The h Bar

OBE

1:22 AM

0celou7?

whaaa

Physics Meta

3:38 AM

0

Q: Posting questions with math symbols

I did a search and came up empty on how to use mathjax on this site. Do I need to use a third party or is a math symbols program built into the physics site?

0celou7

5:39 AM

@OBE I embraced the French in me

4

Kenshin

6:22 AM

Hello

skill patrol

Hi

user228700

8:44 AM

@JohnR: Morning :-)

John Rennie

Morning :-)

user228700

Fancy lunch today?

John Rennie

I haven't decided.

user228700

Say, I never did find out what you did with those parathas...

John Rennie

Well, yes, it will be fancy, but what i mean is that I haven't decided what to have for dinner yet.

user228700

8:45 AM

Riight.

John Rennie

Some weekends I have everything planned, and others I find myself with no big ideas.

I'll wander along to the shops ina bit and just see what catches my eye.

user228700

Sounds good, seeing as u can buy whatever you want :-)

John Rennie

There's a posh supermarket in the UK called Waitrose.

user228700

OK..?

John Rennie

I don't normally shop there because their food is a lot more expensive and only a little bit better than the mainstream supermarkets.

But ... I recently fixed a computer for a friend and she gave me a Waitrose voucher as a present.

user228700

8:48 AM

Oh, cool!

user228700

How much stuff dyou reckon you could buy?

John Rennie

So the plan (only a vague plan) for today is to go to Waitrose and look for interesting stuff to spend my voucher on.

user228700

Lol, OK!! (:-P)

user228700

Haha, did u hit caps by mistake? x'D

John Rennie

Ah, you spotted that :-)

user228700

8:50 AM

I thought that u were just that excited about going to Waitrose :-P

John Rennie

:-)
I'm actually feeling a bit laid back this morning.

I have a bit of a cold and I'm not feeling my usual energetic self.

user228700

Ohh, OK :-|

user228700

Is it still cold over there?

John Rennie

But that's OK. I don't have anything special to do today so I'll just chill and take it easy.

user228700

:-) OK. I hope it goes away quickly.

John Rennie

8:52 AM

@Kaumudi.H We're well into spring now but we still get occasional cold nights when the sky is clear. Last night the temperature dropped below zero.

user228700

Sheesh, OK...

skill patrol

Try vitamin C?

John Rennie

When I got up there was frost everywhere, though the Sun is out now and the frost has melted.

user228700

Ah, frost :-)

user228700

It's late afternoon here; warm and sticky as always.

John Rennie

8:53 AM

@skillpatrol I'm currently eating some chocolate mints. They are just as effective as vitamin C at curing colds (i.e. completely ineffective) but much nicer to eat :-)

user228700

I'm waiting for my friend so we can have lunch but I'm starving because I haven't eaten any food in the past 16 hours.

John Rennie

Cool :-) Are you going out in Chennai for lunch?

user228700

Nope, I've ordered pizza :-P

John Rennie

Mmm, pizza :-)

Not chocolate pizaa?

user228700

Pizza, stuffed garlic bread, some tacos... :-P

user228700

8:55 AM

@JohnRennie NO.

John Rennie

@Kaumudi.H :-)

I find being hungry always makes food taste wonderful. It's almost worth putting up with the hunger for the extra spice it brings to the meal :-)

skill patrol

Me too.

user228700

But this is torture. The food has come, I'm waiting for the friend who's so late >.<

John Rennie

Ah, yes, I feel your pain!

user228700

...and everything's probably getting cold.

skill patrol

8:57 AM

I like cold food.

John Rennie

Put the oven on really hot. Then you can zap the pizzas for just a minute or two when your friend gets there.

user228700

I don't know how to work the oven :-/

John Rennie

????????

user228700

:-P It's true.

John Rennie

You got age 18 without knowing how to work an oven

user228700

8:58 AM

...because we bought an oven only two months ago and I haven't screwed around with it yet.

John Rennie

Wow, you really need to work on your survival skills :-)

3

user228700

Lol.

user228700

Oh, @Sir: The book?!

user228700

Today I am going to kill a human being.

user228700

...I don't plan on eating it but I do plan on enjoying the act of killing him.

user228700

9:02 AM

I mean, the food is literally right here >.<

John Rennie

@Kaumudi.H I guess a lot of Indian cooking is done on a hob rather than in an oven.

user228700

@JohnRennie Yep.

user228700

My friend is here! Ciao! :-)

John Rennie

Bye, enjoy the meal!

user228700

Thanks!

Ben Niehoff

9:44 AM

a lot of apartments here don't come with ovens!

only hobs

I thought it was very strange

skill patrol

Yeah, that is strange.

Why no microwave ovens @BenNiehoff?

Leonhard Euler

11:32 AM

Hello @JohnRennie

Slereah

12:52 PM

I know roughly properties of spacetimes in 1 to 4 dimensions, but are there specific things above

I know that at $n > 4$ you get non-spherical black holes

Do you get weird shit for $n=279$ or something

Yashas Samaga

people are having problems on my question which talks about sexual reproduction using $n$ human beings at biology SE :D

where n > 2

downvotes vs upvotes

This was the question

Ben Niehoff

1:23 PM

So, $E_8$, apparently, is the isometry group of the coset Spin(16)/SU(8)

that's a much easier definition than any other I've seen

@Slereah I know of one interesting thing that happens around 13 or 14 dimensions

for $5 \leq d \leq 13$, black strings are Gregory-Laflamme unstable, and they tend to break up into a fractal "string of beads" if you follow the time evolution

but for $d \geq 14$, the instability instead reaches an endpoint: a stable, non-uniform black string

also Roberto Emparan has a program where he takes the infinite-d limit of Einstein gravity and shows that a lot of calculations become simple

0celou7

Black strings?

Meraj99

Hello guys, I have a quick fun question. So today, we had heavy rain in my area, which usually doesn't happen. There was a lot of water clogged on the roads, and getting to the point... no rare experience with rain is complete without driving your car over like 10-15cm of water. Long story short, our license plate got bent. Any idea why this could've happened?

0celou7

@BenNiehoff what the hell?

Meraj99

There's also fold marks like the ones you see when you try to fold a really hard cardboard sheet.

Fawad

@YashasSamaga do you have biology subject in 12th?

Yashas Samaga

1:29 PM

Yes but I don't study biology.

Ben Niehoff

@0celou7 the simplest black string in 5d is the Schwarzschild geometry times a circle

it's a black hole that is extended along one direction

Secret

so like a donut?

Ben Niehoff

a black ring is like a donut

a black string is usually not contractible

Fawad

@YashasSamaga you are from which board? You don't study because it's uninteresting or not in syllabus?

Yashas Samaga

National Institute of Open Schooling

Ben Niehoff

1:31 PM

but black rings are also unstable, as was shown last year I think!

to see a super sweet movie of a black string undergoing GL instability, look up Lehner and Pretorius

Yashas Samaga

@Fawad I am not interested in high school biology but I'm interested in the stuff above high school :P

Fawad

Good then,just keep the limit :D

0celou7

@BenNiehoff Not trying to troll, what does contractible mean for a physicist?

I'm always curious when physicists use words

Ben Niehoff

same thing it means for a mathematician, I think

0celou7

Which is?

Ben Niehoff

1:34 PM

a black ring is, say, a circle just sitting inside R^4, supported by its own angular momentum

whereas a black string is wrapping the S^1 in R^3 x S^1

Secret

@Meraj99 Some possibilities: It is possible there are strong currents in the water that push or pull the plate in an asymmetic manner result in the bending, or that the plate bump into something underwater

Ben Niehoff

(I'm ignoring the time direction here)

0celou7

Or aliens @Secret

Personally, I never rule them out.

Secret

Aliens seems pretty implausible for that scenario, but ok

@BenNiehoff I found a 2016 paper and a 2010 paper but no movie, any links to the movie?

Ben Niehoff

1:49 PM

@Secret livescience.com/…

the top and bottom are periodically identified

Secret

hmm, it keeps forming beads

Ben Niehoff

indeed

and you can show that something catastrophic has to happen in a finite amount of time

this paper is pretty famous

Secret

Naive intuition agree with me here: With so many black hole beads packing close together, they are just going to merge into one spherical black hole

Ben Niehoff

ah, but it's a theorem in GR that you can't break horizons!

and since the black string is wrapping a non-contractible circle, you have a problem

Secret

Hmm, so that means the topology of the horizon cannot change, and thus we will expect the outcome will have something similar to the topology of a string?

Ben Niehoff

2:04 PM

well, the problem is the cascade keeps going, and in fact it is accelerating exponentially, so it must reach an end state in finite time

but we can't say what that could be!

actually, I think "theorem" was the wrong word to use

basically, this is the cosmic censorship hypothesis

Secret

btw, what number of dimensions will allow so called hollow black holes, that is a black hole with a spherical shell shaped horizon?

Ben Niehoff

that isn't possible

you can have "ringoids" with horizon topology S^m x S^n

but you can't use S^0 as one of the factors

@ACuriousMind would be happy to know that Jose Figueroa-O'Farrill is in fact on MathOverflow, so maybe he can ask him about Majorana spinors!

21

Q: Isometry group of a homogeneous space

Background Let $(M,g)$ be a finite-dimensional riemannian (or more generally pseudoriemannian) manifold. Suppose that I know that a certain Lie group $G$ acts transitively and isometrically on $M$ and after a little bit more work I exhibit $M$ as a homogeneous space $G/H$. Let $$ \mathfrak{g} ...

dg.differential-geometry lie-groups lie-algebras homogeneous-spaces

Sir Cumference

3:13 PM

@Kaumudi.H Well, it didn't come yesterday, and now it's not a business day D:

vzn

@bolbteppa garrett lisi is quite a character, collected a bunch of links on him once... amusing google fodder. made big waves in popsci press years ago... with lubos a case of hating someone rather similar to himself... o_O

0celou7

3:34 PM

@BernardoMeurer I seem to be finding myself at a contemporary music festival

Help

Transcript for