The h Bar

Hey guys, small question. When calculating electric dipole moments, is it valid to place the reference point on a charge. I.e. consider q1 and q2 separated by r, the dipole moment is given by q1(r1-r)+q2(r2-r), is it ok to put r such that it is located at q1 (or q2)?

John Rennie

A multipole expansion can be calculated about any point, in the same way that angular momentum can be calculated about any point. The point you choose for the calculation is generally selected for its physical relevance.

Secret

I see, thanks

user228700

6:40 AM

@JohnR: Morning :-)

John Rennie

Morning :-)

Indeed it's a good morning as I've just finished that big chunk of code I was working on.

dmckee

Painfully true:

smbc-comics.com/comic/why-do-you-want-to-work-here

John Rennie

Although there is considerable testing yet to be done.

user228700

@JohnRennie Nice! :-)

John Rennie

@dmckee In my later years at Unilever I used to do preliminary interviews for applicants. I never asked them why they wanted to work for Unilever :-)

paracetamol

6:43 AM

@JohnRennie Why not?

John Rennie

@paracetamol because it's a stupid question

paracetamol

._.

John Rennie

@paracetamol the answer is (if the candidate was speaking truthfully): I need to feed my family and pay my mortgage

paracetamol

Say @JohnR , the other day...didn't you say there was no such thing as a "stupid question"?

Apr 18 at 11:11, by John Rennie

@Fawad I disagree, though I probably wouldn't use the words good and bad. A question is useful if you will learn something from the answer. If you're just going to write that answer in your homework book then it wasn't a useful question.

;D

John Rennie

@paracetamol that is not the same as saying there are no stupid questions.

paracetamol

6:50 AM

...true...

But close! :D

John Rennie

In fact I was explicitly disagreeing with the claim that there no bad questions.

paracetamol

Oh.

John Rennie

Fawad said:

Apr 18 at 11:10, by Fawad

@paracresol sorry but no question is bad

paracetamol

Yeah...

John Rennie

And I said: I disagree

paracetamol

6:51 AM

Yep...

John Rennie

Anyway I need to work now for a few minutes. Back in a bit ...

paracetamol

Oi! How the heck did something like this happen without me! :(

17 hours ago, by Physics Guy

Better than being a christ (or whatever)

@JohnRennie o/

17 hours ago, by Jim

@PhysicsGuy Please avoid conversations that focus on bashing religious beliefs. This is Physics.SE, not Offensive.SE

:(

Slereah

user466672

8:10 AM

Hey guys

I have a problem with xrd

Can anyone help

2

Q: Powder diffraction - spikes on diffractogram

When a powder sample of a crystal is used to known its crystalline structure, on the diffractogram there are spikes corresponding to the constructive interference, at specific angles, by different crystallography planes. Why each spike is associated with a single family of planes? If the particl...

x-ray-crystallography

Slereah

9:01 AM

Reeb says the branching real line has two non-isomorphic differential structures

End my life

user228700

9:15 AM

Apr 13 at 7:08, by John Rennie

Given what I've read recently I think I would be a bit concerned if my daughter attempted the trip on her own ...

user228700

@JohnR: Eh, do u really think I should wait till I find someone?

John Rennie

@Kaumudi.H Your father is the best person to judge.

user228700

Right, but he only said "Maybe you shouldn't go on your own" :-/

John Rennie

All I know is what I see in the UK papers, and I doubt that gives a complete picture.

I'd guess that maybe means I would be worried about you but I wouldn't stop you

user228700

That's what I thought as well!

user228700

9:18 AM

Eh. I wish he'd let me go. I'm going to push the matter once more when he's around.

Slereah

"A differential structure of class $C^r$ on a manifold $M$ is called regular if for every $C^r$ function defined on the neighbourhood of $x \in M$, there exists a $C^r$ function $f'$ defined on $M$ such that $f$ and $f'$ coincide on a neighbourhood of $x$."

One of the differential structure of the splitting line is not regular

The horror

Reeb says a function is "of rank 1" (de rang 1)

What does it mean

it's no matrix

"For $M$ a manifold with a regular differential structure of class $C^r$, if a $C^r$ function $f$ is of rank $1$ at a point $x$ of $M$, it is also of rank $1$ at any point $y$ not separated from $x$"

"In mathematics, the rank of a differentiable map f : M → N between differentiable manifolds at a point p ∈ M is the rank of the derivative of f at p."

Ah, there we go

ACuriousMind

10:12 AM

@0celouvskyopoulo7 Yes, by teaching as much of it as possible in the finite-dimensional setting.

AccidentalFourierTransform

10:53 AM

-2

A: Gravitational waves as dark energy?

Gravitational radiation is simple general orbital of electron of dark mattere 313 in general garvitaional galaxies orbital 3559. Is the relativity before the Big-Bang in quantum atom 263.

@ACuriousMind ^ please :-P

John Rennie

@AccidentalFourierTransform Hmm, I clicked the delete link and it was immediately deleted. It now says "Deleted by community". I'm not sure what happened there.

AccidentalFourierTransform

what happened here is that an AI was faster than the other one

Secret

[Trigger happy] I want to see a crank post downvoted every second for 15 seconds

(this is impossible, however)

AccidentalFourierTransform

it used to be possible but now He is suspended so...

Slereah

Heyoooo

Do you think he will come back after his suspension

he's pretty old, he might die

AccidentalFourierTransform

10:59 AM

of course he will

from time to time he wanders over here

we have our very own chat ghost

Secret

(To time travel researchers in h bar, I have news for you in a few seconds. Please wait for a few moments ago)

AccidentalFourierTransform

he observes us from the shadows, unable to interact with the alive

he's not dead, yet he's not alive

Slereah

His blood boils whenever we discuss of GR

ACuriousMind

@JohnRennie I spam-deleted the user which also deletes all of their posts.

John Rennie

@ACuriousMind Ah. That's a shame - I was wondering if I had acquired a new superpower :-)

Slereah

11:04 AM

Hm

ACuriousMind

@AccidentalFourierTransform No AIs here :)

Slereah

trying to think of a single Killing vector with a flow that doesn't generate hypersurfaces

Take the torus

Take a single curve that is a space filling curve

The tangent vector to that curve will be a vector field over $M$, but will it be 1) smooth 2) a Killing vector field

I think it wouldn't be smooth, for a start

Possibly not even continuous

Wait

On the Clifford torus, there is a space filling curve that's just a single line of the same slope over the fundamental polygon

The vector field would be perfectly well defined

If we take the Lorentz torus and make the slope $< 45°$ that's a Killing vector field

So Geroch's argument doesn't apply on it, although in that case, we still have $ds^2 = - g_{tt} dt^2 + g_{xx} dx^2$

So not the best counterexample

Although of course the problem is that this spacetime also has Killing fields that don't have space filling integral curves

It slightly bothers me that such a common theorem of GR is not necessarily true

ACuriousMind

Five answers to this question and none correctly explains why $\mathrm{SU}(2)$ appears in quantum mechanics. Sigh.

Slereah

11:20 AM

something something complex Clifford algebra

John Rennie

@ACuriousMind I think the group names are beautifully formatted though

Yashas

8

Q: Functional difference of Benedict's solution and Fehling's solution

Chemically, Benedict's solution and Fehling's solution are very similar, with copper sulfate as a source of copper (II) ions, sodium carbonate and sodium hydroxide respectively as alkalis, and sodium citrate and sodium potassium tartarate respectively as chelators. They are also both used to test...

analytical-chemistry

AccidentalFourierTransform

@ACuriousMind I tried. Is my answer correct?

Slereah

11:38 AM

IIRC Hawking has an imprisonned timelike curve spacetime that is causal but I don't think it has any time symmetry

ACuriousMind

@AccidentalFourierTransform Yes

AccidentalFourierTransform

cool

ACuriousMind

I was already writing my own, though, spreading the gospel of projective representations seems to be my shtick here ;)

AccidentalFourierTransform

6 answers

if the question gets a couple of upvotes, it will probably end up in the hot list

ACuriousMind

It would be a better HNQ than many of the other PSE questions that land there, imo :P

Bernardo Meurer

11:42 AM

@AccidentalFourierTransform @ACuriousMind I need physics halp

AccidentalFourierTransform

shoot

Bernardo Meurer

AccidentalFourierTransform

find $x$?

Slereah

It's right there on the diagram, above the rectangle

Bernardo Meurer

The oscillator in the figure is composed of an ideal spring of constant $k=20N/m$, by a viscous dampener that applies a force given by $F_x = -5.0 v_x$, and by a mass $m$ that slides over the plane with no friction.
a) It is intended that the movement is (translation warning) oscillating and sub-critically damped. What conditions must $m$ obey?

b) Consider that $m=0.5 Kg$. What will be the oscillation's "period"

c) If the oscillator is pushed from it's equilibrium position, and then released, after how long will the amplitude reduce to half?

How the hell do I even do this?

AccidentalFourierTransform

11:46 AM

Harmonic oscillator

In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: F → = − k x → {\displaystyle {\vec {F}}=-k{\vec {x}}\,} where k is a positive constant. If F is the only force acting on the system, the system is called a simp...

the condition is $\zeta<1$

Bernardo Meurer

The hell is $\zeta$

AccidentalFourierTransform

the damping ratio

its defined in the wiki page

actually, all you need is to compute $\omega_0$ and $\zeta$, where these parameters are defined in the wiki page

once you have them, you just use

$$x(t) = A \mathrm{e}^{-\zeta \omega_0 t} \ \sin \left( \sqrt{1-\zeta^2} \ \omega_0 t + \varphi \right)$$

Bernardo Meurer

The heck is that?

AccidentalFourierTransform

the solution of the equations of motion

Bernardo Meurer

Oh jesus

AccidentalFourierTransform

11:50 AM

ok, first: calculate $\zeta$ from the given data

Bernardo Meurer

Ok

ACuriousMind

@BernardoMeurer Um...did you even look at damped harmonic motion before deciding you have no idea how to solve this question?

Bernardo Meurer

@ACuriousMind I don't even know what that is

I have an exam in 6 hours

I know nothing

all of the material is in Comic Sans, I can't read it

ACuriousMind

I can't quite fathom you being asked to solve this exercise without having been taught damped harmonic motion.

Bernardo Meurer

My only notes are, literally, drawings of spinning potatoes

Slereah

11:52 AM

don't let your oscillators get damp

Bernardo Meurer

Apparently that was once and exercise

Slereah

remember to cover them before leaving

Bernardo Meurer

Okay, shit

I have an analysis class

I'lll be right back

sorry

AccidentalFourierTransform

bye

Secret

I wonder if this touched upon how Danielsank often reminded us that complex numbers are often related to dampening processes

Polyaromatic hydrocarbon in forming spin liquids

0celouvskyopoulo7

12:21 PM

@ACuriousMind that's my reaction to all of his school work

Secret

12:51 PM

~~~Beginning transmission~~~

@Slereah Are you aware of this CTC spacetime?

Slereah

Yes, I am

Secret

The arxiv appears to be slightly more up to date despite it is dated 2013 while the actual article is submitted on 2017

Slereah

people do update

Secret

No, I mean the (seemly) more updated version is on 2013 while the less update version is submitted to the journal on 2017

(or perhaps I was wrong, cause in the journal the diagram is actually nicer)

But anyway...

So, out of all the CTC spacetimes, is this seemed more possible than the others such as godel, tipler, etc.?

(even though the WEC is still violated)

The singularities at the kink do look worrying though. What would happen to that particular null curve that hit those singularities at the top and bottom of the inner wall of the bubble

0celouvskyopoulo7

Oh no. I had a proof I mind for something at 1am and now I lost it

Secret

1:06 PM

Another more practical oriented question:

We knew that (the cauchy horizons of) CTC spacetimes tend to impose extra constraints in a cauchy surface at some time in the past.

That means, some events in the cauchy surface will have correlations that otherwise not possible if the CTC region is absent. But then, even if we do found a correlation, is it even possible to tell apart on whether they are due to the information within the cauchy horizon of the CTC, or just they happened to have those correlations and does not have any "future" cause?

Slereah

The big problem of CTCs is mostly the divergence of quantum fields

Emilio Pisanty

chat team, I've got a project for you

The top comment on the MSE proposal definition page reads as follows:

> This was initially proposed by edanm on meta.SE: meta.stackexchange.com/questions/5534/proposal-for-mathematics-se – Dan Dumitru Jun 3 '10 at 23:04

However, MSE/q/5534 leads to this question, with a different title:

6

Q: Does the Stack Overflow software have a name?

Does the software that runs the Stack Overflow sites have a name? I've always thought that, just as Slashdot's software is named "Slash," Stack Overflow's software should be named "Stack" or "The Stack."

discussion stackoverflow so-name

Any ideas on what's going on there?

Yashas

74

Q: Brief outage planned for Wed, May 3, 2017 at 0:00 UTC, 8pm US/Eastern (like a fire drill for computers)

MicroVersion: Planned service degradation: All Stack Overflow/Stack Exchange sites read-only for 20 minutes on Wed, May 3, 2017 shortly after midnight UTC (8PM US/Eastern). If you blink, you'll miss it. Short version: There will be a service degradation for up to 20 minutes shortly after midnig...

discussion featured announcements

Emilio Pisanty

@Yashas boring

=P

up to 20 min

ACuriousMind

@EmilioPisanty That comment likely refers to the old meta.stackexchange for the 1.0 SE sites. The current meta.SE was not around back then.

I have no idea what happened to the old meta.SE posts, though

Emilio Pisanty

1:22 PM

@ACuriousMind You mean, an SE-1.0-specific meta that was separate from meta.stackoverflow?

Xasel

Hello Everybody!

AccidentalFourierTransform

@Yashas the top answer to that question has gathered a ridiculous amount of upvotes

Emilio Pisanty

the mother meta used to be at MSO and split in two in 2014

Yashas

@AccidentalFourierTransform It's yours, isn't it? :P

Xasel

I am going to resume the Matrices/LA course and just for the inspiration for subject

ACuriousMind

1:22 PM

@EmilioPisanty Yes. For evidence of its existence, see the second paragraph here

AccidentalFourierTransform

@Yashas yep

Yashas

And there is a comment from the Review Queue xD

Emilio Pisanty

@ACuriousMind ah, that's mighty interesting

Xasel

I seek few example problems you guys must have encountered in your experience as physicist which are counterintuitve to solve thorugh matrices at first glance but are well tackled with the help of MAtrices/LA?

LA = Linear Algebra

Yashas

This does not provide an answer to the question. If you have a new question, please ask it by clicking the Ask Question button. Include a link to this question if it helps provide context. Just kidding :-P — rand al'thor 14 hours ago

@AccidentalFourierTransform :D

Emilio Pisanty

1:25 PM

@ACuriousMind that presumably explains this, then

AccidentalFourierTransform

@Yashas .-.

ACuriousMind

@EmilioPisanty Yeah

Emilio Pisanty

It's damn hard to read on the wayback machine, though

all the formatting's been stripped bare

web.archive.org/web/20100418194227/meta.stackexchange.com/…

Secret

Hmm...

> Closed causal curves are pretty bad in field theories. There are many configurations in which a field, when confronted with a closed causal curve, will just circle round and round, getting blue shifted with each cycle, and just diverging to hell. This isn't necessarily too terrible, because this may only happen for some configurations of the field.

Is there no known mechanism to make the blueshift bounded, like how when we get into relativity, velocity will be bounded above by the speed of light thus acceleration will only asymptote the velocity towards that limit?

I mean thoeretically, if such mechanism exists, then the field can be blueshifted forever and still be bounded in energy momentum density

Emilio Pisanty

@ACuriousMind done, thanks a bunch.

Secret

1:36 PM

arxiv.org/abs/1311.0268

ACuriousMind

@EmilioPisanty Heh, are you attempting to become the SE chronicler? ;)

Secret

So, if we can design a CTC metric such that the blueshift is bounded in all regions despite increasing forever, then we might be able to get around the quantum field divergence problem

Emilio Pisanty

@ACuriousMind I just got tired of having the same conversation with dewy-eyed idealists over and over again

nothing wrong with idealism

but I would rather have them dry the dew off their eyes and understand the history, instead of projecting their wishes on it

ACuriousMind

I think having all this history in a single place and be able to point to it is a very good idea, and you've invested a lot of effort to find all the relevant original posts

Yashas

Physicists Just Came Up With a Mathematical Model for a Viable Time Machine

Secret

1:43 PM

@Yashas You are too late. THAT is what caused me to stirr the discussion with slereah about time travel today

and in fact, that work is already done since 2013

Emilio Pisanty

@ACuriousMind yeah, it was some work but mostly it's the fact that if e.g. you did it, you wouldn't even know what to look for ;-)

AccidentalFourierTransform

or too early?

Yashas

@Secret You function faster than a bot lol

Secret

Well, you can say it is too early if any one of us just happened to be inside such bubble while typing in this chat (modulo someone else get fried by unruh radiation at the walls)

---
About the notion of time, I actually have a question that is inspired by the following idea explosion:

\begin{align}
& \textrm{Last night dream}\to\textrm{fixed point infinite sums of the form } 2^{\sum^{\infty}=\sum^{\infty}}\\
& \xrightarrow{\textrm{Ted talking about varying $\gamma$ in $\int_\gamma\omega$ which is a functional of $\gamma$}}\xrightarrow{\textrm{infinite sets}}\\
& \textrm{Discrete countable variation idea on optimising}\sum_{\lambda}g(\lambda)\\
& \xrightarrow{\textrm{Google search}}\textrm{Discrete Lagrangian mechanics}
\xrightarrow{\textrm{Bounded sequences}}\\
& \textrm{Sequence variations $\delta \lambda$ have extra degrees of freedom in the form of density of the poin…

And the question is as follows:

How do we experimentally know that time is linear. I.e. when you look at an acceleraitng truck, rather than thinking it is moving faster (covering more distance per unit time), it is equally valid to say that time flows faster while the moving distance are the same?

Both scenarios will look identical to us

Slereah

You can just take an accelerated frame of reference in which the truck is at rest

Secret

1:51 PM

That's true but then no notion of time (linear or not) can be deduced if you rely on the state of the truck to be your clock in that frame

(Of course, one can argue the observer can always measure proper time, and that one take that to be linear for convenience)

I guess I need to think more carefully how will the laws of physics be changed if time is nonlinear

0celouvskyopoulo7

@Slereah how does one say de boglie

AccidentalFourierTransform

2:07 PM

de broy

Slereah

Debrey

Except it's the french e

Which has no english equivalent

AccidentalFourierTransform

is there any straightforward relation between Killing vectors and the Laplacian?

given $K$ Killing, what is $\Delta K$?

0celouvskyopoulo7

@AccidentalFourierTransform Is it $-\mathrm{Ric}(K,K)$?

Slereah

Do you mean divergence

0celouvskyopoulo7

The divergence of a Killing field is zero

Slereah

2:16 PM

Oh delta

AccidentalFourierTransform

@0celouvskyopoulo7 I dont know :-P

perhaps

0celouvskyopoulo7

Actually that doesn't make sense

Slereah

Well

0celouvskyopoulo7

I wrote a scalar but you need a vector

Slereah

What I advise is

0celouvskyopoulo7

2:17 PM

Maybe $-\mathrm{Ric}(K,\cdot)$

Slereah

Take the Killing relation

Apply $\nabla^\mu$ to it

0celouvskyopoulo7

Yeah you're gonna get a curvature term

AccidentalFourierTransform

yep, that's correct

cool!

0celouvskyopoulo7

@AccidentalFourierTransform Note $\nabla^2_{X,Y}K=-R(K,X)Y$

So take the trace of that

@AccidentalFourierTransform what I said?

AccidentalFourierTransform

yes, its $\frac12 \mathrm{Ric}(K,\cdot)$

or with a negative sign

0celouvskyopoulo7

2:19 PM

1/2?

AccidentalFourierTransform

I did what S. said and you get a $\frac12$

I think

$0=\nabla_{(\mu}K_{\nu)}$

apply $\nabla_\mu$:

$0=\Delta K_\nu+\nabla^\mu\nabla_\nu K_\mu$

and commute the derivatives

0celouvskyopoulo7

I dunno. $\Delta K=\sum \nabla^2_{e_i,e_i}K=-\sum R(K,e_i)e_i=-\mathrm{Ric}(K,\cdot)$.

God why use $K$

AccidentalFourierTransform

$0=2\Delta K_\nu+RK$

hmm

0celouvskyopoulo7

Uh, what?

note that $\nabla^aK_a=0$

AccidentalFourierTransform

I...

up to

2 mins ago, by AccidentalFourierTransform

$0=\Delta K_\nu+\nabla^\mu\nabla_\nu K_\mu$

we agree?

0celouvskyopoulo7

2:23 PM

I believe that equation.

Then apply the Ricci identity and note that div K =0

AccidentalFourierTransform

oh fuck

yes

thanks

0celouvskyopoulo7

np

Yashas

In 1975, a Cat Co-Authored a Peer-Reviewed Physics Paper

lol

0celouvskyopoulo7

old (fake) news

@yuggib greetings

@yuggib quick q: In a reflexive Banach space, is a strongly continuous function weakly lower semicontinuous?

Bernardo Meurer

2:38 PM

@0celouvskyopoulo7 Yo

If I launch a projectile with angle $\theta$

0celouvskyopoulo7

oh no

Bernardo Meurer

And I want to to reach $x=12m$ and $y=6m$

0celouvskyopoulo7

are you starting at 0,0

Bernardo Meurer

How do I figure out my launch speed/$v_0$

Yes

0celouvskyopoulo7

Do you know the angle?

You clearly can't do it with every angle

Bernardo Meurer

2:39 PM

Yes

60

This is the figure

and they say the launch angle is 60 degrees

0celouvskyopoulo7

What do you think you should do?

They just want you to reach it? No other stipulation?

@yuggib It might not be true but that's not a problem for what I need, actually

It would be interesting to know when it is true though

I just need it for a function derived from a norm

yuggib

@0celouvskyopoulo7 greetings

so the function is from $X$ to $X$, $X$ reflexive Banach?

or from $X$ to $\mathbb{R}$ or $\mathbb{C}$?

Bernardo Meurer

2:55 PM

@0celouvskyopoulo7 Sorry, had a call

@0celouvskyopoulo7 I tried writing the equations of motion for the projectile

$x(t)=v_0 t \cos\theta$

$y(t) = v_0 t\sin\theta - \frac{gt^2}{2}$

so I know that I need to reach $x=12$ and $y=6$

But I don't know the time so I can't solve those for $v_0$

@0celouvskyopoulo7 Ah, do I do a system? Since I have two equations and two variables?

@Yashas You too since you know stuff

Yashas

$x = x\tan \theta - \frac{gx^2}{2u^2 \cos^2\theta}$

$u$ is the initial velocity

Bernardo Meurer

@Yashas How did you get to that?

Yashas

substitute t as $\frac{x}{v_0 \cos \theta}$ in $y = v_0 t \sin \theta - \frac{gt^2}{2}$

I knew that formula byheart though

dono why I don't forget them

used it over 100 times :P

Transcript for