The h Bar

Semiclassical

2:33 AM

@enumaris magnetohydrodynamics

enumaris

3:09 AM

sounds legit :D

Semiclassical

lol

i dunno much about the subject myself but it's a good name

Gaurang Tandon

@Yashas Hello! I wish to ask about life at IIITH (BTech CSE), because I am thinking of joining it too (this session) :) Are you free for a few minutes? I only have a few questions, won't take much time. Thanks!

Secret

3:31 AM

Last night dream is a wildride (and I think it is the one which is put towards solving problems a lot more than my past dreams). To keep it short:

1. The whole concept of Poliical topology is presented which defines a topology and apply the notion of convergence and limits recursively so that policies get converged to some optimum set, and that is used as the decision

2. An asian girl with glasses called Winstor Eahn doodled something on a blackboard showing there are 6 classes of elementary particles, and on one of the circles on the board it is about something called chrome and antichrome quarks

3. A very lengthy biology class where an E coli sample is prepared on the surface of some capsule thing, and is set for incubation for 75 seconds. The teacher than instructs the class to apply some cranberry colored stain on it, as well to convert a ruler into some kind of foldable tool with a blade at the end for cutting some potassium metal under oil

tandfonline.com/doi/10.1080/0969725042000272834

Political topology in real life, meanwhile, is not very precisely defined

Anonymous

4:11 AM

nytimes.com/2018/06/18/us/politics/…

Anonymous

Lol

Anonymous

@GaurangTandon You can contact him on Facebook, as he doesn't visit this chat often. You can mention me.

Zee

I always wonder what physics people are up to

Jalapeno Nachos

I have some confusion with using a co-rotating reference frame for a falling rigid body. In the co-rotating frames, the x-y solutions are always supposed to be zero, with the center of mass assumed to be at the geometric center of the body. (I know how to convert the solutions to laboratory-frame coordinates.)

Now, if I shift the center of mass of this body and then recompute the quantities of my model to account for a new center of mass location, e.g. compute the moment of inertia about the center of mass, then afterwards, I run my simulations again, and the x-y body frame coordinates are no longer all zero - in fact, the values are really big. Have I done anything "wrong"?

Secret

well, you are on on the wrong time, chat is quiet atm

Zee

4:18 AM

@Secret what do you think about string theory anyways ?

Secret

Not very optimistic compared to back in 2011 due to consistent negative results of supersymmetry

That the mathematics can be translated to condensed matter system seemed to be the only way out if string theory dies

Zee

Could string theory really just die?

As a math guy it feels weird to me that a whole domain of knowledge could just be not true

Secret

I mean, it could die as a physical model, but its mathematics is self consistent, thus the maths will still survivie

There are still some high energy regimes that has not been tested yet, but it will take colliders a while to get there

Zee

I suppose , what would be an example of some of this mathematics ? Mirror symmetry on Calabi-Yau manifolds ?

Secret

yeah, or maybe like duality on calabi yau manifolds, and its associated functional space

Zee

4:25 AM

Ya but it seems to me this stuff in mathematics is also conjuctural

So it may all be false too :p

Secret

I have not followed closely on how much about M-theory and string theory the physics community have uncovered thus I am not sure how many conjectures are still unproven

Zee

Is there research going in gauge theory ?

Secret

There should be, since any high energy physics need some form of gauge theory, but I am unclear about the details. You should ask Acuriousmind or DavidZ about that

Zee

Why is string theory based on complex manifolds ? I realize in even dimensions it makes sense to describe things using complex manifolds

But it seems to make the unfiication with the riemanian geometry of GR harder

Secret

GR is lorentzian, not riemannian

Zee

4:32 AM

As in pseudo riemanian manifolds ?

Couse their almost the same

Secret

yeah, pseudoriemanian

Zee

Ya , pseudo riemanian manifolds. These tend to be almost the same as riemanian manifolds, this is evident from the fact that almost every riemanian geometer that I know knows GR very well

What do you think about the Feynman path integral ?

That shit is crazy

Jalapeno Nachos

How do multidisciplinary scientists with primarily a math / applied math background become researchers in an area like Fluid Dynamics, while feeling like there's a vast amount of fundamental physics that they don't know or are good at?

Zee

Why not ?

Jalapeno Nachos

I typically see a lot of advanced topics courses at our school, on fluid dynamics, that state "on physics background is assumed".

Zee

4:37 AM

You cN take fluid dynamics to a very high level , in par with string theory if you ask me

Jalapeno Nachos

(In our math dept.)

Zee

Oh

Jalapeno Nachos

@Zee without knowing much physics at all?

Zee

Well , am not a fluid dynamics guy but

A lot of that research is done in the math dept

Jalapeno Nachos

on = no up there ^ sorry for the typo

Zee

4:38 AM

you def don’t need to know particle stuff with it

Jalapeno Nachos

I see

Zee

It’s more like PDE stuff then physics frankly

Jalapeno Nachos

@Zee yeah, true

thanks @Zee

Zee

np

For example in my school , the PDE group has continuem mechanics as an area of research

i Would actually say the physics stuff that does not require quantum mechanics is mostly done in math dept now a days

General relativity is done in math dept , fluid dynamics , Schrödinger operators and so on

Physics is more like string theory , quantum stuff , condensed matter and nuclear and cosmology

I would make the following hypothesis , all physics after 100 years becomes an area of research in math

Jalapeno Nachos

@Zee I see

@Zee a friend of mine who's closer to theory said to me something that's interesting: that they don't find simulations trustworthy. I work in mostly modeling and simulations, and are close to the experimentalists, too.

What do you think?

Are the out-of-the-box fancy physical modeling tools, with proprietary Matlab algorithms / source code (not open source) not trustworthy? We don't use the out-of-the-box stuff, we build our own code, but I'm wondering now...

Zee

4:53 AM

Well, the applied math guys that I know all do stimulations, and the pure math guys hate them .

For me , I don’t know enough to say trustworthy or not but

Frankly , am not too interested in them since I don’t like computers much

Jalapeno Nachos

I see

Zee

I don’t think all simulations are equal though

vzn

@Zee hi, welcome to Physics, speaking of that there seems to be some deep connections between fluid dynamics/ deeper physics eg string theory. a key bridge area is "solitons". vzn1.wordpress.com/2018/05/25/fluid-paradigm-shift-2018

Zee

A simulation that behaves by a simple rule that I can understand is much better than a simulation that runs on a large formula , too large for my brain so to speak

vzn

@JalapenoNachos hi, welcome, simulations can be made as trustworthy as any theory, but/ and "the devil is in the details™"... someday simulations will advance the (very deep) theory, that is already happening at a medium level on many fronts...

Zee

4:58 AM

@vzn that does not surprise me , fluid dynamics has deep ties with the deepest areas of mathematics

vzn

@Zee lol, music to my ears! tell that to the physicists in here, many of which have much different ideas/ are averse... :o :(

Jalapeno Nachos

@Zee I agree - I am happy with my own simulations work so far, primarily because my advisor's style is in keeping things as simple as possible. I'm glad I didn't start out using a fancy package that I would never understand and know what's under the hood...

Zee

Well , frankly I don’t blame them , since many people who work in the area don’t explore these deep connections. But these connections are there , for example fluid mechanics and Gromov H principle , or the work of vladmier Arnold or the theory of viscosity solutions in PDE theory

Jalapeno Nachos

@vzn I see - that would be really exciting

vzn

@JalapenoNachos which school? are you talking undergrad or grad classes? what kind of simulations are you working on?

@Zee interesting, thx for tip, have not heard of these & need to look further.

Zee

5:02 AM

I don’t know much about them frankly , but I heard them during some talk I attended

vzn

think the time is ripe for someone to get a major/ top recognition/ award for simulation-oriented or -focused work. there are probably some examples already...

Zee

Oh , ergodic theory also in fluid dynamics

Which uses simulations so that you may like

Jalapeno Nachos

@zee interesting, thanks for that

vzn

@Zee have studied collatz conjecture for many yrs & its considered part of ergodic theory by some, has some connections.

@Zee "simulations" can be seen as part as pure math in the more general area of "algorithmics" making inroads eg "theorem proving" eg consider 4 color conjecture etc...

Jalapeno Nachos

@vzn regarding awards, at our mostly applied math department, a few of our profs. that have won major awards won it only for their theory work, not their applied work. and students generally perpetuate the idea that an applied mathematician is automatically not in the running for a top award...

vzn

5:07 AM

@JalapenoNachos agreed there are more strict boundaries/ distinctions today but they are blurring and will get much blurrier in the future. thats the trend. its a paradigm shift in progress. o_O :)

Jalapeno Nachos

I see

vzn

look at some of the very elite work/ prjs being done in (algorithmic) thm proving eg classification of finite simple groups etc...

Zee

Cédric Villani got a fields medal for work in applied mathematics

vzn

@Zee lol didnt realize it was called "applied," seems some stretch there. just read his excellent book "birth of a thm" & was recommending it in here... there are several professed villani fans around but apparently none deigned to read it yet...

Zee

well he does research in both but I believe his medal was for work in applied math

And that is part of the story , I don’t believe in such a division frankly

vzn

5:12 AM

@Zee he describes it in detail in his book.

> Villani received the Fields Medal for his work on Landau damping and the Boltzmann equation.[4] en.wikipedia.org/wiki/C%C3%A9dric_Villani#Mathematical_work

Zee

these sound like applied math to me :p

vzn

the work is almost oppositely very abstract and "applied" at the same time... few physicists would be able to follow it...

Zee

Yes yes, I agree

The best kinda of work in my opinion

vzn

← huge villani fan! hes even branching into AI lately! =D

Mikhail

Have you ever had to numerically take the Fourier transform of a function with a singularity in it?

vzn

5:15 AM

in theory salon, Mar 29 at 1:05, by vzn

A Math Whiz Has Become a Crucial Political Figure in France (Cedric Villani + AI) / bloomberg

Zee

Ya he’s a cool cat

I actually like his area

vzn

@JalapenoNachos some distinctions/ discriminations are helpful, others are not/ artificial, and sorting that all out is "science/ progress"...

@JalapenoNachos fyi lots of cool collected refs on "experimental math" some very deep vzn1.wordpress.com/2015/12/29/…

Secret

I like algorithms that allow the exploration of high dimensional data sets in a flythrough fashion

frogeyedpeas

6:51 AM

Question: why are observables in quantum mechanics necessarily real functions of the state space? I was reading: en.m.wikipedia.org/wiki/Observable and I found their definition uncomfortably specific, as I feel you can define observables to be much more general things than real numbers.

Also, WiFi is choppy so if anyone tags me, I’ll get notified maybe in a few hours time. As of writing this message I don’t see any messages newer than my first.

John Rennie

An observable is some physical quantity that we can measure experimentally (in principle if not in practice). Experience suggests experiment measurements always return real numbers and never complex ones.

frogeyedpeas

Out of curiosity say I have some state space $S$. And I have a function $f: S \rightarrow \mathbb{Z}/3\mathbb{Z}$, that takes each state and assigns it a value from the cyclic group of order 3. It might be possible to construct an experiment where you generate just one of those values mod 3, and in that case this $f$ would be an observable that surely isn’t a real number

Mithrandir24601

@frogeyedpeas I've been asking this question for a couple of years now... It's a tad complicated - there are systems where you can use what's known as a weak measurement (it's a trick using postselection) to get a complex 'weak value', where the imaginary component gives the change in the conjugate variable. You can get similar ideas in optics, where the change in wavewector upon reflection can be complex, representing a slight transverse shift in position.

Open systems can also be described using effective (non-Hermitian) Hamiltonians with complex values but so far, this has only been (properly) realised in classical systems. Aside from all that, the general idea is that if things were otherwise, Hamiltonians would be non-Hermitian (note that non-Hermiticity does not imply complex eigenvalues) and so, time evolution would be non-Unitary, so this can generally only come up when looking at 'effective' theories of e.g. an open system

The issue with properly recreating a non-Hermitian quantum system (even in theory) is the addition of noise, which makes things a little more complicated

@JohnRennie Well... Give or take... (see example about reflection of light)

Slereah

morning

Mithrandir24601

@Slereah Rytsas Siereah!

Slereah

7:06 AM

@frogeyedpeas just ask yourself what you'll actually see in the experiment

If it's a cyclic group of order 3 you'll just observe one of three result, for which you can assign a real number

Yashas

7:20 AM

@GaurangTandon You can join the IIITH Counselling forum on facebook. You will also find my fb profile. I wonder how you found my SE profile.

frogeyedpeas

I guess I see what you mean, at the same time, instead of real number we could have easily said complex number, or any number of other uncountable sets with countable index. Given that the choice of real number is prettt arbitrary is there a different way to state the theorem that doesn’t talk about real numbers directly?

Well before we entertain that, what I’m trying to accomplish. I have this article, I read it, I felt like what it’s saying could be stated better? But does it matter... probably not. I guess it’s not a big deal and at this point just a personal hunch

I’ll buy it. I’m content now

Slereah

Well Hilbert spaces have a countable basis

if that's what you want

or at least the ones used in QM do

Gaurang Tandon

@Yashas hello :) I found you as RO of PSS room there ;) Okay, I'll msg you with my father's profile on facebook, as I am not yet on fb myself

Slereah

7:58 AM

4

Mikhail

Ugg, I think I need to numerically evaluate an integral over five variables for 64 million points.

Anonymous

@Mikhail What are you trying to do?

Anonymous

8:19 AM

@Slereah Is that a postulate in QM? I had that question too but never found a satisfactory answer. What prevents the existence of non-separable or non-denumerable Hilbert spaces?

Slereah

@Blue It is a postulate, yes

Anonymous

@Slereah I see. It's....physically motivated?

Slereah

I'm not sure

I think it's partly because non-separable Hilbert spaces are awful

Most theorems of separable Hilbert spaces don't apply there

Anonymous

Ah, makes sense

Slereah

in the same way that we generally don't consider non-paracompact manifolds in physics

although for those I do know the physical motivation

(Non-paracompact manifolds don't admit a metric)

Anonymous

8:25 AM

And what is the necessity of a metric (physically)? :P

Anonymous

(I'm a noob in this...so this might be a silly question)

Slereah

@Blue Well in physics we like to have distances

since we're pretty sure that distances exist

Anonymous

Okay, so that's some physical motivation there (though I don't particularly like it :P). Gotcha

Yashas

@GaurangTandon you will require one since the IIIT groups (clubs, ocommunity, etc) are on fb

Slereah

whether or not the space needs to be separable seems to be tricky though

but at least in most of the common cases, the hilbert space is separable

Jack Clerk

8:41 AM

Hello @JohnRennie ! I'm here =D

For the notebook haha

John Rennie

@JackClerk Hi

Jack Clerk

@JohnRennie hey

John Rennie

I'm just looking at the notebook link you gave (Formighieri's notebook) but I only have v10 and it gives errors when I try and evaluate it.

If you just want a basic calculation then I have a notebook someone hereabouts gave me (Danu I think) that calculates pretty much everything from the metric.

I can bung it on a server for you to download if you want ...

Jack Clerk

Yes, please. I appreciate =)

John Rennie

Hang on ...

Jack Clerk

8:47 AM

But given a metric and a program that spits an energy momentum tensor expression, what is the advantages of that?

Slereah

not having to do it by hand?

John Rennie

@JackClerk OK, look on swarchive.ratsauce.co.uk for the file AFT-Schwarzschild.nb

Jack Clerk

THanks!

John Rennie

NB this does the calculation but doesn't print the results. You just need to add the lines to print the various objects it computes.

Jack Clerk

Ok, thank you =)

John Rennie

8:52 AM

@JackClerk e.g. add ArrayRules[ChristoffelSymbolsULL]

The script was written by AccidentalFourierTransform, so you have him to thank

user228700

@JohnR: Oh, there you are!

John Rennie

@KaumudiH Hi :-)

Hangouts?

user228700

OK :-)

Jack Clerk

9:07 AM

0

Q: Question about physical character of 4-tensors in Special Relativity

Consider the following: Any candidate for a physical observable quantity in special relativity must to be expressed as a scalar quantity, constructed from tensorial quantities which describes the physical system and the observer by which the observable quantity is measuared. So, as an ex...

Mithrandir24601

@JohnRennie Oh, how could I forget? There's also particle decay

John Rennie

Observation of particle decay returns complex numbers?

Slereah

My detector just gives me $3 + i7$

2

what do

Jack Clerk

A question eligible for bounty is a good or a bad sing?

sign**

I mean, shows a lack of effort?

Mikhail

@Blue Trying to forward model the 1-st order born approximation for a symmetric potential, but for all incident angles, and colors. So Its really 4 dimensional integral, over the object in x/y, over the object in z, over the incident fields in x/y, and over the colors in \omega.

John Rennie

9:16 AM

@JackClerk eligible for bounty doesn't imply anything about the question.

All questions become eligible for a bounty a set time after they're posted.

@JackClerk downvoting well intentioned answers is not going to earn you friends here

Jack Clerk

@JohnRennie I know....It was too impulsive.

@JohnR You've read my question?

John Rennie

@JackClerk No-one is paid to write answers here. If people are posting frivolous answers then yes I'd downvote, but otherwise you're rewarding someone who is trying to help by a kick in the teeth.

@JackClerk I don't understand what you mean by the exact relationship between the concept of "information of the observer", tetrad basis and projections

Jack Clerk

@JohnRennie Yeah, maybe It was too hard. Thank you for the "social tip".

user929304

9:49 AM

@ACuriousMind Hi, thx :) you're right, but I meant to say, if we have microscopically sized cage, so if you downsize everything to a scale where QM becomes natural setting, do we then expect to find phenomena similar to Faraday cage at that scale?

glS

11:13 AM

7

Q: What is a qubit?

What is a "qubit"? Google tells me that it's another term for a "quantum bit". What is a "quantum bit" physically? How is it "quantum"? What purpose does it serve in quantum computing? (I'd prefer an explanation that is easily understood by laypeople; terms specific to quantum computing should p...

quantum-computer qubit

^if anyone wants to pass by quantumcomputing.SE and take a stab at this question, good answers are needed!

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