The h Bar

yuggib

1:00 PM

they all should deal with strong derivatives of operators

0celo7

Hmm, this prof probably doesn't care about that though

Wonder where he saw it

yuggib

I really don't care about what he cares about ;-P

anyways, strong differentiability may come out in many other situations (probably)

0celo7

Google revealed basically nothing

Secret

1:17 PM

[Sigh...] If you see a popular reading article that quote Albert Einstein and you are lazy, please assume that person is not Albert einstein. Because chance are, being such a universally quotable figure, nearly 99% of such quotes are made up

If however you see his name on some journals, and said paper is not outdated, then it might be true they are quoted something from einstein's research

David Z

We have a chat session coming up and I don't have anything planned to talk about

Secret

Is anything written on the schedule, if not, just start with usual physics research updates you guys have found, and people will fill in the rest of the roll automatically

user116211

@DavidZ Are we done with homework policy?

user116211

If we have none, then the chat can go normally.

David Z

Oh goodness no ;-) I just don't have anything on that prepared, and we're at a point in the process where there isn't anything particularly pressing to discuss about it

user116211

1:25 PM

Maybe we can read Lumo's blog together or do something else.

David Z

@Secret well this is what I always do - the intro for new people and recent topics in physics are fixed, but beyond that it's up to suggestions from the community

user116211

@DavidZ okay.

Secret

(forgot line following my [sigh...] message) paleofuture.gizmodo.com/…

Secret

ok

David Z

@ACuriousMind @rob are you able to be at the chat session? I guess we could have a little mod Q&A thing, if other people don't have anything in particular to discuss

I'll feature this again (now that the election is done) so we can get back to thinking about it, and if people want to discuss that at the session, we can

user116211

1:28 PM

Also, @DavidZ, is the plan of making the election chat transcript into a meta post still in air?

user116211

It would be really good if it happens.

David Z

I won't be doing anything about that, but as I've been saying, if someone else wants to, they can.

user116211

okay, thanks.

ACuriousMind

@DavidZ I'll be there

David Z

cool

If nothing else, I'll just go over things we have to think about for the future, and then it can be open discussion

Secret

1:35 PM

I am a statement version of Spock. If I am emotionally agitated, I make something: secretuniverse.wikia.com/wiki/Fake_Einstein?venotify=created

0celo7

I like how my computer reminds me of my own birthday

user116211

Siri can also do it.

user116211

Don't know about Cortana.

0celo7

@MAFIA36790 Ok I have exam review for analysis

do you want it?

user116211

Go on!

0celo7

1:37 PM

Hmm

No clue how to do any of these

@MAFIA36790 Number 7 isn't too bad

DHMO

proofwiki.org/wiki/…

@0celo7 these two links for q1

user116211

@0celo7 I have seen it somewhere; maybe Royden.

0celo7

@DHMO Huh?

DHMO

@0celo7 the links are not helpful?

rob

@DavidZ Unfortunately I teach a class starting twenty minutes into the chat session, so I can't participate this month or next.

0celo7

1:48 PM

The density of the rationals/irrationals?
No, not at all.

DHMO

@0celo7 alright

Ramanujan

@DHMO then?

DHMO

@Ramanujan ?!

@Ramanujan go back to math room

Ramanujan

@DHMO ok

user116211

Checked Royden.

user116211

1:55 PM

there is Bolzano-Weierstrass Theorem.

user116211

Checked the appendix matched subsequence.

user116211

Got the Arzela-Ascoli Theorem in Chap 10.

user228700

@MAFIA36790 Oh God, how do u even pronounce something like that? ._.

0celo7

Just like it's written?

user116211

Don't know; reading.

user228700

1:57 PM

The second word. Weierstrass.

0celo7

Why-er-strass

user116211

8 or somewhat related question is in the problems of Royden.

user116211

Okay, he then starts Baire Category theorem.

user116211

Nay, it's too far.

user116211

I have to wait.

user228700

2:05 PM

@0celo7 Oh, OK...

user116211

BTW, the downvoter responded; I'm happy he replied.

user116211

But...

user116211

> Note that neither Purcell nor Feynman knew how a magnet works.

user116211

:(

user116211

I don't know what to say ;/

0celo7

2:09 PM

Feynman was probably a crank

John Rennie

@0celo7 Vie-er-strass surely? He was German.

0celo7

@JohnRennie Wahrscheinlich, aber wir sprechen Englisch hier, nicht Deutsch.

I am German you know.

ACuriousMind

@JohnRennie If you pronounce "Why" with a German accent, you get the German pronounciation from what 0celo7 wrote

0celo7

Also that, not Vie

John Rennie

Yes, by Kaumudi isn't German :-)

ACuriousMind

2:10 PM

I.e. use the german "W" not the English "Wh"

0celo7

@ACuriousMind In English the ei and ie are backwards

German is superior in that regard

John Rennie

vie as in the English vie for the leadership.

0celo7

@JohnRennie Wrong.

Sophus Lie is not Sopus lye

dunno how to write that

John Rennie

Lee ?

0celo7

and it's Leibnitz, not Leebnitz

@JohnRennie Yes, that's the correct way to say it

@ACuriousMind Not quite. One has to make the s into an sh to get German

Jim

2:13 PM

@JohnRennie surely, for German, it'd be vie-er-shtrass? Note the important "sh" sound

0celo7

@Jim cf what I just said.

ACuriousMind

@0celo7 Eh, without it it's just Northern German.

0celo7

@ACuriousMind Is st not sht in Hochdeutch

I would say it as sht when speaking properly

John Rennie

I wish I'd never started this. Now if you'll excuse me I'm going to get back to nailing my kneecaps to the desk.

ACuriousMind

@0celo7 Sure, but not all Northerners speak High German.

Jim

2:15 PM

@JohnRennie have fun!

0celo7

@JohnRennie ???

Jim

@ACuriousMind sounds like a "them" problem

0celo7

@ACuriousMind Not all Germans speak High German

ACuriousMind

@Jim I don't know what that means

Jim

it means the problem is with them, not us

ACuriousMind

2:17 PM

@0celo7 Sure. My point was that the lack of the sibilant in pronouncing 'st' is characteristic of Northern Germans

0celo7

@ACuriousMind I spent too much time among southern rural hicks lol

ACuriousMind

@Jim Eh, I don't see it as a problem, mostly, unless the pronounciation is so mangled you can't tell what they're saying

0celo7

BAVARIA

ACuriousMind

Which I've never had happen except for some rural Bavarians :P

Although Swabians can be hard to understand, too

0celo7

Swabians sounds funny

In both languages, really

@ACuriousMind My aphasia makes it hard to understand them

I understand the Grüßdi but after that it goes down hill

@ACuriousMind Apparently my dad can understand them when he's had enough schnapps

He doesn't speak German

ACuriousMind

2:23 PM

Everyone can understand everyone after enough schnapps ;)

0celo7

Not me

Devil's liquid I say

ACuriousMind

Then you haven't had enough schnapps :P

0celo7

@ACuriousMind Did you see the problems above?

THE LONE WOLF.

The question "should zero be followed by units" has been edited,please review it for its reopening.

0celo7

Hmm, the first question isn't that bad

Secret

2:30 PM

Reading my 400+ emails is like entering a city ruins of 1000 years ago. I mean seriously how on earth did my 5 years ago self managed to follow up so many memberships and stuffs?

0celo7

400

what a noob

Secret

how on earth can you have 7000+ (possibly non spam) emails unread, that's insane

0celo7

I just do

user116211

Are they spams?

0celo7

some

i.e. facebook, twitter

Secret

2:36 PM

It also does not help that my own writing is so cryptic and so reliant on pretty much my mental state in my past selves in order to decode, and most of the time I am scratching heads trying to figure out what on earth I am saying

I really need a better system to keep track of all my active memberships and details...

0celo7

@ACuriousMind How many semesters of algebra did you take in undergrad?

ACuriousMind

@0celo7 I took Linear Algebra I & II and Algebra I & II through my first four semesters.

Physics Meta

0

Q: What advice do you have for a physics student who can't think of a topic for his thesis?

I thought this might get closed if I asked on the main site, so I posted it here. If this can actually go on the main site, feel free to let me know. I'm lost without a topic for my thesis. What would you recommend to someone who doesn't have a firm idea for a topic (for a future theorist)?

discussion

0celo7

2:53 PM

@ACuriousMind In machine notation: $\lim_{x\to\infty}f(x)=L\Leftrightarrow (\forall\epsilon>0)(\exists M\ge 0):x> M\implies |f(x)-L|<\epsilon$?

Slereah

Do you mean "in regular mathematical notation"

No

you should

I doubt it

why writing the definition of a limit?

0celo7

2:56 PM

Because I am learning it today

@MAFIA36790 Problem 2 is easy, you should do it.

Slereah

also why are you using $M$ instead of a gentleman's $\delta$

Secret

M is standard for infinite limits

0celo7

@ACuriousMind Is there a notation like $C^0$ for unif cont. functions?

@Secret correct

Slereah

Isn't it like $C^{0+}$ or something

0celo7

I dunno

I'll write it as $\bar C^0$

DanielSank

3:03 PM

Anyone know what the chat session's about today?

Slereah

Baking

We share our cake recipies

David Z

@DanielSank whatever topics people contribute

0celo7

why am I an idiot

Please ban whoever starred that

David Z

@rob OK no problem

Dvij

If a frame is accelerating but the spacetime is flat then what will contribute to the energy-momentum tensor of Einstein Equations to make them produce Christoffel symbols that are non-trivial in that feame?

DanielSank

3:05 PM

@Slereah ok, then I leave this here and probably not come.

Slereah

@Dvij Flatness doesn't depend on the Christoffel symbols

Rindler coordinates have non zero Christoffel symbols but still a zero Riemann tensor

Dvij

Okay, yeah. I presented the question in a bad way.

Secret

[To my dear email system] I like biology, but I don't like stupid debates about how pharmaceutical companies are corrupt!

0celo7

@Slereah but if they are zero in a neighborhood, is spacetime flat in that neighborhood?

Dvij

Einstein Equations will give me zero Riemann tensor and zero ricci tensor, right?

Slereah

3:07 PM

@Dvij yes

Dvij

but then how would I know that the symbols are non-trivial

Slereah

Well, once you have the Christoffel symbols, calculate the Riemann tensor from them

0celo7

@Dvij in a flat spacetime?

Slereah

If it's 0 your spacetime is flat

0celo7

As soon as you have some energy/momentum and you assume it's not a test particle, then you will curve spacetime

John Rennie

3:08 PM

@Dvij I spent a quiet afternoon doing this for the Rindler metric

0celo7

I'm not sure what the exact mechanism is, but you can certainly write down an energy momentum tensor for a point particle

Slereah

The metric of a point particle is just Schwarzschild

John Rennie

Some of the Christoffel symbols are non-zero, but when you compute the Riemann tensor it turns out to be zero (as it should!)

@0celo7 $$ T^{\alpha\beta}({\bf x},t) = \gamma m v^\alpha v^\beta $$

0celo7

@Slereah you're assuming spacetime is R4 without the particle

could be something weird, I dunno

Slereah

The best spacetime

0celo7

3:11 PM

Is $(S^1)^4$ a spacetime?

Dvij

In a flat spacetime, symbols will be non-zero in some frames due to non-homogeneous transformation properties of the symbols. But the curvature tensor and thus all of its contractions will be trivial in all the frames for a flat spacetime. So all I will get is zeros from Einstein equations. Then from Einstein Equations, how can I know whether my frame has trivial symbols or some of them are non zero. that's my question.

0celo7

What's the Euler char

user228700

@0celo7 :-P Why? I did...

Slereah

@0celo7 yes

0celo7

0 because it's odd-dimensional

so yes, that is a spacetime

but is it flat?

Slereah

3:12 PM

4 isn't odd

It's a Torus

It will be flat if it's a Clifford torus

0celo7

@Slereah $\chi((S^1)^4)=\chi(S^1)^4=0$.

@MAFIA36790 problem 3 is ok too

John Rennie

@Dvij I'm still not sure I understand what you are asking. The Christoffel symbols are not tensors so they will vary depending on the coordinate system. Their zero or otherwise values don't stem (just) from the Einstein equation.

0celo7

@JohnRennie e.g. Wald says they are tensors

you physicists need to get it straight

John Rennie

If you take them all together the whole set depends as a tensor in some way I don't really understand. However the individual symbols are not tensors.

Dvij

@JohnRennie My understanding also says they are not tensors and also that I am unable to get them from only Einstein Equations. My question is precisely that how do I get them if not only from Einstein Equations?

John Rennie

3:26 PM

You solve the Einstein equation to get the metric, then choose a coordinate system, then compute the symbols from the metric.

0celo7

Einstein equations + boundary conditions can be solved on certain spacetimes (topologies) in reasonable function spaces

You get the metric, and from that the Christoffels.

Dvij

I thought if I give a computer the energy momentum tensor, Einstein Equations, and Geodesic Equation then I give it all that is needed to compute whatever can be computed. Is it not correct?

0celo7

NO

You need boundary conditions!

John Rennie

@Dvij I'm not sure where the geodesic equation comes in, but from the stress-energy tensor you can in principle use the Einstein equation to get the metric. Then from the metric compute the Christoffel symbols.

user116211

What is Banach?

0celo7

3:29 PM

@MAFIA36790 A complete normed vector space.

user116211

I will get it in chap 10 of Royden.

John Rennie

I say in principle because it's rarely easy :-)

0celo7

@JohnRennie I'm fairly sure it's not true in principle.

There are no general result like that

Not every PDE has a solution

Dvij

@JohnRennie Okay, I get it now! Since the symbol isn't a tensor, although the metric is determined they will depend on the frame I am using in a way that is not covariant and thus I may get non zero symbols in some even if all of them are zero in the others.

0celo7

Ugh, I'm talking to a wall

Whatever

John Rennie

3:31 PM

@Dvij Yes

Dvij

@0celo7 So from your and @JohnRennie 's explanation, I feel that boundary conditions somehow define the frame I am using. Is it?

DHMO

-1

Q: Could Gravity Be Directly Proportional to Mass?

a little background, I'm doing personal research into a theory I believe could unite mass with gravity just as much as mass is with energy. To get to the point, I use several observations of repeatable models within our own solar system. My question is directed towards anyone who may have high-de...

quantum-mechanics particle-physics

Complete bullshit.

0celo7

@DHMO Can you explain your hints from earlier

DHMO

@0celo7 No, those are just my bullshit. I don't know how to do your question at all.

0celo7

Continuity of each function seems too weak to prove what I want.

...

DHMO

3:32 PM

Sorry

0celo7

@Dvij Well, you have to solve your equations in a frame, yes

but I don't think the boundary conditions determine that

John Rennie

God it's useful having a moderator who hangs out in the chat room :-)

Secret

-3

Q: Could Gravity Be Directly Proportional to Mass?

a little background, I'm doing personal research into a theory I believe could unite mass with gravity just as much as mass is with energy. To get to the point, I use several observations of repeatable models within our own solar system. My question is directed towards anyone who may have high-de...

quantum-mechanics particle-physics

Ignoring the fact that this question is related to a construction of a personal theory, he need to clarify what kind of correlation he is on with mind. But this might be off topic considering the question's primary aims

John Rennie

Do we have to see crap questions twice?

ACuriousMind

@JohnRennie I don't know what you're talking about ::whistles::

user116211

3:36 PM

@JohnRennie ACM is the Flash.

0celo7

@JohnRennie why?

What happened?

user116211

@0celo7 Check the post above.

Secret

@JohnRennie I did not know it was referred into the chat before, sorry about that. I have been busy doing calculatiosn in the background thus have not paid much attention to the chat flow

0celo7

I am looking

Oh ACM destroyed that post?

user116211

Yes.

user116211

3:37 PM

A bad post.

0celo7

> =($B$1*$B$2*$B$3*($B$4^2)/2)*((1/(($B$4^2+(A11+($B$4/2))^2)^1.5))+(1/(($B$4^2+(A11-($B$4/2))^2)^1.5)))

ACuriousMind

I closed it, no need to destroy it

0celo7

Excel is a garbage program lol

user116211

@0celo7 Why are you working on it?

0celo7

lab report

user116211

3:38 PM

ohh.

John Rennie

@0celo7 I write chemical engineering software for the nuclear industry using Excel ...

0celo7

not terrible

misspelled measured, whoops

@ACuriousMind Why can't a sequence of polynomials converge uniformly to $\sin(1/x)$ on $(0,1)$? It's continuous and bounded, so nothing is wrong there

Secret

hmm, wonder what kind of system is that, so the magnetic field points one way if you are osmewhere to the left and another way if somewhere to the right, but the experimental result said its symmetrical and depends only with distance

0celo7

@Secret two Helmholtz coils

Secret

i see

0celo7

3:41 PM

in one case, the currents are going the same direction

in the other case, in opposite directions

gotta add the rest of the data to the plot now

Secret

meanwhile having this in the background when following up my various super messy typings and stuffs. It has smooth music

Earth From Space: GoPro ISS Spacewalk | ISS HD Stream - Video Replay Of EVA Russian Cosmonauts 2013

►

user228700

Evening, everyone :-)

user228700

(Or morning or afternoon)

Secret

I have slept too much for 16 hours straight today because I have not sleep on the previous night

user228700

@Secret Omg! 👍

user116211

3:45 PM

@DanielSank o/

DanielSank

@MAFIA36790 \o

user116211

@Secret Take some pills.

Secret

and now it feels like Iam 100 years into the future, as I have mostly no idea what I have wrote in all my past records

user228700

@DanielSank: Hallow from India! XD

Secret

as I follow up memebrship renewal and stuff

0celo7

3:46 PM

oh god do we have a chat session

DanielSank

@Kaumudi Hello from California.

@0celo7 Yes.

0celo7

@JohnRennie Help, you're my only hope

John Rennie

@0celo7 you need a PCIe parallel port card

0celo7

I can put on a white dress if that makes you more inclined to help

user228700

^How I feel most of the time

0celo7

3:47 PM

@JohnRennie :)

No, how do I change the dots on an excel plot

it decided to randomly give me crosses!?

Slereah

They probably ran out of colors

Secret

right click on the crosses, there should be an option called format data series

John Rennie

@0celo7 Double click one one of the dots and it'll open a dilaog for formatting the series. You can choose the marker symbol and size from there.

DanielSank

@0celo7 Like this

0celo7

I don't need your sass Dr. Sank!

DanielSank

3:49 PM

I think you do.

Secret

Fact: When checking a thesis looking thing, it is very very hard to put oneself into a mindset of a reader who does not read in the same way as you

0celo7

@JohnRennie Also how do I turn the values in a whole column negative

@Secret I tend to think such readers are simply wrong.

David Z

@0celo7 the fact that you had to ask how to do it here suggests that you do :-P

j/k

OK, so, before we start (9 minutes), any topics for the chat session?

user228700

I've a quick question; I've always thought about the s-orbital as having lower energy 'cause it's closer to the nucleus and so, there's more attraction b/w the electrons and the nucleus. At least that's the way I've been taught. More attraction, less energy. Is this an OK way to think about it?

0celo7

John Rennie

3:52 PM

@0celo7 I don't know a simple way. I'd set up a parallel column with the values multiplied by -1 then copy that column and paste it back over the originals using Paste Special and pasting only the values.

0celo7

@JohnRennie Help it just turned that one into a dot :(

John Rennie

@0celo7 let me try - I'll just fire up excel ...

0celo7

@DavidZ I don't have a PhD in computer engineering D:

user228700

Anybody?

Secret

@Kaumudi The major reason s orbitals have generally lower energy is because the effective attraction on the electrons are higher due to less screening, which is caused by the orbital have a probability distribution that penetrate to regions closer to the nucleus

0celo7

3:54 PM

@JohnRennie figured it out

thanks for the help

John Rennie

Aha, what was it?

0celo7

I don't know!

But when I clicked on it this time, it said "select data series"

so I formatted all the yellow points

what??

word, why?

Secret

user228700

@Secret Screening being the effect in which the outer electrons aren't attracted as much because of the presence of the inner electrons? I've never really understood why some orbitals have higher screening effect, like the d and f orbitals :/

John Rennie

@Kaumudi it's more complicated than that. For example in hydrogen the s and p orbitals are degenerate

Secret

3:56 PM

noticed how the s orbital have probabilty distrbution closer to the nucleus then the correspnding p orbitals

0celo7

@JohnRennie Flagging as not nice

electrons have feelings too

John Rennie

:-)

user218912

lol

Secret

@Kaumudi d and f orbitals (will dig a pics for you later) penetrate into the nuclear region much less then s orbitals, thus on average electrons will felt more repulsion from the inner electrons and hence higher screening

user228700

@JohnRennie Ohh, yeah :/ OK, great.

user228700

3:58 PM

@Secret Oh, they will feel higher repulsion?!

John Rennie

@Kaumudi It is, as secret says, due the way the inner orbitals screen the outer ones. In effect the outer orbitals see a lower nuclear charge.

Secret

yup

0celo7

I would ask for proof but I'm scared I would get proof

David Z

Shall we get started with the chat session?

Transcript for