I don't understand why some people have a lot of research papers dedicated to calculate various correlators or n-point functions.

is this because the calculation of these correlators really of certain importance?

or is calculating these correlators an easy way to produce papers?'

Adding Greek to my phone's keyboard is proving surprisingly helpful as a physics major

@SirCumference ^ same

did that ages ago

@heather thank you! picked it very spur of the moment, but it's generated a lot of laughs

@danielunderwood lol...

I gave a presentation to my team about the transformer architecture today...pretty much nobody thought it was helpful...=/

Why people haven't made a cheap vr headset (with headphones included) only for 360 movies?

I mean, it doesn't sound like such a bad business idea... and I was wondering why such thing doesn't exist?

I mean, I know about Google Cardboard but that just... made out of cardboard and is not 360 movies specific.

When a lion is desperate, it would take risk to prey on a porcupine. When we are too hungry, we would eat what we don't like to eat. Often we eat some food not because we like to eat it but hunger haunts so hard.

hello @PM2Ring

hello @user8718165

@PM2Ring I need your help. We have a school exhibition...and I want to animate magnetic field lines of a quadruple magnet config and a mag field around a wire...can we do that using python or c++?

I just read "From a conceptual point of view amplitudes are interesting as gedanken experiments which can give insight into the underlying properties of a given theory." It turns out calculating scattering amplitudes can be just one kind of gedanken experiment.

@user8718165 I imagine it's very possible, but it depends on what you mean by "animate" here

You can draw lines easily enough with the right formula, but if you want some interaction of something in that field is going to be a bit tighter

@KyleKanos I want to show like suppose we put a small compass needle...then we can trace out the lines...but it seems too hard to animate

@KyleKanos I know how they'll look like...but have very little knowledge about animation...I locally can't find anyone who can help me with this...what libraries will I have to use...just need a little hint

To do an interactive animation, I haven't a clue. For static ones, it's just a matter of saving the state in a vector and iterating through with a graphics package (e.g., matplotlib.animate or ffmpeg)

@KyleKanos thank you...I'll try to work with these...but I don't think I'll be successful. Moreover there aren't many people around locally whom I can ask for help. I may have to change plans or drop out of the show.

Well there are possibly easier tasks that could be done, depending on whether the show requires some E&M or not

@KyleKanos we have to do something regarding electricity and magnetism. Newton's laws and all those things are for juniors...can you give me an idea?

Lol, younger kids. Adults still use Newton's laws

@KyleKanos sorry...just meant that

You could show the Lorentz force (e.g., particle moving in static magnetic field) without much trouble

$f=qe+qv\times B$ this one

Could also do MHD in 2D with some troubles

Yes, that one

@KyleKanos oh thanks a lot for the idea...I'll have to begin it from today or else it'll be too late

There's also solving Laplace or poisson equations

@KyleKanos currently I don't know much about those stuff

Well, the Wikipedia entries are pretty good

@KyleKanos these are important for animating lorentz force?

No, those are separate from Lorentz force.

@KyleKanos Okay then...I was afraid...I'll focus on lorentz force then

is there anything different between Lorentz force and force due to B: F=qv X B ?

@JMac you'll be able to read all the awful answers there soon enough

@KenWang some people will include the electric force in the Lorentz force i.e. F = qE + qv x B

indeed the Lorentz florce $f^1=-qi_uF^2$, $F$ is the electromagnetic field strength 2-form.

@StanShunpike whatever you're most comfortable with. Any modern system will work just fine.

mathematica will do. so will numpy.

@EmilioPisanty great, so I don't need like CAD software to do that...I wasn't sure because I saw some matlab tutorials and they actually had like, where you could drag and drop linkages and joints. And that would set up your system. and then you could simulate it

but if I have the systems defined as in step 1 i mentioned

then numpy is enough?

that would be great

i love python

@StanShunpike this will obviously depend on the system, and what your expectations are

if you want a system where you "drag and drop linkages and joints" and it automatically simulates the dynamics for you, then that will indeed require more specialized software

but if that's what you're after, what's the physics content of the project?

If you have already worked out the equations of motion, on the other hand, then numpy will be perfectly sufficient to solve them numerically.

Great, I think I'll go for that then.

Thanks @EmilioPisanty !

no worries

I never took a course on ODEs

or PDEs

Every time i open up a text for them tho

I don't really get why a whole course is needed for them

So far, the only ODEs I've used are like

Euler-Lagrange

and the wave equation

Why do schools offer a whole course on them?

Isn't it basically just calc?

@EmilioPisanty Slightly intimidating doing things from "the other side" now, but I'm doing what I can :)

@tpg2114 you'll get the hang of it pretty quick, I should expect

@StanShunpike in a word

No.

@StanShunpike University degrees generally include only one dedicated course on differential equations because there isn't time for more.

Hey @StanShunpike, how've you been?

@ACuriousMind Great! done with econ and back doing physics things. wasn't for me

@ACuriousMind what are u up to these days? what branch of physics?

and ur a mod!

@StanShunpike the branch of No, last we heard

@EmilioPisanty hmm ok I guess I'll just plow through some textbooks then and see what I learn

@StanShunpike No physics, at least not job-wise, I'm a programmer nowadays

@ACuriousMind Interesting! what kind of programming?

@StanShunpike I've been a mod for...2.5 years now! (Gosh time is flying)

@StanShunpike Code analysis (and the framework for running the analyses) for SAP's programming language ABAP

this (ongoing) video series might be a good starting point

@EmilioPisanty great! i'll check it out

particularly as to why it's so hard

And then there's the numerical solution to ODEs/PDEs, which is a whole other area that's not trivial.

@ACuriousMind @StanShunpike ACM is being shy here. He means to say that he maintains COBOL code for a living.

@tpg2114 I imagine Sanderson will get to it in due course on that series.

that video has some intro material

"living" is a kind word for the state of undeath some of this code exists in...

hahahahaha that's my impression based on reading the wikipedia page

though it does bear emphasizing that numerical analysis as pertains ODEs and PDEs is a huge branch of its own

maybe a skim of the relevant chapters of Numerical Recipes will be a good sampling, at least for flavour

::sigh::

25 questions to go

and -522 score to go

why is [optics] so slow?

Clearly that's telling you your answers are too good on average :P

@ACuriousMind well, about 70% of that imbalance is a single question

Ohhh, I'm about half way to a [fluid-dynamics] gold. So in about the year 2026 or so I might get it...

heck, 830 of that score is just my top five answers in that tag

..... all of them via HNQ, of course .....

@EmilioPisanty +368 for 20k views is still excellent

My best is 206 for 63k views

@ACuriousMind well, it's exceptional circumstances, innit

and we're unlikely to see the like again

it's for the better, of course

You just have to wait for the next round of Nobel Prizes, no? :P

but a nine-day streak on HNQ like that is no longer possible

Ah, you mean that

@ACuriousMind also that

but a more accurate description is "the next Nobel Prize that I actually know something about"

which'll be a while

I'm super stoked that they gave it to CPA, but it does come with a bit of a downer - we're much less likely to see a Nobel prize for attosecond science in the next five years or so

anyways, though - any bets on the next Nobel prize?

@EmilioPisanty I think users can no longer choose to make their posts community wiki since it was more often abused than used

@ACuriousMind ah

oh well

Though I've found a friendly moderator will convert a post to Wiki if you ask nicely.

Also, good idea on that post

it's probably a good idea to go through this search and canonize threads wherever appropriate

As you added faq-proposed to it, will you add a link to itself if it becomes faq? :P

@ACuriousMind arguably, it's already linked

@ACuriousMind @tpg2114 @dmckee @DavidZ @Qmechanic @Chris @rob y'all should consider adding it to the Featured tab for a couple of weeks or so. Partly so it acquires enough score that it shines through in search results, and partly so that folks can look in and say "hey, this could really use a FAQ entry on X"

@ACuriousMind first one to read it gets to start the discussion on the mod chat room about whether to do it or not

dammit

@PhysicsMeta good stuff, Meta. You always take ages to chat-bot stuff, though.

hmmmm. Now I'm really curious to see whether this thing gets struck from the record

How to relate the two introduction to surface tension, First one is it is tenacity of surface towards the bulk , the enclosing-type thing; In this picture it is obvious that the force would be perpendicular to the bulk. But In second introduction, An imaginary line and force on both side is taken which have nothing surface and bulk thing.

@tpg2114 hey big congrats on your new position! speaking of that, actually was curious about your fluid-dynamics povs. how did you come to specialize in it? curious, have you heard of Couder-Fort-Bush work? any reaction?

Is that work the droplet on a fluid surface that mimics the particle/wave dynamics? It's pretty awesome, but way outside my area of turbulent combustion and propulsion

I saw one of their early presentations on it at an APS meeting years ago

@tpg2114 cool, yes, there is new controversy on it as if it wasnt already controversial enough. bohrs grandson worked on it. itd be interesting to hear more about your own work sometime. :) quantamagazine.org/…

@EmilioPisanty Good idea! Done.

@vzn my work is not nearly as exciting as trying to prove fundamental parts of reality... Haha.

@tpg2114 lol "exciting" is just a hyped up version of "interesting" ... am sure others would be interested to hear more also :) (many have expressed underwhelment here wrt CFB.) btw EP was citing/ endorsing a semifamous book on rocket propulsion chemistry recently, have to dig it up...

Ignition! is the title you're looking for probably

Is a fantastic book, very entertaining. I've read it a few times over

I've had many discussions about that book with some of my colleagues in the rocket lab.

@tpg2114 yep! thats it thx! apparently a insider favorite among physicists. is your work classified?

Nope. I do basic research. Everything is intended to be open and published

lots of related news in the headlines with musk + bezos these days, plus new NASA probes etc

Yeah, the rocket side captures the imagination. Jet engines are much less interesting or exciting (for most people)

Plus jet engines have been making very small changes for 80 years. Rocket development stopped ages ago and is just now making a big comeback. That means they make big, obvious progress quickly and so it is much more compelling

so do you work on something related to jet engines? its still an outstanding field... have collected a few articles lately on the 737 troubles... o_O

I'm in an air breathing propulsion lab, but I do model/algorithm development for simulating any type of reacting flow. So I work on air breathing, rocket, scramjets, etc

cool :)

Mostly focused right now on how turbulence and chemistry interact under relevant conditions, how to make simulations reliable and predictive, and how to quantify uncertainty and solve stochastic systems

have long thought fluid dynamics is the future of physics in key areas eg cosmology + QM + theories of everything etc...! still waiting for rest of world to catch up o_O

I'm pretty sure all the universe is a second order dissaptive pde

@tpg2114 oh wow found a related paper on that discussed at substantial length in here. "spacetime fabric" Tenev + Horstemeyer arxiv.org/abs/1603.07655 alas again "underwelment" would be an overstatement about the reaction around here lol :(

After three years of it it gets kind of tiresome that you try to plug this paper to every single user you talk to :P

einstein is cited in here a lot too :|

@ACuriousMind I need to boost my h-index, maybe I need to adopt this strategy

@tpg2114 anyway are you talking about spacetime fabric metaphor? big bang/ expansion etc? can you expand on that?

@vzn you give me far too much credit... I just meant that if you look at almost any model we have for how matter behaves - solid, liquid, gas, reacting, mixing, whatever. It's all the same set of pdes

At least on the continuum scale.

@vzn more like debunk-whelment ;)

@tpg2114 agreed! actually am just extending your own observation :)

@bolbteppa lol aka throwing baby out with bathwater™ :P

@bolbteppa did you see @Semiclassicals new idea? just couldnt wait to tell you... o_O

@vzn no, where

I think i actually asked a question on the main site about what the equivalent modulus of elasticity would be for gravitational waves. I don't think it was well received (or well conceived - it's a very uninformed question)

@tpg2114 !!! think it is a very well informed question. did it get deleted? would like to look at it. that is (one) focus of Tenev + Horstemeyer analysis. etc

John linked that very paper on that question 3 years ago :P

@tpg2114 in a material things like stress and strain arise because the particles are bound together due to internal forces (e.g. chemistry etc) and can withstand certain forces, gravity waves like electromagnetic waves shouldn't be expected to have the same properties

Yeah, @ACuriousMind beat me to it

!!! ("the plot thickens™") afaik/ afaict JR seems to have said close to zilch on Tenev + Horstemeyer in the chat room with all the other discussion.

I don't think I really read that paper back then either, it's all above my head

@bolbteppa sure, it may not be physically made of something with internal forces. But that doesn't mean we can't come up with a useful model of how it behaves by approximating it as such

@tpg2114 lol its over almost all physicists heads :P

@tpg2114 which reminds me (have brought this up in past on here, but not a lot of reaction) have you ever heard of dimensional analysis?

I just watched Predestination for the second time and damn this movie is mind-blowing.

hello

@NovaliumCompany That one is definitely a cool movie. If you haven't seen it yet and want another layer of mindbending, watch Primer.

...or go for a classic and watch 12 Monkeys (the movie not the series)

@JMac I've always seen Primer in the movie lists but I've always skipped. I'll definitley watch it.

The trailers of Primer look a bit boring, I hope the story is good tho.

So, I haven't had much exposure to ODEs and PDEs. And I'm trying to learn. So I'm looking at the double pendulum example. And in most cases, I see it just like constantly moving around. And some of the articles I read said this is an example of 'chaos' because small variations to the initial conditions like idk yield radically different trajectories. I also know that there's this thing called the butterfly effect, which I think is another example of chaos theory...

But in all these cases it looks like the system just keeps going and going

Is it part of the nature of a chaotic system that it doesn't settle?

@StanShunpike The standard definition of chaos is just what you just said: Small changes in initial conditions produce wildly different outcomes. A standard measure for this would be the Lyapunov exponent, though it's not perfect and there are many others

@ACuriousMind Interesting, I'll have to read about it. I only recently started getting into dynamical systems

My robotics class covered Kalman filters and we were using the concepts for that

I have also seen them in neuroscience with phase space diagrams for the neural spikes

not chaos specifically, dynamical systems

With regards to "settling", I'm not sure what you mean. If you mean whether it "stops moving", then that's just a characteristc of a system without dissipation (if it can't lose its kinetic energy it has to keep going).

so the double pendulum example is one where it doesn't lose kinetic energy?

If you mean whether it starts repeating its motion, then by Poincaré recurrence almost all trajectories in finite systems eventually repeat themselves.

@StanShunpike Depends simply on whether you neglect friction (e.g. with the air) or not. If you do, the ordinary single pendulum also keeps going for ever without losing energy :P

hahahahaha ok true

(I shouldn't have said "kinetic" energy since the kinetic energy is continually converted to/from potential energy in a pendulum)

Yeah I took it to mean both

@ACuriousMind So like, I'm pretty new to phase space plots and analyzing their geometries

A lot of intro ODE books i see don't have them in it

Why is that?

They seem crucial to understanding the behavior of systems.

Because ODEs don't need to specifically be about phase space or dynamical systems, I guess?

so are phase spaces particular to dynamical systems?

@StanShunpike There is a narrow meaning, in which "phase space" is strictly a physical term for the place where Hamiltonian mechanics takes place (formally the cotangent bundle of the configuration space). And there is a broad meaning, in which "phase space" is the space of states for any dynamical system.

I mean, you can take any ODE as defining the time evolution of some dynamical system, but if it doesn't actually represent time evolution, there doesn't seem to be a point to it

Yeah, the hamiltonian one is related to these things called symplectic manifolds right with 2-forms?

I haven't quite worked it out, but I finally figured out how forms and wedge products work last week

And in any case, the stereotypical "ODE book" in my mind is a math book concerned with how to show the existence and uniqueness of solutions to ODEs (or, if applied math, with numeric algorithms approximating these solutions). It is not concerned with any "real-world interpretation" of the ODEs :P

Ah, that makes sense. I've actually been trying to find "ODEs for Physicsts" or neuroscience or whatever, applied contexts where its not so pure math

to avoid that issue

@StanShunpike Yes, although you can (and most do) do basic Hamiltonian mechanics just in terms of position and momenta without ever involving differential geometry.

@ACuriousMind that's the best way to do it to avoid headaches :D :P

@StanShunpike Thing is, as a physicist you often need to do both: Know how to solve the ODE and know how to qualitatively interpret it for your specific system. And I don't think the two things are very much related.

@NovaliumCompany It can definitely be a bit "boring", especially if you aren't invested in the plot, but as far as time travel storys go, it's really good, just a bit hard to follow

@KyleKanos The leagues of users on this site that are confused about how $q,\dot{q},p$ are related would disagree :P

@ACuriousMind I see, that makes sense.

@ACuriousMind I am confused about the term tensor. do people always mean a multilinear map onto R or can it be a multilinear map onto any vector space?

@StanShunpike Depends on who's talking ;)

@ACuriousMind ok great, that's what I thought. Just wanted to double check.

@ACuriousMind in what context did you run across lyapunov exponents?

Physicists and geometers often mean a "tensor field" when they just say "tensor", which is a function on a manifold that assigns a tensor in the sense of multilinear map/element of the tensor product to every tangent space

Yes! I actually get that now. Somehow, I've gotten better at math over time even though I don't study physics continuously

@StanShunpike Our lecturer in theoretical mechanics mentioned them

I never did anything with them, I just know they exist

Gotcha

(the man was a very typical out-of-touch theorist that taught a heavily formal mechanics course in our first two semesters, and I'm eternally grateful to him for the sheer wealth of stuff he did. I can't count the times where I've been able to recognize something from his lectures years later (and finding out it's typically only content of graduate mechanics courses, if at all :P) )

i love teachers like that. i'm in a robotics class right now and, although the guy sometimes is out of touch, he just routinely breaks how i think about things and it's so cool.

people like that just spark creativity and thought

well, at least half of the students hated him because the exercises and exams were tough :P

Yeah I feel that.

He also once asked everyone who thought he went too fast to raise their hand (with his back turned and an assistant counting). 90% raised their hands, and he vowed to go slower

It lasted half a lecture, then his excitement took over again :D

hahahahaha of course

So I've been reading this book, Quantum Field Theory for the Gifted Amateur

and a lot of the equations I've understood best have been Lagrangians involving scalar fields

and, from what I understand, few particles are actually represented with scalar fields

So where did the idea for a scalar field come from?

Like why would someone say "Hmmm a single number at each point in space/spacetime would be a good way to describe nature"

like how is that different from schrodigners equation?

that's not a scalar field, from what i understand

@StanShunpike It's simply the simplest field: Stuff gets more complicated with higher spin since the corresponding spin representation has $2s+1$ dimensions. Choosing $s=0$, i.e. the scalar field, allows us to focus on all the "non-spin" aspects of the theory

so is the schrodinger equation, when taught at the most basic level, an equation of scalar fields?

like with $\psi(x,t)$

@StanShunpike You can write down an action so that the Schrödinger equation is its equation of motion, but in standard QM, it is simply axiomatically the evolution equation for $\psi$, no action behind it.

Wild, didn't realize that it was an axiom.

Hi. How do I contact a mod not just to flag a post but to expose a recurrent rude behavior?

@StanShunpike Well, as always, it's a bit of a thorny issue. If you postulate the path integral formalism, it's not an axiom but derived. See e.g. physics.stackexchange.com/q/142169/50583

@Exocytosis Just flag any post of the offending user with an "other" flag and explain the issue as clearly as you can in the flag text

I am a human being.

@ACuriousMind Thank you.

Yes, we're all humans here. ::beeps surreptitiously::

@Exocytosis Don't choose a comment if possible, but it doesn't really matter

@ACuriousMind some of you are, at least

@ACuriousMind Sorry I just did, because I already flagged their answer simply as not being an answer. Didn't see your line in time.

@Exocytosis no worries

@ACuriousMind romanized i's!?

@KyleKanos Of course! Constants are romanized (you may not agree :P).

Hey

I know $\mathrm e$ should be Roman, but I've never thought $i$ should...

I can accept not romanizing both, but doing it to one and not the other is just needless cruelty

I had quite the trouble convincing coworkers to write $\mathrm{d}t$. I'll wait to convince them of $\mathrm e$

They also like doing things like $e^{\int f(x)dx}$ which bothers me a lot

I solved the problem by no longer working in an environment where people need to write derivatives or exponentials :P

Yeah, that would be a terrible bank to work at, were that the case for me

We're also terrible with consistent notation as well. Some people use $Z(t)$ to mean discount curves, others use $B(t)$, others use $YC(t)$.

wow, this new monitor is huge...

@enumaris So now you have even more space to view your no data on? ;P

hey hey...

I have some open source data I can work on now

also, I'm slowly gonna get access to 'real world' data

nice :)

I have three 24" monitors to work on :D

i need smart friendz.

I'm running low on real estate space, trying to convince people I should get a fourth

I do fine most of the time with two monitors, but about once a week I'd need ten :P

I just waked from a dream in which I am in a very big classroom for the lecture Classical Mechanics taught by Fon-Zen Lin, who is my undergraduate teacher of Theoretical Mechanics. The classroom is so big that I can't see clearly what the teacher wrote on the blackboard when I was in the middle-to-rear position of that room, which looks like the second floor with respect to the blackboard.

I told my classmate that and moved to a position closer to the blackboard. I also clicked a form to confirm my registration of that course because it seems to be a wonderful course and I haven't had the credit of that course.

> Vigorous effort by several top-flight physicists has produced theoretical evidence that networks of entangled quantum states weave the spacetime fabric.

> Roughly speaking, entanglement is the fabric of spacetime (6, 7).

weee

hey @enumaris have you messed with tensorflow 2.0? It actually looks nice

not yet... it looks like a combination of keras and pytorch api lol

0celo was last seen 2 days ago

He lives

yeah I think they consumed keras or something. There was a tensorflow.contrib.keras module or something previously

I'm too much of a casual to have messed with pytorch, though I hear it's nice

it's quite nice

you make a net

which has to inherit from nn.Module

you can make layers

or w/e

all from nn.Module

then you just have to implement an init and a forward and you're done

turn forward into call

and that's...pretty much the "for experts" api lol

ahh interesting. I recall looking at it, but never doing anything interesting

Is that what you're typically using?

the seats of my aforementioned classroom are arranged with each row higher than its immediately precedent row, so the seats at the middle-to-rear position have been like in the second floor with respect to the blackboard due to the big size of that classroom.