Anyway, our hour is up. Let's keep this in mind for future discussion. See everyone in two weeks!

We will discuss the replacement for the homework policy next time. (Among other things)

@vzn Why global regularity for Navier-Stokes is hard - Terence Tao

@DavidZ Not done yet?

It won't be done for a while

@DavidZ I think there should be a meta chat room for that

@HariPrasad Why

@ACuriousMind This room should be for physics in physic.SE and not about the site itself or any modifications or policy change.

It has long been a fact that there are no topic restrictions in this room, we talk about whatever the users of physics.SE want to talk about

Yep, and in particular, all matters relevant to the site.

If you'd like to make a room for physics discussion only, feel free to do that.

And it's not as if this room was so busy that an occasional meta discussion would noticably disturb any more relevant discussions :P

@ACuriousMind but such meta discussions should be avoided for chat events

@HariPrasad Why

A chat session is exactly the correct moment for such meta discussion because a higher-than-average number of users can be expected to participate

@ACuriousMind Yes but people seek more physics here, especially those who cares about physics and not about the site.

@HariPrasad [citation needed]

@ACuriousMind Sorry. Not available.

Yeah, that's why I think "people seek more physics here" is simply false. People come here to chat. Sometimes about physics, sometimes about the site, sometimes about something totally different. And all that's fine.

@ACuriousMind fine for a few.

users meta-chatting about whether chatting should be restricted to meta or not...

lol

@ACuriousMind Citation: chat.stackexchange.com/search?q=talk+physics&room=71

@HariPrasad lol ctrl-f "John Duffield": 29 times Anybody want to talk physics?

@AccidentalFourierTransform lol lol lol lol lol

I'm vetoing that star

It worth noting that I've seen serious physics be discussed by a few users interleaved with social fluff from a different group of users more than once.

It doesn't seem to bother anyone, and it sometimes results in cross-fertilization of the two lines of inquiry.

I suppose that if the fluff was coming many times faster than the physics it would make the serious discussion hard to follow, but that hasn't been what I've seen.

chat helps to answer questions that are too short for being a question on the main site.

My approach has been basically what dmckee said, that if multiple conversations are interfering with one another, physics discussion gets priority, but that hasn't really been an issue

@ACuriousMind Heya

@BernardMeurer Hey

Hi everybody

@ACuriousMind Tell me something I don't know :)

Dan! Hey man!

Yo

@BernardMeurer The isomorphism classes of oriented circle bundles over a base are given by classes in the integral second cohomology of the base.

::understand some words::

@ACuriousMind Day by day i'll have all your knowledge :v

@BernardMeurer Yesterday I gave you pig orgasms, today I gave you cohomology classes. WHAT WILL TOMORROW BRING?

@ACuriousMind That's the thrill!!

interesting question from stack Overflow that includes physics: Compiling an application for use in highly radioactive environments

@AccidentalFourierTransform : as you can can doubtless deduce, the answer is no.

@ACuriousMind easy way to see it?

Btw I proved the "Five Lemma" today and it's a thrill!

Sudoku for mathematicians ;D

@Danu The sequence $0\to\mathbb{Z}\to\mathbb{R}\to\mathrm{U}(1)\to 0$ (as constant sheaves on $M$) is exact and applying the sheaf cohomology $H^{\bullet}(M,-)$ yields the long cohomology sequence $\dots\to H^1(M,\mathbb{R})\to H^1(M,\mathrm{U}(1))\to H^2(M,\mathbb{Z})\to H^2(M,\mathbb{R})\dots$. But due to partitions of unity, $H^n(M,\mathbb{R})$ vanishes for all $n$, so $H^1(M,\mathrm{U}(1)) = H^2(M,\mathbb{Z})$. That $H^1$ classifies bundles follows from the cocycle construction of them.

@Danu lol

hey guys

@user507974 o/

@user507974 hey guy

i feel so stupid

the only question on a test in an engineering class i left blank was a question on quantum tunneling, cuz i skipped that page

and it was a REALLY easy question

@JohnDuffield welp, they dont want to get in trouble! youre a bad influence ;-)

@AccidentalFourierTransform : not me. I'm one of the good guys.

@ACuriousMind Diagram chases are interesting

(as brain exercises)

im trying to figure out the name for something guys

@user507974 Let's name it Walter

basically its a name of methods that i guess are some kind of convolution

@user507974 then definitely Walter

you insert a signal and that is some how used to get much more precise measurements

@user507974 Uhhh...could you be a bit more precise?

@ACuriousMind well let me see if i can find an example

In other news TERRY TAO NAO

Terry Tao's website has no mention of physics.

Find physics: no matches found.

@ACuriousMind I think this device was kind of inspired by the general idea journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.083002

terrytao.wordpress.com/2008/09/27/what-is-a-gauge

terrytao.wordpress.com/2012/12/29/…

terrytao.wordpress.com/2007/12/28/einsteins-derivation-of-emc2

listed a wrong paper if you clicked in the first five seconds, correct one is (journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.083002)

terrytao.wordpress.com/2009/11/26/…

@JohnDuffield Terry loves physics :D

@AccidentalFourierTransform : oh good, a page that does mention physics. Pity it's from 2008.

Note this: "(Actually, this is only half of Maxwell’s equations, but the other half are a consequence of the interpretation (*) of the electromagnetic field as a curvature of a U(1) connection. Thus this purely geometric interpretation of electromagnetism has some non-trivial physical implications, for instance ruling out the possibility of (classical) magnetic monopoles.)"

@DavidZ Your response on (physics.stackexchange.com/questions/253754/…) still doesn't explain why the question is off-topic. It seems perfectly reasonable for a student (e.g.) to wonder why, if the universe is often described as finite (even only for observability reasons), it's 'boundaries' can't be used as a reference frame. The CMB, in many ways, is such a boundary, and it can be used as a reference frame. Valid question w/ valid answer.

@DanielSank oh hi

@DilithiumMatrix I don't believe what you're saying is true at all. The question shows a complete lack of knowledge of even the basic principles of the subject area being asked about - in particular, I doubt that the asker knows enough to understand a reasonable answer. Those sorts of questions are not suitable for this site.

I could be wrong about the nature of the question, and in that case I trust the community will override my vote.

@user507974 Ojai

Heya guys, anyone familiar with Clifford/Geometric Algebras?

(well, and applications to EM)

@GuillermoAngeris Somewhat. Just ask your question, we'll see what we can do

@GuillermoAngeris I'll take a summer school on it this summer---perhaps ;D

(I'm familiar with the basics of Grassmann algebra somethingsomething, a little bit)

Great! @ACuriousMind @Danu , so I've been working under the GA framework for EM using constant dielectric constant (assuming non-permeable media, throughout)

This is clear, right, as the transformation between Gibbs' vectors and Clifford's algebra is straightforward

It provides a lot of niceties for the invariants, all of which take obvious and relatively simple forms

Now, the question is, when we assume nonuniform media, we can't perform a lot of the reductions

And it seems that GA in this case is mostly an ad-hoc framework to deal with subtleties like the cross-product

So, I was wondering if anyone had any resource exploring EM using GA in materials with non-uniform permittivity

Ugh, I may be able to do GA, but I have no clue at all how dielectrics work. My EM consists of spherical cow charges in vacuum ;)

Jaja, I totally feel you. This is what happens when you start taking applied physics...

Not so much the study of dielectrics, but simplification of the mathematics

Instead of introducing imaginary E,B fields, we just introduce a complete F field

Which has a scalar and bivector part with 90 phase, as we would expect, if we take exp(I\omega t)F_0(q)

So, then we don't have the arbitrary imaginary coefficients to deal with. Additionally, all of the equations take a simple form

As do the boundary conditions

Yeah, like complex fields

Yessir

Spatially varying scalar

The worst of both worlds would certainly be a lot to deal with...

@GuillermoAngeris I, too, don't know anything about the more applied side of EM :/

Hallo

@Danu Eh, it's totally okay

@GuillermoAngeris : please ask a question on the main site. Meanwhile it sounds like you're talking my language. But don't forget that it takes two to tango, and if epsilon varies, what you've got isn't an electromagnetic field. Because c = √(1/ε0μ0) and "the curvature of light rays occurs only in spaces where the speed of light is spatially variable". It's a gravitational field.

@JohnDuffield This question is already on the main site. And, it doesn't require having a gravitational field if we make the approximation that the material itself changes the speed of light (which is mediated by non-classical processes, fine, but it's an approximation)

@user507974 interesting research, do you have some connection to it?

What's going on why's everyone obsessed with Terry Tao?

↑ who was it in this room that said that they werent interested in number theory & that it was boring? ... maybe will not totally like this guest

@vzn ::raises hand::

@BernardMeurer TT was suggested/ recommended as a guest chatter/ invitee

Tao is totally awesome though, I've several of his blog posts bookmarked

@ACuriousMind just for you, check out his fabulous post on collatz conjecture :P =D

Oh that'd be neat

TERRY TAO NAO

wild factoid: Tao recently is the 1st yr winner of a $3M millenium prize, and (so far) also appears to have said absolutely nothing )( about it in his blog or interviews

Tao just waves his hands then he ends up merging like 5 areas of math

@bolbteppa well, to me it does not seem that he waves the hands so much...

@vzn ?

My favourite example is here mathoverflow.net/a/17906/38721 you've got your sobolev spaces, $L^p$ norms, quantum mechanics, calculus, other insane spaces, stunning

@bolbteppa well, it's essentially just analysis... ;-P

@bolbteppa Uh, where is the quantum mechanics there, and why do you think that Sobolev, L^p and calculus are worthy of being listed as distinct objects?

and by the way, the uncertainty principle is intended there as a concept of analysis rather than of quantum mechanics

@yuggib ? lol does anyone around read newspapers anymore?

@vzn link? I'm not aware that the millennium prize is worth 3M (I knew 1M), and I doubt he solved three questions

@yuggib oh! sorry. freudian slip. its called the breakthru prize and yes a single award is worth (a staggering) $3M ie 3x nobel, abel, or turing prizes alone

That was two years ago, you have an interesting idea of "recently" :P

@ACuriousMind "recent" in "evolution of science/ math" pov :)

fyi the Tao link posted earlier on the navier stokes eqn is a millenium prize problem.

claymath.org/millennium-problems/navier%E2%80%93stokes-equation

Do you really think you need to mention here that Navier-Stokes is a millenium problem? :P

That's something both mathematicians and most physicists are certainly aware of

@ACuriousMind maybe you hadnt noticed, youre more well informed than very many :)

on the topic of edu level of "unwashed masses", reminds me of this recent Computer Science hot question

Oh no, flattery! I am powerless against that! ;)

@ACuriousMind re publicity/ awareness, actually it would seem that large segments of the public are not really much aware of the "Big" science prizes eg those just mentioned by me... (not to even say anything about basic science/ math "literacy") & this chat room & SE in general has an extremely wide variance in bkgs... (part of its appeal...) compare this to eg big sports prizes eg world cup etc...

$\sin x /x$ is not defined at zero, right?

@ramsay No, but you can continuously extend $f: \mathbb{R}-\{0\}\to \mathbb{R}, x\mapsto \frac{\sin(x)}{x}$ to a function $\bar{f} : \mathbb{R}\to\mathbb{R}$ by setting $\bar{f}(0) = 0, \bar{f}(x) = f(x)$ for $x\neq 0$.

two minutes

@ACuriousMind but that still make $\frac{\sin (x)}{x}$ undefined

at zero i mean

@ramsay Yes. My point is that the limit $\lim_{x\to 0}\frac{\sin(x)}{x}$ exists, which is the closest you can get for a value of that fraction at zero.

sounds good

if $f(x)=\frac{\sin x}{x}$ then $f(0)=\infty$ , right? @ACuriousMind

@ramsay ...didn't you just tell me that $f(0)$ is undefined?

Why are you now assigning a value to it (even if it is infinity)?

Being undefined and being infinity are two different things.

infinity here means undefined :p

how to make braces very large in mathjax

$\biggl(\biggr)$ = \biggl(\biggr)

But if you just want them to scale to what they enclose, use \left(\right).

no no curvy one

{

Just replace ( by \{, [ or whatever other delimiter you want to use.

thank you

Hey one more question does the derivative of sin x / x exist at 0 (i am getting confused with my book sorry)

If a function is not defined at a point, how could its derivative?

Wth! I am idiot

And my book is stupid

However, you might ask whether the extended function that is zero at zero and $\frac{\sin(x)}{x}$ everywhere else is differentiable at zero. That's probably a nice exercise to figure that out.

@vzn , it was a speaker who came to talk at my department. I am trying to find a general description of methods like this. Here he introduced Bloch oscillations to suppress phase shift, in conjunction with a PI control loop. I am basically looking for a starting point on understanding how this improves sensitivity.

If I get a good grasp on the idea, I want to try implementing similar ideas in other systems

@user507974 yeah it looks very intriguing; do you work in QM or optical experiments?

it reminds me of "weak measurements," do you think any connection?

do you know of an arxiv paper? (ie nonpaywall?)

@vzn for this particular paper?

arxiv.org/abs/1410.8486

I think, it had the same title

yep, they match up, slightly different formatting but same content

@user507974 thx am gonna look it over more carefully

thanks