The h Bar - 2016-10-01 (page 4 of 6)

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4:00 PM

Originally the type A went in to devices and the type B into hub and other things that drove the system. So you could tell by the kind of plug what function (device or controller) the box you were plugging into had.

The ancient tech knowledge is real

user218912

@ACuriousMind idk how you're supposed to take the derivatives like $\partial_\mu$ of $e^{i\mathbf{k \cdot x}}$. this is my first time doing a derivative of a dot product in the exponent.

user218912

can you explain please?

user218912

more specifically I don't know how you get the last equation from the first 2 here

@dmckee Wikipedia seems to use A and B exactly in reverse to your nomenclature, but thanks!

@IceLord It's just the chain rule.

there's no "dot product rule", it's really just the chain rule

4:05 PM

Uhm ... Okay. I'm sure they are right. Because the ancient knowledge gets lost over the years.

Hi, everybody.

@JohnRennie @KaumudiHarikumar I just want to point out that a lot of the important time spent in science, probably most of it, is not flashes of inspiration, but rather methodical, slow progress.

user218912

@ACuriousMind can you show me explicitly the steps for the first one please?

user218912

idk how to treat the dot product.

user218912

do I write it out expanded?

See, I could, but why don't you just try things instead of saying "I don't know how to do it" and giving up?

4:09 PM

^ That

There is nothing to learn here, if you know how a derivative works then you can take this derivative.

@ACuriousMind For what it's worth, I think your generous use of time spent telling others how to solve their problems sort of encourages folks to ask for help instead of trying.

I'm not saying that makes sense.

user218912

@DanielSank fyi ACM barely ever tells me the answer straight up.

...but note how often you get pinged.

user218912

in fact the last 5 times he told me to figure it out myself.

user218912

4:11 PM

which I did.

@IceLord you should have learned this in a course on calculus

I've nearly finished my chapter on monopoles. It's fun.

@DanielSank There's actually another reason. Most of us in this chat are specialised on one aspect of maths and physics, but ACM is pretty all rounded in a lot of maths and physics fields (from high school to research level), thus that massively increase the number of users pinging him for help

user image

@ACuriousMind Ping

4:11 PM

@BernardMeurer Ping

user218912

@0celo7 I know how to take derivatives.

@DanielSank Pong

@Danu ping

@DanielSank Yes, I have noticed that. Aside from the "rubber duck" pings from @0celo7, the questions are mostly genuine and not "tell me how to do this", though. Except for IceLord's case, which I grow increasingly frustrated with.

@JohnDuffield Yes, I've seen that picture. Right hand rule and all that. Not seeing monopoles.

4:12 PM

Ok, what the hell is a rubber duck ping?

@IceLord Because you never ask a question to which you would actually lack the knowledge to figure the answer out.

@DanielSank : exactly.

Rubber duck debugging

In software engineering, rubber duck debugging or rubber ducking is a method of debugging code. The name is a reference to a story in the book The Pragmatic Programmer in which a programmer would carry around a rubber duck and debug their code by forcing themselves to explain it, line-by-line, to the duck. Many other terms exist for this technique, often involving different inanimate objects. Many programmers have had the experience of explaining a programming problem to someone else, possibly even to someone who knows nothing about programming, and then hitting upon the solution in the process...

@JohnDuffield You must be a magnetic monopole, because I can only feel attraction to you

WINK

Stringent application of the basic rules of differentation/integration gives your answers, there are no pitfalls.

user218912

4:13 PM

okay.

@ACuriousMind Heh, well, to be fair, that sort of statement applies to everything.

@DanielSank yes, I know what that is

But what the hell is a rubber duck ping?

@BernardMeurer : sorry, there are no magnetic monopoles. When you understand electromagnetism, you understand why.

@0celo7 Have you tried basic logical extension based on the wiki article and the context of this conversation?

@JohnDuffield I was just hitting on you man :/

4:14 PM

@0celo7 You know those things where you ping me with questions and don't actually expect me to answer them right there and you usually figure out the answer before I ever see them?

2

@DanielSank no, what would hurt

@BernardMeurer : no, I was hitting on you.

@ACuriousMind This is exactly what I'm talking about.

@0celo7 It's when you won't shut up and ask every step of your problem but no one answers and you figure it out anyway

user218912

the answers to my questions are basically always "basic calculus" or "shankar".

4:15 PM

@JohnDuffield WINK

@BernardMeurer That's a legitimate proof strategy

user218912

next time I want to ask a question I should first think, 1. is it basic calculus, 2. is it in shankar.

@JohnDuffield You have a fantastic ability to sound condescending. It's really amazing.

@0celo7 Called rubber ducking

@IceLord So figure them out yourself!!!

ok @DanielSank no need to star every message that's anti-me

4:16 PM

@0celo7 Why do you think I starred that?

Hint: I didn't star that.

Because you think I'm annoying

@ACuriousMind I will stop asking you questions if you want.

@DanielSank That's exactly what someone who starred that would say

@DanielSank : why thank you Danny boy.

user image

@0celo7 That message isn't anti-you. It's my explanation of what "rubber duck pings" was supposed to mean

@0celo7 It's funny how often someone else stars your posts and you think I did it.

4:16 PM

Then why is it starred twice?

Clearly by people who want me to stop, so fess up

@JohnDuffield Oh my goodness. Everyone stop. Magnetic field circles around a wire of current?!

Since these pings usually don't require any action from me, I really don't care that much about them

"that much"

implying you do care

@JohnDuffield Wait, I'm confused. If you have a loop of wire with current, you get magnetic fields in a dipole pattern?!?!

@ACuriousMind I fixed my shelf issue, btw

4:19 PM

@0celo7 Yes, I'm slightly charmed that I'm the first person you think of to ask those questions to and slightly annoyed that you didn't think a bit more about those questions before asking them. Neither feeling is strong, and they more or less balance out

Anyone have any good puzzles etc?

@DanielSank Yes, understanding JD's mind

@ACuriousMind Hahahaha dat ego.

@DanielSank Try Puzzling

@ACuriousMind Those are rarely physics oriented.

4:19 PM

@ACuriousMind Think a bit more? I think some of them are quite difficult problems

Which I don't expect you to know, no, but writing something out helps me 95% of the time

@DanielSank : huh? Yes. As per this image from hyperphysics.

user image

Are you being serious?

@0celo7 Yes, writing things out helps everyone 95% of the time.

yes

@0celo7 Yeah, that's what the rubber duck is all about

^

4:21 PM

Okay bois who's gon' help me with analysis and understanding convergent series & limits

ok so why are you annoyed?

@BernardMeurer ::hides::

@0celo7 Because you machine-gun ping him probably

@BernardMeurer I already said I would

@BernardMeurer maybe

@0celo7 But you'll tell him about sigma algebras etc. instead of getting to the point ;)

4:22 PM

But he said he's annoyed I don't think more, not that the actual pings annoy him.

@0celo7 When do you have the time?

@BernardMeurer Right now.

@DanielSank No, I won't.

@0celo7 Excellent.

Gut, Skype?

I took intro analysis too

4:22 PM

@0celo7 Because seeing a lot of things in my inbox and then finding out it's all now-obsolete pings from you is somewhat underwhelming. Relax, it's not a big deal ::quacks placatingly::

Hmm, ok.

fwiw, I wish I could think things out without pinging you

@0celo7 I'm rather sure you can.

Based on observation, below is a list of h barers and their typical field of expertise:
Acuriousmind: Basically *anything physics and maths related*
Slereah: QFT, CTCs, differential geometry, nonstandard analysis, non archimedian geometry
MAFIA36790: Order theory, topology
0celo7: GR, Proof checking, topology, functional analysis, math rigor
Balerka Sen: Topology, Fiber bundle stuff (that I don't understand), analysis, differnetial geometry
Johnrennie: Physical chemistry, particularly molecular excitations, polymers, general chemistry, newtonian mechanics, thermodynamics, GR

@DanielSank I think out 99% of things without pinging him

@0celo7 Me too, oddly enough.

4:23 PM

that's what annoys me when you say I ping him about "everything"

@0celo7 hyperbole is a common pitfall in language.

I have thousands of thoughts a day, I ping him about one or two things and all of a sudden I'm pinging him about everything

(sometimes it's three or four, I don't know)

@0celo7 Yeah, sounds about right.

Gosh look at the time. Bye all.

user218912

@Secret me lol and bernard

4:25 PM

@Secret I don't know diddly squat about quantum information.

::looks at the time::

::shrugs::

I'm good at quantum mechanics and in particular quantum mechanics in the context of electrical circuits and noise, but I know basically nothing about quantum information.

@Secret :(

ghetto desk shelf is now online

@0celo7 Promotions all around.

@BernardMeurer lololololol

4:26 PM

@DanielSank Ok, I must be misunderstood because of seeing how you seemed to be good at explaining qubits and stuff

@DanielSank Note: Shankar.

@Secret I can explain how they work physically, but I know essentially zilch about actual quantum information.

noted

@0celo7 I find placing "Understanding Analysis" right next to "GR for Mathematicians" somewhat funny ;)

@ACuriousMind :)

4:27 PM

@Secret ACM's should be "Anything and Everything"

It's a really good analysis book

@0celo7 Skype dawg

ok ok

@BernardMeurer Hmmmm, let's test that.

@BernardMeurer Not really, compare to Johnrennie, ACM knew less chemsitry

user218912

4:28 PM

what @0celo7 said he doesn't have skype anymore.

@Secret It's mostly just about the meme

@BernardMeurer Maybe I should stop commenting on anything and everything. :P I should also get back to sorting my things...

@IceLord His sister does

@IceLord I did

wonder if I can log in

oh it's on my phone and Windows partition that it doesn't work

I guess I can still sign on here, huh

That's actually wrong, his sister does not have a Skype

I could've sworn she did

4:29 PM

As for IceLord, to be honest, it might be true that he is an expertise in QFT because he is learning it, but so far his question asking frequency is just slightly less than my super annoying EM question friend

@ACuriousMind That analysis book was for a class

@0celo7 I know

@Secret I'm not there :(

Having said that, I am ok with his questions because they are quite interesting

Hey @ACuriousMind suppose I have a superconducting LC oscillator cooled to the point that it's quantum. The Hamiltonian is $H = \Phi^2 / 2L + Q^2 / 2C$ where $L$ is the inductance, $C$ is the capacitance, $\Phi$ is magnetic flux, and $Q$ is charge. We have $[\Phi, Q] = i \hbar$. If I want to be able to drive this system with a voltage, do I want large $C$ or small $C$, and what dimensionless parameter should I look at?

4:30 PM

Bernard: Get to you shortly...

user218912

@Secret lol what how do I have expertise in QFT? and I don't ask that many questions :p

I remember you telling me to get it so you could ask me questions about the scary $\epsilon$s in there ,)

@ACuriousMind Ok I'm worried why you think it's amusing

@ACuriousMind did you lose an eye :o

This is actually an awesome question that everyone should understand. Note that the fact that it's an LC oscillator is kind of irrelevant and really this is about understanding zero point motion.

@IceLord You might soon in the future, given you are learning and doing a lot of exercise on QFT now as suggested by your questions

4:30 PM

@0celo7 Because one book is for a beginning mathematician and the other for one already reasonably grown

@0celo7 what?

user image

:/

Oh, lol

user218912

@Secret ideally that's the plan.

@ACuriousMind We've been over my unconventional route to geometry before.

user218912

but I would prefer if you associate me with condensed matter @Secret

user218912

4:32 PM

that's what I'm learning right now, even more than qft.

⁰

@DanielSank I'm afraid I don't exactly know what "driving the system with a voltage" means

@0celo7 Sec

I have that analysis book there because a lot of the Banach spaces proofs in my (functional) analysis class are similar to the $\Bbb R$ proofs in there

@0celo7 Sure, doesn't mean I can't still find it somewhat odd and occasionally amusing, right?

4:33 PM

@ACuriousMind How come you didn't notice the book on representation theory?

(which I will read one day, I promise)

@ACuriousMind Ok, it means I try to drive transitions by applying an oscillating voltage to the system.

@DanielSank Yes, so in the formalism, you're adding a time-dependent perturbation to the Hamiltonian, right? What form does that perturbation have?

@ACuriousMind I'm thinking how to answer that without giving away the whole problem.

@BernardMeurer A a-z search does not yield any aspect that particularly stood out. You do ask daniel a coupel of questions, talked about C and python, shared various music videos, make jokes and other things

I guess what I'd say is that the equation of motion for a voltage driven oscillator is $$(C_d / C)\dot{V} = \ddot{\Phi} + \omega_0^2 \Phi$$

4:38 PM

@Secret I'm a specialist in nothing :p

And I guess you could say Python, Linux and that crap

where $C_d$ is something like a coupling strength for the drive.

@DanielSank Mhm, so I'd figure out which Hamiltonian gives that e.o.m., then examine in perturbation theory what effect the parameters have on the transition rate

And to add onto the list:
Obliv: Most analysis related topics
yuggib: Quantum mechanics, distribution theory, analysis and mathematical formulation of quantum
Johnduffield: (For cases where his answers are correct) does appeared to be quite resourceful at answering questions

@ACuriousMind That's the long way, yeah.

@ACuriousMind Skype!

4:42 PM

@DanielSank But I, too, have this idea.

@DanielSank You're saying there's a neat trick?

@ACuriousMind Not so much a trick as that you can just get the answer right by thinking about how the parameters in the Hamiltonian affect the zero point motion.

@0celo7
Is the following a good proof for
x > y and z < 0 => xz < yz?
---
Begin with z<0
Add -z both sides 0 < -z
Multiply -z both sides of the expression x >y => -zx > -zy
Add zx and zy both sides zy > zx
Commutativity of reals yz > xz

user218912

why are square root terms not lorentz invariant?

user218912

like why is $\frac{1}{\sqrt{2\omega_k}}$ not lorentz invariant?

4:44 PM

Well, is $\omega_k$ Lorentz invariant?

user218912

yes.

No, it isn't.

Look at its definition.

user218912

D:

user218912

but I remember my prof saying if the square root wasn't there it would be lorentz invariant.

@IceLord maybe it contains a term like $k_{\mu} k^{\mu}$ and only constants besides?

user218912

4:47 PM

wait let me look at my notes because I might have missed something.

user218912

I can't read my own handwriting.

@IceLord :D

@IceLord That doesn't make any sense. If $s$ is Lorentz invariant, then so is $\sqrt{s}$ and $s^2$.

user218912

@ACuriousMind okay sorry I might be missing a term in that. let me check.

@IceLord dude

4:48 PM

@ACuriousMind any continuous $f(k)$ for a Lorentz-scalar $k$?

he means the integral would not be lorentz invariant

check Weinberg Vol. 1 for a good explanation

chapter 3 probably.

@Sanya Depends on what you mean by $f$ there. For instance, $f(s) = p^0 s$ for a component $p^0$ of a vector $p$ wouldn't be Lorentz invariant.

user218912

@0celo7 ah, you're right.

user218912

I even wrote that down.

@ACuriousMind I meant a continuous function of scalar value I guess ... :D

hmm

4:51 PM

@Secret What is JD's specialty

that's kind of a stupid thing to say

@Sanya Then we're in tautology land because scalar functions of anything should be scalars :D

let's say it's a function \mathbb{R} \to \mathbb{R}

@Slereah The electromagnetic field, Einstein, and the evidence.

2

@Slereah Extremely resourceful for those answers that are actually correct

4:52 PM

@ACuriousMind yeah, I see your point ... I was just wondering why you only listed two exponents ...

Because those two were the currently relevant ones. $s^n$ is a scalar for all n, of course.

There wasn't any deep reason for it

k :D

@Slereah The point is that whenever someone in the chat ask a non QFT SR level question, and for the subet of his answers that are correct, JD often can find highly relevant resources in a short time

Please note that SR is a subset of GR, thus saying non SR automatically excluded GR

user218912

@0celo7 I can't find it, but his book looks weird to me and I don't understand it.

user218912

I can only read modern books because I'm dumb.

4:57 PM

Weinberg is 1995

not old at all

user218912

@0celo7 older than me.

user218912

by 3 years.

user218912

anything before 2007 I consider old.

X_X

I am deeply hurt by this statement

user218912

the < 2007 = old statement?

5:00 PM

yep

user218912

there's just something about books written before 2007, they don't have nice latex fonts and the formulas look bad, they also are difficult to follow. (for me)

What nonsense

I have plenty of pre 2007 LaTeX books

user218912

well they use a bad font package.

user218912

not CMU and amsfonts.

user218912

fine let's move it to 2003.

user228700

5:03 PM

@DanielSank I see :-) Like I said before, I'm looking forward to it anyway. How's it going BTW?

user218912

this is all my opinion btw so don't take it seriously.

user218912

@Sanya are you an undergrad or grad student?

I have a book from 1921 which has decent looking formatting :o

@IceLord I guess I was undergrad until recently

ah no, graduate

depends on whether you use US or British levels

@IceLord I still call bull.

Let's check the shelf.

user image

user218912

@0celo7 fine I'm wrong.

user218912

5:14 PM

mb it's just the books I read.

user116211

Does Valter's post provide complete solution to the homework problem:

user116211

0

A: Wave function in a semi infinite line

I henceforth assume $\hbar=1$. The operator $P=-id/dx$ restricted to the space of smooth functions vanishing at $x=0$ and for $x \to +\infty$ and defined on $[0,+\infty)$ is symmetric. I suspect that there is a self-adjoint extension of $P^2/2M$ with positive purely continuous spectrum, in view ...

user116211

?

user116211

@MAFIA36790 I understand. Do you think I should remove my answer? — Valter Moretti 29 mins ago

user116211

I'm not still competent enough @Valter to judge the content of your post. However, in general, when you answer such post which shows little to no research efforts at all, keep in mind not to completely answer the question and provide complete solution; just show OP the conceptual path; that's it; after-all we are not homework-solving service albeit we have no problem in helping OP to have a clearer insight on the concerned concept(s). — MAFIA36790 17 mins ago

user116211

5:19 PM

@MAFIA36790 Your competence does not matter here. I am just referring to the fact that my answer, correct or not, is complete thus leaving no further efforts to the OP. The policy regarding the questions is clear to me, there are reasons to close this question. But what about the policy about too explicit answers like mine? — Valter Moretti 11 mins ago

user116211

@ValterMoretti: As I said, you shouldn't provide a complete answer to homework problems. Providing an answer that doesn't help a student learn is not in the student's own best interest, and if a solution complete enough to be copied verbatim and handed in is given immediately, it will encourage more people to use the site as a free homework service. — MAFIA36790 6 mins ago

user116211

I helped close the question. I tried to delete my answer, but it is too late. Sorry "to close the stable door after the horse has bolted" — Valter Moretti 4 mins ago

user116211

._.

user116211

I think, after reviewing and giving Valter the link to the homework policy, my job is done; but is Valter's post in par with our policy? Anyways, it has been accepted...

@MAFIA36790 I'm undecided because the question isn't actually a homework question. Asking to "normalize" a sine function on an infinite interval doesn't make sense. Valter took a sensible interpretation of what that was supposed to mean and ran with it, but I would have closed the question as being unclear rather than being homework-like.

user116211

5:26 PM

@ACuriousMind ohh; okay.

His answer would still be a good answer if the question was edited to "Is there a meaningful notion of normalization for this non-normalizable function?", which certainly wouldn't be a homework question.

However, without the asker telling us what they actually mean by "normalization" here, it's just unclear.

user116211

@ACuriousMind Would you like to edit the post to make it less homeworky? That's your call, though.

@MAFIA36790 No, because my edit would impose a particular intent on the question that's not necessarily the intent of the asker. Maybe they already knew the correct notion of normalization and just wanted someone else to compute it, which would be a homework question.

user116211

okay; very, well then.

@KaumudiHarikumar Going well. Checking one of the appendices of my thesis for errors because I'm going to release it on arXiv as a short document.

(I want to be able to easily refer to it)

user116211

5:34 PM

@Sanya I have Hilbert-Courant; one of the oldest literatures in my possession (okay not mine; borrowed from the library)...

My oldest book is probably Spivak 2

user116211

@0celo7 I have Kelley of 1953.

I mean physical age

user116211

ohh.

@MAFIA36790 well, it wasn't about competition, more about stating that even before 2000 people could write decently formatted books

5:35 PM

how old the pieces of papers are

user116211

@Sanya ohh ;D

user116211

I just mentioned it by the by; without any intention of competition ;P

PSA: If you have a quantum harmonic oscillator $H = (1/2) \alpha u^2 + (1/2) \beta v^2$ with $[u,v] = i \gamma$, define $$X \equiv \frac{1}{\sqrt{2\gamma}} \left( \frac{\alpha}{\beta} \right)^{1/4} u$$

and $$Y \equiv \frac{1}{\sqrt{2 \gamma}} \left( \frac{\beta}{\alpha} \right)^{1/4} v$$

Don't pick any other convention (especially factors of 2) or you'll be sad.

Note $[X,Y] = i/2$.

Is that how Shankar does it?

@ACuriousMind USB printer cable. That square connector is a type of USB connector that only seems to be used for printers.

5:39 PM

This is the right choice because then for a coherent state $| \alpha \rangle$ you get $$\langle \alpha | X | \alpha \rangle = \text{Re}(\alpha)$$

@JohnRennie Yeah, dmckee already pointed that out, but thank you, too

There's another convention out there in the world where that equation for the expectation of $X$ is different, which is ridiculously confusing. I think the other convention exists so that $[X,Y]=i$, but I don't think having a factor of two different in the commutator is worth it when that screws up the meaning of $X$.

I realised a few PgDns later that you already had an answer - I'm too late as usual :-)

@0celo7 Not sure. I've never seen any authors parametrize the harmonic oscillator the way I do (i.e. without using a "mass").

I find it very annoying that so many authors write the SHO Hamiltonian as if they're working with a mass on a spring despite the fact that doing so makes $m$ have dimensions of something other than mass.

It's better to write it generically and then write down what $\alpha$, $\beta$, $u$, $v$, and $\gamma$ mean in your particular problem.

(Often $\gamma = \hbar$)

@ACuriousMind sniff

5:44 PM

@Sanya Well, okay, you told me it was for a printer first :D But dmkcee said it was USB

that's obvious :D

PSA: Put all your personal notes in some kind of cloud hosted repository. Doing that has made my life so much easier.

@DanielSank managed to get by without invoking sigma algebras, I even resisted telling him about Banach spaces.

@0celo7 Awesome!

I only wish I had my homework assignments all online.

@DanielSank He did say Hilbert spaces once though

5:49 PM

I think most of his problems are due to a crappy prof and strange notation.

@BernardMeurer I misunderstood you.

@0celo7 Bad notation is responsible for most confusion in physics, and some in math.

Think this will annoy people?

Cartesian product of arbitrary index sets is so mind bogglingly big that diagrams are starting to have not much use. Guess I need to familarise myself more in how to read set builder notation:

user image

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Oct '161

The h Bar

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