The h Bar - 2016-09-22 (page 5 of 5)

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user116211

9:10 PM

Oh, so, can I say $U= U(q_1,\ldots,q_n ; \dot q_1,\ldots,\dot q_n)$ does not explicitly depend on time $t\,?$

::facepalm::

Comment not posted:

user116211

@dmckee :(

@MAFIA36790 Pretty sure he's not talking about you.

> Oh, you've done the [first ultrasimplified model system] problem. Well, that must make you an expert, so I'll take my experience doing [graduate level textbook problem on the same subject], [almost but not quite cutting edge problem], and working on extensions to [best of breed cutting edge simulation of similar problem] and defer to you.

user116211

@ACuriousMind yes, i know...?

9:17 PM

::pounds head on desk::

Sometimes it like pounding sand.

@MAFIA36790 Then I have no idea what your ":(" was supposed to mean, but whatever

user116211

Anyways, @ACuriousMind, I think I got what he wanted to mean.

And there were textual clues that it was going to be that way, but no. I just had to stick my two cents in.

user116211

He meant that since $U$ doesn't depend explicitly on $t$, the total energy remains constant provided $V$ is written like $(18.5).$

user116211

And, yeah, @ACuriousMind, he is talking about scleronomic systems.

user116211

9:20 PM

But @ACuriousMind, where did he get the motivation to write the first term as$\displaystyle\sum_i\dfrac{\partial U}{\partial \dot q_i}~ q_i\,?$

@MAFIA36790 Presumably by computing the total conserved energy for a scler. system?

user116211

@ACuriousMind okay.

user116211

@ACuriousMind Does it involve calculus of variation?

user116211

I would start that chapter tomorrow.

Since the standard method would be to apply Noether's theorem, probably yes.

user116211

9:25 PM

@ACuriousMind okayish!

@ACuriousMind Ok, good news.

Wait, not good news.

i lied

Metric derivative

In mathematics, the metric derivative is a notion of derivative appropriate to parametrized paths in metric spaces. It generalizes the notion of "speed" or "absolute velocity" to spaces which have a notion of distance (i.e. metric spaces) but not direction (such as vector spaces). == Definition == Let ( M , d ) {\displaystyle (M,d)} be a metric space. Let E ⊆ R {\displaystyle E\subseteq \mathbb {R} } have a limit point at ...

@ACuriousMind I need this.

They have the claim of what I need :o

@ACuriousMind I will give you $5 if you can prove it...

Is there a term that experimentalists use when a certain result isn't entirely obvious in supporting a theory? @acuriousmind

like confidence or something?

@0celo7 Can you spell out again what statement exactly you want to prove?

@Obliv I don't understand the question.

Like if the error values are significant (~20%) but you know what the error probably was, is there a term for the confidence you have that the experiment supports the theory?

brb

@ACuriousMind The last line. That the metric derivative is the norm of the Frechet derivative.

@ACuriousMind Exactly:

Let $c$ be a smooth curve with $c(0)=0$, and $c'(0)=v$ (regular derivative). Then $$||v||=\lim_{t\to 0}|t|^{-1}||c(t)||$$

@ACuriousMind We know that $v=\lim_{t\to 0}c(t)/t$, right?

9:48 PM

Wait a moment, can't you just write $\lvert t\rvert^{-1}\lvert\lvert c(t)\rvert\rvert = \lvert\lvert c(t)/t \rvert\rvert$ and then $\lim_{t\to 0} \lvert\lvert c(t)/t\rvert\rvert = \lvert\lvert \lim_{t\to 0} c(t)/t\rvert\rvert$ since the norm is continuous?

@ACuriousMind Yeah. But that's too easy and would mean I spent hours thinking about this for no reason.

I clearly discarded that as a possibility hours ago...

:(

I was just about to write that.

@ACuriousMind Ok, one can generalize that to a Riemannian manifold pretty easily using normal coordinates (in a geodesic ball :D)

So $$||v||=\lim_{t\to 0}\frac{1}{|t|}d(c(t),p)$$

What do you need normality for in this case? Wouldn't any chart suffice?

@ACuriousMind The metric is not nice in a general chart

Ah, yes

and note that $d$ is also not nice in a general chart

@ACuriousMind But in normal coordinates we have $d(c(t),p)=\sqrt{(c^1(t))^2+\cdots+(c^n(t))^2}$

So that's very nice

We need $c$ to stay within a geodesic ball for $|t|<\delta$, but by continuity that's fine.

9:56 PM

Is it just me or my analysis teach looks like he got caught masturbating on his picture

math.tecnico.ulisboa.pt/professor.php?who=fteix

@BernardMeurer That tells us more about how you look when you get caught masturbating than about your teacher, I'd say.

Dude my advisor looks like he's being held at gunpoint and forced to smile

@ACuriousMind Lol, indeed, but isn't there something peculiar about his look?

I'll show later

user116211

@BernardMeurer yeh, constipation, that's it.

9:58 PM

@ACuriousMind weren't you supposed to remind me about something?

Save ~~Marina Joyce~~ 0celo7's advisor

@MAFIA36790 lol

@0celo7 You did not tell me what about.

@ACuriousMind Of course I did

Stupid grad students

Oh, right!

@ACuriousMind remind me in 30 mins, busy rn

10:27 PM

@ACuriousMind has it been 30 mins

No, 19 :P

@ACuriousMind Well I will tell you anyway

there's a group of grad students in my QM course who are really pissing me off

they're foreign students who already know all the material because their undergrad QM courses were good

but we've got a bunch of first year American grad students who either got ruined by Griffiths (prof's words), have never seen QM before, or don't know linear algebra

So the prof is going pretty slow

@BernardMeurer Don't shoot, I'll smile!

@ACuriousMind So these students sit in the back of the room, don't take notes, and always ask pedantic questions

Lol, it looks more like he sat on cold water or something

@ACuriousMind I have no clue why they come to class

Well, neither do I from your description

10:38 PM

@BernardMeurer lol

user218912

@0celo7 sounds like a good description of me. xD

@0celo7 it probably makes them feel very powerful and wise

@Sanya The prof is far more powerful and wise though

That's what I don't get

they never win

so they actually learn something?

The prof isn't some 80yo guy who will let them win. He's a young researcher who destroys them

10:40 PM

but being corrected and losing a scientific/mathematical argument means learning

user218912

I heard from a prof at my university that young prof's like to harass students in areas they're weak but old profs like to work with students on areas they know a lot about @0celo7

@Sanya maybe

Huh. My linear algebra prof studied under Borsuk.

I'm not there so I can't really tell - and if their conduct is detrimental to class progress, the prof should still kick them out

@IceLord I have no idea what that's supposed to mean

My advisor studied under Yau!

user218912

10:44 PM

@ACuriousMind in other words he said young profs like to show people how much they don't know, where as old profs like to show people how much they do know.

user218912

idk how true that is

user218912

@0celo7 @ACuriousMind I finished my problem set and now I have REAL qft related questions from the lectures. can you help me out please?

@IceLord Sounds like nonsense to me

user218912

@ACuriousMind probably.

@IceLord I dunno, my advisor doesn't do either. Maybe the second one

He basically talks about what I want to talk about

user218912

10:47 PM

well he's a good advisor then.

@ACuriousMind Did you do Stone-Weierstrass in analysis?

user218912

@ACuriousMind :(

@0celo7 I'm...not sure, actually

@IceLord ?

@ACuriousMind Do you know what I'm referring to?

user218912

@ACuriousMind can I ask questions please?

10:49 PM

@0celo7 Polynomial approximation of continuous functions

Yes

@IceLord Depends how good they are.

user218912

they will be good now

@IceLord How often do I have to say that people here should just ask their questions and if someone wants to answer they will answer?

I don't commit to answering questions before I have heard them

@ACuriousMind What do you know about Green's functions on sets open in $\Bbb R^n_{1/2}$?

Specifically, do you know anything about their behavior on the boundary?

@0celo7 I don't know what the subscript 1/2 there means, but even without it: Not much, and nothing at all rigorous.

10:52 PM

@ACuriousMind Half-space. I forgot the standard notation.

ok.

user218912

@ACuriousMind in this why is there an $\alpha_{-k}$ shouldn't it be $\alpha_{k'}$?

user218912

what does a negative $k$ subscript mean?

@IceLord Why should there be a $k'$?

@IceLord It's just a label, why does it matter whether it's positive or negative?

user218912

okay got it.

user218912

i thought it meant something

10:55 PM

@0celo7 hi, how do you read this in english ? ($\Bbb R^n_{1/2}$)

@2physics Not sure. I'd say the $n$-half-space.

I think $\Bbb H^n$ is better notation, perhaps.

where is R then?

nowhere

isn't that R sub a half to the n ?

user218912

which means the same thing as what 0celo7 said

10:58 PM

it's just notation

it means the half space

how do you read this $\Bbb R^n$

Are-enn

and this $\Bbb R_{n}$

that's not a thing

how do you read this: $\ a_{1}$

user218912

11:02 PM

a1 or a sub 1.

yea

and this $\phi$

user218912

phi

fee or fai

lol

user218912

doesn't matter? i would say fai though I heard people say fee before.

user218912

11:03 PM

fee sounds weird

in greek it's fee, but in english it's read as fai

@2physics The first one is the original ancient Greek pronounciation, the second one the more common English one.

we used to read the letters in their greek pronunciation here at school as far as I remember.. for example we call $\pi$ p-ee not paI

and also f-ee . but not ks-ee, we read it as ksai

@ACuriousMind yup

That's inconsistent :P

pee is how the Germans say it

11:08 PM

yes and actually it's kinda hard to pronounce some of them, like ksai or psai.. hard to stick k and s together!

I stopped saying "sub" when I learned GR

@2physics That entirely depends on your native language ;)

user218912

I ate 3 donuts today.

user218912

back to back.

yea.. we don't use consonants without vowels between them

user218912

11:11 PM

because I skipped breakfast and just grabbed 3 donuts from the physics lounge just before qft lecture since I was late.

how are you supposed to say $a^n$

a to the n

assuming you want to be consistent with $a_n$ as "sub" n

@2physics not an exponent

user218912

a sup n.

umm

a-n above :D

11:13 PM

@IceLord It's doubtful you can reliably distinguish between the soft and hard sound at the end in quick speech there

@ACuriousMind Clearly you don't speak very clearly

I make sure to spit when I say p so everyone knows.

Try The McGurk Effect! - Horizon: Is Seeing Believing? - BBC Two

The McGurk effect is a perceptual phenomenon that demonstrates an interaction between hearing and vision in speech perception.

interesting. have you ever heard of it?

user218912

yes in grade 8 science we learned it.

sub sub sub.. what i'm saying??

@ACuriousMind If I write a full proof of the Lie-ness of the isometry group, would you read it?

11:18 PM

don't get it wrong, I'm saying sup sup sup :D

@0celo7 yes

@ACuriousMind For real?

I'm considering making it a project for the next weeks/month.

how do you read it in GR : $\ X^2_{1}$

X 1 2

@dmckee If they ask me for how many grams of $^{35}P$ are in a 5 mCi source, do I look the activity up on wiki or where?

ever heard of Q.E.D in logic and set theory?

11:25 PM

nope

of course

user image

huh

Never seen it outside of QFT

this is how they refer to quantum electrodynamics in set theory maybe.. lol

btw is it $\mu$ "m-oo" or "m-ee-u"

myu

it's exactly like the pokemon

11:39 PM

ok thanks

$\frac{\partial u}{\partial t}$

@0celo7 how do you read this please

particularly this: $\ {\partial }$

@0celo7 I used to use ie.lbl.gov/education/isotopes.htm as my go-to source as it has a nice periodic table interface and a wealth of data as you drill down. But it seems to be off-line now.

Alternative include nndc.bnl.gov/nudat2 and nucleardata.nuclear.lu.se/toi/radSearch.asp and there is no reason Wikipedia should have decent numbers, they just aren't neatly organized all in one place.

@dmckee if Ra alpha decays to Rn, and ends up in an excited state, will the alpha have a lower KE as a result?

Trying a ab inito search brings up webelements.com which has some data as you drill down and ptable.com which has lot of general chemistry data and some decay dat all with a grotty interactive interface. Blech!

lower KE as compared to Rn not ending up in an excited state

@0celo7 Yes.

How else could it be?

11:49 PM

I dunno, I'm not a physicist :/

@dmckee So, should i just add the energy of the exited state to the Rn's mass?

treat it as a heavier particle?

@0celo7 'Scuses!

@0celo7 Depends how long that excited state is going to last and why you care. Some decay so quickly that you can simply treat it as a double emission of an alpha and a gamma.

@dmckee I have to find the KE of the alpha directly at emission.

Yeah, treat the excited state as heavier.

We have the equation $E_\alpha=Q_\alpha(\frac{m_D}{m_D+m_\alpha})$, where $m_D$ is the daughter

so I should take $m_D=m_{Rn}+0.241 MeV/c^2$?

Yep.

11:54 PM

@dmckee Should I also account for that when I calculate $Q_\alpha$?

I don't know why they call $\ {\partial }$ , "rond" here! I used to say rond f to rond x for $\frac{\partial f}{\partial x}$ I don't know where this word "rond" comes from.. anybody ever heard of it?

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