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00:07
Conducting on-site interviews is exhausting... I miss being at my desk and getting work done
00:59
The Legendre polynomial generating function comes from the $1/r$ solution to Laplace in spherical coordinates, the Bessel generating function comes from the $e^{ir \cos \theta} f(\phi)$ solution of Laplace (for a choice of parameter), any ideas about other generating functions and their pde's?
@bolbteppa For something in particular? Or just general interest?
General interest I guess
So I've been doing work with polynomial chaos expansions, which allow you to expand stochastic variables in spectral space using orthogonal polynomials (like Legendre, or Hermite, or whatever). For certain distributions of random variables, there is an optimal polynomial set -- this is the Wiener-Askey PCE: maths-people.anu.edu.au/~jakeman/QuantifyingUncertainty/…
Related to this is the Bernoulli polynomial generating function, which seems to naturally arise in discussing the Zeta function as a side aspect, but it too satisfies a pde given here but it's pretty un-motivated e.g. asking for a series solution of that form but then ignoring it in the solution method (or else assuming the unmotivated relation between Bernoulli polynomials at different orders)
Anyway, all of those polynomials listed have generating functions of various complexity
Etc.
01:11
Yeah, my sense is it's the interval that dictates a lot regarding which polynomials you'd use
The easiest way I've come to think of it -- these polynomial expansions are like a Fourier transform, but instead of sin's, you're using some polynomial function. And just like a Fourier transform, you can represent anything with enough polynomials in the family. But some require far fewer if it's an "optimal" one
What I'm hoping for is that e.g. the Laguerre generating function is a solution of some PDE like Laplace in some coordinate system, the way $1/r$ is a solution of Laplace and leads to Legendre polynomials, same thing happens with Bessel as well
Yeah that's the general way to think about it, it seems though that to define the polynomials historically they used certain specific solutions (like $1/r$) to generate them
That's too mathy for me... I'm an end-user of the polynomials, I don't really know where they come from... heh
And if I can't find a good set of polynomials that represent my data in a few terms, I move on to wavelets
Hypergeometric gives all these things, and it has a generating function, would be incredible to find it's PDE as being some natural model, and special cases of it giving all the other stuff
01:28
Even the associated Legendre has a generating function, and it comes by taking derivatives of the Legendre $1/r$ generating function with respect to $\cos \theta$, I have no idea what that means physically
 
1 hour later…
02:55
@bolbteppa I recommend you consult Whittaker & Watson’s Course of modern analysis, or Morse and Feshbach. W&W very likely has material on numerous generating functions for special functions, and M&F might have the physical intuition.
03:21
@ZeroTheHero yeah W&W do it for Bessel/Legendre but not the others, but M&F say that generating functions are related to green functions of PDE's which is awesome and so the question is doing this for the generating functions
 
2 hours later…
05:12
I visited meta on Chemistry.SE and when I tried to upvote the answers there I got this notice: "You haven't voted on questions for sometime, questions need votes too!". Is this a new feature?
Not the exact words.
 
1 hour later…
06:35
@JohanLiebert I've seen that occasionally. It's been a feature of the SE for several years at least.
 
1 hour later…
07:39
The beginning of my pathetic attempts of generating BakuGANs using a GAN.
 
2 hours later…
09:43
Hello everybody
How can I determine the direction of angular velocity and angular acceleration?
Answering my own question: Angular velocity is the rate of angular displacement about an axis. Its direction is determined by right hand rule.

According to right hand rule, if you hold the axis with your right hand and rotate the fingers in the direction of motion of the rotating body then thumb will point the direction of the angular velocity.
Thank you internet :)
10:06
@ICCQBE you can use right hand thumb rule for determining direction of angular velocity, such that curl finger shows direction in which body rotate and thumb show direction of angular velocity or you use cross product $$\omega =v\cross r$$
Mean you can use either $vsin\theta$ or $rsin\theta$.
Similarly rate of change of angular velocity is $\alpha$ you Can you predict the direction of angular acceleration?
1
A: How to Help the Physics.SE grow (member-wise)? Qualitatively as well as Quantitatively

Richter65I debated whether or not to post this; I'm not a long-time user and have little standing in this community, and the question may be inactive anyway. But while I saw bit and pieces (especially from knzhou), I didn't really see the following perspective stated very clearly, so I thought I'd give i...

@JohanLiebert why you post meta question here?
System will automatically transfer the post here.
Since question is already on meta there no need to link the post here.
But I agree with you.
10:43
@JohanLiebert I don't think you should delete the proposal you made for the Welcome Modal just because it is downvoted. It's a useful data point in the discussion to see what people don't want to see, too
"The type III stress-energy tensor: Ugly Duckling ofthe Hawking–Ellis classification"
Good title
@ACuriousMind Did you change the contents of the ask question sidebar yourself or was it done by staff?
@Loong That was done by staff, I don't think moderators can influence that
Okay. I am asking because we have only a simple standard text on Chemistry.
They didn't ask us about it or anything, maybe they're just working through the sites to give them some first-pass customization and didn't reach Chemistry yet
10:55
Hm. I could ask on meta. I am just making sure that I didn't miss a little edit button somewhere.
If so, I've missed it, too :P
:-)
11:22
@YuvrajSingh... that is a new answer (not a question). I thought it would be good if people know that someone has given their price of advice and if possible discuss with them.
@ACuriousMind done!
11:39
@ACuriousMind as ACM said we can, t post the link of answer or the question in the room, and I do not know why you are not saying up!
If I said anything wrong please correct me.
2 days ago, by ACuriousMind
@YuvrajSingh... it's not forbidden to post question links, we just ask people don't advertise their own questions shortly after they're posted
@YuvrajSingh... no that isn't what he said! He says that we shouldn't advertise our own posts immediately after posting them on main site. Though we do can post questions which we find interesting or are in need of moderation, so that others can see them. :-)
12:00
Math.SE
Physics.SE
Why don't we have a custom badge symbol like math, biology and chemistry have?
2
I mean even though it might not be neccessary then too it would look good and give feeling of uniqueness.
@JohanLiebert Presumably because no one requested it when the site got its design and there was no obvious go-to physics-y symbol the designer might have used on their own
@ACuriousMind by any chance would it be possible to request now? :-)
highly unlikely
12:50
One genre of things I enjoy is "Diagrams for measuring devices by theorists who care nothing for implementation of that device"
Give me more gravitational compasses and Schwarzschild probes, please
13:01
@JohanLiebert I am not sure because when I posted the link it was not mine. And ACM has said me. Then what should I say!
13:19
@bolbteppa Good evening sir, I looked into the Einsten's Original Publication of EPR Paradox claiming that QM in incomplete. Then I looked papers of Bell's Inequality. I'm very much confused now. It seems both incomplete and complete.
13:34
@YuvrajSingh... as of now you might have developed conscience as to what is right and what is wrong to do (on PSE). So do the same, as you feel. :-)
It's generally not a good idea to look at the original papers for controversial issues
It's best to look at some overview of the whole issue written later on
so a random question. I am trying to lookup what makes the charm flavor special, but so far I don't see much stuff that stood out. Strangeness does a lot of interesting things because its conservation under electromagnetic and strong interactions means strange quark containing hadrons tends to decay slower, so I am pretty puzzled why charm quarks do not have perks that stood out more in its selection rule
2
13:54
"Synge placed great emphasis on working things out for oneself, writing that “the lust for calculation must be tempered by periods of inaction, in which the mechanism is completely unscrewed and then put together again. It is the decarbonisation of the mind.”"
14:07
I'm not sure I understand what a "decarbonization of the mind" is supposed to be
@ACuriousMind the act of removing all the carbon atoms from the brain! :-)
14:27
20
A: Why would anyone from Japan be in Northern India in early 1900s?

Lars BosteenThere would seem to be quite a number of possibilities, including: Businessmen Japanese business interests in India were extensive between the two world wars. Putting this together with "In the 1880s, 23 percent of prominent Japanese businessmen were from the samurai class; by the 1920s 35% wer...

"Dentists (aka Spies)" 😂 😂 😂
@ACuriousMind remove the carbon
@Slereah but is that good or bad
It doesn't sound good, but Synge seems to be in favor of it
I guess maybe it's supposed to be like removing the bubble from soda
So that your mind is less fizzy
Charms and charmonium do seemed to show up a lot in the known tetraquark and pentaquark states
14:32
removing the bubbles from soda is a bad thing!
Do you really want bubbles in your brain!
Meanwhile, both charm and bottom seemed to be heavily involved in meson mixing and that mixing ratio is important in the strong CP problem
Maybe thought bubbles are good. I'm not convinced
A decarbonated man
Well I was on the brink of falling asleep, but it seems particle physics have renergise me. Now to use that to finish up some powerpoint!
So here's what I had so far:
Up: Lightest of the quarks. It's mass and flavor mixing parameters are still not very precisely measures. Made up the nucleons with the down quarks
Down: Similar to the up quark. No known unique properties
Charm: Has many charmonium states. Play important role in BBar meson mixing and hence a window to the strong CP problem. Seemed to show up very abundantly in exotic hadron states
Strange: Hadrons containing them tend to decay slower than expected
Top: No known hadronisation. Contribute significant mass content by the Higgs boson
I did not find any research that single out up quarks from down quarks (or vise versa)
probably because they interconvert to each other via exchanging W bosons
15:08
"@geocalc33 you can lift the diffeomorphism of that transformation to the frame bundle
I think it just induces a frame rotation" @Slereah thanks, I'm looking into this notion :)
The set of frames is ~ the set of Lorentz transforms
Well, orthonormal frames
okay I'll read more about it
@Slereah do you know if you can have intersecting spacetimes? transversal spacetimes possibly
what do you mean by intersecting
in the transversal manifold sense
If two spaces intersect, they are not locally Euclidian
15:15
specifically, taking say a spacetime $M$ and a spacetime $S$ and maybe taking $M\cup S$ to be the union of the two spacetimes
so it would fail to be a manifold
ie there's no continuous map between a $Y$ shape and $\mathbb{R}^n$
okay I see!
A simple way to see this : Take any open set around the intersection. Homeomorphisms preserve the number of connected components when a point is removed
Y - the intersection is 3 connected components
$\mathbb{R}$ is only two
so what beast would this be?
It is called a branching manifold
In mathematics, a branched manifold is a generalization of a differentiable manifold which may have singularities of very restricted type and admits a well-defined tangent space at each point. A branched n-manifold is covered by n-dimensional "coordinate charts", each of which involves one or several "branches" homeomorphically projecting into the same differentiable n-disk in Rn. Branched manifolds first appeared in the dynamical systems theory, in connection with one-dimensional hyperbolic attractors constructed by Smale and were formalized by R. F. Williams in a series of papers on expanding...
15:20
Ah wow, that's interesting, never heard of a branched manifold till now
It has a rather narrow use in math
like what? dynamics and hyperbolic attractors?
Foliation related things IIRC
I'm gonna check if anyone studies $M \cup S$ and defines $M \star S$ as the convolution of the two
with $G:= M \cup S$ being a branched manifold
real analytic manifolds are so difficult tho lol
15:48
Hello
When considering the equation for a spiral pendulum, why does not one consider according to the second newton axiom the sum of all forces to be ma = mg - dx in the case of movement
it seems to me that the gravitation force is always neglected
and instead we only see the only force working is the force from the spiral pendulum as if the gravitation forcedoes not exit? why so?
My tutor said (because the gravitation force only cause a change of the zero point) but how do you explain this methematically ?
i guess you call it (spring pendulum) in english
So, some metaphysical relevation:
It appears there really isn't a satisfactory solution to the question of why
Consider this. We have so much particle phenomenology to probe quantum field theory
Using the experiment results, we refined the underlying mathematical models of QFT
Usually, that means new symmetries get introduced, and some old rules get tidied up
But if we pause and think of all of this for a second...
Under metaphysics and pataphysics, every single thing are in a sense, unique and also things
That means, at the ground level, there really is no difference between e.g. spacetime, particles, mathematics etc. when you bin them in terms of philosophical categories
But with that, it means all our particle physics experiments are really doing one thing: Mapping out the QFT landscape
So say many years later, we have a reasonably good model that explains most things (and hopefully by then quantum gravity is solved). But then that still left behind a "why" question:
Why do the QFT landscape have the structures and symmetries it does. Or rather, why do our models with these structures corresponds to how reality interacts with our models via experiments
Sure, one can say "God does it", "the multiverse", "it causes itself" etc. and all that
But in the end, that is only pushing the "why" even further back
Thus it seems, all the question of "whys" are really this following root question:
> Why do things have the structure they do, regardless of how high in physics or metaphysics you go
> and is there really an answer to this root Why question
which ultimately boils down to the following:
> Why does existence itself exists?
16:13
@Secret Most esteemed sir. I can not tell you why existence exists. However i am positive you can help me with my question. I am writing a report and i am stuck at this point. Could you bother taking a look? physics.stackexchange.com/questions/535077/…
1
Q: Friction causing an impulse

DarkeninI have a quick question about impulse supplied by friction during a collision. Given this situation: The small bullet hits and sticks to the ball above the center of mass, causing rotation. If there is friction between the surface and mass M, will the change in momentum in the system on the h...

I feel like the fact that this has a diagram is the reason people thought it was a homework problem
The basic answer is that the gravitational force play no role on the maximum displacement of the pendulum. It only affect where $S_0$ is, but the displacement $Delta S$ depends on the spring force alone since it is that one that is the net force
@ACuriousMind My guess is that he's making an analogy to internal combustion engines, which can get a build-up of carbon in the pistons over time.
@Secret I am aware of this fact. I am looking for a mathematical expression. Since when in motion the gravitation force is obviously still there.
16:18
@PM2Ring That makes a bit more sense than the soda theory :P
So you have $-mg + D(x_0-x_1) = ma_y$ where $ma_y$ is actually consists of a reaction force that balances out $-mg$ and then $D(x_0-x_1)$ is the net force, thus in the free body diagram of the block, $mg$ play no role on the acceleration
@AaronStevens Perhaps? You could try asking (some of) them
@Secret Yes this is more the direction of the explanation my mind is looking for. May i ask you politely for an elaboration of the part of "ma is actually consists of a reaction force that balances out -mg"?
@JohanLiebert As ACM said, that's unlikely. All recent UI changes have been to make the sites more uniform so that they are easier to maintain, and to make it easier to roll out changes across the network, and to make the sites run more efficiently. I know that recently they've been trying to simplify the CSS code, which can help pages to load a little faster.
@Loong I'm sure Yaakov Ellis will be happy to respond to your enquiries on MSE about the updated Ask stuff. It's still very new. And he was very quick to respond to suggestions & constructive criticism when they were working on the changes to the close voting interface & notices.
My fourth pathetic attempt of generating bakugans using a GAN has, again, overfitted :P
16:31
@ACuriousMind :) It's not so much of an issue these days, with improved fuels (& fuel additives) & computer controlled ignition. But in the early days, dealing wirh carbon build-up was a problem.
I have only 45 images so I need to find a way to not overfit the model
@Secret I found a video explaining this. Thanks anyway!
@NovaliumCompany Unfortunately, there isn't an active ML chatroom, AFAIK. But a few of the regulars in the Python room do ML, and we don't mind the occasional ML related question there, as long as you're using Python. I don't know if any of those people use GAN, but there's no harm in asking.
16:50
@PM2Ring Well, there are already a few similar overfitting problems so I'll read online, but yeah, ML is AWESOMEEEE.
gotta go
17:02
Man, the SE app is almost unusable
How can it be so much worse than the mobile site
@SirCumference afaik the app is no longer maintained
while they have actively invested in making the mobile site work better, so that's not really a surprise
17:21
@PM2Ring I don't have a Meta SE account anymore. I should ask on Chemistry Meta.
Yes, the app is no longer maintained or recommended for use. However, the mobile site is also going to be retired, when the responsive design is completed across the network. A few of us have pleaded on MSE to keep the mobile site, but to no avail.
@Loong That should be fine. But take a look at the recent posts on MSE about the updated Ask page design, there might be something relevant already.
Yes, I am doing that right now.
No worries. Catch you later.
@ACuriousMind It's a shame, since having HNQ on my home screen is pretty handy
@PM2Ring Wait, what's going on?
How will they be replacing the mobile site?
17:49
This slow-motion is so beautiful. Especially at 0:27!
18:21
an electron in free space has an infinite plane wave wavefunction. however, to be localized it is expressed as a sum of plane waves of different frequencies.

however, for a particle in a box you calculate density of states. that assumes each state can only have one electron, and the same electron can't occupy multiple states.

so can an electron occupy multiple wavefunction solutions. or no?
18:45
"Instead of a mass point, we will then study the simplest massive fermion, the Dirac field in an inertial and a non-inertial reference frame thus taking care of Synge’s verdict “Newton successfully wrote apple = moon, but you cannot write apple = neutron”. "
@Slereah Man IDK who this Synge guy is or even understand what this quote is about; but Synge should really try to disprove his hypothesis sometime. I just took a piece of paper and wrote "apple = neutron" without a single issue.
Try to eat a neutron now
I can write apple = neutron; doesn't mean I'm crazy enough to believe it. I was just making a joke about the word choice.
I assume when he says "successfully write" he means making a coherent scientific paper supported by evidence; which was probably obvious in the context.
Dunno
How can I trust you actually wrote this
You probably don't need to. I bet if you try the experiment yourself you'd get the same result.
You already managed to type it out, which might even be considered writing by some.
19:09
Hm
How does one describe a curve moving with some gauge force acting on it
Is it $\gamma$ being a curve through the principal bundle, but if so, what would be the action it obeys?
What's the action of a curve moving through an EM field in the gauge picture?
I'm not sure what a "curve moving" is. Are you talking about a charged string or something?
I'm talking about a point particle represented by a timelike curve $\gamma$
If I have $$S = \int \| \dot{\gamma} \|$$
for a free particle
What's the action of a charged particle?
You couple the path to the gauge field, i.e. add a term $A_\mu x^\mu$ to the action
If we take a curve $\gamma : I \to \mathcal{P}$, I think you get the Lorentz law by just taking $\nabla_{\dot{\gamma}} \dot{\gamma} = 0$
But what's the action equivalent to that? You can simply take the same action for gravitational forces by replacing the norm of that vector
Can you also do that for EM?
ie is there a norm on $\mathcal{P}$ such that $\delta S$ gives the Lorentz force
Well I guess on $T\mathcal{P}$, more accurately
Or does that trick only work for the Levi-Civita connection
Wouldn't be that weird, since I think it doesn't even work for a general affine connection
Even nlab considers the relativistic action to consist of two distinct terms, kinetic and gauge, so I don't think there's a straightforward way to package it all into one expression like you want
19:21
heh
A good argument
Is there a particular reason why that trick only works on the Levi-Civitta connection?
How is this trick supposed to work exactly? I'm not exactly sure what your $\nabla$ on $P$ is when you write $\nabla_{\dot{\gamma}}\dot{\gamma} = 0$.
The gauge derivative
And what has Levi-Civita to do with that?
The analogy
Oh, you want to know why the gravitational force law is that but it doesn't work for gauge forces?
19:27
ie why does the LC connection can be expressed as the extremization of a curve, but not any other
is it something something solder form natural bundle etc etc
sure, gravity is always special because it's soldered
Although...
Does it work for Kaluza-Klein?
Probably I would guess
I don't know how much KK differs from EM tho
enough that we don't really use it, I suppose
You can tell this article is nlab because "worldvolume is the real line Σ=ℝ\Sigma = \mathbb{R} or the circle Σ=S 1\Sigma = S^1, the worldline;"
Including the case of a closed timelike curve to remain general
19:52
"which will describe the massless fermionic eld and only pure gravtity."
what is the grav titty
 
3 hours later…
22:35
0
Q: Squelched on meta - Homework policies

jmbI need to understand the actual stance of this place on homework type questions. The actual practice as I have recently seen on the Physics site seems unacceptable to me. A homework tagged question was asked about a fluid mechanics problem with which I happen to have significant experience. An a...


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