The h Bar - 2016-04-29 (page 1 of 2)

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1:10 AM

@AccidentalFourierTransform why the name?

4 hours later…

4:58 AM

@ACuriousMind I just realized that I am not sure the statement "excitations in the field give rise to particles" really means. Can you help me with this concept. I have read a bit on it but am not so sure what I think it means is correct.

@Danu any idea to the question I asked AcuriousMind?

6:00 AM

@HDE226868 No worries, it's a great answer :)

6:14 AM

@AlfredCentauri that sounds like a great way to start your day :-) Though possibly less so if it's cold and raining.

0

Q: What are the calculations specifically needed for High Altitude Ballooning?

I want to know the lifting gas needed to launch the vehicle including the positive lift I want to predict the possible retrieval site I want to know the burst altitude AND MAYBE ALL WITHOUT CALCULATORS I want to do it manually while I'm expanding my knowledge to Physics

newtonian-mechanics thermodynamics forces pressure buoyancy

Too broad?

1 hour later…

7:31 AM

There you go, that's the way to build a high rep on the site - answer the really hard questions :-)

10

A: How is 6W equal to 40W in LED bulbs adverts?

A 40W light bulb has a luminous efficiency of 1.9%. That means only 1.9%, or 0.76W, of the energy consumed by the bulb ends up as visible light. LED bulbs have an efficiency of around 10% - the efficiency depends on the design and can be as high as 15% or as low as 8%. So a 6W LED bulb will prod...

8:03 AM

So I have started reading the Joy of Cats

I now know one theorem of category theory

I can probably stop reading

1 hour later…

9:18 AM

@kevinTahN. It means that the Fourier modes of a free field (whose classical value would denote how much of a given oscillation frequency is "excited", and there is oscillation because the K-G equation is a wave equation) become creation/annihilation operators in the quantum theory.

"If you're seeing this on CS or CS Theory, I believe those sites have been subject to a coordinated attack by students from an Indian university." What.

AccidentalFourierTransform

9:54 AM

@user507974 its a reference to xkcd's cat. Why yours? :-P

10:34 AM

@ACuriousMind Epic :P

Oooh a coordinated attack, how exciting :p

India does not possess a monopoly on bad questions. – Shog9♦

@kevinTahN. I'm willing to have a go at explaining this, based on an entirely amateur knowledge of QFT, but you probably want ACM to spectate :-)

11:14 AM

@ACuriousMind It's fun to have access to the Teacher's Lounge at times like these =)

It's just that we express the field operators as Fourier transforms and we pretend the operator for each term is a particle

It's pretty bogus

They are not at all like particles

except for being eigenstates of momentum

@Slereah well yes, but exactly the same applies to the Schrodinger equation. We solve it for a free particle and get momentum eigenstates, then we claim they are particles.

Also totally bogus, yes

They are excitations in the field, yes

We just happen to call them particles

However in QFT the point is that the operator $a^\dagger_\vec{p}$ has a natural interpretation as the creation of a (completely delocalised) particle of momentum $\vec{p}$.

@JohnRennie I think Slereah's issue is that a completely delocalized object is not exactly what we think of when we say "particle"

11:27 AM

Sorry, but @JohnRennie @Slereah for the Schrodinger operator for a free particle you can make wave packets that don't disperse, AFAIK. These do correspond very nicely to what we expect from particles.

@Slereah ACuriousMind gets annoyed with me when I talk about excitations in a quantum field :-)

you totally can

With a gaussian wavepacket

(Schrodinger himself was fooled into thinking $\psi$ was physical, because of this)

And indeed, the dual usage of particle in both classical and quantum mechanics for rather different concepts leads to much confusion

The gaussian wavepacket also happens to be the form of wavefunction with the smallest uncertainty

Which makes it a nice fit for a "classical" notion of particle

11:29 AM

@Slereah WHAT DOES THAT MEAN? The field is an operator, how can it have "excitations"?

@ACuriousMind @Slereah - see, I warned you :-)

I understand the origin of the analogy with the QHO and a classical wave, but this doesn't make any rigorous sense in the QFT

Well it creates excitation in the expectation value of that operator, jeez :p

@Slereah What? The expectation value of the field is typically zero.

Is it?

What about $\langle 1\vert \phi \vert 1 \rangle$

11:31 AM

Also, is "creates excitation" just a fancy word for "changes"?

@Slereah That is not usually an object one looks at

It adds a Fourier mode :p

BUT AM I WRONG

"Expectation value" isn't the same as "vacuum expectation value"

No, sure $\langle 1 \vert\phi\vert 1 \rangle$ is different from $\langle 0 \vert \phi\vert 0 \rangle$. Why would you call $\vert 1 \rangle$ an "excitation"?

You are allowed to look at the EV of anything!

Its expectation value has an additional Fourier mode that is $\approx e^{-kx}$?

@Slereah The point is that the colloquial term "particles are excitations in the field" should correspond to some property of particle states that shows up when you actually do usual QFT, not rely on some arcane derivation you just do to justify that term and then never use again

Well then you do like I do

Just say that it's all bogus

11:36 AM

On this topic: I still haven't given up on writing a "gifted amateur" level description of vacuum fluctuations. Does a free scalar field have vacuum fluctuations i.e. if I convince myself I understand what the term means for a free scalar field would that be a good place to start?

Yes.

If you measure the energy density of the vacuum at two different times, the result will be different

@Slereah Has it? Not obvious to me. You have $\phi = \int_p a(p) + a^\dagger(p)$, and you can act $a^\dagger$ to the right and $a$ to the left. then everything vanishes except for two overlaps $\langle 1 \vert 0\rangle$ and $\langle 0\vert 1 \rangle$ which should be zero, no?

Can you measure the vacuum energy in a free theory? Doesn't that require an interaction?

Hm

Let's see

@JohnRennie The vacuum energy is a renormalization parameter, you have to "measure" it in order to fix its value.

We usually set it to 0.

11:38 AM

@JohnRennie Go back to Mukhanov's book

But, well, its "measurement" would be to measure the cosmological constant.

The one on QFT in curved spacetime

he discusses the concept of vacuum fluctuations extensively

starting from a quantum harmonic oscillator

It's very nice

But when we talk about a vacuum fluctuation aren't we implicitly considering some system interacting with the vacuum. Like the resistor noise that Daniel Sank mentioned as an example.

$\langle 1 \vert \phi \vert 1 \rangle = \langle 0 \vert a_k \phi a^\dagger_k \vert 0 \rangle = \langle 0 \vert a_k \int d^3p (2E_p)^{-1/2}(a_p e^{ipx} + a^\dagger_p e^{-ikx}) a^\dagger_k \vert 0 \rangle$

@JohnRennie I'm not sure, but I thought we'd settled that "vacuum fluctuation" is just a fancy name for the standard derivation of some observables in the vacuum state.

11:40 AM

@Danu "Introduction to Quantum Fields in Classical Backgrounds" ?

@ACuriousMind what I'm curious about is where did the idea of pairs of virtual particles come from? What author first used that metaphor?

$ \langle 0 \vert\int d^3p (2E_p)^{-1/2}( a_k a_p e^{ipx} a^\dagger_k+ a_k a^\dagger_p a^\dagger_k e^{-ikx}) \vert 0 \rangle = \int d^3p (2E_p)^{-1/2}( \langle 0 \vert a_k a_p e^{ipx} a^\dagger_k \vert 0 \rangle+ \langle 0 \vert a_k a^\dagger_p a^\dagger_k \vert 0 \rangle e^{-ikx}) $

Without any actual knowledge, I blame Feynman.

lol

We've discovered that the Hawking radiation virtual particle analogy way first used by Hawking himself, so that's where it came from.

What I would really like is to understand why the particle pairs analogy was introduced and by whom.

Sounds like a case for History of Science and Mathematics.

11:43 AM

Penrose perhaps?

@Danu: would HSM be a good place to post the question, or is it a bit too nerdy for them?

$ \int d^3p (2E_p)^{-1/2}( \langle 1_k \vert [a_p , a^\dagger_k] \vert 0 \rangle e^{ipx}+ \langle 0 \vert [a_k, a^\dagger_p] \vert 1_k \rangle e^{-ikx}) $

@JohnRennie Yes

@JohnRennie Lol

@JohnRennie "I'm voting to close this question as off-topic because it is too nerdy." ?!?!

$\int d^3p ( \langle 1_k \vert \delta(p-k) \vert 0 \rangle e^{ipx}+ \langle 0 \vert \delta(k-p) \vert 1_k \rangle e^{-ikx})$

11:45 AM

Have you even looked at the site before? :D We've got pretty hardcore mathematicians, at least.

$(\langle 1_k \vert 0 \rangle e^{ikx}+ \langle 0 \vert 1_k \rangle e^{-ikx})$

Hm

You're right

It may be 0

@Slereah Told you :P

@Danu OK you've convinced me :-)

good luck

Odd

You'd think the EV would be the classical value of the field you'd expect

11:47 AM

Btw @ACuriousMind there are some divergences in string theory---in the cosmological constant calculations, I found some (doing my problem sheet)

...but maybe that's just because it's bosonic

Wait

I seem to recall

Chapter... 7 of Peskin?

Doesn't he have an argument why the EV of $\phi$ is 0

@Danu Are those calculations in the effective SUGRA or in the full ST?

Might be vacuum though

@ACuriousMind It comes from the one-loop partition function (and it's not supersymmetric...)

Would the field expectation value be always 0 for all states?

11:50 AM

@JohnRennie : Permission to speak, room owner? Vacuum fluctuations are the spatial equivalent of the little ripplets you see on the surface of the sea. The Casimir effect is real, but it's very weak. Virtual particles are not the same thing, they only exist in the mathematics of the model. It's like you divide a field up into abstract chunks and say each is a virtual particle.

Wait I think for a generic state it wouldn't cancel out

Hm

would that only apply to rigged hilbert space rays

12:05 PM

0

Q: Origin of *particle pair* metaphor for vacuum fluctuations

In any layman level description of vacuum fluctuations in quantum field theory the fluctuations are described as a pair of virtual particles spontaneously appearing then disappearing within some short time determined by the particle energies and the uncertainty principle. However this appears to...

quantum-mechanics

Isn't it from Feynman

IIRC Feynman and Hibbs describes path integral interaction processes as particles

Not quite the same but close

or maybe it's just

Dirac sea

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

I must be just a baseless notion arising from the fevered mind of some popular science writer

:D

Well if you do the unrenormalized vacuum as Feynman diagrams

You do get a lot of virtual pairs

@JohnRennie I just remembered this answer, suggesting the issue is older than QFT and even "new" QM.

The particles out of the vacuum thing is suggested by the vacuum diagrams from the naive partition function in QFT

12:17 PM

@ACuriousMind aha, thanks, I'll have a look for the paper that answer links. However I have to dash now so I'll get back to it later this afternoon.

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

cya later

2 hours later…

user116211

2:34 PM

@yuggib: o/

\o

o/

\o/

user116211

What's the difference between symmetric and invariant?

user116211

Hmm... am reading Motl's answer....

user116211

2:48 PM

Never could I get this how they are different :((

Stupid question, but at the top of qutip.org/docs/2.2.0/examples/me/ex-25.html they write that they use the rotating wave approximation to get rid of the sine term. Personally, I don't really see how this works here. What I'm used to is doing some unitary transformation and dropping the additive frequency exponentials, but those aren't even there in the Hamiltonian to begin with here.

Invariant: an object that does not change with respect to given some modification of one of its characterising features.

Symmetric (in physics): an object that is invariant wrt the representation of a given Lie group.

user116211

@yuggib The former sounds good but the later is beyond my scope :((

user116211

However, here is how Feynman defined in his lectures:

user116211

> a thing is symmetrical if one can subject it to a certain operation and it appears exactly the same after the operation.

user116211

2:57 PM

Hmm... the definition looks more like the definition of invariance above.

user116211

Or am I missing something?

user116211

@BernardMeurer: what's up!

@MAFIA36790 Heya

Not much, you?

user116211

Just got the result of JEE, our admission test for college ;)

How did it go?

user116211

3:08 PM

@BernardMeurer not bad.... qualified for the 2nd phase.

Nice! Congrats mate :)

user116211

:D

When's the next phase?

user116211

@BernardMeurer May 22.

Ugh, that's soon :p

user116211

3:12 PM

yup!

user116211

There are other exams also... I'm revising my old books..... and guess what?

user116211

Problems are still popping out, damn ;(

I'm just waiting for the last 3 results

Damn tired of this ass

user116211

@BernardMeurer That's the most boring part at this stage of life, damn, damn!!

It is, you just wait and there's nothing you can do about it and then it drives you completely nuts

4:03 PM

npr.org/sections/thetwo-way/2016/04/29/476154494/…

Fucking woozles

By the sound of it, that particular weasel won't be chewing through any more power cables :-)

4:55 PM

@ACuriousMind HELP!

hey @BernardMeurer

Hey @Obliv!

@Slereah hi... sounds like solitons =D ... apparently very similar mathematics

@vzn Do you know about symbolic mathematics?

@BernardMeurer uh, yeah?

5:04 PM

What is it?

@Slereah lol saw that on reddit too. funny! looks almost like an apr1 joke or onion story... ps have you ever read that paper? seemingly right in line with your tastes :)

@BernardMeurer "huh?"

all of mathematics is symbolic

@vzn Quoting the great nihilist himself?

it has to do with cs i think @skillpatrol

arranging algebraic expressions with code or something?

@BernardMeurer yes, "speak of the devil"! miss him! this place is (far?) less than ½ as lively/ colorful without him, bless his cynical soul

5:06 PM

Got banned for a bloody month he did

And regarding symbolic maths, I just heard someone say that today but I was shy to ask what it was so I'm asking here :v

@BernardMeurer yep, seems to have an "authority problem" along with all his nihilism, although one might say it goes hand in hand

@BernardMeurer who said what about "symbolic maths"?

AccidentalFourierTransform

I need more than 20 close votes a day :-(

@vzn Dunno, I was eating bread at the makerspace

@BernardMeurer huh? you speak in riddles today

and then someone passed by and said "Oh yes, that library for symbolic maths in python is great"

and I was like "Huh, symbolic maths?"

5:09 PM

@BernardMeurer in this room?

nono, in my real life

Not here in my underground world where I'm basically batman

@BernardMeurer oh RL™, whats that? :P

It's a new game, controls are crap but the graphics are AMAZING

@BernardMeurer heh, have you seen the new movie yet? saw it ~3wk ago, wlid stuff, pretty intense violent... reminds me of spock vs kirk in star trek reboot by Abrams...

@BernardMeurer sounds cool, which platform?

@vzn It's a platform called Earth :p

5:11 PM

@BernardMeurer they probably mean SymPy:

sympy.org/en/index.html

2

Which movie?

@BernardMeurer the person may have been talking about symbol manipulation of math eqns, similar to what happens in mathematica, but not sure, dont know which library.

@JohnRennie That's the bugger! You a genius

@JohnRennie wow! hadnt heard of that cool stuff :)

John's trying to steal the title of Jayzuz away from ACM

5:13 PM

@BernardMeurer ?

Ignore that I didn't sleep well

ACM? speaking of batman :P

He's the real Bruce Wayne

@BernardMeurer makerspace? what are ppl working on?

@vzn I manage a workspace here in Rio yeah

All sorts of stuff! I'm working on arbitrary precision algos, some guys are working on a website project, there's a girl doing her PhD in thermodynamics and someone that uses our workshop for developing their dentistry stuff

5:16 PM

@BernardMeurer sounds cool/ awesome, think everyone should descr their prjs on web site or blog, how about that? :)

We've been considering that, we actually even started it but then the guy in charge of that left it away and it hasn't been picked up by anyone yet

and I still need to un-nullify my website

But the work

Ugh, web programming gives me heeby jeebies

@vzn Well obviously not, because solitons are non-linear

lol its heeby jeebies but ok

@Slereah oh whats a little non linear vs linear between friends :P

there seem to be extraordinary (inter)connections between linearity vs nonlinearity in some soliton dynamics...

@vzn Pff, grammar is overrated :p

@BernardMeurer grammar is ok, its strict capitalization/ punctuation that is overrated!

5:21 PM

I like capitalization, I think it's very little effort for a goo aesthetic effect

punctuation I think only semicolons should exist and then everything is always right

@BernardMeurer aw, you went & corrected it, but the mispelling was so cute... reminds me of my latin sig other trying to get rid of her accent in english yet when its something distinct/ likeable/ even attractive :|

can $n$ be in the negative integers too for $(\mathbb{Z}/n\mathbb{Z})^{\times}$ to be an abelian group under multiplication?

Latin sig?

Ah sig other

yes as you surely know latin women ooh la la, aye carumba!

So I got Schwarze in the mail

5:25 PM

oops maybe ooh la la is french. getting my passionate cultures mixed up :P

That book is a brick

Hahaha @vzn

And I have 0 accent btw

I could be from Minnesota

If I didn't have a tan

@Slereah do you read all those books or just use em as "paperweights" :P

I put them in my library to impress guests

think saw one of your bookshelf pics on here awhile back

@Slereah so does it work?

5:27 PM

Well technically, but most people would be impressed by any book really

They do not care for the finer points of physics literature

@vzn Do you happen to own a copy of The Art of Computer Programming?

what's returned by -1 mod -3? it's -1 right?

Hello!

The Art of Computer Programming?

Hi

5:32 PM

Go with the best

Read your SICP today

Structure and Interpretation of Computer Programs

Structure and Interpretation of Computer Programs (SICP) is a textbook aiming to teach the principles of computer programming, such as abstraction in programming, metalinguistic abstraction, recursion, interpreters, and modular programming. It is widely considered a classic text in computer science, and is colloquially known as the wizard book, due to the wizard on the jacket. It was first published in 1985 by MIT Press and written by Massachusetts Institute of Technology (MIT) professors Harold Abelson and Gerald Jay Sussman, with Julie Sussman. It was formerly used as the textbook of MIT ...

@Slereah Yeah, it's supposedly an amazing book

@Obliv I think that's right? Why don't you try it at the Python interpreter?

i just googled it cuz u guys r so slow

yeah its -1

Yep I checked on the Python shell

yeah I have no reason to believe $n$ is restricted to the positive integers for $(\mathbb{Z}/n\mathbb{Z})^{\times}$ to be an abelian group under multiplication

@BernardMeurer famous/ legendary, but never was a big fan myself of that. did cover SICP in "Software Engr101" (surprise!)

5:39 PM

TAOCP is a bible

@BernardMeurer it has some good stuff

@3075 Are you around I need a canadian

@vzn I'm currently reading about the theoretical/mathematical definition of a computer

user image

It's a nice topic

Is that like

5:41 PM

@Slereah My birthday is tomorrow if you want to give it to me :p

Turing machines

Or Von Neumann architecture

Turing machines

@BernardMeurer there was a recent/ rare knuth interview, liked it. he was questioning P vs NP in a subversive way...

I programmed a Turing machine once

I wrote a Brainfuck interpreter on a Turing machine, too

@Slereah TM code on my blog! =D

5:42 PM

Brainfuck is almost a turing machine :p

No, not really

I mean

It is one

But it's not a universal turing machine

Formalisms, formalisms, I said almost :p

in theory salon, May 21 '14 at 23:21, by vzn

20 questions for Donald Knuth / informit

dl.dropboxusercontent.com/u/19940612/Science/…

If you want the code, btw

@vzn Nice! I'll read it :)

@Slereah Lol, you're a programming goon doing things like that

5:44 PM

@BernardMeurer SICP has been free on internet for ages, do recommend it. happy btd :)

I can't read on a bright screen :(

And thanks :)

Well I wrote it like that because I wrote a TM interpreter with that code

BF? this is starting to remind me of these bozos

The Nineteenth Byte

General discussion for codegolf.stackexchange.com | Guidelines: ...

Bozos hahahaha

That's a funny word

so @Slereah do you have any personal physics prjs/ interests

5:48 PM

Sure

I talk about it all the time

@ChrisWhite Did you buy me a ring already?

user54412

@BernardMeurer Has your family agreed to my terms for the dowry?

Although I am not very good at follow through

?!?

I tend to drop projects a lot

I have a folder with my aborted physics papers

Lemme see

5:50 PM

Which terms? I don't think they care much though :D

@Slereah hey how about a tumblr or wordpress site? that would be more fun surely :)

Let's see

There's the closed timelike curve book

This one is actually pretty advanced

40 pages or so

There's the closed timelike curve in Pauli-Fierz formalism

There's the book I wanted to do

ctcs aka "wormholes" :|

"What the fuck is spin"

I accidentally deleted it

Though it wasn't that far along

CTCs aren't wormholes and vice versa

lol ok sounds not much worse than the neologism spintronics, have blogged about it extensively myself :)

5:52 PM

On the other hand, I had a project to write something about superspheroidal wormholes

Then I found out it was a fool's errand because I had no idea how to start

wow quite the mad scientist there, you really fit right in around here :)

I also tried to find out the torsion experienced by a particle going through a solid

And I tried to solve a scalar field in a Ellis-Bronnikov wormhole

that reminds me of bullet art... found it years ago...

I tried to do an actual intrauniverse wormhole solution

hm, what else

lol this is cool its advanced since last looked into it bulletbouquets.com/t/mothers-day

5:55 PM

Well there's the current project

Finding a path integral on a lattice with boundary conditions

Oh right

I also wanted to find an energy condition violation for $\varphi^4$

are you still doing web dev for $?

It is my curse, yes

I really fucking hate it

join the club :(

> "dont quit your day job"™

Oh right

I also tried to find what linearized gravity looked like in a crystal

My book on causality violation is by the way called "CTCs or something.tex"

thx for the resume

5:59 PM

Its real title is actually "CTCs and shit"

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