The h Bar - 2016-10-08 (page 6 of 7)

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user218912

6:23 PM

how come in latex when I want to compile the document to pdf in the viewer the colours look fine but in the actual pdf they are faded/darker?

user218912

anyone know the fix for this?

@bloo You mean a difference between a software PDF viewer and paper printer output, or between two different software viewers?

user218912

@rob no I mean the compiled document in latex program has the right colours which are bright, but when I turn it into a pdf the colours become unsaturated slightly.

user218912

texstudio btw

@bloo The "compiled document" in texstudio is just a different pdf viewer.

6:26 PM

Sounds like a "colors are hard" problem.

If you're making a document to print, worry about how the color specification interacts with the ink in your printer.

Are you viewing both things on the same screen next to each other? If not, it's just differing monitor calibrations

If you're making a PDF to distribute to other people for computer viewing, accept that most other people's computers have crappier colors than yours and they won't duplicate your artistic experience.

user218912

@ACuriousMind yes.

user218912

the document it displays in texstudio beside the code has a different colour than the document that displays when I click the compiled pdf in adobe pdf viewer.

user218912

it's less bright.

6:29 PM

So texstudio vs Acrobat vs xpdf vs foxit vs Imagemagick "convert" vs some other option? All on the same monitor?

user218912

in other words

user218912

I'm having the same problem as this guy

user218912

104

Q: PDF colour model and LaTeX

(Strictly speaking, this is a question about XeLaTeX and the xcolor package but I’m assuming that the issue is actually with the color model of PDF.) I’m having the problem that colours in my PDF don’t match. For example, I screen grabbed a colour value as RGB, declared it in my document and the...

@ACuriousMind I'm burnt out

do you have a hint for "Let $(X,d)$ be a compact metric space. If $(f_n)$ is a sequence in $C_E(X)$ and $f_n\to f_0$ uniformly on $X$, then $\mathscr F=\{f_0,f_1,\dotsc,f_n,\dotsc\}$ has the property that $\mathscr F(X)$ is precompact in $E$. "

$E$ Banach of course

I have already shown that $\mathscr F$ is uniformly equicontinuous

And each $f_\alpha(X)$ is compact I guess

Also $\mathscr F(X)$ is bounded

but that's not helpful because $E$ isn't $\Bbb R^n$

Every time I think I understand a statement "colors are complicated" I get blown away by new information like the answer to the question linked by @bloo

user218912

6:40 PM

I don't get that answer.

user218912

I just want a quick fix.

user218912

like one line of code you add that fixes it.

aha

maybe I should prove that $\mathscr F(x)$ is precompact

then use Arzela-Ascoli

@ACuriousMind thanks

@bloo A commenter suggests that the LaTeX xcolor package has options for specifying the color space when you load the package

You should be able to get a list of them by looking at the xcolor documentation

yasss

I think that works

user218912

6:47 PM

@rob looking now.

texdoc.net/texmf-dist/doc/latex/xcolor/xcolor.pdf

One option is "fixpdftex", try that

perhaps

Could be what you're seeing is that texstudio is using a Postscript color model and Acrobat is using something different in the PDF

But whether it's the viewers or the conversion to PDF where the change occurs is a sneaky under-the-hood TeX question

user218912

@rob I tried that.

user218912

didn't work.

user218912

or maybe I tried it wrong?

If there's a one-line solution, it's going to be something like that, but without access to your setup I can only speculate helplessly

7:01 PM

@ACuriousMind Doesn't it feel good that you don't have to expend any effort to help me sometimes

@bloo time for QM homework now

yikes

want to do it for me?

user218912

@0celo7 I'm already getting fucked by qft.

user218912

but can I see the questions?

lol

my QM prof sent out a not

all the physics profs must make a list of tutors available

he said to just disregard the list, they won't be able to help

user218912

huh?

@bloo university policy, the profs have to provide contact info for tutors

user218912

7:11 PM

@0celo7 like TA's?

so he gave the info, then just wrote underneath they won't be of any help

@bloo no

user218912

@0celo7 because they don't know how to do the problems?

forget it

user218912

I don't want to forget it :(

dropbox.com/s/k74ihei2vpnkn4w/Lectures5%266%282%29.pdf?dl=0

user218912

7:12 PM

it looks doable.

user218912

but right now I'm working on GR and I have to hand in this problem set in 5 days.

doable? the problems have like 6 parts

problem 1 does seem easy

Nilpotent group

In group theory, a nilpotent group is a group that is "almost abelian". This idea is motivated by the fact that nilpotent groups are solvable, and for finite nilpotent groups, two elements having relatively prime orders must commute. It is also true that finite nilpotent groups are supersolvable. The concept is credited to work in the 1930s by Russian mathematician Sergei Chernikov. Nilpotent groups arise in Galois theory, as well as in the classification of groups. They also appear prominently in the classification of Lie groups. Analogous terms are used for Lie algebras (using the Lie bracket...

Why is this stuff so remind of Jordan chains in linear algebra...?

7:36 PM

@ACuriousMind What is special about the momentum space representation of a real wave function?

real in the position basis

@0celo7 Well, what is special about the Fourier transform of a real function?

@ACuriousMind is it odd?

or real

if you complex conjugate it you flip the argument @ACuriousMind

I always look such things up here if I forget them.

And yes, the last one it is.

Ok, so what?

What do you mean, "so what"? you were the one who asked the question!

7:40 PM

I'm do show that the probability current vanishes for a real function

but then they want me to discuss it from a momentum space POV

I know the ev of the momentum operator will vanish

I'm not sure what more they want

Well, what does the derivative in the probability current become under Fourier transform?

$p\phi$

(I think they just want you to derive the vanishing of it in momentum space)

Aha. Now, using the property that $\phi$ is the Fourier transform of a real function, $\phi p \phi^\ast - \phi^\ast p \phi$ gives zero without further computation, right?

uhhh

yes

no

@ACuriousMind I'm pretty sure the fourier transform of $\psi^*\nabla\psi$ is not simply $\phi p\phi^*$

else the fourier transform would always be zero

ohhh

Right

7:45 PM

it will be a convolution actually

Yeah, right

ugh

Well how about this @ACuriousMind

$\langle p\rangle =\int \phi^*p\phi$, right?

yep

so that's equal to $\int p\phi(p)\phi(-p)$

equal to negative itself I think

@ACuriousMind But I still don't know what this has to do with the prob. current

Well, you could argue that the convolutions also cancel each other

But I'm a bit confused at what that question wants you to do, too

7:50 PM

it's due thursday, I'll ask the prof on tuesday

@ACuriousMind When $V(x)=0$, should I just assume the eigenfunctions are 0?

The eigenfunctions of what?

hamiltonian

erm

The Hamiltonian consists of more than just V...

I mean when $V(x)=\infty$

and I'm looking at the eigenfunction at $x$

Yes, $\infty$ marks regions where the particle is forbidden

7:58 PM

@ACuriousMind So there's no tunneling at all?

Tunneling between what?

into the forbidden region

What does it mean intuitively to take the square of the derivative? I know derivative represents the rate of change (i.e., rate of change of position is velocity, and rate of change of velocity is acceleration) but I do not know much actual calculus (i.e., how to calculate derivative).

@heather apply the derivative twice

I can teach you calculus

@0celo7 Uh..."tunnelling" usually denotes that you may find a particle at one side of a classically impassable barrier at one moment and at the other side later.

8:01 PM

right

so what's the answer to my question?

@0celo7, so $d^2$ of position would be acceleration, for example? And yes, calculus would be good to know. But if you undertake teaching me, I'll warn you in advance: I have taken Algebra I but not Algebra II.

@heather Our first topic will be metric spaces

@0celo7, sounds wonderful. What are they?

and yes, that is correct

@0celo7 Please be helpful :P

8:02 PM

=)

@heather I would love to tell you, but ACM has just reminded me that would be most unhelpful.

@0celo7, I will ask the google, then...

Wikipedia says "In mathematics, a metric space is a set for which distances between all members of the set are defined. Those distances, taken together, are called a metric on the set. A metric on a space induces topological properties like open and closed sets, which lead to the study of more abstract topological spaces."

@ACuriousMind Oh, remember that weird inequality with $\frac{x}{1+x}$?

@heather There's a general notational convention that writing $A^2$ for any operator $A$ means applying $A$ twice.

@0celo7 yes

Okay, so a set, a set of numbers, like A = {1, 2, 3}

8:04 PM

@ACuriousMind It's a one-line proof if you cross-multiply :(

@ACuriousMind, good to know - thanks!

I never even thought of that

I don't know what "cross-multiply" means

$a/b=c/d\Leftrightarrow ad=bc$

works with inequalities

@heather A metric space is just a set on which you can define a distance between any two points in the set.

And this distance has certain reasonable properties

they're very useful in topology and analysis

@ACuriousMind Don't you think my avatar is super cute

I think I have a grasp on internal sets now

Robinson has a really awful notation

8:09 PM

why are you reading that book

I guess it's more rigorous since he has to use a lot of metalogic stuff but it's really awful to read

@0celo7 ::shrugs::

Internet cut out for a minute - I'm back.

@0celo7 It's the standard text on the topic

@0celo7, and is distance defined as, say, |x-y|?

8:11 PM

on the real number line, yes

but you can give any set a metric

(distance function)

@0celo7, and that is why it's called a metric space, because the distance can be defined as a function called a metric, I assume?

@0celo7 I JUST NOTICED THAT

BEST AVATAR.

good @NeuroFuzzy

@heather yes

@0celo7, would another way to define distance be through modular arithmetic?

huh?

8:14 PM

Also he uses $\subset$ for logical implication

I do not approve

@Slereah What's wrong with $\implies$?

Errr

Not subset

The one in the other direction

@0celo7, I mean if you translate x into mod whatever and y into mod whatever and see if they are congruent or what the distance is through that? But I guess you can define distance however you want...sorry, that was kind of a weird comment.

$\supset$

$\supset$

8:15 PM

beat you @ACuriousMind

Damn it

:(

yes

Instead of the proper $\to$

Do you think I should learn about calculus before reading Shankar's Quantum Mechanics? Because I'm reading chapter 3 and I'm starting to think so...

Yes

8:16 PM

Because I really have no idea what is going on.

You should also learn basic physics before quantum mechanics

@Slereah, yeah, that might just make sense =)

Sounds like I've got a new project.

If you don't know basic physics but try to learn QM you should also possibly analyze the reasons why you want to learn QM

@heather if you want QM so badly

you can try "quantum mechanics in simple matrix form"

that book itself teaches all the math you need

but still you need to learn some basic physics before reading it

Yeah. The reason I was trying Shankar was because I heard it was a good book and that I should try reading it and because I am interested in quantum mechanics.

All of this is very good advice, thanks.

user116211

8:23 PM

Kelley is getting weird with his absurd definition now.

user116211

For him, ordering is only meant to be transitive and nothing else.

user116211

That means $X\times X$ is ordering.

user116211

But I mentioned it earlier also... okay.

Well $<$ is not symmetric nor reflexive

it is just transitive

what would be a good textbook for learning classical physics necessary for quantum mechanics foundation?

user116211

8:26 PM

@heather Lanczos.

user116211

@Slereah But this is linear ordering.

user116211

He should have defined ordering as partial ordering.

user116211

He takes reflexive in consideration when the linear ordering is $\leqq\,.$

@MAFIA36790, The Variational Principles of Mechanics by Cornelius Lanczos, or something else?

user116211

@heather YES!

8:29 PM

@MAFIA36790 Are you recommending that because you are reading it or because you think it will be useful for heather?

user116211

oops runs away....

user116211

Seriously, though, yes you can learn classical mechanics from Lanczos.

user116211

Although there is no perturbation theory.

@MAFIA36790, and will it be useful for someone who has no idea what they are doing?

I'm not sold on saying that quantum mechanics requires you to know classical physics to begin with. The only useful thing would be Hamiltonian mechanics, which most people seem not to know prior to their first QM course.

8:31 PM

@ACuriousMind, okay, to understand Hamiltonian mechanics do I need to understand anything else?

define "understand"

We learned lagrangian and hamiltonian mechanics at school

user116211

@heather See ACM's comment below; anyways, Lanczos is a self-contained book as he readily clears his intention in the preface of the book.

@Slereah high school?

University

8:32 PM

@ACuriousMind My QM prof says there are 3 types of people in the class: those who do not know linear algebra, those who do not know Hamiltonian mechanics, and those for whom the class is trivial.

@0celo7, know enough to a. not sound like a complete idiot when talking about it and b. solve problems with it

@heather Well, "Hamiltonian mechanics" is as broad a field as (and in fact mostly equivalent to) "Newtonian mechanics".

Mostly equivalent to!?

user116211

Okay, go with the more general and popular Goldstein.

Tell that to symplectic topologists @ACuriousMind

8:34 PM

@0celo7 I don't want to quibble about friction forces...

@ACuriousMind, perhaps I should reformulate my question: what should I read/watch/do to learn this?

you should read Batman

@ACuriousMind I don't either, no clue how those work in Hamiltonian mechanics.

@heather Well, learning calculus and linear algebra would be absolutely necessary to do quantum mechanics. Then one can theoretically learn QM at the typical introductory level. I can't give any book recommendations because I've never self-learned from books.

@heather I can help you.

(god, I sound like JD)

user116211

8:38 PM

@0celo7 is really good in helping @heather.

user116211

@0celo7 NOO!

Well, his book recommendation I would trust.

@ACuriousMind Aww, thanks

What did I do to deserve this

user116211

Let's talk book recommendations

user116211

@0celo7 Recommending me Bredon?

8:39 PM

@ACuriousMind says he would trust my book recommendations

@0celo7 You have learned an impressive amount of things just from books.

oic

@ACuriousMind Classes are 10x better

except for QM, I haven't learned anything there yet

Not because the class is bad, at all

Shankar is just too good

@heather You need to learn calculus through calculus 3.

Some experience with ordinary differential equations won't hurt either

Then you can pick up Shankar and read it, I promise

It's incredibly self-contained

Don't listen to @0celo7, he doesn't believe in QM

I don't believe in QFT

He also doesn't believe in the axiom of choice

8:42 PM

Unfortunate

My current project relies on the axiom of Choice

@ACuriousMind correct

@0celo7, okay. So calculus through calculus 3. Geesh. Any good textbooks to recommend...?

user116211

So 0celo doesn't believe in Zorn's Lemma too?

@heather I learned calculus from, uh

Larson

Larson is very good

I took a class as a sophomore based on the first half

user116211

I should tip the balance towards Apostol.

8:43 PM

that guy?

City Hunter opening 1

taught myself the rest

then I used Paul's online notes, MIT OCW and Khan Academy for calc 3

also the Wiki articles on calc 3 are very good

user116211

@0celo7 Ted of Maths.SE also has written a very good book on multivariable calculus, if Calc 3 means that.

That book is a strange combination of calculus and analysis

I would not have appreciated it back when learning calculus

Obligatory remark that I still don't know the difference

omg ACM

no epsilons, deltas

no open, closed sets

no Banach spaces

the only norm is the Euclidean one

@ACuriousMind clear?

8:48 PM

The latter two things were also the case in all three of my analyisis lectures :P

the latter two?

how the HELL did you prove $\Bbb R^n$ is complete in the Euclidean norm

some PhD level estimation skills right there...

I can't remember, it was almost five years ago now

well the way most people do it is compare the Euclidean metric with the downtown metric

@ACuriousMind partial derivatives are just defined as taking a derivative wrt. one variable, leaving the others fixed

second partials are always assumed to commute

@ACuriousMind still unclear?

limits are computed graphically or with gut feeling

continuous means you can draw the function without lifting the pencil

9:05 PM

Is Weierstrass continuous with that definition?

It's gonna be a lot of effort to draw without lifting

it's probably not even a function

@ACuriousMind yo I have a weird phenomenon

if I have an infinite potential well that's zero on [0,a], then all the eigenfunctions will be sines

so what distinguishes the even and odd modes

What does "even" and "odd" mode mean?

n

even or odd

Why do you think there must be something distinguishing them?

Until now, the weirdest phenomenon here is you saying "yo" ;)

don't bully me

I must be stupid

I can't normalize $\sin(n\pi x/a)$ over $[0,a]$

9:14 PM

Why not

do you not just need $\sqrt {2/a}$ out front?

according to my PDE notes that's indeed what is needed :/

fudge

wonder if my calculator is in degrees or radians

commies

why is my calculator in degrees

user116211

@0celo7 Change it to rad.

user116211

What to do with this:

user116211

-1

Q: How would someone determine the speed of sound in water experimentally?

I'm in highschool so I'm looking for a highschool-level lab idea.

waves experimental-physics acoustics measurement speed

user116211

?

user116211

9:19 PM

Lack of research efforts?

I am guessing it involves a microphone and clocks

@MAFIA36790, actually, it is kind of an interesting question, though it would be nice if the OP has put some more effort into it.

user116211

Well then, I'm skipping it giving OP some time to show his works.

I guess you could also do some interferometer-like experiment

@Slereah Well, it might be easier to get modest precision with a standing wave experiment than an echo one, and that calls for a speaker and microphone.

9:25 PM

well that is what I said

You don't need a fancy interferometer. Just a half-open tube and the ability to control the input frequency.

A speaker is a given, of course

Since you need a sound

Sure, typing over one another.

You can also substitute the ability to control the length of the tube (a piston, perhaps) for frequency control. Which is what I actually do for the speed of sound in air in a lab I give my first year students.

user116211

Anyways, I'm off for now. Night comrades.

Good night

9:42 PM

@dmckee is the number of nodes for a standing wave just $n+1$?

For a half-open pipe and counting the fundamental as $n=0$ and counting the node at the closed end, then I think that is right.

I always have to sketch a few to be sure.

not half open

pinned at both ends

yeah I'm counting the nodes at both ends

better write that in...

user218912

mods will be revealed in a few hours?

no that doesn't make sense

it's $n+2$

@bloo Huh?

9:45 PM

er

user218912

wait

the $n=0$ mode is just nothing I guess

It says "election ends in 3 days" on my end :P

ahhh

user218912

sorry I thought today was 3 days from now.

9:46 PM

@bloo, the election page says the election closes in three days =)

user218912

well that was embarrassing...

Let's say I have a question about quantum computing (this is purely hypothetical). Should it go on physics, computer science, theoretical computer science, or somewhere else?

@0celo7 Yeah. You have to be careful about this because many treatments aren't clear about where they start counting.

rekt

@dmckee I'm counting 0 because that's the lowest energy eigenstate

For reference, I have asked questions about quantum computing on computer science and physics (so I probably should've asked this question earlier, but better now than never).

9:48 PM

it's a legitimate solution of the SE I think

@0celo7 That's how I like to do it, too. But a few treatments differ.

@heather That depends on whether it is primarily about the physical implementation, the operators/states involved, or an algorithm as such. The first two are clearly for physics.SE, but if you want to know something about the algorithm, it's probably rather compsci or theo. compsci than physics.

@ACuriousMind They want me to evolve a sum of two energy eigenstates. To do that I just multiply each summand by the appropriate phase, right?

@ACuriousMind, okay, then say it is about some aspect of a quantum gate, not the physical implementation. Where would that fall?

Depends on the aspect ;)

@0celo7 Why would you do that?

9:51 PM

@ACuriousMind, like my cNOT gate question that I posted here, would that have been better suited on computer science?

@ACuriousMind $|\psi\rangle =\sum c_n|n\rangle$, $U(t)|\psi\rangle =\sum c_nU(t)|n\rangle$

but $U(t)|n\rangle =e^{-iE_nt}|n\rangle$.

@heather I don't think so (but then again, I don't know their scope). The problem with that question isn't that it is off-topic, it's that it's not clear to the reader (or at least to me) what is actually confusing you. I found that out only by your clarifications in chat.

@0celo7 Yes, correct

@ACuriousMind I knew that so why did I have to pull teeth :P

Why did you ask me if you knew?

Confirmation.

user218912

9:59 PM

I just found this old qft book in my room I don't even remember where I got it from.

user218912

by das.

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