The h Bar

Yuvraj Singh...

07:03

Amazing

There's a Lever in Your Ear and It Does Something Amazing

►

user434058

07:53

@ACuriousMind Could you please be kind enough to create a better tag wiki for flux. Thanks! :)

PM 2Ring

08:38

@JingleBells Some Swiss boogie woogie for you: youtu.be/nYPb3dmjg1w

skullpatrol

chat.stackexchange.com/users/37155/john-rennie

John Rennie

@skullpatrol Yes ... ?

skullpatrol

@JohnRennie that^ is an amazingly regular pattern

John Rennie

The votes per day you mean?

skullpatrol

yup

John Rennie

08:49

I think it's just coincidence. I rarely answer questions these days as I'm too busy in the chat. So the only upvotes I get are on old questions.

skullpatrol

Also, your chat appearances.

:^)

Loong

So 1.6 chat messages per week is 1 millirennie?

skullpatrol

7 days a week

JingleBells

@PM2Ring lol thx. You know, I always "Blame it on the boogie"

PM 2Ring

09:22

@JingleBells A good philosophy. :D

skullpatrol

Guru Vishnu

@skullpatrol: Hi. I received your access request to "Particle accelerator". Thanks for your interest! If you have anything to tell me please message me here. That one is protected for storing conversations for easy de-referencing in the future. I use to take a screenshot after the end of the conversation and store it in a word file or bookmark it. It is protected to avoid intermediate messages. Sorry for not accepting your request.

skullpatrol

np, thanks for explaining pal

Guru Vishnu

:-)

Emilio Pisanty

09:42

1

A: Improving Review Queues - Design overview II: Changing review bans and other new features

Thank you for focusing on the filtering system. From what I can see, it's the most under-used feature of the current system, and it could really use some attention. In particular: Allow users to save filters Have the existing filters way up front, together with the button for creating a new one...

↑ in case any of youse want to help support better filtering in the review queues

Luis Miguel Sera que no me amas Original Mix By Mc Jimmy VJ

►

classic.

skullpatrol

niice

@EmilioPisanty one hit wonder?

Charlie

13:05

In the context of particles physics and Feynman diagrams, when we say a particle X "couples" to some other Y particle(s), do we just mean that X and Y are able to interact? As in the photon "couples to" fermions, so the photon is allowed to meet fermions at a vertex on a Feynman diagram?

I don't know any QFT, I'm not so much interested in the origin of the phrase, just if my definition is accurate in this context

Shing

hey guys

I have a question about statistical physics .

When I preparing many ensembles, what exactly is "one of the many possible state" , "states accessible to the system" ?
States that we make no (quantum) measurement? Or actually we can probe more the system without measurement, but we are too lazy to do that, and that's where probability comes in?

*I have read and know what the textbooks said, but I want to know how operationally we make one in real life (at least in principle)

Charlie

"One of the many possible states" I assume is referring to one possible microstate the system can exist in, out of all possible microstates within the phase space

Shing

yes!

Charlie

And "states accessible to the system" means that for instance a particle confined to a container can't be found in a state such that the particle is outside of the container

Shing

yes!

Charlie

13:14

Does that answer your question? I'm not entirely sure what your 3rd line means

Shing

I was thinking the nature of probability in statistical physics.. does probability come from measurement?

or incomplete knowledge regarding quantum state

Charlie

Even classical systems have probability in statistical mechanics

Not the same as the probability in quantum mechanics exactly, but if we freeze a classical gas in time and check what exact microstate the system is is we have a 1/$\Omega$ chance of finding it in one particular state

Shing

@Charlie yeah!! you are right. I see it now. thanks for answering!

Charlie

No problem :)

Emilio Pisanty

13:33

@skullpatrol Luis Miguel?

Luis Miguel

Luis Miguel Gallego Basteri (born 19 April 1970) is a Puerto Rican-born Mexican singer and icon in Latin America, often referred to as El Sol de México (The Sun of Mexico). He is widely regarded by many as the most successful artist in Latin American history, having successfully performed in a wide range of musical styles, including pop, ballads, boleros, tangos, jazz, big band and mariachi. Luis Miguel is also recognized as the only Latin singer of his generation to not crossover to the Anglo market during the "Latin Explosion" in the 1990s.Despite singing only in Spanish, he continued to be the...

hell no.

Yuvraj Singh...

@EmilioPisanty hi sir:-)

Shing

one more question: If I want to know more about the system (since probability in SM comes from incomplete knowledge) , is there any way that I can do so without measurement?

Charlie

I mean there's lots of things you can derive entirely theoretically about the system just from the probabilities

Average quantities for instance

Depends what you're hoping to know

Yuvraj Singh...

I read your papers from your profile

And I have some questions from them

Although my level of knowledge is nothing sir, but I was always wanted to clear some misconception that I have related to that paper, so if you have some time for my silly question, I would be the most happiest one?

@EmilioPisanty

Shing

so suppose I have only knowledge of total energy of three spins under Magnetic field: $-\mu H$

they have three possible states (++-) (+-+) and (-++)

probability comes from incomplete knowledge means I have no idea which one it is

Emilio Pisanty

13:44

@YuvrajSingh... ask away.

Shing

however an actual measurement can tell me or rather force the state to "pick a side"

Charlie

Now you're talking about quantum mechanical probabilities right

Shing

yeah! so I think I was in a false dilemma

Charlie

Someone more qualified will have to answer questions on spin statistics if you have them, I'm not confident I wouldn't accidentally give you bad information

Shing

okay no problem!

thanks anyway!

Charlie

13:50

Might be worth asking a full question on the main site if you have one

no worries :)

user434058

14:33

Yo

John Rennie

14:58

@max hi :-)

max

hi

John Rennie

I'm guessing that you started with the free scalar field, and the particles are the Fock states. Yes?

max

@JohnRennie I might be completely out of my depth here as I have only started learning about QFT about a week ago. My current understanding of creation and annihilation is that it has something to do with the mass-energy equivalence.

I'm essentially wondering how the creation/annihilation operators work big picture

John Rennie

How far into the course have you got?

Are you doing this at university, or is this a self study project?

max

Im self-studying this in order to give an overview presentation on QFT (im in upper 6th)

John Rennie

15:07

Aha, so you're doing your A levels (or would be if they hadn't been cancelled)

max

yeah

John Rennie

Do you know what a simple harmonic oscillator is?

max

I've seen it mentioned, but I don't know much about it

John Rennie

This is going to be a bit tough because you're lacking the basics that are needed to get even a basic idea of how QFT works. As a minimum you really need to have a rough idea of how a quantum simple harmonic oscillator works.

We can talk about that if you want ...

(Why does ACM appear every time I start trying to explain QFT to anyone? It's like trying to explain relativity with Einstein staring at you! :-)

ACuriousMind

Pure coincidence and definitely not a script I've trained to watch this room ;)

John Rennie

15:16

:-)

max

I think I'll have a look into simple harmonic oscillators, currently my understanding of QFT is around how particles are localised vibrations in a quantum field and how the particle's existance is dependant on the energy in the field. What would I need to look into to gain a basic understanding of how creation/annihilaton works in QFT?

Slereah

@ACuriousMind didn't invent QFT

@max How it works in QM, for a start

John Rennie

As a rule QFT is introduced by explaining the free scalar field. This is a very simplified model that can be solved exactly and it gives the states of the field as a set of states called Fock states. The excitations of these states are the particles.

Slereah

also in classical field theories

Try something like classical harmonic oscillator → classic field theory → quantum harmonic oscillator → quantum field theory

Yuvraj Singh...

@EmilioPisanty why overtones of the natural harmonic frequency components result when subject to slightly nonlinear oscillations?

John Rennie

15:21

But the free field has no interactions (that's what free means) and it's the interactions that create and annihilate particles. So while a free field can have particles they don't appear and disappear.

Slereah

(also maybe relativistic quantum mechanics in between)

max

Ok I'll look into that, thanks for your help

John Rennie

@max the trouble is that this is all going to sound like voodoo if you don't have some idea of basic quantum mechanics.

ACuriousMind

not true, voodoo at least makes sense ;P

John Rennie

:-)

Slereah

15:23

That's always the issue

The classical oscillator isn't very exciting

Look kids

A spring!

JingleBells

16:01

Omg I just accidentally deleted a huge chunk of my code and I hit Ctrl + S and I closed the window. My last backup is from 15 days

A) Kill myself B) Kill myself C) Kill myself

I think I'll go with C, seems most reasonable

IT HURTS

SO. F*CKING. MUCH.

AAAAAAAAAAAAAAAAGHHHHHHHHHHHHHHHHH

Pole_Star

ACuriousMind

@JingleBells That's why you should use a version control system, not "backups" ;)

Almost every programmer makes this mistake once. The clever ones don't make it twice.

2

Yuvraj Singh...

Boring

JingleBells

@ACuriousMind aahh.. umbrella after rain. Thanks, what can I do in the future so I don't make this mistake? Maybe github backups something like this?

ACuriousMind

@JingleBells Just using git at all (not necessarily together with github or another host) would have made this mistake reversible

JingleBells

16:14

Okay, I'll use git. It hurts so much, I feel like crying.

I'll be rewriting the code

ACuriousMind

Well, not entirely, you can still shoot yourself in the foot if you don't commit frequently, but commiting frequently is pretty easy

JingleBells

My sibling uses git so I'll ask him for help

Thanks

I thought of making a backup today, but I was like, meh, maybe tomorrow

look at me now

I've learned my lesson. Git, I'm coming

John Rennie

@JingleBells for what it's worth I don't use github for small projects, but I do backup daily to a NAS at my house.

Then from the NAS I backup every second day to Google drive.

This doesn't give me the complete version history that github gives you, but it's enough to recover from the occasional brain fart.

Emilio Pisanty

@YuvrajSingh... that's pretty hard to answer without the full context

John Rennie

16:29

@JingleBells you should bear in mind that:

Emilio Pisanty

But the basic answer is that if you have a nonlinear response, then you are driving your system with monochromatic light, $E(t) = E_0 \sin(\omega t)$, and the response looks something like $P(t) = \chi_1 E(t) + \chi_2 E(t)^2 + \chi_3 E(t)^3 + \cdots$, then the response is going to have terms which look like $\sin^2(\omega t)$ or $\sin^3(\omega t)$, and then there are standard trigonometric identities that tell you that those components can be rephrased as overtones of $\sin(\omega t)$

John Rennie

a) git won't help you if you forget to commit regularly
b) git is the single most hostile bit of software ever written by mortal man

Emilio Pisanty

@JohnRennie (b) is not true.

SVN exists.

however hard git is, SVN is harder.

John Rennie

It is possible, with surprisingly little effort, to corrupt a git repository so thoroughly that only a git sage will be able to recover it.

JingleBells

Can I download the GitHub desktop version and just do manual regular uploads?

John Rennie

16:32

You can, but github is free, so why wouldn't you use github as an offsite backup?

Emilio Pisanty

... though why the system had to settle on an option that is so hard to learn for beginners, when there are hg better options which are much easier to learn, is beyond me

John Rennie

Do you use Windows?

JingleBells

I use windows 10

John Rennie

Then I would use SCM Git.

JingleBells

I mean, I already have 2 commits in GitHub of my code

John Rennie

16:32

git-scm.com

JingleBells

why git-scm and not github?

ACuriousMind

@JingleBells It is worth pointing out that GitHub and git are two different things.

JingleBells

as far as I know, git is the same as github but with commands?

ACuriousMind

GitHub is a hosting site for git repositories. But you can have git repositories without it.

John Rennie

@EmilioPisanty I used SVN for years before I had to switch to git because the project I was working on switched to git. Git is undeniably more powerful than SVN but I found SVN far simpler to use.

JingleBells

16:34

if my pc explodes?

github better?

Emilio Pisanty

@JohnRennie I disagree. For a casual user, SVN is much harder. It starts right off the bat with "OK, sunny jim, which one is your server and which one is your client?"

git is much more chill, and you can shift things around as required

ACuriousMind

GitHub is really just a place to put your git repo in with a lot of whistles and bells for collaborative work on a repo.

Emilio Pisanty

start with a local primary and then shift the primary to a remote once you change your mind, etc

ACuriousMind

If you're just working alone on a project it's not much different from uploading the repo to any other method of file hosting

JingleBells

Isn't it a good idea to download GitHub desktop version and do regular commits there. Why would I use git repos locally? If my PC explodes? I'm the only one working on the project

ACuriousMind

16:37

I have no idea what the "GitHub desktop version" does, honestly

Emilio Pisanty

@ACuriousMind it's just a local GUI client

@JingleBells you are confusing two different products provided by GitHub Inc

they provide (i) the cloud hosting, and (ii) a local GUI client

you can use the cloud hosting they provide using whatever local GUI client you prefer

or, indeed, interfacing directly with a local git back-end over the command line

JingleBells

I'll use their hosting services with their desktop GUI client

Emilio Pisanty

I find their GUI client to be quite uncomfortable, tbh

(I prefer gitkraken and sourcetree where available)

but whatever rocks your boat

JingleBells

Okay, thanks guys. I'mma watch The Girl with the Dragon Tattoo and get back to rewriting the code and most importantly, set up github.

ACuriousMind

I prefer the command line anyway - at least typing an unfamiliar command there actively reminds me that I have no idea what I'm doing :P

doublefelix

17:08

Hey all :) I saw the question on particle creation and annihilation earlier. That brought me here. I was wondering if anyone has some more intution about seeing how decay happens, based on QFT. I have studied QFT, and of course I know the decomposition into annihilation & creation operators of H, but I don't have any physical intuition about how e^itH actually causes particle creation and annihilation.

Can one see the dependence of creation/annihilation on the distance between particles? What kind of physical circumstances increase the likelihood of decay? I can take a guess, but my guesses come from classical intuition, not from the mathematics of QFT

user434058

@ACuriousMind

user434058

9 hours ago, by FakeMod

@ACuriousMind Could you please be kind enough to create a better tag wiki for flux. Thanks! :)

ACuriousMind

@doublefelix The best "intuition" QFT offers you are the Feynman diagrams, just don't take them too literally.

@FakeMod The ping worked the first time. I don't see a need to do so.

doublefelix

The questions i brought up I think are physical questions; if QFT is a complete theory it should have an answer for them, no? Maybe they can be answered with diagrams but how to do so isn't clear to me

ACuriousMind

@doublefelix The "actual" QFT answer is just that decay (or pair creation or whatever else) is just what you get when you apply the time evolution $\mathrm{exp}(-\mathrm{i}Ht)$, and that we can compute the probability of these results pretty well)

I do not believe that physical theories are required to answer "How?" questions with nice stories, see also my little rant in physics.stackexchange.com/a/274374/50583.

doublefelix

17:22

Hmm, maybe I can phrase it in a more experimental way, then. Suppose I have a beam of electrons and a beam of positrons, each directed parallel so that the direction of their motion is the same. The beams are thin, so that the individual particles in the beams are well-separated, and they are well-collimated, so that there isn't too much deviation from the center of the beam. How does the probability of annihilation (e+ e- \to \gamma \gamma) depend on the distance between those parallel beams?

ACuriousMind

@doublefelix That concept (e.g. that the beam shape influences the rate of interaction) is called the "luminosity" of a particle collider setup. E.g. cds.cern.ch/record/941318/files/p361.pdf computes the luminosity for two Gaußian beams as one of its first examples.

But the basic idea is just that you multiply the naive "heads-on" collision cross section from the QFT interaction with the overlap of the beams' spatial probaiblity distributions

user434058