The h Bar

no_choice99

00:04

sup guys

0celo7

The prof should just post all homework for the chapter so I know what problems to do on loose leaf and what to do in my notes...

@dmckee Would it be reasonable for me to email the prof this request?

dmckee

@0celo7 To ask? Sure. To actually get your wish? Well, that depends.

This is the first semester that I'm trying the "post all homework at the beginning" strategy (and only in one class). Up until now I haven't known what I was going to assign until I got almost to that point.

0celo7

Oh, the prof did mention this is his first time teaching this class :(

dmckee

Also, I know some folks don't like the student to start on something until it comes up in class. Not sure why.

@0celo7 He might still have the first N weeks ready.

I'm busy with that for my other classes right now.

0celo7

00:20

@dmckee have fun

meanwhile I keep getting 0=0 or similar :(

ACuriousMind

@0celo7 Well, that's not wrong :)

0celo7

I'm terrible at math D:

the absolute value function is evil

Bernard Meurer

@0celo7 0=0 is my last answer to most difficult questions

0celo7

great I have a chain $a\le b\ge c$

and I need $a\ge c$

waaah

Bernard Meurer

Anyone here up for a fun inequality to solve?

0celo7

00:25

sigh

it was trivial

ACuriousMind

"fun inequality" is an oxymoron :P

Bernard Meurer

Hahahaha

@ACuriousMind I promise you it's a fun one

Only took me a weekend to solve

0celo7

noooooo

there's a second asbolute value in the problem

0537

I'm so sad I have to re-learn quantum mechanics.

Bernard Meurer

@0537 I have to learn it for the first time

0537

00:31

looking through the book and realizing I forgot like 70% of it...

@BernardMeurer really? want to learn it together?

0celo7

I want a strong German man to sweep me off my feet and do these absolute value problems (no homo)

ACuriousMind

@BernardMeurer Okay. Show me your notion of "fun"

0celo7

I wonder where we could find such a person

Bernard Meurer

@0537 Sounds awesome to me, I usually do these things dully by myself

@ACuriousMind let me get my notes

Game:

0537

@BernardMeurer what book?

do you know linear algebra?

0celo7

00:33

@ACuriousMind is it sometimes necessary when proving inequalities involving two numbers to have to go through the 4 scenarios

like if $a,b>0$, $a>0, b<0$, etc.

ACuriousMind

Yeah, we call that Fallunterscheidung

0celo7

is it sometimes necessary to do that AFTER one has used the triangle identity

ACuriousMind

"Sometimes"...sure. But in my experience most things don't actually need the proof-by-cases construction if one is clever

Bernard Meurer

@ACuriousMind Game:
for $abc = 1; a, b, c > 0$ prove that $\sqrt{2}(a+b+c) \geq \sqrt{1+a^2}+\sqrt{1+b^2}+\sqrt{1+c^2}$
Rules: No googling

ACuriousMind

Often there's the "brute force" proof where you just go through all the cases, and the "clever" proof taht somehow sidestep the necessity for that

0celo7

00:36

@ACuriousMind Basically, I have shown $|a|-|b|\le|a-b|$. which is trivial, but the problem wants $||a|-|b||\le|a-b|$. I don't want you to do it for me, just let me know if one has to unterscheid the Falls.

Bernard Meurer

@0537 I had to learn some linear algebra for school but I have not used it in forever

ACuriousMind

@0celo7 No, you don't have to do that

0537

@BernardMeurer so what book are we using?

keep in mind I plan to finish in 15 days from now.

0celo7

lel

Bernard Meurer

@0537 Your pick, I wouldn't know better than you I'm afraid

0celo7

00:40

use Hall

ACuriousMind

@BernardMeurer I don't see any fun

Bernard Meurer

Maybe we should ask @ACuriousMind and @0celo7 for some tips?

0537

I already did.

long time ago.

0celo7

@ACuriousMind no finite group of order one there indeed

Bernard Meurer

@ACuriousMind Already solved?

0537

00:41

@BernardMeurer ACuriousMind doesn't read books (before), and 0celo7 recommended shankar.

I read shankar in like <15 days.

0celo7

I changed my mind to Hall

0537

I forgot all of it though.

0celo7

why would you remember it this time?

ACuriousMind

@BernardMeurer No, I just don't derive pleasure from inequalities ;P

0537

@0celo7 because it's a refresher.

Bernard Meurer

00:43

@ACuriousMind Hahahaha, it's a fun one to solve that one, had me thinking for a while

@0537 Would you rather go with @0celo7's new recommendation or stick to your previous read?

0537

@BernardMeurer shankar is boring as f.

I found some better books.

Bernard Meurer

@0celo7 this one?

0celo7

Yes

It's OK for German middle school level

Bernard Meurer

@0537 Being my first take at it I'm up for anything, I'm not sure I'd be able to discern between options too well

FenderLesPaul

your first tip would be

to not try to learn QM in 15 days

0celo7

00:46

he won't listen to that, don't bother

FenderLesPaul

staring at a concrete wall for 5 days will get you better results

Bernard Meurer

@FenderLesPaul I've tried this concrete wall technique, didn't yield much

0537

@FenderLesPaul I learned it once already though.

and it worked.

I mean I was able to follow qft stuff.

@BernardMeurer I think you should read griffiths qm.

I read it once for my first book.

FenderLesPaul

@0537 sorry but I refuse to believe any deep learning was done in a couple of weeks

Bernard Meurer

this one?

Holy crap these things are expensive

FenderLesPaul

00:53

basic understanding is not the same as learning

Bernard Meurer

Sigh, I don't have money for these books

user54412

@privetDruzia Actually, I'd say at least 95% of astrophysics will not be directly applicable to aerospace engineering. After all, we study the entire universe, and most of it looks nothing like near-earth orbit. That said, astronomers put up a lot of telescopes into orbit, so the instrumentalists who design them would know a thing or two about your interests. But most people studying astro are not instrumentalists; they are observers and theorists and simulators and data reducers.

0537

@BernardMeurer look for a pdf online.

FenderLesPaul

I mean you are free to do what you want but I think @BernardMeurer should take his/her time

in learning QM

QM takes a long time to learn properly

0celo7

@ACuriousMind is there a trick or is it really obvious

ACuriousMind

00:55

@0celo7 There's a really obvious trick

0celo7

fml

is it $a=a-b+b$

user54412

@privetDruzia As for getting into a PhD program, I'd say studying astro-related stuff isn't the most direct way. At your age you should study what's interesting to you. Also, publications are looked upon very favorably by admissions committees, but in most fields they are not expected: most PhD students enter without any publications. That is to say, don't worry so much if you don't have any.

Bernard Meurer

@0celo7 so 1=1 ?

0celo7

@BernardMeurer what

Bernard Meurer

@0celo7 $a= a-b+b$ implies $a=a$ correct?

0celo7

00:58

it also implies $0=0$

Bernard Meurer

@0celo7 Yep, but I like 1's better than 0's

0celo7

this is the hardest problem I've ever done

user54412

@privetDruzia Also, I think every grad student goes through a period of doubt as to whether they're doing the right thing. I'm personally pretty happy with how it turned out, but that's something you'll have to figure out on your own. I will say that there are certainly unhappy people in grad school (in certain fields more than others - be sure to ask how happy students are in any school you're considering).

0celo7

what certain fields are those

user54412

biology for one

user54412

01:01

there's a preponderance of slave-master lab PIs in that field

knzhou

how about within physics?

Mike Miller

i've heard things about astro

user54412

Astrophysics seems relatively happy, probably because it's easier than the rest of physics.

0celo7

holy crap what is the trick???

Bernard Meurer

Where do you guys buy used books? PDFs hurt my eyes

user54412

01:03

@MikeMiller oh?

Bernard Meurer

@0celo7 The hell's going on

0celo7

Amazon

@BernardMeurer me having 80 IQ

Bernard Meurer

@0celo7 What problem is melting you?

0celo7

it's above

Mike Miller

@ChrisWhite astrobetter.com/blog/2012/10/10/not-what-we-want

my impression is that the opinions in that letter are not uncommon for supervisors. in fact, change tht to 'are common'

Bernard Meurer

01:05

$a\leq b \geq c$ proof of $a \geq c$?

0celo7

All I'm able to show is $||a|-|b||\ge |a|-|b|\le|a-b|$

I need $||a|-|b||\le|a-b|$

user54412

@MikeMiller Ah yes, that. That school got a lot of flak in the community for sending that email.

Mike Miller

@ChrisWhite I'm aware of the response.

user54412

Certainly I end up working 80+ hour weeks. Though my adviser has never told me I work too little. He's even suggested I take a vacation now and then.

Bernard Meurer

@0celo7 Papa bless. Have you tried those who know?

0celo7

01:09

@BernardMeurer I'm not making a question about this

Bernard Meurer

@0celo7 </3

0celo7

oh my god now there's like 5 inequalities

I keep proving unrelated crap

FenderLesPaul

I swear my upstairs neighbor walks around in steel clad shoes

Bernard Meurer

I feel your pain @0celo7. I usually start with an inequality and end up proving Schopenhauer was right

0celo7

Do you see the trick?

no_choice99

01:16

did Lev Landau knew about QFT?

FenderLesPaul

@GBeau dude

are you ready for next week?

@no_choice99 yes

he was instrumental in its development

0celo7

Now I have $$||a|-|b||\le |a|+|b|\ge |a-b|\ge|a|-|b|\le||a|-|b||$$

D:

no_choice99

you mean it was very new when Landau was still alive?

new*

FenderLesPaul

no QFT was not new in Landau's time

Bernard Meurer

@0celo7 You're creating a monster

no_choice99

01:18

ah ok

FenderLesPaul

it wasn't as well understood as it is today

but it wasn't new

no_choice99

I see

FenderLesPaul

Dirac formulated QED back in the 30s

no_choice99

i see QFT questions all over hte place, I'd like to learn about it

0celo7

                                   @ACuriousMind am I at least on the right track?

whoa what happened there

Bernard Meurer

01:18

The hell?

no_choice99

what's QED has to do with QFT

0celo7

@ACuriousMind did that still ping you?

FenderLesPaul

QED is QFT of photons

@0celo7 leave the poor man alone

he needs his alone time

no_choice99

oh kind of a sub field or sub topic of QFT?

0celo7

@no_choice99 a specific model of QFT

it is a QFT

no_choice99

01:19

is it common to first learn QED before aft

qft

0celo7

no

no_choice99

ah ok

0celo7

you learn the basic structure of QFT and then apply it to photons + electrons

no_choice99

ok, that sounds interesting

FenderLesPaul

futons

0celo7

01:20

@FenderLesPaul ok then help me out

ACuriousMind

@0celo7 Yes

FenderLesPaul

@0celo7 yes you are on the right track

0celo7

ffs now I have a contradiction

:(

FenderLesPaul

you are a contradiction

an engineer who does pure math for fun

0celo7

this is not fucking fun

FenderLesPaul

01:23

do you even do engineering stuff

0celo7

no

does making a noose count

doing that right now

no_choice99

maths is hard

FenderLesPaul

I guess it's mechanical engineering

no_choice99

abstract maths is torture

0celo7

oh no not a contradiction...but nothing helpful either

welp

Proof: Impossible

FenderLesPaul

01:27

you know what's torture?

life

life is torture

0celo7

this homeowork

FenderLesPaul

you have HW?

when do your classes start?

0celo7

doing it right now

last week

FenderLesPaul

oh shieeet

I had no idea your classes started

0celo7

why else would I be doing this

FenderLesPaul

01:29

boredom

0celo7

I am pretty bored right now doing it...

FenderLesPaul

backreaction

0celo7

@ACuriousMind holy crap what do I do??

I can't do PhD analysis

FenderLesPaul

dude it's easy

if Ph > 7 it's a base

or something

or is it 6

ACuriousMind

Ph neutrality is at 7

0celo7

01:36

:(

you respond to that

ACuriousMind

@0celo7 You said you didn't want me to give you the solution

0celo7

also @ACuriousMind is also a PhD chemist

is there anything he can't do??

@ACuriousMind What's the idea of how to continue?

Is there any clever substitution I need?

FenderLesPaul

@ACuriousMind word

ACuriousMind

@0celo7 Observe a certain...symmetry in $\lvert a\rvert - \lvert b\rvert \leq \lvert a-b\rvert$.

0celo7

@ACuriousMind huh? $|b|-|a|\le|a-b|$ or something?

no_choice99

01:41

there's a question for you guys: physics.stackexchange.com/questions/230160/… that just poped up although the title is strange

ACuriousMind

@0celo7 Yes.

0celo7

@ACuriousMind Well that's trivial.

ACuriousMind

@0celo7 I said the trick was obvious :P

0celo7

aye, but it does not help me

o

I have to underscheid the falls now

depending on whether $|a|-|b|$ is positive or negative

and then applying the definition of abs...yeah, that's stupid

for me to have not seen that...I probably should not be allowed to walk around unsupervised

no_choice99

i have a question that arose from the link i posted above. in EM given an observer and the EM fields he sees , he can deduce the charges and currents (generally althought not 100% of the time).

0celo7

01:46

@ACuriousMind uh, thanks

no_choice99

i'm wondering whether the same holds for shape of objects. can the observer deduce what's making the star not spherical, given the shape he sees on a picture of a star?

like "oh it's not spherical , so this means I'm moving with respect to it" or is there any other explanation to the shape of the star

0celo7

still no response from John Lee

@FenderLesPaul you're a hipster, you probably have friends in Seattle

I need them to find John Lee...

no_choice99

nvm my question doesn't make much sense

Fender are you a hipster?

ACuriousMind

@BernardMeurer Proved it.

FenderLesPaul

@no_choice99 I think I am

no_choice99

02:02

ok i didnt know this :D

Bernard Meurer

@ACuriousMind Cool, would you mind letting me see how you proved it? I made use of $\sqrt{2}x - \sqrt{x^2+1} \geq \frac{\sqrt{2}}{2} lnx$

Which you can prove by derivatives

Physics Meta

0

Q: What is an unupvote and why does it cost more rep points than a downvote?

My first downvote was reported as an unupvote in my rep score. It cost 10 points. I thought that was normal until I received another downvote and discovered that it cost only 2 rep points. What is the difference between a downvote and an unupvote? I Googled it and got nothing.

discussion

ACuriousMind

First, one shows that the function f(a,b,c) = a+b+c has a minimum with respect to the constraint $abc=1$ at $f(1,1,1) = 3$.
This means that $3\leq a+b+c$ for our problem. Now, we observe $$ (\sqrt{1+a^2}+\sqrt{1+b^2}+\sqrt{1+c^2})^2 \leq 1+a^2+1+b^2+1+c^2 = 3+a^2+b^2+c^2$$ using $(x+y)^2\leq x^2+y^2$ twice. Since $abc=1$ implies $a^2b^2c^2=1$, we have $3+a^2+b^2+c^2\leq 2(a^2+b^2+c^2)$. Now take the square root on both sides and obtian the desired inequality after using $\sqrt{x+y}\leq \sqrt{x}+\sqrt{y}$ twice.

The thing with the minimum you do by Lagrange multipliers

Bernard Meurer

@ACuriousMind Very badass method

ACuriousMind

Oops

It doesn't work, $(x+y)^2 \leq x^2+y^2$ isn't true

It's $\geq$. That always happens to me with inequalities, which is why they're no fun :P

Bernard Meurer

02:17

That solution did look too elegant to be true

ACuriousMind

I'm pretty sure though that the $3\leq a+b+c$ should make for a nice solution

0celo7

02:47

How do you prove the square root thing

ACuriousMind

@0celo7 $\sqrt{x+y}\leq\sqrt{x}+\sqrt{y}$?

DanielSank

03:13

Is there a way to search the chat history?

no_choice99

i think so

there's a button at the top "Full transcript"

I guess it gives the full text of the chat

Bernard Meurer

@DanielSank it being open can't you just use google magic?

FenderLesPaul

gewgle

I don't know if I should be happy or disappointed that there's a paper which already does a calculation I need to do

no_choice99

happy...

a step less toward the ultimate goal

0celo7

@ACuriousMind yah

@DanielSank does the search bar not work for you?

make meta post

FenderLesPaul

03:26

@no_choice99 but this means I have less crap to cram into my paper

no_choice99

hmm

doesn't matter much IMO if your conclusion is the same and new

FenderLesPaul

@no_choice99 I suppose

I spose'

Isose

I had too much coffee

no_choice99

lool

DanielSank