The h Bar

Slereah

17:01

I need to harmonize the notation of me book, too

I'm kinda picking up random notations from various papers

0celo7

So where are my proofs

Proof that simply connected means time orientable?

Slereah

you're the math guy man

Just look it up

4

Q: Simply connected manifolds are orientable

For a simply connected $n$-manifold $M\subseteq\Bbb{R}^k$, I want to show that $M$ is orientable. Take a point $p\in M$ and take an $n$-disc, $D^n$, around $p$ (we can take it as small as we please). Since $S^{n-1}$ is orientable and $M$ (and consequently $TM$) is simply connected, the orientati...

differential-topology

bam

0celo7

Time orientable

Slereah

Same thing

The only difference is that the 1-form field has to be timelike

0celo7

@Slereah oh really?

Slereah

17:15

whatever

the proof is in "Line Element Fields and Lorentz Structures on Differentiable Manifolds"

0celo7

Who wrote that

Slereah

Markus

0celo7

People on Reddit saying fallout is 60+ hours

What planet are they living on

Are people doing 20 hours of radiant quests?

ACuriousMind

@0celo7 Maybe they just spend more time managing and organizing their settlements?

Slereah

Maybe they are bad at games

Managing settlements is bullshit though

The controls to do it are awful

Also I'm an intrepid adventurer

I don't want to babysit some settlers

ACuriousMind

17:21

@Slereah That doesn't mean people don't do it :P

0celo7

It's a gimmick

The only settlement I bother with is Sanctuary because it has my adhesive plantation

Slereah

Why even bother

Just pick every bottle of glue and rolls of duct tape

0celo7

I ran out

But I have enough money to buy a few shipments

That would take care of me

Danu

@ACuriousMind Seriously, guys?

No good, IMO :P

I made a macro for \frac

Slereah

Well see

The thing is

You can make a macro for ALL THREE

David Z

17:28

@0celo7 that was kind of inappropriate

for the record

0celo7

Don't know what post you referenced, on mobile.

Slereah

it is the most appropriate response really

0celo7

Ah, I see what people did.

David Z

@0celo7 not a big deal for now, maybe come back and check it when you're on a desktop or laptop

0celo7

They played each side as far as they could until it ruined relations with another.

Like I didn't do any Railroad missions.

Slereah

17:30

yeah that's pretty much what I did

But even then it's a lot of radiant quests

or just boring quests

Most of the railroad quests are "Put a thing on top of the building" or "kill all people in that building"

0celo7

Reddit agrees that the Institute ending is terrible. It really is.

ACuriousMind

@Slereah What did you expect? Skyrim and Fallout3 quests also are almost all of that type?

0celo7

Soooooo anticlimactic, you don't get any answers.

ACuriousMind

And you couldn't expect New Vegas, because that one was written by Obsidian

0celo7

I was so sure that Father would turn out to be a synth, or maybe I'm a synth. Or maybe the synth child is real. Or they try to kill me.

Slereah

17:34

@ACuriousMind Well excuse me for expecting quality!

0celo7

Is New Vegas better?

Slereah

New Vegas is pretty good

ACuriousMind

@Slereah I'm not saying a really good game should have beter quests. But given Bethesda's track record, it is unresonable to expect quality in the quests

dmckee

Some days I think my students are the pits. That you couldn't find a less able body of young folks if you went looking.

Slereah

Why can't I get another Planescape Torment, really

A game where I am searching

FOR MYSELF

dmckee

17:36

Then I come to Physics SE and find a user that restores my faith in my class.

::sigh::

Slereah

@dmckee : There's a hilarious 40's song by a physicist with exactly the same complaints

ACuriousMind

@Slereah Torment: Tides of Numenara coming in early 2016

Slereah

I know

0celo7

@dmckee Link?

Slereah

I contributed $150 to the kickstarter

I hope it will be worth it

ACuriousMind

17:37

I was a bit disappointed by Pillars

Slereah

@dmckee ww3.haverford.edu/physics-astro/songs/roberts/nice.mp3

0celo7

Were the Skyrim quests really that bad?

ACuriousMind

Very solid game in the old BG style, but...it didn't feel really extraordinary

Slereah

But have you sought a physicist and place for him to dwell,

And searched the town in vain to find a vacant dungeon cell

Or tried to teach a thousand students who can't do a sum

The girls who'd like to be Greer Garson finding radium?

dmckee

"You can have whatever can be bought!" Mmmmm ... that's not my institution.

But "research is long but time is short" has always been me.

Slereah

17:39

That was during the early Cold War

Back then was a pretty good time to be a physicist, money wise

People just threw money at you

"Quick make more science"

dmckee

I've been reliable told that right after Sputnik, you could get a professor's position with a pulse and a Ph.D. (or even a pulse and promising ABD).

0celo7

Zee said the term HEP came about because congressmen gave more funding to high energy than particle physics.

Slereah

Oh, if only

When is gonna be the next nuclear escalation

dmckee

@Slereah I'm rooting for "long after I'm dead and gone", and your job prospects aren't going to change my mind. Sorry, man, that's just the way it is.

0celo7

Just run for president

Nuke some place

Then get a professorship

ACuriousMind

17:44

@dmckee "Never" is also a popular choice.

Slereah

Pictured : Me with my new research position

(Also I am Richard Nixon)

What's gonna be the next big technological escalation, I wonder

Probably some boring shit like robots

Can't we go back to the space race

0celo7

I thought you were a roboticist

Slereah

Yes, that is how I know

0celo7

you know, your career is interesting

MS in physics

where you apparently did stochastic processes and path integrals

then an internship doing HEP

then you became a roboticist

Slereah

It was actually

0celo7

17:51

now you're unemployed an a GR maniac

Slereah

First internship as a HEP simulation thing

Second internship on path integrals

One year of math with an internship on meson cross sections (with my old physics professor)

Then two years unemployed

Then software engineering

0celo7

how do you do an internship on path integrals

why didn't you just go for a PhD

Slereah

I did

There's a law in France that says you can't do a PhD without being paid for it

So it's a bit harder to do

it's a well meaning law but basically it just means a lot less people can do PhDs

0celo7

move to Germany?

ACuriousMind

@0celo7 PhD "students" are also paid employees here.

Slereah

17:54

Well not everyone wants to switch country like that :V

0celo7

@ACuriousMind I haven't met a PhD here that wasn't paid

but it's not a law

I'm sure @dmckee will tell me I'm full of sh*t

ACuriousMind

Well, I'm not sure if it is a law, but I'm pretty certain no one will agree to supervise you if they can't get you a position in their research group.

0celo7

@DavidZ I disagree, but whatever.

ACuriousMind

So if Slereah couldn't get someone to do that in France, I don't see why it would be easier here

0celo7

@ACuriousMind Maybe there's more GR groups...

he is interested in some pretty esoteric crap

Slereah

17:58

Eh, the thesis I did were pretty boring stuff for the most part

Meson cross sections were pretty bland

Snore

David Z

@0celo7 I mean, it's not really that bad, but I'm just pointing out that I think saying stuff like that is (part of) why you find your discussions becoming argumentative

0celo7

@DavidZ I'm done with those "discussions" unless he posts something that I want to specifically call out.

Slereah

When I did that thesis, originally it was the $SU(2)$ sigma model

David Z

@0celo7 see, and there you go again

Slereah

Then I told my advisor "I'm almost done!"

0celo7

18:00

you mean $\mathrm{SU}(2)$

Slereah

And he told me "Do $SU(3)$ now!"

And I was like >:|

SU(3) is some shit

0celo7

@DavidZ ...uh, ok?

Slereah

SU(3) spinor decomposition is pretty horrid

ACuriousMind

@Slereah Hahahahaha...I feel for you, retroactively

SU(2) is nice, SU(3) is evil if you want to actually compute stuff

Slereah

0celo7

18:02

proof?

Slereah

Ugh

There's your proof

0celo7

so @ACuriousMind thinks the F4 loading screens are not long

is he crazy

Slereah

yes

ACuriousMind

I'm crazy, but that's completely unrelated to that.

Slereah

he is

The sigma model lagrangian for SU(3)

the horror

0celo7

18:05

I don't see the issue

is that even on curved space

Slereah

it is not

Physics Meta

0

Q: How do I include a picture to a question?

I want to upload an image to support my question from my mobile browser.I cannot see any option to do it. How can I do this?

support

David Z

@0celo7 well, let me put it this way: when your chat personality is such that saying (typing) stuff like these messages I'm pointing out is normal, you tend to let slip things that other people consider abrasive without realizing it. I've seen this many times before, and in many cases what it takes for the person to improve how they get along with others is for someone else to point out what they're saying that can rub other people the wrong way. It's hard to see it by yourself.

Of course in other cases, people who post abrasive things are doing it intentionally.

0celo7

@Slereah why does the Birkhoff theorem imply the shell theorem

apparently this is obvious, but I cannot remember the argument

or figure it out

Slereah

I dunno lol

I mean

Birkhoff theorem for the outside shell theorem is pretty obvious, I guess?

Inside not so sure?

0celo7

18:13

what is the outside shell theorem?

dmckee

@0celo7 I didn't have departmental support the first year. And I knew one guy who was self-funded, but he was independently wealthy and doing the degree for self-actualization reasons.

0celo7

acts like all mass is at the center?

Slereah

Yes

dmckee

Most middle of the line departments and some of the better ones will admit unfunded students, but it is not encouraged.

Slereah

Outside of a spherical static spacetime, in the vacuum, you see Schwarzschild

Inside a spherical shell, you feel nothing

shell theorem is basically Gauss law in classical gravity

0celo7

18:17

@Slereah proof?

Slereah

The proof is Birkhoff

It is trivial!!!

Weren't you reading

0celo7

wtf

where did you prove it

Slereah

use the google m8

just type Birkhoff shell theorem

0celo7

I did

apparently this is an exercise in Straumann

Slereah

I think the inside theorem can be done by proving that the Komar mass inside the shell is 0 implies that it will be flat?

user54412

18:28

@dmckee And I think the further you move away from math/science, the more common self-funding is.

0celo7

@ChrisWhite do you know the proof for the GR shell theorem?

user54412

12 mins ago, by Slereah

The proof is Birkhoff

Slereah

lawl

0celo7

oh ffs

dmckee

@ChrisWhite Oh, yeah. Very common on the humanities side of campus for a grad student to only get a couple of classes of year of grading and to have to provide most of their own funding. But physics and astronomy aren't usually like that.

0celo7

18:30

Birkhoff says that the external solution of stationary mass distribution is Schwarzschild

user54412

@0celo7 Also, I study black holes, not Dyson spheres :p

0celo7

what does this have to do with the inside

Slereah

Minkowski space is Schwarzschild for $m = 0$

0celo7

yeah, so?

Birkoff's theorem is a statement about the exterior solution

Slereah

15

Q: Is spacetime flat inside a spherical shell?

In a perfectly symmetrical spherical hollow shell, there is a null net gravitational force according to Newton, since in his theory the force is exactly inversely proportional to the square of the distance. What is the result of general theory of relativity? Is the spacetime flat inside (given t...

general-relativity spacetime curvature

0celo7

18:33

oh Duffield

@Slereah he doesn't explain it either

Slereah

"All spherically symmetric spacetimes with $R_{ab} = 0$ are static"

Inside the shell the solution is going to be Schwarschild hence

And you can check from the Komar mass that if it is 0, $m = 0$

Which is just Minkowski space

0celo7

@Slereah wtf I thought Schrwarzschild was a statement on the exterior solution

I'm like 99% certain

Slereah

Scharzschild is the generic form of a metric that is spherically symmetric and static in a vacuum

Best reply

ACuriousMind

@dmckee: Sorry, I also drew that exact same freehand circle picture before I noticed the "mobile"

@Slereah That one was great

0celo7

straumann claims Schwarzschild spacetime is nonstatic inside of the horizon

or

Slereah

18:48

Well, inside the horizon $t$ is a spacelike coordinate

0celo7

am I the only one confused here

@ACuriousMind can you explain

or maybe @NeuroFuzzy

@Slereah but that still does not explain why the shell should not contribute

ACuriousMind

@0celo7 I don't see the issue.

0celo7

@ACuriousMind you

you're evil

seriously

how does this follow from the Birkoff theorem

@Slereah if any spherically symmetric mass distribution has a Schwarzschild metric

how can we have a spherically symmetric mass distribution that is part times schwarschild with some nonzero mas

and sometimes minkowski

that's not Schwarzschild

that's patching together two different metrics

Slereah

19:06

Well yes

Schwarzschild is already several different metrics patched together

is it that amazing

0celo7

I don't see how the shell theorem follows at all

hand waving ahoy

@Slereah and the Komar mass is given by an integral at infinity

Slereah

Is it

Iunno

0celo7

so it would take the mass of the shell into account

ACuriousMind

@0celo7 Just because it doesn't say "proof" or cites any other fancy theorems that doesn't mean it's handwaving.

0celo7

@ACuriousMind but he doesn't prove it

he says there cannot be a singularity because it is empty space

proof?

Slereah

19:17

Why a proof

That's just the condition he decided for a spacetime

You could have a shell with a singularity in the middle

He is saying the case of a shell without a singularity

0celo7

this just makes no sense

there is no mass outside of the sphere

but the solution is not flat

so why does not mass inside imply that it must be flat there

Slereah

The important part is what's inside the spherical hypersurface the point is on

0celo7

@Slereah huh?

ACuriousMind

@0celo7: I really don't see your problem: Inside the shell, you have a spherically symmetric vacuum solution (since there's no mass there). So, you invoke Birkhoff's theorem that this is Schwarzschild for the mass enclosed. But the mass enclosed is zero, hence you have Minkowski.

0celo7

I'm not convinced that the shell doesn't have to be taken into account!

I'm very confused

I'm not convinced that Birkoff allows you to " invoke Birkhoff's theorem that this is Schwarzschild for the mass enclosed."

vzn

19:29

@Slereah more on universe as simulation :)

did you say you have a new position? or lost last one? still working in robotics?

ACuriousMind

@0celo7 There is indeed a subtlety that this invokation chooses "unnatural" coordinates which may get discontinuous at the transition between inside and outside. See this paper. The interior solution is Schwarzschild for the mass enclosed (they say "can be transformed", but we all know that coordinate choices don't really change the character of the metric, hm?), but in a time coordinate that is discontinuous when naively stitched to the outside solution.

0celo7

the fact that you're lectuing me on GR shows how far I've fallen

vzn

fallen ~~angel~~!

0celo7

this was an exercise in Straumann that I did a year ago

ACuriousMind

BTW, this is on the first Google page for "Birkhoff theorem Minkowski" :P

0celo7

19:32

@ACuriousMind not for me

ACuriousMind

Ah, sorry, the search was "Birkhoff Minkowski inside shell".

Slereah

that is why I tell you to google :p

0celo7

I did google

@ACuriousMind how can time be discontinuous

ACuriousMind

@0celo7 I linked the paper, why do you ask me?

0celo7

misspelled "discontinuous"

:/

John Duffield

19:47

@Bass : yes, the "coordinate" speed of light is zero at the event horizon. See the Wikipedia article I referred to earlier and note this: "The local instantaneous proper speed of light is always c." But how can you measure the local speed of light in a place where light doesn't move? It takes forever to do this, so you just can't do it.

Slereah

why are convex neighbourhood called "star shaped"

aren't stars concave

0celo7

what the heck does a star shaped neighborhood even look like

we talking a satan star

or a ball

Slereah

0celo7

that's not even connected

Slereah

obviously those are several convex normal neighbourhoods

0celo7

19:58

well

does geodesic ball has constant curvature $\Leftrightarrow$ $M=\mathbb{R}^n$

John Duffield

@0celo7 : what do you mean "Oh Duffield"? There's nothing much wrong with that answer. If you'd like to offer an answer yourself I'm sure everybody will be pleased to see it.

Transcript for