The h Bar

bolbteppa

00:29

God damn representation theory

Alfred Centauri

02:19

@barrycarter you're integrating $v(t)dt$ but that isn't the distance in Carol's coordinates which is the integral of $v(t')dt'$ which, in terms of $t$ is $\tanh(at)\cosh(at)dt$. Also, you want to integrate to Bob's time $t$ and so the integral evaluates to $(\cosh(at) - 1)/a$ as desired.

barrycarter

@AlfredCentauri OK, but why isn't tanh(at) the speed in Carol's frame?

Alfred Centauri

@barrycarter The error is in the differential: $dt' = \cosh(at)dt$. Distance in the primed coordinates is Bob's velocity (in the primed coordinates) integrated with respect to $dt'$, not $dt$

barrycarter

@AlfredCentauri I did think I was off by a differential, but I still don't see exactly what I did wrong. OK, if Carol is observing Bob and is measuring time in her own frame, she see's Bob's velocity as tanh(at) at her time t. Is that correct or not?

Alfred Centauri

@barrycarter Bob's time is $t$, Carol's time is $t'$.

barrycarter

Oh shoot, I think you're right. I was looking to find Bob's velocity as a function of Carol's time. But I think I messed that up.

@AlfredCentauri Thanks, I see what I did. If I wanted to remain in Carol's frame, I'd have to convert Bob's time to Carol's time using Sinh[a t]/a

barrycarter

02:41

@AlfredCentauri Please feel free to answer the question I posted on the main site and I'll approve your answer. My thinking was that velocity is symmetric, which it is, but, as you point out, the 'dt' isn't. A change in Carol's time does not correspond to a fixed change in Bob's time.

FenderLesPaul

@ChrisWhite sweet we'll be able to hang out!

I'm going to get there early August to get a head start on research

I'll probably be spending most of my time in the theory center in Le Conte

Joshua Lin

03:12

I'm heading over to Berkeley too haha
though I

Im just a freshman, never been there before

hopefully itll be an awesome place

user116211

06:14

Is it JD's book?

user116211

amazon.com/RELATIVITY-The-Theory-of-Everything/dp/0956097804

user116211

Atleast the reviews say so ;/

user116211

06:33

@yuggib: o/

yuggib

@MAFIA36790 \o

skill patrol

\o/

@MAFIA36790 yup

yuggib

@skillpatrol \o

skill patrol

How's it going? @yuggib

yuggib

@skillpatrol not bad

working on the train to the U

skill patrol

06:46

o/

user116211

Damn to Amazon; they made this in my recommendation list ;(

user116211

@skillpatrol \o

skill patrol

It's just a recommendation bot ;-)

user116211

@skillpatrol !!gun

skill patrol

wrong subject room

here it would be !!e=mc^2

:D

John Duffield

07:15

@MAFIA36790 : Jesus H Christ, talk about malicious.

skill patrol

don't you @JohnDuffield think you're being a little hypersensitive?

think of it as free advertisement :-)

John Duffield

@skillpatrol : nope.

I don't want any free advertisements. The book's not for sale any more. But I can't stop Amazon providing a platform for silly-money 2nd hand resales, or a platform for dishonesty and slander.

skill patrol

Ahh, such is the life of an author.

John Duffield

@skillpatrol : such is the life of somebody who doesn't like to see physics A-levels going down by 57% in twenty years.

user116211

@JohnDuffield Hmmm... if you are not permitting them, then why are they still promoting it?

user116211

07:25

@skillpatrol Feelings! I know it's not the room...but it's about the feelings.

skill patrol

@JohnDuffield indeed a sad fact all over the world.

Jester Tran

Suppose we have a group of order 8. Is there a general rule that says something about the min/exact/max amount of subgroups in this group?

skill patrol

@MAFIA36790 i just got the inside joke

:-)

@JohnDuffield like I said before, I blame it on the Internet and what its done to kids attention spans

Slereah

The max amount will be bounded by the power set, at least

@skillpatrol Welcome to old age

Your slippers are ready

skill patrol

@Slereah Not to mention their access to pornography

Jester Tran

07:37

@Slereah I guessed this and now I see some intuition

user116211

Missing poor 0celot :(

Slereah

minimum will be at least 1, since the identity is always a subgroup

dunno about finer bounds

skill patrol

Your STD's are ready.

Jester Tran

@Slereah yeah, thanks.

alarge

@barrycarter Obviously it depends on what your application is and how the math functions are used, but for anything high performance I wouldn't use -lm as at least glibc is painfully slow. This is one of the main reasons ICC (Intel's compiler) ends up being faster than gcc in many applications as they give up some accuracy to deliver faster speeds (ICC also defaults to a relaxed floating point "standard" unless I am misremembering).

Slereah

07:40

Up to discrete symmetries of Peskin

He defines right off the bat the parity operator on ladder operators

Which smacks of hubris a bit, to me

But I guess it's faster than showing this to be true from observables

Or whatever theoretical consideration

Slereah

08:45

why does Peskin keep using $\sigma^2$

What does it mean

isn't $\sigma^2$ just $\propto I$

justin

08:59

Could you help me with electrochemistry?I'm stuck at this concept.I think this is a general chat for physics SE isn't it?

skill patrol

The Periodic Table

Haikus are awesome / Chemistry's even better / So pull up a chair

1

Slereah

OH

I just got it

Fucking phew

I almost posted a dumbass question on SE

$\sigma^2$ isn't the square of the sigma 4-vector

It's just the second Pauli matrix

user116211

lol

Slereah

I got stuck way too long on that one

That's the problem with ambiguous notations

user116211

Pauli matrix is written that way?

Slereah

09:06

Yes

user116211

@Slereah Feynman never used that symbol ;(

Slereah

Also nice puppy

And now he's dead

user116211

of course in his lecture (vol 3)

Slereah

Speaks volume, doesn't it

I think the worst notation I ever saw was in Schouten

First page

There's a $n^2$

Can you guess what it means

user116211

The notations he used:

Slereah

09:10

(the $^2$ was a footnote)

user116211

$$\sigma_z,\sigma_y,\sigma_x, \mathbb 1$$

Slereah

Yeah that is a better notation

user116211

@Slereah epic shit!

Slereah

I forget which book that was in but I also recall the Worst Typo

I forget the exact typo but it was diffeomorphism spelt wrong.

Very wrong.

Like

wrong alphabet

It had a $\Gamma$ in it

$\text{dif}\Gamma\text{eomorphism}$

or somesuch

user116211

@Slereah it's a typo; many books contain that....but it's bothering when they abuse notations and symbols.

Slereah

09:19

Yes, but

How do you make that typo

I am curious to know what process caused a gamma to get lost in there

user116211

@Slereah inferior proof-reading.....

Slereah

But this isn't just a slip of the finger

user116211

irresponsible scrutiny.....

Slereah

You don't accidentally press the gamma key

I guess I'd have to find out what process was used to write the book

Might have been pre-computer, in which case

Not a clue

skill patrol

absent mindedness

Slereah

09:21

Fuck I don't remember what book that was

I think I read it in 2008 or so

I should check my files

user116211

@Slereah: Even the Feynman Lectures were infested with typos. You can get more in Berkeley Physics books also, but they are less in that they often meddle SI with Gaussian.

Slereah

Sure, but some typos and errors are easy to make

Forget a letter, press an extra letter, sign errors, factors missing

user116211

@Slereah That's most annoying thing in a student's life ;(

Slereah

I am just unsure what is the process of having a letter from a different alphabet sneaking in

How did people write physics books before computers, anyway?

user116211

@Slereah great curiosity....

Slereah

09:25

mb I should ask the Academia SE

user116211

Let me google a bit....

user116211

@Slereah Ask ACM....

Slereah

ACM

Wait, is ACM even old

user116211

hahahaha....

Slereah

He's just a student

ACuriousMind

09:27

I'm not old, I'm 23. Why?

Slereah

@JohnRennie is the old one

user116211

Where is JD? He may know..... he is older than Einstein ;)

Slereah

JD has never written any physics, though

I am wondering of the process of writing a physics book before computers

Because I am curious to know how someone could spell "diffeomorphism" with a gamma

That typo would be impossible if you were writing in Latex

Maybe possible if you had one of those old timey Space Cadet keyboard, I guess

user116211

3 hours ago, by MAFIA36790

RELATIVITY +: The Theory of Everything

Corella Limited

ACuriousMind

Well, in the really old days where people set the printing types by hand, one could suppose that the printer just didn't look carefully and confused the $\Gamma$ with the $\mathrm f$.

Slereah

09:29

f key is right next to the gamma key, but then again

But was that the case in the mid 20th century?

@MAFIA36790 My point exactly

ACuriousMind

@Slereah I...think not? I don't know, though

Slereah

Do you think that's a decent question for Academia SE

skill patrol

Yes.

ACuriousMind

Doesn't really seem to be about academia, does it?

user116211

@Slereah no.

Slereah

09:31

tough one

ACuriousMind

I'm not sure there is an SE for this

Slereah

Maybe history of science?

user116211

@Slereah maybe... ask Danu.

skill patrol

Better idea ^

Slereah

Is Danu an old fuddy duddy

ACuriousMind

09:32

@Slereah How one typeset formulae before TeX would be a question for them, I think

skill patrol

@Slereah Nah

Perhaps even ask in the LaTeX and friends room

They're very open to inquiries

Slereah

but I don't think they will know those kind of tidbits

skill patrol

They know a lot

$TeX - LaTeX$ TeX, LaTeX and Friends

\begin{chat}

14

22

Good time to ask from the looks of it :-)

user116211

@Slereah: maybe you ask the oldest one here... John, I suppose (or Anna ?)

innisfree

10:00

@mods

Anarchist Birds Worship Fungus, Tokyo

1.4k 1 7 24

PS is there another way to flag a profile description?!

ACuriousMind

@innisfree I think you could have just picked one of their posts at random and explained the issue in a custom flag reason.

John Rennie

10:40

@MAFIA36790 can't help I'm afraid, in my day we were still writing on parchment with quills. What I know about typesetting you could write on a flea's backside - in a large font.

Slereah

10:51

Did you sneer at Gutenberg

"There is an old Egyptian tale of Theuth, the inventor of writing, showing his invention to the god Thamus, who told him that he would only spoil men’s memories and take away their understandings."

Sneering at new technology goes back pretty far

Library Rap

►

John Rennie

2008 isn't that old. Whatever the book was it must surely have been electronically typeset.

Slereah

Oh no

2008 is when I read it

Not when it was published

i remember because it was one of the book I used for my master thesis

John Rennie

If it was manually typeset they might have been intending to use the ﬀ ligature character for the two f's. That doesn't explain how they got a $\Gamma$ instead, but it's one reason why a mistake could have been made with the two f's.

Slereah

11:07

I wonder what book it was

I guess that since it had diffeomorphisms it involved some GR

John Duffield

12:03

@MAFIA36790 : because Amazon can do what they like. I can't tell them they can't sell a 7-year-old secondhand book just because I wrote it.

@skillpatrol : I don't think the internet is to blame for the way people behave badly. I think people are to blame.

innisfree

google.com/search?q=Difγeomorphism

Oh the URL broke. Anyhow, about 3 occurrences of that typo. My guess is that it's just the ff ligature digitalised incorrectly. Those ones are all capital gamma.

Slereah

could be

innisfree

If so, there should be other ff -> fgamma mistakes

Slereah

odd mistake

They don't really look similar

innisfree

google.com/search?q=Difγusion

Also 3.

Slereah

12:13

yeah but

When I check it, it's not actually there

only the OCR does that

in the book, it was actually written

So not too sure what happened

Maybe it was OCRd for a new edition and the mistake popped up?

ACuriousMind

@Slereah Which indicates that the copy of your book was printed from an OCR'd scan, no?

Slereah

could be

Xylius

If you look at the way gamma is drawn/written, you can kinda see how an ff ligature would fit

Especially in the more exotic fonts

However, considering the book was used for your masters, I doubt it would have any strange fonts

Slereah

the mystery is then why do OCRs consider ff to be gamma

FF I'd understand, but ff is kind of weird

innisfree

Pretty dumb OCR. The chances of a gamma in the middle of a word in English is pretty much zero. ff is much more likely, even if it's a worse fit to the image.

ACuriousMind

12:19

Doesn't have to be a weird font, you can see it in this paper. Exactly one of the "diffeomorphism"s shows up when searching for Difγeomorphism, the others are detected correctly as ff. I can't see anything different about the one which registers with gamma

Love Grover

0

Q: What is the meaning of SU(2) triplet scalar field?

The whole analysis in the following is of Left-Right Symmetric model. $SU(2)\otimes SU(2)$ $(2\otimes 2=3\oplus 1)$ will generate a triplet, which in Left-Right Symmetric model is $\vec{\Delta}=\begin{pmatrix}\delta_{1}\\ \delta_{2} \\ \delta_{3} \end{pmatrix}$. The $SU(2)$ quantum number/charge...

quantum-field-theory standard-model group-theory group-representations beyond-the-standard-model

Xylius

True.. That is rather strange

Do OCRs have universal typesetting recognition though? It could be because the first time it encountered the new style and didnt register the character properly

Which may have been followed up by a manual system reconfigure so it recognised the character as a ff ligature

Slereah

Time reversal operator is a bit of an odd duck

Then again I guess that charge conjugation is about the same

Is C also antilinear?

commutatertot

12:44

this question: http://physics.stackexchange.com/questions/250375/translational-invariance

is exercise IA1.1 of Siegel's "Fields" book largely verbatim; is this worth pointing out?

Slereah

apparently not

flag as a homework question

commutatertot

i mean it might be self study i guess

but it feels weird to have it unattributed

Slereah

Still

ACuriousMind

@commutatertot It's worth pointing out - material that is cited should be given a citation.

David Z

@innisfree yeah, do what @ACuriousMind said. @mods in chat doesn't do anything.

@commutatertot doesn't actually matter. It doesn't show any effort.

commutatertot

12:54

aaah ok I was just going to answer it with a hint and include a citation in my answer >_<

thank you though

Slereah

No you must destroy him

ACuriousMind

@DavidZ Except annoy hypothetical users named "mods" ;)

David Z

@commutatertot No problem. For future reference, if you find content that has been copied from somewhere (textbook, website, whatever) and is not attributed, you can either (1) edit in the attribution, or (2) leave a comment with the source so that someone else can edit it in, and also flag for moderator attention.

commutatertot

Alright, will do!

David Z

We'd kind of prefer #1, if you can manage it.

@innisfree BTW I checked it out, I don't see anything flag-worthy on that profile

ACuriousMind

12:59

@DavidZ I think innisfree considers the "fuck you" in the "about me" section to be inappropriate.

David Z

14

Q: Plagiarism checks?

Without mentioning any names, I observe the following. Some answers given on the main site by certain individuals are consistently long and not really relevant to the question, instead giving inappropriate amount of minute and highly technical details related to keywords in the questions - withou...

discussion

@ACuriousMind yeah, I figured. But pretty much anything goes in that section of the profile (other than hate speech and a couple other things, this does not qualify)

The Dark Side

@ACuriousMind I do too. And I've also flagged it, @DavidZ.

@DavidZ Why doesn't it?

ACuriousMind

@TheDarkSide I think about it like this: The "about me" section is for the user telling the reader who they are. In this case, they are telling you they're rude and unfriendly. As the "fuck you" is not directed at someone in particular, it doesn't actually rise to the level of a specific insult, which would be worthy of deletion.

The Dark Side

@ACuriousMind Ah. It is frustrating, how often you make sense.

And that you are too young to be that sane.

David Z

Yeah, what @ACuriousMind said.

The SE team considers the "about me" box to be a sort of "free space" for members to express themselves as they like, and thus it is the least controlled part of the site. Only the most egregiously offensive things would cause a mod to edit that part of a person's profile.

innisfree

13:06

@DavidZ oh right OK. No worries.

The Dark Side

@DavidZ OK. Got it. Thanks.

David Z

FWIW, note that the contents of "about me" doesn't actually show up anywhere else other than a person's profile. You're not going to see it unless you go looking for it. That's why people don't get the same freedom in their choice of display name or avatar, or the content of their posts.

oh wait, I lied: there is that little hovercard that pops up when you hover the mouse over a high-rep user's name in certain places. I guess if someone put something offensive there, we might remove it. I'm not sure offhand; I can't say that it's ever come up on this site as far as I can remember.

The Dark Side

The bar owner has descended :P

innisfree

Ok. No big deal.

ACuriousMind

@TheDarkSide I keep waiting for one of the two appointment notices to get more stars than the other, but apparently we don't choose favourites (or are afraid of their wrath)

The Dark Side

13:14

@ACuriousMind :D But if I remember correctly, some time back, John was one star ahead of Chris!

Probably it was Chris's star on John.

ACuriousMind

Maybe...I'm not watching this chat at all hours ;P

The Dark Side

Yeah, only for like around 23 in a day :P

John Rennie

@ACuriousMind All yesterday afternoon Chris White had five stars and I only had four! I was heartbroken :-)

The Dark Side

@JohnRennie OK. I remembered it to be the other way round.

innisfree

Are mods allowed to be pseudoynmous?

ACuriousMind

13:21

@innisfree Hmmm? What do you think Qmechanic is?

innisfree

The mods I'm aware of seem to use their real names

ACuriousMind

@JohnRennie Oh, I see

innisfree

Oh he's a mod? I see. Still, seems like more mods than regular users use their real name

Loong

@innisfree stackexchange.com/about/moderators

The Dark Side

@innisfree You mean anonymous or pseudonymous? Perhaps a different connotation!

David Z

13:23

@innisfree probably only because there are so many regular users with names like user8675309

or typical forum-type names

innisfree

Maybe. David, what do fellow academics think of you spending time here? A drain on your time?

David Z

I doubt they know

If they did, I suspect they mostly wouldn't care, but it certainly couldn't help

at least, it couldn't help my reputation. Sometimes it does help my research.

innisfree

Hmm OK. Just wondering whether hiring committees are positive, negative or indifferent about it

I guess indifferent

David Z

That's my guess, indifferent to slightly negative

but you check on Academia, I'm sure there's a question about it

ACuriousMind

@Loong Ask Patents is a dictatorship ruled by a single mod? :O

innisfree

13:30

It's outreach, but of the wrong kind. It won't advertise your work or the university.

Loong

@ACuriousMind yes, some guy from the Swiss patent office. His real name is Albert.

2

David Z

@innisfree well, the point of outreach is not to advertise your own work or who you work for. Not in general.

innisfree

You're joking? That's what drives it. That's why universities like it. Don't you think?

Maybe that is too cynical.

David Z

That may be why university administrators like it, but if you ask me, the point of outreach is the reason why the people who actually do outreach do it: to educate people.

Many outreach activities that researchers do are unofficial, not affiliated with the institutions they work for

innisfree

Oh sure. I just mean SE educates, but doesn't advertise. That's why it's not particularly positive to hiring committees. If you reached out to the same number of people about your work and your affiliation to University X, they'd love.

David Z

13:38

Oh, no, I highly doubt that.

innisfree

Really?

David Z

I think the reason SE participation is not positive to hiring committees is simply that it's not research

innisfree

True.

David Z

Consider something like writing a textbook. I bet universities like their faculty to write popular textbooks because that brings publicity and royalty checks to the university. But it's still a negative in the eyes of a faculty hiring committee.

Slereah

is it?

Academia is a strange world

David Z

13:40

Yep, generally

innisfree

Indeed

Slereah

If I ever write a QFT book

All spinors will have indices

and properly placed

Welp

chapter 3 almost finished

only the exercizes remain

goddamn there's a lot

Also after Peskin I should get a better book to explain discrete symmetries :p

Peskin just kinda assumes things and shows that those things are okay

ACuriousMind

14:02

What specifically is there to "explain" about discrete symmetries?

Slereah

Well he starts off shortly by saying what parity and time reversal do

But then he kinda just flat out tells us $T a^s_p = a^{-s}_{-p}$

Which is not necessarily trivial from the definition of T

Jiminion

The weak force is the Euro Disney of forces.

ACuriousMind

@Slereah What is his definition of T? Formally, T and P are the discrete operators you need to get the whole Lorentz group. Since we only fix the representation of the connected component in the usual representations, I don't think the actions of P and T are derived statements.

Slereah

Well the definition is gonna be more along the lines of $T \phi(t,x) \approx \phi(-t,x)$

I think Weinberg spends a while explaining CPT

I should look it up

Hari Prasad

15:00

@dmckee Is there any topic for the upcoming chat session?

Bass

@ACuriousMind I was thinking of the infinite potential well, but you're of course right that it's not about free particles. What's an example for a basis for free particles then?

Hari Prasad

@ACuriousMind I came across this article: That man who ‘deleted his entire company’ with a line of code? It was a hoax

Slereah

Let's discuss our favorite kinds of birds for this chat session

ACuriousMind

@Bass Well, the Hilbert space of free particles is just the space of square-integrable functions, so one possible choice are e.g. the Gaußians multiplied by Hermite polynomials that are the eigenfunctions of the oscillator Hamiltonian. You don't get an eigenbasis of any of the obvious observables for free particles because their spectra are purely continuous.

Hari Prasad

@Slereah Whats your favorite bird?

Slereah

15:16

Owls are pretty neat

ACuriousMind

@HariPrasad Um...okay? Why are you telling me?

ramsay

my friend asked me if $E=mc^2$ is true, it means if he looses 10kgs of weight he will loose around $10^{18}$j of energy

Hari Prasad

@ACuriousMind hmmm... I don't know.

Slereah

Look at this bird

It's a bird

Hari Prasad

@Slereah yah of course

David Z

15:19

@HariPrasad There is at least the replacement for the homework policy, which will come up during the session. Not that we have much to discuss this time; it's mainly to get people to go through the list and vote accordingly.

If anyone else wants something to be discussed during the chat session, ping me. We'll have a schedule with up to 4 or 5 sessions, depending on how many things we have to discuss.

Bass

@ACuriousMind Is that just because the products of Gaussians and Hermite polynomials are dense in $L^2$? Or is there some other requirement?

Oh, and orthogonality..

ACuriousMind

@Bass Yes. A "basis" as usually used in the context of Hilbert spaces is a basis in the sense of linear algebra for a dense subspace

Slereah

@DavidZ why does your avatar look like a libertarian freelance journalist

Writing for the Ron Paul Daily

Bass

@ACuriousMind Okay, so even though we're using the harmonic oscillator Hamiltonian eigenbase to construct a base for the free particles Hilbert space, there's no connection to the HO here. Makes sense, thanks.

Slereah

yes

It's a basis but not an eigenbasis for the theory

so it's not necessarily that useful

ACuriousMind

15:27

Well, there is no eigenbasis for the free theory

Slereah

yep

Secret

16:11

Happy accident:
Landed on this page when I google based on "intuition" gain from naively drawing the indicator distirbutions of the rationals

According to the naive drawing, no matter how far I zoom in, I will always get a segment where the end points are consecutive rational numbers (based on how small I want to make the graduations). I then started to wonder what if I zoom infinitely far, and then starting to wonder how close can 2 neightbouring rational numbers can be marked on the number line. The concept "dense" then pops to mind, which result in the googling

proofwiki.org/wiki/…

Love Grover

16:30

can anybody help me with this question? physics.stackexchange.com/questions/250364/…

ACuriousMind

@LoveGrover I already left a comment that you should ask one question per post. Asking a host of related questions will deter answerers who would like to answer some but not all of those questions, and is not a good fit for the SE model.

Love Grover

16:50

@ACuriousMind physics.stackexchange.com/questions/250433/…

Slereah

blurgh

Trying to find the algebra of the rotation subgroup is not fun

Levi civitta symbols everywhere

ACuriousMind

@LoveGrover Questions should be as self-contained as possible. What do you actually mean by the "left-right symmetric model"? What do you mean by "charges" of the components? (Usually, one means by "charge" something gauge invariant, not something you assign to individual components of a multiplet) What are $T_{3L}$ and $T_{3R}$? Explain your question to us, do not assume we can magically read your mind.

vzn

17:09

@barrycarter nice, just looked it up, looks solid/ innovative... do you have interest/ experience in algorithmic trading? have dabbled in this area myself... eg

Bernard Meurer

@Obliv I'm adding documentation to my code. Try it out, I think it got pretty fast

Slereah

Oh wait there's a quicker way to do it apparently

barrycarter

17:27

@vzn Oh dear god, yes. At one point, I posted implied volatility of NADEX options every hour or so.

@vzn OK, that's technically more Quantitative Finance stuff, but, yes I traded FOREX options both before and after Dodd-Frank.

vzn

17:55

@barrycarter (rats, missed notification) so are you using quantconnect? do you currently trade real (stock) shares? or entirely currency exchange? are you saying algorithmic trading is different than Quantitative Finance? not following on that.

The Periodic Table

TeX, LaTeX and Friends

Transcript for

$TeX - LaTeX$ TeX, LaTeX and Friends