The h Bar

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.

3

That program was designed for desktop use, but it works ok on a phone.

Aaron Stevens

2:37 AM

@NovaliumCompany Simulate or stimulate?

@Knight He deleted his answer

Johan Liebert

5:52 AM

@PM2Ring that is really beautiful.

In the mean time you people can give suggestions on this post:

0

A: How to Help the Physics.SE grow (member-wise)? Qualitatively as well as Quantitatively

One 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...

Novalium Company

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

Novalium Company

6:40 AM

If someone here knows whether the idea in the picture above could work, please tell me :P

Slereah

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

For PHYSICAL THEORY

Such that $$\mathrm{PT} : \mathbf{BDP} \overset{CR}{\to} \mathrm{MT}$$

1010011010

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?

Korra

10:11 AM

4

Q: What is a delta potential?

I 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

John Rennie

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.

Slereah

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

Anti Earth

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| ?

Slereah

Isn't it the outer product?

Anti Earth

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

Slereah

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 $\mathcal{H'} \times \mathcal{H} \to \mathbb{C}$

Anti Earth

test of maths: $x^2$

aw

Slereah

31

A: Any chance of MathJax in chat?

As 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...

Anti Earth

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

Slereah

It's entirely fine, although notation will get complicated

Anti Earth

Thanks very much!

Slereah

It's best to give them symbols

1010011010

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

Slereah

I'm not super good at renormalization I'm afraid

This book uses $\mathrm{Pot}(X)$ for the power set of $X$

odd choice

Slereah

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

1010011010

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

Korra

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

John Rennie

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.

Knight

@JohnRennie Sir please tell me the meaning of "The court of Crimson King"

John Rennie

@Knight I don't think it has a meaning.

Knight

@JohnRennie Sir see these lines

>The rusted chains of prison moons
>Are shattered by the sun

John Rennie

I suspect it has no meaning, it was just Fripp writing stuff that seemed interesting.

Knight

Really?

John Rennie

4:47 PM

It could refer to the sun waking you up - freeing you from the chains of sleep I suppose.

Knight

please sir explain it :(

John Rennie

@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.

Knight

I agree.

John Rennie

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.

Knight

But he looks so sad in the song, I mean his voice

Korra

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?

John Rennie

@Korra exactly, yes.

Korra

Can this wave be 'polarized'?

Or does that not effect the outcome

ACuriousMind

there is no notion of polarization for a generic QM wavefunction.

Novalium Company

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?

Novalium Company

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...?

Novalium Company

8:32 PM

Did you guys know that thinking can actually rewire the brain physically? I just learned this.

Novalium Company

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.

Sir Cumference

my mind is already a pile of mush right now so there's not much to lose

Aaron Stevens

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.

JMac

@AaronStevens A lesson most plumbing systems know all too well.

Aaron Stevens

@JMac I knew it was a bad idea to come into chat haha

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