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1:21 AM
@JohanLiebert You can play with that pendulum effect in a JavaScript program I wrote almost 9 years ago, which is hosted on an old friends site. interactiveds.com.au/images/DP.htm The pendulums use perfect simple harmonic motion, so the pattern doesn't deteriorate like it eventually does with real pendulums.
That program was designed for desktop use, but it works ok on a phone.
1 hour later…
2:37 AM
@NovaliumCompany Simulate or stimulate?
@Knight He deleted his answer
3 hours later…
5:52 AM
@PM2Ring that is really beautiful.
In the mean time you people can give suggestions on this post:
A: How to Help the Physics.SE grow (member-wise)? Qualitatively as well as Quantitatively

Johan LiebertOne of the problems that is raised by such users is that in the recent time the quality of the questions has declined, and since bad questions rarely get good answers therefore it is likely to say that the overall content quality is declining. Poor quality questions are usually asked by new use...

6:17 AM
@AaronStevens Stimulate sorry
I'll draw an image of the idea, one sec
Where the wire is pulled by (supposedly) a strong DC motor.
@AaronStevens Actually, simulate, not stimulate :D
6:40 AM
If someone here knows whether the idea in the picture above could work, please tell me :P
3 hours later…
9:47 AM
Apparently when it comes to rigorous physical theory construction, there seems to be two big schools
The L-program and S-program
You can tell when a paper is about that kind of wankery a lot because they use $\mathrm{PT}$ a lot
Such that $$\mathrm{PT} : \mathbf{BDP} \overset{CR}{\to} \mathrm{MT}$$
Hello there anyone here savvy with the path integral formulation of QFT, and symmetry factors?
I'm looking at ERGE in an exercise, and it's asking me to "not forget the symmetry factors"
My particular example considers phi4 theory so the symmetries are quite obvious; but it begs the question what is meant?
I mean, the symmetries may be omitted, since the task at hand is algebraic, no?
10:11 AM
Q: What is a delta potential?

KorraI know how a delta potential is described mathematically but how can a delta potential be a 'well'? Does it have particles outside the 'well' and 'bind' it or does it somehow have particle inside it? Also, if we consider three dimensions then won't the potential be present at only one point in ...

Can someone look at this
The answers I got didn't really answer my question
10:24 AM
@Korra the answer from yu-v is spot on.
Delta potentials don't exist in the real world. They are a convenient mathematical model for a very short range potential.
10:43 AM
@1010011010 Not forget them for what?
Also which symmetries? There's a few at your disposal
Although not all of them are symmetries of the quantum theory
cf. the axial current anomaly
10:56 AM
Hi all - basic question here, since I've mostly just confused myself.
I'm trying to disambiguate 'product' and 'outer product' in the context of bra-ket notation, and Hilbert spaces.

If I have kets |a> and |b>, what operation is performed in constructing |a><b|?
We call the result an "outer product", but it's incorrect to notate this as |a> (otimes) <b| where (otimes) is the tensor/kronecker product (also referred to as the outer product sometimes, confusingly). Instead, if |a> and |b> were vectors, the matrix |a><b| is just constructed from matrix multiplication of |a> and conjugate-trans
So, in the most abstract sense of vectors, what is the operation OP(|a>, |b>) that produces |a><b| ?
Isn't it the outer product?
Hmm let's say |a> and |b> are rank-1 vectors. I can form |a><b| by matrix multiplication. Is this incidentally ALSO the result of an outer-product for these matrices?
Oh yep, indeed it is
I mean I'd ask you to beware of thinking of it as a matrix product, but yeah
What you have is a linear map $\mathcal{H'} \times \mathcal{H} \to \mathbb{C}$
test of maths: $x^2$
A: Any chance of MathJax in chat?

Ilmari KaronenAs a workaround while this request is pending, there exist several client-side workarounds that can be used to enable LaTeX rendering in chat, including: ChatJax, a set of bookmarklets by robjohn to enable dynamic MathJax support in chat. Commonly used in the Mathematics chat room. An altern...

11:01 AM
Interesting that we notate

|a><b| (otimes) |c><d| when there are really FOUR outer-products present - but I guess that's for clarity
It's entirely fine, although notation will get complicated
Thanks very much!
It's best to give them symbols
11:16 AM
@Slereah ERGE is Wilson renormalisation, so we have
All momentum integrals are understood to have a cuto  invariant under d dimen-
sional rotations. We will try to integrate out the momentum shell in K < |k| < K'.
In order to do that break up the action in a sum of terms. The rst involves only inte-
grals up to K. The rest involve at least one integral in K < |k| < K'. Make
sure you get the symmetry factors correctly in front of each term.
sorry, cutoff
It would appear as if what is asked relates to a sort of diagrammatic/PI hybrid approach to extracting the terms
But I don't see why diagrammatics is of any relevance to the question. Writing phi4 in terms of momentum is so simple I don't see any margin for error in just performing the manipulations as is
Oh, and to avoid confusion, the passage "All momentum... each term." is an extract of what is asked
I'm not super good at renormalization I'm afraid
This book uses $\mathrm{Pot}(X)$ for the power set of $X$
odd choice
11:44 AM
Dude on PSE is being very argumentative with me about what constitutes an axiomatization of physics
He is yelling at me for not including axiomatization of measurement and gets angrier when I tell him that's what axiomatization usually means in physics
Poor move, rly
I'd be glad to discuss axiomatization of measurement all day long but I'm not super motivated by this
12:39 PM
@Slereah No problems. Thanks anyway. I'll ask on the main site maybe, and cross fingers that QMechanic has the time and the inclination to formulate an answer :p
3 hours later…
4:05 PM
@JohnRennie but what my question was that if a particle is in a space near it, then why would the Delta potential effect it? From the explanation of yu-v if delta potential can be seen as a well with its width taken to 0, then just like a well it shouldn't effect the particles outside it. I also do not understand how it acts as a mathematical model for a short range force
4:38 PM
@Korra in QM a particle is not a point. A particle is represented by a wavefunction and that wavefunction always extends over a non-zero region of space. So even though a delta potential has a width of zero the wavefunction representing a particle can overlap it. In that sense it can affect a particle that isn't in the well.
Actually this is true of wells with a finite width as well.
@JohnRennie Sir please tell me the meaning of "The court of Crimson King"
@Knight I don't think it has a meaning.
@JohnRennie Sir see these lines

>The rusted chains of prison moons
>Are shattered by the sun
I suspect it has no meaning, it was just Fripp writing stuff that seemed interesting.
4:47 PM
It could refer to the sun waking you up - freeing you from the chains of sleep I suppose.
please sir explain it :(
@Knight when you read poetry it's always dangerous to try and read too much into it. Some poetry is deep and meaningful, T S Eliot springs to mind, but some might just be words that the poet thought sounded good.
I agree.
Robert Fripp is not one of the world's greatest poets, and I suspect he just wrote interesting sounding lyrics without having any meaning in mind.
But he looks so sad in the song, I mean his voice
5:05 PM
@JohnRennie thank you! That's what I missed out! So, if we consider a wave packet it's not spread out completely but once it gets near enough some part of it gets overlapped with the Delta potential and kiss this effected. Is this correct?
@Korra exactly, yes.
Can this wave be 'polarized'?
Or does that not effect the outcome
there is no notion of polarization for a generic QM wavefunction.
5:32 PM
I'm sorry if this is considered spam (I'm posting it for the second time) but I really need to know if there's any chance of this working. This is a rope attached in the following scenario which is pulled by some (supposedly) strong DC motor (or some alternative). Can it also be done for the tricep?
2 hours later…
7:41 PM
Does anyone know if there's a better way to go about moving the arm (bicep and tricep motion-like)? Like, maybe putting a servo on the rotary part...?
8:32 PM
Did you guys know that thinking can actually rewire the brain physically? I just learned this.
8:50 PM
You can practice a song on the piano and I can imagine practicing that song on the piano and our brains will develop in mostly the same ways.
Soo... yeah, be careful what you let into your mind. Be careful what you think cuz the plasticity can physically alter your brain, and you don't want any toxic stuff in there.
my mind is already a pile of mush right now so there's not much to lose
2 hours later…
10:38 PM
@NovaliumCompany Nothing outside a person can defile them by going into them. Rather, it is what comes out of a person that defiles them.
@AaronStevens A lesson most plumbing systems know all too well.
@JMac I knew it was a bad idea to come into chat haha

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