The h Bar - 2015-08-17 (page 2 of 4)

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3:00 PM

Wow, that's strange

"In 1912 he was awarded the Nobel Prize in Physics for his "invention of automatic regulators for use in conjunction with gas accumulators for illuminating lighthouses and buoys"."

Indeed

user image

Look at that classy guy$

I love those hats

I have way too many of them

I started wearing them in HS because my all-time favorite artist Paul Simon wore them

:p

I don't think he would win the nobel prize today, though

Does the relativity tag usually pick up once the semester starts?

@FenderLesPaul Yeah, with "twin paradox omg!" :P

3:09 PM

:(

What if they are triplets

Octuplet paradfox

"What if one twin got kidnapped by The Death Star and the other twin went to a Starbucks on Jupiter?"

For some reason I don't have any meaningful GR questions so far.

@FenderLesPaul @0celo7 pg. 74 it says $g_{\hat{\mu}\hat{\nu}}(p) = \eta_{\hat{\mu}\hat{\nu}}$, and $\partial_{\hat{\sigma}} g_{\hat{\mu}\hat{\nu}}(p) = 0$ though what properties do the coordinates have that their second derivative does not become 0?

This only applies at point p

The metric isn't Minkowski around it

Ok, my bad.

Thanks.

3:22 PM

Since the derivative is 0, it means the metric is either a local minimum, maximum or stable point there

@obe it's similar to the graph y = x^2

at x = 0

If the second derivative is not 0, it will be a local minimum or maximum

depending on the sign

@obe Carroll and quite a few other texts use the term "local flatness" quite liberally but don't take it too literally

it seems to confuse people more often than not

Local flatness only applies around a point or a geodesic

It is very local

well nothing is flat locally in any strict sense which is why it seems to confuse people

last time I TAed the undergrad GR class here there were a handful of kids who were confused by it

it's just bad terminology

3:29 PM

Really at its core "local flatness" is just a basic linear algebra theorem

All matrices are trigonalizable

Since the metric tensor is symmetric, it is diagonalizable

That's just the statement that $g_{ab}(p) = \eta_{ab}(p)$

there's more to it than that

you also need the first derivative to vanish

True

but even then nothing is flat locally in the mathematical sense

it's just very loose physics terminology that I don't particularly like

seems it was invented to make the whole "Equivalence principle <=> space-time manifold" connection more lucid

Well Einstein didn't invent it

It goes all the way back to Riemann

Is that so? That's interesting. I've never seen a math book use the term.

3:37 PM

Well the term isn't used, I think

@FenderLesPaul What is your problem with it? It just means that there is always a parallel frame in a point

But the definition of a manifold is locally euclidian

I think it's perfectly mathematical.

@Danu Compare it to the usual sense of mathematical "local", which means that there is a neighbourhood in which the statement holds.

That physicists like to extend this to "well-approximated by flat space in regions smaller than the typical curvature scale" may be something different.

3:38 PM

@Danu there's no neighborhood of $p$ in which the manifold is flat

@ACuriousMind Meh, maybe.

i.e. vanishing Riemann

@FenderLesPaul That's not what I said.

@Danu sure but that's what local flatness would mean

and is the question kids kept asking when I TAed

No, only if you want to stick to the "usual" notion of local

3:39 PM

well that's what local means in math

local as in "in a point" is perfectly fine with flat connection

so that's why I like sticking to it

No, that's something that should be specified when context doesn't make it clear IMO

user54412

I'm siding with @Danu here, especially since we have continuity on our side -- anything that holds at a point holds arbitrarily well close enough to the point.

To me, local means "only "senses" the germ"

but not always

It's not a big deal to call this thing local, too.

3:40 PM

@Danu But "in a point" is not the mathematical definition of "local"! Pick a random mathematician and ask him what a local property is. The answer will always be "a property that only needs to be checked on neighbourhoods.

@ACuriousMind As I said, I would mention germs

But maybe

It might just means that there exists an open set around the point such that such quantity is $<\epsilon$

?

@Danu That's at property that holds "on the stalks" - a property on the stalks is different from a local property.

@ChrisWhite Exactly. This is also what @slereah says I guess

3:42 PM

Not like those bloody conifolds

@ACuriousMind I'm gonna go ahead and assert that these matters of terminology are not settled.

Fractal metric shit

the worst

@Danu Ask any geometer. They'll all give my notion of locality.

@ACuriousMind I've seen local be used several times in the way I describe it in my geometry courses.

:O

Physicists :P

3:43 PM

No, nothing like that.

Leeb is a pure mathematician

@ACuriousMind: I agree with you.

4

Q: What's the definition of a "local property"?

Is a property called local if and only if for every point there exists a neighbourhood for which the property is true? For example: Let $X,Y$ be topological spaces. Then $f: X \to Y$ is continuous if and only if for every $x \in X$ there exists a neighbourhood $U$ such that $f \mid_U$ is continu...

general-topology definition

if you want a funnier debate

@ACuriousMind That's in the context of topology anyways

Try asking people what "pathological" means

3:44 PM

I mean... what does a "local operator" mean to you?

@Danu ...a manifold is a topological object?

It is

@ACuriousMind Sure, but it has more of course.

Topology plus a mapping to R^n

@Danu Nothing unless you give a definition :P

3:45 PM

@Slereah No need to recount the basic facts :P

@ACuriousMind MINE, DAMNIT :P

OR IS THERE

I'm no supergenius! Or are I?

ONLY SENSES THE GERM :P

Sigh

user54412

@ACuriousMind But flatness isn't ;P

@ChrisWhite Also this. :D

Meanwhile, can anyone tell me how I can force a program to open?

sudo program

3:46 PM

I have some "control center" that holds the key to my scrolling settings, and it won't open :(

@Danu That's not a definition! :P Also, the germ is different from the point, since it senses the infinitesimal neighbourhood, not the point itself.

@ACuriousMind Yeah, I was just giving some alternative notion of locality that I think of immediately.

@Danu It's not that alternative: A germ is the stalk of a sheaf, and hence the inverse limit of functions on neighbourhoods. Thus, a property that holds "on germs" holds on some neighbourhood, and a property that holds on all neighbourhoods holds on germs.

@ACuriousMind No idea :D

@Danu This year of algebraic geometry I took has finally been good for something! :D

3:53 PM

@ACuriousMind "good" :P

Yeah, really, its main use seems to be to overwhelm people with terminology so far :/

@ACuriousMind That applies to most things we do :)

Heh, true

4:14 PM

So... nobody any clue on how to force a program window to open? :P

@Danu Shutdown-restart

@KyleKanos Doesn't work

Windows 8.1?

10

:\

they inverted my scrolling

why would you get windows 10

That unholy abomination

4:21 PM

@Slereah It's a significant upgrade over 8.1 imo.

@Danu That's customizable, isn't it?

Well that is kind of my point

@KyleKanos ...the program that controls that setting refuses to open. That's the problem.

Stick with windows 7

@Slereah You're falling behind kiddo

4:22 PM

At least you won't get Windows 10's insane privacy policy :p

The program is actually running, but its window won't open.

lack of privacy policy

Alt-tab to it?

@Slereah Meh, it's not that hard to disable most of the problematic things

@KyleKanos No window to alt-tab to.

The program is running but it won't open a window.

Is it in the Task Manager & you need to right click to open it?

4:23 PM

@KyleKanos For instance.

(doesn't work)

Okay, next step is a full wipe of Win10 and replacement with Linux Mint 17.2

@KyleKanos Seems legit.

You don't run into those types of issues on a good OS

@KyleKanos But I've been a bad boy. I think I deserve it.

Well stop being bad

4:26 PM

The first derivative gives you the force of gravity. The second derivative gives you the tidal force, which relates to the spacetime curvature. Your pencil doesn't fall down because "spacetime is curved in the room you're in", but because it's "tilted". See the tilted light cones in this Stanford article. There's flat as in not curved and there's flat as in horizontal.

The Principle of Equivalence refers to an infinitesimal region. To a region of no extent. To no region at all. It's a principle, not an exact correlation. Continually increasing your speed through homogeneous space is like standing still in inhomogeneous space (which is modelled as curved spacetime), but it isn't exactly the same.

@KyleKanos Solid, but boring, advice

It's advice my Freshman-year-of-high-school gym teacher gave to us

And ever since you've been a good boy?

@KyleKanos Because 12 (?) year old kids are such bad-asses :)

13 & 14

4:32 PM

Below standard buttocks

Bob Dylan - Ballad Of A Thin Man (Vinyl rip)

5:06 PM

Win 8.1 master race

I have random touch screen stuff

5:19 PM

10 ftw

Pfft, peasants still relying on Microsoft

lel

5:44 PM

@Huy try DX10 yet

12

how to think with sense? — hilbert 47 secs ago

Good question.

2

Haddaway - What Is Love (Music Video)

I had just got rid of that earworm after I posted it two days ago. Thanks for that!

@ACuriousMind Listen to the song above it :D

@ACuriousMind it never went away for me

6:00 PM

19

Q: Why do we not have to prove definitions?

I am a beginning level math student and I read recently (in a book written by a Ph. D in Mathematical Education) that mathematical definitions do not get "proven." As in they can't be proven. Why not? It seems like some definitions should have a foundation based on proof. How simple (or intuitive...

definition philosophy

...really?

Why do we not define theorems?

@Danu Brilliant follow-up question, it'd certainly get HNQ status, too!

@ACuriousMind Blurgh

6:22 PM

@Slereah as Danu mentioned it is easy to disable the privacy settings

but even so

how many people, in an everyday basis, even cares about the privacy settings?

I care about privacy settings

I feel like a lot of people jump on these "omg privacy violation" bandwaggons but in everyday life just don't give a shit anyways

I couldn't care less; I use like 5 websites frequently anyways

I don't care about the privacy settings.

I don't really care either.

Hmm, (Upvote/Downvote) ~ 12.3271 + 0.0005 * Reputation

6:30 PM

Windows 10 seems pretty egregious, though

The terms include your text and mike input

Ok m8

Slereah just jelly ;D

@KyleKanos ?

Bored waiting for the "You're hired" call, so I decided to look at the Up-Down vote ratio as a function of reputation

@KyleKanos So far off for me lol

~1.2

6:34 PM

@KyleKanos Votes received or votes given?

Oh... received would change the story

Given

@Danu Well it's a good thing it's based on all users with Rep > 10 and Vote Count > 5

user54412

Whenever I plot SE query data, I amuse myself by looking at the outliers and trying to identify the users behind them.

Hm, I'm at 0.43, your function is garbage :P

Again, based on (almost) ALL Users

Not 3 people who frequent this chatroom

6:36 PM

I think that's such a terrible thing, though.

more than 12!

Guess who has a nearly 1000 up/down ratio!?

@KyleKanos Rod Vance

Correct

ding ding ding

lol

6:37 PM

7000+ up, 7 down

I wonder what the 7 posts he voted down were. Spam?

That's just mental

If it were spam, it'd probably be deleted

My guess is he slipped and never bothered to check

Hahaha

I'm also catching up to @ACuriousMind when it comes to total number of DVs

6:42 PM

You're the fourth-most prolific voter of all time, btw.

I'm leading for this year, though

I'm so lazy, lol

You are

Woah Ron Maimon is from NYC

represent

I'm also 3rd most active editor

@Danu 311 votes in 8 months is lazy indeed :P

6:44 PM

Wait...you've not even gotten Electorate badge?

Nevermind

@ACuriousMind How are you getting 311 votes?

@KyleKanos physics.stackexchange.com/…

Oh, for the year

Got it

Compared to our >2k....

This is the query by the way: data.stackexchange.com/physics/query/350959/…

user image

And that's the log-scale plot

(x = rep, y = vote ratio)

The lowest ratio is 0.02 held by a user with 131 rep...

Oooh, I can see me :D

user54412

@KyleKanos And you fit a linear regression to that cloud?!

4

@ChrisWhite Well duh, what else would you use?

6:50 PM

To first order, it should be fine :D

user54412

@ACuriousMind You're a proud member of the convex hull

It came back with P(>t) ~ 0.003

@ACuriousMind Come for the physics, stay for the irrelevant, ridiculous chat.

95% confidence interval was (0.000,0.001) for the slope, (8.870, 15.785) for the intercept

This is the only place in the internet

where a conversation like this could prove so fruitful

6:52 PM

@Danu Haha

131 2 8

That's the user with 131 rep, 4 upvotes & 258 downvotes

Niiiice

@ChrisWhite Isn't that how astronomy works? Huge cloud -> fit with linear regression

I think that's the auto-delete triggering guy from math.SE, he changes his name a lot

It's that guy who cause the MASSIVE debates on math right?

@ACuriousMind This

6:54 PM

@Danu Great minds think alike.

Well if he's triggering auto-deletes, he can keep it up

@ACuriousMind think of any UH trivia yet

In the "new profile", where can we see votes?

Go to the Activity page

@Danu By going to the "old one" by clicking on "activity" in the top left

6:56 PM

@ACuriousMind The German approach

I was sorta thinking of answering Emilio's post about CSS changes (think it's still unanswered) about asking for widescreen, but I imagine it'd be shotdown by SE team

I WANT WIDESCREEEEEN

My PSE layout is royally screwed

I told you how to hack it, but I'd rather like a more uniform approach to it

"They say great science is built on the shoulders of giants. Not here. At Aperture, we do all our science from scratch. No hand holding."

I mean, how many times are there posts in which the equations stick out of the box

6:57 PM

@KyleKanos I'm too noob to actually do that

@0celo7 You could talk to him about the ice troll that spits liquid helium roaming the premises.

@Slereah Love that game

@Danu well if you don't want to toy with it some more, there is a Firefox extension that you can simply install w/o any trouble

@KyleKanos I'm invested in Chrome :P

Emotionally and bookmark-wise :P

@ACuriousMind on a scale of BS to ORNL ca$h, how would you rank that

7:00 PM

Chrome bookmarks can be easily imported into the superior web browser, Firefox

Why is firebox better

@ACuriousMind there's a small error in your answer to that bountied ridiculous weak force question.

I'll edit to correct it

@0celo7 Because it features an extension that allows for widescreen stackexchange while Chrome doesn't

@KyleKanos Hmm, okay.

I can give it a shot, maybe

@KyleKanos So, how? :D

Export bookmarks to HTML file (in Chrome)

7:14 PM

Does Firefox have an iOS app?

Bookmarks -> Import & Backup -> From HTML file (in Firefox)

Oh, upon installation it already imports errrrything

lol

firefox account? :\

I don't have one

Okay, nice, so it's not necessary?

Apparently not

You could always just create a dummy account and never ever use it

7:17 PM

No mobile app, Firefox is trash

Mobile apps are kind of trash

So, bring on the add-ons, @KyleKanos!

@Danu I need cross-platform tab syncing

@Danu addons.mozilla.org/en-us/firefox/addon/…

@Danu Thanks, I caught another silly mistake earlier, but overlooked this one

@KyleKanos Thanks. It's pretty nice. What other things make firefox so good, in your opinion?

7:25 PM

@Danu Comes native to Linux & is in the repos; Chrome isn't.

I only answered that because it deserves at least one correct answer :P

@ACuriousMind Just admit it; you want that bounty!

Oh yeah, @Danu: Thanks for the generically edit

@0celo7 It's total BS, if you can't tell :P

I had meant to punch something like that in there after Timaeus mentioned something about it in the comments

7:26 PM

@Danu Yeah, I'm all about that fat bounty

@KyleKanos No problemo

@ACuriousMind Change a few letters in that last word and I concur

@KyleKanos ^^

Thought so :D

OP to that question, of course, made my answer kinda useless with his edit requesting for examples

@KyleKanos Well, obviously you'd use Chromium, not Chrome, which is in every repo.

@alarge Chromium's inferior to Chrome though

I think

Meh, Firefox is pre-installed on most OSes

I do have Chrome installed on my Linux box, and I use that for Twitch & YouTube (because my Firefox is all screwy and doesn't play videos)

7:31 PM

@KyleKanos Damnit Kyle! :P

It's a Linux problem

The whole Flash & Linux support system is practically non-existant

@KyleKanos If by inferior you mean that Google can't track you as easily, then sure.

I have an android phone, pretty okay with Google tracking everything about me

@KyleKanos Essentially the only flash you have for linux is pepper in chrome. Which I guess you can take and drop into firefox as well.

All hail ~~Skynet~~ Google!

7:34 PM

@alarge Yeah, Chrome installs it correctly, I'd have to do some minor reconfiguring of the Firefox flash to point to pepper instead of whatever it is pointing to now

Kyle, a good adblock for firefox?

Just use NoScript

@KyleKanos Well pepper is part of chrome and they are the developers, so it'd be strange if chrome didn't install it correctly.

Takes a bit of allowing the sites you want to allow, but I don't see any ads with it without any dedicated "adblocker"

@Danu ghostery, same as with chrome

7:36 PM

@KyleKanos Nice. What do you use for automated SE comments?

@Danu addons.mozilla.org/en-US/firefox/addon/se-autoreviewcomments

Oh k just like chrome

Would you happen to know whether there's a tex for facebook thing also? ^^

Don't use FB, so no clue

Fair enough.

...you use TeX on facebook?

7:47 PM

Lol

@ACuriousMind you seriously have an ice troll on campus?

@0celo7 No, but we do have a prof who's called "the Yeti" behind his back

Because he works on low temperature physics and has white hair

Nice

@ACuriousMind when did they start calling him that

Also the ice troll story is better

@0celo7 How should I know that?

I'll stick with that

@ACuriousMind how do you know any of the random stuff you know

7:58 PM

::falls silent for introspection::

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