jimnosperm

It doesn't even remotely answer the question. It's not an answer to the question.

We need a second opinion

Kyle Kanos

Well ACuriousMind voted to delete it too

So we clearly can't ask him

Jimnosperm

No we can't

How about a mod? Is @DavidZ still online?

Or @dmckee, I suppose. What do you think of this answer

dmckee

@Jimnosperm It is certainly an incorrect answer to the question and it certainly contains very little of the poster's own thought.

Jimnosperm

Let's be perfectly clear here. Are you siding with saying it is incorrect but still an answer, or sufficiently unrelated to the question that it constitutes a non-answer?

We're debating if it is related enough to be an answer but wrong for the question, or not even wrong as it's too unrelated to the question

Kyle Kanos

7:20 PM

Well, FWIW, I'm arguing that it does not fit any one of the 5 "Not an answer" reasons ("me too," new question, link-only, comment on another post, saying "thanks") so it shouldn't be deleted as "Not an answer."

Jimnosperm

And I'm saying that writing a comment that it is unrelated to the question and then selecting "No Comment needed" is a valid option

We clearly established that that is a valid option, since the meta posts all agree it is. But now we need to establish that the answer is sufficiently unrelated

or not

Kyle Kanos

@Jimnosperm Except I disagree on that. The few MSO answers we've culled have strings attached that I view as leaning towards me & not you.

Jimnosperm

@KyleKanos You aren't disagreeing that totally unrelated answers can be flagged as NAA and deleted, you only argue that this specific answer is not totally unrelated; that it is subject to one of the strings

Kyle Kanos

No, I think in general that NAA is explicitly for the 5 reasons

If it does not fit one of those 5 reasons, it is definitively not NAA

Jimnosperm

So you're disagreeing in general with the meta posts saying the NAA flag is acceptable for use on answers that are totally unrelated to the question?

Kyle Kanos

7:31 PM

No, I see myself as agreeing with those posts. They all say something to the effect of Do not use this flag for wrong answers

I see you as the one disagreeing with those posts.

Jimnosperm

From the post you linked:

>The "not an answer" flag is for anything posted as an answer which does not constitute so much as an attempt to answer that question, such as

>answers to completely different questions

Kyle Kanos

> Do not use this flag for anything that answers the question or represents a good-faith attempt to answer the question, such as wrong answers

Jimnosperm

There's a follow up about not using the flag on answers that are slightly removed from relevancy due to misunderstandings

Kyle Kanos

> A bad or wrong answer is still an answer

Jimnosperm

but the point remains that flagging answers that are totally unrelated is the first bullet point under what to use the NAA flag for

Kyle Kanos

7:34 PM

> The "not an answer" flag is for posts that are either completely unrelated to the question, a "me too" kind of post, a follow-up question asked in an answer, etc.

Jimnosperm

Highlight the first item in that list ^

completely unrelated to the question

Kyle Kanos

Yes, the following items after that are what she's referring to

The "me too" the "new question" etc

You know, the canonical 5 reasons I've been going on about

Jimnosperm

No, the word "either" indicates that the next thing is the first item in the list

Kyle Kanos

And not also that she doesn't add incorrect answers to that list

she has it before saying that incorrect answer explicitly should not be deleted

Jimnosperm

I agree incorrect answers are not suitable for the NAA flag

Kyle Kanos

7:36 PM

And what constitutes the difference between an incorrect answer and unrelated?

Jimnosperm

But answers that are completely unrelated are not even wrong

Kyle Kanos

How is an unrelated answer not incorrect (to the query) as well?

Jimnosperm

Example:

49 mins ago, by Jimnosperm

Q: What's the square root of 2? A: Bananas are yellow.

Kyle Kanos

Right, I gave the answer as well: Downvote, comment, move on

It is not the correct answer

Jimnosperm

It's not the right answer, but it's not wrong

it's completely unrelated

Kyle Kanos

7:39 PM

So it's an incorrect answer to the query

FreeMind

Most of the books I have been reading in the SR - the first chapter of them - I see no proof for why the $ds^2=c^2dt^2-dx^2-dy^2-dz^2$ why is there a minus sign? Is it related to the geometry of the Minkowski space? I am totally confused.

Jimnosperm

it's not an answer to the query

Kyle Kanos

Sign convention

In physics, a sign convention is a choice of the physical significance of signs (plus or minus) for a set of quantities, in a case where the choice of sign is arbitrary. "Arbitrary" here means that the same physical system can be correctly described using different choices for the signs, as long as one set of definitions is used consistently. The choices made may differ between authors. Disagreement about sign conventions is a frequent source of confusion, frustration, misunderstandings, and even outright errors in scientific work. In general, a sign convention is a special case of a choice of...

You could have $ds^2=-c^2dt^2+dx^2+dy^2+dz^2$

It's somewhat a personal choice for some people to do it one way over another

FreeMind

In addition, why instead of the $t$ in $(t,x,y,z)$ we use $(ct,x,y,z)$? why $ct$? Are my questions rudimentary? If they're, just tell me what's wrong with me?

Jimnosperm

"the square root of 2 is 6" is a wrong answer. "Bananas are yellow" is not an answer

it's essentially an unrelated statement

If I answered with that, people would say "Yes, but what's the answer to my question?"

Kyle Kanos

7:43 PM

And they'd probably downvote it too

(which is what I've been saying gasp !)

Jimnosperm

Downvote and vote to delete as not an answer

I don't care what else is done to it. It's simply not an answer if it's unrelated to the question.

FreeMind

Another question is, why the Lagrangian is defined $L=T-U$ , $T$ being kinetic energy and $U$ being the potential energy. It is extremely bothering me, since it just came out of air ( At least for me, because I do not know the reason ). That's not the end. Why is the action integral defined that way? Specifically, why we consider the integral to have $\delta S = 0$ . I know it is about the calculus of variation it seems I am stupid enough not to understand such simple fact.

I know that for finding critical point we have to put the derivative of function equal to zero, but why action integral? why is it an integral?

Jimnosperm

"Why is the sky blue?" "The Sun appears white because we've evolved to perceive that spectrum and intensity of wavelengths as white".... "Great! But you didn't answer my question". "Yes I did".... ::headdesk::

FreeMind

@KyleKanos Please take a look at two other questions of mine in the chat. Thanks for the link !

Kyle Kanos

9

Q: Is there a proof from the first principle that the Lagrangian L = T - V?

Is there a proof from the first principle that for the Lagrangian $L$, $$L = T\text{(kinetic energy)} - V\text{(potential energy)}$$ in classical mechanics? Assume that Cartesian coordinates are used. Among the combinations, $L = T - nV$, only $n=1$ works. Is there a fundamental reason for it...

classical-mechanics lagrangian-formalism

FreeMind

7:48 PM

@KyleKanos What about the $ct$ instead of $t$ ?

Kyle Kanos

Sorry if I'm blunt, but have you read a textbook on the subject?

I cannot imagine any textbook on SR ignoring the whole $ct$ bit (though perhaps they use $c=1$ and write $t$ everywhere?)

FreeMind

@KyleKanos Not alot, I am just looking at a article.

Kyle Kanos

What article?

FreeMind

minkowski space time

Jimnosperm

@FreeMind Articles usually use the c=1 simplification

Kyle Kanos

7:51 PM

@FreeMind As in the article Space time by Minkowski, or the Wikipedia entry on Minkowski spacetime?

FreeMind

@Jimnosperm The problem is the article has not mentioned anything, I thought $c$ is the speed of light! @KyleKanos Not the wikipedia, article I downloaded from internet.

Jimnosperm

@KyleKanos I find the fact that that confusion exists to be hilarious. Not sure why, but I love it

Kyle Kanos

Yes, $c$ is the speed of light. But one can choose the unit system such that $c=1$ (basically means all your velocities are divided by $c$, though other effects need to be accounted for, such as lengthscales and timescales)

FreeMind

@KyleKanos You know, I am still relying on heuristics and things I learned in past years. When I see a rule or a habit being broken, I get uncomfortable. Now I understand it, thanks. I have not read anything specifically on SR.

@KyleKanos Again, I may seem stupid, but I ask it anyway. Is not the first component of four vector considered to be time quantity? If yes, why $ct$ is considered? $ct$ even if we consider $c=1$ is in $Velocity\times time=length$

Jimnosperm

All components of a vector should have the same dimensions

Kyle Kanos

7:59 PM

if $c=1$, then t is measured in length ;)

Geometrized unit system

A geometrized unit system or geometric unit system is a system of natural units in which the base physical units are chosen so that the speed of light in vacuum, c, and the gravitational constant, G, are set equal to unity. The geometrized unit system is not a completely defined or unique system: latitude is left to also set other constants to unity. We may, for example, also set Boltzmann's constant, kB, and the Coulomb constant, ke, to unity. The reduced Planck constant, ħ, is not equal to 1 in this system, in contrast to Planck units. This system is useful in physics, especially in the special...

FreeMind

@Jimnosperm Then why we call it time? Space-TIME diagram? If it is length?

Kyle Kanos

Well it's not really a length, it's just scaled such that it has units of length

Jimnosperm

@FreeMind What should it be called? Space-length diagram?

FreeMind

@KyleKanos Wikipedia is extremely good resource, but whenever I read it I get headache because I cannot stop the temptation to click on hyperlinks and read the other pages. Finally, I find myself having opened 100 tabs without even understanding what I was looking for!

Kyle Kanos

@FreeMind So stop clicking links

Jimnosperm

8:01 PM

Space-time conveys a better idea of what it really represents

FreeMind

@Jimnosperm Because the extra component added to it is not time but length, anyway, I don't insist on my thought, it is obvious that it is because of my foolishness :|

Jimnosperm

@KyleKanos That's like not reading just one more abstract on viXra. You know you want to stop, but the title is so enticing and hilarious

2

Kyle Kanos

@FreeMind Well $ct$ has units of length. So if you scale (i.e., divide) all terms by $c$ then time must be measured in length.

@Jimnosperm Well I don't actually read anything on vixra, so that is not something I am familiar with

Jimnosperm

Me neither

I avoid the site like the plague

Stan Shunpike

@ACuriousMind Wald says something to the effect that...understanding how to parallel transport is equivalent to understanding how to take the derivative...I get the idea that....if I know how to parallel transport, then I know how to compare to vectors. But I don't really see how the affine connection rules help me parallel transport. What does Linearity over R or the product rule have to do with parallel transporting?

FreeMind

8:04 PM

@KyleKanos So I can say 2 meters is indeed $\frac {2}{c}$ seconds/minutes/hours/etc?

Jimnosperm

no

you can say 2 meters is 2 light-meters * c

NeuroFuzzy

Well you can also measure time in meters (though I wouldn't write that in an answer on this site and I wouldn't write it on an exam!) and that's a fun viewpoint to take

2

you know that probably doesn't help. Forget I said it @FreeMind :P

Jimnosperm

We don't generally use units whenever we set c=1

Kyle Kanos

Well you have to

Even if you use h=k=1 as well

FreeMind

@NeuroFuzzy No problem, indeed, I am happy that you are following me.

Jimnosperm

8:10 PM

you know what I mean

Kyle Kanos

You end up with GeV as the base unit

But everything has a unit of GeV then

FreeMind

I wish someone could give an example

Jimnosperm

@FreeMind an example of what?

FreeMind

@Jimnosperm Nevermind

Kyle Kanos

@NeuroFuzzy Lol...it took me 10 meters to go 3 meters ;)

NeuroFuzzy

8:12 PM

@KyleKanos I see no problem :D

Jimnosperm

In 6 minutes, I'll be 1 minute farther

NeuroFuzzy

Ok...

FreeMind

I am looking at GR without even having mastered EM :|

Indeed, the paradoxes physicists faced at the start was interesting.

T Abraham

10:10 PM

@dmckee it froze again because I didn't know it freezes after one day.. Why don't u wait for me to ask to to unfreeze the room so it won't freeze again before I come back?

dmckee

11:01 PM

From the edit to

-1

Q: Homework questions as a knowledge resource

I'd like to point on the idea of question as a knowledge resource. It has come to my knowledge that understanding a subject, especially Physics, requires experience with as many questions as possible. Reading many solutions of expert members of the Physics site may indeed help many students t...

discussion homework

"Eventually there's a deadline for every exercise, thus time is priceless - therefore a solution is needful."

Did I miss something, or did he just make our point for us?

2

Seriously "I worked hard and I'm running out of time so I deserve an answer."?

tpg2114

11:21 PM

@KyleKanos I like the recent "I saw a cool simulation and I know how to write some code, so I'm gonna go ahead and solve the universe. Why haven't you guys done it yet? It looks so easy!" questions

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