12:00 AM
@obe Also, steve jobs quote timeee: “People think focus means saying yes to the thing you've got to focus on. But that's not what it means at all. It means saying no to the hundred other good ideas that there are. You have to pick carefully. I'm actually as proud of the things we haven't done as the things I have done. Innovation is saying no to 1,000 things.”

indeed

So tossing a book into the donation bin isn't a bad thing.

because out of all the hundred other good ideas
apple thought a pencil was the thing to focus on

are you saying I should donate my GR books
I could donate Weinberg G&C I guess...

12:01 AM
@0celo7 there may have been some chatroom current events in there...

Donate Weinberg

you could donate them to me!!

no, why would I do that
@FenderLesPaul I actually have two copies

The Weinberg foundation
for shitty textbooks

ok, can we not shit on Weinberg so much

12:02 AM
no

Weinberg is like GR for engineers in a sense
it's well written and the math is pretty simple
and the cosmology stuff was apparently excellent for 1972

@FenderLesPaul Yeah I think they could improv ethe pencil a lot.

even Wald cites his book as the "go to for standard cosmology"

holy shit is tomorrow homecoming?

Make the lines much darker... although then an eraser might not work. And a cap so that the stuff doesn't smudge your pockets...

12:03 AM
no wait it's next week
phew

phew?
you going?

maybe even ink and some innovative mechanism so that it flows smoothly

haha no I wouldn't have the opportunity to go even if I wanted to
I was just scared I'd have to meander through armies of alumni
on my way to my office

do you have
100,000
holy crap that's a whole city
it's gonna be nuts tomorrow

thats' a lot for football
bigger than a line for fried ketchup

2 hours later…
1:46 AM
All my courses rn: Clebsch Gordan is love, Clebsch Gordan is life

@0celo7 Can you help me with finite rotations?

2:13 AM
I keep giving downvotes to the questions showing no research effort.
not sure if it is the right way to downvote.

Dammit I need help with a lot more things.

@obe maybe

Chapter 15 is scary.
@0celo7 Do you really know all the stuff in shankar?
Seriously that is impressive... there is a lot of information in there.

anyone is studying Shankar?

I am.

2:15 AM
I will soon start reading Shankar
maybe we can become study peers

I'll be done in 3 days.

discuss whatever we don't understand in the book

Though I'll re-read it, so yeah.

unless you almost finish it
nice.

Enough to learn basic QFT.

2:17 AM
@obe "done"

@0celo7 Right laughable.

@obe not off the top of my head, no

I'm a failure, it's been decided already.

so what do you need help with

Everything.

2:19 AM

That angular momentum and rotations chapter... I barely followed it.
Sorry I don't want to waste your time with things I could have understood if I read more carefully.
I'll re-read... and I wish there was more time.
@0celo7 Can you tell me a few specific topics I can re-read on this weekend?
For being able to do qft from those lecture notes I showed you.

@obe have you learned anything about waves and vibrations?

angular momentum is pretty damn important

D:
fml.
@Shing I believe I saw a lecture course on those a year ago. I barely remember it.

37 mins ago, by ManishEarth
All my courses rn: Clebsch Gordan is love, Clebsch Gordan is life
Learn how angular momentums work in QM, that's all you'll be working with later

2:25 AM
For QFT?

you study physics for becoming a physicist or just for the love for physics?

@obe ... ish

Ok, will do.
@Shing Idk I study it for no reason.

@obe When we did QFT we actually didn't need the CG coefficients much because we weren't dealing with addition of L/S.
But you do need to be good with the spin and L/S stuff for single particles

2:26 AM
you probably won't need CG

Right now I'm doing nuclear structure and it's CG everywhere.

Weinberg uses them but he's weird and has a Nobel prize

lol

lol I just want to say, there is a risk to rush into things.

my prof worked under Weinberg, he can confirm that Weinberg is weird.

2:27 AM
obe is way past rushing

gtg, exam full of CG coefficients

many old tricks in classical physics actually help a lot when you do research.

cya

I am rushing so fast I broke relativity.

but there is also no point at being so damn slow.
G'Hooft has said it is okay to study whatever you want ahead

2:29 AM
ha

but once you are confused or get stuck, you should study things easier/ more elementary.

I picked up a book on twistors
third page ruined me

what book? @0celo7

back to some analysis, quantum computing and SR

@0celo7 Why don't you read penrose RTR for twistors?

2:30 AM
it wasn't the third page per se

The first page is ok.

page 10 is rough

does that mean I'm an expert in twistor geometry now?

it doesn't get better

@0celo7 If I were to buy a mathematical methods book to read, which one do you recommend?
Cahill, or nakahara?

2:33 AM
cahill and nakahara have vastly different scopes

I know though I need something to read in book form.
Which is more useful for me?
it can be a different book too.

shankar
how comfortable are you with complex analysis
i.e. residue, contour integrals

I will this weekend, I know I need it.
@0celo7 that I don't already own.

I wish I had cahill on me
one moment

cahill it is then?

2:36 AM
I'll find some integrals that you should know how to do

thanks.

well you're not planning on doing string theory anytime soon, are you
I think Nakahara is beneficial for that purpose

@0celo7 If I am not dead by the winter, I'll begin ST.
That is after QFT I.

hmm, you should probably be comfortable with renormalization before you start ST
that's what my problem is

we can talk about that later.
if I make it that far.

2:41 AM
you should know how to do $$\int_{\mathbb{R}^3}\frac{\mathrm{d}^3k}{(2\pi)^3}\frac{\mathrm{e}^{\mathrm{i} \vec{k} \cdot\vec x}}{\vec k^2+m^2}$$
oh screw you ChatJax

Idk how to do that.

I took that from Zee's QFT book
it uses contour integration
also a trick from multivariable calc

Did you read the first few pages of the lecture notes?
What are my immediate requirements?

why are you asking me instead of the instructor or a student in the class
and I don't have the notes laying around

Because you're nice.

2:46 AM
ctrl + F "contour" and see if you can follow

That is probably in lecture 2 or 3.
I'll see if I can follow.
Idk where the $2\pi$ comes from in the denominator.

wait sorry.
I looked at the wrong reference.

you better hope I can't find that in Shankar in the next 5 minutes

Yes I follow.

2:49 AM
ok, where does it come from

What does that even mean?

what

It comes from $\langle k | x \rangle$
I remember this.

so you understand the ghost contour

Nope.
I'll learn complex analysis tomorrow.
What is the ghost contour anyway?

2:57 AM
the dashed line

ok.

3 hours later…
5:28 AM
NbCeID 9
http://www.sciencedaily.com/releases/2015/09/150910141238.htm

Can science magazines learn to be more precise?
These are not two photons in free space bound together, they are actually superimposed with Ryberg states such that its potential acts like that of a H2 molecule.
(The conclusion understood by reading the paper with my currently half baked background)
One thing that science magazine love to do in making misleading titles is to enforce the following:

$$\text{Analogue}=\text{Real thing}$$

1 hour later…
6:44 AM
@DavidZ, are there any rules on this site about repeatedly making uninformed and blatantly incorrect statements in comments?
There is a certain user who seems to go about making false assertions about a field he/she apparently knows little about.
Lesser informed readers might mistake this user's apparent confident tone as a sign of expert knowledge.
This would bother me under any circumstances, but since the topic in question is my area of expertise, I am particularly inclined to prevent it.
Here's a link to the latest round of false assertions. It's not the first, hence my decision to bring it to your attention.

0

I, today, saw one new thing below my question cite . I pressed it & this appeared: Example citation: user36790 (http://physics.stackexchange.com/users/36790/user36790), Why does Hamiltonian follow the property $H^*_{ij} = H_{ji}$?, URL (version: 2015-07-31): Why does Hamiltonian follow ...

7:03 AM
Above illustrates how I read some questions

I sometimes hate seeing items with the same nature as those underlined in red but lacking an arrow that direct it to items of nature highlighted in yellow

To me, this question is well answered because:
1. All red lined items terminated at yellow statements
2. Misconceptions (blue) pointed out
MSE sometimes have users saying things of the same nature as those underlined in red, without elaboration, which makes it confusing to work out the problem on how the question or answers went wrong
cyan are just my interpetations, may be right or may be wrong

@DanielSank no, not really. Especially in comments, because they're temporary and can get deleted after they've served their purpose. (Do keep in mind that you can flag extended comment discussions to be moved to chat)

@DavidZ Wait a sec, I can force a comment string into chat?

Yeah. Well, you can always click the link that pops up after enough comments are posted, but also moderators can move comment discussions of any length to chat now. It's a relatively new feature.
Comments are meant for requesting clarification and suggesting improvements. Generally speaking, anything else should be handled in chat. (This is on the main site, not meta)

(cont...)
IMO, a question is optimally answered if:
1. For every question mark that is not rhetorical, it maps to a corresponding yellow statement
2. All red lined statements terminated at yellow (including the comments and answers)
3. If misconception exists, it will be mapped to it from a yellow statement in blue arrow
4. Cyan statements that can be produced by that question for most people will achieve or tends to the theoretical minimum possible for that question

@DavidZ Ugh, yeah I know. The problem is that CuriousOne loves posting random claims in the comments and generally refuses to actually discuss those claims in chat.
I dislike this. Very much.
There are a lot of students around here who probably mistake his assertive tone as a sign of knowledge.

7:13 AM
@DanielSank I will add to my previous statements that even though incorrect statements about physics aren't against the rules, it really does rub us (mods) the wrong way. So it doesn't hurt to bring it to our attention.

I don't want to have to spend time deflecting random unsubstantiated claims that users post in comments, but I also understand that censorship is not a good idea either.
@DavidZ Ok, well, consider this brought to your attention. Again, I realize censorship is really, really dangerous.

Yeah, basically we shouldn't be using moderation actions to enforce correctness. That's what votes are for.

However, as a card carrying PhD holding researcher in experimental quantum computation, it is my opinion that what CuriousOne is saying in the comments there is utter BS.

I moved the comment discussion to chat though. There's no way that belonged in the comment section.

@DavidZ Great. Thanks.

7:15 AM
Answering a question in the comments is a particular pet peeve of mine, and that is actually not allowed (I'm pretty sure...?) so you can flag on that basis.

@DavidZ CuriousOne is now accusing me of not understanding humor.
Because that, apparently, is what he's doing.

I guess it'd be fair to say that anything that comes across as trying to answer the question shouldn't be in a comment.

@DavidZ Lol. Spoken like a true mod.

hehe

7:18 AM
in case there was ever any confusion with all those fake "poser" mods we have running around

(cont.)
Red statements have one thing in common:
To me at least: All I see is emotions + too little concrete infromation

They are loose ends thus they must be mapped to yellow statements else there will be unaddressed issues

@DavidZ I'll keep this in mind. I'm trying not to let emotional response cloud my judgement. CuriousOne posts so many overblown and dramatic content-less claims in comments it's hard not to get frustrated defending against it.

yeah, I know the feeling. We're used to it around here.

@DavidZ Like @Danu?
@DavidZ and that's why I'll never volunteer to be a mod.

@DanielSank lol
(I was joking, of course)

7:21 AM
@DavidZ Me too.
@Danu and I have a... special relationship. It is held together by insulting each other at any opportunity.

Hey

@DanielSank David is one the best :-)

@skillpatrol That's why I pinged him. Very level headed and fair.

7:25 AM
@DanielSank I just happened to analyse it because you guys were talking about it when I was writing how I analyse questions in general

General knowledge of site policy seems great.
@Secret I'm just saying, that's a bad sample. 90% of what he says in comments is devoid of content.

@DanielSank Indeed, just from that example above, all I can see is just emotions (insults, scarsm, rant, arrogance etc.) and no elaboration or useful information what so ever
I usually ignore comments or answers that is loaded with emotions not supported by evidences or reference material

@Secret This is typical. Note how I even linked to a paper directly disproving his claim and he just ignored it.
@Secret Yeah, but he's spreading misinformation about a field I work it. It's not good for the field.

@Slereah CuriousOne.
But Duffield would probably mostly agree.

7:30 AM
@DanielSank I know that feeling. Information integrity is important in all fields of science and I do get furious sometimes when these things happen (sometimes to the extreme when I found I have misinformed people something, that's why I seldom said things in an assertive tone)

Duffield doesn't bug me because his craziness isn't as well shrouded in the appearances of knowing what he's talking about.
I mean, come on, "Einstein and the Evidence"?

What did CuriousOne do

^^^Misinformation (unless in the lucky cases such as accidental discoveries) can often seriously undermine the progress in understand a topic

@DanielSank : He still manages to get quite a few votes

Curiousone give unsupported claims in assertive voice in comments, on the field that daniel is wroking in, which frustrates him

7:34 AM
but which!

@Secret Exactly, the claims themselves are frustrating. It's also frustrating the he refuses to admit he's wrong even after backpedaling so far I thought he'd fall off the edge of the Earth.

### Discussion on question by OrangeCalx0…

Imported from a comment discussion on physics.stackexchange.co...
@Slereah And now he's insulting me directly

crap I posted my analysis in the worng place!!!
and it triggered quite an activity in the chat room that is being referred to
The lesson: I should double check next time my url bar before I post!

8:40 AM
@DanielSank flag his insults and let the mods take care of him pal

Call the police
the internet police

http://www.math.umn.edu/~olver/num.html
Currently self studying this because my honours need it
A very steep learning curve considering I have no backgroudn on numerical methods
A nice take on the answering feature:
http://physics.stackexchange.com/questions/206602/general-relativity-step-by-step-guide#206602

A (forgot term that describe aliteracy device of setting up a question, then answer it shortly) question

9:24 AM
@Shing you decide what is the right way to downvote for you.

9:40 AM
@dmckee I don't understand why you removed my first four comments here. I think they were valuable. Could you consider undeleting them, or provide an explanation as to why they should be deleted?
Yeah
I saw that discussion.

It could be explosive

Also, I can see your removed messages, just FYI.

;-)

@DanielSank I think your "summary" of the behavior of CuriousOne at the end of this chat was really quite unnecessary. Who needs that?
It will just provoke him to talk bad about you, too, presumably.

10:07 AM
I guess @Secret could make a nice picture of it :P

I am biased because I appreciate any source of GR crash course guide
But looking critically (minimizing the bias, the question is not optimally answered

@Secret You favorited that? Yikes!

I favoute for future checking
I learnt that technique from an MSE guy

53

Einstein's equations can be loosely summarized as the main relation between matter and the geometry of spacetime. I will try to give a qualitative description what every term in the equation signifies. I will, however, have to warn potential readers that this will not be a short answer. Furthermo...

@Secret Me too
^^ was my attempt at a similar thing.
I'm missing parallel transport & covariant derivatives, but oh well.

The question is not optimally answered because he mentioned nothign about the more advanced topics such as manifolds, killign vectors and black holes

A question that broad can only be considered optimally answered if the answer effectively compress the entire known knowledge base of GR (and bonus points if you include variants like Shape Dynamics and ADM formalism etc.) into just one page. This is a very difficult task which I am not even sure if it is possible in principle

I'll analyse yours shortly...

10:17 AM
Mine was not exactly aiming to summarize GR, but to summarize the EFE.
I'm pretty sure the former is not possible in one page.
I think this is a really nice way to explain torsion:
39

Here is an example which I found useful when learning about torsion. Consider $\mathbb{R}^3$. Let $X$, $Y$ and $Z$ be the coordinate vector fields, and take the connection for which \begin{matrix} \nabla_X(Y)=Z & \nabla_Y(X)=-Z \\ \nabla_X(Z)=-Y & \nabla_Z(X)=Y \\ \nabla_Y(Z)=X & \nabla_Z(Y)=-...

It's also to what my prof. mentioned in a side-remark one day.

My favorite introduction to GR is Callahan's Geometry of Spacetime
It's a good undergrad intro to GR
It doesn't go that deep into GR, though
A lot of it is just introducing the concepts

I, as a self-learning conservative, don't think it's a good idea to learn GR at all until you're ready for something like Carroll's book.

Like, back when it was just class notes on the internet
But still I found it helpful
Basically the book focuses a lot on what curved spaces are in a language of 2D surfaces embedded in 3D space
It's a nice introduction

^ x 8 I tend to use the favourite feature (like that guy (name forgot) in MSE) to bookmark questions for future reference)
Surely, I can do the same thing with the browser bookmarking system, but stack exchange makes it look tidier

^ x 4 That is currently too high level for me to grasp, I will return that later, though I have seen those equations before

^Caroll is very useful. It introduce me some basic differential geometry as mentioend before, which greatly help in my EFE course

@Secret Just use the "reply" button...

10:31 AM
would be better yes

The down arrow thingy?

it's the week end, I don't want to count
yes

ok, don't let the bump noise get on your nerves, lol

you can turn off the noise, you know

O I see

10:33 AM
So much to learn about chat :P

right, reverting to normal replying then

found this graphical explanation of torsion once, btw

That's okay, too. I like the twisting of extended bodies much more though.
I think my professor said one can formalize it to yield a precise optimization problem that is solved by no-torsion connections only

I tend to remember torsion as the extent of the vector got twisted out of plane as it tries to parallel transport in the manifold

Also
I remember when I was studying quantum mechanics with a friend, when we are investigating commutators [x,d/dx] and attempted to draw out the computation steps, we got something very simailr to the above diagram

Torsion is also sometimes explained in lattice interpretation as lattice defects
For instance in solid state physics
Curvature is deficit angle in the lattice, torsion is lattice elements being joined "wrong"

10:51 AM
We then sort of suspect whether the commutator can be interpreted as some kind of curvature in function space

@Secret I'm not too convinced by this :P

Our reasoning is based on the following:
1. Functions can be considered as vectors in $C(\mathbb{C})$
2. Differential operator is a linear map. Because of fractional calculus, it is defined for all $n \in [0,1]$, thus this abstract diagram is guareenteed to define at all points
3. The diagram is basically each step of the algebra visualised
4. Our result seemed to agree at least for this case and other polynoamials (which I am too lazy to draw
5. The diagram demonstrated exactly what the commutator tries to show mathamtically: How bad is the two arguments failed to commute

You really need to learn to do some latex :p
Rely less on diagrams
A curvature is basically the link between two nearby tangent spaces
I can't really think of why the commutators of QM would be such a thing
Though if you want to learn about what kind of function spaces QM uses that could have curvatures in them
I guess check up on Hilbert bundles
Don't know too much about them
So I can't comment on their connections

@Slereah You mean a connection, I hope? :P

11:06 AM
yes
Hence why I said it

I can type latex, I just cannot type it easily to show a diagram
My algebra is shown in dark grey, thus at least in the usual way of computing the mathematics I know I am doing the correct thing
I heard about something called a Berry curvature in a seminar in my uni, which is used to describe something in superfluids, but I don't understand wnough to see whether there's a connection
Same as you, I have no idea why would a curvature show itself in QM
I think I should look up hibert bundles later (after I got the basics) before revisiting this interpretation

Both the tikzcd and xy packages in latex are very good for drawing (commutative) diagrams; tikzcd is slightly better but maybe a teeny little bit harder to learn.

the problem is, a lot of my diagrams are not necessary commutative diagrams, this is why I often resort to mouse drawn ones to quickly bring up the point of discussion, and then supply the equations in the form of latex afterwards

But sometimes I got soo lazy thus I just decided to write them up instead

"A connection ∇ on a Hilbert bundle H → S associates with every ξ ∈ Vect S a linear map ∇ ξ : C ∞ (S,H) → C∞ (S,H)."

well, that might work for $\frac{d}{dx}$ but not sure about $x$, since this operation is obviously not linear as it depends on x, but maybe I am just not qualified enough at the moment to check rigorously

11:15 AM
It is linear
$\hat{x} (\vert a \rangle + \vert b \rangle) = \hat{x} \vert a \rangle + \hat{x} \vert b \rangle$
All operators are

ah I see
always forgot these basic things...
:24051834
Now I finally understood why people always said that is just a coordinate singularity

@Secret That doesn't actually matter (that they're not commutative)

@Slereah Not all operators are linear, but all the ones we typically care about are :P

just when I thought I have figure out a way to improve the rubber fabric analogy to include intrinsic curvature, it seems John Renie highlighted another factor I need to think about how to put it in

11:29 AM
I don't think that's very important; set $c=1$ at all times :D

$c = \hbar = e = G = k_B = \epsilon_0 = 1$

It just tells you that our units are weird, not that there is some kind of imbalance between spatial/"time" curvature (I highly doubt those can even be separated)
what, $e=1$? No!
:P
Nobody does that

$\pi = 1$
We are all one

@TheDarkSide There's this one girl who keeps on saying she loves how my sweat smells (no joke!)

@Danu I understand that they are noncummatative, but I sometimes like to wonder why it is the case

The common explanation is often given as it is due to the heisenberg uncertainty principle,

but the way the maths show the principle looks really clean and unambigurous, which caused me to wonder whether additional physical insights that helps understanding the why is buried in the intermediate steps in the algebra

thus my visualisation (along with my investgation with my peers, often gone so hardcore as to try to see if there's a physical menaing in every intermediate step of a computation
in fact, a lot of things in quantum are like this, the maths looks really clean and does not carry that notation of "strange, nonintuitive, confusing tc." that most people think of quanutm mechanics

11:32 AM
The non-commutative part is necessary for quantization
It is indeed related to the uncertainty principle

But then that caused me to be interested on how in step by step (allowing cases where multiple steps can occur at the same time) on how the noncommutativity give rise to quantisation

But Acuriousmind will probably said the usual way I try to grasp something in the form of step A, step B step C might be too classical for studying quanutm phenomeon
damn, I really need to read more, but I just love this topic so much
that I just keep saying my thoughts out loud that are possibly incoherent and lacking sufficient support

non-commutativity by itself does not give rise to quantization

^

You can have a theory of Hilbert space rays with commuting operators

http://math.stackexchange.com/questions/1415868/why-in-general-there-is-no-systematic-way-to-find-counterexamples-what-kind-of

In MSE
I often use the answers of these questions (plus reading books and lecture notes on how to construct proofs) in order to generate very crazy counterexamples to cross check my diagrams

For me, I like counterexamples, the crazier the better
Fo counterexamples to a maths thing which has physical interpretation, it is often very sueful to push the limits of how well my interpretation on understanding something
It is true I can do the maths no problem, but since I am so visual in learning (and how diagrams are often very powerful in educating laymens) I often try to turn what I have computed into diagrams

11:45 AM
Systematic way to find counterexamples sounds like the HALTING PROBLEM right there
There are books of counterexamples, if you want
it is nice to have around

I particularly like topology examples, I remember reading things such as the alexander horned sphere, these objects are really crazy to play with

0

It is written that days are counted in UTC. What is this UTC? For example when I am posting the question,the top of my achievement bar reads UTC time 11:31. Can anybody help?

(cont.)
My current interpretation of the rubber sheet analogy which includes intrinsic curvature is shown like this (which you can actually find a piece of cloth to demonstrate that)

I think the rubber sheet analogy should just be ignored :P

11:53 AM
I got this one
It's a pretty broad sample of math counterexamples

Here, you have a very flexible towel with a grid pattern printed on it
If you squish the towel, the gridlines will squash closer

In the absence of mass energy, the gridlines are geodesic in flat space time (only the spatial component) thus all objects free falling will travel along those gridlines (the grid itself is infinitely fine, but for the sake of clarify it is not drawn)

Now when something it put into it, the geodesics are modified so that the object free falls towards it, but they are just travelling along the gridlines
@Slereah should have a look...

well it's a french book
Find one you can read :p

yup of course

If you want to do GR with a grid, look up Regge calculus

(cont.)
The above diagram can be demonstrated to others easily by taking a scarf and then compress it horizontally or pull it apart, you will then see the patterns getting stretched, with no noticeble bending in 3 dimensions

now we just have to figure out how to incoporate dt into this and we have a nearly "lossless illustration" of spacetime curvature

11:58 AM
> lossless illustration of spacetime curvature
> two-dimensional
GG

PS if I say (cont.) it means I am continuing where I last left off because I typed too slow for it to become one message

Well you can have GR in 2D
Not much, but still