The h Bar

0celo7

23:00

@DanielSank why

DanielSank

@FenderLesPaul What does high energy theory have to do with quantum info?

0celo7

@DanielSank wow no imagination

DanielSank

@0celo7 I'm sure you can figure that out yourself.

0celo7

@DanielSank lol

you people have waaaaaayyyyy too much confidence in me

DanielSank

@0celo7 Lol yourself. We just went through the fact that the wave was moving at velocity $\omega/k$ and you said you understood that.

I'm sick of this.

0celo7

23:01

@DanielSank exactly

DanielSank

Bye.

0celo7

so if $k$ is less than $0$, $\omega/k$ is also

ACuriousMind

Draw pictures. If you aren't seeing those waves moving in your head, plot them for different times to see what is happening.

FenderLesPaul

@DanielSank well idk about at ucsb but at Stanford for example there's a lot of communication between the QI people and hep-th people because current research interests in quantum gravity incorporate QI rather fundamentally

0celo7

but that means a negative velocity, i.e. left

FenderLesPaul

23:02

and I was wondering if people at ucsb also did that

0celo7

@DanielSank I'm just confused, why are you upset?

FenderLesPaul

@DanielSank c.f. perimeterinstitute.ca/conferences/it-qubit-summer-school

ACuriousMind

@0celo7 You can't say you know that something is the velocity of a wave and then say you don't understand why the wave is moving. What does "I know that is the velocity" even mean if you don't know that something is moving?

0celo7

@ACuriousMind I know that it's moving!

I'm confused about the direction.

bolbteppa

Hecht Optics first chapter has a good intuitive discussion of wave motion from roughly first principles

0celo7

23:07

@ACuriousMind @DanielSank I'm not trying to piss you off, I'm genuinely confused.

ACuriousMind

Well, I'm confused about what confuses you

0celo7

@ACuriousMind Ok, why is $f(x)\mapsto f(x+c)$ a left translation of the graph?

That's all I'm confused about.

bolbteppa

Hecht discusses this

Anybody have a hand-wavey understanding of why a closed Bosonic string is of the form $H \approx (a_{-n} a_n + \tilde{a}_{-n} \tilde{a}_n)$ while an open string is of the form $H \approx a_{-n} a_n$?

ACuriousMind

@0celo7 Because the point where the graph has the value $f(x_0)$ is now $x_0 -c$, and $x\mapsto x-c$ is a left translation.

0celo7

@ACuriousMind I know that.

DanielSank

23:12

@FenderLesPaul I think there may be an enormous gap between quantum information in the case of cosmology etc. and quantum information as relevant to e.g. quantum computers.

ACuriousMind

@bolbteppa In the closed string, the left- and right-moving modes are independent, while in the open string, the boundary conditions force them to combine into a standing wave, meaning the left- and right-moving modes are related to each other, so you only need to care about one of them.

DanielSank

FenderLesPaul I mean a cultural gap.

ACuriousMind

$a$ and $\tilde{a}$ are the left-/right-movers, right?

FenderLesPaul

@DanielSank I see

ACuriousMind

It's been a while since I looked at that

0celo7

23:14

@ACuriousMind You're right.

ACuriousMind

@0celo7 Then what are you asking?

0celo7

@ACuriousMind I'm just having a crisis.

bolbteppa

Oh wow, cool, thanks a lot

0celo7

:(

@ACuriousMind Do you never get it where you understand all of the equations, can derive and explain everything but don't get it

That's where I'm at

DanielSank

Ok, yes that's an uncomfortable position to be in.

0celo7

23:17

It's the same feeling I get when I disprove something via counterexample.

DanielSank

Look, suppose I have some lumpy shape $f(x)$.

Now I push it to the right a bit by a distance $\delta x$.

I can write that as $f(x-\delta x)$, right?

0celo7

@DanielSank Yes, but I don't get why

DanielSank

The reason this works is that I now have to go a little further in $x$ to recover the same argument in the function.

@0celo7 We're working on that.

0celo7

@DanielSank I understand that

DanielSank

@0celo7 If you get that, then why do you say you do not get why?

bolbteppa

23:19

Pretty sure Hecht explains this with a picture so you'll get it immediately

0celo7

4 mins ago, by 0celo7

@ACuriousMind I'm just having a crisis.

DanielSank

$g(x) \equiv f(x - \delta x)$ is a new function with the same shape of $f$ but it's moved a bit to the right.

@0celo7 That's not a relevant statement.

"I'M CONFUSED" doesn't help.

0celo7

@DanielSank I know.

DanielSank

Anyway... I move my lump to the right a bit... I want to express that in math so I write $f(x-\delta x)$.

I'm not sure I can help you understand that. This may require you to just suffer for a while in the confines of your own brain.

0celo7

I'm pretty sure I understood it when I learned quantum mechanics.

Danu

23:22

Hullo

DanielSank

Leave @Danu, nothing good happening here.

0celo7

@DanielSank ...

You're such a nice guy

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

38 mins ago, by ACuriousMind

@0celo7 Plug the solutions for $t_0$ resp. $x_0$ for the two respective equations Daniel derived into the $f_1 + f_2$ and stare at that until you get it.

Danu

@DanielSank Eh??

What, ocelot homework hour? :)

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

stare at it @0celo7

DanielSank

23:23

@Danu I, uh... how'd you know that?

0celo7

@Danu My homework ended long ago, I'm trying to understand something else

Danu

@DanielSank It's 23 hours per day ;)

0celo7

@Danu Actually, it's me being stupid h---yeah homework hour.

@Sᴋᴜʟʟᴘᴇᴛʀᴏʟ I don't know what was meant by that.

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

51 mins ago, by DanielSank

Read the log.

0celo7

What I gathered was: there are points in space where the wave is zero at all times and there are points in time where the wave is zero for all points of space.

But I'm pretty sure that's just a standing wave.

ACuriousMind

23:28

@Danu "ocelot homework" sounds like were zoologists

0celo7

So I don't know what this business about two standing waves is.

DanielSank

@0celo7 Forget the notion of "two standing waves". That was a misstep on our part.

0celo7

@DanielSank You're...shitting me, right

I've been sitting here for an hour trying to find the standing waves

DanielSank

The point is more that a standing wave has two kinds of nodes: one which is zero at all space for certain times, and one which is zero for all time at certain positions.

@0celo7 No, I said that above already.

ACuriousMind

@DanielSank Nah, I still think that works. One is drawing amplitude against space and varying it with time, the other is drawing amplitude against time and varying that with space

DanielSank

23:31

@ACuriousMind Oh yeah ok good point.

I mean, to me that's one standing wave viewed two different ways.

0celo7

^

ACuriousMind

If I could project GIFs out of the pictures in my head, I could express that better

DanielSank

It sounds like a misrepresentation to say there is a "space standing wave" and a "time standing wave".

0celo7

@ACuriousMind How the heck can you picture this in your head

DanielSank

@ACuriousMind I get it.

@0celo7 It's just two independent variables. Your brain is highly evolved to plot in 3D.

0celo7

23:33

@DanielSank mine apparently isn't

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

@0celo7 because you refuse to sketch it

0celo7

@Sᴋᴜʟʟᴘᴇᴛʀᴏʟ sketch what '-.-

ACuriousMind

@DanielSank Ooooh

DanielSank

@ACuriousMind ?

0celo7

should I try to make a 3D plot of $f_1+f_2$?

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

23:37

Start with 2D

ACuriousMind

@DanielSank I just realized that if you plot the amplitude over the $x$-$t$ plane, the nodes are zero lines parallel to the coordinate axes.

0celo7

I don't know what to plot in 2D

DanielSank

@ACuriousMind Yep.

ACuriousMind

Yeah, now I see why you say "two standing waves" isn't the correct statement.

DanielSank

@ACuriousMind Yeah.

0celo7

23:38

@ACuriousMind ah

DanielSank

The problem is that when we say "standing wave" our brain puts up a gif plotting the wave as a function of position and varying it in time.

There is literally nothing interesting here. We have just come around to realize that our brains are used to plotting a certain slice of a 2D plot.

Actually, I still don't understand why those two solutions to the sine equation gave the space and time nodes...

I guess that is interesting.

0celo7

why would you star that

@DanielSank huh?

you solved a trig equation

ACuriousMind

@DanielSank Hmmm? The equation is searching for the zeros - and the zeros always lie on one of the zero lines, which become nodes when you project on either x or t

DanielSank

@ACuriousMind But why did the $y = x + 2n\pi$ and $y = (2n+1)\pi - x$ solutions separate the lines like that?

I salute you, oh King of removed posts!

4

0celo7

@DanielSank wait, are those supposed to solve $\sin x=\sin y$?

ACuriousMind

23:45

@DanielSank I think because the x-t coordinate system is set up like that (parallel to the zero lines). If you rotate the coordinate system, it doesn't do that anymore.

0celo7

@DanielSank I would salute back you but you have no accomplishments of that magnitude

@DanielSank Also, Black Sheep.

ACuriousMind

Parvitude might be more appropriate :P

0celo7

I don't know that word.

Wow.

ACuriousMind

magnus - big; parvus - small

0celo7

@ACuriousMind Lol you think I remember Latin.

ACuriousMind

23:48

@0celo7 Actually, no, I fully expected you to not understand that :P

0celo7

@ACuriousMind Don't know whether I think you're an ass or happy you finally understand how bad my memory is.

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

Do you want to be put on ignore again for a "week?"

0celo7

@Sᴋᴜʟʟᴘᴇᴛʀᴏʟ No.

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

~~ass~~

0celo7

Huh?

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

23:52

Strike out "ass."

0celo7

If posting Latin adjectives while knowing that the other person will not know the words is not being an ass...don't know where that thought was going

ACuriousMind

@Sᴋᴜʟʟᴘᴇᴛʀᴏʟ I don't care about that.

0celo7

@Sᴋᴜʟʟᴘᴇᴛʀᴏʟ It's also too late.

ACuriousMind

@0celo7 Parvitude is not an adjective ;)

(and while parvus is, I even wrote there what it means)

0celo7

@ACuriousMind Hmm, are "magnus" and "parvus" not adjectives?

ACuriousMind

23:55

magnus and parvus are, but I really wrote that to explain how I came up with parvitude, not to shame you for not knowing them

0celo7

@ACuriousMind There are better things to shame me about.

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

Like your refusal to draw sketchs :P

0celo7

refusal?

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

For this kind of stuff

0celo7

No, I just had no clue what to draw

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

23:58

You gotta start drawing right away pal

0celo7

I'm stuck proving Schur's Lemma, what do I draw there

Danu

arrows

ACuriousMind

I recently met a student of Latin and Ancient Greek at a party. That almost made me wish I could do that without cutting back on physics and math.

0celo7

inb4 ACM tells me to use rank-nullity or some crap

@ACuriousMind That might be even more useless than theoretical physics in the market.

:P

Transcript for