The h Bar

Slereah

20:25

So what is the Cauchy horizon in the Kerr metric, anyway

0celo7

ur mum

Slereah

qph.is.quoracdn.net/…

0celo7

holy god

Slereah

I suspect it's the surface near the new universe since it's above the singularity

But I dunno

Hm, what book has the causal structure of Kerr

HE, probably

Wald also I guess

Danu

@0celo7 That's beyond disgusting.

Slereah

20:30

Wald has the Nordstrom black hole cauchy horizon

Let's investigate

Danu

It's obviously the outside one that will be the Cauchy horizon

There ain't no getting outta that one, AFAIK.

0celo7

@Danu Really?

John Duffield

@0celo7 : you can't. And space isn't a lattice of strings. It's a 3D bulk, something like a ghostly elastic solid. But if it helps any see this article where Einstein referred a field as a state of space.

Slereah

Cauchy horizon, not event horizon :p

0celo7

@JohnDuffield Hmm, I thought you said it was a lattice of strings...

4 hours ago, by John Duffield

See my avatar. You can model Einstein's elastic space as being made up of a lattice of strings. Try to visualize it. Twang a string like you'd pluck a guitar. A photon is a wave propagating on an "open" string.

I must have misunderstood that.

TanMath

20:33

Hello all!

Slereah

Schwarzschild has no Cauchy horizon, right?

Singularity is just at timelike infinity

No worries

Hey @TanMath

ACuriousMind

::finally decides to google what a Cauchy horizon is::

0celo7

@ACuriousMind srs

we've been talking about them for weeks

Slereah

@ACuriousMind : The set of all points that can only be reached by causal curves from the set

TanMath

Hello @Ghost! nice to see you!

ACuriousMind

20:35

@0celo7 So?

Danu

@ACuriousMind The one that is relevant for outside people.

Ghost

@TanMath good to see you too

0celo7

@ACuriousMind strange it took you this long to look them up!

Danu

0

Q: Why is Phys.SE so restrictive in the Q&A it is willing to accept?

There is already a Physics Overflow. Why then not being more lenient in the Q&A to be accepted in Phys.SE? Mathematics SE has 10 times as many questions. SO leads with +10M questions. Those other sites are thriving yet feel way more welcoming. In contrast, here there are way too many questions pu...

I'm starting to get annoyed.

0celo7

@Danu Why?

Danu

20:38

Read the comments over there if you feel so inclined :P

Slereah

" Let us continue this discussion in chat."

Dammit @Danu

Don't we have enough already :p

ACuriousMind

WTF, how did I manage to vote that question up? Damn phone interface with its tiny buttons

0celo7

Oh the JD one, @Danu ?

Danu

@Slereah Step into my domain...

0celo7

Or the whole question?

Danu

20:38

@0celo7 No, I'm not annoyed about that part right now.

ACuriousMind

@Slereah Those "continue" links create their own chatroom separate from this one

Slereah

Ah, good

TanMath

Discussion between Danu and MASL

Imported from a comment discussion on meta.physics.stackexchan...

John Duffield

@0celo7 : no, you can model it as a lattice. Like you draw a line in it and watch how it waves.

ACuriousMind

Is it acceptable to minor-edit a post to reverse one's vote?

Danu

20:41

@ACuriousMind Have done it before.

ACuriousMind

@Danu Well...that's not exactly an answer to my question ;)

Slereah

Can't you just reclick on the button to remove your vote?

Danu

@ACuriousMind Yush?

0celo7

> concise

Danu

@Slereah No.

0celo7

20:41

You're a meanie and everyone knows it!

Danu

@0celo7 I put it in quotation marks :)

ACuriousMind

@Slereah Votes are locked after five minutes unless the post was edited after you voted

Danu

@0celo7 You're just being your usual dramatic self ;)

Slereah

Oh

0celo7

@Danu no

Danu

20:42

@0celo7 Yes.

0celo7

You make me dramatic

TanMath

@ACuriousMind so why didn't you unvote within 5 min?

Danu

@0celo7 Clearly.

But nobody else, as far as I know.

So I tend to see the cause as external to my persona ;)

TanMath

@Danu ignore him, he's the meanie!

0celo7

@Danu Aha!

ACuriousMind

20:43

@TanMath Because I just now noticed that I upvoted it - I was of the firm conviction that I had downvoted it

0celo7

You don't know what the baseline is

Danu

@TanMath He's harmless.

@0celo7 I play tennis. I know what the baseline is.

0celo7

Maybe everyone is isomorphic to a stick in the mud

Except for me

So you are animating them

TanMath

@Danu but annoying!

0celo7

But I am not a stick!

So you animating me makes me "dramatic"

Danu

20:43

animating? What?

TanMath

@0celo7 i think it is time to put you on ignore again... see you later!

Danu

Okay, so the OP declared he doesn't need to read our homework policy to propose changes to our homework policy.

ACuriousMind

Be nice to each other, kids.

0celo7

You haven't said anything particularly interesting / worth responding to

TanMath

@Danu and ignore that guy as well!

Danu

20:45

@0celo7 Stop it

@TanMath You too.

TanMath

Sometimes ignoring is the best solution to everything!

0celo7

@Danu I haven't done anything.

Danu

I'll intervene if you don't cut it out.

0celo7

Intervene in what?

Danu

1 min ago, by 0celo7

Guess who gives a crap, @TanMath ?

That is not nothing.

0celo7

20:46

He wants to ignore me. That's fine, I have no reason to talk to him!

Free market at work.

John Duffield

Come on guys, let's talk physics. OK, have a look at some pictures of the electromagnetic spectrum. What's always the same regardless of wavelength?

TanMath

@Danu what? I am just giving you some advice... you seem annoyed by that guy, so that is why I said that...

@Danu O gosh, he is soooo offensive.. I wish I could flag that post...

0celo7

I don't even know why @TanMath has a problem, lol.

TanMath

@JohnDuffield huh?

0celo7

I don't think he likes GR.

TanMath

20:47

@0celo7 I love having you on ignore!

0celo7

Then why are you responding? You're bad at this.

Slereah

Please people

0celo7

*person

Slereah

Calm your tits.

0celo7

Only one person has uncalm tits.

HDE 226868

20:49

Perhaps introducing a talking stick - where only the person who has the stick gets to talk - would be a good idea here.

It would at least be amusing.

0celo7

Seriously?

Is the stick big enough to take fools out with it?

Slereah

Is it a good idea to give a stick to people fighting

TanMath

@HDE226868 yeah. and @0celo7 should have the least time with the stick!

HDE 226868

Plus, a stick should be homeomorphic to a variety of different things, so perhaps any topologists could come up with creative things to deform it into.

0celo7

I literally have no idea why he's upset, lol.

And he refused to tell me.

ACuriousMind

20:50

@HDE226868 Better use something that cannot be used as an improvised weapon :P

Slereah

A stick is just homeomorphic to a ball in general

Nothing special

John Duffield

@TanMath : it's one of those naïve little questions that's really interesting. If you've ever played a guitar you might appreciate what I'm getting at. Look at the depictions of the electromagnetic spectrum. What's the same regardless of wavelength?

HDE 226868

@ACuriousMind I thought about that, but I figure that someone in here could use nearly anything as a weapon.

TanMath

Let us talk physics now... I don't want to continue talking about @0celo7 's antics...Either somebody take action or let us stop talking about that crazy guy...

2

0celo7

I vote for a talking gun, I will provide ammunition.

ACuriousMind

20:51

@HDE226868 Talking...sponge?

We could call it Bob

3

Slereah

Pillow

ACuriousMind

I'm not ready for pillowtalk appearing in here, I think

TanMath

@JohnDuffield i don't know...

John Duffield

Look at the pictures. It's staring you in the face.

HDE 226868

@ACuriousMind That's reminiscent of the SNL "Bag O' Glass" skit. Soft things are dangerous, because they're choking hazards! A little kid could swallow the sponge and die.

. . . It was funnier when it was acted out with a foam ball.

ACuriousMind

20:52

@HDE226868 No children under 12 allowed in here, I think we're safe

0celo7

@JohnDuffield It's alive??

@ACuriousMind You sure?

Slereah

@0celo7 is still a young one

2

0celo7

There was that Jimmy guy

Slereah

Maybe we could choke him

TanMath

@ACuriousMind you don't know if there are 10 yr olds here

ACuriousMind

20:53

@0celo7 Yes, I am sure that no children under 12 are allowed in here, and you should not joke about being one, because that'll get your account deleted in no time

0celo7

@ACuriousMind Really!

Slereah

Wait, how is time defined for the age limit

0celo7

Cool

Slereah

Is it proper time

0celo7

Wait, is that coordinate or proper time

HDE 226868

20:53

@ACuriousMind Yeah, I'm actually pretty sure that I'm the youngest. :-)

TanMath

@Slereah yes, but it is suprising to think so based on the words he says...

@HDE226868 no, I think I am the yuongest! ;) hehehe...

Slereah

Do you mean like "Homeomorphic"

HDE 226868

@TanMath 17. You?

TanMath

@ACuriousMind BTW, is there really an age limit? i never knew that!

0celo7

@ACuriousMind Wait, by whom

ACuriousMind

20:55

@TanMath Yes, it has to do with US laws

0celo7

Does @dmckee have a duty to delete me if I say I'm 11?

TanMath

@HDE226868 much younger.. I just don't wish to reveal.. I can say I am younger than 16...

ACuriousMind

@0celo7 By the community managers, or really any SE employee who stumbles upon it

TanMath

@ACuriousMind but in stack overflow there are 10 and 11 y olds...

Slereah

Is that the famous script kiddies I hear about

0celo7

20:56

Let's get back to this talking stick idea

TanMath

@ACuriousMind one time we had an 11 yr old asking something about GR...

0celo7

How could it be implemented

HDE 226868

See meta.stackexchange.com/questions/61770/….

Slereah

Julia Stiles is a Hacker

►

^like this

ACuriousMind

5

Q: Are users under the age of 13 now allowed?

I was browsing Stack Overflow, and I came across this question, asked by a user claiming to be under the age of 13, in this case, 7 years old. As far as I know, children under the age of 13 are not allowed to sign up for an account, but upon further investigation, I discovered that the following ...

discussion profile-page users age

HDE 226868

20:56

And meta.stackexchange.com/questions/236710/…

Slereah

Wait

ACuriousMind

Ah, the one HDE posted is better

Slereah

If we are on stack exchange

Shouldn't we use a stack

And

exchange it

0celo7

US Law is Best Law

Qmechanic

3

Q: How can I remove a downvote?

After a clarifying discussion I see that my downvote was not necessary. Do I have to wait until the post is edited and then upvote or is there another possibility?

support editing down-votes voting

@ACuriousMind : Related ^

ACuriousMind

20:58

@Qmechanic Thanks

Slereah

Also we can't call the sponge Bob

Bob is the one who exchange the sponge

And gives it to Alice

0celo7

Hmm

Interesting idea

Are there black holes involved here?

Slereah

You know I haven' seen any paper on CTCs use Alice and Bob

I am somewhat disappointed

Are Alice and Bob still used a lot in physics?

0celo7

bob the talking sponge

Slereah

They used to be used a lot in SR but I haven't really seen them in a while

TanMath

20:59

@Slereah yes...

0celo7

is Alice Bob's sister?

Slereah

I think Alice and Bob do not have a lot of backstory

They are stand ins

But

They like communicating

HDE 226868

Alice and Bob

Alice and Bob are two commonly used placeholder names. They are used for archetypal characters in fields such as cryptography, game theory and physics. The names are used for convenience; for example, "Alice sends a message to Bob encrypted with his public key" is easier to follow than "Party A sends a message to Party B encrypted by Party B's public key." Following the alphabet, the specific names have evolved into common parlance within these fields—helping technical topics to be explained in a more understandable fashion. == OverviewEdit == These placeholder names are used for convenience and...

Slereah

They live in spaceships

And they probably discuss scandalous things because they encrypt a lot

0celo7

I think Alice and Bob are two very adventurous experimentalists

They are married

So my fanfic is less creepy

Slereah

21:01

d22izw7byeupn1.cloudfront.net/journals/PHYSICS/synopses/images/…

I like how they are often drawn like 1950's characters

He's a groovy cat and she's a swell gal

TanMath

hello @Qmechanic

Qmechanic

@TanMath : Hi

TanMath

@Qmechanic you don't come to the bar that often..why?

John Duffield

So, what's the same? It isn't the wavelength. It's the...

Qmechanic

@TanMath : I'm around.

TanMath

21:04

@JohnDuffield spectrum?

John Duffield

@Tanmath : No. Look at the picture. It's obvious once you see it.

TanMath

@JohnDuffield the wave?

John Duffield

@Tanmath : nearly. What attribute of the wave is always the same?

TanMath

@JohnDuffield oh! the amplitude!

John Duffield

@Tanmath : give that man a cigar!

Now, what's the dimensionality of action h?

TanMath

21:08

@JohnDuffield um.. I hope you read the convo...As I am below 16 yrs old, I don't smoke!

;)

@JohnDuffield huh?

John Duffield

@TanMath : what's the dimensionality of Planck's constant?

TanMath

@JohnDuffield what do you mean by dimensionality?

dmckee

@0celo7 Well, I refer it to the team and they delete you. This is one area in which you should leave the sarcasm at home. The COPA is a fairly humorless piece of legislation and the company takes ins requirements very seriously,

John Duffield

@Tanmath : I mean this: "The Planck constant has dimensions of physical action; these are the same as those of angular momentum, i.e., energy multiplied by time, or momentum multiplied by distance." If you've ever played a guitar you will know that you change the wavelength with your left hand and pluck the strings with your right, usually with a plectrum. When you pluck, the amplitude doesn't usually change. The distance doesn't usually change.

TanMath

@JohnDuffield oh, ok...

John Duffield

21:28

@TanMath : sadly I don't think many string theorists are guitarists.

TanMath

@JohnDuffield huh?

0celo7

Ah, back.

Bastian Treichler

@0celo7 you said you read Zee's Einstein Gravity in a Nutshell from front to back. Can I ask you, what is the exact definition of a locally flat coordinate system at a point P? Is it that the metric is equal to delta_ij there, or is it that the metric is diagonal?

Slereah

That's the metric being Minkowski at that point

aka Riemann normal coordinates

0celo7

Metric is delta_ij and the Christoffel symbols vanish.

Bastian Treichler

21:36

He does not clearly define it (at least not in chapter I.6), and in the internet the definitions are not clear

Slereah

Oh, both the metric and the first derivative

0celo7

Which means the first derivative is zero.

TanMath

Hey, who here knows about tensors? (i bet all of you do!)

0celo7

@TanMath I do.

Bastian Treichler

ok, just to be sure, if the metric is C delta_{ij} with a constant C, then the coordinate system is also locally flat?

Slereah

21:37

Tensor? I hardly know 'er

0celo7

@BastianTreichler Yes, note that $C$ can just be absorbed into your basis vectors

Slereah

@BastianTreichler Are we talking GR or generic riemannian manifold

0celo7

So you can always find a basis for which the metric is $\delta_{ij}$ at that point

Bastian Treichler

@0celo7 oh, of course!

thanks

Slereah

In GR it's not $\delta$

0celo7

21:38

At the point!

Bastian Treichler

yes

0celo7

There will be corrections as you move outwards!

Slereah

Yes

Also the second derivative isn't $0$

0celo7

I messed that bit up when I first learned, just making sure you get it.

True, listen to the Frenchman.

Slereah

That's basically the mathematical equivalent of the equivalence principle

0celo7

21:39

@Slereah Sect. 1.6 in Zee is Riemannian.

I have an answer on that somewhere.

obe

Aube - Low Spin Drift

►

0celo7

6

A: Some confusions about principle of equivalence

The statement is better in the following form: The Principle of Equivalence implies that there exists a coordinate system such that the metric of spacetime is locally $\eta_{\mu\nu}$. Note the word locally. If there is real curvature, then there exists no coordinate system $x$ such that $g_...

Bastian Treichler

@0celo7 yep I'm struggling with it at the moment.. tried to calculate the intrinsic curvature of the 2-dim sphere with the metric diag(1, sin^2 theta), but that seems to work at the equator only, because at other places it's not delta_{ij}

right?

0celo7

@obe o.O

Slereah

Don't forget to put dollar signs around your Tex

obe

21:41

@0celo7 my third fav song.

Bastian Treichler

@Slereah ok

0celo7

I'm in physical pain now

My back aches

Slereah

@BastianTreichler : You can always rotate the coordinate system so that your point is at the equator

0celo7

^

Slereah

Since it's a sphere all points are pretty much equivalent

0celo7

21:42

It's homogeneous.

@JohnDuffield

Slereah

That it is

Maximally symmetric and all

Symmetric along the killing vectors $\partial_\theta$ and $\partial_\phi$

And whatever the rotation on a sphere is

Bastian Treichler

@Slereah yep for the sphere that's okay, but if I'm calculating the curvature of the 2-torus, it's a different thing right? I basically need a different coordinate system for each value of $\theta$, to get a locally flat coordinate system for each point

Slereah

Ahahah

The torus

You can also rotate the coordinates of the torus

Bastian Treichler

but only in $\phi$, right?

Slereah

By just doing the coordinate transform $\theta + a$ and $\phi + b$

0celo7

21:45

@Slereah Has a PhD in tori

Slereah

It's a coordinate transformation

You don't have any restriction on it

Bastian Treichler

if I'm rotating it in $\theta$, I'm changing the intrinsic curvature, don't I? I mean that would map a point of negative curvature to one of positive?

TanMath

Nobody knows about tensors?

Slereah

(Except being diffeomorphic)

TanMath

weird!

Slereah

21:45

We know about tensors, yes

0celo7

I do

obe

why would you even answer that.

Slereah

@BastianTreichler : you are just changing the coordinates

0celo7

@obe Huh?

Slereah

Nothing physical

obe

21:46

@0celo7 this one is weird

01 Jana Winderen - Aquaculture [Touch]

►

0celo7

::cringe::

Slereah

@TanMath 0celo7 knows about tensors!

obe

hmm am I the only one who likes this kind of music?

0celo7

@obe yes

John Duffield

@0celo7 : like I said, there's no evidence of any higher dimensions. Space is like the ball, not the uniformly-curved sphere.

Slereah

21:48

Is "trying to drink the last milkshake drop with a straw" a genre of music

Bastian Treichler

@Slereah okay, but for the sphere it's enough to calculate the curvature at one point on the equator, for diag(1, $\text{sin}^2(x)$). Since it's symmetric and all, that curvature (+1) is everywhere the same, . For the 2-torus that's different

0celo7

@JohnDuffield what does that even mean

Slereah

@BastianTreichler : As said, it doesn't matter

Changing coordinates does not change the curvature

0celo7

It matters on the torus.

Slereah

Those are just labels

0celo7

21:49

On the inside you have negative curvature.

On the outside positive.

Slereah

I know

0celo7

And on top it's flat?

Slereah

I CALCULATED IT TWO DAYS AGO

Top and bottom

TanMath

@Slereah him! really?

Slereah

He does!

He scooby-doo

TanMath

21:50

@Slereah so I am having problems understanding how they work

@Slereah I din't want to talk to him, ok?

Slereah

Well ask a question then

TanMath

hello @DanielSank

0celo7

@TanMath Why?

TanMath

@Slereah What is the difference between tensor and vector? How does the notation of tensors work?

Slereah

@BastianTreichler : It's a big thing in GR that scalars are invariant under coordinate changes

@TanMath A vector is a tensor, but not vice versa

ACuriousMind

21:51

@TanMath Have a look at e.g. this answer of mine

0celo7

@ACuriousMind Seriously?

How is that not off-topic?

Bastian Treichler

@Slereah well that's (more or less) clear to me. I just wanted to say the torus is not as simple as the sphere, because on the sphere you calculate the curvature at one point and you're done.

TanMath

@ACuriousMind what is the e stuff? is euler's number used?

Slereah

But finding the Riemann normal coordinates on the torus isn't hard, though

ACuriousMind

@TanMath "We choose a basis $e_i$ for $V$"

Bastian Treichler

21:53

but i'm sure that's also clear to you, so I'm gonna leave that as it is. thanks for your help!

Slereah

Just do a coordinate transform that will put the metric flat at one point

TanMath

@ACuriousMind so it is like a unit vector?

ACuriousMind

@TanMath Do you not know what a basis of a vector space is?

John Duffield

@0celo7 : people refer to the surface of a ball when talking about uniform curvature. Then they say space might be similarly curved, but in a higher dimension. Only there is no evidence whatsoever of any such a higher dimension. Nor is there any evidence that space has some kind of intrinsic curvature or toroidal topology like the old asteroids game.

TanMath

@ACuriousMind A basis? no... I mean, there is a basis vector in QM, is that what you mean?

Bastian Treichler

21:56

@Slereah just one more thing, is it true that if the surface is embedded in Euclidean $E^3$, then the the coordinate lines of a locally flat coordinate system are orthonormal?

Slereah

Not necessarily

ACuriousMind

@TanMath See Wikipedia. You really should learn basic linear algebra before going near either tensors or quantum mechanics.

Slereah

But there exists an orthonormal coordinate system, yes

Bastian Treichler

@Slereah do you have a counterexample?

Slereah

Sure

Take $R^2$

TanMath

21:57

@ACuriousMind so it is like the basis vectors in QM...

Slereah

Do the coordinate change $x \rightarrow x + y$

It isn't orthonormal anymore

Bastian Treichler

forgot to add, "orthonormal at the point where they are locally flat"

and $x \rightarrow x+y$ is locally flat? damn

Slereah

Well if the metric is $\eta$ (or $\delta$), then yes, it is an orthonormal basis

ACuriousMind

@0celo7 I'm lenient on tensor questions because physicists deal with them very differently than most mathematicians. If you think this should be on math, go and vote to migrate

Bastian Treichler

ok (what a relief)

what's $\eta$?

Slereah

21:59

The metric tensor of Minkowski space

0celo7

@ACuriousMind Do they really?

Bastian Treichler

oh I see

Discussion between Danu and MASL

Transcript for