The h Bar - 2017-07-12 (page 3 of 3)

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5:00 PM

I'm not sure they involve that

That I am not sure, but I do read a lot of results that said CTCs breaks unitarity, I might have overlooked a spacetime one among the numerous nonrelativistic quantum ones. The Quantum computing guys love to quote that result as they really want those advantage in computing that arises from CTCs

Let me check again along that direction...

unitarity in QFT depends on time ordering

But there's no time ordering possible in such a spacetime

As u well know, there are no time function on any spacetime with closed timelike curves

yeah because we cannot foilate spacetimes into space like hyersurfaces in any spacetimes that contains a CTC, that prevents assigning a unique time stamp for each slice in an ordered fashion

@Slereah That's clearly just the theorem that all the values of $\zeta$ lies on the $\Re s = 1/2$ line

no other riemann's theorem dude

This 2011 arxiv I am reading seemed quite promising and contains as far stuff I know about time travel in the physics literature. There's a QFT treatment but I have not get to that page yet

5:15 PM

If you want QFT on non causal spacetimes read Bernard Kay instead

This one?

for instance yes

@Jim Just wondering, what areas of cosmology do you deal with?

the cosmos

Cosmological cosmology

5:27 PM

@JohnRennie do you want to know about wave equations on manifolds?

the hypotheses are nice

that's a lie

you can have a well defined wave equation on some CTC spacetimes

it's horrible

For some pretty weird cases

Well, the causality won't be the problem, anyway

@JohnRennie The Cauchy problem with $H^1_0\times H^0$ Cauchy data for the wave equation $$\Box u-mu=f,\quad m\ge 0$$ on $(\Bbb R\times \Sigma, g)$ satisfying the Fundamental Regularity Hypothesis and $f\in L^2(T)$, has a global solution $u$ with $u\in E_1(T)$ for any finite $T$.

What is $E_1$

5:43 PM

@Slereah $C^1([0,T],H^1)$

with the $C^1$ sup norm

@Slereah does AdSS have a horizon?

1) I don't know 2) define "horizon"

2) Something that could cause problems

I'm guessing probably, yes

at least for some solutions

For a large enough AdS spacetime it's gonna be roughly like Schwarzschild anyway

Not sure how to define it, though

You need to define null infinity, first

I'm not sure if it's well defined there

ew.

But I'm guessing you probably have a region for which $I^+(p)$ is entirely within that region

6:02 PM

ACuriousMind has added an event to this room's schedule.

@ACuriousMind what are you up to...

Oh, An AMA..

@0celo7 You could just click on that link and find out...

@Sid is there a reason why you always do two periods?

sorry?

6:05 PM

user image

Oh. That's my obsession with them

No real reason

Also Capital letters on odd Places too

I am going to capitalize Nouns

It is the only good Way to spell

Correct!

I have mixed Feelings about this

the german way

6:12 PM

user image

@ACuriousMind Should i put a \frac or a \tfrac there?

@Slereah more like the german Way

I'm not even sure AdS has a null infinity

I think it only has $i^\pm$

Every point on the conformal compactification boundary can be reached by a timelike curve

6:30 PM

@ACuriousMind what's the sheafy term for being able to extend sections off of closed sets?

soft?

@0celo7 yes

Wait

@Slereah No :P

I'm confusing de sitter and anti de sitter

AdS is the hyperbolic one, right?

@Slereah AdS is hyperbolic

6:33 PM

ah yes

That one probably has a null infinity, yes

So defining a horizon shouldn't be too hard

inspirehep.net/record/1510557/plots

Doesn't Carroll have a conformal diagram of AdS

I forget

Yes it does, p. 327

inspirehep.net/record/879076/plots

"Penrose diagram of Kruskal-extended AdS Schwarzschild black hole."

it looks weird

user image

Thick lines are "AdS infinity", whatever that means

I should read that paper

too much to reeeeeead

Procrastinate. It's a real talent. :P

tell me about it

Try to delay today's work to tomorrow and so on

till you are in a tight deadline. Then, complete the work within a single day

6:48 PM

I'm still not 100% sure of how one defines null infinity, really

@Sid I played TW3 way too much

@ACuriousMind why did you make me play it

I'm not even done

it's so long

I'd guess it would just be the intersection of the boundary of spacetime with infinite null curves

but I'm not 100% sure

I once played GTA 3 and GTA VC for about a month

Ben Crowell gave me a paper he said was related, but while a very interesting paper it only dealt with the construction of the boundary

Nothing else on computer. Just those two games

6:49 PM

not so much the classification of points

@JohnRennie Did you get the latest version of Mathematica to work on Win10?

7:08 PM

@0celo7 win 7 is still the best and the most reliable

Apparently null infinity may be the intersection of the terminal indecomposible future and past set with the boundary

@Mostafa that just shows how terrible Windows is

Ah, there's an Abstract Way to define the completion of the manifold and its topology apparently

where you associate points with their proper indecomposible future and past sets

and then the terminal indecomposible future and past sets are the boundaries of the spacetime

$M^\natural$

Never seen that symbol before

fuuuuuuuuuuck

There's like 10 different definitions for the completion of a spacetime

Have mercy

7:29 PM

@0celo7 Use Linux, then. Or Mac..

why can't spacetime be simple

Why can't we live in $\Bbb R$

7:42 PM

@Sid I have a mac

but you can't play Skyrim on a mac

@Slereah we live in $\Bbb R^4$

Too many dimensions

Hmm... many games tend to work only on Windows. Dunno about Skyrim..

@Sid Please use one or three periods.

Lol.

There you go. One period.

I'm going to cry

I keep breaking latex

@Slereah latex won't compile any more

what do

7:55 PM

what does it say?

Now it says missing { inserted

but nothing is missing

earlier it gave pages and pages of errors

hmm, deleting it and starting a new one fixed it

on the ama, is it am or pm?

@heather It's 12:00 UTC, no idea whether that's am or pm for you.

Hi, everybody.

@ACuriousMind well it could be 12 pm or am UTC, so it depends on that

@DanielSank hello =)

8:06 PM

@heather What's shakin'?

@heather There's no "am" or "pm" in UTC, but it's midday in UTC. Midnight would be 0:00 UTC.

@DanielSank I'm reading about the Kerr effect and trying to figure out what properties are necessary for a Kerr media to have in an optical photon quantum computer.

o_O

@ACuriousMind oh, duh.

i was thinking of a 12 hour system that starts over at noon, as is common in the U.S.

@0celo7 Break your document into manageable pieces so that you can test them independently.

This is the rule #1 of not wasting your life with tex.

8:08 PM

^

rule #2: don't mess with tikz.

@0celo7 I advocate for this package to help you manage your documents in a make-you-happy way.

(I am the author of said package)

The package documentation will show you examples of how to manage your projects.

@DanielSank it's only two pages

@0celo7 So? Put each section in its own file and make sure they compile independently.

That will help you isolate errors.

I fixed it now but I was getting pages and pages of errors before

dunno what happened

To get equations right, pull up an SE page and use mathjax to see your edits in real time.

8:11 PM

I know how to use LaTeX...

I think it was a table that was breaking it

how can you calculate the cross phase modulation abilities of Kerr media? there's no explanation in the book I'm reading.

@heather You have to know the dielectric function.

@heather have you studied nonlinear oscillators at all? I'd start there.

okay

8:30 PM

The stuff you're talking about all comes down to what's called "frequency mixing" (or various other related terms).

At least, I think so. I'm not 100% certain what you're studying, but I'm pretty sure it's related to nonlinear response of materials.

yeah, that sounds right.

Here, let me send you a writeup on how a parametric amplifier works...

god, TexStudio is cancer

Texmaker is pretty good.

I cannot figure out where this thing is saving shit

how is this possible

8:34 PM

Using Windows?

so i'm trying to figure out how all the topics you've listed are connected - looking at the wikipedia article on anharmonic oscillators seems to lead to the example of the electric dipole moment which i think relates to the dielectric function, but the whole thing is rather confusing to me, i'm afraid.

@DanielSank Yes, I am not used to this

@heather Ok, yeah I get why you're confused.

I am used to Windows, but not TeX on Windows

hi alll

8:35 PM

I usually use it on OSX

I would say, start with an example problem of just a plain old nonlinear oscillation problem (i.e. the document I just sent you).

@JohnJack Hello.

okay.

I can't find it

where the hell is it

what does it mean for something to be damped, as in "damped Josephson oscillator"?

@heather "Damped" means "has friction".

8:37 PM

If $| \alpha \rangle$ is a coherent state and $\hat{n} = \hat{a}^{\dagger}\hat{a}$ the number operator, would it be true that $e^{-i \omega t \hat{n}} | \alpha \rangle = |\alpha e^{-i \omega t} \rangle$ ?

Or in other words, has some way of losing energy.

If I make an electrical oscillator and hook it to a resistor, there's "damping".

okay.

@JohnJack How'd you get that?

o/

\o

What are those wacky horizontal dotted lines that show up in chat?

8:39 PM

@ACuriousMind knows all about 'em

@DanielSank I didn't...I am trying to. This is in 'Quantum Optics' by Gerry Knight. Do you agree with it?

I forgot...

@JohnJack You have to expand, apply the number operator (which you should probably know how to do on a coherent state) and then put the sum back into a nice form.

^

It's a pain in the ass but IIRC the result is neat.

Shouldn't it be some infinite sum, still? Idk.

I think the PITA is $a^\dagger | \alpha \rangle$.

That's not a nice thing.

8:41 PM

Just use the expression for how the number operator acts on the coherent state, or equivalently the def. of the coherent state in terms of $n$-particle states.

Yeah, I think you have to expand $|\alpha \rangle$.

...which is why I said it's a PITA.

Fo sho

How u know dat slang?

PITA's are nice doe

@DanielSank How wudn't I?

@Danu heheheh

8:43 PM

@Danu All about whom?

AccidentalFourierTransform

user image

@Danu
Expand using $e^{a \hat{A}} = \sum_{n= 0}^{\infty} \frac{a^n}{n !}\hat{A}^n$.?

@ACuriousMind Deez god dayum linez

Uh

@JohnJack Yes

8:45 PM

I see no lines

You used to insert them by brute force @ACuriousMind

By this

or something

See, that did it

I'm wasting my time

derp

That produces a sort-of line break, but I see no dotted line

really? i can see them

you need more LSD man

Well damnit you once explained this dotted line shit to me so YOU'RE SUPPOSED TO KNOW

8:46 PM

I see dem lines doe

.. what are you doing?

making lines

Take some more LSD

ACM can't trip

sad

making lines?

AccidentalFourierTransform

8:48 PM

$\hrulefill$

\hrulefail

AccidentalFourierTransform

:-/

Kids, this is what happens to people who love +--- signature

Don't do this at home

@BalarkaSen Oh, not you, too :P

I am still trying to understand what is happening here.

8:49 PM

@ACuriousMind u against +++-? u against it bruh?

come at me you

-+++ or bust

Actually, I think I used +--- in my thesis

AccidentalFourierTransform

user image

REKT

Anyone care to explain to me what this is all about?

it's about socialism

8:51 PM

the internet

Metric signatures

Haddaway - What Is Love [Official]

FOR OLD TIMES' SAKE!

@Danu <3

:')

AccidentalFourierTransform

9:02 PM

if $\psi$ is a Weyl field, is it correct to say that $\psi:\mathbb R^4\to\mathbb C^{0|2}$?

@AccidentalFourierTransform I think so, yes

what the hell is that

AccidentalFourierTransform

k thx

$\Bbb C^{0|2}$

AccidentalFourierTransform

Super vector space

In mathematics, a super vector space is a Z 2 {\displaystyle \mathbb {Z} _{2}} -graded vector space, that is, a vector space over a field K {\displaystyle \mathbb {K} } with a given decomposition of subspaces of grade 0 {\displaystyle 0} and grade 1 {\displaystyle 1} . The study of super vector spaces and their generalizations ...

9:11 PM

@AccidentalFourierTransform jeez, what will physicists come up with next

AccidentalFourierTransform

:D

AccidentalFourierTransform

9:23 PM

apparently, countto10 left again :-P

@ACuriousMind Seriously, you use +---?

AccidentalFourierTransform

dude, you're obsessed

@AccidentalFourierTransform how so?

I never bring it up

AccidentalFourierTransform

you always do

e.g.

3 mins ago, by 0celo7

@ACuriousMind Seriously, you use +---?

9:58 PM

@0celo7 That's actually used in most proofs of the index theorem :P

It's my birthday now! <:-)

13

AccidentalFourierTransform

yay

greetings birthday boy

Thanks

@EmilioPisanty I think this was unnecessarily strongly negative language.

@Danu Happy birthday, then! :)

10:36 PM

@DanielSank Possibly, yes, but to be frank that answer is particularly misleading.

Happy birthday pal @Danu

<:-D

PARTY!!!

10:53 PM

@EmilioPisanty Possibly yes, but the comment sounds a lot like an attack on the user rather than a criticism of the post.

@DanielSank I disagree, but ultimately there's very little that can be done either way at this stage

@EmilioPisanty What can be done is discussion and understanding --> future improvements to site.

@DanielSank sure

"complete misunderstanding" suggests that the user has misunderstood something. In this case, yes, the user did misunderstand something. However, I think communication works better when the content of the post is criticized without bringing the user's brain into the picture.

@DanielSank again, I disagree

10:55 PM

@EmilioPisanty Ok.

You can disagree all you want. My opinion should come to you as data.

Nothing more or less.

@DanielSank duly noted ;-)

but I really don't see how that comment is anything other than a criticism of the content of the post

a little rough around the edges, maybe, but it sticks to the post

Simply in that one does not normally think of a piece of text as having or lacking understanding.

A human does or does not have understanding.

It is not a big deal, but when I read it, it felt like a personal criticism.

@DanielSank ¯\ _(ツ)_/¯

would you prefer "misrepresentation"?

it would keep closer to the text

but it would make it much harsher

@EmilioPisanty That would an improvement IMHO and it does not sound harsher to me.

Why not just say "This post assumes an incorrect definition of "coherent state""?

^

Better imho

11:00 PM

@Justwinbaby better than what?

i.e. just how much of the context of this discussion are you factoring into that opinion?

just the overall general sound of the statements made :-)

I really don't want to sound harsh, I really really don't, but you're jumping onto criticism of posts you cannot see with context you're unaware of.

perhaps

@DanielSank Thanks for voicing your opinion. I still disagree but I do appreciate the contact and the tone and I hope you'll do it again if you see a similar example.

@EmilioPisanty Very good.

11:07 PM

@Danu Most?

Now on to really important stuff

37

Q: How do researchers have such beautiful figures in publications, while their homepages are very simple?

I am pretty sure that most of the professors who publish in Nature, Cell or Science have some collaboration with professional designers. The thing is that I have never seen it mentioned. Is it a service offered by the journals? Or do the PIs pay for those services and outsource them?

publications graphics website

EZ

@DanielSank you got any experience publishing in the Nature group?

Figures for publication: Inkscape, GIMP, Photoshop, Python(matplotlib), etc.

I'm pretty sure that they redraw plenty of figures

11:08 PM

Homepage: HTML, Javascript, CSS

You tell me which toolset sounds like more fun.

@EmilioPisanty Nope.

PRL only, baby.

;-)

CPU temp is <30

It's a refrigerator in there

I should pour on some LN2, get it even colder

@DanielSank that's still plenty good ;-)

Cool @0celo7

@0celo7 do it

@DanielSank will it work?

11:11 PM

@0celo7 Of course.

It will also probably crack stuff in your computer, so don't do it.

Don't do it :O

What about putting a bag of ice in there

How do you prevent condensation from killing it

What's the recommended operating temp?

11:33 PM

@Justwinbaby no clue

@0celo7 find out

@Justwinbaby don't know how

Try google

@0celo7^

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