The h Bar

12:59 AM

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Q: Why does improving closed questions + flag to reopen (almost) not work?

Improving closed salvagable non basic non homework questions (homework questions can easily be salvaged by the OP stating his efforts, that does work) does hardly work hear on Physics SE. At least not when I try it. And now, concernig one of the more recent questions I tried to improve and flagg...

tpg2114

1:37 AM

Anybody home?

If anybody has some time and can settle this with an edit, I'd appreciate it rather than the revenge downvote: physics.stackexchange.com/a/67297/6634

My tensor notation is rusty, but I know a^{jj}b_{jj} isn't correct notation. It should be something like a^{ik}b_{jl}\delta_{kl} or something

user54412

3:03 AM

@tpg2114 I'm on your side, but I'm too non-confrontational to edit...

user54412

actually, I'm not sure the expression can be salvaged

user54412

a trace may be basis-independent, but a^{jj} b_{jj} is the trace of a matrix formed by dotting all entries

user54412

("dotting" as in dot-multiplying, a la Matlab notation)

user54412

is that even invariant?

user54412

3:25 AM

btw @tpg2114 Welcome back - you haven't really been around for a while - I almost started answering fluid dynamics questions myself, and that would have been a disaster

tpg2114

4:32 AM

@ChrisWhite I realized it's not the trace of the product but something else and can't for the life of me figure out what it should be... And I've been floating around for a bit, just haven't had much time to answer anything :)

ManishEarth

It's a wrong use of notation

user54412

@ManishEarth Do I remember right - you said you were in the Himalayas recently?

ManishEarth

Yeah

tpg2114

@ManishEarth I didn't think it was Einstein summation but I didn't know what it was really. And I still can't figure out how to represent that same sum in Einstein summation... and it's really bothering me I can't

user54412

One of my profs (and extremely sharp-eyed observer) told me a bunch of Messier objects are visible to the naked eye from there

ManishEarth

4:40 AM

@tpg2114 you can't. You're not supposed to.

tpg2114

That might explain why I can't

ManishEarth

@ChrisWhite At nights I was in a well-lit area, so not really. Still, many more stars than Bombay. However I used to live in Malden, MA before, and the sky is much, much more populated there.

user54412

@tpg2114 It's bothering me too...

user54412

I played around with 2x2 matrices to see if I could construct a simple demonstration of basis-dependence, but I couldn't

user54412

but surely the quantity must be basis-dependent... I think....

tpg2114

4:41 AM

I could only "cheat" and make a new matrix, c_jk = a_ij b_ij and then do c_ll or something. So I could write it in two steps

There's gotta be a way to do it in a single step though

c_kl, not jk... etc. You get the point

Actually that doesn't work either

Sinc a_ij b_ij is a scalar

Maybe it's just a_ij b_ij for all i == j

And that's the only way to write it

Without upsetting Einstein

:)

ManishEarth

Why doesn't the kronecker delta work?

oh wait covariant contravariant

tpg2114

How would you use it? I tried a way to do that but couldn't come up with one that I thought worked

ManishEarth

I suppose $a^{ij}b_{ij}\delta_{ij}$ wouldn't work, right?

Because of covariance/contravariance?

(I'm a bit rusty with tensors)

user54412

@ManishEarth indeed

user54412

well, the delta is special

tpg2114

4:45 AM

That would still have an index repeated three times no?

user54412

it is more amenable to being fast and loose with up/down-ness

tpg2114

Sure, but if we're being fast and loose with notation than a^{jj}b_{jj} is good enough cause it's totally clear what it means :)

ManishEarth

@tpg2114 oh right

user54412

also, for instance, some authors write $\delta^i_j$ without distinguishing between $\delta^i{}_j$ and $\delta_j{}^i$ etc.

Physics Meta

0

Q: Why is Physics Stack Exchange now plain text?

I realised that suddenly, Physics.SE and meta have both gone plain text! Even LaTeX support has disappeared! What has happened? Is this a bug or was this done for a reason?

bug

tpg2114

4:48 AM

On the plus side (for me), in fluids and structures, everything is _ always

I don't deal with tensors with stuff up top

user54412

well if my conjecture is right and a^{jj} b_{jj} is not basis-independent, then necessarily there is no way to express it properly in Einstein notation

user54412

but I'm too tired to figure things out apparently

tpg2114

I think you're right, but I don't really want to get into a pissing match in comments about it... I think I was already downvoted for it

I don't really feel like continuing. I wasn't trying to antagonize, but if the question is about Einstein summation notation, the answers upvoted should have correct notation in them :)

ManishEarth

@ChrisWhite Sounds like a nice question for the site, if phrased properly

user54412

@tpg2114 Yeah, I've been downvoted once or twice in similar situations after pointing out errors

tpg2114

4:51 AM

It's a good thing rep is cheap ;)

ManishEarth

@tpg2114 Looked obvious to me that you weren't, but sometimes a comment on your work gets taken a bit too personally

user54412

sometimes I wish the site didn't tell me about downvotes for a random interval of time, so I couldn't piece together so easily who was behind it

ManishEarth

@ChrisWhite or, you know, you could restrain yourself from piecing it together :P

tpg2114

Or better yet, spiteful people would realize it was easy to put together and staggered it!

user54412

@ManishEarth seriously? trying to get me to ask a second question? :P

ManishEarth

4:52 AM

I agree though, I've seen cases where folks track downvoters and serial-downvote them

user54412

@ManishEarth I don't try - sometimes it's just too easy

ManishEarth

Your case is OK, afaict you only serial downvote them in your mind :P And possibly shoot those telescope lasers at them. But others do use this to track down downvoters.

@ChrisWhite lol. You probably could even ask it on math.SE (not sure though)

user54412

hmmm, I do have a question I want to ask at some point here

user54412

I'll have to double check to make sure it isn't an exact duplicate

ManishEarth

@ChrisWhite what is it?

user54412

4:56 AM

basically I want a canonical question along the lines of "how exactly does faster-than-light travel break causality in SR?"

ManishEarth

Hopefully something awesome like your neutrino one

I was about to say that the answer is pretty easy

user54412

I've seen many bits and pieces of an answer (I've even hinted at it myself), but I don't think we have a good post on it

ManishEarth

Then I read "canonical"

makes sense

user54412

yeah - I want to see the spacetime diagram with A, B, and C

user54412

B is spacelike-separated from A, but "in its future"

user54412

4:58 AM

then in another frame C is in the "future spacelike separted region" of B

user54412

but C is in A's past

user54412

basically, just to drive home the point that any ftl transmission of information leads to closed timelike curves

user54412

and I especially want someone else to make the nice diagram :P

tpg2114

I think the only question I have is how to graduate faster

But not even academia.SE could help me on that one

ManishEarth

@tpg2114 bribery

:P

(their chatroom may help)

tpg2114

5:04 AM

Unfortunately I can't buy results... Once I get results, I might be able to bribe people to ignore the fact they suck... but I need results first :/

user54412

I know that feeling...

ManishEarth

@tpg2114 What exactly is your research about?

tpg2114

I'm working on simulating detonations and surface burning in condensed phase explosives and solid propellants

user54412

@ManishEarth the unfortunate thing about academia is how fast it gets outdated - I wouldn't really trust what many of the regulars on academia.SE (the faculty at least) have to say about finishing grad school

tpg2114

The goal is to come up with a method to bridge the scales from quantum molecular dynamics through the micro-scale to the macro-scale (rocket motors, full sized explosives, etc)

ManishEarth

5:06 AM

@ChrisWhite not all the regulars are faculty

user54412

true

ManishEarth

@tpg2114 Key thing to note when talking to me: Using the word "explosive(s)" will guarantee that I don't read anything else :P

But sounds interesting :P

tpg2114

I'm not a fan of it...

ManishEarth

@tpg2114 a fan of?

tpg2114

How about calling it numerical methods? Multi-scale methods? Fluid-structure interaction?

My work :)

ManishEarth

5:08 AM

oh

I try to work on things I'm interested in, but I've not really worked on anything yet. (Working with a professor on graphene, but I'm still in the learning stage)

user54412

@tpg2114 Sounds complicated - certainly the propagation of detonation/deflagration (is that word used outside astro?) waves in supernovae is one of the biggest computational problems in astrophysics today

user54412

no one knows how to model it correctly

tpg2114

Yeah, deflagration is just the subsonic combustion wave. It's used in our work too :)

Chemistry at the supernovae scale must be difficult since the flame fronts, particularly for a detonation, are so thin compared to the scales of the supernovae

Although I suppose it's similar here. I need to bridge atomistic length scales through to 10's of meters.

user54412

i guess - I never really tried my hand at the numerics

user54412

i don't know if it helps or hurts that everything we consider is not just atomic but fully ionized, and the burning front is based on nuclear fusion rather than chemistry

tpg2114

5:12 AM

It's also way harder for you guys because you have to account for the MHD effects

We have a group in our lab that looks at MHD. Mostly related to turbulence and plasmas

user54412

yeah - that's another huge area in computational astro

tpg2114

We have somebody who is doing nothing but RMI studies with and without MHD

user54412

the keyword these days in astro MHD is "magnetic reconnection" - we have entire conferences based solely on it

tpg2114

The one thing that never ceases to amaze me is that almost all of the universe can be described with the same 2nd order PDE

Fluids, solids... big things, small things, stars down to insects. It's all just a 2nd order PDE

user54412

that is rather cute... and infuriating when you realize all your struggles are about something that sounds so simple

tpg2114

5:16 AM

Yeah. I always get pissed off when I remember that the Navier-Stokes equations really just say F=ma

ManishEarth

@tpg2114 which one?

tpg2114

And you can't create mass or energy

ManishEarth

ah thought so

tpg2114

Things were so much easier when F = ma was all we had to write down... :)

Even Schrodinger's Equation is a 2nd order PDE

If I recall correctly

user54412

yep in most cases it is

tpg2114

5:25 AM

I think I'm going to bed now. Have a good night!

user54412

night :)

Jayesh Badwaik

Anyone around with a QFT background?

user54412

@Jayesh sorry, I probably can't help with anything too advanced :/

user54412

5:40 AM

but if you post something here, others may see it when they stop by

Jayesh Badwaik

Ahh, its okay...
I will just ask my question then. Hoping someone might answer.

I am not really familiar with the domain of quantum field theory and have only an undergraduate knowledge of quantum mechanics. I have a solid state device which I am trying to model. I recently saw a paper which uses quantum field theory to model it. However, as far as I know, quantum mechanics can reliably approximate quantum field theory at low energies.

So, my question is, are there some other limitations apart from the relativistic energies where the quantum mechanics does not work, and instead we need to use QFT?

user54412

@JayeshBadwaik there are several different philosophies on how to go from QM to QFT pedagogically - one of them is indeed trying to have a relativistically correct QM

user54412

another is based on many-particle QM

user54412

it's not so much a need in the latter case as a recasting of the problem to make it tractable

Jayesh Badwaik

I see... I am more for the latter case I guess rather than the former.

user54412

5:51 AM

yeah, my first exposure to QFT was built up from taking ensembles of particles and imposing various (anti-)symmetries on the overall wavefunction

user54412

but QFT is a light hobby of mine - there are definitely more knowledgeable users who visit this room who can say more

Jayesh Badwaik

Ahh, one more very naive question, I am more interested in solving the problem numerically. So, when you say tractable, do you mean, making the problem easier to solve, or easier to model analytically...

user54412

i meant analytically, but I'm reasonably certain there are numerical techniques based on QFT rather than vanilla QM

user54412

but condensed matter computations are something I've never even come close to touching, so I'll stop now before I say anything unreasonably stupid

Jayesh Badwaik

Yes, there are. :-) All of them are currently beyond me (coming from an engineering background). In contrast, I already have a nicely working Schrodinger and Poisson equation FEM solvers, so I was thinking, before letting go of the problem, will it make sense to solve the vanilla QM version.

I hope to learn QFT in my grad school. I will attack the problem again then hopefully.

I think I will give a vanilla QM try anyhow on a weekend.
Thanks for the advice. Gotta go now. See you later... :-)

Physics Meta

6:03 AM

-3

Q: Classification of the questions?

I just realised that the questions on the site are too mixed up together. On the main page, a question about Newtonian Mechanics is often side-by-side with a question on the standard model/general relativity/string theory etc. I think this is quite messy so a classification would be quite good. N...

feature-request

0

Q: How do I see equations in latex with certain add-on softwares with firefox?

I use firefox (latest version) with add-on programs like no-script and adblockplus for security reasons. The problem is that I am unable to see equations written in latex in Physics stack exchange. Even if I let no-script "temporarily allow" the current page, I am unable to see the latex equation...

support

Ϛѓăʑɏ βµԂԃϔ

6:15 AM

@ManishEarth: meta.physics.stackexchange.com/questions/1282/…

@ManishEarth This makes me feel mature :D

ManishEarth

so?

ha

Ϛѓăʑɏ βµԂԃϔ

@ManishEarth Okay..! How exactly should I take that "Ha"... - Is it "Hah..!" that agrees with the msg or "Haaa" which implicitly says WTH? Not in your life or "Ha" which is more like an "Ahhh..."

lot of timeouts - I shouldn't come to chat till I come to colg..! :/

l3win

I'm looking for a flow solver for a turbulent flow field over a flat plate that uses the Cebeci Smith algebraic model.
Is there commercial software out there that utilizes this method?

ManishEarth

@tpg2114 ^^ any ideas?

Brooks

6:48 AM

Hello.

Ϛѓăʑɏ βµԂԃϔ

7:02 AM

@Brooks Hi Brooks... Welcome ;-)

Brooks

7:35 AM

I have some trouble with heat flux, would you mind give me some help?

Brooks

8:28 AM

Is somebody here?

Brooks

8:42 AM

Although I have been asked several ques about heat flux, but I didn't get the contented answer. Help.

Physics Meta

11:04 AM

0

Q: What is required for commenting in someone's question

I am trying to comment on people's question but there is no such option of doing so. Do I need higher reputation ? If yes then how much?

support

tpg2114

3:21 PM

@l3win I don't know of any that has it already implemented, but there are several papers where people put it into Fluent as a UDF. You could also likely put it in OpenFOAM relatively easily if you are comfortable with C++

Ϛѓăʑɏ βµԂԃϔ

4:01 PM

I can't understand why this is happening...

physics.stackexchange.com/review/late-answers/17529

Similar things like these are in Late A review queue..

The engine says it as a new answer.. :O

All 7 posts in the queue come from the same guy... physics.stackexchange.com/users/6032/zephir

Dilaton

4:24 PM

@Ϛѓăʑɏ βµԂԃϔ that is a troll well known in the physics blogosphere, always promoting his personal physically not consistant fringe theory while at the same time he insults main stream physicist and their work. Not exactly the kind of new users we need here ... :-/

Ϛѓăʑɏ βµԂԃϔ

4:50 PM

@Dilaton Well, I can see that :D ... But, the thing that's bugging me - why does this old forsaken post comes under the queue now and says - "This is a new answer posted to an old question" :O

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