The h Bar

Slereah

12:00 AM

proofwiki.org/wiki/…

@IceLord : It's because what you are really calculating is $\partial_t \varphi(x, t) |_{t = 0}$

Obliv

oh my god @slereah is that really necessary to prove

I thought it was just a tautology

user218912

@Slereah oh.

user218912

makes sense, thank you.

0celo7

@Slereah yah so

Slereah

@Obliv will never be a mathematician

0celo7

12:02 AM

then this homework problem is trivial

I have to show that a $\sigma$-compact metric space is separable.

user218912

1 min ago, by Slereah

@Obliv will never be a mathematician

user218912

@Obliv high-five, physicists ftw.

Obliv

@Icelord high-five for being sane

user218912

@Obliv do you play any online games by any chance?

user218912

because I saw someone named Obliv, wondering if that was you.

Obliv

12:05 AM

in what? @iceL

@IceLord not as much anymore but when I do it's usually QL or osu just to unwind. how about you @iceL

user218912

@Obliv It's too embarrassing to reveal.

user218912

and no it's not club penguin or neo-pets.

Obliv

I doubt we crossed paths maybe this name is more popular than I thought.

and my name on steam is currently a gordon ramsay reference lol.

Slereah

there really should be a book on non-standard analysis written by a physicist

It's way too mathy

0celo7

Any $\sigma$-compact metric space is separable.

*Proof.*
Let $(X,d)$ be sigma compact. We showed in class that any compact metric space is separable, that is has a countable dense subset. Let $\{K_i\}_{i\ge 1}$ be a countable union of compact sets whose union is $X$. Each $K_i$ has a countable dense set $D_i$ with respect to the subspace topology. Let $D=\bigcup_{i\ge 1}D_i$. This is countable since a countable union of countable sets is countable. Let $U\subset X$ be open. Then $U$ intersects at least one $K_i$, and $U\cap K_i$ is open in the subspace topology. Thus $D_i$ intersects $U$ s…

Seems legit

@Obliv f^*&ing c#$!sucker?

Slereah

12:13 AM

what is even a non-sigma compact space

The long line?

0celo7

Yes

in fact

a Hausdorff locally Euclidean space is a topological manifold iff it is sigma compact

Obliv

@0celo7 try "WHERE IS THE LAMB SAAAAUUUUUUUUCEEEEEE?" or something like that

0celo7

so pick your favorite non-manifold-almost-manifold

long line works

@Slereah Aha

Slereah

I guess the complete feather is also not sigma compact

0celo7

uncountable union of lines

that's not second countable.

thus not a manifol

thus not sigma compact

Slereah

12:15 AM

is it third countable

Obliv

@0celo7 Okay, calm down. Why don't you just prove that the finite set you just made by combining the countable sets is countable in the same way you prove a normal set is countable? Like, by proving there is a bijection or something right?

0celo7

@Obliv Are you trolling me?

I can't tell

Obliv

@0celo7 I guess I don't have context maybe it's more difficult than you make it out to be.

oh wait no I misread. That's a proof to whatever you were doing in class not the proof of the countable union of countable sets is countable.

0celo7

what the hell are you talking about

Obliv

I don't know. What are YOU talking about?

user218912

12:21 AM

@Obliv set a profile image you look boring.

Obliv

@iceL I kind of like this yellow. Fine, I'll do it when I get home. I have some good images on my desktop.

hey if I'm generalizing a sine wave to multiple parameters such as $r(x,y,z,t) = r_{max}\sin()$ would it look like this: $r(x,y,z,t) = r_{max}\sin(kx+ly + mz + nt)$ where $k = \frac{2\pi}{\lambda}$, $l = \frac{2\pi}{\gamma}$ and so on

0celo7

ugh what is this?

calc 1 homework?

Obliv

I'M JUST CURIOUS GOD @0CELO7

studying for my physics exam tomorrow on waves & thermodynamics

0celo7

ew physics

you need to explain what "generalize a sine wave" means

Obliv

12:37 AM

your attempts at trolling me will not work @0celo7

Hmm to pull off an all nighter or to not pull off an all nighter.. that is the question

Hmm why isn't the kinetic energy in a transverse wave $\frac{\partial y}{\partial x}$ instead of $\frac{\partial y}{\partial t}$ for $y(x,t) = y_m\sin(kx - \omega t + \phi)$

(multiplied by 1/2 and m then squared of course)

0celo7

@Obliv sigh,

you don't get excited enough any more to be worth trolling

not that I would troll ofc

Obliv

Of course not. That post by david z on the sidebar has nothing to do with your flag on my innocent remark.

0celo7

@Obliv Uh, it has to do with JD.

(as do most things in the chat)

@dmckee Can we reduce the number of pinned messages?

I think the fourth has run its course.

Also the second.

Obliv

well I'm going to be up pretty late tonight and need someone to ask random stuff to so I think you're morally obligated to stay up tonight too @0celo7

after what you did to me

0celo7

I've got a nuclear physics exam tomorrow

Obliv

12:51 AM

yeah I know about nuclear physics just ask me stuff

It'll be mutually beneficial I swear (on your grave).

a uranium atom decays into 182 hydrogen atoms. See? I know my stuff.

0celo7

proof?

Obliv

1+1+1+1+....+1 = 182.

oh i was a little off uranium has 92 protons

close though

0celo7

you're forgetting the neutrons

Obliv

it's not like hydrogen is made of neutrons @0celo7

(is it? just testing you)

0celo7

I don't know actually

not a physicist

DanielSank

1:00 AM

@Danu Very basic question

0celo7

@DanielSank How come I don't get those pings any more?

DanielSank

@0celo7 Very basic question

Don't know what you're talking about.

0celo7

Danke

@DanielSank We used to ping each other with bad question titles...

DanielSank

@0celo7 Bitte, baby.

@0celo7 You can't expect me not to have other relationships.

user218912

@Obliv where do you live?

DanielSank

1:23 AM

Someone quick, ask me a question about quantum mechanics or noise.

::crickets::

0celo7

@DanielSank I have a joke

DanielSank

Listening.

@0celo7 Dude?

0celo7

1:43 AM

sorry, talking to someone

it's more of a funny story

so I'm in the music library (quieter, less people, not 60 degrees)

and there's this chinese grad student

the library person picks up a half-empty Dr. Pepper (the drink) and asks "whose Dr. Pepper is this"

and the grad student says "who?"

DanielSank

@0celo7 Is 60 degrees hot or cold?

0celo7

shrinkage

60F

DanielSank

@0celo7 So, too cold?

0celo7

yes.

DanielSank

ok

0celo7

1:46 AM

anyway, I thought that was funny

DanielSank

@0celo7 I don't get it

0celo7

"who?"

you don't get it?

DanielSank

@0celo7 Nope.

0celo7

oh well

I gave it a shot

user218912

@0celo7 I don't get it either.

user218912

1:56 AM

do you mean he thought the drink was referring to a real dr.?

DanielSank

The student though Dr Pepper is a person?

@IceLord There's no . in Dr Pepper.

@0celo7 There's no . in Dr Pepper.

user218912

kk

Obliv

@IceLord U.S. far away from @0celo7

Lol I got it. Did you laugh at him out-loud obnoxiously @0celo7

Sir Cumference

Who'd you guys vote for?

0celo7

2:14 AM

@DanielSank oh

I don't drink it

I drink only water, nothing else

@0celo7 You voting?

for?

moderators

yes I voted

for who?

user218912

2:15 AM

ACM duh

user218912

ACM is the obvious choice.

Sir Cumference

^

0celo7

Dave Chappelle Voting

►

Sir Cumference

He's my first choice. Then Alfred. Then Jim.

You Icelord?

user218912

can't vote.

Sir Cumference

2:15 AM

Oh...

0celo7

watch this

Sir Cumference

:(

user218912

but it would be ACM and idc about the others.

Obliv

@0celo7 is it funny

no screw it I have to study

Sir Cumference

@IceLord Ya know once you reach 200 rep on a site, then you immediately get 100 rep on every site you go to?

0celo7

2:16 AM

@Obliv it is very funny

user218912

yes I did know.

Sir Cumference

Ya oughta try to make it there

user218912

in december when I'm done learning QFT.

user218912

I will answer questions

user218912

for now I'm only asking them.

0celo7

2:17 AM

speaking of, where is my Lee?

Obliv

Okay I decided to watch it. @0celo7 fucking hilarious

0celo7

@Obliv exactly

so @SirCumference I will not answer your question

user218912

@0celo7 you want your lee? then help me out

user218912

we're at 7/15 I think.

Sir Cumference

@0celo7 Sigh...ya needed that video to explain it

Fair enough

But this isn't politics.

It's stackexchange.

Obliv

2:19 AM

are you trying to water down the importance of a moderator on an online q&a site @sirC

!?!?!?!

Sir Cumference

@Obliv yes

Funny enough the fate of this site isn't as important to me as whether Trump or Hillary get elected

0celo7

ok @IceLord I will spoon feed you

user218912

@0celo7 ok 8 spoon feeds.

0celo7

give me the spoons

what am I feeding

Sir Cumference

Ah, I think I've invented the ultimate physics joke.

Obliv

2:22 AM

@sirC I'd still prefer the anonymity of who I vote for just until the voting period ends. That way ACM might answer more of my questions.

user218912

@0celo7 spoon 1

Sir Cumference

Why did the particle keep losing weight?

user218912

@0celo7 what happened there.

Sir Cumference

@Obliv You can always retract them.

0celo7

because you're bulimic

user218912

2:23 AM

I understand the $\langle p | x \rangle$

user218912

where did the other things go

Sir Cumference

@0celo7 the particle

user218912

delta function?

0celo7

I don't, where did it go?

user218912

wait what

0celo7

2:23 AM

what's your equation for $\langle p|x\rangle$

I need context.

Sir Cumference

All right, but seriously

Best physics joke ever

Why did the particle keep losing weight?

user218912

it's $e^{-ip\cdot x}$

user218912

is this a typo or am I being dumb.

Obliv

@sirC why?

Sir Cumference

@Obliv He wanted to be light.

user218912

2:24 AM

I remember I understood it during the lecture. @0celo7

Sir Cumference

Get it? Light has no mass.

0celo7

wait no

Obliv

yep as I thought. I will never entertain jokes from @SirC again.

0celo7

oh I'm stupid

Sir Cumference

@Obliv good idea

0celo7

2:25 AM

it's $\langle p|k\rangle$

which is $\delta(p-k)$

user218912

yep delta function.

user218912

knew it.

0celo7

ok but where does the $\omega$ go

user218912

I said it wrong.

Sir Cumference

All right, what did one electron say to the other?

Obliv

2:26 AM

that actually makes me wonder where the word 'light' came from. Whether it was used as a description for the weight of something before the physics term @sirC

user218912

@0celo7 uhm...

user218912

you mean the $\frac{1}{2\omega}$

DanielSank

@0celo7 Neither do I.

0celo7

@Obliv in German the words for light and light are similar, but different.

Sir Cumference

@Obliv They derived from different languages

user218912

2:27 AM

@0celo7 I don't know where the extra $\omega$ term disappeared to.

user218912

spoon feed me please.

DanielSank

I learned it from reddit

0celo7

I don't know

Sir Cumference

The photon-light was derived from the word "white" from some language, and weight-light was derived from the word "lung" in some other language

0celo7

I need the context

Sir Cumference

2:27 AM

But yeah, I got the new joke

What did one electron say to the other?

...anyone?

Obliv

Okay, from that explanation I will give you another chance @sirC What did it say?

Sir Cumference

@Obliv Nothing. It just waved.

Eh? Ehhhh?

._.

user218912

@0celo7 this goes in this, that's all the context.

Obliv

yeah I like the light one better

Sir Cumference

:(

0celo7

2:30 AM

What book is this from?

user218912

@0celo7 class notes.

0celo7

Did you check Tong?

Sir Cumference

I made multiple answers here

13

Q: Astronomy Jokes

What are the best astronomy jokes you've ever heard? I'm looking for images; however, you can share whatever you like.

discussion

Probably not something to be proud of

user218912

we're just showing how when the field operator acts on the vacuum state you get linear combination of momentum eigenstates. and then he takes the inner product with the momentum eigenket but idk where the $\omega$ goes now.

user218912

and no I didn't

user218912

2:32 AM

let me check

Obliv

Hold on just a second..a transverse wave has transverse kinetic energy but not horizontal?

O wait maybe it's referring to the kinetic energy in the string (which is displaced vertically) since the wave itself can't have kinetic energy as it has no mass

yeah it's that one.

0celo7

@IceLord Free field?

user218912

@0celo7 yes

0celo7

well now wait a moment

don't you need a Lorentz invariant measure?

user218912

@0celo7 yes

user218912

2:37 AM

so are you saying it's just part of the measure?

0celo7

you should have notes on what the delta function exactly is

@IceLord I think so.

user218912

yes you're right.

user218912

I remember him saying that.

user218912

okay so that's why it disappears.

user218912

@0celo7 what do you mean?

0celo7

2:37 AM

nvm

user218912

I know what it is.

0celo7

no

user218912

I don't?

0celo7

I mean what it is in this case

you need a different measure

but looking at Weinberg that seems to be right

user218912

okay thanks.

0celo7

2:38 AM

let's keep going

#2?

user218912

I'm basically reviewing all the notes to make sure I get everything and I'm writing out all the steps he skips.

user218912

5 pages left till I catch up to where we are.

user218912

so yeah wait 5 mins

user218912

10 pages actually. so maybe a bit longer.

Obliv

10 pages can take a long time if it's dense

0celo7

2:43 AM

He read a 600 page book in like a week once.

user218912

that is true. but I retained about 30% scattered throughout which by itself has allowed me to progress really far.

user218912

@0celo7 line 3

user218912

how does it become the commutator?

user218912

I bet this is probably really obvious.

0celo7

it is

Is $\phi^+(x)|0\rangle =0$?

Or $\langle 0|\phi^-(y)=0$

user218912

2:49 AM

@0celo7 yes ofc.

0celo7

of course?

I have no clue what these symbols mean

user218912

@0celo7 because it has only the annihilation operator in its definition.

user218912

let me show you the definitions

0celo7

but just expand the commutator and then you'll have $\langle 0|\phi^-(y)\phi^+(x)|0\rangle=0$

plus the term you had originally

user218912

definitions

user218912

2:51 AM

oh

user218912

@0celo7 which term I had originally?

0celo7

the one with $\phi^+$ and $\phi^-$ flipped

user218912

oh I misread what you said.

user218912

yes I get it.

user218912

@0celo7 if you were able to remember that I'm 100% sure you have a really good memory and you still remember QFT.

user218912

2:55 AM

but I guess you could have guessed it too based off what the answer was.

user218912

so idk.

0celo7

@IceLord correct

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