The h Bar

Slereah

00:34

Domain walls are a pretty good example of symmetry breaking

kevin Tah N.

00:59

I concur. Was just reading up on them

AngusTheMan

01:32

@ACuriousMind Great answer on the instanton question! :D

ACuriousMind

01:47

@AngusTheMan thanks, although I only answered the "easy" part. ;) Currently working through Witten's "Topological Quantum Field Theories" to answer the part about Donaldson invariants.

Stan Shunpike

02:54

@ACuriousMind hey! What exactly is a partition function? I think I've learned enough in the last 3 days to perhaps understand it. I understand why $n_i = g_i e^{-\alpha + \beta \epsilon_i}$ maximizes $\Omega$ (Aka $W$) but I don't understand what $Z$ is or what it means.....I know $Z = \sum_i e^{-\beta \epsilon_i}$ but I don't get what this represents. Can ya help me understand this a little better?

dmckee

04:27

@StanShunpike In the simplest sense it's just a weighted count of qualifying states.

Where the choice of weighting makes it a normalizing factor for the odds of a state being occupied.

kevin Tah N.

04:58

@dmckee can you teach me a bit about large N approximation, and planar diagrams. How does it work for say,. . your favorite matrix theory

well for a very simple theory

dmckee

05:30

@kevinTahN. Uhm ... probably not. I mean, large N approximations ar what they are, but I can't even place the context for "matrix theory" here.

0celo7

07:05

@Bass Morgen.

Bass

@0celo7 Morgen :)

Physics Meta

1

Q: arithmetic question

In this question there are eleven downvotes. I know from downvotes I have gotten that each downvote subtracts 2 points. 2x11=22 should be deleted. The user has a remainder of 8. How is this possible? Even if he/she came with the dowry of 100 from another site, still 8 is not what it should be:78...

discussion

kevin Tah N.

hmm , ok I need some help. I want someone to devote a few chat texts to help me understand large n approximation.

0celo7

@Bass Do you happen to know about wave-like solutions to Maxwell's equations in curved spacetime?

Bass

@0celo7 nope, but I doubt you're gonna find exact solutions that fit to every curved spacetime

0celo7

07:11

@Bass I just want to see if the "center" of the wave packets follow null geodesics.

I don't need exact solutions...just some properties of exact solutions.

Secret

Day 1: It survived, for now

0

Q: Electronic spectroscopy question in a more physics oriented view. What is the operator that describes relaxation pathways?

Recently I was studying spectroscopy in Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy p.357-358 as shown in the image https://i.sstatic.net/LSLyZ.jpg The author said the internal conversion (laballed IC in that energy level diagram) is isoenergetic, whi...

quantum-mechanics physical-chemistry

0celo7

In SR one can show, rigorously, that a plane wave solution of Maxwell's equations travels along a null geodesic.

Secret

However there is no answer yet, still in a tumbleweed state

0celo7

@Secret protip: use \text{} when you have words in your TeX

Secret

done

0celo7

07:14

@Bass However, in GR, one must ignore a term in the Maxwell equations to get the same result.

@Secret protip 2: use \lvert and \rvert in bra-kets: these extend all the way down and have the same overall height as \langle and \rangle

Secret

---
done

0celo7

@Bass Basically, try plugging in $A^\mu=C^\mu\mathrm{e}^{\mathrm{i}S}$ into Maxwell's equations

$C^\mu$ is a constant

in SR you get $\partial^\mu\partial_\mu S=0,(\partial S)^2=0,C_\mu\partial^\mu S=0$

If you take the derivative of $(\partial S)^2=0$, you get the geodesic equation

kevin Tah N.

ok, I will learn anything anyone tells me to on here :D

0celo7

Ok, I have to re-learn wtf a null hypersurface is

wald seems to think $\epsilon^{abcd}k_c\nabla_d(k_ek^e)=0,k=\mathrm{d} S$ is important

Bass

07:32

@0celo7 sorry, can't help you there..

Physics Meta

08:14

0

Q: Can we get the hat frequencies?

I get the impression that the Darth Vader hat is the most popular. I can't imagine why, anyone would think that (a) physicists are all sci-fi geeks and (b) there's a new Star Wars film out. Anyhow, is there a way to find the numbers of Physics SE members currently wearing each hat?

discussion

John Rennie

08:37

Being downvoted by John Duffield because my (conventional) answer does not agree with his idiosyncratic views on relativity is getting to be something of an occupational hazard. I suppose it's just another of the hazards of life, like hemorrhoids or genital herpes.

0celo7

@JohnRennie ...

Have your star

John Rennie

08:55

@0celo7 :-) Though I'd sooner have a Physics SE free of zealotry.

0celo7

@JohnRennie currently formulating a question about solutions of Maxwell's equations outside of the geometric optics approximation

you might find it interesting

(in GR)

John Rennie

I'll have a look, though I was always profoundly uninterested in electrodynamics at school/university. It seemed to be just an excuse for examiners to think up unlikely scenarios for exam questions involving lots of vector calculus.

0celo7

@JohnRennie this is certainly more interesting

basically, I'm wondering what happens to wave packets at distance scales where the curvature can go crazy

do they follow null geodesics?

John Rennie

Be sure to put in something that contradicts a statement by Einstein. You wouldn't want to deprive John Duffield of his fun.

kevin Tah N.

@JohnRennie can you teach me large N approximation?

Stan Shunpike

09:05

@dmckee is that all there is to it? why is it so important?

@JohnRennie hello, long time no speak!

kevin Tah N.

been hanging out here for a while trying to trick people into teaching me large N approximation technique :~)

Stan Shunpike

People here are rather hard to trick

Besides me

John Rennie

@kevinTahN. what is the large N approximation technique?

@StanShunpike Hi Stan. You'll only find me in the chat when I'm pinged or when I want to bitch about something :-)

Stan Shunpike

Naturally, hence I take the time to say hi when u are here ;)

Omg u can wear the hats on ur profile pics?!?

That's what those are for! eureka

kevin Tah N.

I don't know much about it but I think it is some technique used in qcd or something, but you can also use a version of it on simpler theories. You can also may be draw planar diagrams and stuff. Honestly I don't quite know, but . . . . also I think it might actually be called large N expansion . . .

some links to accompany this include books.google.com/…

and one more link comming up

@JohnRennie In Juan Maldacena's Tasi ads/cft lectures he begins with a disquisition of large n and planar diagrams , then precipitates a technique of how to guess some "equivalent string theory" and how to create a dictionary

John Rennie

09:17

Ah ok, you mean letting N go to infinity in an SU(N) representation. I'm afraid that's far outside my skill set.

kevin Tah N.

@JohnRennie the tasi lectures I mentioned arxiv.org/pdf/hep-th/0309246v5.pdf

I see. . .

John Rennie

Unless you already have a good knowledge of quantum field theory it's going to be far outside your skill set as well.

@StanShunpike Keep up at the back there Shunpike! :-)

kevin Tah N.

I have been told it can be treated outside of su(n) for some simple matrix models

Stan Shunpike

Hahahahaha

kevin Tah N.

In its full glory it is definitely outside my skill set lol

@StanShunpike can you discuss it in the scope of tractable simplified matrix models?

I would really love some input on this

@JohnRennie Is there a way we could formulate this within the bounds I mentioned and pose it here or may be to the Q&A section?

Stan Shunpike

09:22

I am not knowledgeable about anything physics related that would be useful to u. Lol my skill set is amateur level. As in I own QCD books but can't understand them.

kevin Tah N.

lol

John Rennie

@kevinTahN. No idea. To be honest, unless you understand QFT well enough to figure it out for yourself I doubt you'd find any answer here very useful, or indeed comprehensible.

kevin Tah N.

I don't have a grasp on qcd either but was assuming the technique could be explored outside of qcd on simple models

@JohnRennie yeah, I read a bit of it in the context of qcd but there were other subtleties that prevented me from getting the general concept (may be algorithm) for doing it. Was hoping Santa would point me to a simple model I could study this for Christmas

this physics.stackexchange.com/questions/123416/… is somewhat close, but might need an even easier toy model and more explanation lol

0celo7

10:54

0

Q: Do light waves precisely follow null geodesic paths in General Relativity?

In special relativity one may show that a plane wave solution of Maxwell's equations (in a vacuum), of the form $A^a=C^a\mathrm{e}^{\mathrm{i}\psi(x)}$ has the following properties: The normal $k:=\mathrm{d}\psi$ to the surfaces of constant $\psi$ is a null vector and the integral curves of $k$ a...

electromagnetism general-relativity maxwell-equations geometric-optics geodesics

anyone is welcome to take a crack at it

0celo7

11:11

Hmmm, looks like @JohnDuffield 's answer on that natural units question is gone

@DanielSank I've heard VA Tech's technical writing class tells people that real scientists use $*$ for multiplication. cf. physics.stackexchange.com/questions/215408/…

(They will take off points if you use $\cdot$ or try to imply multiplication.)

@Huy how was the movie

pls spoil

Huy

@0celo7 much better than I could have ever imagined

0celo7

would an SJW like it

Huy

@0celo7: hard to say without spoiling too much

0celo7

@Huy pls answer my question above

ask one of your high school students who like physics

Huy

sorry that's trivial

won't bother my students with that

they have serious problems to work on

0celo7

11:26

D:

dude how the fuck do they get so smart

Huy

it's a joke

they don't know GR

0celo7

bullshit

Huy

one of the two girls cried today

the other one didn't

I was glad only one did

0celo7

stop trying to unhurt my feelings

just give them my question and make it extra credit or something pls

@Huy why

they're doing PhD level math and are crying...wtf

Huy

cuz she got rekt at my exam

she said she prepared like a lot

and asked how this could be

and I said "idk"

0celo7

11:30

because it's PhD level math and she's not autistic...

@Huy why don't they know GR

Are they the kind of people who think physics is too trivial?

@Huy do you know the proof of $$K(p)=\lim_{r\to 0}\frac{3}{\pi}\frac{2\pi-L}{r^3}$$

the thing with the Gaussian curvature and the circle

Huy

11:49

@0celo7 sorry i have no idea what u mean

0celo7

12:27

@Huy third from the bottom en.wikipedia.org/wiki/Gaussian_curvature#Alternative_formulas

Huy

sorry I need to write some exam questions

I think I have a good one

but I need to see how to properly translate and rotate it in R^3 now so that the numbers are a bit more difficult

0celo7

post here

Huy

(but not too difficult)

trivial stuff

0celo7

you know what the 3D rotation matrices are, right

Huy

yea

0celo7

12:36

ok

then pick some nice angles and multiply away

Huy

I had to prove $S^3/ \{\pm 1\} \cong SO(3)$ on my diffgeo exam

I hate calculations

0celo7

@Huy what's the difference between a space-filling curve and a curve dense in some set

@Huy you should have used Clifford algebra

Huy

not my thing

0celo7

then you just show that $\mathrm{Spin}(3)=\mathrm{SU}(2)$

Huy

that I know

I think space filling curve is usually specifically about $[0,1]^n$

Danu

12:40

No, space filling curves fill space.

Huy

oh yeah

Danu

They're surjective.

Huy

yea

0celo7

huh

does a dense curve not fill the thing

Huy

no

dense means the closure is the thing

0celo7

12:40

yeah

Huy

$Q$ is dense in $R$

Danu

sigh

0celo7

$Q$ is not a curve

Danu

Why would there be a different word for dense if it was the same thing?

0celo7

@Danu that's what I'm asking

I don't see the difference

Danu

12:42

Google

Secret

Analysing some 4 years ago stuff from a user named Mrrl:

It is known whatever "?" is, there's a continuous map that can transform from "?" to the cross section immediately before and after "?"

0celo7

@Danu does not help

Danu

It's just a bit annoying to you that you're constantly filling chat with your questions. But I guess that's my problem, not yours.

0celo7

@Danu what?

Danu

Just exactly what I said.

AngusTheMan

12:45

@Qmechanic Hi, with regard to your answer here, what is a 2-cocycle obstruction? Can we have symlectomorphisms give rise to this problem, if so what are their implications for the physics? Thanks :) physics.stackexchange.com/questions/138099/…

0celo7

beats me how my questions annoy you, sorry sir

I'll go fuck off then

bye

Danu

jeez, such drama

Secret

@Danu do you know spectroscopy to help me out on a conceptual question?

Danu

I don't know any spectroscopy.

Secret

ok nvm

AngusTheMan

12:50

@Secret what kind of spec?

Secret

virbonic spectroscopy

vibronic*

AngusTheMan

go on..

Secret

1

Q: Electronic spectroscopy question in a more physics oriented view. What is the operator that describes relaxation pathways?

Recently I was studying spectroscopy in Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy p.357-358 as shown in the image https://i.sstatic.net/LSLyZ.jpg The author said the internal conversion (laballed IC in that energy level diagram) is isoenergetic, whi...

quantum-mechanics physical-chemistry

AngusTheMan

interesting :) I'll have a look over some books and see if they help!

Huy

@0celo7 halp with my problem in math chat

Secret

12:58

ok thanks

John Duffield

@0celo7 : yep. And your name isn't in the list.

skull petrol

Where can I find the book you authored @JohnDuffield

Danu

13:15

@skullpetrol It was not super interesting.

Too elementary; it was very pedagogical and qualitative.

Emilio Pisanty

13:48

@0celo7 Out of curiosity, which question?

Secret

This is going nowhere, I have sent him a message to ask for clarification

But logically, the boundary of al the slides should be homeomorphic to a circle

John Duffield

14:07

@skullpetrol : you can find some second hand copies on Amazon.co.uk. Ouch, £44 thru £88? That's ridiculous, and nothing to do with me. Aw, I imagine I've got a spare copy lying around. Email me at mynamealloneword at btconnect dot com and I'll send you one. There's a few things I'd change if I were writing it now, like calling a gravitational field a tension gradient instead of a pressure gradient. But there's nothing that has me wincing.

3

John Duffield

14:25

@JohnRennie : you were downvoted because your "conventional" answer did not agree with Einstein. I don't have idiosyncratic views, I referred to Einstein's E=mc² paper and energy in the form of radiation. And to other material re gluons and virtual particles. In fact your answer has so many issues. I'm rather surprised it got six upvotes. I'm also surprised the question was closed as "unclear what you're asking".

Danu

5 hours ago, by John Rennie

Be sure to put in something that contradicts a statement by Einstein. You wouldn't want to deprive John Duffield of his fun.

So predictable.

ACuriousMind

@StanShunpike: (outside QFT) the partition function is little more than the normalisation of your probability density - when divided by it, $\exp(-\beta H) $ becomes a proper density (classical) or a proper density matrix (quantumly)

John Duffield

@JohnRennie : if you contradict Einstein, a) you should be aware of it and b) you should be able to explain why. Trying to wriggle out of it by carping about me cuts no ice.

OK work calls, talk later.

AccidentalFourierTransform

17:21

quick question: is there any constant tensor $a^{\mu\nu}_{\alpha\beta}$ constructed from $\eta^{\mu\nu}$ and $\varpesilon_{\alpha\beta\mu\nu}$, such that it is symmetric in its lower indices and skew-symmetric in its upper indices? I believe there is not, but I couldnt prove it. does any simple argument occur to someone here?

dmckee

17:33

My favorite beef recently: people who tell new users that homework is OK if they show effort. The very first sentence in the meta answer we point to in the close reason contradicts that:

> It's not enough to just show your work and ask where you went wrong. If you just need someone to check your work, you can always seek out a friend, classmate, or teacher.

Emilio Pisanty

@dmckee Any recent examples?

Or maybe not, no reason to shame anyone

dmckee

Yeah. I don't want to call out the user who triggered my rant.

That's good policy in general and I think this user in particular is a good one.

Emilio Pisanty

But yeah, I agree that effort and on-topicness are pretty orthogonal dimensions.

Re: this one

1

Q: Are orbitals physical quantities?

Orbitals, both in their atomic and molecular incarnations, are immensely useful tools for analysing and understanding the electronic structure of atoms and molecules, and they provide the basis for a large part of chemistry and in particular of chemical bonds. Every so often, however, one hears ...

quantum-mechanics atomic-physics quantum-chemistry orbitals

Open to suggestions on how best to phrase the question

dmckee

@EmilioPisanty Not off the top of my head. Since I started teaching I have gained a renewed respect for the importance of the words we use, but at the same time they are often a matter of convention.

The result is that these matter feel simultaneously arbitrary and important to me.

Which feels like a recipe for pointless dissension.

Emilio Pisanty

@dmckee Heh.

Yeah, I'll rephrase it a bit to emphasize the technicalness and manybodiness of the argument.

0celo7

17:59

@JohnDuffield Huh?

Emilio Pisanty

@dmckee Just rephrased it.

0celo7

@EmilioPisanty Here you go

@Huy ok

Had a glass of mead today. That stuff is way too sweet. Dunno how the Nords in Skyrim do it.

0celo7

18:22

@JohnRennie My question is up, check the GR thingie. Would appreciate at least some thoughts in the comment section.

John Rennie

@0celo7 Suppose you zoom in to a small part of the wavefront, then even if the wavefront is curved it's going to look like a plane wave. In that case wouldn't the Poynting vector always point in the null direction?

0celo7

How is the Poynting vector defined again?

I thought it was $E\times B$ or something...

John Rennie

B cross erm, Jesus I've forgotten.

0celo7

I've never seen the Poynting vector appear in SR/GR.

John Rennie

E cross H - I wasn't even close :-)

John Duffield

18:25

@0celo7 : the list of people who voted to delete my answer.

0celo7

@JohnRennie In any case, I'm more interested in the global properties of the wave.

@JohnDuffield I did not vote to delete it.

John Duffield

I know. That's why I said you weren't in the list.

John Rennie

Globally the wave is going to be focussed by the curvature. I'm not sure you can usefully talk about a direction globally.

0celo7

Yeah, your point?

John Duffield

Forget it.

0celo7

18:27

@JohnRennie Isn't $H$ just $B$ up to some factor or something.

@JohnDuffield ok.

John Rennie

Have I mentioned I always hated electrodynamics? :-)

0celo7

yeah

But why does the Poynting vector point in the null direction

John Duffield

Here's the Poynting vector for an electromagnetic wave:

John Rennie

@0celo7 Just a convenient way to define the direction

0celo7

@JohnDuffield indeed...

John Duffield

18:29

And here's the Poynting vector for a static field.

0celo7

Is the direction of the Pointing vector the same as for $\vec A$?

Wait, @JohnDuffield , how is the Poynting vector defined in relativity?

John Duffield

Poynting vector

0celo7

Because the Poynting vector as shown in @JohnDuffield 's pic is not pointing in the null direction.

@JohnDuffield Force of habit.

John Rennie

Yes, but you asked about plane waves

0celo7

No

I said wave-like solutions

John Duffield

18:31

I don't think the Poynting vector is defined any different in relativity.

0celo7

@JohnDuffield how is the cross product defined in SR/GR

John Rennie

To be honest I have no chance of providing a useful answer to your question.

0celo7

@JohnRennie but you're the GR dude

John Duffield

Cross product of what?

0celo7

@JohnDuffield the definition of the Poynting vector requires a cross product

TanMath

18:33

where is @MarkMitchison and @DanielSank?

dmckee

@0celo7 Your not looking for a cross-product exactly. You're looking for a contraction of the electromagnetic field tensor that reduces to the appropriate cross-product in the flat-space case. Right?

Hmmm ... and presumable the time-like component of that result will be, what, the energy density?

0celo7

@dmckee Can you find me one?

John Rennie

Curvilinear coordinates

In geometry, curvilinear coordinates are a coordinate system for Euclidean space in which the coordinate lines may be curved. These coordinates may be derived from a set of Cartesian coordinates by using a transformation that is locally invertible (a one-to-one map) at each point. This means that one can convert a point given in a Cartesian coordinate system to its curvilinear coordinates and back. The name curvilinear coordinates, coined by the French mathematician Lamé, derives from the fact that the coordinate surfaces of the curvilinear systems are curved. Well-known examples of curvilinear...

0celo7

@dmckee Ah!

So something like $T^{00}$?

but that's not a vector

dmckee

@0celo7 Guess so. I'm a bit out of my depth on this stuff.

John Duffield

18:35

@0celo7 : see Space and Time where Minkowski said this: "In the description of the field caused by the electron itself, then it will appear that the division of the field into electric and magnetic forces is a relative one with respect to the time-axis assumed; the two forces considered together can most vividly be described by a certain analogy to the force-screw in mechanics; the analogy is, however, imperfect". The field of the electron is the electromagnetic field.

E and B denote the forces that result from electromagnetic field interactions.

And I have to go. Back later.

Stan Shunpike

@ACuriousMind so what about within QFT? rhat was why i took the time to learn about it

0celo7

@JohnRennie wassat

@dmckee would e.g. Jackson have this

ah!

$F_{0\mu}F_i{}^\mu=S_i$, @dmckee @JohnRennie

that's $T_{0i}$

dmckee

18:51

@0celo7 Looks promising.

0celo7

@dmckee now what

that's some crazy 3-vector, which are not useful in GR

dmckee

@0celo7 Well, there is a time-like part that comes out of your contraction, so the whole thing is still a four vector right?

0celo7

@dmckee No...$F_{0\mu}F_i{}^\mu=\epsilon_{ijk}E^jB^k$

if you get the summation notation

dmckee

Now you try to make that four vector play the same part as the plain Poynting vector plays in E&M the Mawell, not Einstein way. The spacial part is going to point in the direction of energy flow.

0celo7

well I wonder what $F_{0\mu}F_0{}^\mu$ is

@dmckee yes...

time to read the Wiki article on the Faraday tensor

dmckee

18:56

Ah, yes. latin versus greek indicies. I don't do that stuff very often.

0celo7

@dmckee Note that in my question I use abstract Latin indices ;)

which are equivalent to Greek ones :)

@dmckee self-educated guess: $F_{0\mu}F_0{}^\mu=|\vec E|^2$

Stan Shunpike

Do Newton's laws imply "conservation of energy"? Or is that principle a separate thing from those?

0celo7

@dmckee what's the significance of that in EM

@StanShunpike They do!

You want the proof?

Stan Shunpike

Yes! I'd love it

0celo7

Note: this is also useful for solving any ODE of the form $y''(x)=f(y)$

$F=ma=\ddot x$ (set $m=1$ for sanity)

Let $U=-\int F\,\mathrm{d}x$, i.e. $U'=-F$

Let $E:=\dot x^2/2+U$

then $\dot E=\dot x\ddot x+U'\dot x=\dot x(\ddot x+U')=\dot x(\ddot x-F)=0$

Stan Shunpike

19:03

wow

0celo7

chain rule used to take the time derivative of $U$

Stan Shunpike

yeah keep going

0celo7

...

that's it

$\dot E=0$

@StanShunpike now the trouble is when you have $F\ne F(x)$ but instead $F=F(x,\dot x)$

i.e. nonconservative forces

Stan Shunpike

remind me what that is

0celo7

friction

@dmckee could explain better

basically a force that is not the derivative of something

Stan Shunpike

19:07

Wow, that's a really simple result to prove. Interesting!

Cool!

0celo7

it's more difficult in 3 dimensions

dmckee

Newton's law imply the simple conservation of mechanical energy directly.

0celo7

you need some vector calc...but that's about it

dmckee

You have to be a little clever to include thermal energy (mechanical equivalent of heat and/or kinetic theory as a starting place).

0celo7

so not "difficult," just more machinery

dmckee

19:08

Then you have to start generallizing to field energy in E&M and the like.

Or at that point you can just appeal to Noether's theorem, which gives a much less muddled foundation for the whole thing.

0celo7

@dmckee at that point why not just use Noether's theorem

heh

dmckee

Gat minds. Or at least twisted ones.

0celo7

well great mind tell me what significance $|\vec E|^2$ has pls

Stan Shunpike

@0celo7 Yeah I wouldn't put that in the category of difficult. More tedious.

dmckee

@0celo7 It's the energy density of the field to withing some constant factors of $c$ and 2 and maybe some $\pi$s depending on your units.

0celo7

19:11

@dmckee energy density also has a $+|\vec B|^2$

@dmckee brb solving Maxwell's equations

dmckee

@0celo7 But in free space the fields are proportional to one another. So you can express it in terms of just one of them.

0celo7

also note that that's in your precious flat space

I'm looking at GR scales where everything we know and love ought to break

dmckee

Npote that I said "free" (as in matter free) not "flat". I don't know anything about what significance it might have in strongly curved space.

0celo7

yes, $|\vec E|=|\vec B|$ in flat space and in a vacuum

@dmckee because Maxwell's equations are completely different in curved space!

if you read my question you would know that :(

no_choice99

I'd like @DanielSank to have a look at the comments of John Rennie's answer of physics.stackexchange.com/questions/224845/… , I think Daniel could make valuable comments.

0celo7

19:43

@BalarkaSen what are you doing here

Balarka Sen

Looking for Danu

0celo7

@BalarkaSen hmm, you don't want to do some physics

Balarka Sen

too busy hat-hunting

0celo7

uhhhhh

no_choice99

hats are going to disappear in january i think

i haven't figured out how to use more than 1 hat at once

0celo7

19:59

@no_choice99 why do you think there's a way

no_choice99

because I think I had seen John Rennies with several at once, but I may be wrong

0celo7

@no_choice99 he's magic tho

Stan Shunpike

20:32

@0celo7 Suppose I am considering a box resting on the surface of the earth. In the context of GR, is it appropriate to say that the box is in the earth's gravitational "field"? I forget if the fact that gravity "isn't a force" means that it also isn't a field.

I suppose EFE implies field

0celo7

yes

note that it is always in the Earth's field

since gravity never "dies"

Stan Shunpike

what do you mean?

0celo7

define "gravitational field" first ;P

@StanShunpike the box always feels gravity due to the earth

Stan Shunpike

Well, I always think of the Newtonian gravity because I use it most frequently.

0celo7

it's not like you can actually go to $r=\infty$

Stan Shunpike

20:34

@0celo7 How do we know we can't go to $r = \infty$

?

or is that just an assumption

0celo7

@StanShunpike what

$r=\infty$ is not a place

it's a limit

a limit in which an asymptotically flat gravitational field will "die off"

Stan Shunpike

ah okay

i get it

0celo7

@StanShunpike in any case, the wording is not important

hell, I can call gravity a force all day long

doesn't change my math ;)

Stan Shunpike

really?

I thought one of the points about GR was it wasn't a force

0celo7

define force

vzn

21:22

@dmckee wondering more the connection between instantons & solitons... re wikipedia article on instantons:

> Note that there is sometimes also a corresponding soliton in a theory with one additional space dimension.

AngusTheMan

21:52

the shocking moment when you see other members of the PSE on your suggested linkedin contacts! small world eh!

Mr. Y

hey guys

I feel a little bit embarassed to join up a conversation like that and asking for some suggestions on physics textbooks oriented on problem-solving ,but that's what I've been told to do...so if you can help I will be really gratefull....

alarge

@Mr.Y What level and topic did you have in mind?

0celo7

0

Q: Is there a Maupertuis principle for General Relativity?

The motion of a point particle in classical mechanics is given by Newton's equation, $\mathbf{F}=m\mathbf{a}$. Suppose all forces considered are conservative and we have a constant total energy $h$. Let $M$ be the configuration space of our system, $T^*M$ its cotangent bundle, $(\mathbf{q},\mathb...

general-relativity lagrangian-formalism hamiltonian-formalism action geodesics

Random thought turned into a question

Mr. Y

@Alarge Well the standard curriculum used at high school level(I finished high school and am taking gap year )

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