Weak anthropic principle, okaly dokaly

strong anthropic principle, no thanks

Here's what it looks like on the other side of the flat, computer for scale

(and if someone starts invoking the inflationary multiverse then I just lose interest entirely)

pretty big

shine it on a slit!

That's the plan

woo

although I'm gonna get some goggles before I blind myself

makes sense.

As poorly calibrated as it is it still packs a punch

shouldn't lasers be more focused?

Any idea how narrow the wavelength range is?

Not in China, i guess!

The website was pretty sparse as far as technical specifications went

can't say i'm surprised

Well it will have to do for now

i don't remember how one experimentally tests that, though

I don't have 300 bucks for a real laser

i guess one way would be to shine the laser through a prism and see how much divergence you get

but somehow that seems too primitive

That's probably a fairly small divergence if everything is in the red range

I think you need a spectrophotometer for lasers to measure both power output and wavelength

Oh wait

It does say

$660 \pm 10 \mathrm{nm}$

Though I don't know if it is accurate

hmm

It's pretty hard to find cheap goggles for that wavelength

"he National Institute of Standards and Technology, (NIST), has endorsed a laser beam divergence measurement technique that involves measuring the beam diameter at the ideal focal point of an imaging lens. A number of diameter measurement techniques can be used for focal plane divergence measurements including variable apertures, scanning pinholes, knife edges, and CCD arrays. U.S. Laser has adopted the CCD method for diameter measurements, in conjunction with the focal plane technique."

more details here, though the pictures don't work for me: aries.ucsd.edu/LMI/TUTORIALS/diverge.html

@Semiclassical like carroll

oh, here

amazon.com/Safety-Glasses-492nm-770nm-Protection-Goggles/dp/…

It will do

Oh wait they don't ship to France

Dang it

@JohnRennie I'm going to try to make flour chicken Schnitzel tonight.

@JohnRennie wish me luck

Pariser schnitzel!! :-)

apinex.com/det/GS-LPG600.html

That should work

@0celo7 I think i asked you this before, but i don't remember if you replied

did your fluids course do any Lagrangian hydrodynamics stuff?

apinex.com/det/SG02-06.html

@0celo7 If you get the coating nice and crispy it's delicious, so I hope it goes well.

or maybe this

@Semiclassical Entirely possible I didn't see it

@Semiclassical nope

Bit more expensive but more attenuation

mmkay

I am just mere minutes away from completing my masterpiece - risotto with chorizo!

@Semiclassical we're doing exact solutions of the NS equations right now

$0$ is an exact solution

fun fact, doing Lagrangian hydrodynamics was one of the big starting points in numerical GR

DeWitt's work in particular

(I saw that in a random history of science talk)

Also the laser's back has some threads to put screws in

i should make a little stand for it

I guess the more descriptive name for it is the convective form of the Euler equations

Goggles ordered

@Slereah mount this on a robot with a high resolution cam, with mosquito face recognition software

@0celo7 oh. any progress on the ridic pressure problem?

I'd rather not put a camera near that laser :p

It could fry the sensors

would this laser fry a mosquito?

I don't think so

It claims that it could burn paper, I tried and it does not seem to be the case, or very slowly

Maybe it can if well collimated

It could probably blind a mosquito though

@Semiclassical I just turned it in

ah kk

No one else thought to put the Qdot term in

Idk, I have two answers and a note

what I was wondering, thinking back to it

The TA is pretty merciful

what you're including is the contribution from heat entering from the sides of the tube?

Yeah

but, if the water enters at one temperature and leaves at another, won't that also carry energy out of the system?

@peterh what @ACuriousMind said.

i.e. there's heat flowing into the control volume from the sides of the pipe, but there's also a net heat flow out of the system since the water which leaves is hotter than the water which came in

ww3.haverford.edu/physics-astro/songs/roberts/Cyclo.mp3

Possibly the best experimental HEP song

@Semiclassical I don't know. He's posting solutions later

mmkay

The laser has an adjustable aperture

oh nice

Hey, any interest in a "vacuum systems stackexchange"? (Mainly so I can stop cluttering up this chatroom with turbopump questions) area51.stackexchange.com/proposals/114195/…

It looks a bit better if I set it correctly

But then there's interferences

Also my head hurts with all that laser so I might want to wait for those goggles

@Blue May well be - there's a few similar forums on the net that're pretty popular

(fusor.net)

Just thought I'd give it a shot

If only to have a group to commiserate with on the cost of Apiezon

@0celo7

Riddle me this

Can a static spacetime have arbitrary topology

I don't think it's the case

I think you need a foliation

@Slereah Never knew theoretical physics was so into skin care

@Giskard42 The term is actually taken from botany, not skin care

Foliation as in leaves

Ah, interesting!

@Slereah Presumably you'll always be able to foliate along the timelike Killing direction?

Well yes, that's my thought

@Slereah yeah it probably has to split

but that's not much of an argument

I can think more about it later

Which makes me think it's limited to $\mathbb{R}\times \Sigma$ and $S \times \Sigma$

Still pretty broad

But no weird trousers

Although...

I think the Klein bottle spacetime also works

So maybe it needs to be a fiber bundle

It likely follows from the Frobenius theorem for involutive distributions.

Also there's a non-time orientable version of Minkowski space

Ask @BalarkaSen

So definately some weird shit

@BalarkaSen halp

That might only be true for stationary spacetimes. God, I can't remember what these words mean.

@0celo7: there you go. Risotto with kabanos sausage! I did say chorizo earlier but I was mixing up my sausages.

Note the presence of vegetables.

@JohnRennie please delete that. I had to skip breakfast

I'm in pain

oh wait, by definition a static spacetime has spacelike hypersurfaces

But what about a stationary spacetime then D:

This is in Straumann

@JohnRennie you're Not Nice

Also I think the original article of static spacetimes is by Levi-Citvitta too

Statica einsteiniana

Nota del Socio

@ACuriousMind Well, what I have in mind is the discussion of Kant's views on metaphysics in this paper

Mi propongo di studiare sia quel caso particolare delle equazioni gravitazionali di Einstein, che corrisponde a fenomeni staticin sia (sempre dal punto di vista della relatività generale) il movimento di un punto materiale entro un campo statico etc etc

it is a very spicy meatball

(which I find an interesting read, though as with any random philosophy paper there's a risk that it's just wankery)

@Slereah racist.

Mama mia

Archive footage of Levi-Civitta

as far as GR goes he also wrote "Sulla espressione analitica spettante al tensore gravitazionale nella teoria di Einstein"

@ACuriousMind is this allowed?

I'm not sure what this is about

My Italian heritage is offended

I thought you were of german origin

The romans were in Germany

Sorry, the holy roman empire was a lie

So what spacetime topology admits spacelike hypersurfaces

@Slereah R4

I think it needs to be some fiber bundle with base space $\mathbb R$ or $S$ and fiber $\Sigma$

The only really realistic topology tbh

According to Schwarzschild the realistic topology for the universe is $RP^3$

ww3.haverford.edu/physics-astro/songs/Halpern/ket.htm

oh god

looking at the selected books of his shown

"The Cyclical Serpent: Prospects for an Ever-Repeating Universe"

:S

your mouth looks like a serpent

yes, but not a cyclical one

no ouroboros here

Jesus Christ Gromov, $vol$ and $dist$?

How does not that not make his eyes bleed?

@Slereah Gromov is insane. I need a "function" that takes an R-fold to a continuous function on it

How do I write that and what is the correct terminology?

That's clearly a functor ;P

Not a clue

Functor? I hardly know 'er

@ACuriousMind Yeah something like that

What's the target category?

Hm, it's a bit odd. You could just define tupels of a manifold and a function on it as the target category, I guess

@ACuriousMind I want a "thing" that assigns to a manifold its scalar curvature

I don't think that's exactly a functor

It doesn't do anything to the morphisms in $\mathsf{Riem}$

Hm, yeah, maybe it's not usefully phrased as a functor

@ACuriousMind So...what the hell is it

it's a map from $\mathsf{Ob(Riem)}$ to the class of continuous functions on manifolds

that's not very nice

@ACuriousMind So it's $\mathsf{Ob(Riem)}\to \mathsf{Ob(TopMan)}$

o/

hey-o

hi

@0celo7 what variety is that in your picture?

@Danu I don't know what a variety is.

You looking at Gromov's new paper? ^^

@Danu Yes

I started looking at it but was overwhelmed by a sense of disgust at all these estimates :D

@Danu Well that's why you're an algebraist and I'm an analyst...

I'm more disgusted by the typesetting

Also what'd the morphisms be in Riem? Smooth maps such that the pullback of the metric is the metric?

Who is

Top Man

is he some superhero

the superest

@Danu Are there any maps that do that that aren't isometries? I guess you mean local isometries?

I actually have no idea, Riemannian manifolds are probably too concrete for a categorical approach

Yeah idk what I'm saying

But Gromov has a functorial definition of scalar curvature

I'm considering putting it in my Yamabe talk to make myself look smart

some axioms or something right? I forgot already

@Danu Yeah. Additivity under products, volume growth of balls, scaling, and normalization

I guess it's not a "functorial" definition.

Just an axiomatic characterization

It's messy, but a hell of a lot better than $R^i{}_{ji}{}^j$

@Danu the product thing is functorial

So yeah, axiomatic and with a functorial condition

Like Chern class, etc.

Wait

The delivery of MTW is planned between november 25th and december 11th

Characteristic classes have truly functorial definition as certain natural transformations between cohomology and vector bundle functors

I think it might be faster to just order it from another store and cancel that order

guys, is there a sort of justification (apart from experiments) that we assume constraint forces to be perpendicular to the constraint surface? Like, is it that we don't want the constraint forces to do work?

@Slereah loooool

Hm

There's a shop that will deliver faster

But 20€ more

@Slereah Is Amazon.fr sold out?

I think so yeah

@Danu I know what those words mean!

Or maybe they never had it

and they order it on demand

@0celo7 :D

I think I'll cancel it

It is cancelled

Let's order a real thing

there we go

you know I kinda hate the names "static" and "stationary"

I always confuse the two

@Danu I wish I had the time/energy to learn the functorial approach to geometry.

Learn real geometry

The Euclid type

I do know real geometry

@Semiclassical Soo...the useful part of "metaphysics" is recognizing that many of the problems of traditional metaphysics dissolve once we give up our insistence on forcing ontology onto a scientific formalism? I don't see metaphysics here, it just appears to drop metaphysics but oddly enough calls that process "reflective metaphysics".

Do u

I want to learn the abstract crap

Can you prove things from Euclid's axioms

It might be to do with the name, yeah.

I can prove Euclid's axioms from Riemannian geometry

That's easy

the hard part is proving things the other way 'round

is it?

you have to interpret them correctly

well it's different

lines aren't even really defined

Euclidian geometry doesn't have real numbers or anything

You need to do everything with congruences and such

ok wtf mathematica

@ACuriousMind What are some good games coming out this season?

@ACuriousMind Also please suggest a side dish for Schnitzel

@0celo7 I don't know... I've never seen much reason to except for the ability to repackage certain constructions in useful words like "natural transformation" so that you don't have to memorize things and just know there is some commuting diagram

@0celo7 Classic is fries or otherwise baked potatoes

french fries

the initial condition I gave you has one peak

@0celo7 Dunno, not excited for any new game currently

@0celo7 Pommes

baked potatoes is always a good idea

@ACuriousMind What about Monster Hunter World

@ACuriousMind Pommes ist zu hard to mach at home

so why are you acting like the initial condition has three peaks

I tried playing the new South Park game but it was so buggy I had to get a refund

@Danu There's a classification program for natural vector bundle functors. In theory it seems interesting but it's rather difficult

@Danu Never heard of it before (not a console gamer)

@0celo7 Then Bratkartoffeln

...okay, now it's not doing it

I don't see a reason to learn it either -- hence I do not know it -- but it's interesting nonetheless.

w/e, i'll take it

@ACuriousMind Can one bake those somehow? I don't want to be frying two things at once

@ACuriousMind It's coming out on PC! First MH game on PC doe

@0celo7 Any link?

@0celo7 In the oven, with some spices, a bit of water, etc

Should be easy enough to find several recipes online too

@0celo7 Sure you can bake them, but then you get baked potatos, not Bratkartoffeln. Which is not necessarily bad.

@Danu springer.com/us/book/9783540562351

@ACuriousMind If I put some oil on them they become roasted potatoes. Which is necessarily good.

Oh, that Kollar book

@Danu Hmmm...might be interesting

I absolutely loved that shit on the Wii

Very basic set-up but such great depth

Was awesome for 100+ hours online after the single player, too

@Slereah Did you need help?

@Slereah did you check Norbert

Why does ACM care about that?

it's a timed link; click it

Ah ahaha I remember that

@lılostafa Internet communities have a tendency that they become increasingly closed with time. This is the "good old guy" problem.

@lılostafa I think it is some natural social evolution mechanism, with what the SE can't or won't deal.

@lılostafa Earlier, previous systems lacking a top-level control, went on this way much faster. I think if the SE doesn't start some really revolutionary, it will likely kill the SE.

@lılostafa Although I see clear traces that the top-level of the SE knows the problem and it tries to do something about it.

@0celo7 you do analysis, right? - Do you know of any good intro-level resources to Riemann surfaces/sheets (for a physicist if possible)?

@Mithrandir24601 I like analysis real, not complex. Ask @Semiclassical

@0celo7 that's good enough, thanks :) @Semiclassical ^ ^

Alekseev has notes given by Arnol'd on Riemann surfaces that I found to be helpful once upon a long time ago

@Semiclassical On hold trying to call the DMV. Is driving worth this crap?

@BalarkaSen found them, thanks :)

@Mithrandir24601 you're heading down a dark path

@0celo7 all to understand 1 equation...

@Mithrandir24601 What kind of things do you want to do with Riemann surfaces?

@Semiclassical I am convinced they are trying to get me to hang up

the hold music is screeching noises

@Danu look at exceptional points where eigenvalues coalesce (this is obviously the short version - the long version involves trying to explain two different papers in order to understand one equation in a third)

sounds like branched covers

It does, but then again maybe not

@Mithrandir24601 perturbation theory?

Not sure a textbook on Riemann surfaces would be helpful

@BalarkaSen I'm lost on the Wiki page, but it's plausible from what I do get

@0celo7 It's occasionally applicable here

@JohnRennie How is Slater pronounced? Is it /sleɪtər/ or something like /slatər/? In some lectures, I've heard the former, but I've seen people with the same name, pronounced like the latter.

Like later not latter

sl8er

@BalarkaSen what on earth is gyazo.com/077f65ca1fe6640363cc3d498901de48 about

@0celo7 I have never seen this before but the definition is pretty